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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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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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I n f l u e n c e o f s o i l b u l k d e n s i t y and m a t r i c p r e s s u r e on s o i l r e s i s t a n c e t o p e n e t r a t i o n . Can. J . S o i l S c i . 52: 477-483. N i c h o l s , M. L. 1932. The dynamic p r o p e r t i e s of s o i l . I I I . Shear v a l u e s o f uncemented s o i l s . A g r . Eng. 1 3 ( 8 ) : 201-204. 68 Panwar, J . S. and J . C. Siemens. 1972. Shear s t r e n g t h and energy o f s o i l f a i l u r e r e l a t e d t o d e n s i t y and m o i s t u r e . T r a n s . ASAE 15: 423-427. Raghavan, G. S. V. et at. \977. P r e d i c t i o n o f c l a y s o i l c o m p a c t i o n . J . T e r r a m e c h a n i c s . V o l . 14, No. 1. pp. 31-38. Pergamon P r e s s . Reeve, R. C. and N. R. F ausey. 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 . Agronomy 17: 55-66. Amer. Soc. A g r o n . , M a d i s o n , Wis. Rush, E. S. 1968. T r a f f i c a b i l i t y t e s t s w i t h a t w o - w h e e l - d r i v e i n d u s t r i a l t r a c t o r . T r a n s . ASAE 11: 778-782. 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 t h e w e a t h e r 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 t e i n h a r d t , R. 1974. E v a l u a t i n g p e n e t r a t i o n r e s i s t a n c e and w h e e l s i n k a g e r e s p o n s e t o s o i l w a t e r s u c t i o n changes i n a d r a i n i n g c l a y s o i l . S o i l S c i . Soc. Amer. P r o c . , V o l . 38: 518-522. 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. T e r z a g h i , K. 1943b. T h e o r e t i c a l s o i l m e c h a n i c s . 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