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UBC Theses and Dissertations

Model study of sloped tailings deposits Stuckert, Brian John-Adam 1982

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MODEL STUDY OF SLOPED TAILINGS DEPOSITS by BRIAN JOHN-ADAM STUCKERT B . A . S c , 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 , 1979 A THESIS SUBMITTED I N PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF A P P L I E D SCIENCE i n THE FACULTY OF GRADUATE STUDIES i n t h e D e p a r t m e n t o f C i v i l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY (c) B r i a n OF B R I T I S H COLUMBIA S t u c k e r t , 1982 AUTHORIZATION In presenting t h i s thesis in p a r t i a l fulfilment of the require-ments 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 Library s h a l l make i t freel y a v a i l -able for reference and study. I further agree that permission for extensive copying of thi s thesis for scholarly purposes may be granted by the Head of my department or by his or her repre-sentatives. It i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. ABSTRACT Present t a i l i n g s disposal methods are subject to l i m i t a t i o n s that warrent the development of alternative disposal techniques. A recently proposed method is the thickened discharge disposal method, which involves sloping t a i l i n g s towards a downstream embankment, thereby reducing the height of the embankment required to store a given volume of waste material. The seismic s t a b i l i t y of such slopes is a concern and model studies were performed to investigate t h e i r s t a b i l i t y . The model was composed of a sloped deposit of fine sand 81 cm long, 20 cm wide and a downstream bar r i e r 14 cm high. Slopes ranging from 4 to 14 percent were subjected to base accelerations ranging from .025 g to .10 g. Test deposits subjected to accelerations above a c r i t i c a l acceleration, dependent on the slope angle, were observed to liq u e f y and flow. These test deposits came to rest at a f i n a l slope of approximately one percent. Model deformations were recorded and l i q u e f i e d deposits were observed to behave s i m i l a r l y to a viscous f l u i d . A viscous f l u i d model was found to predict actual p a r t i c a l displacements reasonably well. Although application of test results may be limited to size e f f e c t s , i t seems appropriate to analyze l i q u i f i e d cohesionless material as a viscous f l u i d . - i i -2 It i s suggested that a s t a t i c a l l y stable sloped t a i l i n g s deposit, upon liq u e f a c t i o n to a s i g n i f i c a n t depth, may become unstable. The resulting flow, governed by post-liquefaction mechanical properties, could overtop a downstream embankment. - i i i -TABLE OF CONTENTS Chapter Page Table of Contents iv L i s t of Tables v i L i s t of Figures v i i Acknowledgments x 1 INTRODUCTION 1 2 METHODS OF DISPOSAL 4 2-1 Introduction 4 2-2 Present Disposal Methods 6 2-2-1 Upstream Construction Method 6 2-2-2 Downstream Construction Method 8 2-2-3 Comparison with Conventional 12 Earth Dam 2- 3 Recently Proposed Method of Disposal - 15 Thickened Discharge Disposal 2-3-1 Advantages of Thickened 16 Discharge 2- 3-2 Limitations of Thickened 19 Discharge 3 PRELIMINARY TESTING 22 3- 1 Test Material 22 3-2 Comparison of Test Sand with Typical 29 Tail i n g s Material 3- 2-1 Grain Size D i s t r i b u t i o n 30 3-2-2 Permeability and Compressibility 33 3-2-3 Relative Density 35 3-2-4 Liquefaction Resistance 36 iv TABLE OF CONTENTS (Cont'd) Chapter Page 4 MATERIAL BEHAVIOUR 41 4-1 L i q u e f a c t i o n 41 4 -2 P o s t L i q u e f a c t i o n 53 4 - 2 - 1 V i s c o s i t y 57 4 - 2 - 2 Y i e l d Shear S t r e n g t h 62 5 DEPOSITIONAL BEHAVIOUR OF TAILINGS COMPARED 66 TO THE RESPONSE OF A LIQUEFIED DEPOSIT 6 REVIEW OF PREVIOUS MODEL STUDIES 71 7 TESTING PROGRAM 75 7-1 Model T e s t Equipment 78 7-2 T e s t P r o c e d u r e s 84 8 TEST RESULTS 85 9 FLUID ANALYSIS 100 10 PREDICTED PARTICLE DISPLACEMENTS 107 11 FLOW FAILURES 111 1'2 CONCLUSIONS 115 R e f e r e n c e s 118 Appendix 124 v LIST OF TABLES Table Page I Volume Ch a r a c t e r i s t i c s for Test Sand 35 Versus Thickened Discharge Deposit II Correction Factors for I n i t i a l Slope 49 III Summary of Testing Program 87 vi L I S T OF FIGURES F i g u r e Page 1 F l o w C h a r t f o r T y p i c a l D i s p o s a l S y s t e m 5 2 . U p s t r e a m C o n s t r u c t i o n M e t h o d 7 3 S a f e Dam U s i n g U p s t r e a m C o n s t r u c t i o n 9 4 Downstream C o n s t r u c t i o n M e t h o d 1 0 5 C e n t e r l i n e C o n s t r u c t i o n M e t h o d 1 3 6 C o n v e n t i o n a l E a r t h Dam 1 4 7 T h i c k e n e d D i s c h a r g e M e t h o d 1 7 8 E x p a n s i o n o f S t o r a g e F a c i l i t y 1 8 9 G r a i n S i z e D i s t r i b u t i o n 2 3 1 0 e v s . l o g ( k ) R e l a t i o n s h i p 2 4 1 1 S c h e m a t i c L a y o u t o f C y c l i c T r i a x i a l 2 6 T e s t i n g A p p a r a t u s 1 2 L i q u e f a c t i o n R e s i s t a n c e C u r v e 2 7 1 3 T y p i c a l C y c l i c T r i a x i a l T e s t R e c o r d 2 8 1 4 T a i l i n g s G r a i n S i z e D i s t r i b u t i o n s 3 1 1 5 T a i l i n g s G r a i n S i z e D i s t r i b u t i o n s 3 2 1 6 V a r i a t i o n o f P e r m e a b i l i t y w i t h D - | Q 3 4 1 7 E f f e c t o f P l a s t i c i t y on L i q u e f a c t i o n 3 7 R e s i s t a n c e 1 8 L i q u e f a c t i o n R e s i s t a n c e o f T a i l i n g s 3 9 1 9 C y c l i c S t r e n g t h v s . V o i d R a t i o f o r Low 3 9 P l a s t i c i t y T a i l i n g s 2 0 R e s p o n s e o f Medium Dense S a n d U n d e r 4 2 M o n o t o n i c L o a d i n g 2 1 R e s p o n s e o f L o o s e Sand U n d e r M o n o t o n i c 4 4 L o a d i n g v i i L I S T OF FIGURES ( C o n t ' d ) F i g u r e Page 22 R e s p o n s e o f G a r n e t T a i l i n g s U n d e r 45 M o n o t o n i c L o a d i n g 23 a) R e s p o n s e o f L o o s e S a n d To C y c l i c L o a d i n g 46 b) R e s p o n s e o f L o o s e S a n d t o C y c l i c L o a d i n g - 47 E f f e c t i v e S t r e s s P a t h 24 C y c l i c S h e a r S t r e s s R e q u i r e d t o D e v e l o p 10% 49 S h e a r S t r a i n 2.5 L i q u e f a c t i o n R e s i s t a n c e C u r v e C o r r e c t e d f o r 50 S t a t i c S h e a r 26 T e r m i n a l R e s i d u a l P o r e P r e s s u r e s 52 27 B i n g h a m R h e o l o g i c a l M o d e l 55 28 Bingham M o d e l F l o w C u r v e 55 29 A G e n e r a l P o r e P r e s s u r e ( W a t e r C o n t e n t ) 56 D e p e n d e n t B i n g h a m P l a s t i c M o d e l 30 V a r i a t i o n o f P l a s t i c V i s c o s i t y w i t h W a t e r 58 C o n t e n t 3 1 Dependence o f V i s c o s i t y on S o l i d s 60 C o n c e n t r a t i o n 32 Dependence o f L a b o r a t o r y S l o p e on S o l i d s 60 C o n c e n t r a t i o n 33 V i s c o s i t y S p e c t r u m 61 34 ( a ) Rheogram f o r W a t e r S u s p e n s i o n o f F i n e l y 63 D i v i d e d G a l e n a (b) E f f e c t o f S o l i d s C o n c e n t r a t i o n on "C v 63 35 Y i e l d S h e a r S t r e n g t h S p e c t r u m 64 36 O b s e r v e d P o r e P r e s s u r e s f o r a S h a k i n g 73 T a b l e T e s t 37 M o d e l T e s t D e p o s i t D i m e n s i o n s 76 38 B o u n d a r y C o n d i t i o n s f o r S t e a d y S t a t e S e e p a g e 76 v i i i LIST OF FIGURES (Cont'd) F i g u r e Page 39 Flow Net f o r 6° S l o p e 77 40 S c h e m a t i c o f T a b l e Loop 79 41 T y p i c a l T a b l e Response at 5 Hz 81 42 Model C o n t a i n e r 82 43 Breakdown of T e s t i n g Program 86 44 R e s u l t s o f T e s t #7 - 88 45 R e s u l t s o f T e s t #12 89 46 R e s u l t s o f T e s t #13 90 47 R e s u l t s o f T e s t #14 91 48 R e s u l t s of T e s t #16 92 49 R e s u l t s of T e s t #17 93 50 E f f e c t of I n i t i a l S l o p e A n g l e on F i n a l S l o p e 95 Angle 51 A c c e l e r a t i o n L e v e l V e r s u s F i n a l S l o p e A n g l e 96 52 T h r e s h o l d A c c e l e r a t i o n v e r s u s I n i t i a l S l o p e 97 Angle 53 E f f e c t of S l o p e d Base cn Model Response 99 54 E f f e c t of V i s c o s i t y on Boundary L a y e r T h i c k n e s s 103 55 Comparison of P r e d i c t e d and Observed P a r t i c l e 108 D i s p l a c e m e n t s f o r an 8" S l o p e 56 Comparison of P r e d i c t e d and Observed P a r t i c l e 110 D i s p l a c e m e n t s f o r an 4° S l o p e 57 T a i l i n g s Flow F a i l u r e Case H i s t o r y - G r a i n 113 S i z e D i s t r i b u t i o n 58 T a i l i n g s Flow F a i l u r e Case H i s t o r y - D e p o s i t 114 C o n f i g u r a t i o n i x ACKNOWLEDGEMENTS I would l i k e to express my sincere thanks to Dr. Peter M. Byrne and Dr. Yogi P. Vaid for their guidance and support throughout t h i s study. I would also l i k e to thank my wife, Linda, for her patience and moral support, without which this e f f o r t would not have been possible. F i n a l l y , I would l i k e to thank Ertec Western for assistance in typing the f i n a l manuscript. - x -1 CHAPTER 1  INTRODUCTION The d i s p o s a l of mine waste m a t e r i a l i n a safe and cost e f f e c t i v e manner i s a subject that has received considerable a t t e n t i o n i n recent years. Present commonly used d i s p o s a l techniques are subject to l i m i t a t i o n s , as discussed i n Chapter 2, such that the development of a l t e r n a t i v e techniques i s warranted. Any such proposed a l t e r n a t i v e methods must be c r i t i c a l l y evaluated to assess t h e i r v i a b i l i t y as an economical, safe solution to the t a i l i n g s disposal problem. One such alternative i s the thickened discharge disposal method (Robinski, 1975, Robinski, 1978). This method involves c r e a t i n g a m i l d l y sloped c o n i c a l l y shaped t a i l i n g s d e p o s i t , thereby eliminating the need for a high and costly embankment as required for conventional t a i l i n g s disposal systems. The thick-ened discharge method, as discussed i n Chapter 2, has proven to be an economical s o l u t i o n t o the t a i l i n g s d i s p o s a l problem. However, the s t a b i l i t y of a sloped deposit during and a f t e r an earthquake i s a design consideration that must be assessed to determine i f the thickened discharge method i s a viable a l t e r -native appropriate for widespread use. A shaking table model testing program was developed to help to assess the seismic s t a b i l i t y of sloped t a i l i n g s deposits. Due to the wide degree of v a r i a b i l i t y of t a i l i n g s material pro-duced i n the mining industry, and problems associated with the uniform deposition of fine-grained materials in the laboratory, 2 a u n i f o r m f i n e - g r a i n e d sand was s e l e c t e d t o model t a i l i n g s be-h a v i o u r . The t e s t m a t e r i a l i s d e s c r i b e d i n Chapter 3, where i t i s compared t o a v a r i e t y o f t y p i c a l t a i l i n g s m a t e r i a l s . Of i n t e r e s t i n t h i s study i s the response o f a s o i l d e p o s i t t o c y c l i c l o a d i n g . Not o n l y i s the response o f the m a t e r i a l up t o the p o i n t o f l i q u e f a c t i o n o f i n t e r e s t , even more p e r t i n e n t i s the p o s t - l i q u e f a c t i o n r e s ponse o f the d e p o s i t . Fundamentals o f l i q u e f a c t i o n and p o s t - l i q u e f a c t i o n m a t e r i a l b e h a v i o u r are p r e -s e n t e d i n Ch a p t e r 4. P o s s i b l e r a m i f i c a t i o n s o f m a t e r i a l be-h a v i o r fundamentals w i t h r e g a r d t o s e i s m i c s t a b i l i t y o f a s l o p e d t a i l i n g s d e p o s i t are d i s c u s s e d i n C h a p t e r 5. The development o f model t e s t equipment and p r o c e d u r e s i n -c l u d e d a r e v i e w p f p r e v i o u s model s t u d i e s , d i s c u s s e d i n Ch a p t e r 6. The t e s t i n g program, i n c l u d i n g equipment and p r o c e d u r e s , i s d i s c u s s e d i n Ch a p t e r 7. T e s t r e s u l t s are p r e s e n t e d and d i s -cussed i n C h a p t e r 8. The r e s u l t s o b t a i n e d l e d t o the e m p i r i c a l m o d i f i c a t i o n o f a f l u i d a n a l y s i s d e s c r i b i n g t h e p a r t i c l e movements w i t h i n a v i s c o u s f l u i d body. The f l u i d a n a l y s i s i s d e s c r i b e d i n Chapter 9, and p r e d i c t e d p a r t i c l e d i s p l a c e m e n t s a r e p r e s e n t e d i n Ch a p t e r 10, a l o n g w i t h measured p a r t i c l e d i s p l a c e m e n t s f o r comparison. The consequences o f a t a i l i n g s d i s p o s a l f a c i l i t y f a i l u r e can be enormous, and s e v e r a l case h i s t o r i e s a re b r i e f l y d i s -cussed i n C h a p t e r 11. A p a r t i c u l a r l y r e l e v a n t case h i s t o r y , r e s u l t i n g i n l i m i t e d damage, i n v o l v e d t h e l i q u e f a c t i o n and 3 p o s t - l i q u e f a c t i o n f l o w o f s l o p e d t a i l i n g s m a t e r i a l , a s d i s c u s s e d i n C h a p t e r 11. C o n c l u s i o n s drawn a r e p r e s e n t e d i n C h a p t e r 12. 4 CHAPTER 2 METHODS OF DISPOSAL 2-1 Introduction T a i l i n g s are waste material r e s u l t i n g from the grinding and mineral e x t r a c t i o n processes of a mining o p e r a t i o n . A flow chart for a t y p i c a l t a i l i n g s production and disposal system i s shown in F i g . 1. The disposal of t a i l i n g s i s generally con-sidered to be a c a p i t a l expenditure re s u l t i n g in no economic gain. The interest of the mining company i s therefore to mini-mize the c a p i t a l outlay for t a i l i n g s disposal. The v i a b i l i t y of marginal mining properties can depend upon economical t a i l i n g s disposal. The economic f e a s i b i l i t y of mining low grade ore has resulted in a large increase in the amount of waste material to be disposed of (Wahler and Schlick, 1976). Compounded with the fact that many open p i t mining operations are situated in mountainous t e r r a i n with narrow v a l l e y s , higher r e t a i n i n g embankments were required using the present t a i l i n g s disposal methods. The construction of t a i l i n g s dams has t r a d i t i o n a l l y been e m p i r i c a l and the mining in d u s t r y in general did not recognize the inherent s t a b i l i t y problems in the d i s p o s a l methods in use u n t i l several catastrophic f a i l u r e s occurred. Several of these f a i l u r e s are b r i e f l y discussed in Chapter 11. Hoare (1974) estimated that 90% of Canada's large mining operations have suffered i n s t a b i l i t y of some kind, while only 26% have performed s t a b i l i t y analyses ( M i t t a l , 1974). As a Ore Body Crushing Grinding Concentrating Tailings (about 99%) Mineral Concentrate (about X %) Stored Tailings Tailings Dam FLOW CHART FOR TYPICAL DISPOSAL SYSTEM (After Jeyapalan,1980) FIGURE 1 6 r e s u l t governmental regulatory agencies have begun to take a much more active role in governing the disposal of t a i l i n g s . The monetary, environmental and s o c i a l costs of f a i l u r e s as experienced at Buffalo Creek, Aberfan and during the Chilean earthquake of 1965 were enormous and s t r i c t regulations are now being enforced throughout North America and in many parts of the world. These r e s t r i c t i o n s govern s t a b i l i t y and control of contaminants during the mine's l i f e as well as for the abandon-ment of the f a c i l i t y . Abandonment alone poses major design problems (D'Appolonia et a l . , 1972 ). 2-2 Present Disposal Methods There are several t a i l i n g s disposal methods in use. The most common procedure involves the construction of an embankment behind which the waste material i s stored. The performance of the embankment i s greatly affected by the method of construc-t i o n . There are t y p i c a l l y three methods of embankment construc-t i o n . They are the upstream, downstream and centreline con-struction methods. In a l l three the dam i s i n i t i a t e d by the construction of a pervious toe, or s t a r t e r , dam. 2-2-1 Upstream Construction Method The upstream construction method i s depicted in Fig. 2. This method was widely used p r i o r to the aforementioned catas-trophic f a i l u r e s . It was considered to be the most economical storage f a c i l i t y . For an incremental height increase required f o r t a i l i n g s d i s p o s a l a r e l a t i v e l y small dyke, which can be raised quickly using a low volume of construction material, i s Dyke raised by scooping Coarse T a i l i n g s from beach UPSTREAM CONSTRUCTION FIGURE S 8 required. The dyke is generally constructed using the coarse t a i l i n g s , which separate h y d r a u l i c a l l y from the fines near the point of discharge. The progression of the dam's centreline i s in the upstream d i r e c t i o n . The s t a b i l i t y of dams constructed by t h i s method has been found to be inadequate in many instances. As the embankment progresses upstream, subsequent dykes must be constructed over fine waste material. These fines are often underconsolidated and exhibit very low shear strength. For t h i s reason, there i s a l i m i t i n g height to which the structure can be b u i l t for s t a t i c s t a b i l i t y to be s a t i s f i e d . The fine material i s also highly susceptible to liquefac-tion owing to i t s loose, saturated state. Liquefaction can be induced by earthquake, blasting or construction vibrations. It can also occur i f the slimes are subjected to r e l a t i v e l y rapid shear s t r a i n r a t e s r e s u l t i n g i n s u f f i c i e n t cumulative pore pressure buildup. Upon l i q u e f a c t i o n , the outer s h e l l i s incap-able of supporting the l i q u e f i e d mass and f a i l u r e would re s u l t . A safe upstream dam can be constructed i f proper monitoring (Nyren et a l . , 1978), basic earth dam engineering and proper material handling i s employed ( M i t t a l , 1974). A safe upstream t a i l i n g s dam i s depicted in F i g . 