Continuous Failure State Direct Shear Tests

7/29/2019 Continuous Failure State Direct Shear Tests

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Rock Mechan ic sand R ock Eng inee r ing 17, 83--95 (1984)R o c k M e c h a n i c sa n d R o c k E n g i n e e r i n g9 by Springer-Verlag 1984

C o n t i n u o u s F a i lu r e S ta t e D i r e c t S h e a r T e s t sB y

A . T i s a a n d K . K o v f i r iSwiss Fede ra l In s t i tu t e o f Tech no logy , Z ur i ch , Swi tze r land

S u m m a r yTh e pape r dea ls w i th a new t e s t ing p rocedure a imed a t de te rmin ing the fa ilu reenvelopes for peak and residua l s t rength in d i rec t shear tes ts . In a "cont inuousfa i lu re s t a t e " d i rec t shea r t e s t t he spec imen i s ma in ta ined in a s t a t e o f pe rmanen ts l id ing whi le the shear and normal s t ress a re be ing s teadi ly changed. Fi rs t of a l l ,t he spec imen is b roug h t to a s t a t e o f f a ilu re in the conven t iona l mann e r a t a chosencons tan t no rm a l s tr ess. The n unde r m ono tono us ly inc rea s ing shea r d i sp lacemen t thenorma l s t r e ss i s con t inuous ly ad jus t ed so tha t a s t r a igh t l i ne i s p roduced in theshear s t ress-d isp lacement p lane . Both an increase in the s t resses and a decrease is

possible. Th e p rope r se lec t ion of the inc l ina t ion of the s t ra ight l ine ma y involve ast ress pa th which corresponds c lose ly to the fa i lure enve lope of the spec imen. Inthe ca se o f smoo th jo in t su r face s o r in the re s idua l s t r eng th s t a t e o f rough su r face si t i s possib le to de te rmine exac t ly the fa i lure enve lope wi th the a id of a s ingle tes tspec imen. The paper a lso descr ibes a newly deve loped shear tes t appara tus su i tablefo r combina t ion w i th soph i s t i c a t ed se rvo -con t ro l l ed load ing mach ines gene ra l lyava i lable in rock mechanics labora tor ies .1. In t roduc t i on

I f o n e c o m p a r e s t h e r e s u l t s o f c o n v e n t i o n a l t r i a x i a l t e s t s w i t h t h o s e o fd i r e c t s h e a r t e s t s o n j o i n t s o r p l a n e s o f w e a k n e s s a c o n s i d e r a b l e s i m i l a r i t ym a y b e o b s e r v e d . T h e c u r v e s r e p r e s e n t i n g t h e r e l a t i o n s h i p b e t w e e n a x i a ls t re s s a n d a x i a l s t r a i n i n t h e t r i a x i a l t e s t e x h i b i t b a s i c a l l y t h e s a m e f o r m a st h o s e f o r s h e a r d e f o r m a t i o n a n d s h e a r f o r c e i n t h e d i r e c t s h e a r t e s t , i n c l u d -i n g t h e c h a r a c t e r i s t i c s f o r p e a k a n d r e s i d u a l s t r e n g t h ( Fig . 1 ). T h e s e s i m i-l a ri ti e s a r e n o t m e r e l y s u p e r f i c ia l b u t a r e c a u s e d b y t h e n a t u r e o f c e r ta i np h y s i c a l p ro c e s s e s t a k i n g p l a c e in t h e t e st s. A l t h o u g h t h e a n a l o g y h a s i t sl i m i t s , i t i s j u s t i f i e d t o i n v e s t i g a t e t h e p o s s i b i l i t y o f a p p l y i n g a n a l o g o u s t e s tp r o c e d u r e s . I t is in f a c t e v i d e n t f r o m t h e l i t e r a tu r e t h a t p r o c e d u r e s a r e o c -c a s i o n a l l y a p p l i e d i n t h e d i r e c t s h e a r t e s t c o r r e s p o n d i n g t o t h o s e i n t h ec o n v e n t i o n a l a n d m u l t i - s t a g e t r i a x i a l t e s t s .

