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UBC Theses and Dissertations

The seismic cone penetrometer Rice, Anthony Henry 1984

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THE SEISMIC CONE PENETROMETER by ANTHONY HENRY RICE B.A.S.c.,the U n i v e r s i t y Of B r i t i s h Columbia, 1980 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES Department Of C i v i l E n g i n e e r i n g We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA December 1984 © Anthony Henry R i c e , 1984 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l ' f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree that p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of C i v i l E n g i n e e r i n g The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date: 10 December 1984 0 • 1 1 ABSTRACT A s t a t i c cone penetrometer was m o d i f i e d to i n c o r p o r a t e a small diameter t r i a x i a l geophone package. T h i s instrument was e v a l u a t e d by o b t a i n i n g downhole shear wave v e l o c i t y p r o f i l e s at s e l e c t e d r e s e a r c h s i t e s on the F r a s e r R i v e r D e l t a near Vancouver B.C. f o r the purpose of determining i n - s i t u dynamic shear moduli. A second geophone package was added to the instrument to assess the accuracy of shear wave v e l o c i t y measurements, and to determine a p r a c t i c a l instrument c o n f i g u r a t i o n f o r r o u t i n e f i e l d use. Shear wave c h a r a c t e r i s t i c s , downhole v e l o c i t i e s , s i g n a l r e p e a t a b i l i t y and s i g n a l amplitude were studied, to e v a l u a t e the energy source and downhole r e c e i v e r response, and to develop a r a t i o n a l t e s t i n g procedure. Plank type s i g n a l sources were used e x c l u s i v e l y and were found to be both s a t i s f a c t o r y and convenient. Geophone response and r e c e i v e r to s o i l c o u p l i n g were e x c e l l e n t and i d e n t i f i a b l e shear waves were obtained to depths of 40 metres. Down rod s i g n a l t r a n s m i s s i o n , s i g n a l r e p e a t a b i l i t y and problems with high background n o i s e l e v e l s were overcome by s e l e c t i v e t e s t i n g procedures and i n t e r p r e t a t i o n techniques. The r e s u l t s of t h i s study i n d i c a t e t hat a s t a t i c cone penetrometer c o n t a i n i n g j u s t a s i n g l e geophone can p r o v i d e a r a p i d and accurate method f o r c a r r y i n g out a downhole shear wave v e l o c i t y survey. The cone t e s t i n g procedure a l l o w s s u p e r i o r shear wave records s i n c e i t minimizes s o i l d i s t u r b a n c e , p r o v i d e s e x c e l l e n t s o i l to r e c e i v e r c o u p l i n g and c o n t r o l l e d r e c e i v e r o r i e n t a t i o n . Combined with data from the cone b e a r i n g , f r i c t i o n and pore pressure measurement elements, the i n t e r p r e t a t i o n of i n - s i t u s t a t i c and dynamic s o i l p r o p e r t i e s , using a s i n g l e t e s t h o l e , i s g r e a t l y enhanced. i i i i TABLE OF CONTENTS PAGE ABSTRACT i i TABLE OF CONTENTS i i i i LIST OF FIGURES vi ACKNOWLEDGEMENTS v i i i 1.0 INTRODUCTION 1 1.1 R a t i o n a l e f o r Seismic Cone Development 1 1.2 Shear Modulus as a G e o t e c h n i c a l Parameter 2 1.3 T h e s i s O r g a n i z a t i o n 6 2.0 IN-SITU SHEAR MODULUS DETERMINATION 8 2.1 Seismic Wave Phenomenon 8 2.1.1 Types of Seismic Waves 8 2.1.2 Shear Wave C h a r a c t e r i s t i c s 10 2.2 Determination of E l a s t i c Parameters 12 2.3 Conventional I n - S i t u Seismic Measurement 17 2.3.1 Conventional Crosshole T e s t i n g 18 2.3.2 Conventional Downhole T e s t i n g 19 3.0 CPT DOWNHOLE SEISMIC EQUIPMENT AND PROCEDURES 2 5 3.1 I n t r o d u c t i o n 25 3.2 Seismic Cone Penetrometer 27 3.3 Shear Wave Generation 30 3.3.1 S i g n a l Source 30 3.3.2 Source L o c a t i o n 31 3.4 Shear Wave D e t e c t i o n 34 3.4.1 Geophone Response 35 3.4.2 S i g n a l F i l t e r i n g 40 3.4.3 O s c i l l o s c o p e R e s o l u t i o n 41 3.5 T r i g g e r i n g Systems 44 3.6 F i e l d Procedure 45 3.7 Summary 48 4.0 DATA INTERPRETATION AND ANALYSIS 50 4.1 I n t r o d u c t i o n 50 4.2 A r r i v a l Time Measurement 50 4.2.1 Shear Wave I n t e r p r e t a t i o n 51 4.2.2 S t a t i s t i c a l E r r o r Assessment 54 4.3 Shear Wave V e l o c i t y Determination 62 4.3.1 T r a v e l Time E f f e c t s 63 4.3.2 T r a v e l Path E f f e c t s 65 4.4 Dynamic Shear Modulus C a l c u l a t i o n 69 4.5 Summary 74 V PAGE 5.0 DYNAMIC SHEAR MODULUS FIELD MEASUREMENTS 76 5.1 I n t r o d u c t i o n 76 5.2 CPT Seismic F i e l d T e s t i n g C a p a b i l i t y 76 5.2.1 McDonalds Farm S i t e 79 5.2.2 F o r t Langley Freeway S i t e 81 5.2.3 Annacis North P i e r S i t e 83 5.3 Comparison of I n - S i t u Moduli Measurements 85 5.3.1 S e l f - B o r i n g Pressuremeter Moduli 85 5.3.2 CPT Cone Bearing C o r r e l a t i o n s 87 5.3.3 Conventional Crosshole T e s t i n g 87 5.4 Conventional E m p i r i c a l R e l a t i o n s h i p s 92 5.5 Summary 97 6.0 CONCLUSIONS 98 6.1 Summary of Research F i n d i n g s 98 6.2 Fu r t h e r Research 101 6.3 C o n s i d e r a t i o n s f o r P r a c t i c a l A p p l i c a t i o n 103 REFERENCES 106 v \ LIST OF FIGURES FIGURE PAGE 1 S t r e s s , S t r a i n and Shear Modulus R e l a t i o n s h i p 3 2 V a r i a t i o n of Shear Modulus with Shear S t r a i n 5 3 Types of Seismic Waves 9 4 Shear Wave C h a r a c t e r i s t i c s 1 1 5 V a r i a t i o n i n S t r e s s Across a Small S o i l Element 14 6 Conventional Crosshole T e s t i n g 20 7 Conventional Downhole T e s t i n g 22 8 CPT Seismic Equipment Layout 26 9 15 cm 2 Seismic Cone 28 10 S i g n a l Strength Versus Depth 33 11 Geophone C o n s t r u c t i o n 36 12 Geophone S p e c i f i c a t i o n s 37 13 Observed Shear Wave Traces from a S i n g l e Hammer Impulse 38 14 E f f e c t s of S i g n a l Enhancement 43 15 E l e c t r i c a l Step T r i g g e r C i r c u i t 46 16 P o l a r i z e d Shear Wave Traces from Transverse Geophones 52 17 Comparison Between True and Pseudo I n t e r v a l Measurement 55 18 Comparison Between True and Pseudo I n t e r v a l T r a v e l Times f o r Unmatched Geophones 60 19 Comparison Between True and Pseudo I n t e r v a l T r a v e l Times f o r Matched Geophones 61 20 V e l o c i t y Measurement Accuracy as a Fun c t i o n of Measurement P r e c i s i o n and Geophone Separation 64 21 E f f e c t s of S i g n a l R e f r a c t i o n on Wave T r a v e l Path 66 22 Comparison Between Conventional and I t e r a t i v e V e l o c i t y C a l c u l a t i o n s 70 v i i FIGURE PAGE 23 Sand D e n s i t y - Cone Bearing C o r r e l a t i o n 73 24 Research S i t e L o c a t i o n Map 78 25 McDonalds Farm Cone P r o f i l e 80 26 F o r t Langley Cone P r o f i l e 82 27 Annacis North P i e r Cone P r o f i l e 84 28 Comparison Between CPT Seismic and S e l f - B o r i n g Pressuremeter, McDonalds Farm 86 29 Comparison Between CPT Seismic and CPT Bearing P r e d i c t i o n , McDonalds Farm 88 30 Comparison Between CPT Seismic and CPT Bearing P r e d i c t i o n , Annacis North P i e r 89 31 Comparison Between Downhole and Cros s h o l e V e l o c i t y Measurements, Annacis North P i e r 91 32 Comparison Between CPT Seismic and E m p i r i c a l Dynamic Shear Modulus, McDonalds Farm 94 33 Comparison Between CPT Seismic and E m p i r i c a l Dynamic Shear Modulus, F o r t Langley 95 34 Comparison Between CPT Seismic and E m p i r i c a l Dynamic Shear Modulus, Annacis North P i e r 96 v i i i ACKNOWLEDGEMENTS The w r i t e r wisher to acknowledge the support, suggestions and c r i t i c a l comments from Dr. R.G. Campanella dur i n g the course of t h i s study. Information p r o v i d e d by E r t e c Western on CPT downhole s e i s m i c equipment research and development was extremely u s e f u l . The a s s i s t a n c e and suggestions p r o v i d e d by Dr. Peter K. Robertson a l s o proved i n v a l u a b l e . The p r o j e c t would not have been p o s s i b l e without the f i n e t e c h n i c a l support and workmanship p r o v i d e d by Messers A r t Brookes, Gary Burlock, Guy K i r s c h , Dick Postgate, and Don Smythe. The a s s i s t a n c e of Mr. Andy N i c h o l s , Mr. Steve Brown, Mr. C l i f f o r d Tsang, Mr. Jim G r e i g , Ms. Nancy Laing, and Ms. T r i c i a Cook d u r i n g the data a q u i s i t i o n phase of the p r o j e c t was a p p r e c i a t e d . The c o o p e r a t i o n of the B r i t i s h Columbia M i n i s t r y of T r a n s p o r t a t i o n and Highways, and Golder A s s o c i a t e s , Vancouver, fo r access to and inf o r m a t i o n on the Annacis North P i e r S i t e was a l s o a p p r e c i a t e d . S p e c i a l thanks a l s o go to Ms. T r i c i a Cook who typed the t h e s i s . F i n a n c i a l support was pro v i d e d by the N a t u r a l Science and Eng i n e e r i n g Research C o u n c i l of Canada. 1 1 .0 INTRODUCTION 1.1 R a t i o n a l e f o r Seismic Cone Development Accurate assessment of dynamic s o i l p r o p e r t i e s i s an e s s e n t i a l component of foundation design f o r machines and s t r u c t u r e s s u b j e c t e d to o s c i l l a t o r y v i b r a t i o n and i n the response a n a l y s i s of ground s u b j e c t e d to earthquake shaking. One of the most important dynamic s o i l p r o p e r t i e s i s the s o i l shear modulus, G. The simple r e l a t i o n s h i p TVs 2 G = pVs 2 = 1 g from e l a s t i c theory r e l a t e s the shear modulus G to the s o i l d e n s i t y , p ( t o t a l u n i t weight of the s o i l , 7 , d i v i d e d by the a c c e l e r a t i o n due to g r a v i t y , g) m u l t i p l i e d by the square of the shear wave v e l o c i t y , Vs. Shear wave v e l o c i t y and hence shear modulus can be determined using i n - s i t u seismic methods. The two most commonly used i n - s i t u s e ismic methods f o r determining s o i l shear wave v e l o c i t y are the downhole and c r o s s h o l e methods (Stokoe and Abdel-razzak, 1975). However these c o n v e n t i o n a l techniques r e q u i r e s p e c i a l t e s t h o l e p r e p a r a t i o n and the use of s p e c i a l equipment. They can be time consuming and expensive to perform. The recent acceptance of the s t a t i c cone penetrometer as the premier l o g g i n g t o o l f o r i n - s i t u i n v e s t i g a t i o n s i n s o f t s o i l d e p o s i t s (Campanella and Robertson, 1982) makes the 2 i n c o r p o r a t i o n of dynamic t e s t i n g c a p a b i l i t y w i t h i n the cone extremely a t t r a c t i v e . T h i s t h e s i s p resents the r e s u l t s of re s e a r c h c a r r i e d out using a geophone instrumented s t a t i c cone penetrometer. By p l a c i n g geophone v e l o c i t y t r a n s d u c e r s i n c y l i n d r i c a l c a v i t i e s behind the cone t i p and performing a m o d i f i e d downhole seismic survey, shear wave v e l o c i t y versus depth p r o f i l e s were determined. The o b j e c t i v e s of the re s e a r c h were to modify e x i s t i n g cone equipment, and to develop r a t i o n a l t e s t i n g procedures and data a n a l y s i s techniques so that r a p i d , a c c u r a t e shear wave v e l o c i t y and thus shear modulus p r o f i l e s c o u l d be obtained from cone p e n e t r a t i o n t e s t h o l e s . In p a r t i c u l a r i t was con s i d e r e d d e s i r a b l e to enhance the s t r a t i g r a p h i c l o g g i n g c a p a b i l i t y of. the cone penetrometer. A technique was developed whereby shear wave v e l o c i t y and shear modulus c o u l d be measured over 1 metre depth increments. An important aspect of the r e s e a r c h was to assess the r e p e a t a b i l i t y and accuracy of the measurements and to compare the advantages and l i m i t a t i o n s of the se i s m i c cone method with c o n v e n t i o n a l methods of shear wave v e l o c i t y measurement. 1.2 Shear Modulus as a G e o t e c h n i c a l Parameter Shear modulus i s a s o i l parameter which r e l a t e s shear deformation to shear l o a d i n g as shown in Fig u r e 1a. The r e l a t i o n s h i p T = Gy 2 FIG. 1. S T R E S S , S T R A I N AND S H E A R M O D U L U S R E L A T I O N S H I P 4 where r i s t h e a p p l i e d shear s t r e s s , G i s the shear modulus, and 7 i s the r e s u l t i n g shear s t r a i n , i s a f a m i l i a r one. The shear modulus of a s o i l i s , however, not an easy parameter t o determine p r e c i s e l y . I t i s dependent on numerous f a c t o r s . H a r d i n and D r n e v i c h (1972) c l a s s i f i e d t h e s e f a c t o r s and d e t e r m i n e d t h a t among the most imp o r t a n t were s t r a i n a m p l i t u d e , e f f e c t i v e mean p r i n c i p a l s t r e s s , v o i d r a t i o , and degree of s o i l s a t u r a t i o n . D u p l i c a t i o n of i n - s i t u e f f e c t i v e mean p r i n c i p a l s t r e s s e s on s o i l samples i n the l a b o r a t o r y i s a d i f f i c u l t p r o p o s i t i o n . The e f f e c t s of d i s t u r b a n c e d u r i n g s a m p l i n g , h a n d l i n g and t e s t i n g p a r t i c u l a r l y on a c o h e s i o n l e s s s o i l c o m p l i c a t e a c c u r a t e d u p l i c a t i o n of i n - s i t u v o i d r a t i o s . L i k e w i s e f o r p a r t l y s a t u r a t e d s o i l s i t may be d i f f i c u l t t o m a i n t a i n i n - s i t u s a t u r a t i o n l e v e l s i n l a b o r a t o r y samples. The most i m p o r t a n t of the f o u r f a c t o r s a f f e c t i n g shear modulus i s the s t r a i n l e v e l a t which the s o i l i s t e s t e d . Because most s o i l s have a c u r v i l i n e a r s t r e s s s t r a i n r e l a t i o n s h i p as shown i n F i g u r e 1b, the shear modulus, shown as the s l o p e of the secant l i n e , shows s i g n i f i c a n t r e d u c t i o n w i t h i n c r e a s i n g s t r a i n l e v e l . A s t u d y by Seed and I d r i s s (1970) p r e s e n t e d q u a n t i t a t i v e l y the e f f e c t s of s t r a i n a m p l i t u d e on shear modulus. In F i g u r e 2a and 2b, the r e d u c t i o n of shear modulus w i t h i n c r e a s i n g shear s t r a i n i s shown f o r sands and c l a y s r e s p e c t i v e l y . At s t r a i n s of 10- U % or l e s s however, the shear modulus i s a maximum and l i t t l e a f f e c t e d by s t r a i n a m p l i t u d e . The maximum shear modulus i s g e n e r a l l y known as Gmax or the dynamic shear modulus. The s e n s i t i v i t y of the shear modulus parameter t o the above a) SANDS 0 1 1 1 r 1 1 r I 0 - 5 ICT 4 K T 3 K T 2 KD-1 I 10 SHEAR STRAIN % -PERCENT F I G . 2. V A R I A T I O N O F S H E A R M O D U L U S W I T H S H E A R S T R A I N 6 f a c t o r s i s the primary reason t h a t i n - s i t u t e s t i n g u s i n g low s t r a i n s e i s m i c waves i s p r e f e r r e d over l a b o r a t o r y t e s t i n g f o r the d e t e r m i n a t i o n of Gmax. 1.3 T h e s i s O r g a n i z a t i o n T h i s t h e s i s d i s c u s s e s the development and use of a se i s m i c cone penetrometer. Presented are d e t a i l s of the i n - s i t u t e s t i n g equipment, an o u t l i n e of r a t i o n a l t e s t i n g procedures, and a d i s c u s s i o n of the i n t e r p r e t a t i o n techniques r e q u i r e d to permit accurate i n - s i t u shear wave v e l o c i t y measurement and dynamic shear modulus d e t e r m i n a t i o n . Chapter 2 presents a b r i e f d i s c u s s i o n on seismic wave phenomenom and c o n v e n t i o n a l methods of determining shear wave v e l o c i t y i n - s i t u . The advantages and disadvantages of these c o n v e n t i o n a l methods are assessed. A b r i e f l i t e r a t u r e review i s i n c l u d e d . Chapter 3 d i s c u s s e s development of the se i s m i c cone t e s t i n g equipment and the r a t i o n a l t e s t i n g procedures used i n t h i s study. Emphasis i s p l a c e d on i d e n t i f y i n g the t e c h n i c a l l i m i t a t i o n s of the a v a i l a b l e equipment and measurement techniques. Chapter 4 d i s c u s s e s the i n t e r p r e t a t i o n and a n a l y s i s techniques r e q u i r e d to permit a c c u r a t e i n - s i t u shear wave v e l o c i t y d e t e r m i n a t i o n and dynamic shear modulus c a l c u l a t i o n s . In p a r t i c u l a r an e f f o r t i s made to assess both q u a l i t a t i v e l y and q u a n t i t a t i v e l y the magnitude of e r r o r a s s o c i a t e d with dynamic moduli d e t e r m i n a t i o n s . 