3. 2-2-2 Downstream Construction Method The downstream method, F i g . 4, i s considered to result in a more stable structure than the upstream method. This gain i s balanced by the increased expenditure required to ach'ieve t h i s SAFE DAM USING UPSTREAM METHOD (After Casagrande and Maclver,1971) FIGURE 3 Cyclone Starter dam DOWNSTREAM METHOD FIGURE <4 11 end. The downstream method requires a much larger volume of construction material. This material i s generally obtained by cycloning the t a i l i n g s , thus separating the coarser and fi n e r fractions of the material, the coarse f r a c t i o n , or sands being used for construction. The cyclone operation must be c a r e f u l l y c o n t r o l l e d to ensure the proper gradation i s obtained f o r construction material. The sand y i e l d i s a c r i t i c a l factor i n the downstream method, as a low y i e l d r e s u l t s in a slow r i s i n g crest combined with a larger volume of material or be stored. If cycloning does not y i e l d s u f f i c i e n t volumes of sand, borrow material must be used. The fines are usually spigotted o f f the upstream face of the dam, while the sands are spigotted o f f the downstream face, r e s u l t i n g in the centreline progressively moving downstream. The p o s s i b i l i t y of li q u e f a c t i o n of the sands can be eliminated through compaction. Drainage f a c i l i t i e s can also be incorpor-ated in the design to l i m i t the degree of saturation, and hence, the deposit's s u s c e p t i b i l i t y to li q u e f a c t i o n . Either or both methods can be used to guard against l i q u e f a c t i o n and the needs must be determined for individual operations. In s i t u t e s t i n g at Brenda mines ( M i t t a l , 1974) indicates that compaction i s not necessary i f proper drainage i s ensured. Besides the increased cost of this method, another major disadvantage e x i s t s . The downstream face of the s t r u c t u r e changes as the crest elevation i s raised. Construction takes place over many years, and no erosion protection can be applied 1 2 u n t i l the f i n a l crest elevation has been reached. The down-stream face i s therefore subjected to surface erosion for long periods of time, and erosion channels can develop. Another c o n s t r u c t i o n method commonly described i n the l i t e r a t u r e i s the the centreline method. This method, F i g . 5, i s e s s e n t i a l l y a modified downstream construction technique. The crest of the structure r i s e s v e r t i c a l l y as i t is raised. 2-2-3 Comparison with Conventional Earth Dam A n a l y t i c a l and design procedures for a t a i l i n g s dam are s i m i l a r to those used for conventional water retention earth dams. There are, however, inherent d i f f e r e n c e s due to the construction method and nature of the material being stored that preclude the s t r u c t u r e from being considered a conventional earth dam, depicted in F i g . 6. Under s t a t i c loading conditions the slimes have a low shear strength that contributes to the s t a t i c s t a b i l i t y of the struc-ture. This material i s highly susceptible to l i q u e f a c t i o n and, for design purposes, i t i s usually considered the entire deposit i s in a l i q u e f i e d state (Klohn, 1979, Finn and Byrne, 1976). Under these conditions the s p e c i f i c weight of the f l u i d i s con-siderably greater than that of water and there i s an increased h y d r o s t a t i c design load. The f i n e s are l i k e l y to l i q u e f y q u i c k l y and r e s u l t in a r a p i d shear s t r e s s a p p l i c a t i o n that could be considered undrained (Klohn and Maartman, 1972). This could r e s u l t in increased pore pressures and corresponding decreased strengths within the embankment. Cyclone Underdrains CENTRELINE CONSTRUCTION FIGURE 5 CONVENTIONAL EARTH DAM FIGURE e 15 The t a i l i n g s material, when spigotted from the crest of the dam, separate h y d r a u l i c a l l y r e s u l t i n g i n a beach made up of coarser material near the dam. This has the e f f e c t of lowering the p h r e a t i c s u r f a c e w i t h i n the dam. The a n a l y s i s of flow through a t a i l i n g s dam i s much more complex due to the excess pore pressures in the underconsolidated slimes. M i t t a l (1974) provides a detailed seepage analysis and design c r i t e r i a for a t a i l i n g s structure. The dam i t s e l f i s usually homogeneous, with the exception of p o s s i b l y blanket and toe d r a i n s . V e r t i c a l drains are generally not i n s t a l l e d and therefore control over the phreatic surface i s not as e f f e c t i v e as in a conventional structure. Another major d i s t i n c t i o n l i e s i n the fact that construc-t i o n of a t a i l i n g s dam takes place over a much longer period of time. The construction i s performed at a rate that s u i t s the requirements of the mining operation. Also, construction i s not as well controlled and the construction material i s obtained from the t a i l i n g s . T h i s m a t e r i a l i s defined by the m i l l i process, rate of feed, grade of ore and separation technique. The c o n s t r u c t i o n m a t e r i a l can change considerably over the mine's l i f e . 2-3 Recently Proposed Method of Disposal-Thickened Discharge The thickened discharge method has been proposed as an alternative to the construction of high, and costly, embankments (Robinski, 1975, R o b i n s k i , 1978). T h i s method was becoming operational at 13 mines as of 1978, and implementation was being considered at several others. ng ue. 1 6 E s s e n t i a l l y , t h e method i n v o l v e s t h e d e w a t e r i n g , o r t h i c k -e n i n g , o f t h e t o t a l t a i l i n g s t o a p r e - d e t e r m i n e d w a t e r c o n t e n t t h a t w i l l r e s u l t i n a c o n i c a l d e p o s i t w i t h an o u t e r s l o p e o f a p p r o x i m t e l y 6%. A t y p i c a l d e p o s i t i s d e p i c t e d i n F i g . 7. I t i s a l s o p o s s i b l e t o implement t h e t h i c k e n e d d i s c h a r g e method a t a s i t e t h a t has been u s i n g c o n v e n t i o n a l d i s p o s a l t e c h n i q u e s , as shown i n F i g . 8. T h e s y s t e m c a n be a d a p t e d t o a l m o s t any t e r r a i n . 2-3-1 A d v a n t a g e s o f T h i c k e n e d D i s c h a r g e T h e u s e o f t h i c k e n e d d i s c h a r g e r e s u l t s i n c o n s i d e r a b l e c a p i t a l s a v i n g s , b o t h i n i t i a l l y and o v e r t h e mine's l i f e . A much s m a l l e r impoundment f a c i l i t y i s r e q u i r e d t o s t o r e a g i v e n volume o f waste m a t e r i a l . The e l i m i n a t i o n o f l a r g e t a i l i n g s dams e l i m i n a t e s t h e i n h e r e n t c o s t s and p r o b l e m s i n t h e d e s i g n , c o n s t r u c t i o n and m a i n t e n a n c e o f s u c h s t r u c t u r e s . The f a c i l i t y , as p r o p o s e d , e l i m i n a t e s t h e need f o r a d e c a n t s y s t e m . A r e l a t i v e l y s m a l l p o n d a t t h e t o e o f t h e t a i l i n g s d e p o s i t i s r e q u i r e d . T h i s pond i s d e s i g n e d t o r e c e i v e n a t u r a l and t a i l i n g s r u n o f f . I t has been f o u n d t h a t o n l y a v e r y s m a l l amount o f f i n e t a i l i n g s e v e r r e a c h t h e pond a r e a . The pond i s s i t u a t e d i n t h e t o p o g r a p h i c a l low by d e s i g n , a n d a l l r u n o f f r e a c h e s i t w i t h o u t c o n s t r u c t i n g s p e c i a l d r a i n a g e s y s t e m s . B e c a u s e t h e t a i l i n g i s d e p o s i t e d c o n t i n u o u s l y o v e r t h e e n -t i r e d e p o s i t , t h e s u r f a c e i s c o n s t a n t l y w e t t e d . T h i s e l i m i n a t e s w ind e r o s i o n and e n v i r o n m e n t a l p r o b l e m s a s s o c i a t e d w i t h d u s t i n g . Discharge Line Conical deposit of total tailings Tailings pond Reclaim pond THICKENED DISCHARGE METHOD (Robinski,1978) FIGURE 7 1 8 Existing Pond -90 foot dam Thickened Discharge System -additional 20 year capacity -6% tailings slopes -Fixed discharge points Conventional Expansion —additional 20 year capacity -requires 225 foot dam EXPANSION OF STORAGE FACILITY (Robinski,1978) FIGURE S 19 Although the thickened material i s more d i f f i c u l t to pump, cos t s f o r energy used f o r pumping i s reduced from that of a conventional system because of the reduced volume to be pumped. It has also been found that smaller pipelines are required for both the t a i l i n g s and the return l i n e for recycled water. A major advantage of t h i s system i s the inherent proper-t i e s of the deposit that f a c i l i t a t e abandonment. Appropriate chemical neutralizers and f e r t i l i z e r s can be added to the waste m a t e r i a l to provide a uniformly treated surface l a y e r . The surface slope also provides good surface drainage. Robinski also states that the s t a b i l i t y , both s t a t i c and under earthquake conditions, i s superior to that of conventional systems. This statement i s questionable and i s discussed i n the next section. 2-3-2 Limitations of Thickened Discharge The deposit formed by the thickened discharge method i s considered to be s t a t i c a l l y safe. One must be s k e p t i c a l , however, of the reasoning behind assessing the deposits as safe under earthquake conditions. Robinski (1978) states that the slope i s "determined by i t s angle of internal f r i c t i o n ( v i s -c o s i t y ) . " Consolidation w i l l then res u l t in a two fold increase in v i s c o s i t y , which provides a large reserve angle of f r i c t i o n to oppose movement during earthquakes. Although an increase in the angle of internal f r i c t i o n w i l l r e s u l t from consolidation, the increase may not be s u f f i c i e n t to 20 r e s i s t flow upon re l i q u e f a c t ion. The above reasoning does not account for the fact that an earthquake w i l l l i k e l y r e s u l t in a f l u i d mass of much greater depth than present during deposition. The shear stresses at the base of the f l u i d increase with f l u i d depth, and i f the increased shear stresses exceed the increased angle of internal f r i c t i o n angle, flow w i l l r e s u l t . T a i l i n g s are generally a fine, cohesionless material i n a loose, saturated state and are considered to be highly suscep-t i b l e to liquefaction, as discussed in Section 3-2-4. Robinski (1978) states that the low p r o f i l e of the h i l l ensures s t a b i l i t y and reduces the dangers of l i q u e f a c t i o n . Although the s t a t i c shear stresses w i l l l i k e l y increase the deposit's resistance to l i q u e f a c t i o n (Vaid and Finn, 1979), Chern, 1981), considering the nature of the material i t i s l i k e l y that liquefaction would occur during a s i g n i f i c a n t seismic event. It would be prudent to assume that the d e p o s i t could l i q u e f y to a c o n s i d e r a b l e depth. Due to the low p r o f i l e of the h i l l , shear stress rever-sal would be l i k e l y , and large deformations could re s u l t i f the gradient were large enough to cause shear stresses greater than the shear strength of the l i q u e f i e d material. A detailed discussion of the conditions for flow f a i l u r e i s presented in Chapter 5. However at t h i s point, i t should be mentioned that the geometry of the deposit as determined by the depositional behavior of the material i s not necessarily a stable geometry for a considerable depth of l i q u e f i e d material that could r e s u l t from an earthquake. The shear stresses at the 21 b a s e o f t h e l i q u e f i e d m a t e r i a l w o u l d l i k e l y be much g r e a t e r t h a n t h o s e p r e s e n t d u r i n g d e p o s i t i o n . I f t h e y a r e g r e a t e r t h a n t h e s h e a r s t r e n g t h o f t h e l i q u e f i e d mass, f l o w w o u l d r e s u l t . I n summary, e x i s t i n g methods o f t a i l i n g s d i s p o s a l a r e s u b -j e c t t o l i m i t a t i o n s . The d o w n s t r e a m method o f c o n s t r u c t i o n i s v e r y e x p e n s i v e , h o w e v e r t h e r e s u l t a n t s t a b i l i t y o f t h e d e p o s i t i s m a x i m i z e d . The s t a b i l i t y o f t h e u p s t r e a m method h a s been q u e s t i o n e d a s embankment f a i l u r e s , and r e s u l t i n g f l o w f a i l u r e s , h a v e r e s u l t e d i n c a t a s t r o p h i c damage. The t h i c k e n e d d i s c h a r g e m e t h o d , a l t h o u g h v e r y c o s t e f f e c t i v e , may a l s o be o f q u e s t i o n -a b l e s e i s m i c s t a b i l i t y , a n d t h e p o s s i b i 1 i t y o f s i g n i f i c a n t d amage d o e s e x i s t . T h e m o d e l t e s t i n g p r o g r a m d i s c u s s e d i n C h a p t e r 7 w a s d e s i g n e d t o a s s e s s t h e r e s p o n s e o f s t a t i c a l l y s t a b l e s l o p e d m a t e r i a l u n d e r c y c l i c l o a d i n g c o n d i t i o n s . T e s t r e s u l t s , p r e s e n t e d i n C h a p e r 8, i n d i c a t e t h a t f i n a l s l o p e a n g l e s a r e one p e r c e n t o r l e s s , and t h e r e f o r e c o n c e r n w i t h t h e s e i s m i c s t a b i l i t y o f t h i c k e n e d d i s c h a r g e d e p o s i t s may be w a r r a n t e d . 22 CHAPTER 3 PRELIMINARY TESTING P r e l i m i n a r y t e s t i n g was performed to d e f i n e the model t e s t m a t e r i a l , and p r o v i d e a b a s i s f o r comparison with t y p i c a l t a i l -ings m a t e r i a l . 3-1 T e s t M a t e r i a l The m a t e r i a l used i n t h i s study was a uniform f i n e - g r a i n e d Ottawa s i l i c a sand. T h i s type of sand i s a subrounded, quartz sand with a s p e c i f i c g r a v i t y , G s = 2.67. S e v e r a l standard t e s t s were performed a c c o r d i n g to pro-cedures o u t l i n e d by Lambe (1951). A standard s i e v e a n a l y s i s was performed and the r e s u l t a n t g r a i n s i z e d i s t r i b u t i o n curve i s shown i n F i g . 9. The s o i l has a c o e f f i c i e n t of u n i f o r m i t y , Cu = D60/D10 = 1 - 8 -V a r i a b l e head p e r m e a b i l i t y t e s t s were performed at a va-r i e t y of v o i d r a t i o s to produce the e vs. l o g ( k ) p l o t shown i n F i g . 10. For the h y d r a u l i c a l l y d e p o s i t e d m a t e r i a l used i n the model t e s t , v o i d r a t i o s ranging from .75 to .78 were obtained, corresponding t o a range i n p e r m e a b i l i t y from .0105 to .0145 cm/s. S u c c e s s i v e t e s t s were performed at each v o i d r a t i o to ensure data r e l i a b i l i t y . The p e r m e a b i l i t y compares w e l l with t h a t p r e d i c t e d by Hazen's e m p i r i c a l formula, K = 100 D-|g2. T e s t s were run f o r the d e t e r m i n a t i o n of minimum and maxi-mum v o i d r a t i o s . The maximum v o i d r a t i o was obtained using the standard dry method and was compared to the r e l a t i v e l y quick 23 GRAIN SIZE DISTRIBUTION FIGURE S .002 C o e f f i c i e n t of H y d r a u l i c C o n d u c t i v i t y K(cm/s) . 0 0 5 , .01 .02 V o i d R a t i o - Q Q vs. L o g ( K ) .8 FIGURE 10 and simple methods proposed by Burmister (1970) and Yemington (1970). Agreeable re s u l t s were obtained, y i e l d i n g e m a x = .86. A minimum void r a t i o , e m i n = .56, was obtained. The r e l a t i v e density, Dr, for t h i s sand i s therefore found by: Dr = (.86-e)/.3. C y c l i c t r i a x i a l tests were performed using the equipment and procedures described in d e t a i l by Chern (1981). The system i s shown schematically in F i g . 