Prediction of trafficability of tile-drained farmland

Paul, Compton Laurence

doi:10.14288/1.0094554

UBC Theses and Dissertations

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UBC Theses and Dissertations

Prediction of trafficability of tile-drained farmland Paul, Compton Laurence 1978

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Title	Prediction of trafficability of tile-drained farmland
Creator	Paul, Compton Laurence
Publisher	University of British Columbia
Date Issued	1978
Description	This thesis is in four parts that report on the development of two procedures for the prediction of trafficability of tile-drained lowland soils in the Lower Fraser Valley of British Columbia. By these procedures one can predict trafficability either from soil water tension in the plough layer or from water table depth. A mathematical model developed in the Netherlands for the simulation of vertical non-steady flow of water in soils was tested by data collected in Spring from farmers' fields. The model predicted depth to the water table and, less accurately, tension in the top 15 cm in fields drained by tiles spaced at 30.5 m and 15.2 m. Cone penetration resistance (an index of soil strength) of soil in the field was found to be linearly dependent upon soil water tension between 0 and 100 - 175 cm of water. It was possible to predict the slope of this relationship for two mineral soils, but not for an organic soil. Trafficability tests with typical farm vehicles were carried out on one organic and two mineral soils at various degrees of wetness. Soil structure was significantly damaged after the first and third passes of the vehicles. The damage was greatest when the soil was near saturation. However, indices of structure could not be used per se as criteria for trafficable conditions. For each soil a relationship was established between soil strength and traction efficiency measured by wheelslip. A critical value of strength for trafficability was then obtained by using 20% wheelslip as a limiting value for traction efficiency. Reference to known strength-tension curves yielded critical tensions for trafficability. Soil strength was linearly dependent upon water table depth in Spring when evapotranspiration was small and when water table depth was less than 80 cm. Critical water table depths for trafficability inferred from this relationship were 53, 45, and 60 cm for Lumbum muck, Hallart silty clay loam (grassland), and Hallart silty clay loam (cultivated land), respectively. The effect of tile spacing on trafficability in Spring was assessed over a 2-year period. Evidence to support increased benefits due to the closer spacing was inconclusive in both soil types. When compared to undrained land the larger spacing of 30.5 m resulted in a significant increase in the number of trafficable days. Tile drainage was twice as important for the attainment of trafficable conditions in the muck as in the mineral soils. Two procedures for predicting trafficability of tile-drained farmland were described. Predictions by these procedures for two tile spacings in both soil types in Spring 1977 were within 3 days of one another and compared extremely well with dates on which measured water table depth and tension were considered adequate for trafficability.
Subject	Drainage -- Research Trafficability
Genre	Thesis/Dissertation
Type	Text
Language	eng
Date Available	2010-03-01
Provider	Vancouver : University of British Columbia Library
Rights	For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use.
DOI	10.14288/1.0094554
URI	http://hdl.handle.net/2429/21258
Degree	Doctor of Philosophy - PhD
Program	Soil Science
Affiliation	Land and Food Systems, Faculty of
Degree Grantor	University of British Columbia
Campus	UBCV
Scholarly Level	Graduate
AggregatedSourceRepository	DSpace

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PREDICTION OF TRAFFICABILITY OF TILE-DRAINED FARMLAND by COMPTON LAURENCE PAUL B.Sc. ( A g r i c ) , U n i v e r s i t y o f t h e West I n d i e s , T r i n i d a d , 1969 M.Sc. ( A g r i c . ) . , U n i v e r s i t y o f t h e West I n d i e s , T r i n i d a d , 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF SOIL SCIENCE We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA March, 1978 © Compton L a u r e n c e P a u l , 1978 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date 6 i i ABSTRACT T h i s t h e s i s i s i n f o u r p a r t s t h a t r e p o r t on t h e development o f two p r o c e d u r e s f o r the p r e d i c t i o n of t r a f f i c a b i l i t y o f t i l e -d r a i n e d l o w l a n d s o i l s i n t h e Lower F r a s e r V a l l e y o f B r i t i s h C o l u m b i a . By t h e s e p r o c e d u r e s one can p r e d i c t t r a f f i c a b i l i t y e i t h e r f r o m s o i l w a t e r t e n s i o n i n t h e p l o u g h l a y e r o r f r o m w a t e r t a b l e d e p t h . A m a t h e m a t i c a l model d e v e l o p e d i n t h e N e t h e r l a n d s f o r t h e s i m u l a t i o n o f v e r t i c a l n o n - s t e a d y f l o w o f w a t e r i n s o i l s was t e s t e d by d a t a c o l l e c t e d i n S p r i n g f r o m f a r m e r s ' f i e l d s . The model p r e d i c t e d d e p t h t o t h e w a t e r t a b l e and, l e s s a c c u r a t e l y , t e n s i o n i n th e t o p 15 cm i n f i e l d s d r a i n e d by t i l e s spaced a t 30.5 m and 15.2 m. Cone p e n e t r a t i o n r e s i s t a n c e (an i n d e x of s o i l s t r e n g t h ) o f s o i l i n t h e f i e l d was found t o be l i n e a r l y dependent upon s o i l w a t e r t e n s i o n between 0 and 100 - 175 cm o f w a t e r . I t was p o s s i b l e t o p r e d i c t the s l o p e o f t h i s r e l a t i o n s h i p f o r two m i n e r a l s o i l s , b u t n o t f o r an o r g a n i c s o i l . T r a f f i c a b i l i t y t e s t s w i t h t y p i c a l f a r m v e h i c l e s were c a r r i e d o u t on one o r g a n i c and two m i n e r a l s o i l s a t v a r i o u s d e grees of w e t n e s s . S o i l s t r u c t u r e was s i g n i f i c a n t l y damaged a f t e r t h e f i r s t and t h i r d p a s s e s o f t h e v e h i c l e s . The damage was g r e a t e s t when t h e s o i l was n e a r s a t u r a t i o n . However, i n d i c e s of s t r u c t u r e c o u l d n o t be used per se as c r i t e r i a f o r t r a f f i c a b l e c o n d i t i o n s . F o r each s o i l a r e l a t i o n s h i p was e s t a b l i s h e d between s o i l s t r e n g t h and t r a c t i o n e f f i c i e n c y measured by w h e e l s l i p . A c r i t i c a l v a l u e of s t r e n g t h f o r t r a f f i c a b i l i t y was t h e n o b t a i n e d by u s i n g 20% w h e e l s l i p as a l i m i t i n g v a l u e f o r t r a c t i o n e f f i c i e n c y . R e f e r e n c e t o i i i known s t r e n g t h - t e n s i o n c u r v e s y i e l d e d c r i t i c a l t e n s i o n s f o r t r a f f i c a b i l i t y . S o i l s t r e n g t h was l i n e a r l y dependent upon w a t e r t a b l e d e p t h i n S p r i n g when e v a p o t r a n s p i r a t i o n was s m a l l and when w a t e r t a b l e d e p t h was l e s s t h a n 80 cm. C r i t i c a l w a t e r t a b l e d e p t h s f o r t r a f f i c a b i l i t y i n f e r r e d from t h i s r e l a t i o n s h i p were 53, 45, and 60 cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) , and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d l a n d ) , r e s p e c t i v e l y . The e f f e c t of t i l e s p a c i n g on t r a f f i c a b i l i t y i n S p r i n g was a s s e s s e d o v e r a 2-year p e r i o d . E v i d e n c e t o s u p p o r t i n c r e a s e d b e n e f i t s due t o t h e c l o s e r s p a c i n g was i n c o n c l u s i v e i n b o t h s o i l t y p e s . When compared t o u n d r a i n e d l a n d t h e l a r g e r s p a c i n g o f 30.5 m r e s u l t e d i n a s i g n i f i c a n t i n c r e a s e i n t h e number of t r a f f i c a b l e days. T i l e d r a i n a g e was t w i c e as i m p o r t a n t f o r t h e a t t a i n m e n t o f t r a f f i c a b l e c o n d i t i o n s i n t h e muck as i n t h e m i n e r a l s o i l s . Two p r o c e d u r e s f o r p r e d i c t i n g t r a f f i c a b i l i t y o f t i l e - d r a i n e d f a r m l a n d were d e s c r i b e d . P r e d i c t i o n s by t h e s e p r o c e d u r e s f o r two t i l e s p a c i n g s i n b o t h s o i l t y p e s i n S p r i n g 1977 were w i t h i n 3 days o f one a n o t h e r and compared e x t r e m e l y w e l l w i t h d a t e s on w h i c h measured w a t e r t a b l e depth and t e n s i o n were c o n s i d e r e d adequate f o r t r a f f i c a b i l i t y . i v LIST OF SYMBOLS d r a i n a g e i n t e n s i t y (day d i a m e t e r o f cone base (cm) e q u i v a l e n t d e p t h o f an impermeable b a r r i e r below d r a i n d e p t h (cm) _2 s o i l c o h e s i o n (kg cm ) -2 c o h e s i o n i n terms o f e f f e c t i v e s t r e s s (kg cm ) a p p a r e n t c o h e s i o n due t o p l a s t i c v i s c o s i t y o f s o i l -2 p r o p o r t i o n a l t o t h e r a t e of l o a d i n g (kg cm ) _2 c o h e s i o n of u n l o a d e d s o i l a t s a t u r a t i o n (kg cm ) a v e r a g e t h i c k n e s s o f t h e w a t e r - t r a n s m i t t i n g l a y e r o f s o i l (cm) d e p t h i n t e r v a l ( z ^ - z^) (cm), p o s i t i v e base o f n a t u r a l l o g a r i t h m _2 s o i l w a t e r t e n s i o n (cm o f w a t e r o r kg cm ) i n i t i a l and f i n a l h e i g h t s o f w a t e r t a b l e above d r a i n d e p t h mid-way between t i l e s a t s t a r t and end of drawdown p e r i o d , r e s p e c t i v e l y (cm) d i m e n s i o n l e s s m o d i f y i n g w e i g h t , c o h e s i o n , and s u r c h a r g e f a c t o r s , r e s p e c t i v e l y s o i l h y d r a u l i c c o n d u c t i v i t y (cm day ^) y i n t e r c e p t of t h e R i j t e m a e x p r e s s i o n (cm day ^) s a t u r a t e d s o i l h y d r a u l i c c o n d u c t i v i t y (cm day ^) number o f l a y e r s w i t h i n w h i c h u n s a t u r a t e d f l o w t a k e s p l a c e V y ' C ' q d i m e n s i o n l e s s m o d i f y i n g w e i g h t , c o h e s i o n , and s u r c h a r g e f a c t o r s , r e s p e c t i v e l y , dependent o n l y on t h e a n g l e o f s h e a r i n g r e s i s t a n c e , <J> N' N r e d u c e d f o r l o c a l s h e a r f a i l u r e c c P I p l a s t i c i t y i n d e x (% d r y w e i g h t ) PR cone p e n e t r a t i o n r e s i s t a n c e o r s o i l s t r e n g t h -2 (kg cm ) S d r a i n s p a c i n g (cm) degree of s a t u r a t i o n ( d i m e n s i o n l e s s ) _2 s s o i l s h e a r s t r e n g t h (kg cm ) t t i m e (day) t ' l e n g t h o f drawdown p e r i o d (day) t ^ , t ^ t i m e s o f measurement o f w a t e r c o n t e n t and t e n s i o n p r o f i l e s (day) v v e r t i c a l f l u x (cm day "*") , n e g a t i v e f o r downward f l o w V Q i n f i l t r a t i o n v e l o c i t y (cm day , n e g a t i v e v v e r t i c a l f l u x between d e e p e s t u n s a t u r a t e d c e n t r e o f n r a l a y e r and t h e w a t e r t a b l e (cm), n e g a t i v e f o r d r a i n a g e v^ v e l o c i t y o f d r a i n a g e d i s c h a r g e (cm day "*") , n e g a t i v e WT ground w a t e r t a b l e d e p t h b e l o w - s o i l s u r f a c e (cm) x h o r i z o n t a l d i s t a n c e (cm) z d e p t h below s o i l s u r f a c e (cm), n e g a t i v e downward v i d e p t h o f cone base i n s o i l (cm), p o s i t i v e downward w a t e r t a b l e d e p t h (cm), n e g a t i v e s o i l s u r f a c e and b o t t o m o f l a y e r , r e s p e c t i v e l y (cm) d e p t h o f c e n t r e o f t h e d e e p e s t u n s a t u r a t e d l a y e r (cm), n e g a t i v e d r a i n d e p t h (cm), n e g a t i v e exponent o f t h e K(h) r e l a t i o n s h i p (cm "*") c o n s t a n t o f i n t e g r a t i o n i n s t r e n g t h - t e n s i o n model (kg cm 2 ) _3 u n i t w e i g h t o f s o i l o r wet b u l k d e n s i t y (kg cm ) r e s p e c t i v e changes i n c o h e s i o n and s o i l w a t e r t e n s i o n due t o s o i l d e f o r m a t i o n w h i l e l o a d e d -2 (kg cm ) t i m e s t e p (day) 3 -3 w a t e r c o n t e n t change i n l a y e r (cm cm ) d e p t h i n t e r v a l (cm), p o s i t i v e 3 -3 s o i l w a t e r c o n t e n t (cm cm ) 3 -3 d r a i n a b l e p o r e space (cm cm ) _3 d r y s o i l b u l k d e n s i t y (g cm ) -2 a p p l i e d t o t a l s t r e s s (kg cm ) _2 e f f e c t i v e s t r e s s (kg cm ) i n t e r n a l , c o n s o l i d a t e d , d r a i n e d , a n g l e o f s h e a r i n g r e s i s t a n c e (deg.) a n g l e o f s h e a r i n g r e s i s t a n c e i n terms o f e f f e c t i v e s t r e s s (deg.) v i i d i m e n s i o n l e s s f a c t o r r e l a t e d t o t h e degree s a t u r a t i o n p r e s s u r e , head a t d r a i n l e v e l midway betwe th e d r a i n s (cm of w a t e r ) , p o s i t i v e (=-h) v i i i TABLE OF CONTENTS Page ABSTRACT i i LIST OF SYMBOLS i v LIST OF TABLES x i i LIST OF FIGURES x i i i L IST OF PLATES x v i ACKNOWLEDGEMENTS x v i i INTRODUCTION 1 CHAPTER 1 - APPLICATION OF A MATHEMATICAL MODEL FOR INVESTIGATING THE INFLUENCE OF TILE SPACING ON SOIL WATER CONDITIONS 5 A b s t r a c t . 6 1. I n t r o d u c t i o n 1 2. The S i m u l a t i o n Model 8 3. L a y o u t and D e s i g n o f E x p e r i m e n t 16 4. Measurement o f Model I n p u t P a r a m e t e r s 19 4.1 S o i l Water R e t e n t i o n C h a r a c t e r i s t i c 19 4.2 Water Fl o w C h a r a c t e r i s t i c 21 4.3 D r a i n a g e I n t e n s i t y 27 5. R e s u l t s and D i s c u s s i o n 29 6. C o n c l u s i o n s 35 7. R e f e r e n c e s 37 i x Page CHAPTER 2 - PREDICTION OF SOIL STRENGTH FROM HYDROLOGIC AND MECHANICAL PROPERTIES 41 A b s t r a c t 42 1. I n t r o d u c t i o n 43 2. T h e o r e t i c a l C o n s i d e r a t i o n s 44 2.1 E v a l u a t i o n o f S o i l S t r e n g t h by t h e Cone P e n e t r o m e t e r 44 2.2 I n f l u e n c e o f S o i l Water T e n s i o n on S o i l S t r e n g t h 45 3. In Situ Measurements o f H y d r o l o g i c and S t r e n g t h R e l a t i o n s h i p s f o r t h e E s t a b l i s h m e n t o f a S o i l S t r e n g t h - T e n s i o n Model 46 3.1 S m a l l - s c a l e F i e l d E x p e r i m e n t s 46 3.2 L a r g e - s c a l e F i e l d E x p e r i m e n t s 50 3.3 R e s u l t s and D i s c u s s i o n 50 4. T e s t o f t h e E m p i r i c a l Model f o r a P h y s i c a l B a s i s . . . 55 4.1 A T h e o r e t i c a l M odel 55 4.2 E v a l u a t i o n o f t h e I n p u t P a r a m e t e r s 57 4.2.1 The a n g l e o f s h e a r i n g r e s i s t a n c e , <f> . . . 57 4.2.2 The f a c t o r x 59 4.2.3 K and N 60 c c 4.3 A n a l y s i s 61 5. C o n c l u s i o n s 64 6. R e f e r e n c e s 65 X Page CHAPTER 3 - EFFECT OF SOIL WATER STATUS AND STRENGTH ON TRAFFICABILITY 70 A b s t r a c t . . . 71 1. I n t r o d u c t i o n 72 2. M a t e r i a l s and Methods 73 2.1 E x p e r i m e n t a l D e s i g n and L a y o u t 73 2.2 T e s t V e h i c l e s 78 3. R e s u l t s and D i s c u s s i o n 78 3.1 E f f e c t o f T r a f f i c on S o i l P h y s i c a l C o n d i t i o n s . . 78 3.2 E f f e c t o f S o i l S t r e n g t h on W h e e l s l i p . . ...... 86 3.3 E f f e c t o f Water T a b l e Depth on T r a f f i c a b i l i t y . . 89 4. C o n c l u s i o n s 93 5. R e f e r e n c e s 94 CHAPTER 4 - INFLUENCE OF TILE DRAINAGE ON SOIL TRAFFICABILITY IN SPRING 98 A b s t r a c t 99 1. I n t r o d u c t i o n 100 2. E x p e r i m e n t a l 101 3. R e s u l t s and D i s c u s s i o n 102 3.1 E f f e c t of. T i l e D r a i n a g e on S o i l T r a f f i c a b i l i t y i n S p r i n g 102 3.2 I n f l u e n c e o f T i l e D r a i n a g e I n t e n s i t y on T r a f f i c a b i l i t y . . . . . 107 3.3 P r e d i c t i o n P r o c e d u r e s f o r T r a f f i c a b i l i t y o f T i l e - d r a i n e d F a r m l a n d 109 4. C o n c l u s i o n s 115 5. R e f e r e n c e s 116 xi Page SUMMARY 118 APPENDIX A - SOIL PROFILE DESCRIPTIONS. 122 APPENDIX B - SOIL PHYSICAL CHARACTERISTICS 125 APPENDIX C - RESULTS OF WATER TABLE RECESSION MEASUREMENTS AT MATSQUI AND SURREY DURING SPRING 1977 128 APPENDIX D - RESULTS OF DIRECT SHEAR TEST MEASUREMENT 130 APPENDIX E - CONVERSION FACTORS FROM MISCELLANEOUS METRIC TO SI UNITS 132 x i i LIST OF TABLES Table Page CHAPTER 1 1. P a r a m e t e r s e v a l u a t e d f o r use i n t h e model ( S t a n d a r d e r r o r o f K measurements g i v e n i n b r a c k e t s ) 26 s a t 2. P r e s s u r e head a t d r a i n l e v e l a t t i l e l o c a t i o n a t M a t s q u i and S u r r e y d u r i n g t h e p e r i o d 1 3 t h - 2 7 t h A p r i l , 1977 36 CHAPTER 2 1. P l a s t i c i t y i n d e x ( P I ) , and a n g l e o f s h e a r i n g r e s i s t a n c e , ((>, f o r 0-15 cm l a y e r . ( S t a n d a r d e r r o r shown i n b r a c k e t s ) . . 58 CHAPTER 4 1. P r e d i c t i o n o f t r a f f i c a b i l i t y o f t i l e - d r a i n e d l o w l a n d s o i l s i n S p r i n g 1977 by u s i n g a w a t e r t a b l e c r i t e r i o n ( p r o c e d u r e 1) and a s o i l w a t e r t e n s i o n c r i t e r i o n ( p r o c e d u r e 2) 113 x i i i LIST OF FIGURES Figure Page CHAPTER 1 1. Schematic representation of the flow of water i n the Wind and van Doorne (1975) model 10 2. Experimental layout 17 3. S o i l water retention c h a r a c t e r i s t i c s of 0-15 cm layer. . . . 20 4. Schematic diagram of gamma r a d i a t i o n attenuation equipment used for measurement of water flow c h a r a c t e r i s t i c s . . . . 22 5. Water flow c h a r a c t e r i s t i c s of Lumbum muck and H a l l a r t s i l t y clay loam within 0-100 cm tension range. The — c t h s t r a i g h t l i n e s represent the Rijtema expression K = K^e Saturated hydraulic c o n d u c t i v i t i e s shown are r e s u l t s of in s i t u measurements by the auger-hole method. S o l i d l i n e s represent f i r s t attempt at f i t t i n g the data 24 6. Comparison of measured and calculated water table depth for 2 t i l e spacings i n Spring 1977 30 7. Comparison of measured and calculated s o i l water tension i n 0-15 cm layer for 2 t i l e spacings i n Spring 1977. (Bars i n d i c a t e standard error > 3 cm) 32 CHAPTER 2 1. Experimental layout for small-scale f i e l d i n v estigations of strength-tension r e l a t i o n s i n Alouette s i l t loam and H a l l a r t s i l t y clay loam 47 2. Gouda type H.S.A.-5 cone penetrometer 49 x i v Figure Page 3. Changes i n s o i l w a t e r t e n s i o n and s t r e n g t h i n A l o u e t t e s i l t loam a t v a r i o u s w a t e r - t a b l e l e v e l s 51 4. Changes i n s o i l w a t e r t e n s i o n and s t r e n g t h i n H a l l a r t s i l t y c l a y loam a t v a r i o u s w a t e r t a b l e l e v e l s 52 5. Dependence of s o i l s t r e n g t h on s o i l w a t e r t e n s i o n i n t h e 0-15 cm l a y e r o f f o u r l o w l a n d s o i l s . ( s t r e n g t h was d e t e r m i n e d by a 60° cone p e n e t r o m e t e r ) 54 6. Comparison of c a l c u l a t e d and measured s l o p e o f s o i l s t r e n g t h - t e n s i o n r e l a t i o n s h i p i n 0-15 cm l a y e r o f f o u r l o w l a n d s o i l s . ( S t a n d a r d e r r o r b a r s a r e r e l a t e d t o measurement o f <j>.) 62 CHAPTER 3 1. P l a n o f t e s t s t r i p r e p r e s e n t i n g one degree o f wetness f o r t r a f f i c a b i l i t y t r i a l s 75 2. E f f e c t s o f t r a f f i c on s o i l p h y s i c a l c o n d i t i o n s i n p l o u g h l a y e r . B u l k d e n s i t y and a e r a t i o n p o r o s i t y were measured b e f o r e t r a f f i c and a f t e r 3 r d p a s s ; r u t d e p t h a f t e r 1 s t and 3 r d p a s s e s . Mean t e n s i o n of 0-15 cm l a y e r (above l i n e ) and w a t e r t a b l e d e p t h (below l i n e ) w i t h i n t h e t e s t s t r i p s a r e shown a t b o t t o m o f d i a g r a m 81 3. Changes i n p o r e s i z e d i s t r i b u t i o n due t o t r a f f i c a t d i f f e r e n t degrees o f w e t n e s s . Numbers i n d i c a t e mean s o i l w a t e r t e n s i o n i n 0-15 cm l a y e r o f t e s t s t r i p s . P o r e d i a m e t e r s were c a l c u l a t e d f r o m w a t e r d e s o r p t i o n d a t a ( V o m o c i l , 1965) . 82 XV Figure Page 4. E f f e c t o f s o i l s t r e n g t h ( b e f o r e t r a f f i c ) on w h e e l s l i p d u r i n g t h e 1 s t p a s s . A w h e e l s l i p o f 20% was u s e d as a c r i t i c a l v a l u e f o r t r a c t i o n . e f f i c i e n c y 87 5. Cone p e n e t r a t i o n r e s i s t a n c e p r o f i l e s i n g r a s s l a n d and c u l t i v a t e d f i e l d s o f H a l l a r t s i l t y c l a y loam. Measurements were made a t s i x l o c a t i o n s i n t h e u n d r a i n e d p l o t s i n m i d - A p r i l 1977 when t h e w a t e r t a b l e d e p t h i n b o t h p l o t s was 16 cm. S t a n d a r d e r r o r b a r s a r e shown f o r measurements below 10 cm d e p t h 90 6. E f f e c t o f w a t e r t a b l e d e p t h on s o i l s t r e n g t h o f 0-15 cm l a y e r . ( R e g r e s s i o n l i n e s a r e f o r WT between 0 and. 80 cm) . 91 CHAPTER 4 1. E f f e c t o f 3 d r a i n a g e i n t e n s i t i e s on w a t e r t a b l e d e p t h midway between t i l e l i n e s and i n u n d r a i n e d p l o t s i n S p r i n g . C r i t i c a l w a t e r t a b l e d e p t h s f o r t r a f f i c a b i l i t y a r e s u p e r i m p o s e d . R a i n f a l l d i s t r i b u t i o n s d u r i n g t h e s a m p l i n g p e r i o d s a r e shown. Dates on w h i c h t h e f i e l d s were p l o u g h e d a r e i n d i c a t e d by a r r o w s 103 2. I n f l u e n c e o f t i l e d r a i n a g e i n t e n s i t y (= t i l e s p a c i n g ) on t h e number o f days r e q u i r e d t o a t t a i n t r a f f i c a b i l i t y f r o m a f u l l y s a t u r a t e d c o n d i t i o n d u r i n g a r a i n l e s s p e r i o d . 108 3. Schematic r e p r e s e n t a t i o n o f p r o c e d u r e s f o r p r e d i c t i o n o f s o i l t r a f f i c a b i l i t y f r o m b a s i c s o i l p r o p e r t i e s 110 xv i LIST OF PLATES Plate Page CHAPTER 3 1. Rut d e p t h measurements i n p r o g r e s s a f t e r 1 s t p a s s a t M a t s q u i s i t e ( c u l t i v a t e d f i e l d ) 77 2. F i e l d m a c h i n e r y - M a t s q u i s i t e ( g r a s s l a n d and c u l t i v a t e d f i e l d s ) 79 3. F i e l d m a c h i n e r y - S u r r e y s i t e 80 CHAPTER 4 1. R e s u l t o f 33 days' d e l a y i n seed-bed p r e p a r a t i o n and p l a n t i n g i n Lumbum muck due t o l a c k o f t i l e d r a i n a g e . Corn p l a n t s on l e f t a r e g r o w i n g i n an a r e a d r a i n e d by t i l e s s p a c e d a t 30.5 m; t h o s e on r i g h t a r e i n u n d r a i n e d a r e a 105 x v i i ACKNOWLEDGEMENTS I w i s h t o e x p r e s s my deep a p p r e c i a t i o n f o r t h e g u i d a n c e , a s s i s t a n c e and f r i e n d s h i p of my s u p e r v i s o r Dr. J a n de V r i e s whose unen d i n g w i l l i n g n e s s t o h e l p and encourage was n e c e s s a r y t o b r i n g t h i s s t u d y t o c o m p l e t i o n . I g r a t e f u l l y acknowledge t h e h e l p and a d v i c e g i v e n by D r s . C. A. Rowles, L. M. L a v k u l i c h , T. A. B l a c k , R. G. C a m p a n e l l a , T. Podmore, P. B y r n e , and Mr. M a r t i n D r i e h u y z e n , who s e r v e d on my d o c t o r a l committee. I t i s a p l e a s u r e t o acknowledge t h e c o o p e r a t i o n o f t h e E n g i n e e r i n g B r a n c h o f t h e B r i t i s h C o l u m b i a M i n i s t r y o f A g r i c u l t u r e i n a l l o w i n g me t o c a r r y o u t t h i s s t u d y on t h e s i t e s o f t h e ARDA Land D r a i n a g e R e s e a r c h P r o j e c t . I n p a r t i c u l a r , M e s s e r s . B r i a n Baehr and M i c h a e l Hoare who were a l w a y s r e a d y t o a s s i s t w i t h t he f i e l d o p e r a t i o n s and t o s u p p l y w h a t e v e r h e l p o r i n f o r m a t i o n t h a t was r e q u i r e d . A l l o w i n g me t h e use of t h e i r 1976 w a t e r t a b l e d a t a i s g r a t e f u l l y acknowledged. S i n c e r e t h a n k s a r e due t o famers G e r r y H o l t , A l b e r t H a l l , and S t e v e and Douglas de Jong f o r t h e i r a s s i s t a n c e and t o t h e P i t t P o l d e r Company f o r e x t e n d i n g t o me t h e use of i t s f a r m l a n d f o r t h e s t u d y on A l o u e t t e s i l t loam. I am g r e a t l y i n d e b t e d t o Dr. N. N a g p a l and t o M e s s e r s . R. Bhim, A. P i e d r a h i t a , H. F e l d e r h o f , M. G r e v e r s , C. W a l l i , and K. L i p i n s k i f o r t h e i r h e l p i n t h e f i e l d phase o f t h i s s t u d y . x v i i i The a s s i s t a n c e of Mr. B e r n i e von S p i n d l e r i n t h e d r a f t i n g and p r e p a r a t i o n o f t h e diagrams and p l a t e s and t h a t o f J o y c e H o l l a n d s f o r t y p i n g t h e t h e s i s i s d e e p l y a p p r e c i a t e d . Thanks a r e e x p r e s s e d t o t h e B r i t i s h C o l umbia M i n i s t r y of A g r i c u l t u r e f o r f i n a n c i a l s u p p o r t and t o t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a f o r t h e award o f a g r a d u a t e f e l l o w s h i p i n t h e 1977-'78 academic y e a r . F i n a l l y , the u n d e r s t a n d i n g and p a t i e n c e shown by my w i f e , J e n i f e r , and my sons t h r o u g h o u t t h e c o u r s e of my s t u d i e s a r e a p p r e c i a t e d w i t h deep a f f e c t i o n . 