7 C h a p t e r 5 p r e s e n t s t h e r e s u l t s of f i e l d measurements u s i n g t h e s e i s m i c cone p e n e t r o m e t e r . Dynamic s h e a r m o d u l i d e t e r m i n a t i o n s a r e p r e s e n t e d f o r s o i l s a t s e l e c t e d r e s e a r c h s i t e s i n t h e F r a s e r R i v e r D e l t a n e a r V a n c o u v e r B.C. C o m p a r i s o n s between m o d u l i v a l u e s c a l c u l a t e d f r o m d i f f e r e n t i n - s i t u s e i s m i c v e l o c i t y measurements and m o d u l i from o t h e r i n - s i t u measurement c o r r e l a t i o n s a r i s i n g o u t o f c o m p l e m e n t a r y r e s e a r c h a t 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 , a r e a l s o p r e s e n t e d . The s e i s m i c c one d a t a i s a l s o compared w i t h e x i s t i n g e m p i r i c a l r e l a t i o n s h i p s . C h a p t e r 6 c o n c l u d e s t h e t h e s i s w i t h a r e v i e w o f t h e most i m p o r t a n t a s p e c t s of t h e s e i s m i c cone r e s e a r c h . A d i s c u s s i o n o f p o t e n t i a l a d d i t i o n a l r e s e a r c h u t i l i z i n g t h e s e i s m i c cone p e n e t r o m e t e r i s a l s o p r e s e n t e d . The c h a p t e r c l o s e s w i t h a d i s c u s s i o n o f c o n s i d e r a t i o n s f o r p r a c t i c a l a p p l i c a t i o n of t h i s e q u i p m e n t . 8 2.0 IN-SITU SHEAR MODULUS DETERMINATION 2.1 Seismic Wave Phenomenon Since the measurement of seismic wave v e l o c i t i e s i s an i n t e g r a l component of i n - s i t u dynamic modulus de t e r m i n a t i o n , a b r i e f review of wave phenomenon i s p r o v i d e d i n t h i s chapter. Mooney (1974) and Borm (1977) provide e x t e n s i v e d i s c u s s i o n s on seismic wave p r o p e r t i e s and c h a r a c t e r i s t i c s . T h e i r papers have been used e x t e n s i v e l y i n p r e p a r i n g the f o l l o w i n g d i s c u s s i o n . 2.1.1 Types of Seismic Waves Four types of seismic waves propagate through s o i l media: Compressional (P), Shear (S) , R a y l e i g h (R) and Love ( L ) . The f i r s t two are body waves which penetrate i n t o the i n t e r i o r of r the e a r t h and the l a s t two are s u r f a c e waves which t y p i c a l l y propagate w i t h i n one or two wavelengths of the s u r f a c e depending on the l a y e r i n g i n the system (See F i g u r e 3). I t i s the propagation of body waves that i s important i n i n - s i t u modulus determinat i o n . The compression (P) waves are c h a r a c t e r i z e d by p a r t i c l e v i b r a t i o n which l i e s p a r a l l e l to the ray path f o r the wave. The d i r e c t i o n of p a r t i c l e v i b r a t i o n i s uniquely determined by the d i r e c t i o n of the ray. Compression (P) waves are a l s o r e f e r r e d to as l o n g i t u d i n a l or i r r o t a t i o n a l waves and propagate through FIG. 3. TYPES OF SEISMIC WAVES 10 s o l i d s and f l u i d s . They propagate at a higher v e l o c i t y than shear waves. The shear (S) waves are c h a r a c t e r i z e d by p a r t i c l e v i b r a t i o n p e r p e n d i c u l a r to the ray path. The d i r e c t i o n of p a r t i c l e motion may l i e anywhere i n the plane p e r p e n d i c u l a r to the ray, depending mainly on the d i r e c t i o n of motion a t the wave source. The d i r e c t i o n i s not uniquely determined by the d i r e c t i o n of the ray. Shear (S) waves are a l s o r e f e r r e d to as t r a n s v e r s e or r o t a t i o n a l waves and are unable t o propagate through f l u i d s . A l l e n et a l . (1980) has shown that i n a f u l l y s a t u r a t e d s o i l , the compression (P) waves t r a v e l through the pore water i n the s o i l v o i d s at the v e l o c i t y of compression waves i n water. Shear waves t r a v e l only through the s o i l s k e l e t o n . T h e r e f o r e f o r g e o t e c h n i c a l e n g i n e e r i n g purposes at s i t e s where the water t a b l e i s near the s u r f a c e , i t i s the shear wave measurement which i s important i n a s s e s s i n g s o i l p r o p e r t i e s . 2.1.2 Shear Wave C h a r a c t e r i s t i c s For measurements near the s u r f a c e of the e a r t h , the true d i r e c t i o n of the S p a r t i c l e v i b r a t i o n can be c o n v e n i e n t l y r e s o l v e d i n t o a component p a r a l l e l to the s u r f a c e (SH) and a component i n the v e r t i c a l d i r e c t i o n (SV) as shown i n F i g u r e 4a. Seismic sources f o r shear wave g e n e r a t i o n i n e n g i n e e r i n g i n v e s t i g a t i o n s are o f t e n designed to generate e i t h e r dominantly P and SV or dominantly SH waves. The reason l i e s i n the fundamentally d i f f e r e n t behavior of SV and SH waves at a boundary. I f an SH wave s t r i k e s a h o r i z o n t a l g e o l o g i c INCIDENT S-WAVE O) COMPONENTS OF SHEAR WAVE MOTION b) SHEAR WAVE POLARIZATION FIG. 4. S H E A R W A V E C H A R A C T E R I S T I C S 12 d i s c o n t i n u i t y , p a r t of the energy i s t r a n s m i t t e d through and p a r t i s r e f l e c t e d back, but both of the outgoing waves are of the SH type. In c o n t r a s t , an SV wave s t r i k i n g a h o r i z o n t a l g e o l o g i c d i s c o n t i n u i t y w i l l produce four outgoing waves; SV and P; r e f l e c t e d and t r a n s m i t t e d . S i m i l a r l y a P wave s t r i k i n g a d i s c o n t i n u i t y w i l l a l s o produce four outgoing waves. Most seismic sources designed to produce SV waves w i l l a l s o produce s u b s t a n t i a l P waves and most SV d e t e c t o r s w i l l a l s o d e t e c t P waves. Thus the observed seismic wave form may i n c l u d e a complicated sequence of a r r i v a l s c o n s i s t i n g of d i r e c t and converted P and SV waves. In c o n t r a s t , c a r e f u l design of an SH type seismic source should minimize the i n t e r f e r e n c e of other wave a r r i v a l s . Shear waves d i s p l a y a unique c h a r a c t e r i s t i c which allows f o r t h e i r a c c u r a t e i d e n t i f i c a t i o n from other wave types, p a r t i c u l a r l y compressional waves (Schwarz and Musser, 1972). By r e v e r s i n g the d i r e c t i o n of the energy impulse at a b i -d i r e c t i o n a l s i g n a l source, o p p o s i t e l y p o l a r i z e d shear waves can be obtained. T h i s c h a r a c t e r i s t i c , i l l u s t r a t e d i n F i g u r e 4b, i s not u s u a l l y d i s p l a y e d by compressional waves, and i s one of the fundamental reasons that the seismic shear wave survey i s so u s e f u l . 2.2 Determination of E l a s t i c Parameters The equations of motion of body waves i n an e l a s t i c media can be developed by c o n s i d e r i n g the v a r i a t i o n i n s t r e s s across a sma l l r e c t a n g u l a r p a r a l l e l e p i p e d as a s t r e s s wave passes through 13 i t (Kolsky, 1963). As can be seen i n F i g u r e 5, s i x separate f o r c e s act p a r a l l e l to each of the three axes. The r e s u l t a n t f o r c e a c t i n g in the x - d i r e c t i o n i s equal to daxx \ / drxy [ffxx + 5x)6y5z - axx Sy5z +[rxy + 5y|5x5z - rxy 5x6z dx / \ dz which s i m p l i f i e s to ( daxx drxy drxz + + j 6x5y5z dx dy dz By Newtons second law of motion ( n e g l e c t i n g g r a v i t a t i o n a l f o r c e s ) the equation equals d 2 u p[5x5ySz V / d t 2 )a - u d t 2 where p i s the d e n s i t y of the element and u i s the displacement in the x - d i r e c t i o n . T h e r e f o r e d 2 u daxx drxy drxz o = + + 6 d t 2 dx dy dz S i m i l a r l y i n the y and z d i r e c t i o n s d 2 v P d t 2 dryx dx dayy + dy dryz + dz d 2w drzx drzy dazz P d t 2 dx dy "t* dz Assuming the m a t e r i a l i s i s o t r o p i c , Hookes Law may be used to c a l c u l a t e s t r a i n s using the r e l a t i o n s V A R I A T I O N IN S T R E S S A C R O S S A S M A L L SOIL E L E M E N T 15 axx = Xev + 2Gexx rxy = ryx = G7xy 9a ayy = Xev + 2Geyy ryz = rzy = G7yz 9b azz = Xev + 2Gezz rzx = rxz = G7ZX 9c where X Is known as Lame's constant and G i s o f t e n r e f e r r e d to as the modulus of r i g i d i t y or the shear modulus. Lame's constant and the shear modulus are of t e n expressed i n terms of Poisson's r a t i o v, and Young's modulus E, where i>E 7 = 10 (\+v)(\-2v) E G = 1 1 2(1+*) S t r a i n s and r o t a t i o n s i n the m a t e r i a l are determined as f o l l o w s du dv du exx = 7xy = + 12a dx dx dy dv dw dv eyy = 7yz = — + 12b dy dy dz dw dv dw ezz = 7 Z X = + 12c dz dz dx ev = exx + eyy + ezz 13 dw dv 2coxx = — - — 1 4a dy dz dv dw 2wyy = — - — 1 4b dz dx dv du 2 C J Z Z = - 1 4c dx dy 16 Using the L a p l a c i a n operator d 2 d 2 d 2 V 2 = + + 15 dx 2 d y 2 d z 2 the equations of motion s i m p l i f y to d 2 u dev p = 7 + G + GV 2u 16 d t 2 dx d 2 v dev p = 7 + G + GV 2v 17 d t 2 dy d2w dev p = 7 + G + GV2w 18 d t 2 dz The above equations may be sol v e d i n terms of an equation which d e s c r i b e s the propagation of an i r r o t i o n a l wave, and i n terms of an equation which d e s c r i b e s the propagation of a pure r o t a t i o n wave. The f i r s t s o l u t i o n i s obtained by d i f f e r e n t i a t i n g equations 16, 17, and 18 with respect to x, y, and z, r e s p e c t i v e l y , and adding the expressions together. T h i s g i v e s d 2 e v p = 7 + 2G V 2 ev 19 dt which i s known as the wave equation. T h i s i n d i c a t e s t h a t a d i l a t i o n or compression wave propagates with a v e l o c i t y 7 + 2G E( 1-j>) B V c 2 = = = — 20 P p(1+f)(1-2f) p where B i s the bulk modulus. The other s o l u t i o n can be obtained by d i f f e r e n t i a t i n g equation 17 with respect to z and equation 18 with r e s p e c t to y and then e l i m i n a t i n g ev by s u b t r a c t i n g the two e x p r e s s i o n s t o 17 g i v e d 2wxx p = GV2GL>XX 21 dt S i m i l a r e x p r e s s i o n s can be obtained f o r coyy and u z z . These equations i n d i c a t e that a r o t a t i o n a l shear wave propagates with a v e l o c i t y G V s 2 = — 22 P which i s e q u i v a l e n t to equation 1. The compression wave and the shear wave v e l o c i t i e s are r e l a t e d by the equation Vc 2 1 - v = 2 23 V s 2 1-2* except i n the undrained c o n d i t i o n when v i s equal to 0.5 and the r e l a t i o n s h i p i s indeterminent. 2.3 Conventional I n - S i t u Seismic Measurement Over the l a s t f i f t e e n years a good d e a l of l i t e r a t u r e has been p u b l i s h e d concerning the g e o t e c h n i c a l a p p l i c a t i o n s of i n -s i t u s e ismic wave v e l o c i t y measurement. Recently P a t e l (1981) i n a t h e s i s on c o n v e n t i o n a l downhole t e s t i n g prepared a comprehensive l i t e r a t u r e review and ext e n s i v e b i b l i o g r a p h y on the s u b j e c t . His work was a u s e f u l r e f e r e n c e i n the p r e p a r a t i o n of t h i s t h e s i s . 18 As mentioned i n Chapter 1, the most f r e q u e n t l y used methods f o r determining shear wave v e l o c i t y i n - s i t u are the c o n v e n t i o n a l c r o s s h o l e and downhole methods. Both of the methods r e q u i r e the measurement of shear wave t r a v e l times over known t r a v e l d i s t a n c e s through s o i l media. G e n e r a l l y these waves are generated by hammer blows and propagate at shear s t r a i n l e v e l s of 10-" per cent or l e s s . Recently however, l a r g e s t r a i n i n - s i t u shear wave v e l o c i t y measurements have been p o s s i b l e (Wilson Brown and Schwarz, 1978). Large s t r a i n i n - s i t u t e s t i n g was not addressed as a re s e a r c h t o p i c i n t h i s t h e s i s . In the f o l l o w i n g s u b s e c t i o n s each of the c o n v e n t i o n a l t e s t methods i s d e s c r i b e d . A b r i e f l i t e r a t u r e review i s provided and the advantages and disadvantages of each method are presented. 2.3.1 Conventional C r o s s h o l e T e s t i n g One of the e a r l i e r comprehensive s t u d i e s i n t o the g e o t e c h n i c a l aspects of c r o s s h o l e seismic t e s t i n g was prepared by Stokoe and Woods (1972). They r e p o r t e d on the use of hammer induced low s t r a i n l e v e l shear waves to measure shear wave v e l o c i t y i n an i s o l a t e d t e s t f a c i l i t y and at three f i e l d t e s t s i t e s . Wilson et a l (1978) in t r o d u c e d a m o d i f i e d c r o s s h o l e t e s t i n g technique. Using a s p e c i a l l y designed mechanical impulse hammer and d e t e c t o r s l o c a t e d i n c l o s e p r o x i m i t y to the source, shear wave v e l o c i t y was measured at s t r a i n l e v e l s of 10- 1 to 10-3 per cen t . A v i b r a t o r y c r o s s h o l e s i g n a l source was d i s c u s s e d by B a l l a r d (1976) and deep c r o s s h o l e p r o d u c t i o n t e s t i n g was 19 d i s c u s s e d by Auld (1977). The c o n v e n t i o n a l c r o s s h o l e shear wave v e l o c i t y survey method i s i l l u s t r a t e d i n F i g u r e 6. The t e s t i n g procedure u s u a l l y e n t a i l s the placement of three or more boreholes i n a l i n e as shown. The holes are cased and grouted to ensure good s i g n a l t r a n s m i s s i o n and they are surveyed f o r v e r t i c a l l i t y to ensure that the d i s t a n c e between the h o l e s i s known. The s i g n a l source i s s t r u c k v e r t i c a l l y to p r e f e r e n t i a l l y generate SV waves i n one borehole. These are d e t e c t e d at two or more adjacent h o l e s . An o p p o s i t e l y p o l a r i z e d SV wave can be obtained by p u l l i n g with an upward impulse on the s i g n a l source. I n t e r v a l t r a v e l times between adjacent r e c e i v e r h o l e s are used to determine shear wave v e l o c i t y . The most s e r i o u s shortcomings of the c r o s s h o l e t e s t are the need to prepare m u l t i p l e boreholes, the u n c e r t a i n t y about hole s p a c i n g , and the need f o r s p e c i a l i z e d i n - h o l e s i g n a l sources to generate p o l a r i z e d SV waves. The method has s e v e r a l advantages i n that s t r a i n l e v e l can be c o n t r o l l e d and t e s t s can be performed to c o n s i d e r a b l e depth u s u a l l y without problems with s i g n a l a t t e n u a t i o n and r e f r a c t i o n . 2.3.2 Conventional Downhole T e s t i n g Schwarz and Musser (1972) were among the f i r s t to r eport on the use of p o l a r i z e d shear waves i n downhole t e s t i n g . Subsequently numerous i n v e s t i g a t o r s have r e p o r t e d the r e s u l t s of work with the c o n v e n t i o n a l downhole technique. Some of the more notable c o n t r i b u t i o n s to the l i t e r a t u r e are Warrick (1974), 20 Receiver Boreholes 12 ft (3.7m) Source Borehole 7ft(2.lm)-*-a.-PLAN VIEW Vertical Velocity --Vertical Transducer^. j Impulse Generation of Body Waves (Not to Scale) b.-CROSS-SECTIONAL VIEW FIG. 6. CONVENTIONAL C R O S S H O L E TESTING 21 B a l l a r d and McLean (1975), Auld (1977), Beeston and M c E v i l l e y (1977), and Wilson et a l . (1978). P a t e l (1981) t e s t e d v a r i o u s types of s i g n a l sources f o r downhole t e s t i n g and pres e n t s a w e l l documented d i s c u s s i o n of h i s f i n d i n g s . Hoar and Stokoe (1978) have prepared an o u t l i n e of recommended t e s t i n g procedures f o r both the downhole and c r o s s h o l e techniques, f o r the American S o c i e t y f o r T e s t i n g M a t e r i a l s . In a separate paper, (Stokoe and Hoar, 1978) they have prepared an e x t e n s i v e d i s c u s s i o n on the v a r i a b l e s which can a f f e c t the accuracy of i n - s i t u seismic wave measurement. The c o n v e n t i o n a l downhole shear wave v e l o c i t y survey i l l u s t r a t e d i n F i g u r e 7 i n v o l v e s d r i l l i n g and c a s i n g a t e s t h o l e . A t r i a x i a l geophone d e t e c t o r package i s lowered i n t o the hole, and a shear wave i s generated at the s u r f a c e . The shear wave generation process t r i g g e r s an o s c i l l o s c o p e r e c o r d i n g device on which the s i g n a l t r a c e i s s t o r e d . The most commonly used shear wave s i g n a l source c o n s i s t s of a wooden plank h e l d f i r m l y to the ground by d r i v i n g a v e h i c l e onto i t . The plank i s impacted with a sledge hammer, f i r s t at one end and then at the other to generate a p o l a r i z e d shear wave. The t r i a x i a l geophone d e t e c t o r package i s a three component de v i c e c o n s i s t i n g of a r a d i a l , a t r a n s v e r s e and a v e r t i c a l geophone. The geophones generate an e l e c t r i c c u r r e n t when p a r t i c l e a c c e l a r a t i o n s cause a moving c o i l to o s c i l l a t e w i t h i n the geophone. D e t e c t i o n of SH waves i s most e f f e c t i v e when the d e t e c t o r geophone i s h o r i z o n t a l and o r i e n t e d with i t s a x i s p a r a l l e l t o the a x i s of s i g n a l source hammer impacts as shown. The v e r t i c a l l y o r i e n t e d geophone i s intended to p r e f e r e n t i a l l y Receiver Borehole Cost-tn-Place-Concrete Block (0.6m) 2ft 20 ft (6.1m) 1 2ft (0.6m) -PLAN VIEW Oscilloscope Trigger Electrica •Embedded Angle Iron Hammer Generation of Body Waves 3-D Velocity Transducer Wedged in Place (Not to Scale) -CROSS-SECTIONAL VIEW FIG. 7. CONVENTIONAL DOWNHOLE TESTING 23 d e t e c t compression waves generated from v e r t i c a l hammer impacts at the s i g n a l source. There are two fundamental methods of o b t a i n i n g shear wave v e l o c i t y i n a downhole t e s t . The v e l o c i t y may be determined by measuring the increment of shear wave t r a v e l time by e i t h e r a pseudo i n t e r v a l t r a v e l time method or a true i n t e r v a l t r a v e l time method. The pseudo i n t e r v a l method i s c a r r i e d out by advancing a s i n g l e geophone to v a r i o u s depths i n a t e s t h o l e and measuring the t r a v e l time i n t e r v a l between depths from separate energy impulses. The true i n t e r v a l method i n v o l v e s simultaneously m onitoring two geophones with a known s e p a r a t i o n i n a t e s t h o l e and c a l c u l a t i n g the t r a v e l time i n t e r v a l from a s i n g l e energy impulse. These methods are d i s c u s s e d f u r t h e r i n S e c t i o n 4.2. The c o n v e n t i o n a l downhole s e i s m i c survey has s e v e r a l shortcomings. The process of d r i l l i n g and c a s i n g the hole can be time consuming and expensive. The d r i l l i n g process can a l s o cause c o n s i d e r a b l e d i s t u r b a n c e to the s o i l immediately adjacent to the h o l e . The t r i a x i a l geophone package i s commonly lowered u s i n g f l e x i b l e c a b l e s so geophone o r i e n t a t i o n cannot be maintained. Even though wedges or i n f l a t a b l e packers are used to h o l d the package to the c a s i n g , good shear wave t r a n s m i s s i o n through the c a s i n g i s not guaranteed. T h i s can s e v e r e l y l i m i t the e f f e c t i v e survey depth. The main advantages of the downhole technique are that i t i s l e s s expensive than the c r o s s h o l e method and the equipment r e q u i r e d i s e a s i e r to operate. A l s o the shear waves generated and d e t e c t e d d u r i n g the t e s t are v e r t i c a l l y p r opagating, s i m i l a r 24 to those caused by a deep earthquake. The CPT seismic cone penetrometer d i s c u s s e d i n t h i s t h e s i s i s p r i m a r i l y a downhole t e s t instrument at i t s present stage of development. I t i s a r e l a t i v e l y new development ( E r t e c , l 9 8 l , Campanella and Robertson, 1982), however much of the l i t e r a t u r e on c o n v e n t i o n a l g e o t e c h n i c a l seismic methods i s p e r t a i n e n t . The res e a r c h and development work d e s c r i b e d i n t h i s t h e s i s was c a r r i e d out r e c o g n i z i n g the advantages of downhole seismic t e s t i n g i n an e f f o r t to develop an instrument f r e e of some of the shortcomings inherent to the c o n v e n t i o n a l t e s t methods. 