11. A l i q u e f a c t i o n resistance curve was established for i s o t r o p i c consolidation conditions and i s shown in F i g . 12. Relative densities for each test are shown on- the p l o t . Load, pore pressure and s t r a i n s were measured simultaneously on a s t r i p chart recorder. A t y p i c a l record i s shown in F i g . 13. The range of void r a t i o s used was represen-tative of model test conditions. In a l l tests, care was taken to ensure sample saturation (B = 1.00) and tests were performed undrained at 1 Hz. Note that very large strains resulted a f t e r i n i t i a l l i q u e f a c t i o n . Liquefied samples exhibited unlimited s t r a i n potential associated with a contractive flow structure. This is reasonable for the low r e l a t i v e densities used (Dr = 30%). A series of standard t r i a x i a l tests were performed to i n -vestigate the shear strength c h a r a c t e r i s t i c s of the material. Both drained and undrained tests were performed on loose sam-ples. A f t e r the application of Bishop's energy corrections to the drained test results, to correct to constant volume condi-tions, the r e s u l t s y i e l d 0' = 30.5°. Function Generator V Electro-pneumatic Transducer •Back Pressure j — ^Regulator j — p LVDT To Recorders •Double-acting Air Piston Load Cell Eyed Connecting Ring -® Porewater Pressure Transducer To Recorder Cell Pressure Transducer CYCLIC TRIAXIAL TEST APPARATUS (After Chern,1981 j FIGURE 11 .4 .3 b" .2 6 0 34.8 % 3 1 . 5 % 1 ^ 2 8 . 5 % 26 . 5 % • 1 2 5 10 20 50 100 Cycles to Liquefaction LIQUEFACTION RESISTANCE CURVE F I G U R E 1 2 1^ 29 In both c y c l i c and s t a t i c t r i a x i a l testings, a l l samples were consolidated to an i s o t r o p i c e f f e c t i v e stress of .5 kg/cm2. Ideally, testing should have been performed at lower stresses, corresponding to those found i n the model test, however experi-ence has shown that lower c o n f i n i n g pressures may not y i e l d r e l i a b l e r e s u l t s (Vaid, 1981). The results of the tests at the confining pressure used are f e l t to be reasonably representative of the material response under model test conditions. The c o e f f i c i e n t of consolidation was estimated from c y c l i c t r i a x i a l test i n i t i a l consolidation data. The values obtained showed considerable scatter, but were reasonably close to those estimated using r e s u l t s from Yoshimi (1975), who studied the compressibility of a very similar sand at very low confining p r e s s u r e s . The average C v value, C v = 3 f t ^ / s , a l s o agreed f a i r l y well with t y p i c a l values from Lambe and Whitman (1969) and Byrne ( 1980), using test sand permeability, and i s f e l t to be appropriate for test conditions. 3-2 Comparison of Test Sand with Typical T a i l i n g s Material The material used in the testing program was chosen by i t s grain size to simulate t a i l i n g s material as closely as possible. The main constraint on the material beyond resembling t a i l i n g s was that i t be p r a c t i c a l from a t e s t i n g p o i n t of view. The sand was deposited h y d r a u l i c a l l y through water, and was chosen to do so in a reasonable length of time, and be reproducible over a number of t e s t s . A uniform f i n e sand was chosen to s a t i s f y t esting requirements. I t is recognized that t a i l i n g s material i s generally a more well graded material with a high 30 degree of s i l t s i z e p a r t i c l e s but the d e p o s i t i o n of such a material in the laboratory was deemed impractical. The c h a r a c t e r i s t i c s of t a i l i n g s materials are highly v a r i -able, depending on the m i l l i n g process, host rock and type of ore being mined. Due to the g r a v i t a t i o n a l separation inherent in many disposal schemes, the material c h a r a c t e r i s t i c s can be highly variable within a given deposit. The thickened discharge method, however, r e s u l t s i n a r e l a t i v e l y uniform deposit as there i s l i t t l e separation during deposition (Robinski, 1978). The model deposit was also uniform. The following information regarding various t a i l i n g s is for the t o t a l t a i l i n g s produced by the m i l l i n g operation. Corresponding properties of the test sand are included for comparison. 3-2-1 Grain Size Distributon Typical grain size d i s t r i b u t i o n s are shown in Figures 14 and 15. Most mining operations r e s u l t in a s i l t to fine sand t a i l i n g . For coal t a i l i n g s , coarse p a r t i c l e s in the coarse sand to gravel sizes are produced by crushing operation, while clay size to fine sand r e s u l t from the f l o t a t i o n process. I t i s often considered that the s u s c e p t i b i l i t y to lique-faction i s dependent on the grain size d i s t r i b u t i o n . Typical bounds are shown in F i g . 15(d) (Ishihara, 1980). In the case of some t a i l i n g s , many cohesionless clay size p a r t i c l e s are pre-sent. I f one were to use F i g . 15(d) as a s t r i c t guide, error could re s u l t in the assessment of the deposit. The cohesionless f i n e s are in a loose, saturated s t a t e h i g h l y s u s c e p t i b l e to 100 Percent Finer By Weight 8 0 60 40 J 20 J 100 Percent Finer By Weight 80 -I 60 -J 4 OH 2 0 H 1.0 Test sand Mittal Girucky 1.0 . 5 1—• 1 T-Particle S i ze"^mm) 0 5 a) Copper Tailings Test sand Nyren&Mittal i Tj r Particle Size 1 ( m m ) . 0 5 —i— .02 •>) Oil Sand Tailings .02 01 .01 Percent Finer By Weight 100 8 0 - J 60 H 4 0 2 0 1.0 100 Percent Finer By Weight 80 60 A 4 0 20 H 1.0 * • — Murthy ' " " Jeyapalan \ Guerra Test sand Vs 1 -T— 1 1-Particle Size (mm).05 c) Iron Tailings Jeyapalan Sandic Sandic Test sand — i • r Particle Size ( m m ) - 0 5 d) Lead Tailings 02 .01 .02 .01 TAILINGS GRAIN SIZE DISTRIBUTIONS FIGURE 14 100 Percent Finer By Weight 8 0 40 -I 20 4 •» Uranium Tailings P a r ^ l e Size (mm) . 0 ! 100 Percent Finer By Weight 80 6 0 H 40 20-1 Test Sand 1 0 -5 .2 b> Coal Tailings Particle Size (mm) 100 Percent Finer By Weight 1.0 5 2 1 O Molybdenum'Tailings"05 Particle Size (mm) 100 Percent Finer By Weight 80 H Boundaries for most liquefiable soil Boundaries for potentially liquefiable soil -» r — 1 r 2 ' Particle Size (mm) 0 1 d) Liquefaction Susceptible Soils (Ishihara) TAILINGS GRAIN SIZE DISTRIBUTIONS FIGURE 15 33 l i q u e f a c t i o n . I f t h e f i n e s a r e m o d e r a t e l y p l a s t i c , t h e y t e n d t o be l e s s s u s c e p t i b l e t o l i q u e f a c t i o n . The s a n d u s e d i n t h e m o d e l s t u d y h a s a D50 t h a t i s f a i r l y r e p r e s e n t a t i v e f o r most o f t h e t a i l i n g s c o n s i d e r e d . I t i s c o n -s i d e r a b l y more u n i f o r m t h a n most t y p i c a l t a i l i n g s . 3-2-2 P e r m e a b i l i t y and C o m p r e s s i b i l i t y The p e r m e a b i l i t y o f s e v e r a l t a i l i n g s i s shown i n F i g . 16. N o t e t h a t t h e y f a l l r e a s o n a b l y c l o s e t o t h e l i n e d e t e r m i n e d by H a z e n ' s e m p i r i c a l f o r m u l a , K = 100 D]q2, f o r g r a n u l a r s o i l s . The t e s t s a n d i s a l s o shown p l o t t e d and i s c o m p a r a b l e t o some c o p p e r t a i l i n g s , b u t f o r t h e most p a r t h a s a p e r m e a b i l i t y much h i g h e r t h a n most t o t a l t a i l i n g s . T h i s h i g h p e r m e a b i l i t y , c o u p l e d w i t h a r e l a t i v e l y l o w com-p r e s s i b i l i t y , r e s u l t s i n a C v v a l u e c o n s i d e r a b l y h i g h e r t h a n most t a i l i n g s m a t e r i a l . As m e n t i o n e d i n S e c t i o n 3-1, C v was e s t i m a t e d t o be 3 f t ^ / s f o r t h e t e s t s a n d u n d e r t e s t c o n d i -t i o n s . T y p i c a l v a l u e s o f C v f o r t a i l i n g s a r e i n t h e o r d e r o f .0001 f t 2 / s . The r e l a t i v e l y h i g h C v v a l u e o f t h e t e s t s a n d i s i n d i c a t i v e o f a r a p i d l y d r a i n i n g m a t e r i a l . T h i s i m p l i e s t h a t m ost c o h e s i o n l e s s t a i l i n g s w o u l d be more e a s i l y l i q u e f i e d d u r i n g c y c l i c l o a d i n g i n w h i c h d r a i n a g e i s a l l o w e d a t i t s b o u n d a r i e s , and w o u l d r e m a i n l i q u e f i e d f o r a l o n g e r p e r i o d o f t i m e . The Cv v a l u e h a s b e e n f o u n d t o be t h e m o s t i m p o r t a n t p a r a m e t e r i n d e -l a y i n g p o r e p r e s s u r e r e s p o n s e l e a d i n g t o l i q u e f a c t i o n ( S e e d e t a l . , 1 9 7 6 ) . © Lead (Williams et al,1978) ' *\ 't 1 1 1 1— 1 0 6 10-5 1 0 - 4 1 0 . 3 1 0 2 K (cm/s) VARIATION OF PERMEABILITY WITH D 1 0 FIGURE 16 35 3-2-3 Relative Density A r e l a t i v e density of approximately 30% was obtained for the model tests. A r e l a t i v e density of 25-45% i s representative of most t a i l i n g s deposits (Finn & Byrne, 1976). Void r a t i o , rather than r e l a t i v e density., may serve as a more useful parame-ter to describe the denseness or looseness of a t a i l i n g s deposit because of the wide range in grain sizes, from clay to coarse sand in a t y p i c a l t a i l i n g s deposit (Ishihara, 1980). Typical values for a deposit formed by the thickened discharge method (Robinski, 1978) are compared to that of the test sand i n Table 1.- Note the s i m i l a r i t y in s o l i d s f r a c t i o n , Cs, between the test sand and a t y p i c a l thickened discharge deposit. This parameter exhibits considerable control over the post-liquefaction be-havior of a material, as discussed in Section 4.2. The test sand should t h e r e f o r e be reasonably r e p r e s e n t a t i v e of post-l i q u e f a c t i o n behaviour. TABLE 1 Parameter Test Sand Deposit Thickened Discharge Deposit S p e c i f i c Gravity, Gs 2.67 2.9 Void Ratio, e .77 .95 Percent by Weight 77 75 Solids Fraction, Cs . 56 .54 Water Content, w .29 .32 36 3-2-4 Liquefaction Resistance The interest in the li q u e f a c t i o n resistance of t a i l i n g s m a t e r i a l s has been l a r g e l y confined to the m a t e r i a l used i n embankment construction. This i s generally the coarse f r a c t i o n of the t a i l i n g s and behaves much l i k e a natural sand of sim i l a r gradation (Byrne, 1980). Of p a r t i c u l a r interest in the present study i s the c y c l i c response of t a i l i n g s material p r i o r to separation of the coarse and fine components. Unfortunately, no published data e x i s t s . The response of f i n e grained t a i l i n g s i s assumed to be more representative of the t o t a l t a i l i n g s than the response of the coarse grained discard. It is generally assumed that the fine material stored be-hind an embankment i s very susceptible to liquefaction (Byrne, 1980, Klohn, 1979, Ishihara, 1980, Wahler and Schlick, 1976, Guerra, 1972, Taylor and Morrell, 1979). I t s loose, saturated, cohesionless, underconsolidated state i s the basis of th i s con-clusion, however, very l i t t l e published data is available to confirm t h i s notion. Taylor and Morrell (1979) performed c y c l i c t r i a x i a l tests on fine coal-mine discard. These t a i l i n g s had sim i l a r grain-size c h a r a c t e r i s t i c s to most t y p i c a l t o t a l t a i l i n g s from other mining operations. Some of the t a i l i n g s exhibited p l a s t i c i t y , and their resistance to li q u e f a c t i o n was found to be dependent on the p l a s t i c i t y index (Fig. 17). The pore pressure buildup in p l a s t i c specimens was much more gradual than in non-plastic EFFECT OF PLASTICITY ON LIQUEFACTION RESISTANCE (TaYlor&ft1orrel,1979) FIGURE 17 38 samples. Tests were carried out both on remolded specimens and reasonably undisturbed ones. Those non-plastic specimens pre-pared in the laboratory were found to be more susceptible to l i q u e f a c t i o n than undisturbed samples. Most of the non-plastic specimens l i q u e f i e d in less than 20 cycles at a stress r a t i o , "C/0"o' = .15, which is s i m i l a r to the resistance of the test sand. T a y l o r , Kennedy and MacMillan (1979) performed a study on the s u s c e p t i b i l i t y of coarse grained c o l l i e r y d i s c a r d to l i q u e f a c t i o n . The material tested was considerably coarser than t y p i c a l t a i l i n g s discussed in Section 3-2-1. The material was generally found to be more r e s i s t a n t to l i q u e f a c t i o n , t h i s being attributed, by the authors, to i t s high shear strength and rapid equalization of pore pressure throughout the sample. Although these f i n d i n g s agree with the concepts of l i q u e f a c t i o n and appear c o n c l u s i v e , the d i s a s t r o u s t i p f a i l u r e at Aberfan i s b e l i e v e d to have r e s u l t e d from the l i q u e f a c t i o n of s i m i l a r coarse discard. I s h i h a r a (1980) performed a s e r i e s of c y c l i c t r i a x i a l tests on t a i l i n g s materials. The slimes tested were consider-ably f i n e r than t y p i c a l t o t a l t a i l i n g s . The l i q u e f a c t i o n resistance curves were s i m i l a r to that obtained for the test sand, Fig. 18. The effect of void r a t i o on the l i q u e f a c t i o n resistance i s shown in F i g . 19. The test sand i s shown plotted, and compares f a i r l y well to a variety of t a i l i n g s . From th i s p l o t , one can see that deposits at void r a t i o s similar to that obtained in the thickened discharge method of disposal (e = .95) Cyclic Stress Ratio Cyclic Stress Ratio r-D c cr m Tl m AC o o H O" O z 6 D. m Q. •z. CO < c CO CD 3 TA CO ber 3 o o ro m o O yc Tl —1 tn > •z. o CO o o cr D O •U C L < CD CO Cyclic Stress Ratio causing 5% D.A. Strain in 20 Cycles L> N> u in. I 1 1 ; t. o Sri o CO H rn > CO H O H -< H > LT) CO O —I X < < o o > o < o 33 X O • < £ p — I O O — 171 o o cr O a. < T T re O o cr o_ C L C L •< re D c a. 40 w o u l d be a t l e a s t as s u s c e p t i b l e t o l i q u e f a c t i o n as t h e t e s t s a n d . N e i t h e r o f t h e above s t u d i e s a c c o u n t e d f o r t h e p o s s i b l e i n c r e a s e i n r e s i s t a n c e t o l i q u e f a c t i o n d u e t o s t a t i c s h e a r . N o r d i d t h e y a c c o u n t f o r any d e g r e e o f u n d e r c o n s o l i d a t i o n t h a t e x i s t s i n t h e d e p o s i t due t o r e l a t i v e l y r a p i d r a t e s o f d e p o s i -t i o n . I t i s b e l i e v e d t h a t t h i s f a c t o r c o u l d s u b s t a n t i a l l y r e d u c e t h e l i q u e f a c t i o n r e s i s t a n c e o f a n o n - p l a s t i c t a i l i n g s d e p o s i t . I t i s f e l t t h a t t h e t e s t s a n d i s p r a c t i c a l f r o m a m o d e l t e s t i n g s t a n d p o i n t , and a t t h e same t i m e r e a s o n a b l y r e p r e s e n t s t h e c y c l i c r e s p o n s e , a s w e l l a s t h e p o s t - l i q u e f a c t i o n b e h a v i o r , o f t a i l i n g s m a t e r i a l . 41 CHAPTER 4 MATERIAL BEHAVIOUR 4-1 L i q u e f a c t i o n E x t e n s i v e r e s e a r c h has been performed i n the l a s t 15 y e a r s t o u n d e r s t a n d the phenomenon of l i q u e f a c t i o n . The s t a t e - o f - t h e -a r t has p r o g r e s s e d c o n s i d e r a b l y i n t h a t t i m e , and t h e mechanisms l e a d i n g t o l i q u e f a c t i o n are w e l l u n d e r s t o o d (Seed, 1979). How-e v e r , v a r y i n g t e r m i n o l o g y e x i s t s i n the l i t e r a t u r e and ambi-g u i t i e s may a r i s e i f the t e r m i n o l o g y used i n i n c o n s i s t e n t . The d e f i n i t i o n of l i q u e f a c t i o n adhered t o i n t h i s t h e s i s i s c o n s i s t e n t w i t h t h a t put f o r w a r d by Youd (1973). " L i q u e f a c -t i o n i s the t r a n s f o r m a t i o n o f a g r a n u l a r m a t e r i a l from a s o l i d s t a t e i n t o a l i q u e f i e d s t a t e as a consequence o f i n c r e a s e d pore p r e s s u r e s . " T h i s i s e q u i v a l e n t t o i n i t i a l l i q u e f a c t i o n as d e f i n e d by Lee and Seed (1967). Upon r e a c h i n g l i q u e f a c t i o n , a r e a s o n a b l y dense sand ex-h i b i t s d i l a t i v e b e h a v i o u r , and f u r t h e r s t r a i n i n g r e s u l t s i n r e d u c e d pore p r e s s u r e s and i n c r e a s e d s t r e n g t h and r e s i s t a n c e t o shear d e f o r m a t i o n . T h i s s t r a i n h a r d e n i n g type of m a t e r i a l e x h i b i t s l i m i t e d f l o w p o t e n t i a l upon l i q u e f a c t i o n . T y p i c a l b e h a v i o u r under monotonic l o a d i n g i s shown i n F i g . 