I n t h e fo l l o w i n g i t w i l l b e s h o w n h o w t h e c o n c e p t o f th e s o - ca ll ed" c o n t i n u o u s f a i l u r e s t a t e t r i a x i a l t e s t " ( K o v ~ i r i e t a l ., 19 83 ) m a y b e d i r e c t l y6 Rock Mechan ics, VoI. 17/2


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84 A. Tisa and K. Kov~ri :ad ap t ed t o t h e d i r ec t s h ea r t e s t . I n t r i ax i a l t e s t s t h e co n f i n i n g p r e s s u r e an dt h e ax i a l s t r e s s a r e ap p l i ed s u ch a s t o cau s e t h e t e s t s p ec i men t o b e p e r -man en t l y i n a s t a t e o f f a i l u r e . I n t h i s w ay i t i s p o s s i b l e w i t h t h e a i d o f as in g le s p e c i m e n t o o b t a i n a t le a s t p a r ts o f th e f a i lu r e e n v e l o p e f o r b o t h t h ep e ak a n d r e s i d u a l s t r en g t h . S i mi l a rl y , th e n e w t y p e o f d i r ec t s h ea r te s t a i msa t d e te r m i n i n g p o r t i o n s o f t h e c o r r e s p o n d i n g f a i l u r e s u r fa c e s , r a th e r t h a nj u s t s o m e s in g le p o i n t s o n t h e m . S i nc e i t is a d v a n t a g e o u s t o h a v e s e r v o -c o n t r o l o f t h e s h e a r d e f o r m a t i o n w h e n d e t e r m i n i n g t h e p e a k s t r e n g th , f i rs t

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F i g . 1 . C h a r a c t e r i s t i c r e s u l t s o f d i r e c t s h e a r t e s t s o n r o c k j o i n t s i n d i c a t i n g s i m i l a r i ty t ot r i a x i a l t e s t r e su l t s

o f al l t h e c o n s t r u c t i o n o f a n a p p a r a t u s s u i t a b le f o r c o m b i n a t i o n w i t h a s ti ffs e r v o - c o n t r o l l e d l o a d i n g m a c h i n e is d e s c ri b e d . B y t h is m e a n s a n e x i s t in gv e r y ex p en s i v e s y s t em, a s s u ch a mach i n e i s , may b e u t i l i z ed f o r d i r ec t s h ea rt es t s as wel l .

2 . T h e D i r e c t S h e a r A p p a r a t u sL o m b a r d i a n d D a l V e s c o (1966) h a v e r e p o rt e d o n a s h ea r a p p a r a tu sw h i c h w o r k s t o g e t h e r i n c o m b i n a t i o n w i t h a n o r d i n a r y i n s i t u c o n c r e t e t e s t -i n g m a c h i n e . T h e y h a v e a l s o d i s c u s s e d a t e s t i n g p r o c e d u r e a i m e d a t d e t e r -mi n i n g t h e f r i c t i o n co e f f i c i en t o f a j o i n t b y mean s o f a s i n g l e s h ea r t e s t .T h e a p p a r a t u s d e s c r i b e d i n t h i s p a p e r i s b a s e d o n a s i m i l a r p r i n c i p l e , b u tb e c a u s e o f u s e i n l a b o r a t o r y t e s t i n g i t s d e t a i l s m a t c h t h e h i g h e r t e c h n i c a l

an d s c i en ti fi c s t an d a r d s r eq u i r ed .F ig . 2 s h o w s t h e te s t s p e c im e n c o n t a i n e d i n a s h e a r b o x . T o t r a n s m i t

f o r c e t o t h e s p e c i m e n i t is e n c a s t r e d i n e p o x y r e s in i n t h e u s u a l m a n n e r .T h e s h e a r b o x f i t s e x a c t l y b e t w e e n t w o m a s s i v e L - s h a p e d s t e e l p i e c e s . T h en o r ma l f o r ce ( h y d r au l i c j a ck ) an d t h e s h ea r f o r ce ( t e s t i n g mach i n e ) a r e ap -p l i e d t h r o u g h o n e o f th e s e L - s h a p e d s te e l p i ec e s . T h e o t h e r L - s h a p e d s t e e l


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Continuous Failure State Direct Shear Tests 85p i e c e p r o v i d e s t h e r e a c t i o n s , w h i c h a r e t r a n s m i t t e d t o a s t i f f h o r i z o n t a l l yl y in g f r a m e a n d t o t h e l o w e r l o a d i n g p l a t e o f t h e s e r v o - c o n t r o l l e d p r e ss .Th i s i s a ch i ev ed b y r e s t i n g t h e L- s h ap ed s t ee l p i ece ag a i n s t t h e f ro n t s i d eo f t h e f r a m e a s w e l l a s o n a c y l i n d r i c a l b e a r i n g w h i c h r e st s o n t h e l o w e rl o a d i n g p l a te . T h e s e t ti n g - u p o f th e s h e a r b o x w i t h t h e e n c a s t r e d te s t s pe c i-m e n i n t h e L - s h a p e d s te e l p i ec e s to g e t h e r w i t h t h e s t if f f r a m e is c a r r i e d o u t