1 INTRODUCTION S o i l t r a f f i c a b i l i t y may be d e f i n e d as t h e a b i l i t y o f s o i l t o s u p p o r t t r a f f i c w i t h o u t b e i n g damaged s t r u c t u r a l l y beyond l i m i t s f o r good c r o p growth. The t r a f f i c a b i l i t y o f s o i l i s c o n s i d e r e d adequate i f t h e s o i l has s u f f i c i e n t s t r e n g t h i n terms o f b e a r i n g c a p a c i t y and t r a c t i o n c a p a c i t y t o s u p p o r t a p a r t i c u l a r v e h i c l e and e n a b l e i t t o d e v e l o p t h e f o r w a r d t h r u s t n e c e s s a r y t o overcome i t s r o l l i n g r e s i s t a n c e ( G i l l and Vanden B e r g , 1968). P e r f o r m i n g f a r m i n g o p e r a t i o n s when a s o i l i s u n t r a f f i c a b l e o f t e n r e s u l t s i n s t r u c t u r a l damage, low t r a c t i o n e f f i c i e n c y w i t h l o s s o f e n e r g y , and equipment becoming s t u c k . T i m e l i n e s s i n p e r f o r m i n g f a r m i n g o p e r a t i o n s i s one i m p o r t a n t consequence of i m p r o v i n g t r a f f i c a b l e c o n d i t i o n s . I t has a d i r e c t b e a r i n g on j o b e f f i c i e n c y , t h e number o f a v a i l a b l e workdays, r i s k c o n c e n t r a t i o n , and A g r i c u l t u r a l p r o d u c t i v i t y . T i m e l i n e s s i s a major o b j e c t i v e o f d r a i n a g e . D r a i n a g e of e x c e s s w a t e r f r o m a s o i l i mproves i t s s t r e n g t h and hence i t s t r a f f i c a b i l i t y (Reeve and Fausey, 1974). T r a f f i c a b i l i t y i s an i m p o r t a n t c r i t e r i o n i n t h e d e s i g n o f d r a i n a g e systems i n A g r i c u l t u r e . M o r e o v er, t h e p r e d i c t i o n of t r a f f i c a b i l i t y can s e r v e as an i m p o r t a n t t o o l f o r t h e p l a n n i n g of d r a i n a g e schemes on a r e g i o n a l l e v e l . The c u r r e n t s t a t e o f t r a f f i c a b i l i t y p r e d i c t i o n i n t h e w o r l d t o d a y c a n be summed up i n t h e c o n c l u d i n g remarks of t h e Chairman o f a N a t i o n a l C o n f e r e n c e on " t h e P r e d i c t i o n o f F i e l d Workdays" h e l d i n E n g l a n d j u s t r e c e n t l y : "... i t seems that we 2 have a long way to go before we can achieve -prediction of workdays with any degree of accuracy" (Agr. Eng., 1977). I n 1974 a p r o j e c t t o i n v e s t i g a t e and d e m o n s t r a t e t h e o v e r a l l b e n e f i t s of s u b s u r f a c e t i l e d r a i n a g e i n l o w l a n d a r e a s o f t h e Lower F r a s e r V a l l e y o f B r i t i s h C o l umbia was s e t up under terms of t h e F e d e r a l - P r o v i n c i a l A g r i c u l t u r a l and R u r a l Development A c t Agreement (ARDA). C o o p e r a t i o n between t h e U n i v e r s i t y of B r i t i s h C o l u m b i a and ARDA r e s u l t e d i n a two-year s t u d y (1975-1977) of t h e s o i l t r a f f i c a b i l i t y a s p e c t s o f t h e p r o j e c t i n t h e m u n i c i p a l i t i e s o f M a t s q u i and S u r r e y . The s t u d y was s u p p o r t e d by g r a n t s from t h e B r i t i s h C o l umbia M i n i s t r y o f A g r i c u l t u r e and a g r a d u a t e f e l l o w s h i p from t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a . T h i s t h e s i s r e p o r t s on t h i s s t u d y w h i c h has as i t s o v e r a l l o b j e c t i v e s (1) t h e development of p r o c e d u r e s f o r t h e p r e d i c t i o n o f s o i l t r a f f i c a b i l i t y o f t i l e - d r a i n e d l o w l a n d s o i l s i n S p r i n g and (2) t h e e v a l u a t i o n of t h e e f f e c t s o f t i l e d r a i n s i n s t a l l e d a t two s p a c i n g s on t r a f f i c a b i l i t y . C h a p t e r 1 r e p o r t s on t h e t e s t o f a m a t h e m a t i c a l model f o r the s i m u l a t i o n o f t h e t r a n s i e n t - s t a t e d r a i n a g e p r o c e s s by f i e l d d a t a c o l l e c t e d i n S p r i n g 1977. Problems o f e v a l u a t i n g i n p u t d a t a from b a s i c s o i l p r o p e r t i e s and s e n s i t i v i t y o f o u t p u t t o s u c h d a t a a r e d i s c u s s e d . The model was seen as a t o o l f o r t h e p r e d i c t i o n o f w a t e r t a b l e d e p t h and s o i l water, t e n s i o n i n t h e p l o u g h l a y e r i n S p r i n g . C h a p t e r 2 d e a l s w i t h t h e development of a s e m i - e m p i r i c a l model t h a t r e l a t e s s o i l s t r e n g t h , r e p r e s e n t e d by cone p e n e t r a t i o n 3 r e s i s t a n c e , t o t e n s i o n i n t h e p l o u g h l a y e r . T h i s model s e r v e s t o e s t a b l i s h a t r a f f i c a b i l i t y l i m i t i n terms o f s o i l w a t e r t e n s i o n . I n C h a p t e r 3 t h e o b j e c t i v e s were (1) t o examine t h e e f f e c t s o f t r a f f i c a t v a r i o u s d e grees o f w etness on s o i l s t r u c t u r e , (2) t o e s t a b l i s h a r e l a t i o n s h i p between s o i l s t r e n g t h and t r a c t i o n e f f i c i e n c y as measured by w h e e l s l i p , and (3) t o d e t e r m i n e t h e l e v e l o f d r a i n a g e t h a t i s adequate t o a l l o w s o i l s t o e f f i c i e n t l y s u p p o r t f a r m t r a f f i c t y p i c a l l y e n c o u n t e r e d i n t h e Lower F r a s e r V a l l e y . These o b j e c t i v e s were r e a l i z e d by c a r r y i n g out t r a f f i c a b i l i t y t e s t s w i t h f a r m e r s ' equipment a t v a r i o u s d e grees o f w e t n e s s . T r a f f i c a b i l i t y l i m i t s and t h e f a c t o r s w h i c h i n f l u e n c e them a r e d i s c u s s e d . I n C h a p t e r 4 t h e e f f e c t s o f t i l e d r a i n a g e a t two s p a c i n g s a r e e v a l u a t e d i n terms o f s o i l t r a f f i c a b i l i t y i n S p r i n g 1976 and 1977 u s i n g t h e w a t e r t a b l e c r i t e r i o n s e t i n C h a p t e r 3. Recommendations a r e made w i t h r e s p e c t t o t i l e s p a c i n g r e q u i r e d f o r e a r l y t r a f f i c a b i l i t y i n S p r i n g . E f f e c t o f d r a i n a g e i n t e n s i t y on t r a f f i c a b i l i t y i s i n v e s t i g a t e d by s i m u l a t i n g w a t e r t a b l e drawdown c u r v e s w i t h a range o f chosen d r a i n a g e i n t e n s i t i e s . F i n a l l y , t h e development of two p r o c e d u r e s f o r p r e d i c t i n g t r a f f i c a b i l i t y b ased on t h e e n t i r e s t u d y i s d i s c u s s e d and t h e p r o c e d u r e s compared. References A g r i c u l t u r a l E n g i n e e r . 1977. J o u r , and P r o c . I n s t . Agr. Engs. V o l . 32, No. 4: 107. 4 G i l l , W. R. and G. E. Vanden B e r g . 1968. S o i l dynamics i n t i l l a g e and t r a c t i o n . Hdbk. 316 A g r . Res. S e r v . USDA. 516 pp. Reeve, R. C. and N. R. Fausey. 1974. D r a i n a g e and t i m e l i n e s s o f f a r m i n g o p e r a t i o n s . In: J a n van S c h i l f g a a r d e ( e d . ) . D r a i n a g e f o r A g r i c u l t u r e . A g r o n. Monograph. 17. pp. 55-66. Amer. Soc. A g r o n . , M a d i s o n , Wis. 5 CHAPTER 1 6 APPLICATION. OF A MATHEMATICAL MODEL FOR INVESTIGATING  THE INFLUENCE. OF TILE SPACING ON  SOIL WATER CONDITIONS ABSTRACT A m a t h e m a t i c a l model d e v e l o p e d i n t h e N e t h e r l a n d s f o r t h e s i m u l a t i o n o f v e r t i c a l n o n - s t e a d y f l o w o f w a t e r i n s o i l s was t e s t e d by d a t a c o l l e c t e d i n S p r i n g f r o m two f a r m e r s ' f i e l d s i n t h e Lower F r a s e r V a l l e y . The model was fou n d t o be u s e f u l f o r p r e d i c t i n g d e p t h t o the w a t e r t a b l e and, t o a l e s s e r e x t e n t , t e n s i o n i n t h e t o p 15 cm i n f i e l d s d r a i n e d by t i l e s spaced a t 30.5 m and 15.2 m. However, t h e r e were p r o b l e m s i n e v a l u a t i n g i n p u t d a t a . The most d i f f i c u l t p r o b l e m was t h a t o f o b t a i n i n g t h e d r a i n a g e i n t e n s i t y w h i c h was fou n d t o have a g r e a t e f f e c t on model o u t p u t . A s o l u t i o n o f t h e m o d i f i e d Glover-Dumm t r a n s i e n t - s t a t e d r a i n a g e e q u a t i o n p r o v i d e d a means o f o b t a i n i n g d r a i n a g e i n t e n s i t y d i r e c t l y f r o m w a t e r t a b l e r e c e s s i o n . E v a l u a t i o n o f K and a f r o m t h e w a t e r f l o w c h a r a c t e r i s t i c measured o on u n d i s t u r b e d columns o f s o i l i n t h e l a b o r a t o r y was n o t v e r y s u c c e s s f u l ; o n l y p a r t o f t h e d a t a a p p e a r e d t o d e s c r i b e u n s a t u r a t e d f l o w i n t h e f i e l d . 7 1. INTRODUCTION The c o s t o f d r a i n i n g l a n d by s u b s u r f a c e means i s i n v e r s e l y r e l a t e d t o d r a i n s p a c i n g . The economic r e t u r n s t o be a c h i e v e d from a d r a i n a g e system a r e dependent upon c o s t / b e n e f i t r a t i o s o f t h e system. Bouwer (1974) has f o r e s e e n t h e need t o o b t a i n adequate i n f o r m a t i o n on t r a f f i c a b i l i t y i n r e l a t i o n t o s o i l w a t e r s t a t u s ' as a means o f c a l c u l a t i n g t h e maximum economic r e t u r n s f r o m t h e d e s i g n of d r a i n a g e s y s t e m s . F o r a g i v e n s o i l , changes i n s t r e n g t h a r e a s s o c i a t e d a l m o s t e n t i r e l y w i t h changes i n s o i l w a t e r s t a t u s . Boersma et al. (1970) c o n c l u d e d t h a t i f t h e w a t e r s t a t u s o f a s o i l was p r e d i c t a b l e i t s c a p a c i t y t o w i t h s t a n d v e h i c u l a r t r a f f i c c o u l d be p r e d i c t e d f r o m known r e l a t i o n s between s o i l w a t e r s t a t u s and s t r e n g t h . The p r e d i c t i o n o f s o i l w a t e r s t a t u s a f f e c t e d by t i l e d r a i n a g e systems has been t h e o b j e c t i v e o f a number o f r e s e a r c h s t u d i e s i n c l u d i n g t h o s e of Bouwer and v a n S c h i l f g a a r d e , 1963; Hoffman and Schwab, 1964; V a i g n e u r and J o h n s o n , 1966; T a y l o r and W a t t s , 1967; Boersma et al., 1970; Young and L i g o n , 1972; Skaggs, 1974; and Wind and v a n Doorne, 1975. I t a p p e a r s t h a t t h e p r o b l e m o f p r e d i c t i n g s o i l w a t e r s t a t u s r e q u i r e s knowledge of c e r t a i n s o i l p r o p e r t i e s , boundary c o n d i t i o n s o f t h e s o i l , and w e a t h e r d a t a s u c h as r a i n f a l l . However, l a c k o f u n d e r s t a n d i n g o f t h e i n t e r a c t i o n s o f t h e s e v a r i a b l e s and t h e i r e f f e c t on s o i l w a t e r s t a t u s has impeded p r o g r e s s i n o b t a i n i n g s o l u t i o n s . 8 The p r e d i c t i o n method o f Wind and van Doorne (1975) seemed t o o f f e r p o s s i b i l i t i e s f o r t r a f f i c a b i l i t y p r e d i c t i o n because i t n o t o n l y p r e d i c t s d e p t h t o t h e w a t e r t a b l e but a l s o c a l c u l a t e s w a t e r t e n s i o n i n t h e t o p s o i l . A l s o , t h e model has been s u c c e s s f u l l y a p p l i e d by Wind (1976) t o t h e c a l c u l a t i o n o f w a t e r s t a t u s i n a t o p s o i l o v e r a p e r i o d of 23 y e a r s . The o b j e c t i v e o f t h e s t u d y d e s c r i b e d i n t h i s c h a p t e r was t o t e s t the model under f i e l d c o n d i t i o n s f o r a s i l t y c l a y loam w i t h p e a t s u b s t r a t u m and an o r g a n i c s o i l o f v a r i a b l e d e p t h o f d e p o s i t i o n ( A ppendix A ) . The model was d e v e l o p e d f o r homogeneous s o i l s . I t s use i n t h i s s t u d y was b a s e d on t h e s i m p l i f y i n g a s s u m p t i o n t h a t t h e s o i l s were homogeneous i n t h e r e g i o n o f u n s a t u r a t e d f l o w . I f f o u n d a p p l i c a b l e t h e n i t s use c o u l d be an i m p o r t a n t i n p u t f o r t h e o v e r a l l p r e d i c t i o n o f t r a f f i c a b i l i t y o f t i l e - d r a i n e d f a r m l a n d i n t h e Lower F r a s e r V a l l e y and o t h e r a r e a s w i t h s i m i l a r s o i l and c l i m a t i c c h a r a c t e r i s t i c s . 2. THE SIMULATION MODEL The model used was t h a t o f Wind and van Doorne (1975). I t i s b ased on n o n - s t e a d y v e r t i c a l u n s a t u r a t e d f l o w f r o m t h e s o i l s u r f a c e t o t h e w a t e r t a b l e . Below t h e w a t e r t a b l e d i s c h a r g e t o t h e d r a i n s i s by s a t u r a t e d h o r i z o n t a l f l o w . The a n a l y s i s i s c a r r i e d out f o r a s o i l p r o f i l e midway between two p a r a l l e l d r a i n s . The model i s a p p l i c a b l e d u r i n g p e r i o d s when e v a p o t r a n s p i r a t i o n i s s m a l l enough t o be i g n o r e d . I t c a l c u l a t e s t h e m o i s t u r e c o n d i t i o n i n a s o i l a t e v e r y d e p t h and t i m e by c o m b i n a t i o n o f t h e s t o r a g e f u n c t i o n ( water r e t e n t i o n c h a r a c t e r i s t i c ) , and t h e t r a n s p o r t f u n c t i o n ( f l o w c h a r a c t e r i s t i c ) o f u n s a t u r a t e d s o i l w i t h a d r a i n a g e f u n c t i o n (Wind, 1975). The a n a l y s i s w h i c h f o l l o w s i s t a k e n from Wind and van Doorne (1975). I t i s p r e s e n t e d h e r e i n a s i m p l i f i e d f o r m as an a i d t o the i n t e r p r e t a t i o n o f t h e r e s u l t s o f t h e s t u d y d e s c r i b e d i n t h i s c h a p t e r . F i g . 1 i s a s c h e m a t i c r e p r e s e n t a t i o n o f t h e f l o w o f w a t e r i n t h e model. W i t h i n t h e u n s a t u r a t e d l a y e r , t h e v e r t i c a l f l u x , v , o v e r a chosen d i s t a n c e , d, between t h e c e n t r e s o f c o n s e c u t i v e l a y e r s of e q u a l t h i c k n e s s i s g i v e n by D a r c y ' s e q u a t i o n f o r v e r t i c a l f l o w , viz: where v i s n e g a t i v e f o r downward f l u x and z i s n e g a t i v e i n t h e downward d i r e c t i o n . The v a l u e of K t o be used i n e q u a t i o n (1) i s o b t a i n e d from t h e R i j t e m a (1965) e x p r e s s i o n w h i c h i s v a l i d o n l y w i t h i n a t e n s i o n range o f a p p r o x i m a t e l y 20 - 200 cm.. T h i s l i m i t s use o f t h e model t o wet c o n d i t i o n s . The r e l a t i o n between s o i l h y d r a u l i c c o n d u c t i v i t y , K, and t e n s i o n , h, w i t h i n t h i s range i s e x p r e s s e d by: (2) 10 d = Az o -z„ - z , y 2 v 2 - V ]_ -At' Az Boundary between l a y e r s C e n t r e o f l a y e r -z n-1 -z n -z : 8 V = V D n Ae n - l V V - V T n n n - l At Az IMPERMEABLE BARRIER T I L I 7T7777777 \ //////S///////////////7/////////////SJ/SS'/f FIG. 1: S c h e m a t i c r e p r e s e n t a t i o n o f f l o w o f w a t e r i n t h e Wind and van Doorne (1975) model. 11 The v e r t i c a l f l u x , v, i s o b t a i n e d by assuming h t o be a d i f f e r e n t i a b l e f u n c t i o n o f z and t h a t v i s c o n s t a n t o v e r t h e d e p t h i n t e r v a l , d, d u r i n g t h e t i m e i n t e r v a l , A t , used i n t h e model. F i r s t , d i f f e r e n t i a t i n g (2) w i t h r e s p e c t t o z y i e l d s : dK Tjr -ah dh — = K e • - - a - j — dz o dz dz S o l v i n g f o r dh/dz and s u b s t i t u t i n g i n t o (1) g i v e s dK 1- av + aK = 0 dz The s o l u t i o n o f e q u a t i o n (3) f o r boundary c o n d i t i o n s (z = z^, K = K 1 ) and (z = z^, K = K 2 ) i s K2 - K l v i = 7 \ " K, 1 ea ( z l " z2> _ ! 1 ad _ ~1 E q u a t i o n (4) a l s o governs t h e f l u x , v , between t h e o s o i l s u r f a c e and t h e c e n t r e o f t h e uppermost l a y e r . I t i s assumed t h a t d u r i n g r a i n f a l l t h e r e i s a v e r y t h i n s a t u r a t e d l a y e r a t t h e s o i l s u r f a c e as a r e s u l t o f s t r u c t u r a l i n s t a b i l i t y , c r u s t i n g and s w e l l i n g 12 of the s o i l at the surface. Therefore, during r a i n f a l l , the value of the s o i l water tension at the surface i s set at zero. As water flows from the centre of one layer to that of the next at a constant r a t e , v, storage or release of water takes place within the layers according to the equation of continuity, 9 9 a V dV 3t 8z (5) From the differ e n c e of two fluxes at the boundaries between the layers the change i n water content, AG^, i s given by AG = (v - v , , ) ^ f i = l , 2 , . . . . n (6) i to i - 1 where n i s the number of layers within which unsaturated flow takes place. Every change i n water content causes a change i n tension, so both p o t e n t i a l gradient and hydraulic conductivity change. As water moves down the p r o f i l e , there i s a f l u x , v , n between the deepest unsaturated centre (depth z ) and the water table (depth z o ) . Applying equation (4) y i e l d s K - K v = — i 2 _ K ( 7 ) n e c * ( z n - Z g ) _ ± n A saturated flow, Vp, with the same v e l o c i t y e x i s t s between the water table (z = z g , h = 0, K = K G = K q ) and the drainage depth 13 (z = z D > h = ~ ^ D ) • Because h = 0 a t t h e w a t e r l e v e l i n t h e d r a i n s , must be > 0 f o r a f l o w f r o m s o i l t o d r a i n s . As downward f l o w i s t a k e n n e g a t i v e , f l o w t o t h e d r a i n s w i l l a l s o be n e g a t i v e . So, vn = VD = -Aip (Wind and van Doorne, 1975) (8) where A can be a p p r o x i m a t e d by a c o n s t a n t and i s c a l l e d t h e d r a i n a g e i n t e n s i t y . I t i s t h e d r a i n d i s c h a r g e p e r u n i t h e i g h t o f t h e w a t e r t a b l e above d r a i n d e p t h midway between t h e d r a i n s . I t has u n i t s o f day ^ ( t h a t i s , cm day ^ cm "*"). A i s t a k e n f r o m a s e r i e s s o l u t i o n o f t h e e q u a t i o n f o r t h e t r a n s i e n t c a s e o f a moving w a t e r t a b l e h a v i n g an i n i t i a l shape o f a f o u r t h degree p a r a b o l a (Dumm, 1954; L u t h i n , 1973; W e s s e l i n g , 1973). By t h i s s o l u t i o n , t h e w a t e r t a b l e h e i g h t , h t , midway between d r a i n s (x = S/2) can be r e p r e s e n t e d a t t i m e , t , by K DTr2t' s a t  h t ( x , t ) = 1.16 h Q e Vs2 (9) w h i c h i s b e t t e r known as t h e m o d i f i e d Glover-Dumm t r a n s i e n t s t a t e d r a i n s p a c i n g e q u a t i o n . I n t h i s e q u a t i o n t h e d r a i n a g e i n t e n s i t y , A, i s r e p r e s e n t e d by ( W e s s e l i n g , 1973): K DTT2 A = S a t (10) us z and D = b + h o/2 r e p r e s e n t s t h e ave r a g e t h i c k n e s s o f t h e w a t e r t r a n s m i t t i n g l a y e r . 14 A p p l y i n g e q u a t i o n (1) w i t h c o n s t a n t dh/dz below t h e w a t e r t a b l e g i v e s f r o m e q u a t i o n ( 8 ) : ty v = v = -Atp = K ( ) - K (11) n D D o z - z o 8 D S o l v i n g e q u a t i o n (11) f o r ty y i e l d s K (z Z j J o 8 D = A ( z -V) + K < 1 2> g D o S u b s t i t u t i o n i n t o e q u a t i o n (8) and r e p l a c i n g v n by t h e r i g h t hand s i d e o f e q u a t i o n (7) g i v e s : K - K AK (z - z„) ° n o g T>J a ( z n - z ) . n A ( z - z „ ) + K U J ; e n g - 1 g D o By means o f e q u a t i o n (13) t h e v a l u e o f z i s d e t e r m i n e d . Hence, t h e v a l u e of z - z„ = \l> i s c a l c u l a t e d . F i n a l l y , v = v g D D n D i s o b t a i n e d f r o m e q u a t i o n ( 8 ) . The w a t e r t a b l e , t h e r e f o r e , moves i n r e s p o n s e t o a d i f f e r e n c e i n f l u x e s between depths z and z n . S i n c e t h e f l u x , g n-1 v^ , a t Zg i s e q u a l t o t h e f l u x , v ^, o r i g i n a t i n g a t d e p t h z^, t h e change i n s t o r a g e i n t h e l a y e r i m m e d i a t e l y above t h e w a t e r t a b l e i s r e p r e s e n t e d by: (14) 15 t h where Az i s t h e d i s t a n c e between t h e l o w e r boundary o f t h e (n - 1) l a y e r and t h e w a t e r t a b l e . I t s h o u l d be n o t e d t h a t m o i s t u r e i s d i s t r i b u t e d u n i f o r m l y i n t h e l a y e r s e x c e p t i n t h e r e g i o n i m m e d i a t e l y above t h e w a t e r t a b l e where t h e i n c r e m e n t o f m o i s t u r e i s added t o o r s u b t r a c t e d f r o m t h e w a t e r t a b l e h e i g h t . S t a b i l i t y C r i t e r i o n The v a l u e o f At i n e q u a t i o n s (6) and (14) s h o u l d n o t be chosen t o o s m a l l because o f h i g h c o m p u t a t i o n a l c o s t ; t o o l a r g e a v a l u e f o r At l e a d s t o i n s t a b i l i t y c aused by e r r o r a m p l i f i c a t i o n . The c o r r e c t c h o i c e o f At f r o m a s t a b i l i t y s t a n d p o i n t was o b t a i n e d by Wind and v a n Doorne (1975) f r o m e r r o r a n a l y s i s . They c o n c l u d e d t h a t t h e s t a b i l i t y c o n d i t i o n had t h e fo r m d K e a d + 1 A t < l (15) d9 ad , d v e - 1 where dK/d6 i s t h e s l o p e o f t h e K(6 ) c u r v e a t s a t u r a t i o n , v v In t h e p r e s e n t s t u d y At was found by e q u a t i o n (15) t o be 0.002 day. A l s o , a v a l u e o f d = 10 cm was used f o r c o n v e n i e n c e . Model Input S o i l p r o p e r t i e s needed as i n p u t s i n t o t h e model a r e t h e s o i l w a t e r r e t e n t i o n and f l o w c h a r a c t e r i s t i c s . O ther i n p u t s i n c l u d e d r a i n a g e d e p t h , d r a i n a g e i n t e n s i t y , p o n d i n g d e p t h , l a y e r t h i c k n e s s , t i m e s t e p , r a i n f a l l d i s t r i b u t i o n , and t h e i n i t i a l m o i s t u r e p r o f i l e . 16 Model Output The model s i m u l a t e s t h e d r a i n a g e p r o c e s s . A main program c o n t r o l s two subprograms w h i c h c a l c u l a t e and p r i n t d e p t h t o t h e wa t e r t a b l e , t i l e d i s c h a r g e , p o n d i n g d e p t h , and w a t e r c o n t e n t , w a t e r t e n s i o n , and f l o w v e l o c i t y p r o f i l e s . The program, w r i t t e n i n FORTRAN, i s p r e s e n t e d i n t h e paper by Wind and van Doorne (1975). 3. LAYOUT AND DESIGN OF EXPERIMENT The d a t a p r e s e n t e d i n t h i s c h a p t e r were o b t a i n e d from e x p e r i m e n t a l s i t e s w h i c h were p a r t o f t h e A g r i c u l t u r a l and R u r a l Development A c t (ARDA) l a n d d r a i n a g e p r o j e c t . D e s i g n and i m p l e m e n t a t i o n of t h e e x p e r i m e n t a l p l a n were n o t under t h e c o n t r o l o f t h e a u t h o r b u t were done by t h e E n g i n e e r i n g B r a n c h o f t h e B r i t i s h C o l u m b i a M i n i s t r y of A g r i c u l t u r e . Two e x p e r i m e n t a l s i t e s were l o c a t e d on f a r m e r s ' f i e l d s i n t h e Lower F r a s e r V a l l e y , one (3.84 ha) a t M a t s q u i and t h e o t h e r (1.42 ha) a t S u r r e y . D e t a i l s o f t h e e x p e r i m e n t a l l a y o u t a r e p r e s e n t e d i n F i g . 2. A t M a t s q u i t h e s o i l was H a l l a r t s i l t y c l a y loam. One p a r t (1.95 ha) was p l a n t e d w i t h g r a s s i n 1974 w h i l e an a d j o i n i n g f i e l d o f 1.89 ha c a r r i e d c r o p s o f c o r n (Zea mays L.) i n 1975 and 1976 and beans (Phaseolus vulgaris L.) i n 1977. T h i s l a t t e r f i e l d , h e r e a f t e r r e f e r r e d t o as c u l t i v a t e d l a n d , was n o t p a r t o f t h e s t u d y d e s c r i b e d i n t h i s c h a p t e r and r e f e r e n c e t o i t i s m e r e l y f o r c o n v e n i e n c e i n t h e c o n t e x t o f t h e o v e r a l l s t u d y . FIG. 2: E x p e r i m e n t a l l a y o u t . 