20 ( C a s t r o , 1969. T y p i c a l l y , a s u f f i c i e n t l y l o o s e m a t e r i a l , c o n t r a c t i v e by n a t u r e , w i l l e x h i b i t u n l i m i t e d f l o w d e f o r m a t i o n . I n the l i q u e -f i e d c o n d i t i o n , d i l a t i v e t e n d e n c i e s t o reduce pore p r e s s u r e s are i n s u f f i c i e n t and d e f o r m a t i o n c o n t i n u e s u n t i l the a p p l i e d shear Axial Strain (%) RESPONSE OF MEDIUM DENSE SAND (After Castro) FIGURE SO 4 3 stresses are reduced to a l e v e l compatible with the low shear strength of the l i q u e f i e d material (Youd, 1973). Typical be-haviour under monotonic loading i s shown in F i g . 21 (Castro, 1969). The response of garnet t a i l i n g s under monotonic loading i s shown in F i g . 22 for comparison (Highter and Tobin, 1980). The response i s very similar to that of sand. The response of saturated c o h e s i o n l e s s m a t e r i a l to un-drained c y c l i c loading was also studied by Castro (1969). A recent study (Chern, 1981) investigates the e f f e c t i v e stress c o n d i t i o n s w i t h i n each c y c l e , and l i q u e f a c t i o n was found to coincide with a c r i t i c a l e f f e c t i v e stress r a t i o , ^*1 ,/^3 , f inde-pendent of c y c l i c stress r a t i o and anisotropic consolidation r a t i o . Typical deformation and stress path c h a r a c t e r i s t i c s for loose, i s o t r o p i c a l l y consolidated material are shown i n F i g . 23 (Chern, 1981). Note that large flow deformations occurred in loose samples. Large. (> 20%) deformations occurred i n c y c l i c t r i a x i a l tests performed in the present study, and any indica-tions of s i g n i f i c a n t d i l a t i v e behaviour at large strains were not evident. Unlimited' flow at low r e l a t i v e densities i s con-s i s t e n t with other work (Castro, 1969, De Alba et a l . , 1976). Although there are many variables affecting the poten t i a l f o r l i q u e f a c t i o n of a given deposit, the most important are generally regarded to be the r e l a t i v e density, or void r a t i o , angularity of the particles,, the compressibility and drainage c h a r a c t e r i s t i c s , the i n i t i a l stress state of the material and the nature and duration of the s t r e s s e s to which i t i s sub-jected. RESPONSE OF LOOSE SAND (After Castro) FIGURE S1 RESPONSE OF GARNET TAILINGS (AFTER HIGHTER ANDTOBIN, 1980) FIGURE 2 2 RESPONSE OF LOOSE SAND TO CYCLIC LOADING (AFTER CHERN. 1981) FIGURE S3A 08 1.0 1.2 . 1.4 (q'+03')/2 (kg/cm2) ^Ts2atreSS P 3 t h ° f 0 7 0 1 1 0 r - ° a d i l i g T G S t °" Is°tropically Consolidated RESPONSE OF LOOSE SAND TO CYCLIC LOADING (After Chern,1981) FIGURE S3 B 48 Of p a r t i c u l a r interest in t h i s study i s the i n i t i a l stress state of the material. The ef f e c t of s t a t i c shear on liquefac-tion resistance has been found to be dependent on the r e l a t i v e density, l e v e l of i n i t i a l shear stresses and the degree of shear stress reversal (Vaid & Finn, 1979, Chern, 1981). Loose samples were found to be both more and less resistant to liquefaction depending on the magnitude of the s t a t i c shear stress r a t i o . Chern (1981) found that high s t a t i c shear stress levels resulted in a less resistant material, and postulated that the reason l i e s in the fact that the i n i t i a l state of stress l i e s closer to the c r i t i c a l e f f e c t i v e stress r a t i o l i n e at higher i n i t i a l s t a t i c stress l e v e l s . Vaid and Finn (1979) performed a similar study on a uniform Ottawa sand of d i f f e r e n t gradation than the present test sand. Results from t h e i r study are shown in F i g . 24. Correction factors r e f l e c t i n g the e f f e c t of s t a t i c shear on l i q u e f a c t i o n resistance were obtained from F i g . 24 for slopes used i n the model study and are t a b u l a t e d i n Table I I . The l i q u e f a c t i o n resistance curves adjusted for s t a t i c shear are shown in F i g . 25. The predicted increase in resistance with i n c r e a s i n g slope i s c o n s i s t e n t with r e s u l t s obtained i n the model te s t , as discussed in Chapter 8, however these results must be considered q u a l i t a t i v e as the r e l a t i v e densities used by Vaid and Finn (50%) were considerably higher than those used in the model test. The r e s u l t s of Vaid and Finn were used for lack of more appropriate available data. The corrected curves are used,in an analysis in Appendix 1. 2 Dr=50% avo'=2Kg/cm2 N=10 Dr=50% a v o 1=2Kg/cm 2 ^ N=30 1 1 t s / a v o • CYCLIC SHEAR STRESS REQUIRED TO DEVELOP 10% SHEAR STRAIN a) 10 Cycles (Loose Ottawa Sand) b) 30 Cycles (Loose Ottawa Sand) FIGURE 2 4 Slope Angle F (10 cycles) F (30 cycles) 2° .07 1.1 1.1 4° .14 1.4 1.4 60 .21 1.-7 2.0 . 8° .28 2.0 j 2 . 3 CORRECTION FACTORS FOR INITIAL SLOPE T A B L E 22 .4 u n O <N D 0 •H 4 J «f >-( W V ) w u •H u u 0 Adjusted f o r simple shear conditions 5 10 20 C y c l e s t o l i q u e f a c t i o n i LIQUEFACTION RESISTANCE CURVES CORRECTED FOR STATIC SHEAR FBGURE S S 51 I n i t i a l s t a t i c shear r e s u l t s i n a t e r m i n a l v a l u e of r e s i -d u a l p ore p r e s s u r e l e s s than the i n i t i a l e f f e c t i v e c o n f i n i n g p r e s s u r e . The maximum r e s i d u a l pore p r e s s u r e i s independent o f the r e l a t i v e d e n s i t y and c y c l i c s t r e s s r a t i o , and can be shown t h e o r e t i c a l l y t o v a r y l i n e a r l y w i t h s t a t i c shear s t r e s s l e v e l , as shown i n F i g . 26 (Chern, 1981). Chern's d a t a i s shown t o compare f a v o r a b l y w i t h t h a t t h e o r e t i c a l l y p r e d i c t e d . The a f f e c t o f a r e s i d u a l pore p r e s s u r e r a t i o l e s s than u n i t y on the p o s t -l i q u e f a c t i o n f l o w c h a r a c t e r i s t i c s has not been q u a n t i f i e d i n t h e p r e s e n t s t u d y , o t h e r than t o s t a t e t h a t the m a t e r i a l w i l l e x h i b i t e n c h a n c e d v i s c o s i t y and y i e l d s h e a r s t r e n g t h i n t h e l i q u e f i e d s t a t e o v er t h a t p r e s e n t f o r r e s i d u a l pore p r e s s u r e r a t i o s o f u n i t y . The m a t e r i a l used i n t h i s study was a u n i f o r m , f i n e sand. T a i l i n g s m a t e r i a l i s g e n e r a l l y more w e l l - g r a d e d w i t h the p r e -sence of c o h e s i o n l e s s f i n e s . Y o s h i m i (1977) r e p o r t s c o n f l i c t i n g r e s u l t s r e g a r d i n g the a f f e c t o f t h e s e f i n e s on the l i q u e f a c t i o n r e s i s t a n c e o f the m a t e r i a l . F i e l d s t u d i e s i n d i c a t e t h a t the r e s i s t a n c e i s d e c r e a s e d , w h i l e l a b o r a t o r y i n v e s t i g a t i o n s i n d i -c a t e t h e o p p o s i t e . Y o s h i m i f e e l s t h a t t h e d i s c r e p e n c y i s a f u n c t i o n o f the d e p o s i t i o n a l environment. I t i s a l s o l i k e l y t h a t the e f f e c t o f f i n e s i n the f i e l d i s t o s uppress d r a i n a g e s i g n i f i c a n t l y . I n the l a b o r a t o r y , t r i a x i a l t e s t i n g was con-d u c t e d u n d r a i n e d and t h e s u p p r e s s i o n o f d r a i n a g e i s n o t a f a c t o r . Because the f i n e m a t e r i a l i s c o h e s i o n l e s s i t i s con-s i d e r e d t h a t i t w i l l have a d e t r i m e n t a l e f f e c t on the l i q u e -f a c t i o n r e s i s t a n c e of a d e p o s i t where d r a i n a g e i s a l l o w e d . 52 o -P < .8 .6 .4 P r e d i c t e d value ($=30°) 0.1 0.2 T s/a3c' _J l 0.3 1.0 1.2 1.4 1.6 TERMINAL RESIDUAL PORE WATER PRESSURE (After Chern,1881) FIGURE 26 53 The presence of p l a s t i c fines a f f e c t s the liquefaction re-sistance, and the nature of the affect i s dependent upon the p l a s t i c i t y index. I f the p l a s t i c i t y index i s high the re-s i s t a n c e to l i q u e f a c t i o n increases s i g n i f i c a n t l y . The pore pressure increase i s more gradual for the highly p l a s t i c slimes. This i s i l l u s t r a t e d by Ishihara (1980), who tested a variety of t a i l i n g s m a t e r i a l . Note that at high void r a t i o s , or low r e l a t i v e densities, sand and low p l a s t i c i t y slimes have similar c y c l i c strengths, while the highly p l a s t i c slimes are s i g n i f i -c a n t l y stronger. The p o s t - 1 i q u e f a c t ion behaviour of these m a t e r i a l s i s assumed to be the same. For a l i q u e f i e d mass, "there i s not tangible evidence suggesting that the non-plastic materials should move faster as grain flows than the p l a s t i c fine discards" (Taylor and M o r r e l l , 1979). 4-2 Post Liquefaction Due to the loose, saturated s t a t e of most t a i l i n g s de-p o s i t s , they are susceptible to l i q u e f a c t i o n . Many deposits have l i q u e f i e d , several of which are discussed in Chapter 11. In a l l cases where f a i l u r e s occurred, the t a i l i n g s were ob-served to behave as a viscious f l u i d . It is therefore consi-dered app r o p r i a t e to d i s c u s s the p o s t - 1 i q u e f a c t ion behavior in terms of f l u i d behavior. To f a c i l i t a t e the discussion, i t w i l l be made within the framework of a rheological model gen-e r a l l y accepted as describing the behaviour of l i q u e f i e d t a i l -ings. 54 L i q u e f i e d t a i l i n g s b e h a v e i n a n o n - N e w t o n i a n f a s h i o n . T h e r e a r e a l a r g e number o f m o d e l s a v a i l a b l e t o d e s c r i b e n o n -N e w t o n i a n f l u i d b e h a v i o u r . T h e m o s t common m o d e l s u s e d t o d e s c r i b e l i q u e f i e d t a i l i n g s a r e t h e B i n g h a m p l a s t i c m o d e l and t h e power l a w f l u i d m o d e l (Wasp e t a l . , 1 9 7 7 ) . Of t h e s e , t h e B i n g h a m model i s t h e s i m p l e s t and h a s been u s e d i n many p r e v i o u s s t u d i e s . I t i s b e l i e v e d t h a t t h e B i n g h a m m o d e l a c c u r a t e l y d e s -c r i b e s t h e f l o w c h a r a c t e r i s t i c s o f h o m o g e n o u s t a i l i n g s f l o w ( J e y a p a l a n , 1980, Wasp e t a l . , 1977) and n a t u r a l m u d f l o w s ( E n o s , 1 9 7 7 ) . I f t h e s o l i d s c o n c e n t r a t i o n i s f a i r l y c o n s t a n t t h r o u g h -o u t t h e body o f t h e f l u i d , i t c a n be c o n s i d e r e d homogenous (Wasp e t a l . , 1 9 7 7 ) . The l i q u e f i e d t e s t s a n d i s c o n s i d e r e d a s a homo-g e n o u s s l u r r y , and c a n t h e r e f o r e be d e s c r i b e d by t h e B i n g h a m m o d e l . T h e B i n g h a m model i s shown s c h e m a t i c a l l y i n F i g . 27 and a t y p i c a l f l o w c u r v e i s shown i n F i g . 28. The two p a r a m e t e r m odel c a n be e x p r e s s e d a s : ^ = t y + 77p£ f o r 71 >~Cy and £ = 0 f o r ~C < "Cy w h e r e b y r e p r e s e n t s t h e t h r e s h o l d s h e a r s t r e s s r e q u i r e d t o i n i -t i a t e movement and \ p i s t h e p l a s t i c v i s c o s i t y , o r c o e f f i c i e n t o f r i g i d i t y , w h i c h i s f u l l y a n a l a g o u s t o t h e N e w t o n i a n v i s c o s i t y a b ove t h e t h r e s h o l d s h e a r s t r e s s . The m o d e l c a n be e x t e n d e d t o t h r e e p a r a m e t e r s t o i n c l u d e t h e e f f e c t o f , w a t e r c o n t e n t , a s shown i n F i g . 29. T h i s c o u l d BINGHAM RHEOLOGICAL MODEL FIGURE 2 7 Bingham f l u i d S t r a i n rate, £ BINGHAM MODEL FLOW CURVE FIGURE 2 8 A GENERAL PORE PRESSURE (WATER CONTENT) DEPENDENT BINGHAM PLASTIC MODEL FIGURE 29 (After Jeyapalan, 1980) 57 be used to account for consolidation e f f e c t s . Also, the quanti-tat i v e impact of the maximum pore pressure r a t i o being less than unity during c y c l i c loading, for a n i s o t r o p i c a l l y consolidated material, i s d i f f i c u l t to ascertain and i s beyond the scope of the present work, other than to say that for pore pressures less than the i n i t i a l c o n f i n i n g pressure the m a t e r i a l would have higher shear strength and v i s c o s i t y than i f the pore pressure equalled the confining pressure. 4-2-1 V i s c o s i t y The v i s c o s i t y i s defined as the r a t i o of the shear stress to the rate of shear s t r a i n produced by a given shear stress. The v i s c o s i t y depends on the nature and extent of mechanical inte r a c t i o n between the p a r t i c l e s . The p r i n c i p a l factors that govern the v i s c o s i t y of a slurr y are the p a r t i c l e size, d i s t r i -b u tion and shape, and the s o l i d s c o n c e n t r a t i o n . The s o l i d s concentration, used by Robinski (1978) to describe the f l u i d behaviour, determines the degree of mechanical i n t e r a c t i o n . F i g . 30 shows data for a variety of materials, indicating the dependence of v i s c o s i t y upon the water content, or s o l i d s concentration. Using a water content, w = .66 at the time of disposal, and w = .32 in the deposit (from Robinski (1978), one could expect an increase in the v i s c o s i t y of roughly 10 times due to consolidation. This neglects excess pore pressure due to overburden. Chong (1971) performed a study on s i l i c o n beads, and showed that the r e l a t i v e v i s c o s i t y , 1 r = 1 / 1 0 , i s independent of par-t i c l e s i z e , shape and d i s t r i b u t i o n . The results are shown in 58 107 a o CL cr -P o o Ul •H > u -H +J Ul (0 106 105 104 103 102 ©Curry (1966)-Debris flow CP i i d a (1938)-Tailings A Sharp & Nobles (1953)-Debris flow @ C a s t i l l o & Williams (1979) Glycerian suspension of c o i l s l u r r y $ Aber£on (1967)-Tailings Q B l i g h t (1980)-Tailings ^ Jeyapalan (1980)-Tailings! 100 10 Water content (%) VARIATION OF PLASTIC VISCOSITY WITH WATER CONTENT FiGURE 30 (After Jeyapalan) 59 F i g . 31. Note that the absolute v i s c o s i t y i s determined by ^o, the v i s c o s i t y for no p a r t i c l e interaction, and $4, , the asymp-t o t i c solids concentration, which i s dependent on the nature of the m a t e r i a l . In F i g . 32, r e s u l t s from Robinski (1978) are provided for comparison. Note the s i m i l a r i t y in shape of re-s u l t s presented in Figures 31 and 32, indicating the control of solids concentration, vi a the v i s c o s i t y , on the slope angle. This control i s dependent on the depositional c h a r a c t e r t i s t i c s discussed e a r l i e r . Although cohesive clay p a r t i c l e s are generally thought to increase v i s c o s i t y , Taylor and Morrell (1979) caution that i t i s the e f f e c t i v e stress condition that controls the v i s c o s i t y and the e f f e c t of p l a s t i c i t y may not be s i g n i f i c a n t . Chong's (1971) r e s u l t s i n d i c a t e that n o n - p l a s t i c f i n e s may r e s u l t i n a de-crease in v i s c o s i t y . Extensive research of flow s l i d e s , and subsequent back-calculations allowed the determination of a range of v i s c o s i t i e s for l i q u e f i e d t a i l i n g s (Jeyapalan, 1980), as shown in F i g . 33. The v i s c o s i t y of the t e s t sand, as determined from F i g . 30, and that corresponding to a t y p i c a l thickened discharge deposit A. (w = 29% and 32% respectively) i s approximately 5 x 1 0 cps, which i s s l i g h t l y lower than the lower bound suggested in F i g . 33. The value used in the analysis described i n Chapter 6 was chosen to be 1 x 105" cps for the l i q u e f i e d test sand, and pro-vided reasonable predictions of f l u i d behaviour. This value provided the best correlation between predicted and observed model test deformations. DEPENDENCE OF VISCOSITY ON SOLIDS CONCENTRATION FIGURE 31 (After Chong, 1971) DEPENDENCE OF LABORATORY SLOPE ON SOLIDS CONCENTRATION (After Robinski) FIGURE 32 61 iQrm — Dow-corning grease Probable Range for Liquefied Tailings —-I 10 6 Pi -V i s c o s i t y (cps) lcps=2x!0~ 5 l b - s e c / f t 2 lcps=.001 Pascal-sec 10 10' AberCan t a i l i n g s V i s c o s i t y used i n analysis Natural mudflow T a i l i n g s sand ^ 1 A s p e r F i g < 3 0 T a i l i n g s d e p o s i t ) Wet cement mortar Machine o i l Water @ 20°C VISCOSITY SPECTRUM (After Jeypalan) FIGURE 33 62 4-2-2 Y i e l d Shear Strength The y i e l d shear s t r e n g t h , Ty, i s the shear s t r e s s r e -quired to i n i t i a t e movement of a Bingham f l u i d . As a material approaches l i q u e f a c t i o n , the y i e l d strength decreases consid-e r a b l y , and a f t e r l i q u e f a c t ion "^y remains a constant f i n i t e value. y increases with solids concentration as a res u l t of increased p a r t i c l e i n t e r a c t i o n , as can be seen i n F i g . 