25 3.0 CPT DOWNHOLE SEISMIC EQUIPMENT AND PROCEDURES 3.1 I n t r o d u c t i o n The impetus f o r development of the CPT downhole seismic equipment at the U n i v e r s i t y of B r i t i s h Columbia came out of res e a r c h and development work c a r r i e d out by E r t e c Western (1981) i n C a l i f o r n i a . P r e l i m i n a r y i n v e s t i g a t i o n s by that group using a t r i a x i a l geophone package c o n t a i n i n g m i n i a t u r e 28 Hz. v e l o c i t y transducers to perform downhole seismic surveys were very s u c c e s s f u l . Among the recommendations r e s u l t i n g from t h e i r r e s e a r c h were that the t r i a x i a l geophone package should be coupled to a cone penetrometer t i p , and that the accuracy and r e l i a b i l i t y of v e l o c i t y measurements made us i n g t h i s equiment be eva l u a t e d . These recommendations formed the b a s i s f o r t h i s study. During the progress of t h i s r e s e a r c h p r o j e c t i t was determined that c e r t a i n equipment c h a r a c t e r i s t i c s played an i n t e g r a l p a r t i n a s s e s s i n g f i e l d measurement accuracy and o p e r a t i o n a l l i m i t a t i o n s . Equipment m o d i f i c a t i o n s and v a r i a t i o n s i n t e s t i n g procedures were r e q u i r e d d u r i n g the course of the study to develop an optimum f i e l d t e s t i n g method. The l a y o u t of the CPT downhole se i s m i c equipment developed f o r t h i s study i s shown i n F i g u r e 8. The equipment c o n s i s t e d of a geophone instrumented 15 cm 2 s t a t i c cone penetrometer, a N i c o l e t 4094 d i g i t a l o s c i l l o s c o p e with 15 b i t (100 KHz) analog to d i g i t a l r e s o l u t i o n and fl o p p y d i s k c a p a b i l i t y . A 7 Kg hammer, a plank type s i g n a l source and an e l e c t r o n i c t r i g g e r switch were 26 U.B.C. IN-SITU TESTING VEHICLE OSCILLOSCOPE a TRIGGER "7 S olGNAL SOURCE GROUND SURFACE • HAMMER IMPULSE  ASSUMED SHEAR WAVE TRAVEL PATH -4- RADIAL ^ T R A N S V E R S E f ^ V O R A B L Y , VERTICAL •CONE PUSHING RODS TRIAXIAL G BO PHONE PACKAGE ADDITIONAL TRIAXIAL GEOPHONE PACKAGE (FOR TRUE INTERVAL MEASUREMENT} ° | — T R I A X I A L GEOPHONE PACKAGE I5cm^ CONE FIG. 8. C P T S E I S M I C E Q U I P M E N T L A Y O U T 27 used. T h i s chapter d i s c u s s e s those aspects of equipment response which d i r e c t l y a f f e c t e d the accuracy of shear wave v e l o c i t y and dynamic shear modulus d e t e r m i n a t i o n s . The chapter a l s o d e t a i l s the r a t i o n a l f i e l d procedures which were developed i n order to o b t a i n a c c u r a t e CPT downhole seismic data. 3.2 Seismic Cone Penetrometer The cone used i n the downhole seismic i n v e s t i g a i o n i s shown in F i g u r e 9. I t was very s i m i l a r i n design to the 5 channel cones i n s e r v i c e at the U n i v e r s i t y of B.C. (Campanella & Robertson, 1981). The cone was advanced using the U.B.C. i n - s i t u t e s t i n g v e h i c l e u s i n g standard 20T c a p a c i t y Dutch Cone rods. These rods are hollow (16 mm I.D. X 36 mm O.D.) and 1.0 metres long. The hollow design permits the i n s t a l l a t i o n of a 14 conductor c a r r i e r c a b l e through the pushing rod anulus to t r a n s m i t e l e c t r i c a l s i g n a l s from the cone to r e c o r d i n g equipment at the s u r f a c e . The cone was l a r g e r than c o n v e n t i o n a l cones having a 15 cm 2 c r o s s s e c t i o n a l area (44 mm O.D.), and an o v e r a l l l e n g t h of 50 cm. The cone t i p p r o v i d e d an automatic f r i c t i o n reducer by d e v e l o p i n g an o v e r s i z e hole through which the s m a l l e r diameter pushing rods c o u l d pass f r e e l y . The cone was equipped with a bearing r e s i s t a n c e l o a d c e l l , a pore pressure measurement tran s d u c e r , a f r i c t i o n l o a d c e l l to measure r e s i s t a n c e along an equal end area f r i c t i o n s l e e v e (225 cm 2 s u r f a c e a r e a ) , a slope sensor and a t r i a x i a l geophone package. The geophone package 28 -~o : FRICTION LOAO CELL EQUAL END AREA 225m2 FRICTION SLEEVE PORE PRESSURE TRANSDUCER 14 CONDUCTOR ELECTRICAL CABLE ADDITIONAL 28Hz TRIAXIAL GEOPHONE PACKAGE (For true interval measurement) -28Hz TRIAXIAL GEOPHONE PACKAGE SLOPE SENSOR BEARING LOAD CELL POROUS PLASTIC DISK 60° CONE TIP, (43.7mm o.d.) FIG. 9. 15 c m 2 CPT SEISMIC CONE 29 c o n t a i n e d one v e r t i c a l l y o r i e n t e d and two p e r p e n d i c u l a r h o r i z o n t a l l y o r i e n t e d GSC-14-L3 mi n i a t u r e v e l o c i t y transducers which have a resonant frequency i n the order of 28 Hz. The o r i e n t a t i o n of the geophone package was c l e a r l y marked on the e x t e r i o r of the cone. The v e l o c i t y t r a n s d u c e r s , manufactured by Geo Space C o r p o r a t i o n , were 1.7 cm i n diameter, 20 cm high, and weighed 19 grams. During the f i e l d t e s t i n g procedure the h o r i z o n t a l geophones were o r i e n t e d i n r a d i a l ( p e r p e n d i c u l a r ) and t r a n s v e r s e ( p a r a l l e l or f a v o r a b l e ) o r i e n t a t i o n to the s i g n a l source. The t r a n s v e r s e geophone was p l a c e d to d e t e c t SH shear wave a r r i v a l s and the v e r t i c a l geophone was p l a c e d to d e t e c t SV shear wave a r r i v a l s i f the cone was to be used in c r o s s h o l e c o n f i g u r a t i o n . The r a d i a l phone was p l a c e d f o r convenience and acted as a t r a n s v e r s e geophone i f the cone or s i g n a l source was r e o r i e n t e d . The design and c o n s t r u c t i o n of the geophone c a r r i e r p r o v i d e d a snug s e a t i n g f o r the t r i a x i a l geophone package. The method of advancing the s t a t i c cone penetrometer p r o v i d e d continuous f i r m mechanical c o n t a c t between the geophone c a r r i e r and the surrounding s o i l . T h i s allowed f o r e x c e l l e n t s i g n a l t r a n s m i s s i o n . In a d d i t i o n , geophone o r i e n t a t i o n c o u l d be c o n t r o l l e d and extremely accurate depth measurements c o u l d by o b t a i n e d . T h i s instrument worked w e l l f o r c a r r y i n g out pseudo i n t e r v a l v e l o c i t y surveys (see S e c t i o n 2.2.2) but as research progressed, q u e s t i o n s arose whether b e t t e r r e s u l t s might be obtained using a true i n t e r v a l survey method. In order to address t h i s q u e s t i o n the cone was modified to i n c o r p o r a t e a second t r i a x i a l geophone package. The geophone packages were 30 s i m i l a r l y a l i g n e d with the a x i s of each geophone e x a c t l y 1.000 metres from i t s c o u n t e r p a r t . The r e s u l t i n g comparison of pseudo and true i n t e r v a l survey data i s d i s c u s s e d i n S e c t i o n 4.2. 3.3 Shear Wave Generation As d i s c u s s e d i n Chapter 2, a s u i t a b l e downhole seismic s i g n a l source should p r e f e r e n t i a l l y generate l a r g e amplitude shear (SH) waves with l i t t l e or no compressional wave component. The s i g n a l s should be re p e a t a b l e , d i r e c t i o n a l and r e v e r s i b l e , and the source should be r e l a t i v e l y p o r t a b l e . A number of d i f f e r e n t types of sources have been evaluated by other i n v e s t i g a t o r s (See P a t e l , 1981). The r e s u l t s of t h e i r s t u d i e s i n d i c a t e that an e x c e l l e n t downhole seismic shear wave source c o n s i s t s of a plank, weighted to the ground and struck with a sledge hammer. Such a source was u t i l i z e d e x c l u s i v e l y in t h i s CPT downhole seismic study. 3.3.1 S i g n a l Source The s e i s m i c s i g n a l source c o n s i s t e d of a 3 metre long laminated wood plank. The ends of the plank were covered with s t e e l p l a t e to provide a good s t r i k i n g s u r f a c e . The plank was p o s i t i o n e d on l e v e l mineral s o i l with ends e q u i d i s t a n t from the t e s t h o l e and a v e h i c l e was d r i v e n onto i t to provide a normal f o r c e to h o l d the plank i n p l a c e . A 7 kilogram sledge hammer was used to induce a p o l a r i z e d shear wave by s t r i k i n g o p posite ends 31 of the plank with a moderate blow. The hammer was dropped from approximately the same height f o r each impulse. A 1 metre long plank source was t e s t e d but i t was found i n c o n v e n i e n t to use as i t was d i f f i c u l t to h o l d s t i l l , and produced lower energy S waves of poor q u a l i t y . S i nce the seismic waves were generated manually, the s i g n a l s were not e n t i r e l y r e p e a t a b l e . However p r o c e d u r a l and i n t e r p r e t a t i o n techniques d i s c u s s e d i n S e c t i o n s 3.6 and 4.2 were used to reduce these e f f e c t s . The p o r t a b i l i t y and s i m p l i c i t y of the s i g n a l source appeared to out-weigh these shortcomings. I t was noted that the s i g n a l q u a l i t y was a f f e c t e d by the c o n d i t i o n of the s t e e l s t r i k i n g p l a t e s . A f t e r e x t e n s i v e use, the p l a t e s became dented and chipped, and r e p e a t a b l e s i g n a l t r a c e s were more d i f f i c u l t to produce. To ensure good s i g n a l q u a l i t y , the s t e e l s t r i k i n g p l a t e s were r e p l a c e d when worn. 3.3.2 Source L o c a t i o n The d i s t a n c e from the t e s t h o l e at which the plank source was p l a c e d ( s i g n a l source o f f s e t ) was c o n s i d e r e d an important f a c t o r . I t has been suggested by P a t e l (1981) that a p r e f e r e n t i a l SH wave r a d i a t i o n window e x i s t s . He has shown that when s i g n a l s t r e n g t h i s normalized f o r t r a v e l path l e n g t h , maximum amplitude i s recorded f o r SH waves t r a v e l l i n g at a 45 degree angle of i n c i d e n c e to the ground s u r f a c e . Other i n v e s t i g a t o r s ( E r t e c , 1981) have suggested that i f the s i g n a l source i s too c l o s e to the t e s t h o l e , down rod s i g n a l t r a n s m i s s i o n may occur. T h i s p o s s i b i l i t y may e x i s t i f the rods 32 come i n c o n t a c t with the s i d e of the hole due to poor v e r t i c a l l i t y , hole squeezing or c a v i n g . Our o b s e r v a t i o n s i n d i c a t e t hat down rod s i g n a l t r a n s m i s s i o n was not a s e r i o u s problem. I t appears that i t would be advantageous to p l a c e the downhole source at a d i s t a n c e from the t e s t h o l e so that maximum amplitude s i g n a l s are o b t a i n e d . However, as P a t e l (1981) has p o i n t e d out, the f u r t h e r from the t e s t h o l e the source i s l o c a t e d the more l i k e l y r e f r a c t i o n i s to occur through the l a y e r i n g at the s i t e . The problem of r e f r a c t i o n may be overcome by using an i t e r a t i v e data a n a l y s i s technique, (See S e c t i o n 4.5) but there remains the o p e r a t i o n a l inconvenience of c o n s t a n t l y r e p o s i t i o n i n g the s i g n a l source as r e c e i v e r depth i s changed. In a d d i t i o n , the f u r t h e r the source i s l o c a t e d from the t e s t h o l e , the g r e a t e r the s i g n a l a t t e n u a t i o n and the lower the s i g n a l to noise r a t i o . F i g u r e 10 shows the e f f e c t of s i g n a l source o f f s e t and depth on s i g n a l s t r e n g t h i n a normally c o n s o l i d a t e d c l a y d e p o s i t . The shear wave t r a c e s i n the lower r i g h t hand p o r t i o n of the f i g u r e provide a q u a l i t a t i v e comparision of raw t e s t data. A l l three s i g n a l s were r e c e i v e d at 18.0 metres depth from a s i g n a l source l o c a t e d at 5.5 metres, 10.6 metres, and 19.8 metres from the t e s t h o l e . For p r o d u c t i o n t e s t i n g purposes the plank source was p l a c e d as c l o s e to the cone hole as p r a c t i c a l to minimize s i g n a l r e f r a c t i o n . I t was determined that small o f f s e t s more c l o s e l y d u p l i c a t e d the v e r t i c a l propagation of h o r i z o n t a l shear waves. Because of equipment c o n f i g u r a t i o n , the c l o s e s t the source c o u l d 33 MAXIMUM GEOPHONE RESPONSE m V i 1 1 1 1—•— 120 140 160 ISO 200 TIME m sec. "* RECEIVER DEPTH »meters FIG. 10. SIGNAL STRENGTH VERSUS DEPTH 34 be p l a c e d was 2.5 m e t r e s from t h e t e s t h o l e . Some n o i s e t r a n s m i s s i o n t h r o u g h t h e r o d s was o b s e r v e d and d i f f i c u l t y was e x p e r i e n c e d i n a c c u r a t e l y i d e n t i f y i n g c o m p r e s s i o n wave a r r i v a l s . However, t h e n o i s e p r o b l e m was n o t s e r i o u s and down r o d s i g n a l t r a n s m i s s i o n d i d n o t a p p e a r t o a f f e c t a c c u r a t e s h e a r wave a r r i v a l i d e n t i f i c a t i o n . S i n c e t h e p l a n k t y p e s o u r c e has a f i n i t e l e n g t h , t h e c a l c u l a t e d wave t r a v e l p a t h l e n g t h w i l l depend on w h i c h p o i n t on t h e s o u r c e i s u s e d t o measure t h e s i g n a l s o u r c e o f f s e t d i s t a n c e f r o m t h e t e s t h o l e . The e f f e c t on t r a v e l p a t h l e n g t h c a n be q u i t e l a r g e when s h o r t s i g n a l s o u r c e o f f s e t s and s h a l l o w r e c e i v e r d e p t h s a r e u s e d . Wave t r a v e l p a t h l e n g t h s w i l l be l o n g e r i f i t i s assumed t h a t t h e s i g n a l e m i n a t e s from t h e ends of t h e s i g n a l s o u r c e t h a n i f i t i s assumed t h a t t h e s i g n a l e m i n a t e s from t h e c e n t r e of t h e s i g n a l s o u r c e . The e f f e c t on wave t r a v e l p a t h l e n g t h s d i m i n i s h e s r a p i d l y w i t h i n c r e a s i n g d e p t h . In o r d e r t o m a i n t a i n c o n s i s t e n c y t h r o u g h o u t a s u r v e y , s i g n a l s o u r c e o f f s e t s were a l w a y s m easured from t h e p o i n t o f i m p a c t , s i n c e b o t h ends o f t h e p l a n k s o u r c e were e q u i d i s t a n t f r o m t h e t e s t h o l e . 3.4 S h e a r Wave D e t e c t i o n The s i g n a l d e t e c t i o n equipment c o n s i s t s o f t r i a x i a l geophone p a c k a g e s c o n t a i n i n g t h r e e m u t u a l l y p e r p e n d i c u l a r 28 Hz v e l o c i t y t r a n s d u c e r s . The geophones a r e c o n n e c t e d t h r o u g h a 1000 Hz low p a s s s i g n a l f i l t e r c i r c u i t t o a 15 b i t r e s o l u t i o n N i c o l e t 4094 d i g i t a l o s c i l l o s c o p e w i t h f l o p p y d i s k c a p a b i l i t y . D e t a i l s 35 of each component are p r o v i d e d in the f o l l o w i n g s u b s e c t i o n s . 