18 A t S u r r e y t h e s o i l was Lumbum muck and was p l a n t e d w i t h c o r n . P r o f i l e d e s c r i p t i o n s o f the two s o i l t y p e s a r e g i v e n i n A p p e n d i x A. P h y s i c a l c h a r a c t e r i s t i c s a r e g i v e n i n A p p e n d i x B. A t e a c h s i t e 10 cm d i a m e t e r f l e x i b l e t i l e l a t e r a l s were spaced a t 30.5 m and 15.2 m a d j a c e n t t o an a r e a w i t h no t i l e s . The a v e r a g e d e p t h o f l a t e r a l s was 1.22 m a t M a t s q u i and 1.27 m a t S u r r e y . A t t h e f o r m e r s i t e a 10 cm t h i c k l a y e r o f s o f t c l a y was o b s e r v e d a p p r o x i m a t e l y 3 cm below t h e l a t e r a l s w h i l e a t S u r r e y l a t e r a l s l a y above a s i l t loam s t r a t u m o f v a r i a b l e d e p t h and u n d e t e r m i n a b l e -t h i c k n e s s . M a i n t i l e s d r a i n e d i n t o a c o n c r e t e sump fr o m w h i c h w a t e r was pumped a u t o m a t i c a l l y i n t o a n e a r b y d i s c h a r g e d i t c h . The e x p e r i m e n t a l l a y o u t f a c i l i a t e d c o m p a r i s o n o f t h r e e d r a i n a g e i n t e n s i t i e s . U n f o r t u n a t e l y , i s o l a t i o n o f p l o t s f r o m each o t h e r and fr o m s u r r o u n d i n g open f i e l d d i t c h e s was poor e s p e c i a l l y s i n c e b o t h s o i l t y p e s had peat s u b s t r a t a o f h i g h s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s (Appendix B ) . Sa m p l i n g l o c a t i o n s a r e i n c l u d e d i n F i g . 2. A t each l o c a t i o n r a i n f a l l , w a t e r t a b l e d e p t h and s o i l w a t e r t e n s i o n were m o n i t o r e d d u r i n g t h e S p r i n g o f 1977. T e n s i o n was measured i n t r i p l i c a t e a t d e p t h s o f 1, 5, and 15 cm w i t h tensiometer-manometer systems. The por o u s cups o f t h e t e n s i o m e t e r s were c o n s t r u c t e d f r o m f r i t t e d g l a s s beads ( a i r e n t r y v a l u e * 220 cm o f w a t e r ) f o r q u i c k r e s p o n s e . F i n a l r e s u l t s were e x p r e s s e d as a mean o f t h e measurements t a k e n a t t h e t h r e e d e p t h s . Water t a b l e d e p t h was r e c o r d e d i n 1.2 m deep auger h o l e s encased w i t h 1.9 cm b o r e open-ended g a l v a n i z e d p i p e s . G r a v e l 19 was p l a c e d a t t h e bo t t o m o f the h o l e s and around t h e p i p e s . The below-ground s e c t i o n o f each p i p e was p e r f o r a t e d e v e r y 1 cm w i t h 3 mm h o l e s . A f l e x i b l e p l a s t i c b low tube was used t o l o c a t e the p h r e a t i c s u r f a c e i n t h e open w e l l s . By t h i s method measurement o f w a t e r t a b l e d e p t h was a c c u r a t e t o w i t h i n 1 cm. A l l measurements o f w a t e r t a b l e were n o r m a l i z e d w i t h r e f e r e n c e t o a f i e l d datum. A t each s i t e r a i n f a l l was c o l l e c t e d w i t h a 10 cm d i a m e t e r f u n n e l and r e c o r d e d as a t o t a l f o r t h e p r e c e d i n g 24 h o u r s . 4. MEASUREMENT OF THE MODEL INPUT PARAMETERS 4.1 Soil Water Retention Characteristic 7.5 cm x 7.2 cm c o r e samples were c a r e f u l l y t a k e n a t random f r o m t h e 0 - 15 cm l a y e r o f t h e e x p e r i m e n t a l s i t e s . A l t o g e t h e r 30 samples o f H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) , 15 o f H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) and 22 o f Lumbum deep muck were c o l l e c t e d . The method d e s c r i b e d by V o m o c i l (1965) was used f o r t e n s i o n s l e s s t h a n 100 cm. The c o r e s were mounted on 79 um g l a s s bead p o r o u s p l a t e s and s a t u r a t e d . By i n c r e a s i n g t h e l e n g t h o f a h a n g i n g w a t e r column i n s t a g e s t h e s o i l s were s u b j e c t e d t o a range o f t e n s i o n s f r o m 0 cm t o 100 cm. Water c o n t e n t was measured from t h e a c c u m u l a t e d o u t f l o w a f t e r e q u i l i b r i u m a t each a p p l i e d s u c t i o n . P r e s s u r e p l a t e a p p a r a t u s was used ( R i c h a r d s , 1965) on d i s t u r b e d s o i l f o r t e n s i o n s g r e a t e r t h a n 100 cm. The r e s u l t i n g s o i l w a t e r r e t e n t i o n c h a r a c t e r i s t i c s a r e shown i n F i g . 3. F IG. 3: S o i l water r e t e n t i o n c h a r a c t e r i s t i c s of 0-15 cm l a y e r . 4. 2 Water Flow Charaoteristio The w a t e r f l o w c h a r a c t e r i s t i c was measured on u n d i s t u r b e d s o i l columns by t h e i n s t a n t a n e o u s p r o f i l e method of Rose et al. (1965). A s c h e m a t i c d i a g r a m o f t h e i n s t r u m e n t a t i o n used f o r t h i s measurement i s shown i n F i g . \ 4 . Two columns o f u n d i s t u r b e d s o i l 10 cm x 18 cm x 25 cm deep were t a k e n f r o m t h e H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) 0 - 25 cm l a y e r and two f r o m a s i m i l a r d epth i n Lumbum muck. Each column was sampled w i t h t h e a i d of an a c r y l i c open-ended box w i t h removable s i d e s . I n t h e l a b o r a t o r y t h e column was t r a n s f e r r e d t o a n o t h e r a c r y l i c p l a s t i c box o f s l i g h t l y l a r g e r c r o s s - s e c t i o n and s e a l e d i n t o p o s i t i o n w i t h m o l t e n p a r a f f i n wax. The box was t h e n mounted on t o a 79 um u n c o n s o l i d a t e d g l a s s bead p o r o u s p l a t e and a t t a c h e d t o a h a n g i n g w a t e r column. S a t u r a t i o n was e f f e c t e d by r a i s i n g t h e w a t e r t a b l e i n s t a g e s o v e r a p e r i o d of 72 h o u r s . D r a i n a g e f r o m s a t u r a t i o n was t h e n c o n t r o l l e d by l o w e r i n g t h e h a n g i n g column i n s t a g e s . S o i l w a t e r c o n t e n t was measured w i t h a gamma r a d i a t i o n a t t e n u a t i o n d e v i c e a t d e p t h s o f 1, 5, 10, and 15 cm. S o i l w a t e r t e n s i o n was measured a t d e p t h s of 1, 10, and 15 cm w i t h a t e n s i o m e t e r - p r e s s u r e t r a n s d u c e r s y s t e m a t t a c h e d t o a s t r i p - c h a r t r e c o r d e r via a s t e p p i n g s w i t c h and t i m e r . These measurements were c a r r i e d o u t i n a manner s i m i l a r t o t h a t d e s c r i b e d i n d e t a i l by Chow and de V r i e s (1973). I n t h e i n s t a n t a n e o u s p r o f i l e method t h e a v e r a g e f l u x i s g i v e n by PRESSURE TRANSDUCERS I 2 3 ® ® ® TENSION/1 ETERS Cs 137 SOURCE SOURCE, COLLIMATOR OUTLET Jll STEPPING SWITCH a TIMER '..SOIL / PLASTIC LID PARAFFIN WAX SCINTILLATION DETECTOR' A V vDETECTOR COLLIMATOR ^ 2 9 pm GLASS BEAD POROUS PLATE STRIP CHART RECORDER COUNTING EQUIPMENT FIG. 4: Schematic diagram of gamma r a d i a t i o n a t t e n u a t i o n equipment used f o r measurement o f w a t e r f l o w c h a r a c t e r i s t i c s . 23 z i de - ~ d z d t / ( t 2 - t ) (16) z o I t was o b t a i n e d by n u m e r i c a l i n t e g r a t i o n between two w a t e r c o n t e n t p r o f i l e s a t t ^ and fr o m Z q and z^. The c o r r e s p o n d i n g w a t e r p o t e n t i a l g r a d i e n t was o b t a i n e d f r o m g r a p h i c a l d i f f e r e n t i a t i o n o f w a t e r p o t e n t i a l p r o f i l e s . The u n s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y was t h e n c a l c u l a t e d by ft2 f Z l x de J t l J z - d T d z d t / ( t 2 - V K(h) = -± ^ : ( 1 7 ) + i r az where z was t a k e n as p o s i t i v e downward. The w a t e r f l o w c h a r a c t e r i s t i c s o f H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) and Lumbum muck a r e p r e s e n t e d i n F i g . 5. R e s u l t s f r o m b o t h columns a r e p l o t t e d t o g e t h e r . The R i j t e m a ( 1 9 6 5 ) e x p r e s s i o n ( e q u a t i o n ( 2 ) ) was a p p l i e d t o a l l o f t h e d a t a p o i n t s e x c e p t t h e s a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s . T h i s i s i n d i c a t e d by t h e s o l i d l i n e s i n F i g . 5. V a l u e s o f K and a were t h e n o b t a i n e d and o used i n a t e s t r u n o f t h e model. The o u t p u t showed much h i g h e r d r a i n a g e r a t e s t h a n t h o s e measured i n t h e f i e l d . I n s t e a d , t h e a p p r o a c h o f R i j t e m a , 1 9 6 5 ; Feddes, 1 9 7 1 ; Wind and van Doorne, 1 9 7 5 ; and Wind, 1 9 7 6 , was used. I n t h i s a p p r o a c h v a l u e s o f K q and a were s c a l e d down t o y i e l d a p r e d i c t i o n c o n s i s t e n t w i t h measured v a l u e s o f w a t e r t a b l e d e p t h and s o i l w a t e r t e n s i o n . I t i s o f i n t e r e s t t o n o t e t h a t t h e s e r e d u c e d v a l u e s o f K and a b o r e some r e l a t i o n t o t h e 24 SOIL WATER TENSION , cm w a t e r FIG. 5: Water f l o w c h a r a c t e r i s t i c s o f Lumbum muck and H a l l a r t s i l t y c l a y loam w i t h i n 0-100 cm t e n s i o n r a n g e . The s t r a i g h t l i n e s r e p r e s e n t t h e R i j t e m a e x p r e s s i o n K = K 0 e - a n . S a t u r a t e d h y d r a u l i c c o n d u c t i v i t i e s shown a r e r e s u l t s o f in s i t u measurements by t h e a u g e r - h o l e method. S o l i d l i n e s r e p r e s e n t f i r s t a t t e m p t . a t f i t t i n g t h e d a t a . 25 measured d a t a i n t h a t t h e y c o u l d be o b t a i n e d f r o m r e g r e s s i o n l i n e s drawn j u s t w i t h i n t h e s c a t t e r o f t h e d a t a p o i n t s ( b r o k e n l i n e s i n F i g . 5 ) . The r e g r e s s i o n e q u a t i o n s d e s c r i b i n g t h e b r o k e n l i n e s a r e : H a l l a r t s i l t y clay loam . l o g K = -0.0094 h + l o g 3.11; 35 cm < h < 96 cm Lumbum muck l o g K = -0.0104 h + l o g 23.55; 27 cm < h < 89 cm The v a l u e s of K and a o b t a i n e d f r o m t h e s e e q u a t i o n s and a l s o used o i n t h e model a r e p r e s e n t e d i n T a b l e 1. E v i d e n c e i n f a v o u r o f t h e above a p p r o a c h was p r o v i d e d by t h e av e r a g e r a i n f a l l i n t e n s i t i e s d u r i n g t h e p e r i o d under s t u d y . On H a l l a r t s i l t y c l a y loam t h e av e r a g e r a i n f a l l i n t e n s i t y f o r A p r i l - M a y 1977 was 0.64 cm day ^; on Lumbum muck i t was 0.57 cm day A t s t e a d y s t a t e t h e s e i n t e n s i t i e s s h o u l d be e q u a l t o t h e s o i l s ' h y d r a u l i c c o n d u c t i v i t i e s . S i n c e t e n s i o n s c o r r e s p o n d i n g t o t h e s e c o n d u c t i v i t i e s were g r e a t e r t h a n 35 cm and 27 cm f o r H a l l a r t and Lumbum, r e s p e c t i v e l y , t h e R i j t e m a e x p r e s s i o n was thought t o be a p p l i c a b l e w i t h i n t h e t e n s i o n r a n g e s shown above. I t s h o u l d be n o t e d t h a t t h e K(h) f u n c t i o n was measured o n l y t o a d e p t h o f 25 cm. A p p l i c a t i o n o f t h e d a t a t o t h e e n t i r e p r o f i l e was a p r o b a b l e s o u r c e o f e r r o r e s p e c i a l l y i n t h e H a l l a r t s i l t y c l a y loam w h i c h had a m i n e r a l - p e a t i n t e r f a c e a t t h e 60 cm d e p t h . However, u n s a t u r a t e d f l o w was thought t o be dominant i n t h e upper 60 cm TABLE 1: Parameters e v a l u a t e d f o r use i n t h e model. ( S t a n d a r d e r r o r o f K measurements g i v e n i n b r a c k e t s ) S o i l K s a t 0-100 cm l a y e r (cm d a y - 1 ) V 0-25 cm l a y e r (cm day--'-) a 0-25 cm l a y e r ( c m - 1 ) P 0-15 cm l a y e r (cm3 G m - 3 ) A ( d a y " 1 ) T i l e s p a c i n g 15.2 m 30.5 m H a l l a r t s i l t y 379 (35) 3.11 0.0213 . 0.052 0.025 0.022 c l a y loam Lumbum muck 450 (42) 23.55 0.0237 0.133 0.011 0.009 27 o f t h e p r o f i l e d u r i n g t h e p e r i o d under s t u d y . Water t a b l e measurements t o be p r e s e n t e d l a t e r i n t h i s c h a p t e r s u p p o r t t h i s v i e w . S a t u r a t e d h y d r a u l i c c o n d u c t i v i t y , K , was measured i n t h e S 3 . L muck and s i l t y c l a y loam f i e l d s in situ a t s i x l o c a t i o n s by t h e a u g e r - h o l e method d e s c r i b e d by Boersma (1965). The r e s u l t s a r e p r e s e n t e d i n T a b l e 1 and i n c l u d e d i n F i g . 5. They agree w e l l w i t h s i m i l a r measurements made by D r i e h u y z e n (1975) a t t h e same s i t e s . 4. 3 Drainage Intensity C a l c u l a t i o n o f d r a i n a g e i n t e n s i t y by e q u a t i o n (10) a t f i r s t p o s e d a s e r i o u s p r o b l e m . D r a i n a b l e p o r e space was d e t e r m i n e d a t 60 cm t e n s i o n ( T a y l o r and W a t t s , 1967; L u t h i n , 1966) fr o m t h e r e t e n t i o n d a t a p r e s e n t e d i n F i g . 3. The v a l u e s o b t a i n e d a r e shown i n T a b l e 1. K was measured as d e s c r i b e d above. However, t h e e q u i v a l e n t d e p t h , b, of an impermeable l a y e r b e l o w d r a i n d e p t h c o u l d n o t be d e t e r m i n e d a c c u r a t e l y . O b s e r v a t i o n s i n d i c a t e d t h a t a c l a y l a y e r a p p r o x i m a t e l y 10 cm t h i c k e x i s t e d a t a d e p t h o f a p p r o x i m a t e l y 125 cm w i t h i n H a l l a r t s i l t y c l a y loam ( d r a i n d e p t h ~ 122 cm). A l s o , a t t h e S u r r e y s i t e t h e t i l e l i n e s were f o u n d t o have been l a i d above a 130 cm deep s i l t loam s u b s t r a t u m o f Lumbum muck ( d r a i n d e p t h ~ 126 cm). Use of t h e s e v a l u e s y i e l d e d v e r y h i g h i n t e n s i t i e s f o r t h e t i l e s p a c i n g s a t b o t h s i t e s . R e s u l t s o f t e s t r u n s made w i t h t h e s e i n t e n s i t i e s r e s u l t e d i n w a t e r t a b l e d e p t h s much g r e a t e r t h a n t h e measured v a l u e s . 28 One p o s s i b l e e x p l a n a t i o n i s t h a t t h e r e was a h i g h r e s i s t a n c e t o f l o w somewhere w i t h i n t h e w a t e r - t r a n s m i t t i n g l a y e r i n t h e f i e l d s . Such a h i g h r e s i s t a n c e c o u l d have been caused by r e s t r i c t i o n o f f l o w a t t h e t i l e - s o i l i n t e r f a c e . H i g h r e c h a r g e of t h e systems by r a p i d l a t e r a l seepage t h r o u g h t h e p e a t f r o m d i s c h a r g e d i t c h e s and s u r r o u n d i n g f i e l d d i t c h e s c o u l d have a l s o c o n s t i t u t e d an e f f e c t i v e r e s i s t a n c e t o f l o w . These d i t c h e s u s u a l l y r u n a l m o s t f u l l i n S p r i n g because o f poor c o n t r o l o f d i t c h l e v e l s on a r e g i o n a l s c a l e . Seepage t h r o u g h t h e 60 cm deep p e a t s u b s t r a t u m o f H a l l a r t s i l t y c l a y loam and t h r o u g h Lumbum muck had been o b s e r v e d i n t h e f i e l d . I n s t e a d , d r a i n a g e i n t e n s i t i e s used i n t h e model were o b t a i n e d by a d i f f e r e n t a p p r o a c h w h i c h c o n s i s t e d of s o l v i n g t h e m o d i f i e d t r a n s i e n t s t a t e Glover-Dumm e q u a t i o n ( e q u a t i o n 9) f o r t h e d r a i n a g e i n t e n s i t y , A. Thus, A D l n ( 1 . 1 6 ^ ) A = — ^ f - = [ p - * - ] <: PS Z Hence, i n s t e a d o f c a l c u l a t i n g A d i r e c t l y f r o m i n d i v i d u a l measurements o r e s t i m a t e s . o f K , D, S, and u by e q u a t i o n (10) i t was c a l c u l a t e d S SL U w i t h o u t a knowledge o f t h e s e p a r a m e t e r s . I n each d r a i n a g e p l o t v a l u e s of h and h were o b t a i n e d f r o m w a t e r t a b l e r e c e s s i o n measurements o t (Appendix C) t a k e n d u r i n g a p e r i o d , t ' , o f n e g l i g i b l e p r e c i p i t a t i o n f o l l o w i n g heavy r a i n f a l l i n S p r i n g 1977. These v a l u e s were t h e n used f o r t h e c a l c u l a t i o n of A by e q u a t i o n ( 1 8 ) . The r e s u l t s a r e p r e s e n t e d i n T a b l e 1. 29 T h i s method was f o u n d t o be u s e f u l o n l y when r e c e s s i o n was c o n s i d e r e d o v e r a f a i r l y l o n g p e r i o d . A p e r i o d o f s i x weeks was c o n s i d e r e d a t b o t h s i t e s . D u r i n g t h i s t i m e r a i n f a l l was v e r y low, b e i n g 0.65 mm day ^ a t M a t s q u i and 0.84 mm day a t S u r r e y . T h i s a p p r o a c h o f p r e d i c t i n g w a t e r t a b l e drawdown f r o m f i e l d measurements o f w a t e r t a b l e r e c e s s i o n has been used by a number o f i n v e s t i g a t o r s i n c l u d i n g Young and L i g o n (1972) and Skaggs (1976). The l a t t e r a u t h o r was o f t h e o p i n i o n t h a t t h e use of p o i n t measurements t o d e f i n e h y d r a u l i c p r o p e r t i e s o f f i e l d - s i z e u n i t s l e d t o s e r i o u s l i m i t a t i o n s i n d e s c r i b i n g r e a l i t y . He f u r t h e r s t a t e d t h a t i n s i t u a t i o n s where i t was d e s i r a b l e t o measure " f i e l d e f f e c t i v e " v a l u e s o f s o i l p a r a m e t e r s f o r use i n p r e d i c t i n g w a t e r t a b l e drawdown, e t c . , t h e most r e l i a b l e r e s u l t s would n o r m a l l y be o b t a i n e d when t h e p a r a m e t e r s were d e t e r m i n e d f r o m measurements o f t h e phenomena t o be u l t i m a t e l y p r e d i c t e d . 5. RESULTS AND DISCUSSION• Water t a b l e d e p t h s measured a t M a t s q u i ( g r a s s l a n d ) and S u r r e y d u r i n g A p r i l 1977 t o g e t h e r w i t h t h o s e c a l c u l a t e d by t h e model a r e p r e s e n t e d i n F i g . 6. A l s o shown a r e r a i n f a l l d i s t r i b u t i o n s d u r i n g t h e same p e r i o d . The model was a b l e t o p r e d i c t w a t e r t a b l e d e p t h s q u i t e w e l l i n b o t h t y p e s o f s o i l . S l i g h t l y b e t t e r p r e d i c t i o n was o b t a i n e d w i t h t h e c l o s e r s p a c i n g . C a l c u l a t e d v a l u e s were w i t h i n 5% of o b s e r v e d v a l u e s i n most c a s e s f o r t h e 3-week s a m p l i n g p e r i o d . FIG. 6: Comparison of measured and c a l c u l a t e d w a t e r t a b l e d e p t h f o r 2 t i l e s p a c i n g s i n S p r i n g 1977. 31 Depth t o t h e w a t e r t a b l e d i d n o t d i f f e r g r e a t l y between s p a c i n g s e s p e c i a l l y i n t h e s i l t y c l a y loam. However, t h e c l o s e r s p a c i n g r e s u l t e d i n a c o n s i s t e n t l y deeper w a t e r t a b l e by about 5 - 15 cm a t b o t h s i t e s . The model i n d i c a t e d p o n d i n g on A p r i l 12 a t M a t s q u i w h i l e no p o n d i n g was measured. T h i s d i s c r e p a n c y c o u l d have been c a u s e d by a mismatch between t i m e o f p o n d i n g and t i m e of w a t e r - t a b l e measurement o r by t o o l a r g e an i n f i l t r a t i o n r a t e , .v , computed by t h e model. I n t h e model t h e i n f i l t r a t i o n r a t e i s governed by e q u a t i o n ( 4 ) . I n p u t o f r a i n f a l l on p e r h a p s a 6 - h o u r l y b a s i s m i g h t p r o v i d e a b e t t e r match between r a i n f a l l r a t e and i n f i l t r a b i l i t y . The r a t e o f drawdown measured o v e r p e r i o d s w i t h o u t r a i n was s u r p r i s i n g l y s m a l l a t t i m e s . I t seemed t h a t a l t h o u g h t h e h y d r a u l i c c o n d u c t i v i t i e s o f t h e s o i l s m i ght have been l a r g e t h e h y d r a u l i c g r a d i e n t s were s m a l l and l e d t o o n l y s l o w d r a i n a g e out o f t h e f i e l d s . Such s m a l l g r a d i e n t s c o u l d have r e s u l t e d f r o m b a c k seepage f r o m t h e d i s c h a r g e d i t c h and s u r r o u n d i n g f i e l d d i t c h e s . Measured and p r e d i c t e d s o i l w a t e r t e n s i o n i n t h e t o p 15 cm i n A p r i l 1977 a r e shown i n F i g . 7. The o u t p u t f r o m t h e computer i n c l u d e d t e n s i o n s a t d e p t h s o f 5 and 15 cm whereas t e n s i o n was measured i n t h e f i e l d s a t 1, 5, and 15 cm and a v e r a g e d o v e r t h e s e 3 d e p t h s . To compensate somewhat f o r t h e absence o f a v a l u e f o r t h e 1 cm d e p t h i n t h e model o u p t u t , 4 cm of t e n s i o n was added t o t h e v a l u e c a l c u l a t e d f o r t h e 5 cm d e p t h and t h e r e s u l t used t o i n d i c a t e a t e n s i o n a t t h e 1 cm d e p t h . By t h i s means t h e model o u t p u t was 32 160 140 HALLART SILTY CLAY I QAM (GRASSLAND) 120 100 15.2 m _30.5m CALCULATED MEASURED • o 80 u <u 4-1 E u z g to z LU (£ U l I— J o CO FIG. 7: Comparison o f measured and c a l c u l a t e d s o i l w a t e r t e n s i o n i n 0-15 cm l a y e r f o r 2 t i l e s p a c i n g s i n S p r i n g 1977. ( B a r s i n d i c a t e s t a n d a r d e r r o r >• 3 cm) . 33 a v e r a g e d o v e r t h e same de p t h i n t e r v a l s as t h o s e measured i n the f i e l d . The model f a i l e d t o p r e d i c t s o i l w a t e r t e n s i o n i n t h e 0 - 15 cm l a y e r a c c u r a t e l y . G e n e r a l l y , c a l c u l a t e d v a l u e s were w i t h i n 16% o f measured v a l u e s f o r j u s t o v e r o n e - h a l f o f t h e s a m p l i n g p e r i o d . However, i t gave an a c c e p t a b l e i n d i c a t i o n o f t h e upward o r downward t r e n d i n t e n s i o n . O m i s s i o n o f e v a p o t r a n s p i r a t i o n f r o m t h e model p r o b a b l y c o n t r i b u t e d t o t h e l a c k o f a c c u r a c y . T h i s a l o n e would have c a u s e d s i m u l a t e d v a l u e s t o be l o w e r t h a n measured v a l u e s as on t h e v e r y sunny day o f A p r i l 25 a t M a t s q u i . But t h i s was n o t t h e case f o r any s i g n i f i c a n t p a r t o f t h e s t u d y p e r i o d . D i u r n a l v a r i a t i o n i n r a i n f a l l seemed t o b e t t e r e x p l a i n t h e l a c k o f f i t . On some days t e n s i o n measurements were made soon a f t e r l i g h t r a i n . T h i s had t h e e f f e c t o f s h a r p l y l o w e r i n g t e n s i o n i n t h e t o p 1 cm l a y e r and hence y i e l d e d an o v e r a l l low t e n s i o n i n t h e 0 - 15 cm l a y e r . On s u c h days t h e model w o u l d n o t p r e d i c t so low a t e n s i o n s i n c e t h e ave r a g e d a i l y r a i n f a l l measured a t a p p r o x i m a t e l y 8 a.m. was n o t enough t o a l l o w i t t o do so. When a t t e m p t s a r e made t o p r e d i c t so f l u c t u a t i n g a v a r i a b l e as s o i l w a t e r t e n s i o n d i u r n a l v a r i a t i o n s a r e l i k e l y t o p r o v i d e l i m i t a t i o n s e s p e c i a l l y when r a i n f a l l i s r e c o r d e d as an ave r a g e f o r t h e p r e c e d i n g 24 hour p e r i o d . G e n e r a l l y , p e r f o r m a n c e o f t h e model was found t o be dependent upon t h e d r a i n a g e i n t e n s i t y , A, w h i c h governed d r a i n 34 d i s c h a r g e . C a l c u l a t i o n o f A d i r e c t l y f r o m K , D, S, and u w i t h S c i L e q u a t i o n (10) posed a p r o b l e m because o f K v a r i a t i o n i n t h e f i e l d S a L and t h e l a c k o f d e f i n i t i o n o f t h e l o w e r boundary o f each d r a i n a g e system s t u d i e d . I t s h o u l d be n o t e d t h a t t h e p r o p o s e r s o f t h e model made no a t t e m p t t o e v a l u a t e A f r o m b a s i c s o i l p r o p e r t i e s b u t r a t h e r , assumed A v a l u e s c o n s i s t e n t w i t h t h e i r r e q u i r e m e n t s . A l s o , t h e model uses t h e v a l u e o f K r a t h e r t h a n K f o r t h e h i g h e s t v a l u e o f o s a t h y d r a u l i c c o n d u c t i v i t y . The s u c c e s s f u l use o f t h e model i n t h e p r e s e n t s t u d y was i n p a r t due t o t h e use of A v a l u e s o b t a i n e d by a s o l u t i o n o f t h e m o d i f i e d Glover-Dumm d r a i n a g e e q u a t i o n . A c c o r d i n g t o e q u a t i o n (8) t h e r e i s a l i n e a r p r o p o r t i o n a l i t y between d i s c h a r g e , v Q , and p r e s s u r e head, . A l t h o u g h t h e r e a r e s i t u a t i o n s i n w h i c h (8) does n o t h o l d because o f n o n - l i n e a r r a d i a l and e n t r y r e s i s t a n c e s , i t g i v e s a good a p p r o x i m a t i o n i n many ca s e s (Wind, 1972). The y i n t e r c e p t o f t h e R i j t e m a e x p r e s s i o n , K q , and t h e v a l u e of a o b t a i n e d f r o m t h e K(h) f u n c t i o n g r e a t l y a f f e c t e d model o u t p u t . These p a r a m e t e r s governed f l o w o f w a t e r i n t h e u n s a t u r a t e d l a y e r . T h e i r e v a l u a t i o n i n t h i s s t u d y was based on model o u t p u t . A c c u r a t e measurement of t h e K.