34 (Govier and A z i z , 1972). A s a r e s u l t , c o n s o l i d a t i o n a f t e r d e p o s i t i o n can be expected to i n c r e a s e ^"y. However, l i t t l e quantitative data of t h i s nature exists. During flow £"y i s constant. Upon c e s s a t i o n of flow, a t h e o r e t i c a l Bingham f l u i d ' s y i e l d structure reforms (Wasp et a l . , 1977). Note that without a f i n i t e y i e l d strength, flow would continue i n d e f i n i t e l y . The y i e l d stress during l i q u e f a c t i o n i s a d i f f i c u l t parame-ter to assess. Jeyapalan (1980) estimates the range shown in F i g . 35, based on Castro's (1969) data and the behaviour of soft marine sediments. Ponce and B e l l (1971) performed t r i a x i a l tests at very low confining pressures, over a wide range of re-l a t i v e densities (5-95%) and obtained values for the apparent cohesion of .14 to .22 p s i . This corresponds to the lower end of data presented by Jeyapalan and i s shown in F i g . 34. Extra-polating the r e s u l t s of F i g . 34 to so l i d s concentrations of i n -terest in t h i s work (Cs = .55) r e s u l t s in a y i e l d strength of approximately 10 psf, which is s l i g h t l y lower than the lower bound suggested by Jeyapalan. The material used to produce Fig. 63 a) Rheogram for Water Suspension of Finely Divided Galena (D5Q=50 microns) (After Govier&Aziz,1972) Volume Fraction of Solids,Cs EFFECT OF SOLIDS CONCENTRATION ON FIGURE 34 Probable Range for Liquefied Tailings 8000 psf 4000 psf 2000 psf 1000 psf 500 psf 250 psf 150 psf 20 psf Hard clay Very stiff clay Stiff clay Medium clay Soft clay Very soft clay *~ j - Range from Ponce&Bell,1971 10 psf U Extrapolation of Figure 34 YIELD SHEAR STRENGTH SPECTRUM FIGURE 35 65 34 was s i m i l a r i n g r a i n s i z e t o t y p i c a l t a i l i n g s , h o w e v e r e x t r a p o l a t i o n i n v o l v e d o n l y two known p o i n t s , a n d may n o t p r o v i d e r e l i a b l e r e s u l t s a t h i g h s o l i d s c o n c e n t r a t i o n s . T he Bingham r h e o l o g i c a l model r e a s o n a b l y d e s c r i b e s t h e f l o w o f l i q u e f i e d , l o o s e , s a t u r a t e d m a t e r i a l . I t i s u s e f u l i n t h e d i s c u s s i o n o f t h e d e p o s i t i o n a l b e h a v i o u r o f t a i l i n g s , as com-p a r e d t o t h e p o s t - l i q u e f a c t i o n b e h a v i o u r o f t h e r e s u l t i n g s t a t i c a l l y s t a b l e d e p o s i t , a s i s d i s c u s s e d i n t h e f o l l o w i n g c h a p t e r . > 66 CHAPTER 5 DEPPSITIONAL BEHAVIOUR OF TAILINGS COMPARED TO  THE RESPONSE OF A LIQUEFIED DEPOSIT In order to assess the s t a b i l i t y of a l i q u e f i e d , sloped deposit, one must f i r s t analyze the depositional c h a r a c t e r i s t i c s of the f l u i d . I t is believed by the present author that the slope angle, and length of slope, of a thickened discharge de-posit i s determined by the rheological properties of the f l u i d as well as by the pattern of flow during deposition, whereas Robinski (1978) discusses only the f l u i d properties, neglecting the depositional pattern inherent in the disposal method. As discussed in Section 4-2, the flow of t a i l i n g s can be adequately described using the Bingham f l u i d rheological model. I t i s a two parameter model, the flow being described by a threshold y i e l d shear stress, "C"y, and a p l a s t i c v i scos i ty, *? p. These properties and the f l u i d c h a r a c t e r i s t i c s on which they depend are discussed in Sections 4-2-1 and 4-2-2. An applied shear stress must be greater than y for flow to occur. The flow i s then described by the p l a s t i c v i s c o s i t y . When the t a i l i n g s are deposited, they flow down the slope, and the flow i s unsteady, as i t eventually comes to rest. The dri v i n g forces are the i n i t i a l v e l o c i t y and gravity. The f l u i d has a boundary layer, which is a c r i t i c a l aspect of the deposi-t i o n a l behaviour. The boundary layer i s the region next to the base of the flow in which the f l u i d has had i t s v e l o c i t y dimin-ished because of shearing resistance created at the boundary due 67 to a no-slip condition. The boundary layer thickness i s i n the order of one foot for a ty p i c a l l i q u e f i e d t a i l i n g s , as deter-mined using formulae presented by S c h l i c t i n g , 1955. As the material flows from the point of discharge, i t then spreads out due to lack of l a t e r a l confinement. For conserva-ti o n of mass, the f l u i d layer must therefore become thinner, and the boundary l a y e r e f f e c t becomes i n c r e a s i n g l y dominant, i n accordance with boundary layer theory for laminar flow ( S c h l i c t -ing, 1955, Roberson and Crowe, 1975). Most t a i l i n g s and debris flows are characterized by laminar flow (Enos, 1977, Jeyapalan, 1980 ). The shear stress within the boundary layer varies from a maximum at the base of the f l u i d to zero at the surface. The maximum shear stress i s proportional to the thickness of the f l u i d , and therefore decreases from the point of deposition. This i s one of the mechanisms leading to cessation, or "freez-ing", of the flow. The v i s c o s i t y manifests i t s e l f in the development of the boundary layer. The boundary layer dissipates the f l u i d momen-tum and hence r e t a r d s the flow c o n s i d e r a b l y . The amount of momentum decay i s a function of the v i s c o s i t y . Momentum decay t r a n s l a t e s to a re d u c t i o n in v e l o c i t y , with which the shear s t r e s s at the base also v a r i e s . Therefore the shear s t r e s s decreases in the downstream d i r e c t i o n due to the viscous nature of the material. Both the effect of v i s c o s i t y and the f l u i d spreading r e s u l t in decreased shear stresses downstream. When 68 t h e s h e a r s t r e s s a t t h e b a s e e q u a l s t h e t h r e s h o l d s h e a r s t r e s s , m o t i o n c e a s e s . A l s o , any p o r e p r e s s u r e d i s s i p a t i o n d u r i n g f l o w w o u l d r e s u l t i n a h i g h e r y i e l d s t r e s s and v i s c o s i t y and f l o w w o u l d c e a s e s o o n e r . I f t h e f l u i d d i d n o t e x h i b i t a y i e l d s t r e s s , f l o w w o u l d c o n t i n u e i n d e f i n i t e l y . I f a g r e a t e r d e p t h , o r v o l u m e , o f m a t e r i a l i s d e p o s i t e d , i t w o u l d f l o w f u r t h e r f o r a g i v e n s l o p e , a s t h e d r i v i n g s h e a r f o r c e s due t o g r a v i t y w o u l d be i n c r e a s e d . The p i p e e x i t v e -l o c i t y , i f i n c r e a s e d , w o u l d a l s o r e s u l t i n h i g h e r b a s e s h e a r s t r e s s e s . A g i v e n f l u i d w i l l come t o r e s t a t a v a r i e t y o f s l o p e s , w i t h t h e i n i t i a l d e p t h , v e l o c i t y and s l o p e d e t e r m i n i n g t h e e x t e n t o f f l o w , w h i l e t h e f i n a l d e p t h o f t h e m a t e r i a l d e p e n d s on t h e s l o p e . H a v i n g e s t a b l i s h e d t h e m e c h a n i s m s t h a t r e s u l t i n t h e f l o w " f r e e z i n g " , one must now a c c o u n t f o r c h a n g e s t h a t o c c u r a f t e r d e p o s i t i o n . R o b i n s k i s t a t e s t h a t t y p i c a l l y 2 t o 4 f e e t o f mate-r i a l i s d e p o s i t e d u n i f o r m l y p e r y e a r . When t h e f l u i d c e a s e s m o t i o n , i t i s assumed t o h a v e v e r y h i g h e x c e s s p o r e p r e s s u r e s . T h e s e p o r e p r e s s u r e s w i l l d i s s i p a t e , w i t h t h e m a t e r i a l c o n s o l i -d a t i n g , r e s u l t i n g i n a h i g h e r s o l i d s c o n t e n t . U p o n r e 1 i q u e f a c t i o n t h e m a t e r i a l w o u l d e x h i b i t an i n -c r e a s e d v i s c o s i t y o v e r t h a t p r e s e n t d u r i n g d e p o s i t i o n . T h e y i e l d s t r e n g t h u p o n l i q u e f a c t i o n may i n c r e a s e , h o w e v e r no q u a n t i t a t i v e s t u d y has been p e r f o r m e d t o d e t e r m i n e t h e e x t e n t o f c h a n g e . R o b i n s k i ( 1 9 7 8) f e e l s t h a t t h e e n c h a n c e d f l u i d p r o p e r -t i e s r e s u l t i n a r e s e r v e a n g l e o f f r i c t i o n a v a i l a b l e t o o p p o s e 69 movement d u r i n g an e a r t h q u a k e . A l t h o u g h t h e f l u i d i t s e l f has i m p r o v e d , t h e s h e a r s t r e s s c o n d i t i o n s o f t h e b a s e o f t h e f l u i d have changed d r a s t i c a l l y , h a v i n g a d e t r i m e n t a l e f f e c t on t h e s t a b i l i t y o f t h e d e p o s i t . I n t h e e v e n t o f a s i g n i f i c a n t s e i s m i c e v e n t , i t i s c o n s i -d e r e d t h a t t h e d e p o s i t i s l i k e l y t o l i q u e f y t o a c o n s i d e r a b l e d e p t h . A l t h o u g h t h e r h e o l o g i c a l p r o p e r t i e s a r e i n d i c a t i v e o f a f l u i d more a b l e t o r e s i s t a g i v e n s h e a r s t r e s s t h a n t h e f l u i d p r e s e n t a t t h e t i m e o f f r e e z i n g , t h e b o u n d a r y s h e a r s t r e s s c o n -d i t i o n s o f t h e f l u i d a r e much d i f f e r e n t t h a n a t t h e t i m e o f f r e e z i n g . The d e p t h o f t h e f l u i d , and hence t h e s h e a r s t r e s s e s a t t h e b a s e o f t h e f l u i d w i l l have i n c r e a s e d d r a m a t i c a l l y and w i l l l i k e l y e x c e e d t h e y i e l d s h e a r s t r e n g t h o f t h e l i q u e f i e d m a t e r i a l . The bo u n d a r y l a y e r i s o n l y a f r a c t i o n o f t h e f l u i d d e p t h , and hence e x h i b i t s much l e s s c o n t r o l o v e r t h e f l o w o f t h e e n t i r e f l u i d l a y e r . The f l u i d o u t s i d e o f t h e b o u n d a r y l a y e r b e h a v e s l i k e an i n v i s c i d f l u i d . F l o w i s l i k e l y t o r e s u l t i n t h e down s l o p e d i r e c t i o n and t h e d i s t a n c e o f f l o w i s a f u n c t i o n o f d e p t h o f l i q u e f a c t i o n . I t i s s u s p e c t e d t h a t any d o w n s t r e a m embankment wo u l d be o v e r t o p p e d u n d e r t h e s e c o n d i t i o n s , l i k e l y r e s u l t i n g i n f a i l u r e . The r e s u l t s o f t h e model s t u d y , d i s c u s s e d i n C h a p t e r 8, i n d i c a t e t h a t t h e v i s c o u s f l u i d r e s u l t i n g f r o m l i q u e f a c t i o n a c t s as a s t a n d i n g wave, o v e r t o p p i n g t h e downstream b a r r i e r c o n s i s t e n t l y and r e s u l t i n g i n a f i n a l s l o p e o f a p p r o x i -m a t e l y one p e r c e n t . The d e p o s i t i o n a l f l o w s t r u c t u r e and bo u n d a r y s h e a r s t r e s s e s c a n n o t be c o n s i d e r e d t o g o v e r n t h e f l o w o f a s l o p e d d e p o s i t 70 l i q u e f i e d to any s i g n i f i c a n t depth. I t is doubtful that any enhanced f l u i d properties res u l t i n g from consolidation after deposition would be able to r e s i s t the much larger shear stres-ses that would exist at the base of the l i q u e f i e d layer. For t h i s reason the s t a b i l i t y of a deposit formed by the thickened discharge method i s questionable during a s i g n i f i c a n t seismic event. 71 CHAPTER 6 REVIEW OF PREVIOUS MODEL STUDIES Model studies have been performed by many authors on the c y c l i c response of s o i l deposits and earth embankments (Yoshimi, 1977; Prakash, 1977). The majority of these were designed to study the s t r u c t u r a l response of embankments and retaining walls for purposes of design (Prakash, 1977). Much of the model te s t -ing performed has been done with either dry sand, at extremely high a c c e l e r a t i o n s or with imposed boundary c o n d i t i o n s that render the re s u l t s inapplicable to t h i s study. Other studies observed the pore pressure response within a s o i l deposit subjected to c y c l i c loading (Finn et a l . , 1970, I s h i h a r a , 1967, De Alba et a l . , 1976, Kawakami, 1966, Goto, 1966, Tanimoto, 1967). These studies provide both q u a l i t a t i v e and quantitative descriptions of the response leading the lique-f a c t i o n . S i m i l i t u d e requirements f o r a shaking t a b l e model study are o u t l i n e d by I s h i h a r a . He concluded that s t r i c t s i m i l i t u d e requirements cannot, and need not, be s a t i s f i e d as p a r t i a l similitude should be s u f f i c i e n t . Yoshimi (1977) sug-gests that model studies be used to v e r i f y a n a l y t i c a l tools, which in turn can be used to predict f i e l d behaviour, thereby enabling one to neglect sim i l i t u d e requirements. Results of studies using pore pressure monitoring at v a r i -ous depths i n d i c a t e that l i q u e f a c t i o n occurs simultaneously over the depth of the deposit (Finn et a l . , 1970, De Alba et a l . , 1976, Yoshimi, 1967). No such monitoring was used in the 72 present study and i t i s assumed that these results are a p p l i -cable. This assumption was confirmed by v i s u a l observation. The previous studies were also used to select test parame-ters in the present study. Accelerations, frequency and s o i l depths were selected with due regard to previous studies. Finn et a l . , showed that for a- sample of roughly the same depth there existed a frequency at which the acceleration d i s t r i b u t i o n was not uniform over the depth of the sample. For frequencies g r e a t e r than 20 Hz, t h i s behaviour became more pronounced. There also existed a natural frequency at which acceleration response was greatly enchanced. The implications of t h i s are discussed further in Appendix 1 . Most previous studies were performed at similar accelerations, i f s l i g h t l y higher, than the present program. Threshold accelerations were not being sought in previous studies. The pore pressure response of l i q u e f i a b l e shaking table specimens has been found to be very similar to that in c y c l i c t r i a x i a l and simple shear tests (Finn, 1970, Yoshimi, 1967), Fi g . 36. The pore pressure t y p i c a l l y rose to approximately 60% of the e f f e c t i v e confining pressure, then exhibited a rapid pore pressure increase leading to l i q u e f a c t i o n . These studies were designed to observe the response within a horizontal deposit. Anisotropic pore pressure response within sloping deposits has not been studied, but is assumed to be similar to that found in c y c l i c t r i a x i a l or simple shear tests conducted on specimens exhibiting a s t a t i c bias. Zircon Sand p=150cm H 2 0 a=440 cm/sec2 - i 1 r 40 60 80 Time ,t,(sec) OBSERVED PORE PRESSURES FOR A SHAKING TABLE TEST FIGURE 36 Transducer Depth PC1 40 cm P62 140 cm PC3 240 cm (After Yoshimi,1967) 74 No tests have been performed to describe the post-lique-faction behaviour in a sense that would be applicable to the present model study. Most of the work on slurry flow has been with regard to assessing pipe flow and are therefore not con-sidered here. Jeyapalan (1980) performed an inundation study of l i q u e f i e d t a i l i n g s using o i l as the viscous material. This work produced re s u l t s that support the concept of "freezing" flow discussed i n Chapter 5. Robinski (1978) performed laboratory studies to determine the residual angle of t a i l i n g s at various s o l i d s concentration, but provided no information on the test equipment or depositional features that would be of interest here. No model study has been performed to observe threshold accelerations and subsequent behaviour of a l i q u e f i e d deposit. i 75 CHAPTER 7 TESTING PROGRAM The testing program comprised 18 model tests and was de-signed to determine the response of sloped, cohesionless, sat-urated deposits subjected to horizontal c y c l i c accelerations. Horizontal accelerations simulate v e r t i c a l l y propagating h o r i -zontal shear waves produced during an earthquake. This assump-ti o n i s common i n earthquake related analyses. The sloped deposit was used to simulate a sloped t a i l i n g s d e p o s i t as obtained using the thickened discharge method of disposal. Fine sand, simulating t a i l i n g s , was deposited hy-d r a u l i c a l l y i n t o a p l e x i g l a s s container to obt a i n a loose, uniform deposit. The r e l a t i v e density was e s s e n t i a l l y the same f o r a l l t e s t s , and equal to 30% +_ 5%. Model dimensions are given i n Figure 37. Steady state seepage conditions were i n -duced by a constant head upstream boundary c o n d i t i o n . The i n i t i a l pore pressures were governed by the boundary conditions shown in Figure 38, and a flow net showing equipotential values i s shown in F i g . 