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' - - - - ' - . . . . . . . .- - / , , ,_ _ _ ,~DRAUL,O~. . . . .ACK -- /- - i 'LO W ER AC LELOWERABLE BALL , CALLCASTOR A ) ~ ' ~ SPHERI~CAL O INTI ' ~ ' ~ I I 'LOWERABLE ZYLINDRICALj R O L L E R S [ C ) II J O I N T . M O V A B L E A S S A M B L Y T A B L EF i g . 2 . S c h e m a t i c a l v i e w o f th e s h e a r a p p a r a t u s u s e d in c o m b i n a t i o n w i t h a l o a d i n g m a c h i n e

o n a n a s s e m b l y t a b l e b e s i d e t h e p r e s s . A t t h e s a m e t i m e t h e t r a n s d u c e r s f o rm e a s u r i n g s h e a r a n d n o r m a l d e f o r m a t i o n s a r e m o u n t e d . W i t h t h e a i d o f t h el o werab l e b a l l c a s t o r s (A) an d (B ) a s we l l a s t h e ro l l e r s (C ) t h e s h ea r ap p a -r a t u s , w h i c h i s n o w r e a d y f o r t es t in g , c a n b e e a s i ly i n s e r te d b e t w e e n t h el o a d i n g p l a t e s a n d t h e c o l u m n s o f t h e t e s t i n g m a c h i n e . A f t e r l i f t i n g u p t h eb a ll c a s t o r (A ) n o f u r t h e r f o r c e t r a n s m i s s i o n c a n t a k e p l a c e a t t h i s p o i n t.T h e c y l i n d r ic a l j o i n t t r a n s m i t s t h e v e r t i c a l l o a d i n g t o t h e l o w e r l o a d i n g p l a t eo f t h e p r e ss . S i nc e t h e b e a r i n g (B ) c a n o n l y t r a n s m i t c o m p r e s s i v e f o r c e st h e a x is o f t h e c y l i n d r i c a l j o i n t a n d t h e c e n t r e o f t h e s p h e r i c a l j o i n t ( a bo v e )m u s t l ie in a v e r t i ca l p la n e . A s a n a d e q u a t e m e a s u r e t o p r e v e n t l i f ti n g - o ff o ft h e f r am e a t t h e b a l l j o i n t (B ), a s m a l l e ccen t r i c i t y o f t h e cen t r e o f t h es p h e r i c a l j o i n t ( to t h e r i g h t o f t h e v e r t ic a l p l a n e t h r o u g h t h e c y l i n d e r j o in t )is f o r e s e e n . A n e c c e n t r i c it y o f 5 m m f o r e x a m p l e is s u f f ic i e n t a n d d o e s n o ti n f l u e n c e t h e d i s t r i b u t io n o f t h e n o r m a l s t re s s o n t h e j o i n t s u r f a c e t o a n ya p p r e c i a b l e e x t e n t .T h e s h e a r a p p a r a t u s is d e s i g n e d i n s u c h a w a y t h a t b o t h p a r t s o f t h et e st s p e c i m e n c a n d i s p la c e a n d r o t a t e w i t h r e s p e c t t o e a c h o t h e r w i t h o u tc o n s t r a i n t s . T h i s i s m a d e p o s s i b l e b y t h e l o a d t r a n s f e r t h r o u g h t h e c r o s s r o l la n d t h e s p h e r i c a l j o i n t a r r a n g e m e n t . T h e c e n t r e o f t h e s p h e r e i s a l i g n e dw i t h t h e c e n t r e o f t h e j o i n t s u r f a c e a t t h e s t a r t o f t h e t e s t .

A f a i r ly l o n g h y d r a u l i c ja c k w a s c h o s e n i n o r d e r t o m i n i m i z e t h e c o m -p o n e n t o f t h e j a c k f o r c e d u e t o i t s r o t a t i o n u n d e r t h e a c t i o n o f t h e s h e a rd e f o r m a t i o n s . T h e h i n g e d lo a d c e ll p l a c e d b e t w e e n i t a n d t h e s p e c im e n w a ss e l e c t e d o n a c c o u n t o f i t s h i g h a c c u r a c y , s i m p l e c a l i b r a t i o n a n d i t s d i r e c t6 *


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86 A.T isa and K. KovSri:e lectrical s ignal . The measuring system consists of load cel ls (S and N) and6 d isp lacement t ransducers . The measurement of the re la t ive d i sp lacementof the joint surfaces in the direct ions of N and $ is carried out direct ly a tthe join t i tse lf, on bot h sides of the test specimen, i~ut a lso b y observ ing th e

Fig. 3. The shear apparatus resting on the movable assembly table

Fig. 4. The shear apparatus together with the servo-controlled stiff loading m achine duringa testre la t ive movements of the loading pla tes and the L-shaped steel pieces. I t iskn ow n tha t tim tw o me a sur ing a r ra ll geme n t s g ive d i f fe re n t r e suks ( C r a w -f o r d , 1 9 80 ; L e i c h n i t z , 1981).