(h) r e l a t i o n s h i p , a l t h o u g h d i f f i c u l t t o make ( S i n c l a i r et at., 1 9 7 4 ) , i s n e c e s s a r y f o r s u c c e s s f u l a p p l i c a t i o n o f the model. D i f f i c u l t i e s i n a p p l y i n g t h e model i n t h i s s t u d y were p r o b a b l y r e l a t e d t o l a y e r i n g i n t h e s i l t y c l a y loam and t o h e t e r o g e n e i t y 35 o f b o t h s o i l t y p e s . The a s s u m p t i o n o f K b e i n g a d i f f e r e n t i a b l e f u n c t i o n of z ( e q u a t i o n ( 4 ) ) depends o n homogeneity w i t h i n t h e u n s a t u r a t e d l a y e r s o f s o i l . The c a l c u l a t i o n o f i n f i l t r a t i o n r a t e s was based on t h e as s u m p t i o n o f z e r o t e n s i o n a t t h e s o i l s u r f a c e . A l t h o u g h Colman and Bodman (1945) p r e s e n t e d d a t a w h i c h s u p p o r t s s u c h an a s s u m p t i o n t h e work o f D a v i d s o n et al. (1963) has c a s t some doubt as t o whether t h i s a s s u m p t i o n i s v a l i d . H i g h w a t e r l e v e l s i n e x t e r n a l f i e l d d i t c h e s appeared t o i n f l u e n c e w a t e r t a b l e r e c e s s i o n . A l s o , t h e v a l u e s o f v ^ and i p ^ i n e q u a t i o n (8) were based on t h e a s s u m p t i o n of z e r o p r e s s u r e head a t t h e t i l e l i n e s . T h i s was n o t a l w a y s t h e case i n t h i s e x p e r i m e n t as e x e m p l i f i e d i n T a b l e 2 w h i c h shows p r e s s u r e heads a t t h e p o s i t i o n o f t h e t i l e l i n e s a t M a t s q u i and S u r r e y d u r i n g t h e p e r i o d 1 3 t h - 2 7 t h A p r i l , 1977. T h i s o b s e r v a t i o n meant t h a t was s m a l l e r t h a n c a l c u l a t e d by t h e model and p a r t l y e x p l a i n e d why a r e d u c t i o n i n A by c o n s i d e r i n g a l o n g r e c e s s i o n p e r i o d was n e c e s s a r y . The model was f a i r l y e c o n o m i c a l t o r u n , c o s t i n g a p p r o x i m a t e l y US $0.20 p e r c a l c u l a t e d day. 6. CONCLUSIONS The most i m p o r t a n t r e s u l t o f t h e i n v e s t i g a t i o n was t h a t f o r t h e two s o i l t y p e s t h e model a c c u r a t e l y p r e d i c t e d w a t e r t a b l e d e p t h i n S p r i n g b u t d i d n o t g i v e as good an i n d i c a t i o n o f s o i l w a t e r TABLE 2: P r e s s u r e ; head* a t d r a i n l e v e l a t t i l e l o c a t i o n a t M a t s q u i a n d - S u r r e y d u r i n g t h e p e r i o d 1 3 t h - 2 7 t h A p r i l , 1977. M a t s q u i S u r r e y S o i l w a t e r p r e s s u r e head (cm) D r a i n s p a c i n g (m) 13/4 14/4 15/4 16/4 17/4 18/4 19/4 20/4 21/4 22/4 23/4 24/4 25/4 26/4 27/4 15.2 53 34 29 — 39 40 33 29 — 27 23 — 18 21 33 30.5 68 62 56 -- 65 64 50 45 — 44 39 — 28 33 37 15.2 85 58 80 57 85 83 75 50 50 62 50 51 56 52 73 30.5 78 59 94 58 77 75 72 52 51 69 51 51 63 52 63 *Recorded as w a t e r l e v e l i n p i e z o m e t e r 37 t e n s i o n i n t h e p l o u g h l a y e r . Model o u t p u t was dependent upon t h e d r a i n a g e i n t e n s i t y , A, and upon th e K(h) f u n c t i o n . E x p e r i m e n t s of t h i s n a t u r e s h o u l d be d e s i g n e d so t h a t d r a i n a g e t r e a t m e n t s a r e p r o p e r l y i s o l a t e d f r o m e a c h o t h e r and f r o m e x t e r n a l f i e l d d i t c h e s . T h i s i s p a r t i c u l a r l y i m p o r t a n t i n f i e l d s w i t h p ermeable s u b s o i l s . An i m p o r t a n t consequence of t h e r e s u l t s r e p o r t e d h e r e i n i s t h a t t h e model c o u l d be used as a t o o l i n t h e p r e d i c t i o n o f t r a f f i c a b i l t i y i n S p r i n g . F u r t h e r work on t h e p r e d i c t i o n o f s o i l w a t e r t e n s i o n f o r t r a f f i c a b i l t i y p r e d i c t i o n i s n e c e s s a r y . 7. REFERENCES Boersma, L. 1965. F i e l d measurements o f h y d r a u l i c c o n d u c t i v i t y above a w a t e r t a b l e . In: B l a c k ( e d . ) . Methods o f s o i l a n a l y s i s , P a r t I . Agronomy 9: 234-252. Amer. Soc. A g r o n . , M a d i s o n , Wis. Boersma, L. et al. 1970. C h a r a c t e r i z a t i o n of w a t e r t a b l e s i n Oregon s o i l s w i t h r e f e r e n c e t o t r a f f i c a b i l i t y . C o n t r a c t R e p o r t M-70-1. U.S. Army Eng. Waterways.Expt. S t a . , V i c k s b u r g , M i s s i s s i p p i . Bouwer, H. 1974. 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M. et al. 1963. The measurement and d e s c r i p t i o n -of w a t e r f l o w t h r o u g h C o l u m b i a s i l t loam and H e s p e r i a sandy loam. H i l g a r d i a 34: 601-617. D r i e h u y z e n , M. 1975. U n p u b l i s h e d d a t a . Dumm, L. D. 1954. D r a i n s p a c i n g f o r m u l a . A g r . Eng. f o r O c t o b e r 1954. pp. 726-730. Feddes, R. A. 1971. Water, h e a t , and c r o p growth. Meded. Landbouwhogeschool Wageningen 71-12. Hoffman, G. J . and G. 0. Schwab. 1964. T i l e s p a c i n g p r e d i c t i o n based on d r a i n o u t f l o w . T r a n s . ASAE 7: 444-447. L u t h i n , J . N. 1973. D r a i n a g e E n g i n e e r i n g . K r i e g e r P u b l . Co., New Y o r k . R i c h a r d s , L. A. 1965. P h y s i c a l c o n d i t i o n of w a t e r i n s o i l . In: B l a c k ( e d . ) . Methods o f s o i l a n a l y s i s . P a r t I . Agronomy 9: 128-152. Amer. Soc. A g r o n . , M a d i s o n , Wis. 39 R i j t e m a , P. E. 1965. An a n a l y s i s o f a c t u a l t r a n s p i r a t i o n . A g r . Res. Rept. no. 659. I n s t . Land and Water Man. Res., Wageningen, N e t h e r l a n d s . Rose, C. W. et al. 1965. D e t e r m i n a t i o n o f h y d r a u l i c c o n d u c t i v i t y as a f u n c t i o n o f d e p t h and w a t e r c o n t e n t f o r s o i l in s i t u . A u s t . J . S o i l Res. 3: 1-9. S i n c l a i r , L. R. et at. 1974. P e r m e a b i l i t y o f u n s a t u r a t e d f i e l d s o i l s c a l c u l a t e d f r o m l a b o r a t o r y d e s a t u r a t i o n d a t a . T r a n s . ASAE 17: 399-405. Skaggs, W. R. 1974. The e f f e c t o f s u r f a c e d r a i n a g e on w a t e r t a b l e r e s p o n s e t o r a i n f a l l . T r a n s . ASAE 17: 406-411. Skaggs, W. R. 1976. D e t e r m i n a t i o n o f t h e h y d r a u l i c c o n d u c t i v i t y -d r a i n a g e p o r o s i t y r a t i o f r o m w a t e r t a b l e measurements. T r a n s . ASAE 19: 723-729. V a i g n e u r , H. 0. and H. P. Johnson. 1966. D r a i n a g e d e s i g n based on m o i s t u r e b a l a n c e . T r a n s . ASAE 9: 764-767. V o m o c i l , J . A. 1965. P o r o s i t y . In: B l a c k et al. ( e d s . ) . Methods of s o i l a n a l y s i s . Agronomy No. 9. P a r t I I : 299-314. Amer. Soc. Ag r o n . , M a d i s o n , Wis. W e s s e l i n g , J . 1973. S u b s u r f a c e f l o w t o d r a i n s . In: D r a i n a g e P r i n c i p l e s and A p p l i c a t i o n s . P u b l . 16 - V o l . I I . I n t . I n s t . Land R e e l , and Improv., Wageningen, t h e N e t h e r l a n d s . Wind, G. P. 1972. A h y d r a u l i c model f o r t h e s i m u l a t i o n o f n o n - h y s t e r e t i c v e r t i c a l u n s a t u r a t e d f l o w o f m o i s t u r e i n s o i l s . Tech. B u l l . 79: 226-246. I n s t . Land and Water Man. Res., Wageningen, The N e t h e r l a n d s . 40 Wind, G. P. 1973. W o r k a b i l i t y and d r a i n a g e . N o t a 890. I n s t i t u u t v o o r C u l t u u r t e c h n i e k i n W a t e r h u i s h o u d i n g , Wageningen, t h e N e t h e r l a n d s . Wind, G. P. 1976. A p p l i c a t i o n o f a n a l o g and n u m e r i c a l models t o i n v e s t i g a t e t h e i n f l u e n c e o f d r a i n a g e on w o r k a b i l i t y i n S p r i n g . N e t h . J . A g r i c . S c i . 24: 155-172. Wind, G. P. and W. v a n Doorne. 1975. A n u m e r i c a l model f o r t h e s i m u l a t i o n o f u n s a t u r a t e d v e r t i c a l f l o w o f m o i s t u r e i n s o i l s . J . Hydr. 24: 1-20. Young, T. C. and J . T . L i g o n . 1972. Water t a b l e and s o i l m o i s t u r e p r o b a b i l i t i e s w i t h t i l e d r a i n a g e . T r a n s . ASAE 15: 448-451. 4 1 C H A P T E R 2 42 PREDICTION OF SOIL STRENGTH FROM HYDROLOGIC AND  MECHANICAL PROPERTIES ABSTRACT Cone p e n e t r a t i o n r e s i s t a n c e (an i n d e x o f s o i l s t r e n g t h ) was found t o be l i n e a r l y dependent upon s o i l w a t e r t e n s i o n w i t h i n a l i m i t e d range o f t e n s i o n s c l o s e t o s a t u r a t i o n . T h i s r e l a t i o n s h i p was e s t a b l i s h e d w i t h d a t a c o l l e c t e d f r o m t h r e e s o i l t y p e s i n f a r m e r s ' f i e l d s d u r i n g l a t e W i n t e r and S p r i n g o v e r a two-year p e r i o d . F o r t h e m i n e r a l s o i l s t h e s l o p e o f t h e r e l a t i o n s h i p was p r e d i c t e d q u i t e w e l l by a t h e o r e t i c a l l y - b a s e d e q u a t i o n . The s l o p e c o u l d n o t be p r e d i c t e d f o r an o r g a n i c s o i l . S o l u t i o n o f t h e e q u a t i o n , however, r e q u i r e d a number of s i m p l i f y i n g a s s u m p t i o n s and d i r e c t measurement o f t h e a n g l e o f s h e a r i n g r e s i s t a n c e , <f>. T h i s e q u a t i o n was i n t e g r a t e d and t h e c o n s t a n t o f i n t e g r a t i o n w h i c h depended upon t h e p a r t i c u l a r d e p t h under s t u d y was r e p l a c e d by t h e i n t e r c e p t f r o m t h e e m p i r i c a l s t r e n g t h - t e n s i o n r e l a t i o n s h i p . I n t h i s way, a s e m i - e m p i r i c a l model was d e v e l o p e d f o r t h e p r e d i c t i o n o f s o i l s t r e n g t h f r o m s o i l w a t e r t e n s i o n , o r viae versa, f o r t h e m i n e r a l s o i l s . P r e d i c t i o n f o r t h e o r g a n i c s o i l c o u l d be c a r r i e d out w i t h t h e e m p i r i c a l r e l a t i o n s h i p . 43 1. INTRODUCTION I n l o w l a n d a r e a s s o i l w a t e r s t a t u s o f t e n d e t e r m i n e s t h e l e n g t h o f t h e g r o w i n g s e a s o n and the economic r e t u r n s t h a t can be o b t a i n e d f r o m f a r m i n g . The d e s i g n o f d r a i n a g e systems i n t h e s e a r e a s must t a k e i n t o a c c o u n t s o i l t r a f f i c a b i l i t y as a d e s i g n c r i t e r i o n . I n t h i s c o n t e x t B ekker (1960) r e c o g n i z e d t h e need f o r r e l a t i n g s o i l w a t e r t e n s i o n i n t h e t o p s o i l t o v e h i c u l a r t r a c t i o n . U.S. Army E n g i n e e r s a t t h e Waterways E x p e r i m e n t S t a t i o n have i d e n t i f i e d s o i l s t r e n g t h as one o f t h e main s o i l c h a r a c t e r i s t i c s w h i c h a f f e c t t r a c t i o n (Reeve and F a u s e y , 1974). A c o n s i d e r a t i o n of t h e r e l a t i o n s h i p between the s o i l s ' w a t e r s t a t u s and t h e i r s t r e n g t h w o u l d , t h e r e f o r e , seem t o be i m p o r t a n t i n t h e d e s i g n of d r a i n a g e systems i n t h e a r e a s i n q u e s t i o n . S i m i l a r a p p roaches i n t h i s d i r e c t i o n i n c l u d e t h e s t u d i e s o f W i l l i a m s and Shaykewich, 1970; M i r r e h and K e t c h e s o n , 1972; Towner and C h i l d s , 1972; S t e i n h a r d t , 1974; Wind, 1975; and Feddes and van W i j k , 1976. Feddes and van W i j k (1976) r e c o g n i z e d t h a t t h e b e a r i n g s t r e n g t h of a s o i l depended on i t s w a t e r m e n i s c i . t e n s i o n and b u l k d e n s i t y but t h a t t h e n a t u r e o f t h e e x a c t r e l a t i o n s h i p was s t i l l unknown. However, r e s e a r c h has shown t h a t a s o i l ' s r e s i s t a n c e t o d e f o r m a t i o n i s made up o f f r i c t i o n a l f o r c e s a t t h e i n t e r - p a r t i c l e c o n t a c t a r e a s t h a t r e s i s t s l i d i n g o f p a r t i c l e s and c o h e s i v e f o r c e s t h a t h o l d p a r t i c l e s t o g e t h e r ( T e r z a g h i and P eck, 1967). W h i l e f r i c t i o n a l p r o p e r t i e s a r e a c h a r a c t e r i s t i c of t h e s o i l m a t e r i a l , an 4 4 i n c r e a s e i n w a t e r m e n i s c i t e n s i o n s e r v e s t o i n c r e a s e c o h e s i v e f o r c e s and so i n c r e a s e s a s o i l ' s s t r e n g t h ( A i t c h i s o n , 1961; W i l l i a m s and Shaykew i c h , 1970; Towner and C h i l d s , 1972). I n s w e l l i n g and s h r i n k i n g m a t e r i a l s , b u l k d e n s i t y i s some f u n c t i o n o f t e n s i o n and i t s r e s u l t a n t i n f l u e n c e on s t r e n g t h i s super i m p o s e d on t h e t e n s i o n - s t r e n g t h r e l a t i o n s h i p ( W i l l i a m s and Shaykewich, 1970). S i n c e a s o i l ' s w a t e r t e n s i o n i s pomewhat p r e d i c t a b l e (Chapter i t s c a p a c i t y t o w i t h s t a n d v e h i c u l a r t r a f f i c can be p r e d i c t e d from known r e l a t i o n s between s o i l w a t e r t e n s i o n and s t r e n g t h . The main o b j e c t i v e i n t h i s c h a p t e r i s t o e s t a b l i s h such a r e l a t i o n s h i p f o r t h r e e m i n e r a l s o i l s and a muck and t o t e s t i t f o r a p h y s i c a l base. 2. THEORETICAL CONSIDERATIONS 2.1 Evaluation of Soil Strength by the Cone Penetrometer I n e n g i n e e r i n g p r a c t i c e , p i l e f o r m u l a e a r e u s u a l l y based on t h e t h e o r y o f b e a r i n g c a p a c i t y o f s t r i p f o u n d a t i o n s ( T e r z a g h i , 1947); t h e s o i l i s assumed t o be a r i g i d m a t e r i a l and t o s u f f e r g e n e r a l s h e a r under l o a d i n g w i t h a s l i d i n g s u r f a c e d e s c r i b e d by a p l a n e and a l o g a r i t h m i c s p i r a l , t h e o r i g i n o f w h i c h l i e s under t h e m i d p o i n t o f t h e f o u n d a t i o n . C o m p r e s s i b i l i t y o f t h e s o i l i s sometimes t a k e n i n t o a c c o u n t by an e m p i r i c a l r e d u c t i o n o f t h e s t r e n g t h p a r a m e t e r s , t h e c o h e s i o n , c, and t h e a n g l e o f s h e a r i n g r e s i s t a n c e <j> ( T e r z a g h i , 1943b; Meyerhof, 1951). Based on P r a n d t l ' s t h e o r y o f p l a s t i c f a i l u r e i n s o i l , T e r z a g h i (1943b) showed t h a t t h e f o r c e s a c t i n g on t h e base o f a p i l e 45 s u c h as a cone p e n e t r o m e t e r a r e g i v e n by PR = K yBN + K cN + K yz. N (1) Y Y c c q b q T h i s t h e o r y i s o r i e n t e d toward t h e s t a t i c b e a r i n g c a p a c i t y o f f o o t i n g s and does n o t c o n s i d e r dynamic e f f e c t s ( F r i e t a g , 1968). However, t h e r e seems t o be a s e v e r e l a c k o f a t h e o r y w h i c h b e t t e r d e s c r i b e s t h e p e n e t r a t i o n r e s i s t a n c e t o a cone i n terms o f s o i l s t r e s s - s t r a i n p r o p e r t i e s . F r i e t a g (1968) was o f t h e o p i n i o n t h a t w h i l e t h e cone p e n e t r o m e t e r was a s u r p r i s i n g l y a c c u r a t e and e f f i c i e n t t o o l f o r m e a s u r i n g i n - p l a c e s t r e n g t h of s o i l s , i t was n e c e s s a r y t o r e l y h e a v i l y on e x p e r i m e n t a t i o n and e x p e r i e n c e t o i n t e r p r e t cone p e n e t r a t i o n r e s i s t a n c e t e s t r e s u l t s . N e v e r t h e l e s s t h e cone p e n e t r o m e t e r has become a b a s i c i n s t r u m e n t f o r t h e assessment o f s o i l t r a f f i c a b i l i t y ( K n i g h t and F r i e t a g , 1962; Rush, 1968; S t e i n h a r d t and T r a f f o r d , 1974). 2. 2 Influence of Soil Water. Tension on Soil Strength The d i r e c t c o n t r i b u t i o n t h a t s o i l w a t e r t e n s i o n makes t o s o i l s t r e n g t h i s t h r o u g h i t s c o n t r i b u t i o n t o t h e i n t e r g r a n u l a r o r e f f e c t i v e s t r e s s o p e r a t i n g between s o l i d s o i l p a r t i c l e s ( B i s h o p and E l d i n , 1950). C h i l d s (1955) showed t h a t t e n s i o n had t h e e f f e c t o f an e q u i v a l e n t i s o t r o p i c l o a d r e s i s t i n g s h e a r f o r c e s . A i t c h i s o n (1961) and B i s h o p (1961) m o d i f i e d t h e T e r z a g h i e f f e c t i v e s t r e s s e q u a t i o n t o make i t a p p l i c a b l e t o u n s a t u r a t e d c o n d i t i o n s . T h e i r e q u a t i o n f o r t h e e f f e c t i v e s t r e s s , a', i s a' = a + X h (2) 46 The f a c t o r x r e p r e s e n t s t h e p r o p o r t i o n o f t h e t e n s i o n t h a t c o n t r i b u t e s t o t h e e f f e c t i v e s t r e s s . A l t h o u g h x = 1 when t h e degree o f s a t u r a t i o n = 1 and, X = 0 when S = 0, x i s n o t l i n e a r l y r e l a t e d t o S r ( B i s h o p , 1960; W i l l i a m s and Shaykewich, 1970). I n t h e Mohr-Coulomb s h e a r s t r e n g t h e q u a t i o n , t h e s o i l s h e a r s t r e n g t h , s, i s g i v e n by s = c* + (a + xh) t a n <j>' (3) t h e t e r m (a + xh) i s t h e e f f e c t i v e s t r e s s n o r m a l t o t h e p l a n e o f f a i l u r e . I t i s a p p a r e n t f r o m t h i s e q u a t i o n t h a t an i n c r e a s e i n s o i l w a t e r t e n s i o n l e a d s t o an i n c r e a s e i n s h e a r s t r e n g t h . 3. IN SITU MEASUREMENTS OF HYDROLOGIC AND STRENGTH  RELATIONSHIPS FOR THE ESTABLISHMENT OF A SOIL  STRENGTH-TENSION MODEL 3.1 Small-scale Field Experiments I n 1976 s m a l l s c a l e f i e l d e x p e r i m e n t s were c a r r i e d o u t on A l o u e t t e s i l t loam and H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) . A t each l o c a t i o n a s o i l b l o c k 345 cm x 345 cm was i s o l a t e d by f i r s t c u t t i n g t h e g r a s s c o v e r t o a n k l e h e i g h t and d i g g i n g a 30 cm wide t r e n c h t o a d e p t h o f 120 cm ( F i g . 1). A pump f a c i l i t a t e d movement o f w a t e r i n t o o r out of t h e t r e n c h . A n e a r b y d i t c h was used as a w a t e r s o u r c e and s i n k . P o l y e t h y l e n e s h e e t i n g p l a c e d on t h e o u t e r s u r f a c e o f the t r e n c h s e r v e d t o c o n t a i n w a t e r a r o u n d t h e b l o c k . Two b a t t e r i e s . of s e n s i t i v e mercury manometer t e n s i o m e t e r s . w e r e i n s t a l l e d 47 P E G POLY ETHYLENE SHEET TENSIOMETERS DEAD WEIGHT OPEN WELL f i 4 TRENCH LEVEL INDICATOR 30 em 345 cm 9 • 9 9 9 • I l l - — > CROSS-SECTION SCALE M30 PLAN TENSIOMETER BATTERY -• OPEN WELL BULK DENSITY SAMPLING SITE MEASURING STATION FOR PENETRATION RESISTANCE AND SOIL WATER CONTENT RAIN GAUGE PUMP POLYETHYLENE SHEET DITCH TRENCH LEVEL INDICATOR FIG. 1: E x p e r i m e n t a l l a y o u t f o r s m a l l - s c a l e f i e l d i n v e s t i g a t i o n s o f s t r e n g t h - t e n s i o n r e l a t i o n s i n A l o u e t t e s i l t loam and H a l l a r t s i l t y c l a y loam. 48 w i t h t h e i r f r i t t e d g l a s s bead cups l o c a t e d a t dep t h s o f 1, 5, 15, 30, and 50 cm. Open w e l l s encased w i t h p e r f o r a t e d 1.9 cm b o r e g a l v a n i s e d p i p e s were augered a t 3 l o c a t i o n s a l o n g one d i a g o n a l o f t h e b l o c k t o i n d i c a t e w a t e r t a b l e d e p t h . A t s e l e c t e d e l e v a t i o n s o f t h e w a t e r t a b l e , p e n e t r a t i o n r e s i s t a n c e was measured w i t h a Gouda t y p e HSA-5, 5 cm 2 b a s e , 60° t i p , cone p e n e t r o m e t e r ( F i g . 2) a t 6 l o c a t i o n s . A t each l o c a t i o n measurements were t a k e n a t dept h s o f 0, 1, 5, 10, 15, 20, 25, 30, 40, 50,and 60 cm. Some i n v e s t i g a t o r s , and e s p e c i a l l y t h o s e i n v o l v e d i n m i l i t a r y s t u d i e s ( K n i g h t and F r i e t a g , 1962; G i l l and Vanden B e r g , 1968; Rush, 1968) have i d e n t i f i e d t h e 15-30 cm l a y e r , t h e s o - c a l l e d " c r i t i c a l l a y e r " w i t h s o i l t r a f f i c a b i l i t y . However, i n t h e p r e s e n t s t u d y emphasis was p l a c e d on t h e 0-15 cm l a y e r s i n c e t h e p r o p e r t i e s o f t h e p l o u g h l a y e r a r e c e r t a i n l y v e r y dynamic and i m p o r t a n t i n A g r i c u l t u r e . S o i l s t r e n g t h was, t h e r e f o r e , e x p r e s s e d as t h e mean p e n e t r a t i o n r e s i s t a n c e o f s i x r e p l i c a t i o n s o f v a l u e s r e c o r d e d a t l e v e l s o f 0, 1, 5, 10,and 15 cm. A t t h e s t a r t o f t h e e x p e r i m e n t t h e s o i l was s a t u r a t e d by r a i s i n g t h e w a t e r t a b l e i n t h e t r e n c h t o t h e s o i l s u r f a c e i n s t a g e s o v e r a p e r i o d o f 7 days t o m i n i m i z e a i r entrapment. S a t u r a t i o n was c o n f i r m e d by t e n s i o m e t e r o u t p u t . L o s s o f w a t e r f r o m the t r e n c h by deep seepage was s i g n i f i c a n t o n l y w i t h i n t h e s i l t y c l a y loam w i t h a pe a t s u b s t r a t u m . The w a t e r l e v e l was c o n t r o l l e d by pumping_ a t a r a t e e q u a l t o t h e seepage r a t e . 49 FIG. 2: Gouda t y p e H.S.A.-5 cone p e n e t r o m e t e r . 50 3.2 Large-scale Field Experiments D u r i n g t h e l a t e W i n t e r and S p r i n g o f 1976 and 1977 cone p e n e t r a t i o n r e s i s t a n c e and s o i l w a t e r t e n s i o n were p e r i o d i c a l l y measured a t 3 l o c a t i o n s midway between and o v e r t i l e s on t h e e x p e r i m e n t a l s i t e s d e s c r i b e d i n C h a p t e r 1 ( F i g . 2 o f t h a t C h a p t e r ) . Measurements were a l s o t a k e n a t t h r e e l o c a t i o n s w i t h i n t h e u n d r a i n e d a r e a s . A l t o g e t h e r 70 p e n e t r a t i o n r e s i s t a n c e p r o f i l e s o f 6 r e p l i c a t e s e ach were o b t a i n e d by t h i s means f o r H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) ; 60 p r o f i l e s f o r H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) and, 71 f o r Lumbum muck. S o i l w a t e r t e n s i o n p r o f i l e s were measured i n c o n j u n c t i o n w i t h each s t r e n g t h p r o f i l e . The methods o f measurement and e x p r e s s i o n o f t h e r e s u l t s were d e s c r i b e d i n t h e p r e c e d i n g s e c t i o n . I n t h e s e l a r g e - s c a l e e x p e r i m e n t s t e n s i o n was e x p r e s s e d as a mean of t h e measurements t a k e n a t 1, 5,and 15 cm. 3. 3 Results and Discussion R e s u l t s f r o m t h e s m a l l - s c a l e f i e l d e x p e r i m e n t s a r e shown i n F i g s . 3 and 4. S o i l s t r e n g t h , s o i l w a t e r t e n s i o n , and d e p t h t o t h e w a t e r t a b l e a r e p l o t t e d o v e r t h e p e r i o d s o f c o n t r o l l e d d r a i n a g e . A l s o shown a r e r a i n f a l l d i s t r i b u t i o n s d u r i n g t h e e x p e r i m e n t s . F o r b o t h A l o u e t t e s i l t loam and H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) l o w e r i n g of t h e w a t e r t a b l e caused an i n c r e a s e i n s o i l w a t e r t e n s i o n and a c o r r e s p o n d i n g i n c r e a s e i n t h e s t r e n g t h o f t h e upper l a y e r s o f t h e s o i l . F i g . 3: Changes i n s o i l w a t e r t e n s i o n and s t r e n g t h i A l o u e t t e s i l t loam a t v a r i o u s w a t e r - t a b l e l e v e l s . 52 EIG. 4 : Changes i n s o i l w a t e r t e n s i o n and s t r e n g t h i n H a l l a r t s i l t y c l a y loam a t v a r i o u s w a t e r t a b l e l e v e l s . 53 A f t e r 7 days o f d r a i n a g e , heavy r a i n f a l l f o r c e d t h e d e c i s i o n t o r e s a t u r a t e t h e s i l t loam by r a i s i n g t h e w a t e r t a b l e t o t h e s o i l s u r f a c e i n s t a g e s . As a r e s u l t , s o i l w a t e r t e n s i o n d e c r e a s e d and cau s e d a d e c r e a s e i n t h e s o i l ' s s t r e n g t h . The s t r e n g t h r e c o r d e d a f t e r r e s a t u r a t i o n was t h e same as t h a t f o u n d a t i n i t i a l s a t u r a t i o n . D a t a o b t a i n e d from b o t h s m a l l - s c a l e and l a r g e - s c a l e f i e l d e x p e r i m e n t s were used t o e s t a b l i s h a r e l a t i o n s h i p between s o i l w a t e r t e n s i o n and s o i l s t r e n g t h w i t h i n t h e 0-15 cm l a y e r . The r e s u l t s a r e p r e s e n t e d i n F i g . 5. A d i r e c t l i n e a r r e l a t i o n s h i p was found f o r a l l 4 s o i l s . The e q u a t i o n s were as f o l l o w s : H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) : PR = 0.048 h + 4.73; , r 2 = 0.90 T e n s i o n range o f v a l i d i t y = 0.