39. Slopes of 2°, 4° and 8° were subjected to base accelera-tions ranging from .03 g to .10 g. I n i t i a l l y , varying down-stream boundary heights were used to produce the desired slope, and in the l a t t e r portion of the program a constant downstream boundary height was used, while varying the upstream boundary height. A number of tests were performed using a sloped base to simulate a sloping valley f l o o r . A l l tests were performed at a BOUNDARY CONDITIONS FOR STEADY STATE SEEPAGE F I G U R E 3 8 Seepage Analysis Slope=6 ° 22.5 cm 21.6 20.8 19.9 19.1 18.2 17.4 16 FLOW NET FOR 6°SLOPE '* "i - - • . | F I G U R E 3 9 78 frequency of 5 Hz, with steady state seepage and sim i l a r r e l a -t i v e d e n s i t i e s . A discussion of the selection of test parame-ters can be found in Appendix 1. 7-1 Model Test Equipment An MTS Earthquake Simulator console was used to provide the base a c c e l e r a t i o n s . The system i s a closed-loop servo-controlled system. This type of system involves the generation of the d e s i r e d motion and monitoring of the response of the t a b l e . Any d i f f e r e n c e between the input and the output i s relayed as a corresponding voltage to the servo valve, which adjusts the hydraulic flow accordingly. Monitoring at the table i s by way of an LVDT located at the piston that moves the table. A schematic of the loop i s shown in F i g . 40. The command signal was chosen to be a sine wave, provided by an Exact 340 function generator. This function generator was found to have less d i s t o r t i o n at the peak of the sine wave than other function generators tested. This reduced a spike in the acceleration response of the table at i t s maximum displacement. The spike i s an inherent feature of the hydraulic system. Pressure accumulators did not eliminate the occurrence of the spike. This undesirable feature has been obtained in si m i l a r test equipment (Ramsay, 1981). Although a spike was unavoid-able, i t could be reduced substantially by operating the hydrau-l i c s at low pressure (150-250 p s i ) . This adjustment did not affec t the frequency response of the system. Function Generator Servo Controller V, V1-V2 Displacement Feedback Oil Supply 150-250 p: Servo Valve Hydraulic Piston LV DT SCHEMATIC OF TABLE LOOP FIGURE 4Q 80 Several frequencies were used in preliminary t e s t s . It was found that a frequency of 5 Hz led to reasonable results, imparting v i r t u a l l y undrained conditions, and provided repro-ducible response over the range of accelerations used. Typical table response i s shown in F i g ; 41. The table i t s e l f i s 4 feet by 9 feet and i t weighs approxi-mately 1 000 lbs. I t i s made of cast aluminum with bracing to provide the necessary r i g i d i t y . I t moves in one h o r i z o n t a l d i r e c t i o n and i s supported by two v-slotted needle bearings and two f l a t bearings. In t h i s way proper alignment i s maintained without obtaining unevenly di s t r i b u t e d loads on the bearings due to bending or temperature changes. The table response was monitored at i t s surface by a K i s t l e r Model 305 accelerometer. The a c c e l e r a t i o n s were recorded by a V i s i g r a p h cathode ray recorder. The model container was constructed using 1/2 inch p l e x i -glass. The container i s shown in F i g . 42. The box was designed to provide a constant head boundary condition at the upstream end and a variable downstream boundary height to simulate a dam at the toe of the deposit. It i s assumed in t h i s study that the dam provides a r e l a t i v e l y r i g i d boundary in the f i e l d . The upstream constant head boundary condition was maintained by a drainage l i n e fixed at the height required for a given slope and downstream boundary height. The water was separated from the deposit by a.baffle and a pervious p l a s t i c wall. This elimin-ated erosion due to splashing during shaking. The base of the TYPICAL RESPONSE AT 5 Hz FIGURE 41 CO 20.4 cm (top) 20.35 cm (bottom) 20.1 cm (top) 20.35 cm (bottom) 20.3 cm (top) 20.4 cm (bottom) —r r~ Inlet a Overflow Variable Height Of Bottom Weir 81.0 cm Height to Bottom of lnlet=16 3 cm Height to Overflow=16.6 cm PLAN VIEW Outlet Scraper Guide Pervious Plastic SIDE VIEW MODEL CONTAINER FIGURE 4S Hopper Outlet tp Inlet 83 box was sanded to provide f r i c t i o n a l resistance. The container was designed so that i t could be t i l t e d to simulate a sloped base boundary condition. A hopper was designed to deposit the test sand evenly over the width of the box. Smooth runners, a smooth p l e x i g l a s s surface and lubricants were used to minimize disturbance during deposition. The hopper outlet could be closed during f i l l i n g and upon opening had a reasonably even flow over i t s width. The outlet space was 1/8" and the volume of each pass resulted in a sand layer approximately 1 cm thick. When the desired slope was approximated, a scraper was used to provide an even slope. The scraper was cut to just over half the width of the box to prevent scour at i t s base and edges dur-ing s c r a p i n g . The scraper was braced f o r s t i f f n e s s and was a d j u s t a b l e to d i f f e r e n t slopes. The runners were smooth to reduce distrurbance during scraping. P a r t i c l e movements were monitored with silica.beads placed in the sand against the p l e x i g l a s s wall. P a r t i c l e movements were recorded on p l a s t i c sheets attached to the p l e x i g l a s s . It was determined that side e f f e c t s were minimal by observing transverse l i n e s on the surface of the deposit. The glass beads were placed i n l a y e r s on a g r i d p a t t e r n . The placement of l a y e r s , and subsequent s c r a p i n g , were observed to have no noticeable affect on layers that had been previously placed. 84 7-2 T e s t P r o c e d u r e s The m o d e l c o n t a i n e r was c l e a n e d p r i o r t o e a c h t e s t . I t was t h e n b l o c k e d i n p l a c e t o p r e v e n t movement r e l a t i v e t o t h e t a b l e s u r f a c e . The d o w n s t r e a m b o u n d a r y was p l a c e d , u s i n g s i l i c o n e g r e a s e t o s e a l i t s e d g e s . A g r i d was p l a c e d on t h e o u t s i d e t o f a c i l i t a t e p l a c e m e n t o f t h e s i l i c a b e d s and t o r e c o r d t h e i r d i s -p l a c e m e n t s . A l l e l e c t r o n i c e q u i p m e n t was c a r e f u l l y c a l i b r a t e d p r i o r t o t h e f i r s t t e s t . The box was t h e n f i l l e d w i t h d e - a i r e d w a t e r w i t h i n l e t s and o u t l e t s c l o s e d . S a n d was t h e n d e p o s i t e d i n 1 cm l i f t s t h r o u g h a p p r o x i m a t e l y 6" o f w a t e r . The h e i g h t o f w a t e r was m a i n t a i n e d by d r a i n i n g w a t e r i n t e r m i t t e n t l y a s n e c e s s r y . S i l i c a b e a d s were p l a c e d a c c o r d i n g t o a p r e - d e t e r m i n e d g r i d p a t t e r n . The oven d r i e d s a n d was w e i g h e d p r i o r t o p l a c e m e n t f o r t h e v o i d r a t i o d e t e r m i n a t i o n . When t h e s l o p e was a p p r o x i m a t e d , t h e s c r a p e r was u s e d t o remove t h e e x c e s s s a n d , w h i c h was d r i e d and w e i g h e d . The d o w n s t r e a m o u t l e t was t h e n o p e n e d t o d r a i n o f f e x c e s s w a t e r . W i t h t h e u p s t r e a m o v e r f l o w and i n l e t o p e n e d , s t e a d y s t a g e s e e p -age was e s t a b l i s h e d . No p i p i n g a s o b s e r v e d a t t h e d o w n s t r e a m b o u n d a r y , a s p r e d i c t e d i n p r e l i m i n a r y c a l c u l a t i o n s . The h y d r a u l i c s w e r e t h e n s w i t c h e d o n , t h e i n l e t and o u t l e t s c l o s e d , and t h e t e s t w o u l d be r u n f o r 20 c y c l e s u s i n g a s t o p w a t c h . T a b l e a c c e l e r a t i o n s , p a r t i c l e d i s p l a c e m e n t s , f i n a l s l o p e c o n f i g u r a t i o n and o v e r t o p p i n g v o l u m e were r e c o r d e d . 85 CHAPTER 8 TEST RESULTS The t e s t i n g program was designed to study the response of various of test model i n i t i a l surface slopes to a range of base accelerations. The primary purpose was to determine the f i n a l slope to which a l i q u e f i e d -deposit would come to rest. Of secondary inter e s t was the determination of a c r i t i c a l accelera-tion required to induce l i q u e f a c t i o n throughout a given test model configuration. The e f f e c t of a sloping base on test re-s u l t s was also studied. Several tests were duplicated to estab-l i s h r e p r o d u c i b i l i t y of r e s u l t s . A t o t a l of 18 shaking table tests were performed. Figure 43 provides a breakdown of the t e s t s i n terms of degree of l i q u e f a c t i o n throughout the sample, base slope and a c q u i s i t i o n of p a r t i c l e displacements. Table I I I provides a summary of individual test d e t a i l s . Figures 44 through 49 depict the p a r t i c l e displacements that resulted in tests for which a s i g n i f i c a n t portion of the deposit was assumed to have l i q u e f i e d . Shown in each of these figures are the i n i t i a l and f i n a l surface configuration, as well as the displacement of v e r t i c a l lines over the length of the sample. Note that in the center of the sample, where the end boundary e f f e c t s are minimized, i n i t i a l l y v e r t i c a l lines assume a parabolic shape, as might be expected from a viscous f l u i d . The f i n a l slopes obtained are very shallow, a l l being less than 1%. There appears to be a trend, as shown in Figure 50, 9 Tests Exhibited Complete Liquefaction -(used in particle displacement prediction comparisons and final slope determinations) 8-Horizontal Base-•6-Obtained Particle Displacements 1 2-Did Not Obtain Particle Displacements 1-Sloped Base-Obtained Particle Displacements ? Tests Exhibited artial Liquefaction "•(used in shakedown and for ustablishing threshold accelgrati ons) 8-Horizontal Base .1-Sloped Base BREAKDOWN OF TESTING PROGRAM F I G U R E 4 3 TEST INITIAL SLOPE AMAX RELATIVE DENSITY CYCLES FREQUENCY LIQUEFACTION PARTIAL TOTAL FINAL SLOPE 1 4.2° • 05g 28% 28 5Hz X — 2 4° .05g 29% 20 5Hz X — FIXED 3 8° .05g 31% 18 5Hz X — UPSTREAM 4 8° .05g 27% 21 5Hz X — BOUNDARY 5 10° .05g 35% 22 5Hz X — HEIGHT 6 8° .05g 25% 33 20Hz X = 16.2 cm 7 4.8° .05g 26% 20 5Hz X .5% 8 8° .05g 38% 22 5Hz X — 9 8° .025g 33% 20 5Hz — — 10 4° .05g 29% 21 5Hz X Sloped Base 11 8° .05g 29% 20 5Hz X Sloped Base FIXED 12 8° . lOg 30% 20 5Hz X .7% DOWNSTREAM 13 8° .08g 37% 20 5Hz X .6% BOUNDARY 14 8° .06g 31% 21 5Hz X 3.0% HEIGHT 15 4° .04g 31% 21 5Hz X — = 14.0 cm 16 4° •045g 37% 21 5Hz X ' .5% 17 2° .04g 28 20 5Hz X .3% 18 2° .03g 35 20 5Hz X — SUMMARY OF TESTING PROGRAM TABLE III co RESULTS OF TEST 7 F I G U R E 4 4 SCALE: 1 inch RESULTS OF TEST 12 F I G U R E 4 5 CO SO SCALE: 1inch= 10 cm RESULTS OF TEST 13 F I G U R E 4 6 o Final Slope 1 Displacement of Vertical Line 1 / / ' , icJ 1 / SCALE: 1 inch=10cm RESULTS OF TEST 14 F I G U R E 4 7 / SCALE : 1 inch=10cm t 1 RESULTS OF TEST 16 F I G U R E 4 8 \ SCALE: 1 inch=10 cm RESULTS OF TEST 17 F I G U R E 4 9 SD 94 whereby i n c r e a s i n g the i n i t i a l surface slope r e s u l t s in an increased f i n a l slope of the l i q u e f i e d material. This could be the r e s u l t of lower induced maximum pore pressures due to higher i n i t i a l s t a t i c shear induced i n the steeper samples, as discus-sed i n S e c t i o n 4-1. The lower maximum pore pressures would re s u l t in a higher v i s c o s i t y and higher threshold shear stress, as discussed in Section 4-2. The net r e s u l t would be increas-ing f i n a l slope angle for increasing i n i t i a l surface slopes. Another contributing factor could be the affect of d i l a t i o n , and a r e s u l t i n g increase i n shear strength of the m a t e r i a l . Increasing the surface slope implies that higher shear strains were imposed, and the e f f e c t of d i l a t i o n increases with increas-ing s t r a i n . Although c y c l i c t r i a x i a l tests indicate no s i g n i f i -cant d i l a t i o n up to 10% a x i a l s t r a i n , i t i s f e l t that a small degree of d i l a t i o n could account for some of the increase in f i n a l slope obtained. I t should be noted that i n a l l model tests, movement ceased p r i o r to eliminating the base accelera-tions. Several samples at a given i n i t i a l surface slope angle were subjected to a range of base a c c e l e r a t i o n s to determine the rela t i o n s h i p between the base acceleration and the f i n a l slope angle. It was determined that a c r i t i c a l acceleration l e v e l existed, above which the l i q u e f i e d deposit assumed a constant f i n a l slope angle. Figure 51 shows the effect of increasing acceleration on f i n a l slope angle. The c r i t i c a l acceleration l e v e l was'obtained for 2°, 4° and 8° and Figure 52 depicts the r e s u l t s . Increasing the i n i t i a l surface slope angle results in 1-0 i All Tests at Cs=56.5%t1% amax£(amax)crit EFFECT OF INITIAL SLOPE ON FINAL SLOPE F I G U R E 5 0 2 H 0 2 4 6 Final Slope Angle (Degrees) ACCELERATION LEVEL VERSUS FINAL SLOPE ANGLE F I G U R E 51 9 7 8-i 6 H Threshold Acceleration (%g) 4 -2 H 0 2 4 6 8 Initial Slope Angle (Degrees) THRESHOLD ACCELERATION VERSUS INITIAL SLOPE ANGLE F I G U R E 5 2 98 an i n c r e a s e d c r i t i c a l a c c e l e r a t i o n l e v e l . T h i s i s c o n s i s t e n t w i t h t h e e f f e c t o f s t a t i c s h e a r s t r e s s on l i q u e f a c t i o n r e s i s -t a n c e as d i s c u s s e d i n S e c t i o n 4-1. The e f f e c t o f a s l o p i n g b a s e ( s i m u l a t i n g a s l o p i n g v a l l e y f l o o r ) was s t u d i e d t o d e t e r m i n e t h e a p p l i c a b i l i t y o f t h e h o r i -z o n t a l b a s e t e s t r e s u l t s f o r s l o p e d b a s e c o n d i t i o n s . F i g u r e 53 shows t h e r e s u l t s f o r T e s t 10, i n w h i c h a s l o p e d b a s e was i m -p o s e d . A l s o shown f o r c o m p a r i s o n a r e t h e d e f o r m a t i o n s o b t a i n e d f o r t e s t 16, i n w h i c h a h o r i z o n t a l b a s e was u s e d . B o t h t e s t s , h a d i d e n t i c a l i n i t i a l s u r f a c e s l o p e s (4°) a n d s i m i l a r b a s e a c c e l e r a t i o n s (.05g and 0.45g, r e s p e c t i v e l y ) . I t was d e t e r m i n e d f r o m T e s t 4 ( h o r i z o n t a l b a s e ) and T e s t 11 ( s l o p e d b a s e ) , t h a t a s l o p i n g b a s e had l i t t l e e f f e c t o n t h e t h r e s h o l d a c c e l e r a t i o n and f i n a l s l o p e a n g l e f o r a g i v e n i n i t i a l s u r f a c e s l o p e . I n g e n e r a l , t e s t o b s e r v a t i o n s , b o t h v i s u a l and m e a s u r e d , i n d i c a t e d t h a t t h e l i q u e f i e d d e p o s i t s were b e h a v i n g s i m i l a r t o w hat m i g h t be e x p e c t e d f r o m a h i g h l y v i s c o u s f l u i d . F o r t h i s r e a s o n , a v i s c o u s f l u i d m o d e l was u s e d t o p r e d i c t t h e p a r t i c l e d i s p l a c e m e n t s o b s e r v e d . T h i s m o d e l i s d i s c u s s e d i n t h e f o l l o w -i n g c h a p t e r . 99 LEGEND Horizontal Base — — Sloped Base EFFECT OF SLOPED BASE ON MODEL RESPONSE F I G U R E 5 3 100 CHAPTER 9 FLUID ANALYSIS An a n a l y s i s was performed assuming l i q u e f i e d t e s t sand ac t s as a viscous f l u i d , c o n s i s t e n t with t e s t o b s e r v a t i o n s . For l i q u e f i e d test deposits that experienced a marked reduc-t i o n in the surface slope, i t i s assumed that the s t a t i c shear s t r e s s at the base due to the slope exceeded the t h r e s h o l d shear stress required to cause flow. This has been the assumed mechanism in flow f a i l u r e s in nature (Castro, 1969; Casagrande, 1971). Upon exceeding t h i s shear s t r e s s , the m a t e r i a l i s assumed to behave as a viscous Newtonian f l u i d u n t i l such time as the driving shear stress equals the threshold shear stress, at which point the f l u i d ceases motion, as depicted i n Figure 28. Over the p e r i o d of motion i t i s assumed that a unique p l a s t i c v i s c o s i t y governs the deformation of the m a t e r i a l . The material appeared to liquefy simultaneously throughout. It then appeared to behave as a viscous f l u i d , and rather than the surface material flowing downstream, i t exhibited motion s i m i l a r to a standing wave. A preliminary analysis treated the problem as a standing wave in water. Surface displacements were close to those obtained experimentally but horizontal displace-ments near the base were g r o s s l y o v e r - p r e d i c t e d , due to the neglect of v i s c o s i t y in water wave theory. Introduction of v i s c o s i t y imposes a no-slip base boundary c o n d i t i o n . Lambe (1945) provides a q u a l i t a t i v e d e s c r i p t i o n of the motion of a v e r t i c a l l i n e due to a standing wave in a 101 viscous f l u i d . Motions of the boundary layer are very similar to those obtained i n the model te s t . No rigorous, closed form solution could be found for a standing wave in a highly viscous f l u i d . L i u and Davis (1977) study a standing wave in water and account for the v i s c o s i t y of the water. Their model e s s e n t i a l l y i n v o l v e s two boundary l a y e r s capable of t r a n s f e r r i n g shear, bounding an i n v i s c i d i n t e r i o r f l u i d . The lower and upper boundary layers are required to s a t i s f y the constraints of no-s l i p at the base and zero shear at the free surface. It i s assumed that t h i s t h e o r e t i c a l model of the boundary layer could be representative of the l i q u e f i e d test model. The thickness of the lower boundary layer was determined to be very close to the thickness of the test deposit. The theory c a l l s for the use of a transformed, or "stretched", coordinate system in the boundary layer. V e r t i c a l displacements at the edge of the boundary layer, in the i n t e r i o r f l u i d , are assumed to be small. The "stretched" normal coordinate i s required to obtain correct displacements r e l a t i v e to the i n t e r i o r f l u i d . Having no i n t e r i o r f l u i d , the tr a n s f o r m a t i o n became unnecessary. A l l predictions were made using "unstretched" coordinates and the accuracy of the predictions empirically substantiate neglecting the transformation. The primary objective of this analysis was to show that the model test material could be described as a viscous f l u i d , not to propose that t h i s model be used to des-cribe f i e l d behavior. 