3.4.1 Geophone Response Each 28 Hz v e l o c i t y transducer used i n the t r i a x i a l geophone packages c o n s i s t s of a permanent magnet and a copper wire c o i l suspended by l e a f s p r i n g s as shown i n F i g u r e 11. The geophones generate e l e c t r i c c u r r e n t when p a r t i c l e a c c e l e r a t i o n s cause the geophone housing and magnet to o s c i l l a t e r e l a t i v e to the c o i l which r e s i s t s movement because of i t s i n e r t i a . The geophones are very s m a l l , extremely rugged and designed to operate i n any o r i e n t a t i o n . The manufacturers s p e c i f i c a t i o n s are reproduced i n Figure 12. The geophones showed good response to s i n g l e hammer impacts to a depth of 40 metres. F i g u r e 13 p r o v i d e s a q u a n t a t i v e comparison of geophone response amplitude and r e l a t i v e shear wave t r a v e l times with depth. The geophone output v o l t a g e i s d i r e c t l y r e l a t e d to the p a r t i c l e o s c i l l a t i o n v e l o c i t y as shown on the i n s e t s c a l e s . The s t r a i n l e v e l caused by the shear waves can be estimated at any depth d u r i n g the CPT downhole seismic survey. The r e l a t i o n s h i p between shear s t r a i n 7xy, shear wave v e l o c i t y Vs, and peak s h e a r i n g v e l o c i t y uy MY 7xy = 24 Vs ( a f t e r White, 1965) can be used. The shear wave v e l o c i t y i s c a l c u l a t e d as o u t l i n e d i n S e c t i o n 4.3, and the s h e a r i n g p a r t i c l e v e l o c i t y i s estimated from the peak geophone output v o l t a g e on 36 FIG. 11. GEOPHONE CONSTRUCTION GSC-14-L3 SEISMOMETER The GSC-1^-13 i s a very s m a l l , extremely rugged seismometer. It is designed and b u i l t to maintain performance c h a r a c t e r i s t i c s even a f t e r being subjected to high shock f o r c e s . P r i n c i p a l a p p l i c a t i o n s include i n t r u s i o n d e t e c t i o n , m i l i t a r y use and v i b r a t i o n monitoring. Standard natural frequency i s 28 Hz, wit h t i l t angle of operation up to 180 . SPECIFICATIONS Standard Natural Frequency 28 Hz * 5 Hz Standard C o i l Resistance <s> 25°C 570 Ohms t 5% I n t r i n s i c Voltage S e n s i t i v i t y . 29 V/in/sec t 15% Normalized Transduction Constant .012 >/ Rc (V/in/sec) T i l t Angle of Operation 180° Open C i r c u i t Damping 18 of C r i t i c a l t ,0k Moving Mass 2.15 g Operating Temperature -30°F to +160°F Dimensions: Diameter .66 in (1.7 cm) " Height .70 in (1.8 cm) Height With Terminals .80 in (2.0 cm) Weight 19 g FIG. 12. GEOPHONE SPECIFICATIONS FIG. 1 3 . O B S E R V E D S H E A R W A V E T R A C E S F R O M A S I N G L E H A M M E R I M P U L S E 39 F i g u r e 13. A n a l y s i s of f i e l d data gathered f o r t h i s study i n d i c a t e s that shear s t r a i n amplitudes caused by sledge hammer blows on the s u r f a c e plank source vary between 10- 4 per cent near the sur f a c e and 10- 6 per cent at depth. Each geophone has i t s own unique o s c i l l a t i o n c h a r a c t e r i s t i c s and i t s own fundamental frequency. When a geophone i s e x c i t e d by an impulse i t begins to o s c i l l a t e and the o s c i l l a t i o n s tend to occur a t the n a t u r a l frequency of the phone. When pseudo i n t e r v a l v e l o c i t y surveys are c a r r i e d out (see S e c t i o n 2.2.2) the same geophone i s used to determine a l l wave t r a v e l time measurements, the f r e e o s c i l l a t i o n c h a r a c t e r i s t i c s are not a s e r i o u s f a c t o r f o r c o n s i d e r a t i o n . When two geophones are monitored s i m u l t a n e o u s l y , as i n the case of a true i n t e r v a l v e l o c i t y survey, the geophone o s c i l l a t i o n c h a r a c t e r i s t i c s can be q u i t e important. In order to compare the response of two adjacent geophones t h e i r response c h a r a c t e r i s t i c s and n a t u r a l f r e q u e n c i e s should be s i m i l a r . The geophones used i n the s e i s m i c cone f o r true i n t e r v a l surveys were i n i t i a l l y s e l e c t e d randomly, but were l a t e r p a i r e d on the b a s i s of t h e i r n a t u r a l f r e q u e n c i e s and t h e i r f r e e response t o impulse v i b r a t i o n s . The importance of geophone response c h a r a c t e r i s t i c s w i l l be d i s c u s s e d f u r t h e r i n S e c t i o n 4.2. 40 3.4.2 S i g n a l F i l t e r i n g P r e f e r e n t i a l g e n e r a t i o n of SH shear waves and shear wave p o l a r i z a t i o n are two key components to ensure accurate shear wave i d e n t i f i c a t i o n . Another important f a c t o r was noise r e d u c t i o n . Some i n v e s t i g a t o r s (Stokoe and Hoar, 1978) suggest that s i g n a l f i l t e r s should not be used s i n c e they can cause phase s h i f t s and s i g n a l d e l a y . Other i n v e s t i g a t o r s (Beeston & M c E v i l l y , 1977) suggest that low pass f i l t e r s can be used f o r ambient noise l e v e l r e d u c t i o n . Our experience i n d i c a t e d that c l e a r shear wave t r a c e s are obtained by using 1000 Hz low pass Butterworth f i l t e r s (-6 db/1 KHz) i n the shear wave d e t e c t i o n c i r c u i t . When f i l t e r s were not used h i g h frequency background noi s e masked the shear wave s i g n a l and made p r e c i s e a r r i v a l time i n t e r p r e t a t i o n very d i f f i c u l t . The e f f e c t of these f i l t e r s was assessed using formulae p r o v i d e d by H a l l et a l (1981). The time s h i f t , At, i n t r o d u c e d by a low pass Butterworth f i l t e r can be c a l c u l a t e d u s ing the equation A t = a r c t a n ( f / f c ) / 2 i r f 25 where f i s the s i g n a l frequency i n h e r t z and fc i s the f i l t e r c u t o f f frequency. Based on the o b s e r v a t i o n that shear waves t y p i c a l l y propagate at f r e q u e n c i e s l e s s than 100 h e r t z , the induced time l a g would be l e s s than 0.16 msec. The e f f e c t s of p o s s i b l e f i l t e r induced e r r o r were minimized by s e l e c t i n g only one f i l t e r s e t t i n g throughout ah e n t i r e survey. 41 3.4.3 O s c i l l o s c o p e R e s o l u t i o n Seismic wave t r a c e s d e t e c t e d by the t r i a x i a l geophone packages at depth were recorded on a N i c o l e t 4094 d i g i t a l o s c i l l o s c o p e with f l o p p y d i s k c a p a b i l i t y . T h i s u n i t has a 15 b i t analog t o d i g i t a l s i g n a l r e s o l u t i o n , very accurate timing c a p a b i l i t y and t r i g g e r d elay c a p a c i t y . Seismic wave forms were d i s p l a y e d on a CRT screen and d e s i r e d s i g n a l s were s t o r e d on double s i d e d double d e n s i t y magnetic f l o p p y d i s k s f o r l a t e r data r e d u c t i o n . The data c o u l d be r e c a l l e d to the screen at any time. The data manipulation f e a t u r e s of the scope allowed s i g n a l enhancement and a m p l i f i c a t i o n in the f i e l d or o f f i c e . A s e r i e s of d i s k programme packages allowed s i g n a l smoothing, averaging, i n t e g r a t i o n , frequency a n a l y s i s and other o p e r a t i o n s . Conventional i n - s i t u seismic a n a l y s i s has used data from photographic records of analog o s c i l l o s c o p e wave t r a c e s or s t r i p c h a r t r e c o r d i n g s . Since seismic wave t r a v e l time measurements are i n the m i l l i s e c o n d range the s i g n a l r e c o r d i n g device must p r o p e r l y respond i n t h i s range (Hoar and Stokoe, 1978). Through the use of preset t r i g g e r d e l a y s , the d i g i t a l o s c i l l o s c o p e allowed t i m i n g to the nearest 0.02 msec or 20 usee i n t h i s downhole se i s m i c work. T h i s provided ten to one hundred times the t i m i n g r e s o l u t i o n that c o u l d be obtained using c o n v e n t i o n a l analog data. For purposes of comparison a 12 b i t analog to d i g i t a l s i g n a l r e s o l u t i o n o s c i l l o s c o p e was a l s o used i n the f i e l d . The l o s s of r e s o l u t i o n equated to an e i g h t f o l d decrease i n v o l t a g e s e n s i t i v i t y . I t was found that the low v o l t a g e response of the 42 g e o p h o n e s a t d e p t h s g r e a t e r t h a n t e n m e t r e s became an i m p o r t a n t c o n s i d e r a t i o n i n a s s e s s i n g t h e s u i t a b i l i t y o f t h i s u n i t f o r r e s e a r c h p u r p o s e s . The 1 5 b i t o s c i l l o s c o p e was c a p a b l e o f r e c o r d i n g c l e a r s h e a r wave t r a c e s f r o m s i n g l e hammer i m p u l s e s t o d e p t h s o f 4 0 m e t r e s a t a s a m p l i n g r a t e o f 1 0 0 Msec p e r p o i n t . The 1 2 b i t o s c i l l o s c o p e was c a p a b l e o f r e c o r d i n g c l e a r s h e a r wave t r a c e s f r o m s i n g l e hammer i m p u l s e s t o o n l y 1 5 m e t r e s a t t h e same s a m p l i n g r a t e . The d i g i t a l o s c i l l o s c o p e i s b a s i c a l l y an e l e c t r o n i c c l o c k a n d a s e n s i t i v e v o l t m e t e r . An a n a l o g v o l t a g e s i g n a l i s s a m p l e d a t d i s c r e t e t i m e i n t e r v a l s and t h e s i g n a l i s s t o r e d i n a d i g i t a l memory a n d d i s p l a y e d on t h e s c o p e s c r e e n . The 1 5 b i t o s c i l l o s c o p e c a n s a m p l e v o l t a g e d i f f e r e n c e s o f j u s t 6 . 2 5 0 uV w h i l e t h e 1 2 b i t o s c i l l o s c o p e c a n s a m p l e t o 5 0 . 0 M V d i f f e r e n c e s . The v e r y s m a l l a m p l i t u d e r e s p o n s e o f t h e g e o p h o n e s a t d e p t h i s t h e r e f o r e more c l e a r l y d i s p l a y e d by t h e 1 5 b i t u n i t . S i g n a l s f r o m t h e 1 2 b i t u n i t a p p e a r e d s m e a r e d and were d i f f i c u l t t o i n t e r p r e t p r e c i s e l y . The p r o b l e m o f p o o r s i g n a l r e s o l u t i o n w i t h a 1 2 b i t u n i t c a n be o v e r c o m e u s i n g a t e c h n i q u e known a s s i g n a l e n h a n c e m e n t . S i g n a l e nhancement i n v o l v e s t h e s t a c k i n g a n d a d d i t i o n o f m u l t i p l e s i g n a l t r a c e s t o p r o v i d e s u f f i c i e n t s i g n a l a m p l i t u d e f o r r e c o g n i t i o n a n d i n t e r p r e t a t i o n . T h i s t e c h n i q u e i s w i d e l y u s e d i n c o n v e n t i o n a l r e f r a c t i o n s e i s m i c where 8 b i t a n a l o g t o d i g i t a l r e s o l u t i o n e q u i p m e n t i s common. The e f f e c t s o f s i g n a l e nhancement a r e i l l u s t r a t e d i n F i g u r e 1 4 . I n o r d e r t o u n a m b i g u o u s l y d e t e r m i n e s h e a r wave a r r i v a l 1 S E T O F P O L A R I Z E D H A M M E R B L O W S 20-I TIME, msec FIG. 14. E F F E C T S OF SIGNAL E N H A N C E M E N T 44 times, the 15 b i t analog to d i g i t a l c a p a b i l i t y was considered necessary f o r resea r c h purposes. Since the r e p e a t a b i l i t y of i n d i v i d u a l s i g n a l t r a c e s was under study, s i g n a l enhancement was avoided. For p r a c t i c a l e n g i n e e r i n g a p p l i c a t i o n s lower r e s o l u t i o n equipment might be c o n s i d e r e d s u i t a b l e . By using s i g n a l enhancement and a c c e p t i n g a lower l e v e l of measurement p r e c i s i o n , s a t i s f a c t o r y downhole data c o u l d be obtained. The q u e s t i o n of measurement accuracy and p r a c t i c a l c o n s i d e r a t i o n s w i l l be d i s c u s s e d f u r t h e r i n Secton 4.3. 3.5 T r i g g e r i n g Systems Proper o s c i l l o s c o p e t r i g g e r i n g i s a very important part of acc u r a t e shear wave v e l o c i t y d e t e r m i n a t i o n . Three t r i g g e r systems were assessed i n t h i s i n v e s t i g a t i o n . The f i r s t system simply c o n s i s t e d of a geophone p l a c e d on the ground adjacent to. the s i g n a l source. When the s i g n a l source was s t r u c k , ground v i b r a t i o n s would cause e x c i t a t i o n of the geophone, and genera t i o n of an e l e c t r i c c u r r e n t which would t r i g g e r the o c s i l l o s c o p e . U n f o r t u n a t e l y the r i s e time of the geophone was f i n i t e and v a r i e d c o n s i d e r a b l y . F r e q u e n t l y spurious ground v i b r a t i o n s would cause premature t r i g g e r i n g and i n a c c u r a t e wave t r a v e l time measurements. The second system c o n s i s t e d of a s o l i d s t a t e i n e r t i a a c t i v a t e d switch manufactured by 5th Dimension Inc., P r i n c e t o n , New J e r s e y . The switch i s designed to c l o s e f o r a small f i n i t e p e r i o d as the r e s u l t of i n e r t i a l f o r c e s on d e c e l l e r a t i o n a f t e r s t r i k i n g the s i g n a l source. The switch then allowed the passage 45 of 12 V DC to the t r i g g e r p o r t on the o s c i l l o s c o p e . The switch c i r c u i t r y was designed to cause a s t e p - l i k e i n c r e a s e i n v o l t a g e over a p e r i o d of l e s s than 1 Msec. T h i s c h a r a c t e r i s t i c was observed but i t was a l s o observed that some delay o c c u r r e d between hammer impact and scope t r i g g e r i n g . I t i s not important that a delay occurs but i t i s important that i t s magnitude be known and that the delay be c o n s i s t e n t (Hoar & Stokoe, 1978). The i n e r t i a switch was c r o s s checked with an e l e c t r i c a l step t r i g g e r and i t was found that the t r i g g e r delay v a r i e d approximately i n v e r s e l y with the s t r e n g t h of the hammer blow. The delay was measured at 300 Msec p l u s or minus 50 Msec. T h i s delay exceeded the maximum recommended by Hoar and Stokoe (1978), and the v a r i a b i l i t y of the delay c o u l d have l e a d t o i n c o r r e c t v e l o c i t y d e t e r m i n a t i o n s . The t h i r d system, which was used to gather a l l the f i e l d data presented i n t h i s t h e s i s , c o n s i s t e d of an e l e c t r i c a l step t r i g g e r of the type i l l u s t r a t e d in F i g u r e 15 (Hoar & Stokoe, 1978). The MC1455 l i n e a r i n t e g r a t e d c i r c u i t has a s i g n a l r i s e time of l e s s than 1 usee and use of the device has been h i g h l y recommended by other i n v e s t i g a t o r s . The t r i g g e r system was checked before each survey to ensure proper response and r e p e a t a b i l i t y . 3.6 F i e l d Procedure The downhole seismic survey c o u l d be c a r r i e d out during rod advance or du r i n g rod withdrawl. G e n e r a l l y , the seismic cone penetrometer was advanced i n the c o n v e n t i o n a l manner, at a V o l t * Hammer I m p u l s e Tim* 1 * U I R H C ) 0-TR IGGER SIGNAL 1000 •*JWWV\r To Hammer 10/iF —p 9V ~ • . 5 »   — i — r Typt 555' Lintor Inltgrgltd Circuit 4 e I — ^ To Impultt Rod b - C I R C U I T D I A G R A M 1 » To< Oicilloicop* FIG. 15 . E L E C T R I C A L S T E P T R I G G E R CIRCUIT 47 constant rate of 2 cm/sec and b e a r i n g , f r i c t i o n and pore pressure p r o f i l e s were obtained (Campanella and Robertson, 1981; G i l l e s p i e , 1981). The seismic survey was then performed at s e l e c t e d depth i n t e r v a l s d u r i n g rod changes. I t was found most c o n v i e n i e n t to c a r r y out the s e i s m i c survey d u r i n g rod withdrawl, a f t e r the s t a t i g r a p h y had been i d e n t i f i e d . Because of the low amplitude of the shear wave s i g n a l s and the high amplitude v i b r a t i o n s eminating from the i n - s i t u t e s t v e h i c l e , i t was necessary to decouple the rods from the truck d u r i n g t e s t i n g . T h i s r e q u i r e d that a r e l a t i v e l y s t r a i g h t h o l e be advanced. Decoupling was achieved by removing the pushing rod guide bushing and l e t t i n g the rods stand f r e e i n the rod w e l l . The reasonably r a p i d advance and withdrawl procedure using the CPT seismic cone allowed f o r a two man crew to e a s i l y complete a 40 metre hole i n a 10 hour f i e l d day. Accurate depth determination c o u l d be made at any time by measuring the rod l e n g t h . Geophone o r i e n t a t i o n was e a s i l y maintained throughout the survey s i n c e the rod advance and withdrawl mechanism a p p l i e s no r o t a t i o n a l f o r c e to the rods. Hole v e r t i c a l l i t y c o u l d be e a s i l y assessed u s i n g the continuous data from the cone t i p slope sensor d i s c u s s e d i n S e c t i o n 3.2. The CPT downhole seismic survey procedure appearred to work best between depths of 3 and 30 metres, though s i g n a l d e t e c t i o n was p o s s i b l e to 40 metres. Above 3 metres the e f f e c t s of s i g n a l source o f f s e t measurement as d i s c u s s e d i n S e c t i o n 3.3 were s i g n i f i c a n t . Below .30 metres the s t r e n g t h of i n d i v i d u a l energy impulses was attenuated such that c l e a r i n d i v i d u a l shear wave exc u r s i o n s were not always i d e n t i f i a b l e . 