- 175 cm H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) : PR = 0.054 h + 3.13; r 2 = 0.91 T e n s i o n range o f v a l i d i t y = 0 - 135 cm A l o u e t t e s i l t loam: PR = 0.024 h + 4.52; , r 2 = 0.96 T e n s i o n range of v a l i d i t y = 0 - 110 cm Lumbum muck: PR = 0.035 h + 0.91; r 2 = 0.81 T e n s i o n range o f v a l i d i t y = 0 - 100 cm FIG. 5: Dependence of s o i l s t r e n g t h on s o i l w a t e r t e n s i o n i n t h e 0-15 cm l a y e r of f o u r l o w l a n d s o i l s . ( s t r e n g t h was d e t e r m i n e d by a 60° cone p e n e t r o m e t e r ) 55 Lumbum muck was o f t h e l o w e s t s t r e n g t h a t any t e n s i o n . The g r a s s f i e l d o f H a l l a r t s i l t y c l a y loam was o f s l i g h t l y g r e a t e r s t r e n g t h t h a n t h e c u l t i v a t e d f i e l d t h r o u g h o u t t h e t e n s i o n range i n v e s t i g a t e d . The s t r e n g t h o f A l o u e t t e s i l t loam a t s a t u r a t i o n ( z e r o t e n s i o n ) was i n t e r m e d i a t e between t h o s e shown by t h e two s i l t y c l a y loams. However, i t s s t r e n g t h i n c r e a s e d a t a s l o w e r r a t e as s o i l w a t e r m e n i s c i t e n s i o n i n c r e a s e d . S t u d i e s by W i l l i a m s and Shaykewich (1970) and, S t e i n h a r d t and T r a f f o r d (1974) have i n d i c a t e d l i n e a r r e l a t i o n s h i p s between s o i l s t r e n g t h and w a t e r t e n s i o n i n t h e v e r y wet range. The e f f e c t t h a t s o i l b u l k d e n s i t y has on s o i l s t r e n g t h ( M i r r e h and K e t c h e s o n , 1972) and p o s s i b l y on t h e s t r e n g t h - t e n s i o n r e l a t i o n s h i p has n o t been i n v e s t i g a t e d i n t h i s s t u d y because o n l y a s m a l l r a n g e o f d e n s i t y l e v e l s has been e n c o u n t e r e d f o r e a c h s o i l under f i e l d c o n d i t i o n s i n S p r i n g . 4. TEST OF THE EMPIRICAL MODEL FOR A PHYSICAL BASIS 4.1 A Theoretical Model S t e i n h a r d t (1974) r e v i e w e d a s p e c t s o f s o i l s t r e n g t h t h e o r y f o r u n s a t u r a t e d s o i l s and e x p r e s s e d t h e c o h e s i o n , c, by c = c + x t a n d> (h - Au) + c + Ac o A Y n o As a w o r k i n g h y p o t h e s i s he assumed t h a t w i t h i n a r e s t r i c t e d t e n s i o n range o f t h e w a t e r c h a r a c t e r i s t i c s and l o a d i n g c o n d i t i o n s , h and x ( a s a f u n c t i o n o f h ) , i n a d r a i n i n g s o i l , change i n d e p e n d e n t l y o f a l l o t h e r v a r i a b l e s on t h e r i g h t hand s i d e o f e q u a t i o n ( 4 ) . The 56 a s s u m p t i o n t h a t and <(> a r e i n d e p e n d e n t of h i s based on l a b o r a t o r y r e s u l t s o f F a r r e l l and G r e a c e n , 1966; W i l l i a m s and S h a y k e w i c h , 1970; and E l - D o m i a t y and C h a n c e l l o r , 1970. He f u r t h e r s t a t e d t h a t w i t h i n t h e v e r y wet r ange A C q and Au c o u l d be c o n s i d e r e d n e g l i g i b l e and t h a t x remained n e a r l y c o n s t a n t . F o r t h e s h r i n k a g e and s w e l l i n g c h a r a c t e r i s t i c s o f t h e s o i l s used i n t h e p r e s e n t s t u d y y i n e q u a t i o n (1) can be t r e a t e d as c o n s t a n t o v e r t h e t e n s i o n range of i n t e r e s t . By i n s e r t i n g e q u a t i o n (4) i n t o e q u a t i o n ( 1 ) : PR = K yBN + K N (C + X tan<(>(h - Au) + c + Ac ) + K yzji (5) Y Y c e o n o y q 1 b q D i f f e r e n t i a t i n g w i t h r e s p e c t t o h y i e l d s ^ s K c N c X t a n * (6) where h . < h < h mxn max E q u a t i o n (6) was o b t a i n e d by S t e i n h a r d t (1974) and t e s t e d s u c c e s s f u l l y w i t h i n the t e n s i o n range 0 - 5 0 cm. The o b j e c t i v e o f t h e p r e s e n t s t u d y was t o use t h i s s i m p l e model t o p r o v i d e some i n d i c a t i o n as t o t h e p h y s i c a l b a s i s o f t h e e m p i r i c a l models i n F i g . 5, b e a r i n g i n mind t h e a s s u m p t i o n s w h i c h a r e most v a l i d c l o s e t o s a t u r a t i o n . I f was f e l t t h a t as t h e s o i l s e x h i b i t e d f l a t r e t e n t i o n c u r v e s ( t h a t i s , t h e degree o f s a t u r a t i o n changed o n l y s l i g h t l y w i t h i n c r e a s i n g t e n s i o n w i t h i n t h e wet range) e q u a t i o n (6) m i ght be a p p l i c a b l e f o r a p p r o x i m a t e p r e d i c t i o n s o f cone p e n e t r a t i o n r e s i s t a n c e w h i c h i s an i n t e g r a t e d measure o f s o i l s t r e n g t h . 57 4.2 Evaluation of the Input Parameters 4.2.1 The angle of shearing resistance, $ U s i n g t h e method d e s c r i b e d by Lambe (1951) p l a s t i c i t y i n d i c e s were o b t a i n e d f o r H a l l a r t s i l t y c l a y loam ( g r a s s l a n d and c u l t i v a t e d f i e l d s ) and A l o u e t t e s i l t loam. R e s u l t s o f p l a s t i c and l i q u i d l i m i t measurements a r e summarized i n A p p e n d i x B. P l a s t i c i t y i n d i c e s a r e p r e s e n t e d i n T a b l e 1 below. The a n g l e o f s h e a r i n g r e s i s t a n c e , <j), was t h e n o b t a i n e d f r o m an e m p i r i c a l r e l a t i o n between <J> and p l a s t i c i t y i n d e x , P I , r e p o r t e d by T e r z a g h i and Peck (1967) i n t h e i r F i g . 18.1. T h i s a p p r o a c h o f r e c o g n i z i n g a r e l a t i o n s h i p between P I and <j) i s n o t new ( N i c h o l s , 1932; R u t l e d g e and McHardy, 1968; S t e i n h a r d t , 1974; Raghavan et al., 1977). S i n c e T e r z a g h i and Peck ' s (1967) r e l a t i o n s h i p was r e p o r t e d l y a p p l i c a b l e t o s e n s i t i v e c l a y s a more d i r e c t means o f m e a s u r i n g <j> was sought. I n a c c o r d a n c e w i t h the a p p r o a c h o f Panwar and Siemens (1972) t h e d i r e c t s h e a r t e s t method d e s c r i b e d by Lambe (1951) was c a r r i e d out on t r i p l i c a t e d c o m p o s i t e samples o f 2 mm s i e v e d a i r - d r y s o i l . Lumbum muck was a l s o a n a l y s e d by t h i s p r o c e d u r e . The s o i l s were compacted w i t h a m e t a l p l u n g e r t o f i e l d b u l k d e n s i t y l e v e l s and s h e a r e d under the a p p l i c a t i o n o f -2 n o r m a l s t r e s s e s o f a p p r o x i m a t e l y 0.10 and 0.35 kg cm . The d a t a a r e p r e s e n t e d i n A p p e n d i x D. Peak s h e a r s t r e s s was p l o t t e d a g a i n s t n o r m a l s t r e s s t o o b t a i n <(). The r e s u l t s a r e summarized i n T a b l e 1. The two methods y i e l d e d i d e n t i c a l r e s u l t s f o r t h e s i l t y c l a y loam. D i r e c t measurement y i e l d e d a much l o w e r v a l u e f o r t h e s i l t 58 TABLE 1: P l a s t i c i t y i n d e x , ( P I ) , and a n g l e o f s h e a r i n g r e s i s t a n c e , cf), f o r 0 - 15 cm l a y e r ( S t d . e r r o r shown i n b r a c k e t s ) S o i l P l a s t i c i t y I n d e x , ( P I ) (% w e i g h t ) f r o m P I <j>, deg D i r e c t s h e a r t e s t H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) 14 33 33 (0.9) H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) 34 34 (0.9) A l o u e t t e s i l t loam 34 27 (2.6) Lumbum muck 29 (1.2) 59 loam. A <f> o f 29 f o r Lumbum muck compared w e l l w i t h v a l u e s r e p o r t e d by Bekker (1969) and H e l e n e l u n d et al. (1972) f o r s u r f a c e l a y e r s of p e a t s o i l s . The v a l u e s o b t a i n e d by d i r e c t s h e a r were used i n t h e model. I t i s a p p r e c i a t e d t h a t a more a c c u r a t e and a p p l i c a b l e method o f m e a s u r i n g <j), perhaps c a r r i e d out in situ, might have been more d e s i r a b l e . However, o b v i o u s d i f f i c u l t i e s would have been e n c o u n t e r e d because o f t h e h e t e r o g e n e i t y of t h e p l o u g h l a y e r o f A g r i c u l t u r a l s o i l s . " U n d i s t u r b e d " samples f o r l a b o r a t o r y measurements o f cj) a r e u n l i k e l y t o y i e l d u s e f u l r e s u l t s because of t h e mass o f s h e a r p l a n e s o c c u p y i n g s u c h samples. I t must a l s o be r e a l i z e d t h a t no s i m p l e law w i l l d e s c r i b e t h e s t r e n g t h b e h a v i o r o f s u c h h e t e r o g e n e o u s and dynamic s o i l s as t h o s e used f o r A g r i c u l t u r e ( L a r s o n and A l l m a r a s , 1971). 4.2.2 The factor x B i s h o p and B l i g h t (1963) f o u n d x t o be > 0.85 w i t h i n t h e 0 - 50 cm t e n s i o n range f o r f o u r u n s a t u r a t e d c l a y s . W i l l i a m s and Shaykewich f o u n d x > S w i t h i n t h e v e r y wet range. I n t h e p r e s e n t s t u d y S^ , w i t h i n t h e t e n s i o n range o f i n t e r e s t was > 0.79 f o r t h e f o u r s o i l s . A x v a l u e o f 0.85 was used i n a c c o r d a n c e w i t h B i s h o p and B l i g h t ' s (1963) work. 60 4.2. 5 K and N —e a A major f a c t o r t h a t must be c o n s i d e r e d when e s t i m a t i n g t h e s e p a r a m e t e r s i s t h e mode o f sh e a r f a i l u r e . I n s o i l s t h a t a r e f a i r l y s t i f f t h e s t r a i n s p r i o r t o f a i l u r e a r e r e l a t i v e l y s m a l l and t h e t y p e o f s h e a r f a i l u r e t h a t o c c u r s i s r e f e r r e d t o as g e n e r a l s h e a r f a i l u r e . On t h e o t h e r hand, i n l o o s e s o i l s , t h e s t r a i n s p r i o r t o f a i l u r e a r e r e l a t i v e l y l a r g e and t h e mode o f f a i l u r e i s termed l o c a l s h e a r f a i l u r e ( C r a i g , 1974). A consequence o f l o c a l s h e a r f a i l u r e i s an e m p i r i c a l r e d u c t i o n i n t h e sh e a r s t r e n g t h p a r a m e t e r s , c and <j) , t o t w o - t h i r d s o f t h e v a l u e s used f o r g e n e r a l s h e a r f a i l u r e ( T e r z a g h i , 1943b). The v a l u e s o f must the n be r e p l a c e d by N ^ . B o t h N c and a r e o b t a i n e d f r o m <|>-Nc and < ( > - N ^ r e l a t i o n s h i p s f o r s h a l l o w f o o t i n g s p r e s e n t e d i n F i g . 33.4 of T e r z a g h i and Peck (1967). F o r a 60° cone S t e i n h a r d t (1974) o b t a i n e d e s t i m a t e s f o r o f 1.8 f o r g e n e r a l s h e a r f a i l u r e and 1.2 f o r l o c a l s h e a r f a i l u r e . These v a l u e s were, t h e r e f o r e , used i n t h e p r e s e n t i n v e s t i g a t i o n . G e n e r a l s h e a r f a i l u r e was assumed f o r A l o u e t t e s i l t loam and b o t h f i e l d s o f H a l l a r t s i l t y c l a y loam. T h i s a s s u m p t i o n f o r t h e c u l t i v a t e d f i e l d was thought v a l i d on t h e grounds t h a t i n S p r i n g t h e s o i l has a l r e a d y c o n s o l i d a t e d s i n c e t h e p r e v i o u s y e a r ' s o p e r a t i o n s . S i n c e o r g a n i c s o i l s a r e l o o s e and s e t t l e e x c e s s i v e l y under l o a d ( T e r z a g h i and Peck, 1967) o n l y 61 l o c a l s h e a r f a i l u r e c a n be assumed. H o l l i n g s h e a d and Raymond (1972) were o f t h e o p i n i o n t h a t l o c a l s h e a r f a i l u r e was common i n o r g a n i c s o i l s . 4. 3 Analysis U s i n g t h e d a t a f o r H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) as an example, e q u a t i o n (6) g i v e s , assuming g e n e r a l s h e a r f a i l u r e , d f f R ) = (1.8) (48) (.85) (.6494) = 47.7 dn The v a l u e o f 48 f o r Nc was o b t a i n e d from t h e <I>-N r e l a t i o n s h i p f o r s h a l l o w f o o t i n g s p r e s e n t e d i n F i g . 33.4 of T e r z a g h i and Peck (1967). When h i s e x p r e s s e d i n cm ( t h a t i s , u n i t s o f t e n s i o n ) r a t h e r t h a n s t r e s s u n i t s , t h i s becomes ~ 0.048 k g cm" 3 w h i c h d e s c r i b e s t h e s l o p e o f the p e n e t r a t i o n r e s i s t a n c e - t e n s i o n c u r v e . P r e d i c t e d and measured v a l u e s o f d(PR)/dh f o r t h e f o u r s o i l s a r e compared i n F i g . 6. S t a n d a r d e r r o r s o f the p r e d i c t i o n s based on t h e e r r o r i n m e a s u r i n g <j) a r e a l s o i n d i c a t e d . V e r y good co m p a r i s o n s were o b t a i n e d f o r t h e s i l t loam (agreement w i t h i n 6%) and t h e s i l t y c l a y loams ( w i t h i n 0.7%). The b e h a v i o r o f the muck was n o t p r e d i c t a b l e . The r e a s o n f o r t h i s c o u l d n o t be a c c o u n t e d f o r by t h e a s s u m p t i o n o f too s m a l l a v a l u e f o r x n o r by t h e e r r o r i n m e a s u r i n g <j>. I t i s n o t w i t h i n t h e scope o f t h i s s t u d y t o o f f e r e x p l a n a t i o n s 0.06 r 0.04 H PREDICTED d(PR) dh 0.02 h HALLART SILTY CLAY LOAM (CULTIVATED) HALLART SILTY CLAY LOAM (GRASSLAND) 0.02 MEASURED 0.06 FIG. 6: Comparison o f c a l c u l a t e d and measured s l o p e o f s o i l s t r e n g t h - t e n s i r e l a t i o n s h i p i n 0-15 cm l a y e r o f f o u r l o w l a n d s o i l s . ( S t a n d a r d e r r o r b a r s a r e r e l a t e d t o measurement o f <j>.) 63 w i t h r e s p e c t t o mechanisms o f s t r e n g t h b e h a v i o r . F u r t h e r work on t h e r e a c t i o n o f o r g a n i c s o i l s t o a p p l i e d s t r e s s e s seems n e c e s s a r y . S i n c e t h e t h e o r e t i c a l l y - b a s e d e q u a t i o n (6) s a t i s f a c t o r i l y p r e d i c t e d t h e s l o p e o f t h e s t r e n g t h - t e n s i o n r e l a t i o n s h i p i n t h e m i n e r a l s o i l s one can combine i t w i t h t h e e m p i r i c a l model t o p r e d i c t s o i l p e n e t r a t i o n r e s i s t a n c e f r o m t e n s i o n d a t a , o r viae versa, w i t h i n t h e v e r y wet range. I n t e g r a t i o n o f (6) y i e l d s PR - K N hx t a n cj> + 3 c c where t h e c o n s t a n t o f i n t e g r a t i o n , 3, i s t h e i n t e r c e p t o b t a i n e d f r o m t h e e m p i r i c a l r e g r e s s i o n e q u a t i o n . 3 depends on t h e p a r t i c u l a r d e p t h i n t e r v a l s o v e r w h i c h t h e v a l u e s o f cone p e n e t r a t i o n r e s i s t a n c e a r e a v e r a g e d . F o r H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) 3 = 4.73; f o r H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) 3 = 3.13; and, f o r A l o u e t t e s i l t loam 3 = 4.52. The s e m i - e m p i r i c a l models o b t a i n e d i n t h i s s t u d y c a n , t h e r e f o r e , be summarized as f o l l o w s : H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) : PR = 0.048 h + 4.73; 0 < h < 175 cm H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) : PR = 0.055 h +-3.13; 0 < h < 135 cm 64 A l o u e t t e s i l t loam: PR = 0.023 h + 4 . 5 2 ; . 0 < h < 110 cm (10) F o r Lumbum muck o n l y t h e e m p i r i c a l e q u a t i o n can be used: PR = 0.035 h + 0.91; 0 < h < 100 cm (11) A l o u e t t e s i l t loam was i n c l u d e d i n t h i s s t u d y m e r e l y t o t e s t t h e s t r e n g t h - t e n s i o n r e l a t i o n s h i p o v e r a w i d e r range of s o i l t e x t u r e s . 5. CONCLUSIONS S o i l s t r e n g t h and s o i l w a t e r t e n s i o n d a t a f o r t h e 0 - 15 cm l a y e r o b t a i n e d d u r i n g l a t e W i n t e r and S p r i n g 1976 and 1977 were used t o e s t a b l i s h a l i n e a r r e l a t i o n s h i p between t h e two v a r i a b l e s f o r t h r e e s o i l t y p e s . The t e n s i o n range of v a l i d i t y was fo u n d t o be between 0 cm and 100 - 175 cm depending on s o i l t y p e . A s i m p l e t h e o r e t i c a l l y - b a s e d model p r o p o s e d by S t e i n h a r d t (1974) t o d e s c r i b e t h e s l o p e o f t h e p e n e t r a t i o n r e s i s t a n c e - t e n s i o n r e l a t i o n s h i p c l o s e t o s a t u r a t i o n was found t o be a p p l i c a b l e t o A l o u e t t e s i l t loam and H a l l a r t s i l t y c l a y loam. The model c o u l d n o t p r e d i c t t h e s l o p e f o r Lumbum muck and an e n t i r e l y e m p i r i c a l r e l a t i o n s h i p was o b t a i n e d f o r t h i s s o i l . A p a r t f r o m t h e a s s u m p t i o n s made t o ' f o r m u l a t e t h e model, i t s a p p l i c a b i l i t y was dependent upon e m p i r i c a l e v a l u a t i o n o f t h e a n g l e o f s h e a r i n g r e s i s t a n c e , <j>, a f a c t o r , x> r e l a t e d t o t h e degree o f 65 s a t u r a t i o n , and t h e a s s u m p t i o n o f t h e mode o f s h e a r f a i l u r e . F u r t h e r work on t h e s t r e n g t h b e h a v i o r o f o r g a n i c s o i l s seemed n e c e s s a r y . By c o m b i n i n g t h e model w i t h t he e m p i r i c a l l y - d e r i v e d s t r e n g t h - t e n s i o n r e l a t i o n s h i p an e q u a t i o n w h i c h d e s c r i b e s t h e dependence o f s o i l s t r e n g t h on t e n s i o n c l o s e t o s a t u r a t i o n was o b t a i n e d f o r .the m i n e r a l s o i l s . Such an e q u a t i o n i s seen as b e i n g a u s e f u l t o o l f o r p r e d i c t i n g s o i l s t r e n g t h and hence, s o i l t r a f f i c a b i l i t y f r o m t e n s i o n d a t a i n v e r y wet s o i l s . A l s o , i f . a c r i t i c a l s o i l s t r e n g t h f o r t r a f f i c a b i l i t y were known, a c r i t i c a l t e n s i o n c o u l d be o b t a i n e d . T r a f f i c a b i l i t y o f Lumbum muck c o u l d be p r e d i c t e d w i t h t h e e m p i r i c a l model. 6. REFERENCES A i t c h i s o n , G. D. 1961. 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New Y o r k . John W i l e y and Sons. 510 pp. 69 T e r z a g h i , K. and R. B. Peck. 1967. S o i l mechanics i n E n g i n e e r i n g p r a c t i c e . 2nd Ed. John W i l e y and Sons. 729 pp. Towner, G. D. and E. C. C h i l d s . 1972. The m e c h a n i c a l s t r e n g t h o f u n s a t u r a t e d p o r o u s g r a n u l a r m a t e r i a l . J o u r . S o i l S c i . , V o l . 23, No. 4, 481-498. W i l l i a m s , J . and C. F. Shaykewich. 1970. The i n f l u e n c e o f s o i l w a t e r m a t r i c p o t e n t i a l on t h e s t r e n g t h p r o p e r t i e s o f u n s a t u r a t e d s o i l . S o i l S c i . Soc. Amer. P r o c . 34: 835-844. Wind, G. P. 1975. W o r k a b i l i t y and d r a i n a g e . Nota 890. I n s t i t u u t v o o r C u l t u u r t e c h n i e k en W a t e r h u i s h o u d i n g , Wageningen, t h e N e t h e r l a n d s . 70 CHAPTER 3 71 EFFECT OF SOIL WATER STATUS AND  STRENGTH ON TRAFFICABILITY ABSTRACT T r a f f i c a b i l i t y t e s t s w i t h t y p i c a l f a r m v e h i c l e s were c a r r i e d o ut on t h r e e l o w l a n d f i e l d s a t v a r i o u s d e grees o f we t n e s s . S t r u c t u r a l damage a f t e r t h e f i r s t and t h i r d p a s s e s was a s s e s s e d i n terms o f b u l k d e n s i t y , a e r a t i o n p o r o s i t y , p o r e - s i z e d i s t r i b u t i o n and r u t d e p t h . These i n d i c e s c o u l d n o t be used per se as c r i t e r i a f o r t r a f f i c a b l e c o n d i t i o n s because o f l a c k o f i n f o r m a t i o n c o n c e r n i n g w e l l - d e f i n e d s t r u c t u r a l l i m i t s f o r p l a n t g r o w t h . F o r each s o i l a r e l a t i o n s h i p was e s t a b l i s h e d between i t s s t r e n g t h ( a s s e s s e d w i t h a cone p e n e t r o m e t e r ) and t r a c t i o n e f f i c i e n c y measured by w h e e l s l i p . A c r i t i c a l v a l u e o f s t r e n g t h f o r t r a f f i c a b i l i t y was i n f e r r e d f r o m t h i s r e l a t i o n s h i p . T h i s was t h e n used t o o b t a i n . s o i l w a t e r t e n s i o n l i m i t s f o r t r a f f i c a b i l i t y f r o m known r e l a t i o n s between t e n s i o n and s t r e n g t h . S o i l s t r e n g t h was f o u n d t o be l i n e a r l y dependent upon w a t e r t a b l e d e p t h i n S p r i n g when e v a p o t r a n s p i r a t i o n was s m a l l and when t h e w a t e r t a b l e d e p t h was l e s s t h a n 80 cm. C o n s i d e r a t i o n o f t h e s e r e l a t i o n s h i p s l e d t o t h e e s t a b l i s h m e n t o f c r i t i c a l w a t e r t a b l e l i m i t s f o r t r a f f i c a b i l i t y . These were 53, 45, and 60 cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) , and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , r e s p e c t i v e l y . 1. INTRODUCTION S i n c e W o r l d War I I when ground f o r c e s s u f f e r e d .. . c o n s i d e r a b l e l o s s e s due t o poor ground c o n d i t i o n s s t u d i e s i n t o t r a f f i c a b i l i t y o f s o i l s have become i n c r e a s i n g l y i m p o r t a n t on a w o r l d - w i d e b a s i s . U n f o r t u n a t e l y , most o f t h e s e s t u d i e s have been o f a m i l i t a r y n a t u r e and t h e development o f equipment f o r t r a f f i c a b i l i t y assessment has been i n t h a t d i r e c t i o n . T h i s has r e s u l t e d i n a l a c k o f p r o p e r s o i l / m a c h i n e r y management g u i d e l i n e s i n a l a r g e p o r t i o n o f the w o r l d ' s f a r m l a n d s . I n A g r i c u l t u r e , s o i l t r a f f i c a b i l i t y may be d e f i n e d a s t h e a b i l i t y o f a s o i l t o s u p p o r t t r a f f i c w i t h o u t b e i n g damaged s t r u c t u r a l l y beyond l i m i t s f o r good c r o p growth. The t r a f f i c a b i l i t y o f a s o i l i s c o n s i d e r e d adequate i f t h e s o i l has s u f f i c i e n t s t r e n g t h i n terms o f b e a r i n g c a p a c i t y and t r a c t i o n c a p a c i t y t o s u p p o r t a v e h i c l e and e n a b l e i t t o d e v e l o p t h e f o r w a r d t h r u s t n e c e s s a r y t o overcome i t s r o l l i n g r e s i s t a n c e ( G i l l and Vanden B e r g , 1968). S o i l c o m p a c t i o n due t o t r a f f i c i s a l s o a f a c t o r t o c o n s i d e r when a s s e s s i n g t r a f f i c a b i l i t y . The i m p o r t a n c e o f t h e t r a f f i c a b i l i t y p r o b l e m around t h e w o r l d has prompted r e c e n t a c t i v i t y . A l d a b a g h (1971) and Reeve and Fa.usey (1974) i n t h e U n i t e d S t a t e s ; R u t l e d g e and McHardy ( 1 9 6 8 ) , H a r r i s o n and C e s s f o r d ( 1 9 6 9 ) , Feldman and Dorm i e r ( 1 9 7 0 ) , and S e l e r i o and Brown (1972) i n Canada; Wind ( 1 9 7 5 ) , and Feddes and v a n W i j k (1976) i n t h e N e t h e r l a n d s ; Soane (1970) and S t e i n h a r d t and T r a f f o r d (1974) i n E n g l a n d a r e some o f t h e r e s e a r c h e r s who have a d d r e s s e d t h e m s e l v e s t o v a r i o u s a s p e c t s o f t h e p r o b l e m o f m a n i p u l a t i n g wet s o i l s f o r A g r i c u l t u r a l p u r p o s e s . However, q u e s t i o n s o f optimum f i e l d w a t e r s t a t u s f o r s o i l m a n i p u l a t i o n , t h e e f f e c t s o f o p e r a t i n g t r a c t o r s a t h i g h s o i l w a t e r c o n t e n t s on t r a c t i o n e f f i c i e n c y and th e r e s u l t a n t e f f e c t s on A g r i c u l t u r a l p r o d u c t i v i t y due t o s o i l c o m p a c t i o n s t i l l r e m a i n l a r g e l y unanswered. S t u d i e s w h i c h p r o v i d e i n f o r m a t i o n on m a c h i n e - s o i l i n t e r r e l a t i o n s h i p s a r e , t h e r e f o r e , u r g e n t l y r e q u i r e d . The o b j e c t i v e s o f t h i s s t u d y a r e (a) t o examine t h e e f f e c t s of t r a f f i c on s o i l s t r u c t u r e w i t h a v i e w t o e s t a b l i s h i n g t r a f f i c a b i l i t y c r i t e r i a , (b) t o e s t a b l i s h a r e l a t i o n s h i p between s o i l s t r e n g t h and t r a c t i o n e f f i c i e n c y measured by w h e e l s l i p , and (c) t o d e t e r m i n e the l e v e l o f d r a i n a g e t h a t i s adequate t o a l l o w s o i l s t o e f f i c i e n t l y s u p p o r t f a r m t r a f f i c t y p i c a l l y e n c o u n t e r e d i n t h e Lower F r a s e r V a l l e y . 2. MATERIALS AND METHODS 2.1 Experimental Design and Layout The e f f e c t s o f s o i l w a t e r s t a t u s and s t r e n g t h on t r a f f i c a b i l i t y were s t u d i e d on s e l e c t e d a r e a s l o c a t e d on t h e e x p e r i m e n t a l f i e l d s d e s c r i b e d i n C h a p t e r 1. D i f f e r e n t d r a i n s p a c i n g s p r o v i d e d s t r i p s o f l a n d p a r a l l e l t o t i l e l i n e s a t v a r i o u s s o i l w a t e r t e n s i o n s and w a t e r t a b l e d e p t h s . Three t e s t s were r u n on H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) , f o u r on H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , and f o u r on Lumbum muck. A l l t e s t s on a p a r t i c u l a r s o i l were c o m p l e t e d w i t h i n f o u r h o u r s . Each t e s t c o n s i s t e d o f r u n n i n g a t r a c t o r 74 w i t h a t t a c h m e n t s a l o n g a 30.5 m s t r i p o f l a n d . The l a y o u t o f each t e s t i s shown d i a g r a m a t i c a l l y i n F i g . 1. Because o f t h e need t o m i n i m i z e s o i l v a r i a b i l i t y o v e r t h e t e s t s t r i p s and t h e work l o a d o v e r t h e s h o r t t i m e of e x p e r i m e n t a t i o n o n l y two r e p l i c a t e s of each t e s t were a t t e m p t e d . A l o n g each t e s t p l o t and b e f o r e each t e s t , s o i l w a t e r t e n s i o n was measured a t d e p t h s o f 1, 5, and 15 cm w i t h tensiometer-manometer systems. Cone p e n e t r a t i o n r e s i s t a n c e used as an i n d e x o f s o i l s t r e n g t h was measured a t d e p t h s o f 0, 1, 5, 10, and 15 cm. Depth t o t h e w a t e r t a b l e was measured i n open w e l l s l o c a t e d w i t h i n t h e t e s t p l o t s . D e t a i l s o f t h e manner o f measurement of t h e s e p r o p e r t i e s were g i v e n i n C h a p t e r 1. L o c a t i o n s o f s a m p l i n g s t a t i o n s f o r t h e s e measurements a r e shown i n F i g . 