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Continuous Failure State Direct Shear Tests 87In Fig. 3 the shear test apparatus ready for use and resting on the

movable assembly table is shown. Fig. 4 shows the apparatus in combina-tion with the loading machine. The stiff four columned loading press witha capacity of 3000 kN may be seen on the left side of the figure. To itsright is the electronic control and recording unit. In the foreground the stiffframe resting upon the movable assembly table and containing the relativelylong hydraulic jack can be seen. Fig. 5 shows a close-up photo of the detailsof the shear box between the two L-shaped steel pieces with the arrange-ment of the three inductance type transducers for measuring shear and

Fig. 5. Close-up view of the shear box with the displacement gauges mounted directlyon the jointnormal deformation as well as any rotation. At the top of the picture is thespherical joint, having two parts, as well as the upper part of the cross-rolljoint. Since the assembly table used for setting-up the test as well as the stiffframe which rests on it can be moved very easily, a shear test can be pre-pared very quickly and after its execution the shear apparatus may be re-moved so that the loading press is free for other purposes. The mobility ofthe shear testing apparatus is also very advantageous from the point of viewof efficient use of laboratory space. The maximum normal stress and shearstress which can be applied to a joint surface of the size of 24 x 12 cm are20 N/ mm 2 and 100 N/m m 2 respectively.


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88 A. Tisa and K. Kov dri:3 . J o i n t s w i t h N o D r o p i n S h e a r S t r e n g t h D u e t o D i s p l a c e m e n t

J o i n ts w i t h a s m a l l a m o u n t o f s u r f a c e r o u g h n e s s o r jo i n ts f o r w h i c ht h e r e s i d u a l s t r e n g t h h a s b e e n r e a c h e d d u r i n g t h e c o u r s e o f a s h e a r t e s t d on o t a s a r u l e s h o w a n y d r o p i n s t r e n g t h w i t h i n c r e a s i n g d e f o r m a t i o n a tc o n s t a n t n o r m a l s tr e ss . T h e i d e a li z e d f o r c e d e f o r m a t i o n d i a g r a m o f s u c h j o in t sis i l lu s t r a t ed i n F i g . 6 w i t h a p h y s i ca l m o d e l o f a s li d in g b l o ck c o u p l ed i ns e r i e s w i t h a s p r i n g . Th e co e f f i c i en t o f s l i d i n g f r i c t i o n i s # an d t h e s p r i n gst i f fness i s K.

T #!// O=const .~ . . . . .

dP 2 . . . . . . . . . . . . : / ~ ! ) L ~ K / , K a r c il lndp d

Fig. 6 . S implif ied model for smooth jo ints or rough joints in residual s trength s ta te

(d)

S H E AR D I S P L A C E M E N T d

-TA Z \F(O ,~:) : o

IN O R M A L S T R E S S ( ~Fig. 7. Diffe rent shear-displacement path s (a, b an d [ (d)) resulting in por tion s of the

failure envelopeT h e d i s p l a c e m e n t o f t h e e n d o f t h e s p r i n g d is m a d e u p o f th e s p r i n gc o m p r e s s i o n d e a n d t h e d i s p l a c e m e n t o f t h e r i g id b l o c k d r . I f o n e a s s u m e s

t h a t t h e n o r m a l s t re s s ~r i s k e p t co n s t a n t t h e n o n e o b t a i n s t h e s t r e s s - d is -p l a c e m e n t d i a g r a m , s h o w n o n t h e r ig h t o f F ig . 6 , a f t e r o n e c y c l e o f l o a d i n ga n d u n l o a d i n g . T h e c o n d i t i o n f o r i ni ti a l sl ip o f t h e b l o c k a n d t h u s f o rr each i n g t h e l i m i t in g s t re s s r * is t h u s g i v en b y d ~ > 0 . A s l o n g a s t h e n o r m a ls t re s s ~r i s k e p t c o n s t a n t t h e s h ea r s t r e s s - d i s p l acemen t r e l a t i o n can o n l yt a k e t h e f o r m o f t h e h o r i z o n t a l (r* = c o n s t . ) o r t h e d e s c e n d i n g s t r a i g h t l in eo f s l o pe K w h i c h m e a n s n o s lip ( - 3 * < r < ~ * ) . S lip of t h e b lo c k , h o w e v e r ,c a n a l s o o c c u r u n d e r t h e a c t i o n o f c o n t i n u o u s l y c h a n g i n g n o r m a l a n d s h e a rs t r e s s e s . I n F i g . 7 t h e p o i n t A o n t h e l e f t d i ag r am can b e t ak en a s t h e s t a r t -i n g p o i n t f o r a n e w s h e a r te s t. T h e s t re s s e s r a n d r ~ a r e s u c h th a t s l ip m a y