48 3.7 Summary The f o l l o w i n g i s a step by st e p summary of the procedures used to c a r r y out the CPT Downhole Seismic Survey. a. P o s i t i o n the plank source and c l e a r any d e b r i s beneath i t . Place v e h i c l e on the plank. b. Connect the cone and the hammer to the o s c i l l o s c o p e p l u g - i n , and i s o l a t e the pushing rods from mechanical c o n t a c t with the t r u c k . Switch n o i s e f i l t e r s on. c. A l i g n the CPT Seismic Cone with the plank s i g n a l source and advance to the d e s i r e d depth. d. S e l e c t the d e s i r e d switch p o s i t i o n s and a c t i v a t e the t r i g g e r on the o s c i l l s c o p e . e. Induce a shear wave s i g n a l i n t o the ground by a p p l y i n g a h o r i z o n t a l hammer blow to the plank source. f. Store the d e s i r e d s i g n a l on the d i s k r ecorder and r e f e r e n c e the i n f o r m a t i o n on the data form. g. P o l a r i z e the s i g n a l by a p p l y i n g a h o r i z o n t a l hammer blow to the opposite end of the plank source and st o r e the s i g n a l . h. Enhance the s i g n a l or repeat the above procedures d. to 49 g. as necessary. i . Advance or withdraw the cone to the next s e l e c t e d depth and repeat procedures d. to h. 50 4.0 DATA INTERPRETATION AND ANALYSIS 4.1 I n t r o d u c t i o n The procedure r e q u i r e d to o b t a i n dynamic shear moduli va l u e s from CPT downhole se i s m i c data i n v l o v e s three b a s i c s t e p s . These are; measurement of shear wave a r r i v a l times, d e t e r m i n a t i o n of shear wave v e l o c i t i e s over s e l e c t e d depth i n t e r v a l s and use of complementary or supplementary s o i l d e n s i t y data to c a l c u l a t e the dynamic shear modulus from e l a s t i c theory. At each step i n the data r e d u c t i o n process v a r i o u s u n c e r t a n t i e s a r i s e and assumptions may r e s u l t i n a cumulative e r r o r i n the c a l c u l a t e d dynamic shear modulus v a l u e s . T h i s chapter p r e s e n t s a d e t a i l e d d i s c u s s i o n of the i n t e r p r e t a t i o n and a n a l y s i s procedures used at each stage of the data r e d u c t i o n f o r t h i s study. A q u a n t i t a t i v e assessment of data u n c e r t a i n t y and cumulative e r r o r i s a l s o presented. In a d d i t i o n , suggested f i e l d and i n t e r p r e t a t i o n procedures which may be employed to reduce the cumulative e r r o r s are d i s c u s s e d . 4.2 A r r i v a l Time Measurement The most important step i n the i n t e r p r e t a t i o n of CPT downhole seismic data i s a c c u r a t e measurement of shear wave a r r i v a l times. U n f o r t u n a t e l y a c c u r a t e d i r e c t t r a v e l time measurements are extremely d i f f i c u l t to make. In Chapter 3 r e f e r e n c e was made t o . three e x t e r n a l f a c t o r s ( t r i g g e r 51 r e p e a t a b i l i t y , s i g n a l r e p e a t a b i l i t y , and geophone r e p e a t a b i l i t y ) which can s i g n i f i c a n t l y a f f e c t a r r i v a l time measurement. Chapter 2 d i s c u s s e d the unique p o l a r i z a t i o n c h a r a c t e r i s t i c of shear waves and it* was i n d i c a t e d that p r e f e r e n t i a l shear wave ge n e r a t i o n c o u l d be achieved. The recommended f i e l d procedures a l l o w easy q u a l i t a t i v e r e c o g n i t i o n of the shear wave, but q u a n t i t a t i v e i n t e r p r e t a t i o n of the shear wave a r r i v a l time can r e q u i r e concerted a n a l y t i c a l e f f o r t . The ac c u r a t e q u a n t i t a t i v e i n t e r p r e t a t i o n of shear wave a r r i v a l times i s d i s c u s s e d i n the f o l l o w i n g two s u b s e c t i o n s . 4.2.1 Shear Wave I n t e r p r e t a t i o n F i g u r e 16 shows t y p i c a l p o l a r i z e d shear wave t r a c e s d e t e c t e d by the t r a n s v e r s e geophone of a t r i a x i a l geophone package p o s i t i o n e d at two depths. I d e a l l y the shear wave a r r i v a l would be i d e n t i f i e d as the i n i t i a l l a r g e amplitude wave e x c u r s i o n at p o i n t S. Numerous i n v e s t i g a t o r s ( P a t e l , 1981; Hoar and Stokoe, 1978; Schwartz and Musser, 1972) have d i s c u s s e d t h i s method of i d e n t i f y i n g a r r i v a l times from p o l a r i z e d shear wave t r a c e s . The presence of noise and i n t e r f e r e n c e of other waves such as the compressional wave t r a i n , combined with the r a t h e r slow response of the geophone i n the v i c i n i t y of t h i s p o i n t make p r e c i s e and repeatable a r r i v a l time i d e n t i f i c a t i o n r a t h e r d i f f i c u l t . I t was c o n s i d e r e d d e s i r a b l e to s e l e c t some subsequent p o i n t on the shear wave t r a c e to overcome these problems. The most r e a d i l y i d e n t i f i a b l e p o i n t s on the shear wave 52 FIG. 16 . P O L A R I Z E D S H E A R W A V E SIGNAL T R A C E S F R O M T R A N S V E R S E G E O P H O N E S 53 t r a c e from the t r a n s v e r s e ( f a v o r a b l y o r i e n t e d ) h o r i z o n t a l geophones are the zero v o l t a g e c r o s s o v e r p o i n t s . T h i s was rec o g n i z e d by Shannon and Wilson (1976) who suggested that s i n c e p a r t i c l e v e l o c i t y was changing most r a p i d l y at the zero c r o s s o v e r p o i n t s the shear wave t r a c e was l e s s a f f e c t e d by minor d i s t o r t i o n s and noise than at other p o i n t s on the wave form. The i l l u s t r a t e d wave forms i n F i g u r e 16 d i s p l a y the v o l t a g e response of e l e c t r o m e c h a n i c a l v e l o c i t y t r a n s d u c e r s e x c i t e d by a shear wave impulse. As d e s c r i b e d p r e v i o u s l y , each v e l o c i t y t r a nsducer has i t s own n a t u r a l o s c i l l a t i o n c h a r a c t e r i s t i c s . Once the shear wave energy impulse has d i s s i p a t e d the geophones tend to o s c i l l a t e at t h e i r n a t u r a l frequency u n t i l i n t e r n a l damping causes the motion to cease. T h i s means that the shear wave s i g n a l t r a c e i s more a f f e c t e d by the geophone o s c i l l a t i o n c h a r a c t e r i s t i c s than by the shear wave impulse f u r t h e r along the t r a c e . For the purposes of t h i s study i t was concluded that the f i r s t zero v o l t a g e c r o s s o v e r p o i n t would p r o v i d e an e a s i l y i d e n t i f i a b l e and reasonable r e p r e s e n t a t i v e r e f e r e n c e p o i n t f o r shear wave a r r i v a l time i n t e r p r e t a t i o n . A r r i v a l times obtained i n t h i s manner however, do not r e f l e c t the t r u e t r a v e l time of shear waves between the source and r e c e i v e r . The f i r s t zero c r o s s o v e r r e f e r e n c e p o i n t s can only be used to o b t a i n i n t e r v a l t r a v e l times and i n t e r v a l v e l o c i t i e s over s e l e c t e d depth increments. I t was found that the measured f i r s t zero c r o s s o v e r time on a s i g n a l t r a c e from one hammer blow d i d not n e c e s s a r i l y correspond with the times measured on t r a c e s from subsequent blows. Small d i f f e r e n c e s i n hammer energy and s t r i k i n g 54 o r i e n t a t i o n between blows, and the r e p e a t a b i l i t y l i m i t a t i o n s of the e l e c t r o m e c h a n i c a l geophone sensors caused a r r i v a l time measurements to vary. During the i n i t i a l stages of the study, data was gathered using a s i n g l e geophone package, and t r a v e l times were determined using a pseudo i n t e r v a l method. As d i s c u s s e d i n S e c t i o n 2.2.2 t h i s method i n v o l v e s d e t e c t i o n of shear waves generated from separate hammer blows at a s i n g l e geophone as i t i s p o s i t i o n e d at adjacent depths, as i l l u s t r a t e d i n F i g u r e 17a. In an e f f o r t to overcome the d i f f i c u l t i e s of hammer s i g n a l r e p e a t a b i l i t y , the cone was m o d i f i e d to i n c o r p o r a t e a second t r i a x i a l geophone package (see S e c t i o n 3.2). T h i s equipment m o d i f i c a t i o n allowed true i n t e r v a l surveys to be c a r r i e d out. As d i s c u s s e d i n S e c t i o n 2.2.2, t h i s method i n v o l v e s simultaneous d e t e c t i o n of shear wave t r a c e s from a s i n g l e energy impulse, and i s i l l u s t r a t e d i n F i g u r e 17b. Even with t h i s change, s i g n a l v a r i a b i l i t y was s t i l l observed. To assess the v a r i a b i l i t y of the a r r i v a l time measurements, m u l t i p l e shear wave t r a c e s were obtained from m u l t i p l e i n d i v i d u a l hammer blows at one metre depth increments over a 20 metre cone h o l e . The a r r i v a l time data was then analyzed assuming normal s t a t i s t i c a l d i s t r i b u t i o n s a t each depth i n t e r v a l . T h i s a n a l y s i s i s d i s c u s s e d i n the f o l l o w i n g s u b s e c t i o n . 4.2.2 S t a t i s t i c a l E r r o r Assessment In order to o b t a i n some l e v e l of c o n f i d e n c e i n the T r a v e l t i m e c a l c u l a t e d by m o n i t o r i n g s h e a r wave a r r i v a l from s e p a r a t e energy i m p u l s e s . O P S E U D O V I N T E R V A L roi V a) T r a v e l t i m e c a l c u l a t e d by s i m u l t a n e o u s l y m o n i t o r i n g two shear wave a r r i v a l s from a s i n g l e energy i m p u l s e . IO] T R U E I N T E R V A L ol V b) FIG. 1 7 . C O M P A R I S O N B E T W E E N T R U E AND P S E U D O I N T E R V A L M E A S U R E M E N T 56 m e a s u r e m e n t s b e i n g made, a n d i n o r d e r t o q u a n t i t a t i v e l y a s s e s s t h e e f f e c t s o f measurement v a r i a b i l i t y , t h e a r r i v a l t i m e d a t a was a n a l y z e d s t a t i s t i c a l l y . The f i r s t z e r o c r o s s o v e r a r r i v a l t i m e s a n d t h e t r u e i n t e r v a l t r a v e l t i m e s w e re assumed t o be n o r m a l l y d i s t r i b u t e d random v a r i a b l e s . A n o r m a l random v a r i a b l e h a s a d i s t r i b u t i o n w h i c h i s d e f i n e d by t h e c l a s s i c b e l l s h a p e d n o r m a l o r G a u s s i a n c u r v e . T h i s c u r v e i s d e f i n e d by t h e f u n c t i o n 1 n(x;u,o) = e- 0' 5 (U~u)/o)2 26 2TTO Where TT=3. 14159 e=2.71828 M=mean and a = s t a n d a r d d e v i a t i o n The mean, u, and s t a n d a r d d e v i a t i o n , a, o f a s e t o f a r r i v a l t i m e o r i n t e r v a l t r a v e l t i m e m e a s u r e m e n t s a r e e a s i l y c a l c u l a t e d u s i n g t h e f o r m u l a e n X i 27 i = 1 '-23 a 4X)Xi2-(l]Xi) 28 i=1 i=1 n ( n - l ) 57 The goodness of f i t of the a r r i v a l time data and the i n t e r v a l t r a v e l time data to the assumed normal d i s t r i b u t i o n was checked u s i n g the Chi-squared (X 2) goodness of f i t t e s t . The observed d i s t r i b u t i o n of measurements Gi was compared with an expected d i s t r i b u t i o n of normally d i s t r i b u t e d measurements E i using the f u n c t i o n If the X 2 value i s s m a l l , a good f i t i s i n d i c a t e d . If the X 2 value i s g r e a t e r than 10, the f i t to the assumed d i s t r i b u t i o n i s poor. In order to guarantee good r e s u l t s when a s s e s s i n g the goodness of f i t of a number of measurements to the normal d i s t r i b u t i o n , sampling theory d i c t a t e s the number must be gre a t e r than 30. For t h i s reason, the f i e l d survey procedure u s u a l l y i n v o l v e d the genera t i o n and r e c o r d i n g of 30 to 40 shear wave t r a c e s at each depth increment. The degree of confidence with which the mean a r r i v a l times or mean t r u e i n t e r v a l t r a v e l times c o u l d be determined was assessed u s i n g the f u n c t i o n k ( e i - E i ) 2 X 2 = 2 9 E i i = 1 30 58 Where e i s the range of the mean, a i s the standard d e v i a t i o n , n i s the number of measurements and Z A/ 2 i s the con f i d e n c e i n t e r v a l c o e f f i c i e n t . The mean pseudo i n t e r v a l t r a v e l times u were e a s i l y c a l c u l a t e d u s ing the r e l a t i o n s h i p yx,-x, = M,- M, 31 where n, i s the mean r e f e r e n c e a r r i v a l time of a set of shear wave s i g n a l s at one depth, and u, i s the mean r e f e r e n c e a r r i v a l time of a set of shear wave s i g n a l s from the same geophone at a subsequent depth. The standard d e v i a t i o n of the pseudo i n t e r v a l t r a v e l times were c a l c u l a t e d using °*t-x, = / — + — 32 The c o n f i d e n c e i n t e r v a l s were determined using the formula where Z */2 i s the c o n f i d e n c e i n t e r v a l c o e f f i c i e n t (1 .960 f o r 95%). During the i n i t i a l p o r t i o n of t h i s study the geophones used in the cone with dual geophone packages were s e l e c t e d randomly and t h e i r n a t u r a l frequency and o s c i l l a t i o n c h a r a c t e r i s t i c s were unmatched. Both pseudo and t r u e i n t e r v a l t r a v e l times were ev a l u a t e d i n a s i n g l e survey. The r e s u l t s were compared and data from a t y p i c a l survey using unmatched geophones i s p l o t t e d i n 59 F i g u r e 18. The X 2 v a l u e s c a l c u l a t e d as d i s c u s s e d e a r l i e r i n t h i s s e c t i o n , f o r t h i s d a t a averaged between 4 and 6 i n d i c a t i n g a r e a s o n a b l e f i t of the d a t a t o an assumed normal d i s t r i b u t i o n . The comparison shown i n F i g u r e 18a between t r u e and pseudo i n t e r v a l t r a v e l t i m e s u s i n g unmatched geophones i s poor. The s t a n d a r d d e v i a t i o n s p l o t t e d i n F i g u r e 18b i n d i c a t e r e a s o n a b l y t i g h t d i s t r i b u t i o n s . F i g u r e 18c i n d i c a t e s t h a t one s t a n d a r d d e v i a t i o n i s g e n e r a l l y l e s s than 1.5% of the mean f o r both pseudo and t r u e i n t e r v a l measurements. In o r d e r t o a s s e s s the e f f e c t s of o s c i l l a t i o n c h a r a c t e r i s t i c s , geophones were p a i r e d so t h a t t h e i r n a t u r a l f r e q u e n c i e s were i d e n t i c a l and t h e i r f r e e o s c i l l a t i o n response were v e r y s i m i l a r . The r e s u l t s from a t y p i c a l s u r v e y u s i n g matched geophones a r e p l o t t e d i n F i g u r e 19. Aga i n t h e X 2 v a l u e s c a l c u l a t e d f o r t h i s d a t a averaged between 4 and 6 i n d i c a t i n g a r e a s o n a b l e f i t t o the assumed normal d i s t r i b u t i o n . In F i g u r e 19a i t can be seen t h a t the pseudo i n t e r v a l t r a v e l t i m e s and the t r u e i n t e r v a l time d a t a comparison i s b e t t e r than when unmatched geophones were used. F i g u r e s 19b and 19c i n d i c a t e t h a t the c h a c t e r i s t i c s of the d i s t r i b u t i o n s a r e s i m i l a r (magnitude of s t a n d a r d d e v i a t i o n ) t o t h o s e o b s e r v e d u s i n g unmatched geophones. I d e a l l y the pseudo i n t e r v a l d a t a and the t r u e i n t e r v a l d a t a s h o u l d be i d e n t i c a l . However the range of t r a v e l t i m e s i s g e n e r a l l y much l e s s than 0.5 msec. The e f f e c t of t h i s v a r i a t i o n on the c a l c u l a t e d shear wave v e l o c i t y w i l l be d i c u s s e d i n the f o l l o w i n g s e c t i o n . The advantages of t r u e i n t e r v a l method a r e t h a t the M E A N I N T E R V A L S T A N D A R D T R A V E L TIME D E V I A T I O N T R A V E L TIME E R R O R % m s e c m s e c FIG. 18. C O M P A R I S O N B E T W E E N TRUE AND P S E U D O I N T E R V A L T R A V E L TIMES FOR U N M A T C H E D G E O P H O N E S • 6 hi E I Si «H a 12 16 M E A N I N T E R V A L T R A V E L TIME m s i c S 6 7 8 — T R U E I N T E R V A L •- P 8 E U D 0 I N T E R V A L J - 4 T R U E I N T E R V A L 2 e^ E a Q P 8 E U D 0 I N T E R V A L 12 8 ) 18-1 S T A N D A R D D E V I A T I O N m s e c .1 .2 .3 .4 0 — i • i » o T R A V E L TIME E R R O R % m E X H Q. LU 9 o 12 b) 1B-> 2 S 4 6 C ) F I G . 1 9 . C O M P A R I S O N B E T W E E N T R U E A N D P S E U D O I N T E R V A L T R A V E L T I M E S F O R M A T C H E D G E O P H O N E S I—' 62 geophones are p o s i t i o n e d at a f i x e d known spacing and the s i g n a l which a r r i v e s at each geophone o r i g i n a t e d from the same hammer blow so that t r i g g e r e f f e c t s are minimized. The advantages of the pseudo i n t e r v a l method are that only one geophone i s r e q u i r e d and i t appears t h a t u n l i k e the true i n t e r v a l method, geophone response c h a r a c t e r i s t i c s are l e s s of a f a c t o r . As a r e s u l t of t h i s s t a t i s t i c a l assessment i t was determined that a s i n g l e geophone and pseudo i n t e r v a l measurements were adequate. A f i e l d procedure i n v o l v i n g the generation and r e c o r d i n g of 10 shear wave t r a c e s at each depth i n t e r v a l d u r i n g a survey should be used. T h i s would provide s u f f i c i e n t data to determine a 95% confidence i n t e r v a l f o r the mean of each pseudo and tr u e i n t e r v a l t r a v e l time measurement of pl u s or minus 0.125 msec. (Th i s c a l c u l a t i o n was c a r r i e d out using Eqs. 30 and 33 assuming a known standard d e v i a t i o n of 0.2 msec f o r the t r a v e l time measurements). 