1. D u r i n g t h e f i r s t p a s s , w h e e l s l i p , used as an i n d e x o f t r a c t i o n e f f i c i e n c y , w a s r e c o r d e d by p l a c i n g a marker on t h e c i r c u m f e r e n c e o f one o f t h e d r i v e wheels and c o u n t i n g t h e number o f r e v o l u t i o n s c o m p l e t e d by t h e w h e e l as t h e t r a c t o r t r a v e r s e d t h e e n t i r e l e n g t h o f t h e t e s t s t r i p . P o s i t i o n o f t h e marker was r e a d r e l a t i v e t o t h e f a c e of an i m a g i n a r y c l o c k . T h i s f i x e d t h e p r e c i s i o n of measurement t o 1/48 o f a r e v o l u t i o n . The c i r c u m f e r e n c e of the d r i v e w h e e l s was measured when t h e t r a c t o r s t o o d on a f l a t c o n c r e t e s u r f a c e . W h e e l s l i p was t h e n e x p r e s s e d by ( G i l l and Vanden B e r g , 1968): % w h e e l s l i p = a ~ b 100% a where a = d i s t a n c e t r a v e l l e d by p o i n t on w h e e l and b = d i s t a n c e t r a v e l l e d by v e h i c l e 75 REP. 2 30.5m REP. I 30.5 m x SAMPLING STATION AFTER PASSES SAMPLING STATION BEFORE PASSES WHEELTRACKS t TRACTOR APPROACH FIG. 1: P l a n o f t e s t s t r i p r e p r e s e n t i n g 1 one degree o f wetness f o r t r a f f i c a b i l i t y t r i a l s . 76 T h i s method o f w h e e l s l i p measurement i s p r o b a b l y n o t as a c c u r a t e as more s o p h i s t i c a t e d methods a v a i l a b l e (Lyne and M e i r i n g , 1 9 7 7 ) . I t was, however, t h e b e s t t h a t c o u l d be found w i t h i n t h e r e s t r i c t i o n s o f t i m e and a v a i l a b l e r e s o u r c e s . There a r e many e r r o r s a s s o c i a t e d w i t h w h e e l s l i p measurements. These i n c l u d e f l e x i n g o f t i r e s d u r i n g t r a v e l , d i f f e r e n t i a l s i n k a g e a l o n g t h e t r a v e l p a t h and w i d e f l u c t u a t i o n o f s t r e s s e s a t t h e w h e e l - s l i p i n t e r f a c e . U n f o r t u n a t e l y , one can do v e r y l i t t l e about t h e s e e r r o r s a t t h e p r e s e n t t i m e ( G i l l and Vanden B e r g , 1 9 6 8 ; Lyne and M e i r i n g , 1 9 7 7 ) . A f t e r t h e f i r s t and t h i r d p a s s e s , r u t d e p t h was measured a t e ach s t a t i o n by p l a c i n g a l o n g p l a n k a c r o s s t h e w h e e l t r a c k s and r e c o r d i n g t h e v e r t i c a l d e p t h o f r u t below t h e bottom s t r a i g h t edge o f t h e p l a n k as i t l a y on t h e n a t u r a l s o i l s u r f a c e . Rut d e p t h measurement a t one of t h e M a t s q u i s i t e s i s shown i n P l a t e 1. Core samples 7.5 cm x 7.2 cm d i a m e t e r were t a k e n a t each s t a t i o n b e f o r e p a s s a g e and a f t e r t h e t h i r d p a s s . From t h e s e s a m p l e s , b u l k d e n s i t y , a e r a t i o n p o r o s i t y a t 60 cm t e n s i o n and p o r e s i z e d i s t r i b u t i o n were d e t e r m i n e d by t h e methods d e s c r i b e d by B l a k e ( 1 9 6 5 ) and V o m o c i l ( 1 9 6 5 ) . Core samples were n o t t a k e n a f t e r t h e f i r s t p a s s f o r f e a r o f i n t e r f e r e n c e w i t h subsequent passage. A l s o , whenever b o g g i n g o c c u r r e d i t was n o t v a l i d t o t a k e c o r e samples f r o m t h e w h e e l r u t s because o f g r e a t d i s t u r b a n c e and l o s s o f t o p s o i l ( b u l l d o z i n g ) . 7 7 78 2.2 Test Vehicles The t e s t v e h i c l e used a t t h e M a t s q u i s i t e s i s shown i n P l a t e 2. I t was a 65 HP Massey F e r g u s o n f a r m t r a c t o r p u l l i n g a Loewen 4500 l i t r e s f u l l manure s p r e a d e r . The ground p r e s s u r e a t -2 t h e d r i v e w h e e l s was e s t i m a t e d as 0.8 kg cm fro m measurements o f wheel c o n t a c t a r e a on h a r d ground and w e i g h t s o f t h e v e h i c l e s s u p p l i e d by t h e m a n u f a c t u r e r s . The w e i g h t o f t h e f u l l manure s p r e a d e r was c a l c u l a t e d t o be a p p r o x i m a t e l y 6,500 kg. The t e s t v e h i c l e used on Lumbum muck a t S u r r e y i s shown i n P l a t e 3. I t was a D a v i d Brown 990 S e l e c t a m a t i c 48 HP far m t r a c t o r w i t h f r o n t and back manure s c r a p e r s . I t s ground p r e s s u r e a t t h e -2 d r i v e w h e e l s was a p p r o x i m a t e l y 0.5 kg cm The v e h i c l e s were o p e r a t e d i n low gear w i t h e n g i n e s r u n n i n g a t f u l l t h r o t t l e . Speeds o f 2-3 km h ^ were m a i n t a i n e d as t h e y were d r i v e n i n a s t r a i g h t l i n e p a t h a l o n g t h e s t r i p s . Because o f p o s s i b l e i n t e r f e r e n c e w i t h t h e f a r m e r s ' s c h e d u l e s , , t e s t v e h i c l e s were a c c e p t e d and used as s u p p l i e d by t h e f a r m e r s . 3. RESULTS AND DISCUSSION 3.1 Effect of Traffic on Soil Physical Conditions . Changes i n b u l k d e n s i t y , a e r a t i o n p o r o s i t y , r u t d e p t h , and p o r e s i z e d i s t r i b u t i o n w i t h i n t he 0-15 cm l a y e r due t o passage o f the f i e l d m a c h i n e r y a r e shown i n F i g s . 2 and 3. A l s o shown a r e t h e ave r a g e s o i l w a t e r t e n s i o n (numbers above l i n e ) and w a t e r t a b l e d e p t h (below l i n e ) i n t h e t e s t s t r i p s d u r i n g t h e e x p e r i m e n t s . 65 hp MF 178 + 4500 l i t r e LOEWEN Manure Spreader 80 PLATE 3: F i e l d M a c h i n e r y - S u r r e y S i t e 45 hp DB 990 S e l e c t a m a t i c + f r o n t and back manure s c r a p e r s BULK DENSITY, g cm"5 AERATION POROSITY, RUT DEPTH, cm LUMBUM MUCK 10 0.6 0.2 20 10 B l a l 20 40 LlLli U BEFORE PASSAGE g AFTER l " PASS • AFTER 3 r f PASS JL s^ Sfe Jkfo HALLART SILTY CLAY LOAM GRASSLAND CULTIVATED mi i l !• 1, ^ ^ a TENSION (em) 60 ! ! 38 | i 27 ! • i : 7 : * t i 63 : i 47 i i 0 i 1 33 1 1 1 : 32 ! j 12 j t t i 3 : WATER TABLE (cm) , 80 • : 45 ; j 35 i i 1 4 ! ' 67 ! 1 1 i 45 | i ' 3 i I 66 ; ; 59 : I 34 ; i '2 i \ FIG. 2: E f f e c t s o f t r a f f i c on s o i l p h y s i c a l c o n d i t i o n s i n p l o u g h . U a y e r . B u l k d e n s i t y and a e r a t i o n p o r o s i t y were measured b e f o r e t r a f f i c and a f t e r 3rd p a s s ; r u t depth a f t e r 1 s t and 3rd p a s s e s . Mean t e n s i o n o f 0-15 cm l a y e r (above l i n e ) and wat e r t a b l e d e p t h (below l i n e ) w i t h i n t h e t e s t s t r i p s a r e shown a t bottom o f diagram. • • 82 PORE VOLUME DRAINED PER 10 cm INCREASE IN TENSION cm* cm"s x I0"5 4 8 4 0 32 24 16 8 0 24 16 2 4 16 I HALLART SILTY CLAY LOAM / - CULTIVATED., LUMBUM / MUCK .•60 BEFORE PASSAGE AFTER 3 r d PASS 38 27cm -J i_ HALLART SILTY CLAY LOAM - GRASSLAND 6 3 47 0 cm •. 33 32 I 12 cm 0 200 400 0 200 400 PORE DIAMETER, jim 200 400 FIG. 3: Changes i n p o r e s i z e d i s t r i b u t i o n due t o t r a f f i c a t d i f f e r e n t d e g rees o f we t n e s s . Numbers i n d i c a t e mean s o i l w a t e r t e n s i o n i n 0-15 cm l a y e r o f t e s t s t r i p s . P o r e d i a m e t e r s were c a l c u l a t e d from w a t e r d e s o r p t i o n d a t a ( V o m o c i l , 1965). 83 S i g n i f i c a n t changes i n s o i l p h y s i c a l properties were recorded i n some tests at a l l 3 s i t e s . In Lumbum muck the greatest increase (24%) i n bulk density, p^ , was recorded i n the test conducted at an average s o i l water tension of 38 cm. This increase was s t a t i s t i c a l l y s i g n i f i c a n t at the 0.10 l e v e l of p r o b a b i l i t y . However, the drop i n aeration p o r o s i t y (P = 0.10) was largest i n the d r i e s t s t r i p (average tension = 60 cm). This was probably a r e s u l t of an i n i t i a l l y high aeration porosity i n t h i s s t r i p . The d r a s t i c reduction i n aeration pores (> 50 ym diameter) due to t r a f f i c w ithin a l l the test s t r i p s i s apparent from the pore-size d i s t r i b u t i o n s before and a f t e r 3 passes shown i n F i g . 3. The implications of such reductions with respect to a i r and water movement can be very important f o r plant growth (Warkentin, 1971). On the grassland f i e l d of H a l l a r t s i l t y clay loam bulk density increased s i g n i f i c a n t l y (P < 0.05) a f t e r 3 passes i n a l l the t e s t s , the la r g e s t increase of 34% (P < 0.001) occurring within the wettest p l o t . Aeration porosity decreased s i g n i f i c a n t l y (P < 0.05) i n a l l p l o t s . The wettest p l o t showed the greatest decrease of 67% (P < 0.01). Pore-size d i s t r i b u t i o n was al t e r e d by t r a f f i c i n a manner s i m i l a r to the muck, large pores occupying a smaller volume a f t e r the t h i r d pass. Bulk density increases due to t r a f f i c were small but s i g n i f i c a n t (P < 0.05) within the c u l t i v a t e d f i e l d of H a l l a r t s i l t y 84 c l a y loam. A t t h e same ti m e a e r a t i o n p o r o s i t y a l s o d e c r e a s e d (P < 0.10) w i t h t h e l a r g e s t drop o f 86% (P = 0.05) o c c u r r i n g w i t h i n t h e 12 cm t e n s i o n t e s t p l o t . A p p a r e n t l y , t o t a l p o r o s i t y d e c r e a s e d o n l y s l i g h t l y w h i l e t h e p o r e - s i z e d i s t r i b u t i o n s h i f t e d towards t h e s m a l l e r p o r e s i z e s d r a i n e d a t t e n s i o n s g r e a t e r t h a n 60 cm. The p o r e s i z e d i s t r i b u t i o n c u r v e s s u p p o r t t h i s o b s e r v a t i o n . Rut d e p t h i n c r e a s e d s i g n i f i c a n t l y (P < 0.05) w i t h wetness a f t e r t h e f i r s t and t h i r d p a s s e s i n a l l t h e e x p e r i m e n t a l f i e l d s . I n t o l e r a b l e r u t d e p t h s (> 10 cm) were o b s e r v e d i n Lumbum muck i n a l l p l o t s and e s p e c i a l l y i n t h e p l o t l o c a t e d i n u n d r a i n e d l a n d where an a v e r a g e v a l u e of 35 cm w i t h a maximum o f 60 cm was r e c o r d e d . A b o g g i n g c o n d i t i o n was r e a c h e d i n t h i s p l o t . D epth o f r u t t i n g was n o t as s e v e r e on t h e s i l t y c l a y loam f i e l d s e x c e p t i n t h e w e t t e s t p l o t l o c a t e d on t h e u n d r a i n e d s e c t i o n of t h e c u l t i v a t e d f i e l d where t h e a v e r a g e r u t d e p t h was 10 cm and a b o g g i n g c o n d i t i o n r e a c h e d . I n t h i s d i s c u s s i o n , i t i s d i f f i c u l t t o i n t e r p r e t r e s u l t s on an a b s o l u t e b a s i s . C r i t i c a l s o i l p h y s i c a l l i m i t s f o r adequate p l a n t g r o w t h have n o t been e s t a b l i s h e d f o r a l l s p e c i e s o f p l a n t s g r o w i n g i n a l l c o n d i t i o n s . I n f a c t , s u c h an u n d e r t a k i n g would be e x t r e m e l y d i f f i c u l t , . i f n o t i m p o s s i b l e . However, i t seems t h a t becomes _3 l i m i t i n g f o r p l a n t g r o w t h a t about 1.20 -.1.50 g cm ( F l o c k e r et al., 1958; Adams et al., 1960; Soane, 1970). A number of p a p e r s d e a l i n g w i t h c r i t i c a l a e r a t i o n p o r o s i t i e s f o r p l a n t growth (Viehmeyer and H e n d r i c k s o n , 1948; F l o c k e r et al., 1959; M e r e d i t h and P a t r i c k , 1960; 85 Soane, 1970; G r a b l e , 1971) i n d i c a t e a w i d e range o f a e r a t i o n p o r o s i t i e s (0 - 15%) w h i c h may l i m i t g rowth depending upon o t h e r growth f a c t o r s . M e r e d i t h and P a t r i c k (1960) o b s e r v e d a c t i v e s u d a n g r a s s r o o t growth i n a c l a y loam h a v i n g an a e r a t i o n p o r o s i t y , d e t e r m i n e d a t 60 cm t e n s i o n , t o be as low as 0%. The t e n s i o n a t w h i c h a e r a t i o n p o r o s i t y i s d e t e r m i n e d i s n o t s t a n d a r d b u t v a r i e s f r o m 40 cm t o 300 cm. T h i s makes d i r e c t c o m p a r i s o n s o f r e p o r t e d v a l u e s d i f f i c u l t . I n l i g h t o f t h e f o r e g o i n g , d a t a f r o m t h e p r e s e n t e x p e r i m e n t ( F i g . 2) i n d i c a t e d t h a t on t h e b a s i s o f and a e r a t i o n p o r o s i t y Lumbum muck was t r a f f i c a b l e a t s m a l l v a l u e s o f t e n s i o n and w a t e r t a b l e d e p t h . However, t h e r e i s an o b v i o u s d i s a d v a n t a g e i n u s i n g (and p o s s i b l y a e r a t i o n p o r o s i t y ) as a c r i t i c a l l i m i t i n o r g a n i c s o i l s . R e s u l t s o f r u t d e p t h seemed more m e a n i n g f u l . T h e r e f o r e , s o l e l y on t h e b a s i s o f a 10 cm r u t d e p t h b e i n g e x c e s s i v e , i t appeared t h a t t h i s s o i l was t r a f f i c a b l e a t t e n s i o n s g r e a t e r t h a n 38 - 60 cm. The w a t e r t a b l e d e p t h s c o r r e s p o n d i n g t o t h i s t e n s i o n range i n t h e t e s t s t r i p s were 45 - 80 cm. I n t h e g r a s s l a n d and c u l t i v a t e d f i e l d s o f H a l l a r t s i l t y c l a y loam, and e s p e c i a l l y i n t h e f o r m e r , a e r a t i o n p o r o s i t y l e v e l s were s u p r i s i n g l y l ow (< 8%) even b e f o r e passage o f t h e v e h i c l e s . O nly b r o a d c o n c l u s i o n s c o u l d be made w i t h r e s p e c t t o s t r u c t u r a l damage by t r a f f i c i n t h e s e f i e l d s . No l i m i t i n g v a l u e s o f p ^ n o r r u t d e p t h were e n c o u n t e r e d e x c e p t i n t h e 3 cm t e n s i o n t e s t s t r i p o f t h e c u l t i v a t e d 86 f i e l d where r u t d e p t h was e x c e s s i v e . However, e x t r e m e l y low a e r a t i o n p o r o s i t y (< 3%) was e n c o u n t e r e d a f t e r 3 p a s s e s i n g r a s s l a n d t e s t p l o t s a t t e n s i o n s l e s s t h a n 47 cm (water t a b l e < 45 cm fr o m F i g . 2). The same was t r u e i n t h e c u l t i v a t e d f i e l d when t h e t e n s i o n was l e s s t h a n 33 cm (w a t e r t a b l e < 60 cm). I n summary, a t t e m p t s t o use s o i l s t r u c t u r a l i n d i c e s as c r i t e r i a f o r t r a f f i c a b l e c o n d i t i o n s have o n l y been p a r t l y s u c c e s s f u l . The r e a s o n s f o r t h i s were t w o f o l d . F i r s t l y , c r i t i c a l s o i l p h y s i c a l l i m i t s f o r p l a n t g r o w t h have n o t been e s t a b l i s h e d w i t h any degree o f c e r t a i n t y . S e c o n d l y , b u l l d o z i n g of s o i l o ut o f r u t s c o m p l i c a t e d i n t e r p r e t a t i o n o f t h e r e s u l t s . The b r o a d c o n c l u s i o n s made h e r e might be s u p p o r t i v e o f t h e e s t a b l i s h m e n t o f a t r a f f i c a b i l i t y c r i t e r i o n s u c h as % w h e e l s l i p as r e l a t e d t o t e n s i o n and w a t e r t a b l e d e p t h , t o be d e s c r i b e d l a t e r i n t h i s c h a p t e r . 3.2 Effects of Soil Strength on Wheelslip The r e l a t i o n s h i p between s o i l s t r e n g t h b e f o r e p a s s and w h e e l s l i p d u r i n g t h e f i r s t pass i s p r e s e n t e d i n F i g . 4. A t a l l t h r e e s i t e s w h e e l s l i p i n c r e a s e d r a p i d l y as s o i l s t r e n g t h d e c r e a s e d i n r e s p o n s e t o p o o r e r d r a i n a g e . A b o g g i n g c o n d i t i o n w i t h 100% w h e e l s l i p was o b t a i n e d i n Lumbum muck and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) i n u n d r a i n e d p l o t s . The h i g h e s t w h e e l s l i p o b t a i n e d on t h e g r a s s l a n d was o n l y 27% and t h i s was o b t a i n e d a t a s o i l w a t e r t e n s i o n o f 0 cm i n t h e 0 - 15 cm l a y e r . The f a r m e r was a b l e t o t r a v e r s e t h i s f i e l d w i t h the same equipment used i n t h e s t u d y 87 4: E f f e c t o f s o i l s t r e n g t h ( b e f o r e t r a f f i c ) on w h e e l s l i p d u r i n g t h e 1 s t p a s s . A w h e e l s l i p of 20% was used as a c r i t i c a l v a l u e f o r t r a c t i o n e f f i c i e n c y . 88 a l t h o u g h t h e w a t e r t a b l e was a t t h e s u r f a c e . However, as p o i n t e d o u t i n t h e p r e c e d i n g s e c t i o n , t r a f f i c a t such a low t e n s i o n c a u s e d a 34% i n c r e a s e i n b u l k d e n s i t y and an a e r a t i o n p o r o s i t y d e c r e a s e o f 67%. The c u r v i l i n e a r n a t u r e o f t h e s o i l s t r e n g t h - t r a c t i o n e f f i c i e n c y r e l a t i o n s h i p has been r e p o r t e d by Hassan (1977) who used t r a c t i v e p u l l as h i s e f f i c i e n c y i n d e x , Rush (1969) who used drawbar p u l l , and K n i g h t and F r i e t a g (1962) u s i n g t o w i n g f o r c e . T here i s g e n e r a l agreement among r e s e a r c h e r s t h a t maximum t r a c t i o n e f f i c i e n c y i s a c h i e v e d a t a w h e e l s l i p o f about 20% ( G i l l and Vanden B e r g , 1968; K i l g o u r , 1976; Lyne and M e i r i n g , 1977; Gee-Clough, 1977). U s i n g 20% s l i p as a c r i t i c a l v a l u e f o r t r a c t i o n e f f i c i e n c y , t h e c r i t i c a l s o i l s t r e n g t h v a l u e s o f t h e 0 - 15 cm l a y e r f o r e f f i c i e n t -2 t r a c t i o n were 2.6, 6.4, and 4.6 kg cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , r e s p e c t i v e l y ( F i g . 4 ) . D i f f e r e n c e s between t h e s e v a l u e s a r e p o s s i b l y r e l a t e d t o d i f f e r e n c e s i n mechanisms g o v e r n i n g t h e s o i l s ' i n t e r a c t i o n s w i t h t he moving v e h i c l e s . These v a l u e s can be used f o r e s t a b l i s h i n g d r a i n a g e d e s i g n c r i t e r i a f r o m known r e l a t i o n s h i p s between s o i l s t r e n g t h and h y d r o l o g i c p r o p e r t i e s . F o r example, f r o m t h e s t r e n g t h - t e n s i o n r e l a t i o n s h i p s p r e s e n t e d i n S e c t i o n 4.3 o f C h a p t e r 2, c r i t i c a l s o i l w a t e r t e n s i o n s f o r t r a f f i c a b i l i t y were 48, 35, and 27 cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , 89 r e s p e c t i v e l y . Why t h e c u l t i v a t e d s i l t y c l a y loam had a l o w e r c r i t i c a l t e n s i o n t h a n t h e g r a s s l a n d i s n o t c l e a r . The added b e a r i n g s t r e n g t h p r o v i d e d by a r e l a t i v e l y dense p l o u g h pan o b s e r v e d a t a p p r o x i m a t e l y 25 cm b e l o w t h e s u r f a c e , i n t h e f o r m e r f i e l d i s one p o s s i b l e r e a s o n . The e x i s t e n c e o f s u c h a pan i s e x e m p l i f i e d by t h e cone p e n e t r a t i o n r e s i s t a n c e p r o f i l e s shown i n F i g . 5. S o i l s t r e n g t h a t p l o u g h s o l e l e v e l was much g r e a t e r i n t h e c u l t i v a t e d f i e l d t h a n i n t h e g r a s s l a n d where no w e l l - d e f i n e d p l o u g h pan was o b s e r v e d . F u r t h e r r e s e a r c h on t h e e f f e c t s o f a p l o u g h pan on s o i l t r a f f i c a b i l i t y i s w o r t h y o f c o n s i d e r a t i o n . 3. 3 Effect of Water Table Depth on Traffioability L o w e r i n g o f t h e w a t e r t a b l e i n c r e a s e s t h e b e a r i n g s t r e n g t h of a s o i l and improves i t s a b i l i t y t o w i t h s t a n d f i e l d o p e r a t i o n s ( A r m s t r o n g , 1977). C o n t r o l o f t h e w a t e r t a b l e i s , t h e r e f o r e , one d i r e c t means o f c o n t r o l l i n g t r a f f i c a b i l i t y i n wet s o i l s . The e f f e c t o f w a t e r t a b l e d e p t h on s o i l s t r e n g t h d u r i n g S p r i n g 1976 and 1977 i s shown i n F i g . 6. Measurements were c a r r i e d out i n t h e e x p e r i m e n t a l f i e l d s a t l o c a t i o n s i n d i c a t e d i n F i g . 2 o f C h a p t e r 1 w h i l e t h e m e a s u r i n g p r o c e d u r e was d e s c r i b e d i n S e c t i o n 3 of t h a t c h a p t e r . The r e l a t i o n s h i p appeared t o be l i n e a r e x c e p t f o r w a t e r t a b l e d e p t h s g r e a t e r t h a n about 80 cm a f t e r w h i c h s o i l s t r e n g t h i n c r e a s e d more r a p i d l y . T h i s d e v i a t i o n was p a r t i c u l a r l y t r u e on some CONE PENETRATION RESISTANCE, kg cm"2 0 2 4 6 8 10 12 DEPTH, cm FIG. 5: Cone p e n e t r a t i o n r e s i s t a n c e p r o f i l e s i n g r a s s l a n d and c u l t i v a t e d f i e l d s of H a l l a r t s i l t y c l a y loam. Measurements were made a t s i x l o c a t i o n s i n t h e u n d r a i n e d p l o t s i n m i d - A p r i l 1977 when t h e wat e r t a b l e depth i n b o t h p l o t s was 16 cm. S t a n d a r d e r r o r b a r s a r e shown f o r measurements below 10 cm de p t h . 91 14 12 A HALLART SILTY CLAY LOAM (GRASSLAND) PR * 0.048 WT + 4.24 r 2 = 0.56 A HALLART SILTY CLAY LOAM (CULTIVATED) PR = 0.031 WT + 2.73 r 2 = 0.58 J- o LUMBUM MUCK PR = 0.039 WT + 0.53 A r 2 = 0.82 A A A C O N E . PENETRATION RESISTANCE kg cm"2 (PR) 20 40 60 80 100 WATER TABLE DEPTH, cm (WT) F i g . 6: E f f e c t o f w a t e r t a b l e d e p t h on s o i l s t r e n g t h o f - 0 - 1 5 cm l a y e r . R e g r e s s i o n l i n e s a r e f o r WT dept h s between 0 and 80 cm. 92 sunny days l a t e r i n S p r i n g when d r y i n g o f t h e s u r f a c e s o i l c a used i n c r e a s e s i n s t r e n g t h o f t h e p l o u g h l a y e r w i t h o u t a p p r e c i a b l y l o w e r i n g t h e a l r e a d y deep w a t e r t a b l e . F o r t h i s r e a s o n i t was tho u g h t v a l i d t o f i t a s t r a i g h t l i n e t o t h e d a t a f o r w a t e r t a b l e s between 0 and 80 cm. The b e s t f i t was o b t a i n e d f o r Lumbum muck 2 (r .= 0.82) i n w h i c h no w a t e r t a b l e d e p t h g r e a t e r t h a n 80 cm was e n c o u n t e r e d d u r i n g t h e s a m p l i n g p e r i o d s . Only f a i r f i t s were d i s p l a y e d 2 2 by the m i n e r a l s o i l s - r ( g r a s s l a n d ) = 0.56; r ( c u l t i v a t e d ) = 0.58. Some o f t h e v a r i a b i l i t y i n t h e d a t a f o r t h e s e two f i e l d s c o u l d be e x p l a i n e d by r e s t r i c t i o n o f v e r t i c a l f l o w a t t h e boundary between t h e m i n e r a l s o i l above and t h e p o r o u s p e a t below due t o t h e i r d i f f e r e n t h y d r a u l i c c o n d u c t i v i t i e s . (See A p p e n d i x B ) . Such a r e s t r i c t i o n c o u l d have u p s e t t h e te n d e n c y o f w a t e r i n t h e m i n e r a l s o i l t o e q u i l i b r a t e w i t h t h e w a t e r t a b l e i n t h e p e a t l a y e r . A l i n e a r r e l a t i o n s h i p between cone p e n e t r a t i o n r e s i s t a n c e and w a t e r t a b l e d e p t h has a l s o been r e p o r t e d by A l d a b a g h (1971) f o r s i x a r e a s i n Iowa. By a p p l y i n g t h e l i n e a r e q u a t i o n s t o t h e c r i t i c a l s t r e n g t h l e v e l s o b t a i n e d i n F i g . 4, i t was p o s s i b l e t o e s t a b l i s h c r i t i c a l w a t e r t a b l e l e v e l s f o r t r a f f i c a b i l i t y . These were 53, 45, and 60 cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) , and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , r e s p e c t i v e l y . These r e s u l t s r e c e i v e good s u p p o r t f r o m t h e b r o a d c o n c l u s i o n s made i n S e c t i o n 3.1 w h e r e i n t h e e f f e c t s o f t r a f f i c on s o i l p h y s i c a l p r o p e r t i e s were d i s c u s s e d . I n t h a t 93 s e c t i o n i t appe a r e d t h a t " c r i t i c a l " w a t e r t a b l e l e v e l s might be 48-80 cm f o r t h e muck, and 45 cm and 66 cm f o r t h e g r a s s l a n d and c u l t i v a t e d f i e l d s , r e s p e c t i v e l y . Of i n t e r e s t h e r e i s t h e work o f S t e i n h a r d t and T r a f f o r d (1974) who recommended w a t e r t a b l e d e p t h s o f 50 - 60 cm as b e i n g n e c e s s a r y t o m i n i m i z e s t r u c t u r a l damage o f c l a y s o i l s i n En g l a n d . A l s o , t h e f i n d i n g s o f S c h o t h o r s t (1970) who showed t h a t adequate b e a r i n g s t r e n g t h was o b t a i n e d i n p e a t g r a s s l a n d i n t h e N e t h e r l a n d s when t h e w a t e r t a b l e was deeper t h a n 30 cm. Even a f t e r t a k i n g i n t o a c c o u n t the a v e r a g i n g method used t o e x p r e s s t e n s i o n i n t h e 0 - 15 cm l a y e r t h e r e seems t o be a mismatch between t h e c r i t i c a l v a l u e s o f t e n s i o n and w a t e r t a b l e d e p t h i n t h e c u l t i v a t e d s i l t y c l a y loam. A p e r c h e d w a t e r t a b l e caused by t h e p l o u g h pan i n t h i s f i e l d c o u l d have d e c r e a s e d t h e s l o p e o f t h e s t r e n g t h - w a t e r t a b l e r e l a t i o n s h i p w i t h o u t i n t e r f e r i n g w i t h t h e s t r e n g t h - t e n s i o n r e l a t i o n s h i p e s t a b l i s h e d above t h e pan. F u r t h e r work on t h e e f f e c t s o f t h e p l o u g h pan on s o i l w a t e r - s t r e n g t h r e l a t i o n s i s n e c e s s a r y . 