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Con tinuous Failure State Direc t Shear Tes ts 89i m m ed i a t e l y o cc u r i . e . th e p o i n t A l ie s o n t h e f a i l u r e en v e l o p e F ( 0, r ) = 0 .I f t h e s t re s s e s o a n d r a r e n o w i n c r e a s e d s i m u l t a n e o u s l y s u c h t h a t i n th e r - dp l an e t h e s t r a i g h t l in e a w i t h a s l o p e 0 < f i < e i s d e s c r i b e d t h e n an y i n c r ea s ei n r is a c c o m p a n i e d b y a sl ip . F o r a m o n o t o n o u s i n c r e a s e o f r * b y A r t h ec o r r e s p o n d i n g p e r m a n e n t d i s p l a c e m e n t is g i v en b y

A d p = A r ( co t f l - c o t e ) i . e . A d v > O .T h e s t r a i g h t l i n e a o n t h e l e f t ( Fig . 7 ) co r r e s p o n d s t o t h e c u r v e a w h i ch i sp a r t o f t h e f a i l u r e en v e l o p e F (0 , r ) = 0 . S i mi l a r co n s i d e r a t i o n s a r e v a l i d f o rt h e s t r a i g h t l i n e b i n v o l v i n g a d ec r ea s e o f t h e s t r e s s e s .

F o r t h e s i m p l e s l i p m o d e l u n d e r t h e c o n d i t i o n o f m o n o t o n o u s l y c h a n g -i n g n o r ma l s t r e s s t h e f o l l o w i n g g en e r a l i s ed r e l a t i o n s a r e v a l i d :- ( H - e ) < f l < e A d > A d ~ A d ~ > O - -* F (0 , r ) = 0 (1 )o r

0 < f l < e F ( G ' c ) = 0 , 0 > 0 A , " r > ' ~ A ( L i n e a )- ( H - e ) < f i < 0 F ( o , r ) = 0 , O


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90 A. Tisa and K. Kov~ri :m a i n l y i n th e t r a n s i ti o n f r o m t h e s t r a ig h t l i n e p o r t i o n o f t h e c u r v e w i t h t h es l o p e ~ t o t h e h o r i z o n t a l p o r t i o n c o r r e s p o n d i n g t o s li p. I n s te a d o f a n a b r u p tt r an s i t i o n (F ig . 6 ) o n e o b s e r v es w i t h ac t u a l m a t e r i a l a r o u n d i n g - o f f o f t h ecu r v e . T h u s i f o n e w a n t s t o o b t a i n accu r a t e l y t h e l i m i t c o n d i t i o n F (( r, r ) = 0t h e n t h e s l o p e s h o u l d b e c o n f i n e d w i t h in t h e l im i ts

- ~ o ( H - a ) < f l < ~o ~,w h er e i n t h e au t h o r ' s ex p e r i en ce t h e v a l u e co = 0 .5 g i v es r e s u l ts w e l l o n t h esafe s ide .

T h e k n o w l e d g e g a i n e d f r o m t h e s i m p l e m o d e l c a n b e p u t i n t o p r a c ti c ew i t h s h e a r t e s t s o n j o i n t s h a v i n g s m o o t h s u r f a c e s o r t o d e t e r m i n e t h e r e -s i d u a l s t ren g t h en v e l o p e . S i n ce t h e s p ec i m en is a l w ay s i n a s t a t e o f s l ipd u r i n g t h is t y p e o f te s t it is a p p r o p r i a t e t o e m p l o y t h e te r m " c o n t i n u o u sf a i l u r e s t a t e " ( C F S ) d i r ec t s h ea r t e s t . Th e ex p e r i men t a l a r r an g emen t r eq u i r edis s h o w n s ch em a t i ca l l y in F i g. 9 . I t i s n eces s a r y t h a t t h e s h ea r s t re s s , n o r m a l

S ~ S E~I X Y

N TFig. 9. Sche ma tic diagram of the test m onitoring arrangem entS Shear force, ST Load transducer, N No rm al force , NT Load transducer, DT Sheardisplacement transducer, XY Recorder

s t r e s s a n d s h e a r d i s p l a c e m e n t s a r e m o n i t o r e d c o n t i n u o u s l y . T h e t w o x - yr e c o r d e r s e n a b l e t h e s e p a r a t e r e c o r d i n g o f t h e s h e a r s t re s s - sh e a r d i s p la c e -m e n t d e v e l o p m e n t a n d t h a t o f t h e c o r r e s p o n d i n g s tr e ss p a t h .