4.3 Shear Wave V e l o c i t y Determination The method of determining shear wave v e l o c i t y from CPT downhole s e i s m i c data b a s i c a l l y i n v o l v e s d i v i d i n g an increment shear wave t r a v e l time i n t o an increment of t r a v e l path l e n g t h . Both these v a r i a b l e s are s u s c e p t i b l e t o e r r o r and an assessment of the e f f e c t of these e r r o r s on the c a l c u l a t e d shear wave v e l o c i t y i s presented i n the f o l l o w i n g s u b s e c t i o n s . 62 geophones are p o s i t i o n e d at a f i x e d known spacing and the s i g n a l which a r r i v e s at each geophone o r i g i n a t e d from the same hammer blow so that t r i g g e r e f f e c t s are minimized. The advantages of the pseudo i n t e r v a l method are that only one geophone i s r e q u i r e d and i t appears t h a t u n l i k e the true i n t e r v a l method, geophone response c h a r a c t e r i s t i c s are l e s s of a f a c t o r . As a r e s u l t of t h i s s t a t i s t i c a l assessment i t was determined that a s i n g l e geophone and pseudo i n t e r v a l measurements were adequate. A f i e l d procedure i n v o l v i n g the generation and r e c o r d i n g of 10 shear wave t r a c e s at each depth i n t e r v a l d u r i n g a survey should be used. T h i s would provide s u f f i c i e n t data to determine a 95% confidence i n t e r v a l f o r the mean of each pseudo and true i n t e r v a l t r a v e l time measurement of plus or minus 0.125 msec. (Th i s c a l c u l a t i o n was c a r r i e d out using Eqs. 30 and 33 assuming a known standard d e v i a t i o n of 0.2 msec f o r the t r a v e l time measurements). 4.3 Shear Wave V e l o c i t y Determination The method of determining shear wave v e l o c i t y from CPT downhole s e i s m i c data b a s i c a l l y i n v o l v e s d i v i d i n g an increment shear wave t r a v e l time i n t o an increment of t r a v e l path l e n g t h . Both these v a r i a b l e s are s u s c e p t i b l e t o e r r o r and an assessment of the e f f e c t of these e r r o r s on the c a l c u l a t e d shear wave v e l o c i t y i s presented i n the f o l l o w i n g s u b s e c t i o n s . 63 4.3.1 T r a v e l Time E f f e c t s The e f f e c t s of u n c e r t a i n t y i n t r a v e l time measurement on v e l o c i t y measurement accuracy are a f u n c t i o n of geophone spacing, the v e l o c i t y of the s o i l being t e s t e d , and the p r e c i s i o n of the t r a v e l time measurement. The s o i l v e l o c i t y i s a f i x e d parameter, however by changing the i n t e r v a l measurement p r e c i s i o n , or the geophone s e p a r a t i o n the shear wave v e l o c i t y measurement accuracy can be improved or reduced. T h i s i s i l l u s t r a t e d i n F i g u r e 20. For example, i f 5 per cent v e l o c i t y measurement accuracy i s c o n s i d e r e d d e s i r a b l e and the a v a i l a b l e measurement equipment and a n a l y s i s procedures allow p r e c i s i o n t o only 0.5 msec (500 Msec), then i n order to determine the v e l o c i t y of a 400 m/sec s o i l to the d e s i r e d accuracy, a geophone spacing of 4 metres must be used. C l e a r l y the o b j e c t i v e s of accurate v e l o c i t y d e termination and d e t a i l e d s t r a t i g r a p h i c d e f i n i t i o n are i n d i r e c t c o n f l i c t . The d e f i n i t i o n of i n d i v i d u a l s t r a t a becomes masked by v e l o c i t y averaging over l a r g e r depth increments. Some compromise i s r e q u i r e d to r e s o l v e t h i s c o n f l i c t , and the d e c i s i o n must be based on the o b j e c t i v e s of the data c o l l e c t i o n . The c o n s i d e r a t i o n s of equipment a v a i l a b i l i t y , time, and c o s t must a l l be assessed when c o n s i d e r i n g use of t h i s measurement system for p r a c t i c a l a p p l i c a t i o n s . For r e s e a r c h purposes i t was co n s i d e r e d d e s i r a b l e to a l l o w the shear wave v e l o c i t y measurement c a p a b i l i t i e s of the seismic cone penetrometer to complement i t s c o n v e n t i o n a l s t r a t i g r a p h i c l o g g i n g c a p a b i l i t y with good accuracy. An o b j e c t i v e of a shear 64 S O I L V E L O C I T Y S O I L V E L O C I T Y C 3 U i K a. u i S u i CE 3 (0 < 111 < > OC U l H / S O I L V E L O C I T Y 1 0 0 m / s 2 0 0 m / 8 ,3 0 0 m / s . 4 0 0 m / s 1 0 % V E L O C I T Y A C C U R A C Y T 3 ~ r 4 5 S O I L V E L O C I T Y 1 0 0 m / s 2 0 0 m / s 3 0 0 m / s 4 0 0 m / s 2 0 % V E L O C I T Y A C C U R A C Y T 3 T " 4 - r -6 G E O P H O N E R E S P O N S E , m G E O P H O N E R E S P O N S E , m F I G . 2 0 . V E L O C I T Y M E A S U R E M E N T A C C U R A C Y A S A F U N C T I O N O F M E A S U R E M E N T P R E C I S I O N A N D G E O P H O N E S E P A R A T I O N 65 wave v e l o c i t y accuracy of p l u s or minus 5 per cent over one metre depth increments was set e a r l y d u r i n g the resea r c h p r o j e c t . As was e x p l a i n e d i n S e c t i o n 4.1, the r e s u l t i n g equipment measurement c a p a b i l i t i e s were not as good as had been a n t i c i p a t e d . Due to the observed s c a t t e r between the true i n t e r v a l and pseudo i n t e r v a l data, i t was determined that a r e a l i s t i c upper bound on t r a v e l time measurement e r r o r would be 10 per cen t . (Based on 1 metre i n t e r v a l measurements and a m u l t i p l e s i g n a l a n a l y s i s p r o c e d u r e ) . 4.3.2 T r a v e l Path E f f e c t s The e f f e c t s of i n a c c u r a t e depth and s i g n a l o f f s e t measurement are p a r t i c u l a r l y important when d i r e c t t r a v e l time measurements (source to r e c e i v e r ) are being made. The e f f e c t s are' l e s s c r i t i c a l when i n t e r v a l measurements are being made, unle s s the probe i s at a very shallow depth. These e f f e c t s were d i s c u s s e d i n S e c t i o n 3.3. Of more importance i n deeper measurements are the e f f e c t s of s i g n a l r e f r a c t i o n through l a y e r i n g at the s i t e . The c o n v e n t i o n a l method of a n a l y s i s f o r both pseudo and true i n t e r v a l downhole se i s m i c surveys i n v o l v e s the conver s i o n of recorded shear wave i n t e r v a l t r a v e l times i n t o c o r r e c t e d i n t e r v a l t r a v e l times. The c o r r e c t i o n i s based on a simple t r i g o n o m e t r i c r e l a t i o n s h i p assuming a d i a g o n a l s t r a i g h t l i n e t r a v e l path from the s i g n a l source to the geophone r e c e i v e r . T h i s method of a n a l y s i s i s i l l u s t r a t e d i n F i g u r e 21a. The c o r r e c t e d t r a v e l time i n t e r v a l T c o r r , equals the • I Q N A L S O U R C E O F F S E T S T R A I G H T LINE T R A V E L P A T H b) I T E R A T I V E T R A V E L P A T H 67 d i f f e r e n c e between t h e c o r r e c t e d t r a v e l t i m e s t o a d j a c e n t d e p t h s . The c o r r e c t e d t r a v e l t i m e s a r e f o r an assumed e q u i v a l e n t v e r t i c a l p a t h a s i f t h e s i g n a l s o u r c e were l o c a t e d i m m e d i a t e l y a d j a c e n t t o t h e t e s t h o l e . The c a l c u l a t i o n s f o r t h i s c o r r e c t i o n a r e s t r a i g h t f o r w a r d and e a s i l y done on a programmable c a l c u l a t o r . When t h e s i g n a l s o u r c e i s o f f s e t o n l y a s m a l l d i s t a n c e from t h e t e s t h o l e t h i s a n a l y s i s i s s u i t a b l e . In S e c t i o n 3.3 i t was d e t e r m i n e d t h a t s i g n a l s o u r c e o f f s e t s s h o u l d be as s m a l l as p o s s i b l e b u t i n c e r t a i n c a s e s l a r g e r o f f s e t s may be n e c e s s a r y . When s i g n a l s o u r c e o f f s e t s a r e l a r g e , and v e l o c i t y c o n t r a s t s e x i s t between m a t e r i a l s a t d e p t h t h e c o n v e n t i o n a l a n a l y s i s a p p r o a c h w i l l n o t be a c c u r a t e b e c a u s e wave r e f r a c t i o n w i l l c a u s e v a r i a t i o n s i n t r a v e l p a t h l e n g t h i n d i f f e r e n t l a y e r s . One a p p r o a c h t o c o r r e c t f o r t h i s e f f e c t ( T e l f o r d e t a l , 1976) i n v o l v e s t h e a p p l i c a t i o n o f S n e l l ' s Law f o r r e f r a c t i o n i n an i t e r a t i v e p a t h a n a l y s i s a s shown i n F i g u r e 21b. The a n a l y s i s assumes t h a t t h e s u b s o i l medium c a n be d i v i d e d i n t o a number of t h i n h o r i z o n t a l beds i n e a c h o f w h i c h t h e s h e a r wave v e l o c i t y i s c o n s t a n t . I f t h e number o f beds was e x t e n d e d t o i n f i n i t y , t h e t h i c k n e s s o f e a c h bed would become i n f i n t e s i m a l and t h e v e l o c i t y d i s t r i b u t i o n w ould become a c o n t i n u o u s f u n c t i o n w i t h d e p t h . R e f e r r i n g t o t h e n t h bed i n F i g u r e 21b, we have s i n I s i n i = p 34 Vn Vo Vn V n ( z ) 35 AXn = AZn t a n i n 36 68 AZn ATn = 37 Vn cos i The raypath parameter p i s a constant which depends upon the d i r e c t i o n i n which the ray l e f t the s i g n a l source, that i s , i t depends upon i°. In the l i m i t when n becomes i n f i n i t e , we get dx dt 1 — = tan i — = 38 dz dz Vcos i By i n t e g r a t i o n n-1 pViAZi pVnAZn X = > , + , 39 ^ ^ 1 - ( p V i ) 2 J l-(pVn) 1=1 n AZi T = y ^ 4 0 V i 1 - ( p V i ) 2 i=1 By reducing the expr e s s i o n f o r x, we have Vn = / / X n 7! , . , . .2 V / • y O J - \ \ - (pVi) ) IS / ^T" p V i A Z i \ \ 2 Zn 69 A value i s s e l e c t e d f o r p and Vn i s c a l c u l a t e d . Vn and p are s u b s t i t u t e d i n t o the equation f o r t and the convergence with the recorded a r r i v a l time i s checked. A new value f o r p i s assumed and numerous i t e r a t i o n s are performed u n t i l the c a l c u l a t e d a r r i v a l time and the measured a r r i v a l time are w i t h i n a predetermined t o l e r e n c e . Changes i n the value of p change the p o s i t i o n of the t r a v e l path, thus the d e s i g n a t i o n , i t e r a t i v e t r a v e l path a n a l y s i s . The i t e r a t i v e a n a l y s i s i s extremely time consuming i f performed by hand. A computer i s necessary to perform the a n a l y s i s . The shear wave v e l o c i t y versus depth p l o t i n F i g u r e 22 d i s p l a y s the r e s u l t s of both c o n v e n t i o n a l and i t e r a t i v e t r a v e l path a n a l y s e s . The p l o t shows that shallow depth v e l o c i t i e s c a l c u l a t e d u s i n g the c o n v e n t i o n a l a n a l y s i s are c o n s i s t e n t l y higher because the assumed s t r a i g h t l i n e t r a v e l paths are too s h o r t . The d i f f e r e n c e s are however small when compared to other sources of e r r o r a s s o c i a t e d with the downhole t e s t . For most p r a c t i c a l purposes a s t r a i g h t l i n e t r a v e l path assumption would be q u i t e s a t i s f a c t o r y . 4.4 Dynamic Shear Modulus C a l c u l a t i o n In S e c t i o n 1.1 and 2.2, the a p p l i c a t i o n of e l a s t i c theory to the study of wave propagation was d i s c u s s e d . I t was shown that the dynamic shear modulus G of a s o i l i s r e l a t e d to the s o i l d e n s i t y ( t o t a l u n i t weight of the s o i l , 7, d i v i d e d by the a c c e l e r a t i o n due to g r a v i t y , g ) m u l t i p l i e d by the square of the 70 SHEAR WAVE VELOCITY, Vs, m/sec 100 I 2 0 0 t 3 0 0 i A O kO O C O N V E N T I O N A L A N A L Y S I S A I T E R A T I V E A N A L Y S I S 5 -» A O Ok S I G N A L S O U R C E O F F S E T 1 0 m E * x H o. iu 10 o A O A O A O AD A O A O 1 5 -O A F I G . 2 2 . C O M P A R I S O N B E T W E E N C O N V E N T I O N A L A N D I T E R A T I V E V E L O C I T Y C A L C U L A T I O N S 71 shear wave v e l o c i t y Vs, by the r e l a t i o n s h i p 7 V S 2 G = pVs 2 = 1 9 In the pr e v i o u s s e c t i o n s of t h i s c hapter, methods f o r determining accurate shear wave t r a v e l times and shear wave v e l o c i t i e s have been d i s c u s s e d . I n s p e c t i o n of the above equation i n d i c a t e s t h at s i n c e the shear wave v e l o c i t y i s squared, the accuracy of the v e l o c i t y measurement i s most important i n a c c u r a t e l y a s s e s s i n g the shear modulus. The accuracy of the s o i l d e n s i t y measurement i s not as important. There are four ways by which i n - s i t u s o i l d e n s i t y can be determined; undisturbed sampling, neutron l o g g i n g , p e n e t r a t i o n r e s i s t a n c e - d e n s i t y c o r r e l a t i o n s or by making an educated estimate. Neutron l o g g i n g equipment i s rather s p e c i a l i z e d and was not co n s i d e r e d f o r t h i s p r o j e c t . Most s o i l s have a d e n s i t y l y i n g between 1600 kg/m3 and 2250 kg/m3, thus even an estimated value of 1900 kg/m3 would probably introduce an e r r o r i n G of no more than 20 per cent (Stokoe and Woods,1972). For re s e a r c h purposes a more ac c u r a t e assessment was c o n s i d e r e d d e s i r a b l e . Cohesive s o i l s such as the c l a y s and s i l t s i n v e s t i g a t e d i n t h i s study are e a s i l y sampled and undisturbed samples can be retu r n e d to the l a b o r a t o r y f o r d e n s i t y d e t e r m i n a t i o n s . C o h e s i o n l e s s s o i l s , p a r t i c u l a r l y coarse sands, are extremely d i f f i c u l t to sample i n an undisturbed c o n d i t i o n . In-s i t u d e n s i t i e s i n these m a t e r i a l s were determined, f o r t h i s study, u s i n g a sand de n s i t y - c o n e bearing c o r r e l a t i o n d e s c r i b e d below. 72 The determination of s o i l d e n s i t y or more c o r r e c t l y r e l a t i v e d e n s i t y , from cone bearing data has been the subject of much recent r e s e a r c h (Robertson, 1983). Though no unique r e l a t i o n s h i p e x i s t s between cone r e s i s t a n c e , i n - s i t u s t r e s s and r e l a t i v e d e n s i t y , the r e s u l t s of c a l i b r a t i o n chamber t e s t s on quartz sands ( B a l d i , 1982) have l e d to the development of the cone bearing - r e l a t i v e d e n s i t y c o r r e l a t i o n shown i n Fi g u r e 23. By o b t a i n i n g d i s t u r b e d sand samples at s e l e c t e d depths, determining maximum and minimum d e n s i t i e s , and e s t i m a t i n g i n -s i t u r e l a t i v e d e n s i t y from the cone bearing c o r r e l a t i o n , the i n -s i t u sand d e n s i t y can be estimated at each depth i n t e r v a l . The v o i d r a t i o of a s o i l e, i s r e l a t e d to the maximum v o i d r a t i o emax, minimum v o i d r a t i o emin, and the r e l a t i v e d e n s i t y Dr, by the equation e=emax-Dr(emax-emin) 42 The t o t a l u n i t weight of the s o i l 7 , i s r e l a t e d to the s p e c i f i c g r a v i t y of the p a r t i c l e s Gs, the degree of s a t u r a t i o n Sr, the v o i d r a t i o e, and the u n i t weight of water 7 W , by the equation Gs + Sr 7S = 7W 43 1 + e From the t o t a l u n i t weight, s o i l d e n s i t y i s e a s i l y determined as shown i n Fi g u r e 23. 73 CONE B E A R I N G , q c , kg/cm* 0 100 200 300 400 500 E u 0.5 cn to LU rr u b. 1.0 1.5 o M rr o x 2.0 D = -r e max - e max min X 100% = Idmax x Yd " Ydmin X 100% dmax dmi n 's S.G. v 1 + w v Y — = X -=—— X 'w g g 1+e r P S.G. 9 relative density bulk wet dens i ty specific gravity gravitational acceleration w -e Yw • water content void ratio unit weight water dry unit weight soi 1 F I G . 2 3 . S A N D D E N S I T Y - C O N E B E A R I N G C O R R E L A T I O N ( U n a g e d u n c e m e n t e d quar tz s a n d s , a f t e r R o b e r t s o n , 1 9 8 3 ) 74 4.5 Summary The d e t e r m i n a t i o n of dynamic shear moduli i n s o i l u s i ng the CPT downhole seismic survey r e q u i r e s a m u l t i s t e p procedure. 1. Measurement of shear wave a r r i v a l times using a convenient r e f e r e n c e p o i n t such as the f i r s t zero c r o s s o v e r p o i n t . 2. Make an assessment of the v a r i a b i l i t y of the measurement by averaging ten or more a r r i v a l times at s e l e c t e d depth increments. 3. V e c t o r a l l y convert a c t u a l t r a v e l times to pseudo i n t e r v a l t r a v e l times, assess the v a r i a b i l i t y of these d e t e r m i n a t i o n s and a r r i v e at a best f i t t r a v e l time e s t i m a t e . 4. Determine the t r a v e l path l e n g t h and determine i n t e r v a l shear wave v e l o c i t i e s using a c o n v e n t i o n a l or an i n t e r a t i v e t r a v e l path a n a l y s i s . 