4. CONCLUSIONS S i g n i f i c a n t changes i n s o i l s t r u c t u r a l p r o p e r t i e s were r e c o r d e d when f a r m equipment was r u n on t h r e e wet l o w l a n d s o i l s . A t t e m p t s t o use b u l k d e n s i t y , a e r a t i o n p o r o s i t y and r u t d e p t h as c r i t e r i a f o r t r a f f i c a b l e c o n d i t i o n s were o n l y p a r t l y s u c c e s s f u l . B u l l d o z i n g o f s o i l o ut o f r u t s by wh e e l a c t i o n c o m p l i c a t e d i n t e r p r e t a t i o n o f t h e d a t a . 94 C r i t i c a l l e v e l s o f s o i l s t r e n g t h f o r e f f i c i e n t t r a c t i o n o f t y p i c a l f a r m v e h i c l e s were found f r o m t r a f f i c a b i l i t y t e s t s t o -2 be 2.6, 6.4, and 4.6 kg cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) and H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , r e s p e c t i v e l y . C o r r e s p o n d i n g c r i t i c a l s o i l w a t e r t e n s i o n s o f 48, 35, and 27 cm were o b t a i n e d f r o m known s t r e n g t h - t e n s i o n r e l a t i o n s h i p s . S o i l s t r e n g t h i n t h e p l o u g h l a y e r was l i n e a r l y dependent upon w a t e r - t a b l e d e p t h s between 0 cm and 80 cm. C r i t i c a l w a t e r t a b l e d e p t h s f o r t r a f f i c a b i l i t y o b t a i n e d by way o f t h i s dependency were 53, 45, and 60 cm f o r t h e muck, s i l t y c l a y loam ( g r a s s l a n d ) , and s i l t y c l a y loam ( c u l t i v a t e d ) , r e s p e c t i v e l y . A p l o u g h pan i n t h e c u l t i v a t e d f i e l d appeared t o i n f l u e n c e s o i l w a t e r - s t r e n g t h r e l a t i o n s and f u r t h e r i n v e s t i g a t i o n s i n t o t h i s i n f l u e n c e seemed e v i d e n t . 5. REFERENCES Adams, E. P. et al. 1960. I n f l u e n c e o f s o i l c o m p a c t i o n on cr o p growth and development. T r a n s . 7 t h I n t . Congr. S o i l S c i . 1: 607-615. A l d a b a g h , A. S. Y. 1971. E f f e c t o f t i l e d r a i n a g e on t r a f f i c a b i l i t y f o r A g r i c u l t u r a l equipment. Ph.D. D i s s . , Iowa S t a t e U n i v . , Ames, Iowa. A r m s t r o n g , A. C. 1977. F i e l d d r a i n a g e and f i e l d workdays: R e s u l t s f r o m a n a t i o n a l e x p e r i m e n t . The A g r . Eng., J o u r , and P r o c . I n s t . A g r . Eng., V o l . 32, No. 4: 93-94. 95 B l a k e , G. R. 1965. B u l k d e n s i t y . In: B l a c k et al. ( e d s . ) . Methods of s o i l a n a l y s i s . Agron. No. 9, P a r t I : 374-390. Amer. Soc. A g r o n . , M a d i s o n , Wis. Feddes, R. A. and A. L. M. v a n W i j k . 1976. I n f l u e n c e o f d r a i n a g e on c r o p y i e l d : an i n t e g r a t e d model a p p r o a c h . I n s t , f o r Land and Water Man. Res. 15 pp. Feldman, M. and K. W. D o r m i e r . 1970. Wheel t r a f f i c e f f e c t s on s o i l c o m p a c t i o n and growth o f wheat. Can. A g r . Eng., V o l . 12, No. 1: 8-11. F l o c k e r , W. J . et al. 1958. Response o f w i n t e r c o v e r c r o p s t o s o i l c o m p a c t i o n . S o i l S c i . Soc. Amer. P r o c . 22: 181-184. F l o c k e r , W. J. et al. 1959. Some growth r e s p o n s e s o f tomatoes t o s o i l c o m p a c t i o n . S o i l S c i . Soc. Amer. P r o c . 23: 188-191. Gee-Clough, D. 1977. Ground d r i v e equipment and a i d s t o e x t e n d t r a f f i c a b i l i t y l i m i t s and m i n i m i z e s o i l s t r u c t u r a l damage. J o u r , and P r o c . I n s t . A g r . Eng., V o l . 32, No. 4: 99-102. G i l l , W. R. and G. E. V a n d e n B e r g . 1968. S o i l dynamics i n t i l l a g e and t r a c t i o n . Hdbk. 316. A g r . Res. S e r v . , USDA. 516 pp. G r a b e l , A. R. 1971. E f f e c t s o f c o m p a c t i o n on c o n t e n t and t r a n s m i s s i o n o f a i r i n s o i l s . In: Barnes et al. ( o r g a n i z e r s ) . Compaction o f A g r i c u l t u r a l s o i l s . ASAE Monograph, pp. 154-164. H a r r i s o n , H. P. and R. G. C e s s f o r d . 1969. E s t i m a t i n g t r a c t o r drawbar p u l l f r o m s o i l p r o p e r t i e s . Can. A g r . Eng., V o l . 11, No. 2: 63-Hassan, A. E. 1977. T r a f f i c a b i l i t y s t u d y o f a c a b l e s k i d d e r . T r a n s . ASAE 20: 26-29. 96 K i l g o u r , J . 1976. A r e s m a l l t r a c t o r s p o s s i b l e ? W o r l d C r o p s . J u l y / A u g u s t 1976: 152-158. K n i g h t , S. J . and D. R. F r i e t a g . 1962. Measurement of s o i l t r a f f i c a b i l i t y c h a r a c t e r i s t i c s . T r a n s . ASAE 5: 121-214, 132. Lyne, P. W. and P. M e i r i n g . 1977. A w h e e l s l i p meter f o r t r a c t i o n s t u d i e s . T r a n s . ASAE 20: 238-242. M e r e d i t h , H. L. and W. H. P a t r i c k , J r . 1960. E f f e c t s o f s o i l c o m p a c t i o n on s u b s o i l r o o t p e n e t r a t i o n and p h y s i c a l p r o p e r t i e s o f t h r e e s o i l s i n L o u i s i a n a . Agron. J o u r . 52: 163-167. Reeve, R. C. and N. R. Fausey. 1974. D r a i n a g e and t i m e l i n e s s o f f a r m i n g o p e r a t i o n s . In: J a n van S c h i l f g a a r d e ( e d . ) . D r a i n a g e f o r A g r i c u l t u r e . Agron. Monograph 17: 55-66. Amer. Soc. A g r o n . , M a d i s o n , Wis. Rush, E. S. 1969. S o f t - s o i l p e r f o r m a n c e o f a f o u r - w h e e l - d r i v e l o g s k i d d e r . T r a n s . ASAE 12: 546-551. R u t l e d g e , P. L. and F. V. McHardy. 1968. The i n f l u e n c e o f the weather on f i e l d t r a c t a b i l i t y i n A l b e r t a . Can. A g r . Eng., V o l . 10, No. 2: 70-73. S c h o t h o r s t , C. 1970. The p o l d e r l e v e l f o r p e a t g r a s s l a n d i n t h e L o p i k e r w a a r d ( D u t c h ) . N o t a 595 1CW. S e l e r i o , I . S. and D. M. Brown. 1972. E s t i m a t i o n o f S p r i n g workdays f r o m c l i m a t o l o g i c a l r e c o r d s . Can. A g r . Eng., V o l . 14, No. 2: 79-81. Soane, B. D. 1970. The e f f e c t s o f t r a f f i c and implements on s o i l c o m p a c t i o n . J o u r . I n s t . A g r . Eng. O c t o b e r 1970. pp. 115-126. 97 S t e i n h a r d t , R. and B. D. T r a f f o r d . 1974. Some e f f e c t s o f s u b s u r f a c e d r a i n a g e and p l o u g h i n g on t h e s t r u c t u r e and c o m p a c t a b i l i t y o f a c l a y s o i l . J o u r . S o i l S c i . V o l . 25, No. 2: 138-152. Viehmey.er, F. J . and A. H. H e n d r i c k s o n . 1948. S o i l d e n s i t y and r o o t p e n e t r a t i o n . S o i l S c i . 65: 487-493. V o m o c i l , J . A. 1965. P o r o s i t y . In: B l a c k et al. ( e d s . ) . Methods o f s o i l a n a l y s i s . Agronomy No. 9, P a r t I I : 299-314. Amer. Soc. A g r o n . , M a d i s o n , Wis. W a r k e n t i n , B. P. 1971. E f f e c t s o f c o m p a c t i o n on c o n t e n t and t r a n s m i s s i o n o f w a t e r i n s o i l s . In: Barnes et al. ( o r g a n i z e r s ) . C ompaction of A g r i c u l t u r a l s o i l s . ASAE Monograph, pp. 126-153. Wind, G. P. 1975. W o r k a b i l i t y and d r a i n a g e . N o t a 890. I n s t i t u u t v o o r C u l t u u r t e c h n i e k en W a t e r s h u i s h o u d i n g , Wageningen, t h e N e t h e r l a n d s . 98 CHAPTER 4 99 INFLUENCE OF TILE DRAINAGE ON  SOIL TRAFFICABILITY IN SPRING ABSTRACT Depth t o t h e w a t e r t a b l e midway between t i l e l i n e s was measured i n S p r i n g 1976 and 1977 on t h r e e f a r m e r s ' f i e l d s d r a i n e d by t i l e s s p aced a t 30.5 m and 15.2 m. Measurements were a l s o made i n a d j o i n i n g a r e a s w i t h no t i l e d r a i n a g e . The e f f e c t s o f t i l e d r a i n a g e on t r a f f i c a b i l i t y were a s s e s s e d by i m p o s i n g an e m p i r i c a l l y - d e r i v e d w a t e r t a b l e l i m i t f o r e f f i c i e n t t r a c t i o n o f t y p i c a l f a r m v e h i c l e s . E v i d e n c e t o s u p p o r t i n c r e a s e d b e n e f i t s due t o t h e c l o s e r s p a c i n g was i n c o n c l u s i v e . When compared t o u n d r a i n e d l a n d t h e 30.5 m s p a c i n g r e s u l t e d i n e a r l i e r t r a f f i c a b i l i t y o f Lumbum muck by 37 days i n 1976 and 36 days i n 1977. F o r H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) t h e c o r r e s p o n d i n g v a l u e s were 13 days i n 1976 and 16 days i n 1977 w h i l e f o r t h e c u l t i v a t e d f i e l d on t h i s s o i l t y p e 25 days were g a i n e d i n each y e a r . A t i l e s p a c i n g o f 30.5 m was, t h e r e f o r e , recommended f o r e a r l y t r a f f i c a b i l i t y o f b o t h s o i l t y p e s i n S p r i n g . D r a i n a g e i n t e n s i t y e x e r t e d i t s g r e a t e s t i n f l u e n c e on t r a f f i c a b i l i t y when t h e i n t e n s i t y was low ( t h a t i s , a t l a r g e t i l e s p a c i n g s ) . Two p r o c e d u r e s f o r p r e d i c t i n g t r a f f i c a b i l i t y o f t i l e - d r a i n e d l a n d were d e s c r i b e d . P r e d i c t i o n s by t h e s e p r o c e d u r e s f o r b o t h 100 t i l e s p a c i n g s i n t h e two s o i l t y p e s s t u d i e d i n S p r i n g 1977 were i n c l o s e agreement w i t h one a n o t h e r and w i t h measured d a t e s on w h i c h t h e two s o i l t y p e s were c o n s i d e r e d t r a f f i c a b l e . 1. INTRODUCTION I n t h e Lower F r a s e r V a l l e y o f B r i t i s h C o l u m b i a , o n e - h a l f t o t h r e e - q u a r t e r s o f a l l t h e f a r m l a n d i s b o r d e r e d by ov e r 520 km of r i v e r dykes and 69 km o f s e a dykes a l t o g e t h e r p r o t e c t i n g some 73,000 ha o f l a n d a g a i n s t f l o o d i n g ( V i c t o r i a , 1968). Removal o f e x c e s s w a t e r becomes a p r i o r i t y i n S p r i n g when d e l a y s i n manure s p r e a d i n g , l a n d p r e p a r a t i o n , and p l a n t i n g can r e s u l t i n re d u c e d y i e l d t h r o u g h a s h o r t e n i n g o f t h e g r o w i n g season. T h i s i s n o t o n l y a l o c a l p r o b l e m . I n r e c e n t y e a r s many i n v e s t i g a t i o n s r e l a t e d t o S p r i n g workdays have been p r o c e e d i n g i n numerous d i s c i p l i n e s (Raghavan et al., 1976; Wind, 1976; E l l i o t t et al., 1977; S m i t h , 1977). D r a i n a g e by t i l e s i s one d i r e c t means o f i n f l u e n c i n g t h e e f f e c t i v e l e n g t h o f t h e g r o w i n g season. However, an e s t i m a t e o f t h e t i m e g a i n e d by t i l e d r a i n a g e i s needed by t h e f a r m e r who must s t r i k e a b a l a n c e between workdays g a i n e d and t h e c a p i t a l i n v e s t m e n t i n h i s d r a i n a g e system. A r e v i e w o f t h e l i t e r a t u r e by S t e i n h a r d t and T r a f f o r d (1974) has i n d i c a t e d a l a c k o f r e l i a b l e q u a n t i t a t i v e d a t a t o e v a l u a t e t h e e f f e c t s o f t i l e d r a i n a g e on s o i l t r a f f i c a b i l i t y . 101 The o b j e c t i v e s o f t h i s c h a p t e r a r e f i r s t , t o e v a l u a t e th e p e r f o r m a n c e o f t i l e d r a i n s i n s t a l l e d a t two s p a c i n g s i n terms o f t r a f f i c a b i l i t y . Second, t o examine t h e e f f e c t s o f d r a i n a g e i n t e n s i t y on t r a f f i c a b i l i t y as an a i d t o p l a n n i n g o f t i l e d r a i n a g e systems. And t h i r d l y , t o d e v e l o p p r o c e d u r e s f o r p r e d i c t i n g t r a f f i c a b l e c o n d i t i o n s o f t i l e - d r a i n e d l a n d f r o m a knowledge o f r a i n f a l l , b a s i c s o i l p r o p e r t i e s and s o i l - m a c h i n e i n t e r a c t i o n s . 2. EXPERIMENTAL A t t h e e x p e r i m e n t a l s i t e s d e s c r i b e d i n C h a p t e r 1 ( F i g . 2 ) , w a t e r t a b l e l e v e l s were m o n i t o r e d a l m o s t d a i l y midway between t i l e l i n e s and i n u n d r a i n e d p l o t s d u r i n g 1976 and 1977. D a t a f o r 1976 were s u p p l i e d by t h e E n g i n e e r i n g B r a n c h o f t h e B r i t i s h C o l u m b i a M i n i s t r y o f A g r i c u l t u r e . The method o f measurement has been d e s c r i b e d i n C h a p t e r 1. A t t h e M a t s q u i s i t e open w e l l s were l o c a t e d o n l y i n t h e g r a s s l a n d f i e l d because o f management c o n s i d e r a t i o n s . As a r e s u l t , w a t e r t a b l e d e p t h i n b o t h f i e l d s was r e p r e s e n t e d by a s i n g l e v a l u e . C o n s e q u e n t i a l e r r o r was c o n s i d e r e d s m a l l because t h e e n t i r e s i t e was e x t r e m e l y f l a t and s p o t c h e c k s o v e r t h e s a m p l i n g p e r i o d i n d i c a t e d s i m i l a r w a t e r t a b l e d e p t h s . 102 3. RESULTS AND DISCUSSION 3.1 Effect of Tile Drainage on Soil Trafficdbility in Spring Water t a b l e depths midway between t i l e l i n e s and i n u n d r a i n e d p l o t s d u r i n g S p r i n g 1976 and 1977 i n Lumbum muck and H a l l a r t s i l t y c l a y loam a r e shown i n F i g . 1. A l s o shown a r e r a i n f a l l d i s t r i b u t i o n s d u r i n g t h e same p e r i o d s . Superimposed on t h e diagrams a r e w a t e r - t a b l e d e p t h s a t w h i c h t h e s o i l s became t r a f f i c a b l e as d e t e r m i n e d i n C h a p t e r 3. Arrows i n d i c a t e d a t e s on w h i c h f a r m e r s p l o u g h e d t h e f i e l d s based on t h e i r own judgements and m a n a g e r i a l c o n s t r a i n t s . I t s h o u l d be n o t e d t h a t t h e s o i l s were p r o b a b l y w o r k a b l e some t i m e a f t e r t h e y became t r a f f i c a b l e d epending p r i m a r i l y upon t h e n a t u r e o f t h e work and d e p t h o f s o i l m a n i p u l a t i o n . Lumbum muck I n S p r i n g 1976 d r a i n a g e by t i l e s s p a c e d a t 30.5 m l e d t o an i n c r e a s e o f 11 t r a f f i c a b l e days o v e r p l o t s d r a i n e d by t h e c l o s e r s p a c i n g . T h i s r e s u l t i s d i f f i c u l t t o e x p l a i n . Seepage i n t o t h e 15.2 m p l o t s w h i c h were l o c a t e d c l o s e t o t h e d i s c h a r g e d i t c h was one p o s s i b l e r e a s o n . S o i l v a r i a b i l i t y w i t h r e s p e c t t o d e p t h o f t h e p e a t d e p o s i t i s a n o t h e r . N e v e r t h e l e s s , u n d r a i n e d a r e a s were t r a f f i c a b l e 37 days l a t e r t h a n p l o t s w h i c h were d r a i n e d by t i l e s spaced a t 30.5 m and 26 days l a t e r t h a n t h e 15.2 m p l o t s . U n d r a i n e d a r e a s were v e r y c l o s e t o b e i n g t r a f f i c a b l e on May 19 and i n t e r p r e t a t i o n o f the d a t a depended upon t h e p r e c i s i o n o f w a t e r t a b l e measurement. As me n t i o n e d i n S e c t i o n 3 o f C h a p t e r 1, t h i s p r e c i s i o n was w i t h i n 1 cm. 103 LUMBUM MUCK UNDRAINED 60 3 RAINFALL, 2 cm j 0 MAY I I .1 UNDRAINED MAY I JUNE I - 30.5 m TILE SPACING — — 15.2 m TILE SPACING WATER TABLE DEPTH, cm f OR ^ DATE FIELD PLOUGHED CRITICAL WATER TABLE DEPTH FOR TRAFFICABILITY HALLART SILTY CLAY LOAM 1977 MAY I JUNE I RAINFALL, 2 cm I 0 MAY I JUNE I i'l- " In 111 ill! - 1-I . 1 • FIG. 1: E f f e c t o f 3 d r a i n a g e i n t e n s i t i e s on w a t e r t a b l e d e p t h midway between t i l e l i n e s and i n u n d r a i n e d p l o t s i n S p r i n g . C r i t i c a l w a t e r t a b l e d e p t h s f o r t r a f f i c a b i l i t y a r e sup e r i m p o s e d . R a i n f a l l d i s t r i b u t i o n s d u r i n g t h e s a m p l i n g p e r i o d s a r e shown. Dates on w h i c h t h e f i e l d s were p l o u g h e d a r e i n d i c a t e d by a r r o w s . 104 I n 1977 p l o t s d r a i n e d by t i l e s spaced a t 15.2 m were t r a f f i c a b l e 4 days b e f o r e 30.5 m p l o t s and 40 days b e f o r e u n d r a i n e d p l o t s . I n b o t h y e a r s a c t u a l d a t e s on w h i c h t h e p l o t s were p l o u g h e d were i n r e a s o n a b l e agreement w i t h d a t e s c a l c u l a t e d f o r t r a f f i c a b l e c o n d i t i o n s a l t h o u g h i n 1977 t h e f a r m e r c o u l d have moved h i s equipment on t h e d r a i n e d p l o t s a p p r o x i m a t e l y two weeks e a r l i e r . Farmers i n v o l v e d i n t h i s p r o j e c t o f t e n t r i e d t o p l o u g h a l l p l o t s i n c l u d i n g u n d r a i n e d a r e a s a t t h e same t i m e . One s u c h a t t e m p t i s i n d i c a t e d i n F i g . 1 f o r S p r i n g 1977. The r e s u l t was a poor one. R e p l o u g h i n g and r e p l a n t i n g were c a r r i e d out 33 days l a t e r on June 9. The r e s u l t o f t h i s d e l a y i n p l a n t i n g t i m e , due t o l a c k of d r a i n a g e i s shown i n P l a t e 1. The c o r n p l a n t s on t h e l e f t were i n t h e 30.5 m p l o t s w h i l e t h o s e on t h e r i g h t were i n t h e u n d r a i n e d p l o t s . D a t a f r o m t h e two y e a r s d i d n o t i n d i c a t e t h a t t h e 15.2 m s p a c i n g was b e t t e r t h a n t h e 30.5 m s p a c i n g f r o m a t r a f f i c a b i l i t y v i e w p o i n t . The b e n e f i t s of one o v e r t h e o t h e r were r e v e r s e d i n t h e two y e a r s . O b v i o u s l y , a l o n g e r p e r i o d o f assessment would have y i e l d e d more c o n c l u s i v e e v i d e n c e . Taken o v e r t h e e n t i r e drawdown p e r i o d s p r e s e n t e d i n F i g . 1 t h e two s p a c i n g s seemed t o have a p p r o x i m a t e l y t h e same i n f l u e n c e . A f a r m e r would be w e l l a d v i s e d t o choose t h e l a r g e r s p a c i n g on t h e e v i d e n c e p r e s e n t e d h e r e i n . B e n e f i t s o f s u b s u r f a c e d r a i n a g e o v e r no d r a i n a g e were more e v i d e n t o v e r t h e 2-year p e r i o d and a t i l e s p a c i n g o f 30.5 m i s c e r t a i n l y recommended f o r e a r l y t r a f f i c a b i l i t y o f t h i s s o i l t y p e i n S p r i n g . 105 Result of 33 days' delay in seed-bed preparation and planting in Lumbum muck due to lack of tile drainage. Corn (Zea mays L.) plants on left are growing in an area drained by tiles spaced at 30.5 m; those on right are in undrained area. 106 Hallart silty clay loam I n t h e g r a s s l a n d f i e l d 30.5 m p l o t s were t r a f f i c a b l e 13 days b e f o r e u n d r a i n e d p l o t s i n S p r i n g 1976 and 16 days b e f o r e i : i S p r i n g 1977. The c l o s e r s p a c i n g l e d t o a f u r t h e r i n c r e a s e of a t Least 6 days i n 1976 and about 1 day i n 1977. F l u c t u a t i o n o f t h e w a t e r t a b l e due t o heavy r a i n f a l l i n l a t e A p r i l 1976 caused some d i f f i c u l t y i n i n t e r p r e t a t i o n o f t h e d a t a . I n t h e c u l t i v a t e d f i e l d i n 1976 t h e 15.2 m s p a c i n g p l o t s became t r a f f i c a b l e 26 days b e f o r e the u n d r a i n e d a r e a and 1 day b e f o r e t h e p l o t s w i t h t h e l a r g e r s p a c i n g . I n 1977 t h e v a l u e s were 30 days and 5 d a y s , r e s p e c t i v e l y . The f a r m e r p l o u g h e d t h e u n d r a i n e d s e c t i o n i n v e r y wet c o n d i t i o n s i n 1976 and p a i d t h e p e n a l t y of p o o r e s t a b l i s h m e n t and y i e l d o f h i s c o r n c r o p . I n 1977 o t h e r m a n a g e r i a l p r o b l e m s f o r c e d him t o p l o u g h t h e e n t i r e f i e l d when t h e u n d r a i n e d p l o t s became t r a f f i c a b l e . There were s m a l l but p o s s i b l y s i g n i f i c a n t d i f f e r e n c e s between t h e two t i l e s p a c i n g s i n b o t h f i e l d s i n b o t h y e a r s . The economic b e n e f i t s t h a t m ight be d e r i v e d f r o m s u c h s m a l l d i f f e r e n c e s cannot be made i n t h i s s t u d y . B e n e f i t s o f t i l e d r a i n a g e o v e r no d r a i n a g e were v e r y e v i d e n t and a t i l e s p a c i n g o f 30.5 m i s recommended i n b o t h t h e g r a s s l a n d and c u l t i v a t e d f i e l d s o f H a l l a r t s i l t y c l a y loam. r 107-3. 2 Influence of Tile Drainage Intensity on Trafficability As an a i d t o p l a n n i n g o f t i l e d r a i n a g e systems w a t e r t a b l e drawdown c u r v e s were s i m u l a t e d by t h e d r a i n a g e model d i s c u s s e d i n Ch a p t e r 1 by c h o o s i n g a range o f d r a i n a g e i n t e n s i t i e s . S i m u l a t i o n was begun w i t h t h e w a t e r t a b l e a t t h e s o i l s u r f a c e . There was no r a i n f a l l t h r o u g h o u t t h e p e r i o d o f s i m u l a t i o n and t i l e d e p t h was f i x e d a t 1 m. Known c r i t i c a l w a t e r t a b l e l e v e l s f o r t r a f f i c a b i l i t y o f t h e two s o i l t y p e s were super i m p o s e d on t h e o u t p u t . The r e s u l t s a r e p r e s e n t e d i n F i g . 2. As d r a i n a g e i n t e n s i t y , A, d e c r e a s e d f r o m a h i g h v a l u e the number o f days r e q u i r e d t o a t t a i n t r a f f i c a b i l i t y a t f i r s t i n c r e a s e d s l o w l y and t h e n more r a p i d l y as A f e l l b elow 0.014 day ^ i n b o t h s o i l t y p e s . T h i s i n d i c a t e d t h a t t i l e d r a i n a g e s p a c i n g e x e r t e d i t s g r e a t e s t i n f l u e n c e on t r a f f i c a b i l i t y when t h e s p a c i n g was l a r g e . When t h e s p a c i n g was s m a l l n o t as much i n f l u e n c e was a p p a r e n t . T h i s was i n agreement w i t h t h e work o f Wind (1975) who found c o n s i d e r a b l e i n f l u e n c e on w o r k a b i l i t y below an i n t e n s i t y o f 0.003 day f o r a d r a i n d e p t h o f 150 cm i n a loam. However, he s t a t e d t h a t s u c h low i n t e n s i t i e s were n o t a p p l i c a b l e i n p r a c t i c e . The i n f l u e n c e o f t i l e s p a c i n g on t r a f f i c a b i l i t y was a p p r o x i m a t e l y t w i c e as i m p o r t a n t f o r t h e a t t a i n m e n t o f t r a f f i c a b l e c o n d i t i o n s i n t h e muck t h a n i n t h e s i l t y c l a y loam g r a s s l a n d . R e f e r e n c e t o e q u a t i o n (10) o f C h a p t e r 1 can p r o v i d e t h e t i l e s p a c i n g a t any i n t e n s i t y i f t h e v a r i a b l e s d e s c r i b e d t h e r e i n a r e d e t e r m i n e d . 108 0.050 0.038 h DRAINAGE INTENSITY (day" 1 ) 0.026 h 0.014 h 0.002 LUMBUM MUCK H A L L A R T SILTY CLAY LOAM (GRASSLAND) 10 15 20 25 NUMBER OF DAYS REQUIRED TO ATTAIN TRAFF ICABLE CONDITION FIG. 2: I n f l u e n c e o f t i l e d r a i n a g e i n t e n s i t y ( E t i l e s p a c i n g ) on t h e number o f days r e q u i r e d t o a t t a i n t r a f f i c a b i l i t y f r o m a f u l l y s a t u r a t e d c o n d i t i o n d u r i n g a r a i n l e s s p e r i o d . 109 3.3 Prediction Procedures for Trafficability of Tile-drained Farmland The p r o c e d u r e s whereby t h e t r a f f i c a b i l i t y o f t i l e - d r a i n e d f a r m l a n d m i ght be p r e d i c t e d w i l l embrace s t u d i e s d e s c r i b e d i n C h a p t e r s 1 t h r o u g h 3. T h i s s e c t i o n , t h e r e f o r e , s e r v e s as a means of c o m b i n i n g t h e v a r i o u s p a r t s o f t h e p r e d i c t i o n p r o c e d u r e s . I n o r d e r t o s i m p l i f y t h e d e s c r i p t i o n an example w i l l be g i v e n u s i n g Lumbum muck d r a i n e d by t i l e s s p a ced a t 15.2 m i n S p r i n g 1977. A s c h e m a t i c r e p r e s e n t a t i o n o f t h e p r o c e d u r e s i s p r e s e n t e d i n F i g . 3. PROCEDURE 1 Step 1: From t r a f f i c a b i l i t y t e s t s ( C h a p t e r 3) an e m p i r i c a l r e l a t i o n s h i p was e s t a b l i s h e d between w h e e l s l i p and s o i l s t r e n g t h as measured by cone p e n e t r a t i o n r e s i s t a n c e . From t h e l i t e r a t u r e i t was f o u n d t h a t maximum drawbar p u l l o c c u r s a t a p p r o x i m a t e l y 20% w h e e l s l i p . T h i s v a l u e o f w h e e l s l i p was t a k e n as a c r i t i c a l l i m i t f o r t r a c t i o n e f f i c i e n c y . By r e l a t i n g 20% w h e e l s l i p t o s o i l s t r e n g t h via t h e e m p i r i c a l -2 r e l a t i o n s h i p a c r i t i c a l s t r e n g t h v a l u e o f 2.6 kg cm f o r t r a f f i c a b i l i t y was e s t a b l i s h e d f o r Lumbum muck ( F i g . 4 -C h a p t e r 3 ) . Step 2: I n t h e S p r i n g o f 1976 and 1977 w a t e r t a b l e d e p t h and s o i l s t r e n g t h were m o n i t o r e d and r e s u l t e d i n a l i n e a r r e l a t i o n s h i p between t h e two v a r i a b l e s ( F i g . 