E x a m p l e s o f C F S - d i r e c t s h e a r t e s t s f o r r e s i d u a l s t r e n g t h d e t e r m i n a t i o na r e g i v en i n F ig . 10 . S aw n p l an a r s u r f ace s o f g r an i ti c s p ec i men s ( 15 x 1 2 cm)w e r e s e l e c t e d . A s s h o w n i n t h e f i g u r e t h e t e s t N o . 1 s t a r t e d a t t h e s e l e c t e dn o r m a l s tr e ss o f (r = 1 . 0 M P a w h i c h w a s k e p t c o n s t a n t u n t il t h e s h e a r s tr e ssh a d r e a c h e d t h e l e v e l o f ~ :~ 0 . 43 M P a ( p o i n t A ) , n e c e s s a ry t o c a u s e p l a s ti cd i s p l a c e m e n t s a l o n g t h e d i s c o n t i n u i t y . H e r e a f t e r t h e s p e c i m e n i s s u b j e c t e dt o a c o n s t a n t r a t e o f s h e a r d i s p l a c e m e n t , w h e r e a s t h e n o r m a l s tr e ss is c o n -t i n u o u s l y in c r e a s e d in s u c h a w a y t h a t t h e p e n o f t h e x - y r e c o r d e r a c t u a l l yf o l l o w s t h e ch o s en l i n e a . A s a r e s u l t t h e cu r v e a i s d e s c r i b ed i n t h e s t r e s sp l an e b e i n g i d en t i ca l w i t h t h e f a i l u r e s u r f ace o f t h e t e s t ed j o i n t s u r f ace . Th et e s t N o . 2 s t a r te d a t t h e s e l e c te d n o r m a l s tr e ss o f 0" = 4 .0 M P a a n d p r o c e e d e df r o m t h e p o i n t B d o w n w a r d s a l o n g t h e li ne b i n v o lv i n g s t e a d y d e c r e a s e o ft h e n o r m a l s t r e s s . T h e a g r e e m e n t o f t h e t e s t s r e s u l t s a r e n o t s u r p r i s i n g c o n -s i d e r i n g t h e l a ck o f a s p e r i t i e s o f t h e a r t i f i c i a l l y p r ep a r ed j o i n t s u r f ace s . Th e


9/13

Continuous Failure State Direct Shear Tests 91e x p e c t a t i o n s f r o m t h e s i m p l if i e d m o d e l b e h a v i o u r (F ig . 6 ) a r e c o m p l e t e l yfu l f i l l ed b y t h e s e t e s t s . Th e b eh av i o u r o f s l i ck en s i d ed j o i n t s u r f aces wh e rea s p e r i t i e s o r t e e t h h a v e a l r e a d y b e e n s h e a r e d t h r o u g h i s s i m i l a r t o t h a t o f~ 3 -D.:E

2 -03

I.-03 1


10/13

92 A.Tisa and K. Kov~iri:of the brick were within _+ 2 mm for a specimen size of 15.0 x 9.5 cm. Thegeometry of the teeth of the cement mortar specimens is shown in Fig. 12and Fig. 14.

Testing Procedure:The experimenting technique is based on the concept of CFS-triaxialtests which has been dealt with in detail elsewhere (Kov~tr i et al., 1983).The CFS-direct shear tests however may be carried out both with increas-ing and decreasing normal stress. In Fig. 11 the results of two such tests

6 ~

o. 4=E

LuCc 3-I-

I11::I : : 2 -

\No .

6 -

5-

I I I I ~0 .4 0 .8

S H E A R D I S P L A C E M E N T ( r a m)

o. 4-

uJ" 3-I,-uJ

wz 2

9 c o n v e n t i o n a l t e a t

I1

A

3 4N O R M A L S T R E S S ( M P a )

Fig. 11. Two CFS-direct shear tests compared with results of conventional single failurestate tests (brick with tensile fracture surface)

carried out with brick specimens are shown. The sample No. 1 reached thefirst failure state at the selected constant normal stress of 2.0 MPa. Sub-sequently, at a given rate of shear displacement the normal stress was in-creased such as to describe a straight line. As a result the line ~ describingthe stress path was obtained. The sample No. 2 reached the first failurestate at a constant norma l stress of 4.0 MPa at point B. Hereafter thenormal stress was continuously decreased causing decrease in shear stress.For the unloading curve b the stress path b was obtained. The line b showsa good agreement with the results of conventional tests whereas the linediffers considerably. The explanation is seen in the larger shear displace-ments per unit shear stress involved in the generation of line a in the shearstress-displacement diagram. For straight lines a having a slope correspond-ing to that of the linear portion of the initial shear stress-displacement dia-