5. C a l c u l a t e the dynamic shear modulus f o r each depth increment u s i n g the e l a s t i c r e l a t i o n s h i p G=pVs 2, where p i s determined from supplimentary or complementary d e n s i t y 75 data. The r e s e a r c h c a r r i e d out f o r the p r e p a r a t i o n of t h i s t h e s i s i n d i c a t e s t h a t the accuracy of the shear wave v e l o c i t y d e t e r m i n a t i o n s i s dependent on the depth i n t e r v a l over which t r a v e l time measurements are made. By f o l l o w i n g a c a r e f u l l y planned f i e l d survey procedure and by using the data a n a l y s i s techniques suggested, shear wave v e l o c i t i e s should e a s i l y be obtained over one metre depth increments with an e r r o r of l e s s than ten per cent. With s u f f i c i e n t a d d i t i o n a l i n f o r m a t i o n such as the cone bearing p r o f i l e and s e l e c t e d samples s o i l d e n s i t y d e t e r m i n a t i o n s c o u l d be made w i t h i n 5 % u n c e r t a i n t y . Since the shear wave v e l o c i t y i s squared i n the dete r m i n a t i o n of shear modulus, i t should be p o s s i b l e to determine Gmax to w i t h i n p l u s or minus 25 % over one metre depth increments. The accuracy of the d e t e r m i n a t i o n s are improved over l a r g e r depth increments although averaging of s o i l p r o p e r t i e s becomes a f a c t o r to c o n s i d e r . 76 5.0 DYNAMIC SHEAR MODULUS FIELD MEASUREMENTS 5.1 I n t r o d u c t i o n The p r e v i o u s chapters have i n t r o d u c e d the concept of u s i n g a geophone instrumented seismic cone penetrometer to determine i n - s i t u dynamic shear moduli. Seismic wave phenomena, e l a s t i c theory, r a t i o n a l downhole seismic f i e l d procedures and data i n t r e p r e t a t i o n techniques have been d i s c u s s e d . T h i s chapter presents the r e s u l t s of i n - s i t u f i e l d t e s t i n g c a r r i e d out at three s e l e c t e d r e s e a r c h s i t e s i n the F r a s e r River D e l t a near Vancouver, B.C. to evaluate these procedures and techniques. T h i s chapter opens with a d i s c u s s i o n of the f i e l d t e s t i n g c a p a b i l i t i e s of the seismic cone and presents cone bearing data p r o f i l e s , shear wave v e l o c i t y p r o f i l e s and dynamic shear modulus p r o f i l e s f o r each r e s e a r c h s i t e . The dynamic shear modulus values determined from seismic wave measurement are then compared with other i n - s i t u moduli d e t e r m i n a t i o n s . Seismic modulus data i s compared with cone bearing - modulus c o r r e l a t i o n s , pressuremeter unload - r e l o a d moduli, and c o n v e n t i o n a l c r o s s h o l e seismic data where a v a i l a b l e . The chapter then c l o s e s by p r e s e n t i n g a comparison of e x i s t i n g e m p i r i c a l shear modulus r e l a t i o n s h i p s with CPT downhole seismic modulus d e t e r m i n a t i o n s . 77 5.2 CPT Seismic F i e l d T e s t i n g C a p a b i l i t y The l o c a t i o n of each of the three r e s e a r c h s i t e s i s shown in F i g u r e 24. These s i t e s were s e l e c t e d on the b a s i s of s o i l c o n d i t i o n s , a c c e s s i b i l i t y , and the a v a i l a b i l i t y of a d d i t i o n a l i n - s i t u t e s t i n g data. The seismic CPT was advanced to 40 metres at the McDonalds Farm S i t e , 20 metres at the F o r t Langley Freeway S i t e and 30 metres at the Annacis North P i e r S i t e . Continuous cone b e a r i n g and pore pressure p r o f i l e s were obtained and from t h i s data the cone bearing p r o f i l e s shown i n F i g u r e s 25, 26, and 27 were obtained. The plank type s i g n a l source was p l a c e d with ends e q u i d i s t a n t w i t h i n 3.00 metres of the cone hole at each s i t e . The cone was then withdrawn and at each metre i n t e r v a l , 30 to 40 shear waves were generated and 30 to 40 s i g n a l t r a c e s were obtained from the t r a n s v e r s e geophone. A post t r i g g e r delay was used to capture only the i n i t i a l shear wave p o r t i o n of each s i g n a l The f i r s t zero c r o s s o v e r p o i n t s were used f o r shear wave a r r i v a l time i n t e r p r e t a t i o n and the true and pseudo i n t e r v a l t r a v e l times were p l o t t e d i n s t a t i s t i c a l d i s t r i b u t i o n as d e s c r i b e d i n Chapter 4. I n t e r v a l shear wave v e l o c i t i e s were c a l c u l a t e d u s ing the mean i n t e r v a l t r a v e l times over each depth increment. The r e s u l t i n g shear wave v e l o c i t y p r o f i l e s f o r each s i t e are shown i n F i g u r e s 25, 26, and 27. The dynamic shear moduli shown i n the f i g u r e s were c a l c u l a t e d using the e l a s t i c r e l a t i o n s h i p G=pVs 2 1 . d i s c u s s e d e a r l i e r . lUxnl F I G . 24. R E S E A R C H S I T E L O C A T I O N M A P SCALE 1:250,000 00 79 5.2.1 McDonalds Farm S i t e The f i r s t s i t e ( r e f e r r e d to here as the McDonalds Farm S i t e ) i s l o c a t e d j u s t north of the Vancouver I n t e r n a t i o n a l A i r p o r t on Sea I s l a n d i n Richmond, B.C. E x t e n s i v e i n - s i t u t e s t i n g has been c a r r i e d out at t h i s s i t e , as i t has been the p r i n c i p a l r e s e a r c h s i t e f o r the U n i v e r s i t y of B r i t i s h Columbia s o i l s group f o r f i v e y e a r s . A complete d e s c r i p t i o n of the s i t e may be found i n Campanella et a l , 1983. The s i t e s t r a t i g r a p h y , shown i n F i g u r e 25, c o n s i s t s of 2 metres of organic s i l t o v e r l y i n g 10 to 12 metres of sand which o v e r l i e s an e x t e n s i v e d e p o s i t of normally c o n s o l i d a t e d c l a y e y s i l t . The water t a b l e depth v a r i e s with seasonal and t i d a l f l u c t u a t i o n s but was g e n e r a l l y w i t h i n 1 to 2 metres of the ground s u r f a c e d u r i n g t e s t i n g . The s i d e by s i d e comparison of the p r o f i l e s i n F i g u r e 25 shows an i n t e r e s t i n g comparison between the cone b e a r i n g p r o f i l e and the incremental downhole shear wave v e l o c i t y measurements. The steady i n c r e a s e i n cone bearing with depth i n the sand and the sudden drop at the s i l t sand i n t e r f a c e at 13 metres are m i r r o r e d by the shear wave v e l o c i t y and dynamic shear modulus p r o f i l e s . The shear wave v e l o c i t y and shear modulus p r o f i l e s then i n c r e a s e with depth and c o n f i n i n g pressure as would be expected i n a normally c o n s o l i d a t e d c l a y e y s i l t . The bulk s o i l d e n s i t i e s shown f o r the sand were determined from s e l e c t i v e d i s t u r b e d sampling and from a s i t e s p e c i f i c cone bearing s o i l d e n s i t y c o r r e l a t i o n as d i s c u s s e d i n s e c t i o n 4.4. Bulk s o i l d e n s i t i e s i n the s i l t were based on one u n d i s t u r b e d SOIL DENSITY SHEOR WAVE VELOCITY 1 7 0 0 , K G / C U ^ 0 0 0 , M / 5 E C ' BEARING RESISTANCE DYNAMIC SHEAR MODULUS SOIL 2SO 0 , K G / S Q C M 1 200 0 , K G / 5 0 C M 1 .250 PROF ILE • o i l t U T PI* • e f t C l a y e y t I L T F I G . 2 5 . M C D O N A L D S F A R M C O N E P R O F I L E 81 sample from 16 metres depth. 5.2.2 F o r t Langley Freeway S i t e The second s i t e ( r e f e r r e d t o here as the F o r t Langley Freeway S i t e ) i s l o c a t e d 50 Km east of Vancouver on the freeway p o r t i o n of the Trans Canada Highway near F o r t Langley, B.C. Although only r e c e n t l y u t i l i z e d , t h i s s i t e shows tremendous p o t e n t i a l f o r i n - s i t u t e s t i n g r e s e a r c h . The s i t e s t r a t i g r a p h y , shown i n F i g u r e 26, c o n s i s t s of an e x t e n s i v e d e p o s i t of s o f t normally c o n s o l i d a t e d s i l t y c l a y and c l a y e y s i l t with o c c a s i o n a l sandy l a y e r s . The water t a b l e f l u c t u a t e s s e a s o n a l l y but was observed to be w i t h i n 2 metres of the s u r f a c e . The s i t e i s l o c a t e d on the shoulder of the . h e a v i l y t r a v e l l e d Trans Canada Highway immediately adjacent to a r a i l w a y overpass. Observations i n the f i e l d i n d i c a t e d that the shear wave d e t e c t i o n was l i t t l e a f f e c t e d by heavy truck t r a f f i c on the freeway. However, l a r g e amplitude, low frequency n o i s e was noted when f r e i g h t t r a i n s c r o s s e d the overpass. The impulse type s i g n a l g e n e r a t i o n procedure allowed f o r operator d i s c r e t i o n . Wave ge n e r a t i o n and d e t e c t i o n was only c a r r i e d out d u r i n g q u i e t p e r i o d s . The p l o t t e d s o i l d e n s i t y values i n F i g u r e 26 were obtained from a set of continuous samples obtained between 2.8 and 14.2 metres depth. Both the shear wave v e l o c i t y and shear modulus p r o f i l e s i n F i g u r e 26 show gradual i n c r e a s e with depth except between 11 and 13 metres. The i n c r e a s e i n value over t h i s increment corresponds F I G . 2 6 F O R T L A N G L E Y C O N E P R O F I L E 83 w e l l with a s u b t l e but observable i n c r e a s e i n cone bearing over the same increment. I n s p e c t i o n of a v a i l a b l e samples i n d i c a t e s that the s o i l i s sandier here than i t i s higher and lower i n the s i t e p r o f i l e . 5.2.3 Annacis North P i e r S i t e The t h i r d s i t e ( r e f e r r e d t o here as the Annacis North P i e r S i t e ) i s l o c a t e d on Annacis I s l a n d adjacent to the main arm of the F r a s e r R i v e r , 5 Km west of New Westminster. E x t e n s i v e s o i l i n v e s t i g a t i o n s have been c a r r i e d out at t h i s s i t e f o r a proposed c a b l e stayed b r i d g e . The s i t e s t r a t i g r a p h y shown i n F i g u r e 27, c o n s i s t s of a 3 metre sand dike o v e r l y i n g a t h i n l a y e r of o r i g i n a l o rganic s i l t t o p s o i l which o v e r l i e s a reasonably continuous sand d e p o s i t . The sand i s u n d e r l a i n by marine s i l t and i n t e r l a y e r e d s i l t and sand at great depth, but only the upper sands were encountered d u r i n g t e s t i n g with the seismic cone. The water t a b l e f l u c t u a t e s with r i v e r l e v e l but was encountered at approximately 4 metres depth at the time of the survey. The shear wave v e l o c i t y p r o f i l e i n F i g u r e 27 s u b t l e y m i r r o r s the cone bearing p r o f i l e with l i t t l e i n the way of dramatic v e l o c i t y c o n t r a s t s . The most notable f e a t u r e s are an in c r e a s e i n shear wave v e l o c i t y at 11 metres which corresponds to a s i g n i f i c a n t i n c r e a s e i n cone bearing peaks. These s t r a t i g r a p h i c f e a t u r e s are n o t i c e a b l y a m p l i f i e d on the dynamic shear modulus p l o t . The bulk s o i l d e n s i t y p l o t i s based on sample averages F I G . 2 7 . A N N A C I S N O R T H P I E R C O N E P R O F I L E 85 obtained from c o n v e n t i o n a l d r i l l i n g and sampling at the s i t e . 5.3 Comparison of I n - S i t u Moduli Measurements Downhole seismic i s only one method by which i n - s i t u shear moduli can be determined. Other methods i n c l u d e d i r e c t d e t e r m i n a t i o n from c r o s s h o l e seismic t e s t i n g , and i n d i r e c t d e t e r m i n a t i o n from cone bearing - dynamic shear modulus c o r r e l a t i o n s or s e l f - b o r i n g pressuremeter unload - r e l o a d t e s t s . Since data from each of these methods i s a v a i l a b l e from one or another of the r e s e a r c h s i t e s , i t i s presented here f o r comparison. 5.3.1 S e l f - B o r i n g Pressuremeter Moduli The dynamic shear moduli values c a l c u l a t e d from seismic shear wave v e l o c i t i e s i n the sands at the McDonalds Farm S i t e are r e p l o t t e d i n F i g u r e 28. They are compared with dynamic shear modulus values determined using data from 10 s e l f b o r i n g pressuremeter t e s t s i n h o l e s adjacent to the seismic cone h o l e . The pressuremeter t e s t s were c a r r i e d out as p a r t of a PhD. Research program (Robertson, 1983). The unload - r e l o a d moduli from each pressuremeter t e s t was m u l t i p l i e d by a f a c t o r of 5 to o b t a i n the p l o t t e d values (based on a suggestion by Byrne and E l d r i d g e (1982), that the i n i t i a l tangent modulus under s t a t i c l o a d i n g c o n d i t i o n s i s about one f i f t h the dynamic modulus). The pressuremeter data l i e s above the seismic cone data but the 86 DYNAMIC SHEAR MODULUS, Gmax, K g / c m 2 5 0 0 1 1 0 0 0 I 1 5 0 0 I O A o • S E L F - B O R I N G P R E S S U R E M E T E R O C P T S E I S M I C D A T A 5 H SAND A A A O o O A O A 1 5 H SILT O o o o Note: SBPMT Unload - Reload moduli mu l t ip l i ed by 5 to determine Gmax. F I G . 2 8 . C O M P A R I S O N B E T W E E N C P T S E I S M I C A N D S E L F - B O R I N G P R E S S U R E M E T E R , M C D O N A L D S F A R M 87 comparison i s reasonably good. The data from t h i s s i t e i n d i c a t e s that s e l f b o r i n g pressuremeter moduli from unload - r e l o a d c y c l e s should be m u l t i p l i e d by a f a c t o r of about 3 to get Gmax. 5.3.2 CPT Cone Bearing C o r r e l a t i o n s In some recent work by Robertson & Campanella (1982) a cone be a r i n g dynamic shear modulus c o r r e l a t i o n was proposed f o r normally c o n s o l i d a t e d uncemented qu a r t z sands. The c o r r e l a t i o n was developed by combining r e l a t i v e d e n s i t y cone r e s i s t a n c e r e l a t i o n s h i p s developed by B a l d i , (1982) with e m p i r i c a l dynamic shear modulus r e l a t i o n s h i p s developed by Seed and I d r i s s (1970), and Hardin and Drnevich (1972). In F i g u r e 29 the seismic shear modulus data from the sand at McDonalds Farm i s compared with the p r e d i c t e d shear modulus value s from the CPT c o r r e l a t i o n . The CPT downhole seismic data l i e s below the cone p r e d i c t i o n but the comparison i s q u i t e good. In F i g u r e 30 the s e i s m i c shear modulus data from the Annacis North P i e r S i t e i s compared with p r e d i c t e d shear modulus va l u e s from the cone b e a r i n g c o r r e l a t i o n . The agreement between the two curves i s e x c e l l e n t . 5.3.3 Conventional Crosshole T e s t i n g The CPT seismic downhole survey at the Annacis North P i e r S i t e was c a r r i e d out approximately 5 metres from a three hole a r r a y used f o r a c r o s s h o l e seismic survey. The c r o s s h o l e data 88 DYNAMIC SHEAR MODULUS,Gmax, K g / c m 2 F I G . 2 9 . C O M P A R I S O N B E T W E E N C P T S E I S M I C A N D C P T B E A R I N G P R E D I C T I O N , M C D O N A L D S F A R M 89 DYNAMIC SHEAR MODULUS, Gmax, Kg / c m * 0 6 0 0 1 0 0 0 1 5 0 0 F I G . 3 0 . C O M P A R I S O N B E T W E E N C P T S E I S M I C A N D C P T B E A R I N G P R E D I C T I O N , A N N A C I S N O R T H P I E R 90 was obtained at 2.5 metre i n t e r v a l s between 5 metres and 110 metres depth. The h o l e s were surveyed a f t e r the c r o s s h o l e t e s t i n g was completed to ensure that the t r a v e l path lengths were a c c u r a t e l y known. In F i g u r e 31 the c r o s s h o l e data i s compared with CPT downhole data between the surface.and 30 metres depth. The downhole data l i e s c o n s i s t e n t l y above the c r o s s h o l e data but g e n e r a l l y the two s e t s of data compare w i t h i n 20 per c e n t . Comparisons between c r o s s h o l e and downhole seismic survey data cannot be c o n s i d e r e d p a r t i c u l a r l y c o n c l u s i v e . Numerous v a r i a b l e s may a f f e c t the r e s u l t s and be used to account f o r d i s c r e p a n c i e s between two s e t s of data. One p o s s i b l e e x p l a n a t i o n of t h i s d i s c r e p a n c y i s that shear wave v e l o c i t i e s are a n i s o t r o p i c . As d i s c u s s e d i n Chapter 2, the shear waves d e t e c t e d in downhole t e s t s are SH waves which propagate v e r t i c a l l y but o s c i l l a t e i n the h o r i z o n t a l d i r e c t i o n . Shear waves detected i n c r o s s h o l e t e s t i n g are SV waves which propagate h o r i z o n t a l l y but o s c i l l a t e i n the v e r t i c a l d i r e c t i o n . L i t t l e data i s a v a i l a b l e i n the l i t e r a t u r e to support t h i s e x p l a n a t i o n , and the data base here i s too l i m i t e d to prove t h i s c o n c l u s i v e l y . A d d i t i o n a l r e s e a r c h i s suggested to determine the magnitude of a n i s o t r o p i c e f f e c t s on shear wave v e l o c i t y and shear modulus. A d d i t i o n a l e x p l a n a t i o n s of the d i s c r e p a n c y i n c l u d e d i f f e r e n c e s i n s t r a t i g r a p h y between the c r o s s h o l e l o c a t i o n and the downhole l o c a t i o n . Borehole d i s t u r b a n c e and probe i n s t a l l a t i o n e f f e c t s , and the cumulative e f f e c t of inherent measurement e r r o r s a s s o c i a t e d with each survey. » D e s p i t e the v e l o c i t y measurement des c r e p a n c i e s , the 91 SHEAR WAVE VELOCITY, m/sec S O I 1 0 0 1 5 0 I 2 0 0 I 2 5 0 — I 3 0 0 3 5 0 6 A A C R O S S H O L E S E I S M I C • D O W N H O L E S E I S M I C 1 0 4 A 5 i«H & 2 0 -| Ul O 2 5 H 3 0 H A A A A A A F I G . 