102 The theory, as proposed by Liu and Davis, enforces the no-s l i p condition of the base through the insertion of a boundary layer of thickness; £ = (2V/w) 1/2 where V= / j " = kinematic v i s c o s i t y f = 19 50 g/cm3 w = k Co = angular frequency k = 2*r/wave length = wave number Co = [g tan h(kd)/k] 1/2 = wave speed Using the length of the model container as 1/2 the wave length and V = .1 m2/s ( *l = 2x10^ cps), one obtains a boundary layer t h i c k n e s s of 20 cm, which i s s l i g h t l y greater than the mean depth of the model deposit. The boundary thickness i s shown in Fi g . 54 as a function of v i s c o s i t y over the range of v i s c o s i t i e s for l i q u e f i e d t a i l i n g s determined in Section 3-2-1. In solving the boundary value problem of the bottom boun-dary layer, using unstretched coordinates, one obtains dimen-sionless v e l o c i t i e s as: u = ^-^-^U s i n ( t ) - e" s i n ( t - _ f ) sin hiB «"a - " l l f + " f / - m e t - / ) -^-J— cos t i t - / - ) + 1/2 cos(t) - 1/2 sin(t )7 Dow-corning grease Aberfan tailings-—i ANALYSIS -1 Natural mudflow ~~| f Probable range for ^ liquefied tailings r Kinematic Viscosity, ,(m2/t) EFFECT OF VISCOSITY ON BOUNDARY LAYER THICKNESS FIGURE 5 4 1 04 where ( A ( t ) a c c o u n t s f o r t h e v i s c o u s d e c a y o f t h e wave a m p l i t u d e , A, w i t h slow t i m e , t = k C Q t B = k 2 d 2 f = k z + v*B I n t e g r a t i n g t h e v e l o c i t i e s w i t h r e s p e c t t o t i m e , one o b t a i n s p a r t i c l e d i s p l a c e m e n t s , e x p r e s s e d d i m e n s i o n a l l y a s : / \ n * s i n ( k x ) a t , b 2 . 1 ,> , a . * x = Co Ao s i n h ^ e (^Tp) [ " b C O S ( b t ) + £ Z s i n ( b t > - e f [- 1 c o s ( b t - / ) + — s i n ( b t - / ) ) ] b 2 *y - c°*° f f f 1 ^ *at i/<- s »•<>>« + ^ •*•><>*; + f - ^ " ( - -J c o s ( b t - / ) + — s i n ( b t - / ) + - — * b b 2 2 ^— s i n (bt-_f) + a ,, , • 1 — c o s ( b t - j - ) + 2 M ( - s i n ( b t ) b b 2 2 b + — c o s ( b t ) ) - 1 ( _ 1 — c o s ( b t ) + a . . . ., - s i n ( b t ) )] b 2 2 b b 2 where a = - 2 k 2 V* b = kCr, 105 For p r e d i c t i o n of p a r t i c l e displacements in the model test, i t is necessary to provide corrections to the theoreti c a l displacements. The f i r s t factor accounts for settlement due to c y c l i c loading. It was assumed that the settlement at any point is related to the settlement at the node in the centre of the deposit. This could be determined simply by observing the post-testing p o s i t i o n of the surface point. The surface settlement at any other point was assumed to be d i r e c t l y proportional to the o r i g i n a l depth at that p o i n t . On a v e r t i c a l l i n e , the settlement was assumed l i n e a r from zero to the base to the surface. Surface settlements at the node were found to be 1-2%, which agress well with the 1-3% estimated by Yoshimi et a l . (1975). Another correction was imposed to account for the necessary assumption of a sinusoidal surface. the correction factor was determined at the surface and decreased l i n e a r l y with depth. Due to the shallow slopes involved in the study, the assumed sinusoidal surface was reasonable and the correction factors r e l a t i v e l y small. The i n i t i a l control p a r t i c l e positions were in v e r t i c a l l i n e s . the coordinates used in the above analysis are the mean p a r t i c l e positions. The i n i t i a l v e r t i c a l lines correspond to the maximum p a r t i c l e displacements. It was therefore necessary to assume a mean p a r t i c l e p o s i t i o n and solve for the p a r t i c l e displacements i t e r a t i v e l y . P a r t i c l e displacements were solved for incrementally over one-quarter of a cycle of wave motion. 106 The a n a l y s i s does not account f o r the f r i c t i o n a l a s p e c t o f the Bingham f l u i d . The a c t u a l m a t e r i a l ceased motion s l i g h t l y b e f o r e 1/4 o f a c y c l e had been completed. It was at t h i s p o i n t t h a t d r i v i n g shear s t r e s s e s e q u a l l e d t h e t h r e s h o l d shear s t r e s s o f the m a t e r i a l . The p a r t i c l e d i s p l a c e m e n t s used i n the p r e -d i c t i o n s i n C h a p t e r 10 were t h o s e c o r r e s p o n d i n g t o t h e t i m e a t which the c o r r e c t e d v e r t i c a l d i s p l a c e m e n t at the upstream a n t i n o d e e q u a l l e d t h e f i n a l d i s p l a c e m e n t i n t h e model t e s t . The v i s c o s i t y used i n the a n a l y s i s was s e l e c t e d as b e i n g r e p r e s e n t a t i v e f o r l i q u e f i e d t a i l i n g s . T h i s a n a l y s i s i s not i n t e n d e d t o p r o v i d e q u a n t i t a t i v e r e s u l t s i n terms of v i s c o s i t y , but i s d e s i g n e d t o i n v e s t i g a t e the o b s e r v e d phenomenon. The o b s e r v e d c h a r a c t e r i s t i c response o f the l i q u e f i e d model d e p o s i t was f o u n d t o be i n a g r e e m e n t w i t h t h e s e l e c t e d v i s c o u s f l u i d model. The s u c c e s s f u l a p p l i c a t i o n o f t h e a n a l y t i c a l model t o p r e d i c t model t e s t b e h a v i o u r does not n e c e s s a r i l y imply t h a t e x t r a p o l a t i o n can be made 'to f i e l d b e h a v i o u r . L i q u e f i e d f i e l d d e p o s i t s would l i k e l y be of much g r e a t e r depth and more complex geometry t h a n p r e s e n t i n the model t e s t . 1 07 CHAPTER 10 PREDICTED PARTICLE DISPLACEMENTS The f l u i d a n a l y s i s p r e s e n t e d i n C h a p t e r 9 c a n be u s e d t o p r e d i c t p a r t i c l e d i s p l a c e m e n t s o b t a i n e d i n t h e m o d e l t e s t s . P r e d i c t i o n s a r e made o n l y f o r t e s t s i n w h i c h t h e e n t i r e d e p o s i t was o b s e r v e d t o l i q u e f y . Due t o t h e c o m p l e x n a t u r e o f t h e b o u n d a r y v a l u e p r o b l e m i n v o l v i n g a s l o p i n g b a s e , p r e d i c t i o n s a r e made o n l y f o r d e p o s i t s w i t h h o r i z o n t a l b a s e s . The p a r t i c l e d i s p l a c e m e n t s f o r i n i t i a l s u r f a c e s l o p e s o f 4° and 8° were o b t a i n e d u s i n g a s i n g l e v a l u e o f k i n e m a t i c v i s -c o s i t y . T h i s v a l u e , V = .1 m^/s, i s w i t h i n t h e ra n g e o f p r o b -a b l e v i s c o s i t i e s f o r l i q u e f i e d t a i l i n g s , as was p o i n t e d o u t i n F i g u r e 47. A c c u r a t e p r e d i c t i o n s a r e n o t meant t o i m p l y t h a t t h e v i s c o s i t y c a n be q u a n t i f i e d . I t i s t h e n a t u r e o f t h e f l u i d movement t h a t i s t h e i n t e r e s t , n o t a b s o l u t e q u a n t i t i e s . T r e a t -i n g t h e l i q u e f i e d t e s t m a t e r i a l a s a v i s c o u s f l u i d a p p e a r s t o g i v e r e a s o n a b l e r e s u l t s f o r a f i r s t a p p r o x i m a t i o n . A l t h o u g h o n l y one v i s c o s i t y was em p l o y e d , v i s c o s i t i e s o f one t e s t r e l a -t i v e t o a n o t h e r c an be q u a l i t a t i v e l y d i s c u s s e d w i t h r e g a r d t o p o r e p r e s s u r e e f f e c t s and s o l i d s c o n c e n t r a t i o n . P r e d i c t e d p a r t i c l e d i s p l a c e m e n t s n e a r t h e downstream boun-d a r y a r e not made due t o d i s t o r t e d d e f o r m a t i o n s c r e a t e d by o v e r -t o p p i n g o f t h e b o u n d a r y . A f i n i t e b o u n d a r y h e i g h t was not c o n -s i d e r e d i n t h e a n a l y s i s . P r e d i c t e d d i s p l a c e m e n t s f o r an i n i t i a l s l o p e o f 8° a r e shown i n F i g u r e 55. A c t u a l d i s p l a c e m e n t s o b t a i n e d i n T e s t #13, 1 0 8 LEGEND — - Actual Displacements Predicted Displacements COMPARISON OF PREDICTED AND OBSERVED SLOPE MOVEMENTS FOR 8° SLOPE t F I G U R E 5 5 1 09 where a m a x = ,08g > ( a m a x ) c r i t , are also shown p l o t t e d f o r comparison. Excellent agreement was obtained. The p a r t i c l e motion appears to be p r e d i c t e d w e l l using the viscous f l u i d standing wave model. P r e d i c t e d displacements f o r an i n i t i a l slope of 4° are shown in Figure 56. Actual displacements obtained in Test #7, where a m a x = . 0 5g >(a r n a x) c r i t , are a l s o shown p l o t t e d f o r comparison. Predictions compare very well with actual p a r t i c l e motions. The fact that the viscous f l u i d model accurately predicts the p a r t i c l e motions lends c r e d i b i l i t y to the treatment of the l i q u e f i e d material as a viscous f l u i d . The analysis used employed a boundary layer, essential in the analyses of viscous f l u i d s . Whether the flow i s in the realm of a standing wave, as i s the case of a l i q u e f i e d sloped model test deposit, or in the case of unrestrained flow as occurs during deposition in the thickened discharge method, the effect of the boundary layer must be considered. This point was discussed in Chapter 5. In summary, the predicted displacements, using a viscous f l u i d stand wave model, accurately describe the shape of actual displaced v e r t i c a l lines as obtained in the model tests. The test r e s u l t s imply that a flow f a i l u r e in the f i e l d could result from l i q u e f a c t i o n of a sloped non-plastic t a i l i n g s deposit. Distance of Section from Upstream Boundary (in cm) 10 cm 20 cm 30 cm 0 5 0 5 0 5 0 Displacement (cm) Maximum Acceleration = .05g LEGEND • Actual Displacements — . Predicted Displacements COMPARISON OF PREDICTED AND OBSERVED SLOPE MOVEMENTS FOR 4° SLOPE F I G U R E 5 6 111 CHAPTER 11 FLOW FAILURES In the event of f a i l u r e of a t a i l i n g s f a c i l i t y , large Q u a n t i t i e s of m a t e r i a l can flow downstream, r e s u l t i n g i n an extensive inundation zone. S e v e r a l c a t a s t r o p h i c f a i l u r e s have considerable attention in the l i t e r a t u r e . The most prom-inent of these are the Aberfan disaster (Jeyapalan, 1980), the Buffalo Creek f a i l u r e (Wahler and Schlick, 1976), and the E l Cobre dam f a i l u r e (Dobry and Alvarez, 1967). These f a i l u r e s a l l inolved the liqu e f a c t i o n and flow of mine t a i l i n g s . A detailed discussion of these f a i l u r e s i s not considered necessary here, and i t su f f i c e s to report that hundreds of l i v e s were l o s t and hundreds of millions of do l l a r s damage resulted. Jeyapalan (1980) used a viscous f l u i d model to accurately predict the flow c h a r a c t e r i s t i c s and extent of flow" involved in these, and other, f a i l u r e s . . The consideration of a boundary layer was fundamental in determining the extent of flow. It was determined that a given f l u i d could come to rest on a variety of slopes, the flow distance being longer for steeper slopes. Of p a r t i c u l a r i n t e r e s t in t h i s study are flow f a i l u r e s occurring at mild slopes. Casagrande (1971), Youd (1973) and Castro (1969) provide limited descriptions of such f a i l u r e s . A f a i l u r e discussed by Williams (1978) i s the most applicable case h i s t o r y to the present study. The t a i l i n g s d i s p o s a l scheme c o n s i s t e d of d e p o s i t i n g coarse t a i l i n g s at slopes of 3 to 4 degrees, s i m i l a r to slopes used i n the present study. The 1 12 m a t e r i a l h a s s i m i l a r g r a i n s i z e c h a r a c t e r i s t i c s t o t h a t o f t h e t e s t s a n d , a s shown i n F i g u r e 57. The t a i l i n g s s a nd i s s l i g h t l y c o a r s e r and l e s s u n i f o r m . The d e p o s i t c o n f i g u r a t i o n i s shown i n F i g u r e 58. N o t e t h e s i m i l a r i t y t o t h e t e s t s a n d d e p o s i t a l s o shown i n F i g u r e 58 ( a s d i s c u s s e d i n C h a p t e r 8, m o d e l t e s t r e -s u l t s f o r s l o p e d b a s e d e p o s i t s e x h i b i t e d no n o t i c e a b l e d i f f e r -e n c e i n e i t h e r f i n a l s l o p e a n g l e o r t h r e s h o l d a c c e l e r a t i o n ) . The t a i l i n g s d e p o s i t h a d e x h i b i t e d p r e v i o u s i n s t a b i l i t y due t o e x c e s s p o r e p r e s s u r e s g e n e r a t e d d u r i n g c o n s t r u c t i o n , a n d r e m e d i a l m e a s u r e s w e r e e m p l o y e d t o o b t a i n a s t a t i c a l l y s t a b l e d e p o s i t . The a p p a r e n t l y s t a b l e d e p o s i t t h e n e x p e r i e n c e d l i q u e -f a c t i o n due t o an unknown s o u r c e , a s e v i d e n c e d by s a n d b o i l s and t h e e x t e n t o f f a i l u r e , and mass movement o c c u r r e d d o w n s l o p e . T h e m a t e r i a l , a l t h o u g h s t a t i c a l l y s t a b l e a t 3 t o 4 d e g r e e s , f l o w e d and p r o d u c e d u p t h r u s t i n g a t t h e t o e o f t h e s l o p e . The m o d e l t e s t d e p o s i t e x h i b i t e d v e r y s i m i l a r b e h a v i o u r , and w o u l d t h e r e f o r e a p p e a r t o be r e a s o n a b l y r e p r e s e n t a t i v e o f f i e l d b e h a v i o u r f o r t h i s p a r t i c u l a r c a s e h i s t o r y . 1 1 3 Percent Finer By Weight 100 0.05 Particle Diameter Sand Silt TAILINGS FLOW FAILURE CASE HISTORY GRAIN SIZE DISTRIBUTION F I G U R E 5 7 FIGURE 5 8 1 1 5 CHAPTER 12  CONCLUSIONS A shaking table model study has been performed to i n v e s t i -gate the s t a b i l i t y of sloped, cohesionless, saturated deposits during and after c y c l i c - l o a d i n g . Of p a r t i c u l a r interest i s the s t a b i l i t y of sloped t a i l i n g s deposits created using the thick-ened discharge disposal method. In addition to model testing, a l i t e r a t u r e review of conventional disposal techniques, t y p i c a l t a i l i n g s material properties and the behaviour of viscous f l u i d s was performed. Based on the r e s u l t s obtained i n t h i s study, the following conclusions are made: 1. Conventional t a i l i n g s disposal techniques are subject to li m i t a t i o n s such that the development of alternative methods of disposal i s warranted. The upstream construction method does not, in general, meet s t a b i l i t y requirements, while the downstream construction method res u l t s in extremely high costs for design, construction and abandonment. 2. The thickened discharge method of disposal has proven to be an economic alternative of questionable s t a b i l i t y . 3. The model test sand used has a liq u e f a c t i o n resistance curve similar to that of ty p i c a l cohesionless t a i l i n g s material. 4. The post-liquefaction response of the model test sand i s reasonably representative of cohesionless t a i l i n g s material under model test conditions. 1 16 5. When subjected to horizontal base accelerations s u f f i c i e n t to cause complete l i q u e f a c t i o n of the model test deposit a s i g n i f i c a n t reduction in the deposit slope was obtained. The f i n a l slope was observed to be approximately one per-cent. A l l model test deposits were formed at a Relative Density of approximately 30 per cent, which translates to a s o l i d s f r a c t i o n s i m i l a r to that obtained i n a t y p i c a l thickened discharge deposit. 6. A c r i t i c a l acceleration was observed for each i n i t i a l slope, above which complete l i q u e f a c t i o n was obtained. The c r i t i -cal acceleration was found to increase with i n i t i a l slope angle, and ranged from approximately .035g, at 2 degrees, to .060g, at 8 degrees. 