11/13

C ont in uou s Fai lu re S tate Direct Shear Tests 93g r a m a g o o d a g r e e m e n t w i t h c o n v e n t i o n a l t e s t r e s u l t s i s e x p e c t e d . I n F i g . 1 2t h e r e c o r d i n g o f a c o m p l e t e C F S - s h e a r t e s t i s p r e s e n t e d i n d i c a t i n g t h e t e c h -n i q u e t o d e t e r m i n e b o t h t h e p e a k a n d t h e r e s i d u a l s t r e n g t h e n v e l o p e w i t ht h e a i d o f j u s t o n e s i n g l e s p e c i m e n . T h e t e s t s t a r t s f r o m t h e i n i t i a l p e a kf a i lu r e s t a t e a t th e s e l e c t e d n o r m a l s tr e ss 3 .0 M P a a t p o i n t B . T h e s h e a r d i s -p l a c e m e n t s m e a s u r e d d i r e c t l y a t t h e j o i n t ( d l ) a n d a t t h e l o a d i n g p l a t e o f

6 1 c on v o n to n a s t

s a /1 i

w 3 - ~ | I X / \ I 2 m m I =

~ I I I I I I I1 2 3 4 5 6 7 1 2 3S H E A R D I S P L A C E M E N T ( m m ) N O R M A L S T R E S S ( M P a )

Fig. 12. Resul ts of a CFS -direct shear test to determine pea k and residual s t rength envelopedl : displacemen ts at joint , d2: displaceme nt of the plates of loading mac hine(cast m ortar with regular teeth)t h e t e s t i n g m a c h i n e ( d 2 ) d i f fe r b y a f a c t o r o f a n o r d e r o f m a g n i t u d e . T h ed i f fe r e n c e is b r o u g h t a b o u t m a i n l y b y t h e d e f o r m a b i l i t y o f t h e e p o x y r e s ins u r r o u n d i n g t h e s p e c i m e n i n t h e s h e a r b o x a n d le ss b y t h e c o m p r e s s i b il it yo f th e s t ee l c o m p o n e n t s o f th e a p p a r a t u s . A l t h o u g h f u r t h e r d i s p l a c em e n t s o ft h e l o a d i n g p l a t e d 2 a t p o i n t B w e r e p r e v e n t e d a d i f f e r e n t i a l m o v e m e n t o ft h e j o i n t s ( d l ) o c c u r r e d d u r i n g t h e c o n t i n u o u s d e c r e a s e o f t h e n o r m a l s t r e s s .T h e d e c r e a s e o f n o r m a l s t r e s s c o u p l e d w i t h c o n t i n u o u s s l i p o n t h e j o i n t sc a u s e d a d e c r e a s e a l s o i n s h e a r s t r e s s . T h e c o r r e s p o n d i n g s t r e s s p a t h s h o w no n t h e r i g h t a g r e e s f a i r l y w e l l w i t h t h e c o n v e n t i o n a l t e s t r e s u l t s . A t p o i n t Ct h e n o r m a l s t re s s w a s a g a i n a b r u p t l y b r o u g h t t o t h e v a l u e o f 3 .0 M P a . A tc o n s t a n t n o r m a l s t r e s s t h e s p e c i m e n w a s s u b j e c t e d t o l a r g e s h e a r d i s p l a c e -m e n t s u n t i l t h e r e s i d u a l s t r e n g t h v a l u e a t D w a s t h o u g h t t o b e r e a c h e d .S t a rt in g f r o m t h is p o i n t t h e r e s id u a l s h e a r s t r e n g t h e n v e l o p e w a s d e t e r m i n e da c c o r d i n g t o t h e p r o c e d u r e o u t l i n e d i n t h e p r e v i o u s s e c t i o n . I t i s w o r t hm e n t i o n i n g t h a t t h e s h e a r d is p l a c e m e n t s d l s t a r t in g f r o m t h e p o i n t B w e r es i m u l t a n e o u s l y a c c o m p a n i e d b y n o r m a l d i s p l a c e m e n t s c a u s i n g d i l a t a n c y . I n


12/13

9 4 A . T i s a a n d K . K o v ~ r i :F i g . 1 3 t h e r e s u l t s o f a se r i e s o f C F S - s h e a r t e s t s c a r r i e d o u t b o t h w i t h i n -c r e a s i n g a n d d e c r e a s i n g s t re s s e s a r e s h o w n . T h e s c a t t e r i n r e s i d u a l s t r e n g t h