3 1 . C O M P A R I S O N B E T W E E N D O W N H O L E A N D C R O S S H O L E V E L O C I T Y M E A S U R E M E N T S , A N N A C I S N O R T H P I E R 92 comparison between the two s e t s of data would f o r most p r a c t i c a l e n g i n e e r i n g purposes be c o n s i d e r e d reasonably a c c e p t a b l e . 5 .4 Conventional E m p i r i c a l R e l a t i o n s h i p s The dynamic shear modulus, Gmax, of a s o i l can be r e l a t e d to the i n - s i t u mean e f f e c t i v e s t r e s s by an e m p i r i c a l r e l a t i o n s h i p of the form Gmax=Kg Pa(om'/Pa) n 44 where Kg i s a dimensionless modulus number, am' i s the mean e f f e c t i v e s t r e s s , Pa i s a re f e r e n c e s t r e s s ( i k g / c m 2 ) , and n i s an exponent. For uncemented quartz sands i t has been shown that n i s approximately 0.5. In normally c o n s o l i d a t e d s i l t s and c l a y s n i s approximately 1.0. (Seed and I d r i s s , 1970; Hardin and Drnevich, 1972). A c o r r e l a t i o n proposed by Robertson (1983) suggests that modulus numbers in uncemented quartz sands are dependent on r e l a t i v e d e n s i t y . The sands at McDonalds Farm have r e l a t i v e d e n s i t i e s which range between 50 and 70 per cent based on the cone b e a r i n g c o r r e l a t i o n i n F i g u r e 23. Robertsons' proposed c o r r e l a t i o n would i n d i c a t e that the modulus number should vary between 1000 and 1250. The sands at the Annacis North P i e r S i t e have r e l a t i v e d e n s i t i e s that range between 30 and 50 per cent based on the cone bearing c o r r e l a t i o n i n F i g u r e 23. Robertsons' proposed c o r r e l a t i o n would i n d i c a t e that the mdulus number should vary between 750 and 1000 f o r these sands. The seismic cone shear modulus data from the McDonalds Farm s i t e has been p l o t t e d i n F i g u r e 32 and compared with shear 93 m o d u l i c a l c u l a t e d u s i n g two d i f f e r e n t modulus numbers. In t h e s a n d an e x p o n e n t o f 0.5 was assumed and i n t h e s i l t an e x p o n e n t of 1.0 was assumed. The r e s u l t i n g p l o t o f s h e a r modulus w i t h d e p t h i n t h e s a n d shows t h a t Kg i n c r e a s e s w i t h d e p t h a s t h e r e l a t i v e d e n s i t y of t h e s a n d i n c r e a s e s . T h e n , a s would be e x p e c t e d , Kg d r o p s o f f i n t h e s i l t below 13 m e t r e s d e p t h . S h e a r modulus numbers f o r t h e n o r m a l l y c o n s o l i d a t e d s i l t a r e r e a s o n a b l y c o n s i s t e n t w i t h d e p t h r a n g i n g between 400 and 500. In a n o r m a l l y c o n s o l i d a t e d homogeneous c l a y e y s i l t , t h i s k i n d o f c o n s i s t e n c y w o u l d be e x p e c t e d . The dynamic s h e a r modulus v a l u e s from t h e F o r t L a n g l e y Freeway S i t e have been p l o t t e d i n F i g u r e 33 and compared w i t h s h e a r m o d u l i c a l c u l a t e d u s i n g two d i f f e r e n t modulus numbers. The above m e n t i o n e d e m p i r i c a l e x p o n e n t i a l r e l a t i o n s h i p has been u s e d a s s u m i n g Gmax i s a l i n e a r f u n c t i o n w i t h d e p t h (n e q u a l s 1 ) . The r e s u l t i n g p l o t i n d i c a t e s t h a t t h e modulus number Kg i s r e a s o n a b l e c o n s i s t e n t w i t h d e p t h , v a r y i n g between 400 and 500. I t i s i n t e r e s t i n g t o n o t e t h a t t h i s i s t h e same r a n g e o b s e r v e d f o r t h e McDonalds Farm d a t a . The s h e a r modulus d a t a from t h e A n n a c i s N o r t h P i e r S i t e h a s been p l o t t e d i n F i g u r e 34. An e x p o n e n t o f 0.5 was assumed f o r t h e s a n d . The r e s u l t i n g p l o t o f s h e a r modulus w i t h d e p t h shows and i n c r e a s e w i t h d e p t h f r o m a v a l u e o f l e s s t h a n 500 n e a r t h e s u r f a c e t o a v a l u e between 750 and 1000 a t 30 m e t r e s d e p t h . As d i s c u s s e d a b o v e , numbers o f t h i s m a g n i t u d e would be e x p e c t e d . F I G . 3 2 . C O M P A R I S O N B E T W E E N C P T S E I S M I C A N D E M P I R I C A L D Y N A M I C S H E A R M O D U L U S , M C D O N A L D S F A R M 95 F I G . 3 3 . C O M P A R I S O N B E T W E E N C P T S E I S M I C A N D E M P I R I C A L D Y N A M I C S H E A R M O D U L U S , F O R T L A N G L E Y 96 F I G . 3 4 . C O M P A R I S O N B E T W E E N C P T S E I S M I C A N D E M P I R I C A L D Y N A M I C S H E A R M O D U L U S , A N N A C I S N O R T H P I E R 97 5.5 Summary The data comparisons i n t h i s chapter lend some measure of con f i d e n c e to the accuracy of the shear wave v e l o c i t y data a q u i r e d , and the shear moduli v a l u e s determined using the CPT seismic cone. Comparisons with cone bearing p r e d i c t i o n s of dynamic modulus i n sand are good. Comparisons with dynamic mouli d e t e r m i n a t i o n s from s e l f b o r i n g pressuremeter t e s t s show s i m i l a r t r e n d s . However i n the s o i l s t e s t e d , the SBPMT values are somewhat higher than the CPT seismic v a l u e s . T h i s may be due, i n p a r t , t o the higher s t r e s s l e v e l at which the s e l f - b o r i n g pressuremeter t e s t unload - r e l o a d modulus was c a r r i e d out. Cro s s h o l e and CPT downhole shear wave v e l o c i t y comparisons appear reasonable, though comparison of data from a d d i t i o n a l s i t e s u s ing the same s i g n a l d e t e c t i o n equipment i s s t i l l r e q u i r e d . Comparisons with e m p i r i c a l shear modulus r e l a t i o n s h i p s i n d i c a t e t h a t the seismic cone can p r o v i d e a reasonably r a p i d and a c c u r a t e method of checking both fundemental and t h e o r e t i c a l dynamic s o i l parameters. 98 6.0 CONCLUSIONS The CPT downhole seismic t e s t has been d i s c u s s e d i n t h i s t h e s i s . The r e s u l t s of the data presented i n d i c a t e that the t e s t can p r o v i d e a r a p i d and accurate assessment of shear moduli at s i t e s where cone p e n e t r a t i o n t e s t i n g can be c a r r i e d out. T h i s chapter summarizes some of the most important c o n c l u s i o n s d i s c u s s e d i n the previous t e x t , p r e s e n t s suggestions f o r f u r t h e r r e s e a r c h using t h i s d e v i ce and b r i e f l y d i s c u s s e s some c o n s i d e r a t i o n s f o r use of t h i s equipment i n p r a c t i c a l e n g i n e e r i n g a p p l i c a t i o n s . 6.1 Summary of Research F i n d i n g s The fundamental o b j e c t i v e of t h i s r e s e a r c h p r o j e c t was to develop the c a p a b i l i t y to d i r e c t l y measure dynamic s o i l p r o p e r t i e s with the s t a t i c cone penetrometer. In order to f u l l y complement the s t r a t i g r a p h i c l o g g i n g c a p a b i l i t i e s of the cone, i t was c o n s i d e r e d d e s i r a b l e to attempt to design an instrument and s u i t a b l e o p e r a t i n g and data a n a l y s i s procedures to measure shear moduli over minimum depth increments. One of the d i f f i c u l t i e s encountered d u r i n g the study was that v e l o c i t y measurement e r r o r i n c r e a s e d as f i n e r s t r a t i g r a p h i c i d e n t i f i c a t i o n ( i e . t r a v e l time measurement over smaller depth increments) was sought. Greater v e l o c i t y measurement ac c u r r a c y i s p o s s i b l e over l a r g e r depth increments, however the dynamic p r o p e r t i e s of i n d i v i d u a l s t r a t i g r a p h i c f e a t u r e s become masked by 99 av e r a g i n g . During the course of t h i s study both pseudo and t r u e i n t e r v a l type measurements were s t u d i e d . Because s i m i l a r t r a v e l time measurement e r r o r s were noted f o r both types of survey method, i t was determined that there was no gre a t advantage to using a t r u e i n t e r v a l (two geophone) method. Since t r i g g e r r e p e a t a b i l i t y was good, r e s u l t s obtained u s i n g the pseudo i n t e r v a l method (one geophone) were s a t i s f a c t o r y . The most important c o n c l u s i o n s to come from t h i s work are 1. The CPT downhole survey has d i s t i n c t advantages over c o n v e n t i o n a l c r o s s h o l e t e s t i n g i n that i t i s much q u i c k e r to run and r e q u i r e s only one t e s t h o l e . 2. The CPT downhole survey has d i s t i n c t advantages over c o n v e n t i o n a l downhole t e s t i n g . I t i s f a s t e r to c a r r y out, does not r e q u i r e a cased and grouted borehole, accurate depth determination i s p o s s i b l e at a l l times d u r i n g the survey, and geophone o r i e n t a t i o n i s c o n s i s t e n t l y maintained. 3. The main sources of measurement uncertanty are due to poor s i g n a l r e p e a t a b i l i t y between the source and d e t e c t o r , and u n c e r t a i n t y about t r a v e l path l e n g t h . The problem of u n c e r t a i n t r a v e l path l e n g t h can be overcome by p l a c i n g the s i g n a l source as c l o s e as p o s s i b l e to the t e s t h o l e . Source 100 r e p e a t a b i l i t y problems c o u l d be overcome by using an automatic rather than a manual energy impulse. Detector r e p e a t a b i l i t y problems c o u l d be overcome by r e p l a c i n g the mechanical geophone with a s o l i d s t a t e p ickup device f r e e of strong s e l f resonance c h a r a c t e r i s t i c s . Down rod s i g n a l t r a n s m i s s i o n i s not a problem with t h i s measurement system. The use of a s u i t a b l y s i z e d f r i c t i o n reducer prevents c o u p l i n g between the s o i l and the rods and prevents down rod s i g n a l t r a n s m i s s i o n . By decoupling the rods from the t e s t v e h i c l e d u r i n g shear wave measurement, problems with mechanical v i b r a t i o n are minimized. Background high frequency n o i s e can s e r i o u s l y a f f e c t the ac c u r a t e i n t e r p r e t a t i o n of low amplitude shear wave s i g n a l s . By u s i n g s e l e c t i v e s i g n a l f i l t e r i n g much sharper s i g n a l t r a c e s can be obt a i n e d . The 1000 Hz f i l t e r s used i n t h i s study caused minimal phase s h i f t s . Any such s h i f t s are reasonably constant and would c a n c e l when i n t e r v a l v e l o c i t y measurement techniques are employed. The a v a i l a b i l i t y of cone bearing data g r e a t l y a s s i s t s i n a s s e s s i n g the dynamic s o i l p r o p e r t i e s . Cone b e a r i n g d e n s i t y c o r r e l a t i o n s can be used to complement p h y s i c a l bulk d e n s i t y d e t e r m i n a t i o n s . 101 7. The CPT downhole seismic survey system works most e f f e c t i v e l y between 3 and 30 metres depth. D e t e c t i o n of s i g n a l s from i n d i v i d u a l hammer blows have been d e t e c t e d to 40 metres. With s i g n a l enhancement i t i s c o n c e i v a b l e that s i g n a l s c o u l d be detected at s t i l l g r e a t e r depth. 8. Dynamic shear modulus determinations from the CPT downhole seismic survey compare w e l l with p r e d i c t i o n s made using other i n - s i t u t e s t methods, and v a l u e s c a l c u l a t e d u sing e m p i r i c a l r e l a t i o n s h i p s . 6.2 F u r t h e r Research During the progress of t h i s i n v e s t i g a t i o n , s e v e r a l a d d i t i o n a l r e s e a r c h t o p i c s u t i l i z i n g the seismic cone penetrometer measurement c a p a b i l i t i e s became apparent. At i t s present stage of development the seismic cone i s p r i m a r i l y an onshore i n v e s t i g a t i o n t o o l . The i n c r e a s i n g use of cone p e n e t r a t i o n t e s t i n g o f f s h o r e makes the development of an o f f s h o r e seismic cone a l o g i c a l e x t e n s i o n of t h i s r e s e a r c h . Development of such a c a p a b i l i t y would d e f i n i t e l y r e q u i r e the development of an a l t e r n a t e s i g n a l source, or use of the equipment i n c r o s s h o l e c o n f i g u r a t i o n . The development of an i n -hole shear wave source ( e i t h e r t o r s i o n a l or v e r t i c a l ) might be pursued, provided the s i g n a l to noise r a t i o i s high enough or s u i t a b l e analog and d i g i t a l f i l t e r s can be used to s o r t the 102 shear wave from other v i b r a t i o n s . S e v e r a l onshore r e s e a r c h t o p i c s r e q u i r e i n v e s t i g a t i o n and c o u l d be pursued immediately. 1. Development of a new shear wave source which w i l l allow c o n t r o l of the shear wave amplitude. The present hammer source does not allow the shear wave amplitude to be maintained with depth. A l a r g e r mechanical source would allow c o n t r o l of s i g n a l amplitude and extend the present depth l i m i t a t i o n s of the se i s m i c cone. In a d d i t i o n an automatic source would reduce d i f f i c u l t i e s a s s o c i a t e d with source r e p e a t a b i l i t y . 2. Use of the seismic cone penetrometer in c r o s s h o l e t e s t i n g a p p l i c a t i o n s . The q u i c k e s t way to i n v e s t i g a t e t h i s p o s s i b i l i t y would be to run the seismic cone beside an advancing Standard P e n e t r a t i o n T e s t h o l e (See Ohta, Goto, Kagami & Shiono, 1978) 3. I n v e s t i g a t e i n - s i t u shear modulus a n i s o t r o p y by comparing downhole determinations with c r o s s h o l e data i n s t a t i s t i c a l l y s i g n i f i c a n t q u a n t i t i e s . 4. Attempt to develop f i n e r s t r a t i g r a p h i c logging c a p a b i l i t y by o b t a i n i n g t r a v e l time measurements over s m a l l e r depth 103 increments ( i e . 0.5 metres). 5. C a r r y out parametric chamber t e s t i n g using the seismic cone to f u r t h e r assess the r e l a t i o n s h i p between cone b e a r i n g , c o n f i n i n g p r e s s u r e , s o i l d e n s i t y and dynamic shear modulus under c o n t r o l l e d c o n d i t i o n s . I t should be recognized however that because of sand composition e f f e c t s , there w i l l be no unique r e l a t i o n s h i p f o r a l l sands. 6. F u r t h e r i n v e s t i g a t e i n - s i t u compressional wave measurements, p a r t i c u l a r l y i n s i l t s and c l a y s to see i f s u b t l e d i f f e r e n c e s i n P wave v e l o c i t y can be c o r r e l a t e d to c o n s o l i d a t i o n c h a r a c t e r i s t i c s . (See Hamdi and T a y l o r Smith, 1981). The e x i s t i n g pore pressure measurement c a p a b i l i t i e s i n the cone (See G i l l e s p i e , 1981) c o u l d be used f o r c o r r e l a t i o n . 7. I n v e s t i g a t e whether the dete r m i n a t i o n of i n - s i t u s o i l damping c o e f f i c i e n t s may be p o s s i b l e using the sei s m i c cone penetrometer. 6.3 C o n s i d e r a t i o n s f o r P r a c t i c a l A p p l i c a t i o n The equipment and procedures d i s c u s s e d through most of t h i s 104 t h e s i s have been d i r e c t e d toward res e a r c h a p p l i c a t i o n of the s e i s m i c cone penetrometer. For r e s e a r c h purposes a v e l o c i t y measurement accuracy of 10 per cent over 1 metre depth i n t e r v a l s was c o n s i d e r e d a d e s i r a b l e o b j e c t i v e . To achieve t h i s end, extremely s e n s i t i v e s i g n a l d e t e c t i o n equipment was r e q u i r e d . Both true and pseudo i n t e r v a l type v e l o c i t y survey data was obtained f o r comparison purposes, and s p e c i a l s t a t i s t i c a l a n a l y s i s of a r r i v a l time data was u t i l i z e d . For p r a c t i c a l e n g i n e e r i n g a p p l i c a t i o n s pseudo i n t e r v a l surveys over l a r g e r depth increments using lower r e s o l u t i o n s i g n a l d e t e c t i o n equipment may be c o n s i d e r e d a c c e p t a b l e . A cone penetrometer f o r such work would r e q u i r e the i n s t a l l a t i o n of only one h o r i z o n t a l l y o r i e n t e d v e l o c i t y t r a n s d u c e r . P r a c t i c a l a p p l i c a t i o n of a geophone instrumented s t a t i c cone penetrometer c o u l d provide a reasonably r a p i d and accurate assessment of shear moduli at s o f t s o i l s i t e s . The major advantages of t h i s method over other downhole seismic techniques a r e : 1. B e t t e r s o i l to r e c e i v e r c o u p l i n g a l l o w i n g g r e a t e r e f f e c t i v e depth of o p e r a t i o n . 2. C o n t r o l l e d r e c e i v e r o r i e n t a t i o n f o r improved shear wave det e c t i o n . 3. Extremely a c c u r a t e depth d e t e r m i n a t i o n . 1 05 Rapid i n s t a l l a t i o n and removal of the probe. Improved c o s t e f f e c t i v e n e s s of g e o t e c h n i c a l i n v e s t i g a t i o n s where cone p e n e t r a t i o n t e s t i n g i s being c a r r i e d out, because of the wealth of a d d i t i o n a l g e o t e c h n i c a l data a v a i l a b l e from one t e s t h o l e . 106 REFERENCES A l l e n , N.F., R i c h a r t , F.E., and Woods, R.D., " F l u i d Wave Propagation i n Nearly S a t u r a t e d Sand," J o u r n a l of the Ge o t e c h n i c a l Engineering D i v i s i o n , ASCE, V o l . 106, no. GT3,' March, 1980, pp. 235-254. 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