7. The model test results cannot be r e l i a b l y extrapolated to the f i e l d u n t i l further studies are made to quantitatively assess model geometry e f f e c t s . 8. Model test deformations can be empirically predicted using a viscous f l u i d model that accounts for boundary layer ef-fects. Based on the success of the viscous model to predict model t e s t r e s u l t s , combined with a review of p e r t i n e n t l i t e r a t u r e , i t is suggested that l i q u e f i e d t a i l i n g s can be viewed as a viscous f l u i d . 9. A sloped deposit l i q u e f i e d to a considerable depth i s not n e c e s s a r i l y s t a b l e j u s t because i t i s s t a t i c a l l y s t a b l e p r i o r to 1 i a u e f a c t i o n . P o s t - l i q u e f a c t i o n s t a b i l i t y i s 1 17 dependent on the depth of l i q u e f a c t i o n as well as the properties of the l i q u e f i e d material. 1 18 REFERENCES 1. Almes, R. 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J . , Maartaman, C. H., Lo, R. C. Y.- and Finn, W. D. L., (1978), "Simplified Seismic Analysis for Ta i l i n g s Dams", ASCE Specialty Conference on Earthquake Engineering and S o i l Dynamics, Pasadena, C a l i f . , pp. 540-556. 30. Klohn, E. J . , (1979), "Design and Construction of T a i l i n g s Dams, "Undergraduate Class Notes", UBC, Vancouver. 31. Kunhausen, G. H., (1978), "Tailings Disposal at the Bear Creek Uranium Project", T a i l i n g s Disposal Today, Vol. 2, pp. 109-124. 32. Lamb, S i r H., (1945), "Hydrodynamics", 6th Edition, Dover Publications, New York, N. Y. 33. Lambe, T. W. , (1951), " S o i l Testing or Engineers", John Wiley & Sons, New York, N. Y. 34. Lambe, T. W. , (1951), " S o i l Testing for Engineers", John Wiley & Sons, New York, N.Y. 35. L i u , A. K. and Davis, S. H., (1977), "Viscous Attentuation of Mean D r i f t in Water Waves", Journal of Flu i d Mechanics, Vol. 81, Part 1, pp. 63-84. 36. M i t t a l , H. K., (1974), "Design and Performance of Ta i l i n g s Dams", Ph. D., Thesis, University of Alberta, Edmonton. 37. Murthy, Y. K., Thomas, K., Rau, S. G. and Srinivosa, A., (1976), "Mine T a i l i n g s Dams in India", Proc. ICOLD, Mexico, Vol. 1, pp. 599-612. 38. Nyren, R. H. , Haakonsen, R. A. and M i t t a l , H. K., (1978), "Disposal of Tar Sand T a i l i n g s at Syncrude Canada Ltd", T a i l i n g s Disposal Today, Vol. 2, pp. 54-74. 39. Ponce, V. M. and B e l l , J . M., (1971), "Shear Strength of Sand at Extremely Low Pressures", ASCE S o i l Mechanics and Foundation D i v i s i o n , SM4, Vol. 97, pp. 625-638. 40. Prakash, S., (1977), "Seismic Response of S o i l Deposits, Embankments, Dams and Structures", Proc. 9th International 121 Conference on S o i l Mechanics and Foundation Engineering, Vol. 2, Tokyo, pp. 624-630. 41. Ramsay, S., (1981), Masters Thesis in progress, UBC, Van-couver. 42. Richart, J r . , F. E., (1977), "Dynamic Stress-Strain Rela-tionships for S o i l s " , Proc. 9th International Conference on S o i l Mechanics and Foundation Engineering, Vol. 2, Tokyo, pp. 605-612. 43. Roberson, J.' A. and Crowe, C. T. , (1975), "Engineering F l u i d Mechanics", Houghton M i f f l i n Co., Boston. 44. R o b i n s k i , E. I., (1975), "Thickened Discharge - A New Approach to T a i l i n g s D i s p o s a l " , B u l l e t i n , The Canadian I n s t i t u t e of Mining and M e t a l l u r g y , D e c , pp. 47-59. 45. Robinski, E. I., (1975), "Tailings Disposal By Thickened Discharge Method for Improved Economy and Environmental C o n t r o l " , T a i l i n g s D i s p o s a l Today, V o l . 2, pp. 75-95. 46. Salazar, R. C. and Gonzales, R. I., (1972), Design, Con-s t r u c t i o n and Operation of the T a i l i n g s P i p e l i n e s and Underwater Disposal System of Atlas Consolidated Mining and Development Corporation in the Phil i p p i n e s " , T a i l i n g s Disposal Today, Vol. 1, pp. 477-511. 47. Sandic, G., (1978)/ "Tailings Dam for Zletoro Mine", T a i l -ings Disposal Today, Vol. 2, pp. 254-268. 48. Sc h l i c h t i n g , H. , ( 1955), "Boundary Layer Theory", McGraw-H i l l Book Co. Inc., New York, N.Y. 49. Seed, H. B. and I d r i s s , I. M., (1970), " S o i l Modulii and Damping Factor or Dynamic Response Analyses", University of C a l i f o r n i a , Report No. EERC 70-10. 50. Seed, H. B., Martin, P. P. and Lysmer, J . , (1976), "Pore-water Pressure Changes During S o i l L i q u e f a c t i o n " , ASCE Geotechnical Eng. Div., Vol. 102, GT4, pp. 323-346. 51. Seed, H. B., (1979), " S o i l Liquefaction and C y c l i c Mobility Evaluation for Level Ground During Earthquakes", ASCE Geo-technical Eng. Div., Vol. 105, GT2, pp. 201-255. 52. Siddharithan, R., (1981), "R. , (1981), " S t a b i l i t y of Buried Pipelines Subjected to Wave Loading", Masters Thesis, UBC, Vancourver. 53. Sorenson, R. M., (1978), "Basic Coastal Engineering", John Wiley & Sons, New York, N.Y. 1 22 54. Tanimoto, R., (1967), "Liquefaction of Sand Layer Subjected to Shock and Vibratory Loads", Third Asian Regional Con-ference on S o i l Mechanics and Foundation Engineering, Vol. 1, pp. 362-365. 55. Taylor, R. K., Kennedy, G. W. and MacMillan, G. L., (1978), " S u s c e p t i b i l i t y of Coarse-grained Coal-mine D i s c a r d to Liquefaction", 3rd International Congress of th Int. Ass'n of Engineering Geologists, Madrid, pp. 91-100. 56. Taylor, R. K., MacMillan, G. I. and Morrell, G. R., (1978), " L i q u e f a c t i o n Response of•Coal-mine T a i l i n g s to E a r t h -quakes", 3rd International Congress of the Int. Ass'n of Engineering Geologists, Madrid, pp. 79-90. 57. Taylor, R. K. and Morrell, G. R. , (1979), "Fine Grained C o l l i e r y Discard and Its S u s c e p t i b i l i t y to Liquefaction and Flow Under C y c l i c Stress", Engineering Geology, No. 14, pp. 219-229. 58." Vaid, Y. P. and Finn, W. D. L. , ( 1979), "Static Shear and L i q u e f a c t i o n P o t e n t i a l , ASCE Geotechnical Engineering D i v i s i o n , Vol. 105, GT10, pp. 1233-1246. 59. Vaid, Y. P., (1981), Personal Communication. 60. Wahler, W. A. and S c h i c k , D. P., (1976), "Mine Refuse Impoundments in the United States", Proc. ICOCD, Mexico, Vol. 1, pp. 279-319. 61. Wasp, E. J . , Kenny, J . P. and Ghandi, R. L., ( 1 977 ), "Solid-Liquid Flow Slurry Pipeline Transportation", Trans Tech Publications, Clausthal, Germany. 62. Watermeyer, P., Williamson, R., (1978), "Ergo T a i l i n g s Dam-Cyclone Separation Applied to a Fine Grind Product", T a i l i n g s Disposal Today, Vol. 2, pp. 369-396. 63. W i l l i a m s , M. P. A., (1978), " T a i l i n g s Dam F a i l u r e Case History", T a i l i n g s Disposal Today, Vol. 2, pp. 428-433. 64. Yemington, E. G., (1970), "Suggested Method of Test for Minimum Density of Nonchoesive S o i l s and Aggregates", ASTM STP 479, p. 125. 65. Yen, B. C , (1967), " V i s c o s i t y of Saturated Sand Near Liquefation", International Symposium on Wave Propogation and Dynamic Properties of Earth Materials, New Mexico, pp. 877-888. 66. Yoshimi, Y., (1967), "An Experimental Study of Liquefaction of Saturated Sands", S o i l and Foundation, Vol. 7, No. 2, pp. 20-32. 1 23 67. Yoshimi, Y., Fumio, K. and Kohji, T. , ( 1975), "One Dimen-sional Volume Change Ch a r a c t e r i s t i c s of Sands at Very Low Confining Pressures", S o i l and Foundation, Vol. 15, No. 3, pp. 51-60. 68. Yoshimi, Y., (1977), "Liquefaction and C y c l i c Deformation of S o i l s Under Undrained C o n d i t i o n s " , Proc. 9th I n t e r -national Conference on S o i l Mechanics and Foundation Engi-neering, Vol. 2, Tokyo, pp. 613-623. 69. Youd, T. L., (1973), " L i q u e f a c t i o n , Flow and A s s o c i t e d Ground F a i l u r e " , Geological Survey C i r c u l a r 688, Washing-ton, D. C. 1 24 APPENDIX 1 Parameter Selection The test parameters considered are the slope, downstream boun-dary height, acceleration and frequency. In selecting these parameters use was made of similitude requirements, previous model studies, available a n a l y t i c a l tools and preliminary tests in the present study. As mentioned p r e v i o u s l y , s i m i l i t u d e requirements need not s t r i c t l y be met, however they can be used as a useful guideline in parameter selection. Similitude requires that both geometric and dynamic s i m i l a r i t y be s a t i s f i e d (Ishihara, 1967). For complete geometric si m i l i t u d e , both deposit dimensions and surface displacements must have s i m i l a r i t y , F i g . A l ( a ) . How-ever, Ishihara shows that very large accelerations are required to s a t i s f y complete geometric s i m i l i t u d e . I t i s t h e r e f o r e p r a c t i c a l to s a t i s f y only incomplete similitude. Where A denotes the scale r a t i o ; A = J? m/^fp = nm/np must be sat-i s f i e d for incomplete geometric si m i l i t u d e , Jl being length and h being height. Incomplete similitude, for a model to f i e l d acceleration r a t i o of 1.0, was s a t i s f i e d for t h i s study. The model test therefore does not s t r i c t l y represent the protype behaviour of the deposit p r i o r to l i q u e f a c t i o n as model surface displacements required for complete similitude are extremely h igh. Dynamic similitude must s a t i s f y e l a s t i c and i n e r t i a l forces in t h i s analysis. .01 El Centro Ap=15 cm T p » . 1—4.0 sec ap-.2g 1 b) Prototype parameter* 1/1000 1/750 1/500 Am/Ap c) Model parameters. SIMILITUDE DATA F I G U R E A 1 126 A i = A ^m/Yp am/ap Ae = A £*m/£p Gm/Gp Similitude requires that Ai = Xe, therefore *m/flp = V A G m/G p y p / d - m <fm/^p Sa t i s f y i n g incomplete geometric similitude, and assuming; £~A/H, G m = Gp and Ym = Yp implies «m / a P = V A * A m/A p T m = A Tp (T = period) Ishihara provides prototype data to use in a similitude study, Fi g . A l ( b ) . This data impies incomplete similitude has been s a t i s f i e d for A = 1/25 in t h i s study. For a m/ap = 1, one obtains Am/Ap = 1/750.. This surface d e f l e c t i o n corresponds to £" m = .1%, which i s the approximate l e v e l of s t r a i n p r i o r to l i q u e f a c t i o n in the c y c l i c t r i a x i a l t e s t s . The above analysis implies that the parameters used i n the model test appear rea-sonable, and the predicted s t r a i n s of .1% implied by incomplete similitude i s reasonable for a t y p i c a l earthquake record. For the geometric configuration of the model test the proba-b i l i t y of piping at the downstream boundary was assessed. The program "SEEPAGE", developed at UBC using f i n i t e element tech-niques, was used to obtain a flow net, p r o v i d i n g a maximum hydraulic gradient of i = .11, well below the value of 1.0 1 27 r e q u i r e d f o r p i p i n g . No p i p i n g occurred during any of the tests. Early tests were performed with a constant upstream s o i l depth and v a r i a b l e downstream depths to obtain the d e s i r e d slope angle. It was found that there existed, for a given accelera-ti o n and slope, a c r i t i c a l depth at which complete liq u e f a c t i o n did not occur.. This phenomenon was investigated using "STAB.W" (Siddharthan, 1981). This program calculates the incremental pore pressure increase due to c y c l i c shearing and allows for d i s s i p a t i o n of incremental excess pore pressures, while chang-ing Mv to account for the reduced e f f e c t i v e stress. Note that t h i s analysis i s necessary to analyze the drained condition of the.model t e s t , whereas a t y p i c a l t a i l i n g s d eposit could be treated as undrained without serious error. Typical i n i t i a l test results are shown in F i g . A2. The abrupt change in slope was a consistent feature in tests where only a portion of the s o i l mass l i q u e f i e d . i t is assumed that s o i l upstream from the change had t o t a l l y l i q u e f i e d , and large par-t i c l e displacements were recorded. Relatively small p a r t i c l e displacements occurred in the nonliquefied material. In order to perform the analysis, the l i q u e f a c t i o n resistance curve, adjusted f o r s t a t i c shear, was used to determine the numbers of cycles to l i q u e f a c t i o n under a /0"1 1 = .18, cor-responding to amax = - u 5 9- T n e i s o t r o p i c pore pressure generation curve shown in F i g . A3(a) was used, and t r i a x i a l test data implied using a value of •& = .7. A back analysis of test TYPICAL PARTIAL LIQUEFACTION RESULTS, FIGURE A 2 00 DC a> 3 0. •2 .4 .6 Cycle Ratio N/NL. .8 1.0 a)Theoretical and Experimental Pore Pressure Generation Curves Depth (cm) b) Analytical Pore Pressure Generation PORE PRESSURE GENERATION. F I G U R E A 3 M VO 1 30 r e s u l t s provided a c o m p r e s s i b i l i t y s l i g h t l y lower than that determined i n S e c t i o n 5-2-2 ( C v = .4 f t ^ / s ) . T h i s could be expected using the i s o t r o p i c pore pressure generation curve, as the pore pressure generation i s more rapid than i f anisotropy were accounted for. It should be noted that the r e s u l t s of the a n a l y s i s confirm q u a l i t a t i v e l y the observations made during testing. Typical, maximum pore pressure r a t i o s obtained for varying depths are shown in F i g . A3(b). This curve implies that there i s indeed a depth of s o i l at which the response of the deposit changes dramatically. This depth corresponds to the abrupt change in the slope obtained in model tests. Although the res u l t s of the model test can be explained by the analysis, parameters, such as compressibility, used may not be correct. The pore pressure generation curve applies to i s o -t r o p i c a l l y consolidated samples, while anisotropic conditions exi s t in the model. This has the affect of "stretching" the pore pressure generation curve, but would not e l i m i n a t e the exi s t e n c e of a c r i t i c a l height at which l i q u e f a c t i o n would occur, i t would merely increase mv required to provide a given c r i t i c a l height. The use of consistent input parameters ( & = .7, mv = .4 p s f - ^ ) s u c c e s s f u l l y p r e d i c t e d the appropriate c r i t i c a l height for va r i o u s slopes, which have d i f f e r e n t l i q u e f a c t i o n p o t e n t i a l curves. The c r i t i c a l heights determined experimentally, coupled with a n a l y t i c a l r e s u l t s , were used to establish a downstream 131 boundary height for which the entire s o i l mass would li q u e f y under the excitations used i n the l a t t e r portion of the testing program. The selected height proved to be satisfactory for t h i s purpose. Another a n a l y s i s was performed to assess the choice of the depth of the deposit. This e l a s t i c analysis involved estimat-ing the frequ e n c i e s of the f i r s t 5 modes of v i b r a t i o n . The analysis requires the selection of a shear modulus and the em-p i r i c a l r e l a t i o n proposed by Seed and Id r i s s (1970) was used, where G m a x = 1000(K2)max( m') 1^ 2 P s f « Appropriate values, corresponding to tes t conditions and various depths were used to 5 o b t a i n G m a x . The value of G m a x = 1 x 1 0 psf i s supported by the equation proposed by Richart (1977). It was established that the table did not impart accelerations near the natural frequency of the deposit. Confidence i n the procedure was es-tablished by predicting the natural frequency obtained experi-mentally by Finn et a l . (1969) to within 10%. The accelerations used (.03 g - .10 g) are f e l t to be reasonably representative of f i e l d accelerations that might be expected. The sinusoidal frequency of 5 Hz was chosen for aforementioned p r a c t i c a l considerations and i s f e l t to be representative of earthquake motion. De Alba et a l . (1976) consider a frequency of 4 Hz to be a representative frequency. A l l tests were run for the a r b i t r a r y number of 20 cycles, which i s considered to be a r e a l i s t i c number of uniform cycles for a se i s m i c event. Seed et a l . (1976) suggest that 20 uniform 1 32 cycles are appropriate for an earthquake of Richter magnitude of 7-1/2. In the determination of the appropriate design earthquake for a given s i t e the s i t e geology, areal attenuation c h a r a c t e r i s t i c s , distance from causitive f a u l t and extent and depth of rupture should be estimated for a r e a l i s t i c evaluation of the s o i l res-ponse to be determined. The a c c e l e r a t i o n s , frequencies and durations of the model test discussed herein are not represen-tative of any p a r t i c u l a r seismic event, rather they are con-sidered to be reasonably representaive of possible conditions and were chosen to produce li q u e f a c t i o n of the model deposit in order to observe the post-liquefaction behaviour of the mate-r i a l . 

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