6 -

o. 4u)o0uJr 3I -~o

o3

~

I1

9 c o n v e n t i o n a l t e s t

I I I I2 3 4 5

N O R M A L S T R E S S ( M P a )Fig. 13. Brick material. Results of conventional tests and CFS-tests

4

g : 3 -

1

o

fc o n v e n t i o n a l t e s t

I L I J1 2 3 4

N O R M A L S T R E S S ( M P a )

Fig. 14. Cement mortar. Results of conventional tests and CFS~testsw a s c o n f i n e d to th e n a r r o w s h a d e d a r e a . T h e p e a k s t r e n g t h o f t h is m a t e r i a l ,h o w e v e r , e x h i b i t s a c o n s i d e r a b l e s c a t t e r i r r e s p e c t i v e o f t h e t y p e o f t h e e x -


13/13

Co ntinuou s Failure State Direct Shear Tests 95p e r i m e n t a l p r o c e d u r e . S in g le f a i lu r e s t a te s o f p e a k s t r e n g t h a r e i n d i c a t e d b yt h e f u l l a n d t h e h o l l o w c i r c l e s , t h e l a t t e r b e i n g t h e s t a r t i n g p o i n t f o r C F S -s h ea r t e s ts . I n F ig . 1 4 t h e f o u r C F S - s h ea r t e s t s ap p l y i n g d ec r ea s i n g s t r e ss e ss h o w r e a s o n a b l e a g r e e m e n t w i t h t h e r e s u l t s o f c o n v e n t i o n a l t e s t s . T h e d a s h e dl in e d if fe r s c o n s i d e r a b l y f r o m t h e o t h e r c u r v e s d u e t o t h e g re a t e r a m o u n t o fs l i p p e r mi t t ed d u r i n g t h e s e t e s t s ( F i g . 1 0 ) . Th e r e s i d u a l s t r en g t h r e s u l t s f r o ma ll o f t h e t es t s a re a g a i n c o n f i n e d t o a n a r r o w b a n d .

Con c l u s i o n sT h e c o n c e p t o f " c o n t i n u o u s f a i l u r e s t a t e " ( C F S ) d i r e c t s h e a r t e s t w a s

s h o w n t o f u r n i s h t h e e x a c t f a il u r e e n v e l o p e f o r t h e r e s id u a l s h e a r s t re n g t h .T h e d e t e r m i n a t i o n o f t h e p e a k s t r e n g th e n v e l o p e o f r o u g h s u r fa c e s o r t h o s ew i t h t e e t h s e e m s a l s o t o b e p o s s i b le p r o v i d e d t h a t o n l y a s m a l l a m o u n t o fs l i p p e r u n i t s h ea r s t r e s s i s p e r mi t t ed d u r i n g t h e t e s t s . A s e r i e s o f t e s t s o nt w o d i f f e r e n t m a t e r ia l s w i t h d i f fe r e n t d e g r e e s o f i n t e rl o c k i n g h a s s h o w n v e r ye n c o u r a g i n g r e s u l t s . T h e n e w s h e a r t e s t a p p a r a t u s f o r c o m b i n e d u s e w i t h as e r v o - c o n t r o l l e d l o a d i n g m a c h i n e w h i c h i s n o w a v a i l a b l e i n m o s t r o c k m e -c h a n i c s l a b o r a t o r i e s h a s p r o v e d t o b e v e r y u se f u l i n c o n d u c t i n g C F S - d i r e c ts h ea r t e s t s .

R e f e r e n c e sC ra w f o rd , A .M . (1980) : Rate Dependen t Behaviour o f Roc k Joint s. Ph . D .Thesis, U niversi ty of To ron to.K ow l r i , K . , T i sa , A . , A t t i ng er , R . O. (1983) : The C oncep t o f "Con t inuous Fa i-lure State" Triaxial Tests. Ro ck Mech. and R oc k En gng . Vol. 16 /2 , 117--131.L e ic h n it z , W . (1981): Mechanische Eigenschaften yon Felstrennfl~ichen im direktenScherversuch. Ver6ff. d. Inst. f. Bodenmechanik und Felsmechanik der Univer-sit,it Fridericiana in Karlsruhe, H ef t 89.L o m b a rd i , G . , D al V es co , E . (1966) : Die experimen teUe Bestimmung der Rei -bungskoeffizienten fiir die Felswiderlager d er Staum auer Co ntra (Verzasca).Proc. 1st Congress of the Int. S oc . R oc k Mec h., Lisbon, 571--576 .

Continuous Failure State Direct Shear Tests

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