6 - C h a p t e r 3 ) . The r e g r e s s i o n e q u a t i o n was 110 ESTABLISHMENT OF CRITICAL SOIL STRENGTH REQUIRED FOR EFFICIENT TRACTION PROCEDURE 1 PROCEDURE 2 ESTABLISHMENT OF EMPIRICAL SOIL STRENGTH - WATER TABLE DEPTH RELATIONSHIP ESTABLISHMENT OF SEMI-EMPIRICAL SOIL STRENGTH - TENSION RELATIONSHIP (EMPIRICAL. FOR MUCK) CRITICAL WATER TABLE DEPTH FOR TRAFFICABILITY CRITICAL SOIL WATER TENSION FOR TRAFFICABILITY THEORETICAL MODEL FOR PREDICTION OF WATER TABLE DEPTH THEORETICAL MODEL FOR PREDICTION OF SOIL WATER TENSION PREDICTION OF TRAFFICABILITY FIG. 3: S c h e m a t i c r e p r e s e n t a t i o n o f p r o c e d u r e s f o r p r e d i c t i o n o f s o i l t r a f f i c a b i l i t y f r o m b a s i c s o i l p r o p e r t i e s I l l PR = 0.039 WT + 0.-53 ; . r 2 = 0.82 From t h i s e q u a t i o n a c r i t i c a l w a t e r t a b l e d e p t h o f 53 cm was o b t a i n e d f o r t r a f f i c a b i l i t y o f the muck s o i l . Step 3: Water t a b l e d e p t h s f o r e a r l y S p r i n g 1977 were p r e d i c t e d by t h e d r a i n a g e model i n C h a p t e r 1 ( F i g . 6 - C h a p t e r 1 ) . R e f e r e n c e t o t h e p r e d i c t e d l i n e f o r the.15.2 m s p a c i n g showed t h a t a w a t e r - t a b l e d e p t h of 53 cm was r e a c h e d on A p r i l 19 and hence t h e s o i l became t r a f f i c a b l e on t h a t d a t e . PROCEDURE 2 -2 Step 1: Same as i n P r o c e d u r e 1. C r i t i c a l PR = 2.6 kg cm Step 2: A s e m i - e m p i r i c a l s o i l s t r e n g t h - t e n s i o n model a p p l i c a b l e t o H a l l a r t s i l t y c l a y loam and A l o u e t t e s i l t loam was d e s c r i b e d i n Ch a p t e r 2 ( e q u a t i o n (7) - Ch a p t e r 2 ) . T h i s e q u a t i o n c o u l d be s o l v e d f o r t h e t e n s i o n , h, u s i n g t h e known c r i t i c a l PR f o r t h o s e s o i l s . F o r Lumbum muck, however, t h e r e l a t i o n s h i p was e m p i r i c a l and a c r i t i c a l t e n s i o n f o r t r a f f i c a b i l i t y o b t a i n e d f r o m . e q u a t i o n (11) o f C h a p t e r 2 was 48 cm. Step 3: S o i l w a t e r t e n s i o n f o r e a r l y S p r i n g 1977 was p r e d i c t e d i n C h a p t e r 1 ( F i g . 7 - Ch a p t e r 1 ) . R e f e r e n c e t o t h e p r e d i c t e d l i n e i n d i c a t e d t h a t a t e n s i o n o f 48 cm was r e a c h e d i n t h e 0 - 15 cm l a y e r o f Lumbum muck on A p r i l 21. The s o i l was t a k e n as b e i n g t r a f f i c a b l e on t h a t day. 112 The two d a t e s p r e d i c t e d by t h e two p r o c e d u r e s were e x t r e m e l y c l o s e . C h o i c e o f p r o c e d u r e depends upon t h e a v a i l a b i l i t y o f i n s t r u m e n t a t i o n f o r t h e measurement o f e i t h e r w a t e r t a b l e d e p t h o r s o i l w a t e r t e n s i o n i n t h e 0 - 15 cm l a y e r o f s o i l . T e s t o f t h e a c c u r a c y o f p r e d i c t i o n by a c t u a l w o r k d a t e was n o t p o s s i b l e because o f a number o f u n c o n t r o l l a b l e f a c t o r s . The 15.2 m p l o t was a c t u a l l y p l o u g h e d on May 7, t h a t i s , 2 weeks a f t e r t h e t r a f f i c a b l e d a t e p r e d i c t e d above. The r e a s o n s f o r t h i s a p p a r e n t d e l a y i n t h e f a r m e r ' s r e c o g n i t i o n o f t r a f f i c a b i l i t y were many. F i r s t l y , t h e s o i l was p r o b a b l y w o r k a b l e some days a f t e r t h e f i e l d became t r a f f i c a b l e . S e c o n d l y , he d e l a y e d w o r k i n g t h e s o i l u n t i l t h e u n d r a i n e d a r e a c o u l d j u s t be p l o u g h e d i n h i s e s t i m a t i o n (as a m a t t e r of f a c t , he d i d , w i t h poor r e s u l t s as d i s c u s s e d i n S e c t i o n 3.1). And t h i r d l y , h i s equipment was b e i n g used e l s e h w e r e on h i s farm. R e s u l t s o f p r e d i c t i o n o f t r a f f i c a b i l i t y o f Lumbum muck and H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) by t h e two p r o c e d u r e s a r e p r e s e n t e d i n T a b l e 1. P r e d i c t i o n was c a r r i e d o u t f o r b o t h t i l e s p a c i n g s i n S p r i n g 1977. R e f e r e n c e was made t o F i g s . 6 and 7 o f C h a p t e r 1. A l s o shown i n T a b l e 1 a r e t h e d a t e s o f w h i c h t r a f f i c a b i l i t y was r e a c h e d from t h e a c t u a l measurements o f w a t e r t a b l e d e p t h and s o i l w a t e r t e n s i o n p r e s e n t e d i n F i g s . .6 and 7 o f C h a p t e r 1. T h i s s e r v e d t o e v a l u a t e p r e d i c t a b i l i t y by t h e d r a i n a g e model i n terms o f a c t u a l t r a f f i c a b l e d a t e . TABLE 1: P r e d i c t i o n o f t r a f f i c a b i l i t y of t i l e - d r a i n e d l o w l a n d s o i l s i n S p r i n g 1977 by u s i n g a w a t e r t a b l e c r i t e r i o n ( P r o c e d u r e 1) and a s o i l w a t e r t e n s i o n c r i t e r i o n ( P r o c e d u r e 2) S o i l T i l e s p a c i n g (m) P r o c e d u r e 1 Date on w h i c h s o i l became t r a f f i c a b l e Measured from w a t e r t a b l e d e p t h * P r o c e d u r e 2 Measured by t e n s i o n i n 0-15 cm l a y e r Lumbum Muck 30.5 15.2 Apr. 21 Apr. 19 Apr. 24 Apr. 20 A p r . 23 A p r . 20 Apr. 23 Apr. 20 H a l l a r t S i l t y C l a y Loam ( G r a s s l a n d ) 30.5 15.2 Apr. -14 Apr. 11 Apr. 13 Apr. 11 A p r . 14 A p r . 14 Apr. 14 A p r . 14 *Taken from F i g . 6 o f C h a p t e r 1. Taken from F i g . 7 of Chap t e r 1. 114 Good co m p a r i s o n s between t h e two p r o c e d u r e s were o b t a i n e d . P r e d i c t i o n was w i t h i n 3 days f o r t h e s i l t y c l a y loam and w i t h i n 2 days f o r t h e muck. I t was i n t e r e s t i n g t h a t t h e muck was p r e d i c t e d as b e i n g t r a f f i c a b l e 1-2 weeks a f t e r t h e s i l t y c l a y loam. T h i s was i n agreement w i t h d a t a p r e s e n t e d i n F i g . 2. Moreover, 1-2 weeks i s a g e n e r a l time span i n w o r k a b i l i t y o f t h e s o i l t y p e s i n d r a i n e d a r e a s o f the Lower F r a s e r V a l l e y . When based on w a t e r t a b l e d e p t h , t r a f f i c a b l e d a t e p r e d i c t e d by t h e d r a i n a g e model was w i t h i n 3 days o f t h e measured d a t e f o r the muck and 1 day f o r t h e m i n e r a l s o i l . On t h e b a s i s o f s o i l w a t e r t e n s i o n p r e d i c t e d d a t e s were i d e n t i c a l t o measured d a t e s f o r b o t h s o i l s . T h i s was u n e x p e c t e d s i n c e t h e model was found t o g i v e a somewhat i n a c c u r a t e s i m u l a t i o n of t e n s i o n i n t h e p l o u g h l a y e r ( C h a p t e r 1 ) . A p p a r e n t l y , t h e a c c u r a c y was good enough f o r t r a f f i c a b i l i t y p r e d i c t i o n s i n t h e p a r t i c u l a r y e a r i n v e s t i g a t e d . The f a c t t h a t t h e two p r o c e d u r e s , based on d i f f e r e n t c o n s i d e r a t i o n s o f s o i l s t r e n g t h b e h a v i o r , were i n s u c h good agreement has g i v e n added c o n f i d e n c e i n t h e p r e d i c t i o n o f t r a f f i c a b i l i t y as o u t l i n e d i n t h i s s t u d y . I t s h o u l d be s t r e s s e d t h a t any model t h a t can p r e d i c t w a t e r t a b l e d e p t h and/or s o i l w a t e r t e n s i o n i n t h e p l o u g h l a y e r can be used i n t h e s e p r o c e d u r e s . One s u c h model was used i n t h i s s t u d y m a i n l y t o d e m o n s t r a t e i t s a p p l i c a b i l i t y . 115 4. CONCLUSIONS D a t a c o l l e c t e d o v e r a two-year p e r i o d f o r Lumbum muck d i d n o t i n d i c a t e t h a t a t i l e s p a c i n g o f 15.2 m l e d t o e a r l i e r t r a f f i c a b i l i t y i n S p r i n g t h a n a s p a c i n g o f 30.5 m. When compared t o u n d r a i n e d l a n d a s p a c i n g o f 30.5 m r e s u l t e d i n e a r l i e r t r a f f i c a b i l i t y by 37 days i n 1976 and 36 days i n 1977. I n H a l l a r t s i l t y c l a y loam s m a l l and p o s s i b l y i n s i g n i f i c a n t d e l a y s i n t r a f f i c a b i l i t y were c a u s e d by d r a i n a g e o f g r a s s l a n d and c u l t i v a t e d f i e l d s i a t a s p a c i n g o f 30.5 m r a t h e r t h a n 15.2 m. U n d r a i n e d g r a s s l a n d was t r a f f i c a b l e 13 days l a t e r t h a n 30.5 m t i l e - d r a i n e d l a n d i n S p r i n g 1976 and 16 days l a t e r i n 1977. I n a c u l t i v a t e d f i e l d t h e d e l a y was 25 days i n each y e a r . A t i l e s p a c i n g o f 30.5 m was recommended f o r e a r l y t r a f f i c a b i l i t y o f b o t h s o i l t y p e s i n S p r i n g . T i l e d r a i n s p a c i n g , i n d i c a t e d by d r a i n a g e i n t e n s i t y , had t h e g r e a t e s t e f f e c t on t r a f f i c a b i l i t y when t h e s p a c i n g was l a r g e . C hanging t h e d r a i n a g e i n t e n s i t y when i t was a l r e a d y h i g h had l i t t l e o r no e f f e c t on t r a f f i c a b i l i t y . T h i s i n d i c a t e d t h a t r e t u r n s on i n v e s t m e n t i n a t i l e d r a i n a g e s y s t e m d e c r e a s e a t an i n c r e a s i n g r a t e as t i l e s p a c i n g i s d e c r e a s e d . T i l e d r a i n a g e was about t w i c e as i m p o r t a n t f o r t r a f f i c a b i l i t y i n t h e muck t h a n i n t h e m i n e r a l s o i l . 116 P r e d i c t i o n s o f t r a f f i c a b i l i t y by two p r o c e d u r e s based on d i f f e r e n t a s p e c t s o f s o i l s t r e n g t h b e h a v i o r were w i t h i n 3 days o f one a n o t h e r . Compared t o t r a f f i c a b l e d a t e s d e t e r m i n e d by a p p l y i n g c r i t i c a l l i m i t s f o r t r a f f i c a b i l i t y t o measured v a l u e s o f w a t e r t a b l e d e p t h and t e n s i o n , b o t h p r o c e d u r e s p e r f o r m e d e x t r e m e l y w e l l i n t h e y e a r under i n v e s t i g a t i o n . 5. REFERENCES E l l i o t t , R. L. et al. 1977. A s i m u l a t i o n model f o r p r e d i c t i n g a v a i l a b l e days f o r s o i l t i l l a g e . T r a n s . ASAE 20: 4-8. Raghavan, G. S. V. et al. 1976. P r e d i c t i o n t e c h n i q u e s f o r t r a c t i o n u s i n g f i e l d and l a b o r a t o r y d a t a . T r a n s . ASAE 19: 405-408. S m i t h , C. V. 1977. Work days f r o m w e a t h e r d a t a . J o u r , and P r o c . I n s t . A g r . Eng. V o l . 32, No. 4: 95-98. S t e i n h a r d t , R. and B. D. T r a f f o r d . 1974. Some a s p e c t s o f s u b s u r f a c e d r a i n a g e and p l o u g h i n g on t h e s t r u c t u r e and c o m p a c t a b i l i t y o f a c l a y s o i l . J o u r . S o i l S c i . , V o l . 25, No. 2: 138-152. V i c t o r i a , B r i t i s h C o l u m b i a . 1968. I n f o r m a t i o n g u i d e - F r a s e r R i v e r f l o o d c o n t r o l program. 27 pp. Wind, G. P. 1975. W o r k a b i l i t y and d r a i n a g e . N o t a 890. I n s t i t u u t v o o r C u l t u u r t e c h n i e k en W a t e r s h u i s h o u d i n g , Wageningen, t h e N e t h e r l a n d s . 117 Wind, G. P. 1976. A p p l i c a t i o n o f a n a l o g and n u m e r i c a l models t o i n v e s t i g a t e t h e i n f l u e n c e o f d r a i n a g e on w o r k a b i l i t y i n S p r i n g . N eth. J . A g r i c . S c i . 24: 155-172. 118 SUMMARY Two p r o c e d u r e s f o r t h e p r e d i c t i o n o f s o i l t r a f f i c a b i l i t y o f t i l e - d r a i n e d f a r m l a n d s i n S p r i n g were d e v e l o p e d f r o m d a t a c o l l e c t e d f r o m f a r m e r s ' f i e l d s i n t h e Lower F r a s e r V a l l e y o v e r a two-year p e r i o d . By t h e s e p r o c e d u r e s one can p r e d i c t t r a f f i c a b i l i t y e i t h e r from s o i l w a t e r t e n s i o n i n t h e p l o u g h l a y e r o r fr o m w a t e r t a b l e d e p t h . The e f f e c t s o f d r a i n i n g t h e s e a r e a s by t i l e s spaced a t 30.5 m and 15.2 m were e v a l u a t e d i n terms o f s o i l t r a f f i c a b i l i t y i n S p r i n g 1976 and 1977. The f i r s t s t e p i n t h e s t u d y was t o t e s t a m a t h e m a t i c a l model p r o d u c e d i n t h e N e t h e r l a n d s f o r t h e c a l c u l a t i o n o f v e r t i c a l n o n - s t e a d y f l o w o f w a t e r i n s o i l s w i t h f i e l d d a t a . The model a c c u r a t e l y p r e d i c t e d w a t e r t a b l e d e p t h b u t d i d n o t g i v e so good an i n d i c a t i o n o f s o i l w a t e r t e n s i o n i n t h e top 15 cm l a y e r . Model o u t p u t was dependent upon d r a i n a g e i n t e n s i t y and upon t h e K.(h) f u n c t i o n g o v e r n i n g f l o w o f w a t e r i n t h e u n s a t u r a t e d zone. S e c o n d l y , a s i m p l e s e m i - e m p i r i c a l r e l a t i o n s h i p between s o i l s t r e n g t h , r e p r e s e n t e d by cone p e n e t r a t i o n r e s i s t a n c e , and s o i l w a t e r t e n s i o n was fo u n d a p p l i c a b l e f o r p r e d i c t i n g t r a f f i c a b i l i t y o f A l o u e t t e s i l t loam and H a l l a r t s i l t y c l a y loam. However, t h e model was n o t a p p l i c a b l e t o Lumbum muck and an e m p i r i c a l e q u a t i o n had t o be used f o r t h i s s o i l . Use of t h e t h e o r e t i c a l l y - b a s e d model f o r t h e m i n e r a l s o i l s depended upon a number o f s i m p l i f y i n g a s s u m p t i o n s i n c l u d i n g t h e mode o f s h e a r f a i l u r e . E v a l u a t i o n o f t h e a n g l e o f s h e a r i n g r e s i s t a n c e and a f a c t o r , x> r e l a t e d t o t h e degree o f s a t u r a t i o n was a l s o r e q u i r e d . 1 1 9 T h i r d l y , t r a f f i c a b i l i t y t e s t s w i t h t y p i c a l f a r m v e h i c l e s were c a r r i e d o u t a t v a r i o u s d e g r e e s o f w e t n e s s on t h e muck and s i l t y c l a y loam f i e l d s under i n v e s t i g a t i o n . C o n s i d e r a t i o n o f t h e e f f e c t s o f t r a f f i c on s o i l s t r u c t u r a l damage under wet c o n d i t i o n s p r o v e d p o s i t i v e b u t f a i l e d t o e s t a b l i s h t r a f f i c a b i l i t y c r i t e r i a on t h e b a s i s o f s o i l s t r u c t u r e . The main c r i t e r i o n f o r t r a f f i c a b i l i t y was o b t a i n e d f r o m t h e t e s t s by u s i n g 20% w h e e l s l i p as a l i m i t f o r t r a c t i o n e f f i c i e n c y . O t h e r c r i t e r i a f o r use on a p r a c t i c a l l e v e l were o b t a i n e d f r o m e s t a b l i s h e d r e l a t i o n s h i p s among w h e e l s l i p , s o i l s t r e n g t h , s o i l w a t e r t e n s i o n , and d e p t h t o t h e w a t e r t a b l e . C r i t i c a l t e n s i o n s f o r t r a f f i c a b i l i t y were 48, 35, and 27 cm f o r Lumbum muck, H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) , a n d H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) , r e s p e c t i v e l y . C o r r e s p o n d i n g l i m i t i n g d e p t h s t o t h e w a t e r t a b l e were 53, 45, and 60 cm. The p l o u g h pan i n t h e c u l t i v a t e d f i e l d o f s i l t y c l a y loam seemed t o i n f l u e n c e c r i t i c a l l i m i t s o b t a i n e d i n t h a t f i e l d and f u r t h e r r e s e a r c h i n t o t h e n a t u r e o f t h e i n f l u e n c e was recommended. E v a l u a t i o n o f t i l e d r a i n a g e systems was t h e n p o s s i b l e by i m p o s i n g t h e w a t e r t a b l e l i m i t s f o r t r a f f i c a b i l i t y upon measured drawdown c u r v e s f o r a 2-year p e r i o d . I n Lumbum muck a d r a i n s p a c i n g o f 30.5 m r e s u l t e d i n e a r l i e r t r a f f i c a b i l i t y i n S p r i n g by 37 days i n 1976 and 36 days i n 1977 when compared t o u n d r a i n e d l a n d . I n H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) u n d r a i n e d l a n d was t r a f f i c a b l e 13 days 120 l a t e r t h a n 30.5 m t i l e - d r a i n e d l a n d i n S p r i n g 1976 and 16 days l a t e r i n 1977. I n a c u l t i v a t e d f i e l d t h e d e l a y was 25 days i n each y e a r . E v i d e n c e o f s i g n i f i c a n t b e n e f i t s due t o t h e 15.2 m s p a c i n g o v e r t h e l a r g e r s p a c i n g was n o t c o n c l u s i v e i n any o f t h e a r e a s s t u d i e d . A d r a i n s p a c i n g o f 30.5 m was recommended f o r a l l t h r e e f i e l d s . A l o n g e r p e r i o d o f i n v e s t i g a t i o n c o u l d l e a d t o a b e t t e r c o m p a r i s o n o f t h e t i l e s p a c i n g s . S i m u l a t i o n o f w a t e r t a b l e drawdown c u r v e s o v e r r a i n l e s s p e r i o d s a t chosen d r a i n a g e i n t e n s i t i e s p r o v i d e d t h e f o l l o w i n g i n f o r m a t i o n : (1) d r a i n s p a c i n g had t h e g r e a t e s t e f f e c t on t r a f f i c a b i l i t y when t h e s p a c i n g was l a r g e r , (2) c h a n g i n g t h e d r a i n a g e i n t e n s i t y when i t was a l r e a d y h i g h d i d n o t i n f l u e n c e t r a f f i c a b i l i t y s i g n i f i c a n t l y and, (3) t i l e d r a i n a g e was t w i c e as i m p o r t a n t f o r t h e a t t a i n m e n t o f t r a f f 1 c a b i l i t y o f t h e muck t h a n t h e m i n e r a l s o i l . F i n a l l y , two p r o c e d u r e s f o r t h e p r e d i c t i o n o f s o i l t r a f f i c a b i l i t y e m bracing t h e e n t i r e s t u d y were p r e s e n t e d . By t h e s e methods one c o u l d p r e d i c t t r a f f i c a b i l i t y d i r e c t l y f r o m s o i l w a t e r t e n s i o n i n t h e p l o u g h l a y e r o r f r o m w a t e r t a b l e d e p t h . The p r o c e d u r e s were a p p l i e d t o t h e p r e d i c t i o n o f t r a f f i c a b i l i t y o f t h e muck and s i l t y c l a y loam g r a s s l a n d a t two t i l e s p a c i n g s i n S p r i n g 1977. P r e d i c t i o n s by t h e two methods were w i t h i n 2 days f o r t h e muck and 3 days f o r t h e m i n e r a l s o i l . T e s t o f t h e p r e d i c t i o n s by t r a f f i c a b l e d a t e s o b t a i n e d by d i r e c t measurement o f s o i l h y d r o l o g i c p a r a m e t e r s i n d i c a t e d e x t r e m e l y good p e r f o r m a n c e o f b o t h methods d u r i n g t h e y e a r under i n v e s t i g a t i o n . 121 G r e a t e r a c c u r a c y i n p r e d i c t i n g s o i l w a t e r t e n s i o n was seen as a f u t u r e r e s e a r c h need. A l t h o u g h t h e i n v e s t i g a t i o n s were a d d r e s s e d t o t h e problems o f t r a f f i c a b i l i t y i n S p r i n g , i t i s p o s s i b l e t h a t some o f the c o n c l u s i o n s made might a p p l y t o t r a f f i c a b i l i t y i n t h e F a l l and W i n t e r months when f i e l d o p e r a t i o n s s u c h as h a r v e s t i n g and manure s p r e a d i n g a r e c a r r i e d o u t . 122 APPENDIX A 123 SOIL PROFILE DESCRIPTIONS Horizon Depth, cm Description H a l l a r t S i l t y C l a y Loam ( O r t h i c Humic G l e y s o l ) Ap 0-25 Grey t o v e r y d a r k g r e y (10YR 3.5/1 m o i s t ) g r a n u l a r b l o c k y s t r u c t u r e ; f r i a b l e ; g r a s s r o o t s common; c r a c k s ; earthworm a c t i v i t y h i g h ; i r o n p i p e s i n o l d r o o t c h a n n e l s ; 20-25 cm l a y e r compact i n c u l t i v a t e d f i e l d . Ah 25-60 V e r y d a r k g r e y (10YR 3/1 m o i s t ) t o v e r y d a r k g r e y i s h brown (10YR 3/2 m o i s t ) p l a s t i c s i l t y c l a y ; s u b a n g u l a r b l o c k y s t r u c t u r e ; some m o t t l i n g ; f r i a b l e ; a few g r a s s r o o t s ; earthworm a c t i v i t y ; c l a y c u t a n s . Om 60-12Q S t r a t i f i c a t i o n s o f d a r k r e d d i s h brown (5YR 2.5/2 m o i s t ) p e a t w i t h i n s i l t y c l a y m e r g i n g w i t h d a r k f i b r o u s p e a t w i t h some s t r a t i f i e d s i l t . Bg 120-130+ S o f t l i g h t - g r e y c l a y m e r g i n g w i t h f i n e sand and c l a y i n r e d u c e d c o n d i t i o n . Lumbum Muck ( T y p i c M e s i s o l ) Op 0-25 Dark r e d d i s h brown (5YR 3/2 m o i s t ) muck and p e a t ; c r a c k s i n s u r f a c e ; g r a s s r o o t s common; p a r t i a l l y decomposed t r a s h from p r e v i o u s c r o p s . 124 Horizon Om I I B g Depth, om Description 25-*120 P e a t y muck w i t h h i g h c o n t e n t o f p a r t i a l l y decomposed sphagnum moss; some t w i g s and o t h e r woody o r g a n i c m a t e r i a l . T h i s h o r i z o n o f v a r i a b l e t h i c k n e s s . =120+ S i l t loam i n r e d u c e d c o n d i t i o n ; o f v a r i a b l e d e p t h . 125 APPENDIX B P h y s i c a l c h a r a c t e r i s t i c s o f t h e 0-15 cm l a y e r o f t h e s o i l s used i n the s t u d y M e c h a n i c a l A n a l y s i s * A t t e r b e r g L i m i t s % / / P a r t i c l e L i q u i d P l a s t i c % % % O r g a n i c d e n s i t y L i m i t L i m i t S o i l C l a y S i l t Sand m a t t e r (g.cm-3) (% wt) (% wt) H a l l a r t s i l t y c l a y loam 35 63 0 10 2.31+.03 74 60±.8 ( g r a s s l a n d ) H a l l a r t s i l t y c l a y loam 37 60 0 10 2.37±.04 66 59±.5 ( c u l t i v a t e d ) Lumbum muck 12 11 0 77 1.56±.01 — — A l o u e t t e s i l t loam 21 64 8 16 2.10±. 02 75 68±.3 $K of 0-50 s a t cm l a y e r of H a l l a r t s i l t y c l a y loam = 277 cm day 0-100 cm l a y e r = 379 cm day *Hydrometer method ( s i x samples) //Pycnometer method u s i n g kerosene ( f o u r samples) $ A u g e r - h o l e method ( s i x s i t e s ) continued. continued P h y s i c a l c h a r a c t e r i s t i c s o f t h e 0-15 cm l a y e r o f t h e s o i l s used i n t h e s t u d y S o i l Max. P r o c t o r d e n s i t y (g cm"3) C o m p a c t i b i l i t y Optimum w a t e r c o n t e n t Max. m o d i f i e d f o r c o m p a c t i o n P r o c t o r d e n s i t y (cm3 cm -^) (gm c m - 3 ) Optimum w a t e r c o n t e n t f o r c o m p a c t i o n i n m o d i f i e d method ( g . c m - 3 ) H a l l a r t s i l t y c l a y loam ( g r a s s l a n d ) 1.05 .52 1.24 .42 H a l l a r t s i l t y c l a y loam ( c u l t i v a t e d ) 1.00 .52 1.14 .46 Lumbum muck 0.39 ,66 0.45 61 A l o u e t t e s i l t loam 128 APPENDIX C R e s u l t s o f w a t e r t a b l e r e c e s s i o n measurements a t M a t s q u i and S u r r e y d u r i n g S p r i n g 1977 L o c a t i o n S o i l D r a i n S p a c i n g (m) Water T a b l e H e i g h t * D r a i n Depth (cm I n i t i a l , h o Above t) F i n a l ; h t R e c e s s i o n P e r i o d , t * ( days) M a t s q u i H a l l a r t s i l t y 30.5 64 29 42 ro c l a y loam 15.2 62 25 42 S u r r e y Lumbum muck 30.5 84 67 42 15.2 85 62 42 *At m i d - p o i n t between t i l e l i n e s . 130 APPENDIX D R e s u l t s o f d i r e c t s h e a r t e s t measurements ( s o i l t a k e n f r o m 0-15 cm l a y e r ) Normal Peak Shear Peak Dry B u l k Water S t r e s s S t r e s s ty D e n s i t y , C o n t e n t S o i l R e p l i c a t e (kg cm - 2) (kg c m - 2 ) (deg) (g cm" 3) (% wt) H a l l a r t s i l t y c l a y 1 .103 .089 .74 loam ( g r a s s l a n d ) .354 .251 33 .76 6 2 .103 .089 .78 .354 .244 32 .76 7 3 .103 .086 .77 .354 .258 35 .76 6 H a l l a r t s i l t y c l a y 1 .103 .103 .76 loam ( c u l t i v a t e d ) .354 .268 34 .77 8 2 .103 .099 .74 .354 .281 36 .78 7 3 .103 .097 .74 .354 .257 33 .78 7 A l o u e t t e s i l t loam 1 .103 .098 .72 .354 .262 32 .80 7 2 .103 .100 .77 .354 .231 27 .77 7 3 .103 .137 .76 .354 .248 23 .77 7 Lumbum muck 1 .103 .099 .38 .354 .226 27 .40 14 2 .103 .094 .41 .354 .248 31 .37 14 3 .103 .097 .41 .354 .236 29 .41 14 132 APPENDIX E 133 C o n v e r s i o n F a c t o r s f r o m M i s c e l l a n e o u s M e t r i c U n i t s t o SI U n i t s The f a c t o r r e l a t i n g t h e v a l u e of a p h y s i c a l q u a n t i t y i n S I u n i t s t o t h e v a l u e o f m i s c e l l a n e o u s m e t r i c u n i t s used i n t h i s t h e s i s i s l i s t e d f o r d i f f e r e n t q u a n t i t i e s i n t h e f o l l o w i n g t a b l e . V a l u e o f p h y s i c a l V a l u e o f p h y s i c a l q u a n t i t y i n m i s c e l l a n e o u s x C o n v e r s i o n f a c t o r = q u a n t i t y i n m e t r i c u n i t s S I u n i t s V a l u e i n P h y s i c a l M i s c e l l a n e o u s C o n v e r s i o n V a l u e i n Q u a n t i t y M e t r i c U n i t s F a c t o r S I U n i t s -3 -.3 _3 D e n s i t y 1 g cm 10 1 kg m -2 -2 -2 P r e s s u r e 1 kg cm 1.020x10 1 k N m 1 cm w a t e r 1.020x10" 2 1 N m~2 2 2 A r e a 1 ha 10 1 km 3 3 Volume 1 l i t r e 10 1 m

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UBC Theses and Dissertations

UBC Theses and Dissertations

Prediction of trafficability of tile-drained farmland Paul, Compton Laurence 1978

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Prediction of trafficability of tile-drained farmland Paul, Compton Laurence 1978

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