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T H E R E S ID U A L S H EA R S TR E N GT H O F F IL L E D J O IN T S IN R O C K

La res is tanc e reslduelle a u c i s ai l le m en t d e s f is s u r es d e r o c h e r em p l i es

Der Res ts c herwiders tand v on gefOl l ten Ges teins trennf lac hen

Prof essor

H. K. KUTTER

A. RAUTENBERG

Di pl . Geol .

Ar bei t sgr uppe Fel smechani k

Ruhr - Uni ver si t ~t Bochum, W. - Ger many

SUMMARY:Thi s paper pr esent s a st udy of t he i nf l uence of f i l l er t hi cknes s, j oi nt r oughness , and t ype off i l l i ng mat er i al on t he r e s i dual s hear s t r engt h of i nf i l l ed j oi nt s . Di r e ct s hear t es t s wer eper f or med on s ands t o ne j oi nt s of di f f er e nt r o ughnes s es and di f f er e nt t hi c k f i l l er s of s and, c l ay,or a mi xt ur e of bot h. A p r el i mi nar y eval uat i on of t he t es t r e s ul t s s hows an i nc r eas e i n s t r engt hwi t h f i l l er t hi c knes s and a dr o p i n s hear s t r engt h wi t h dec r eas i ng r o ughnes s ; f or a s moot h s ur -f ac e i t l i es even bel ow t he s t r engt h of t he s oi l . Af t e r appl yi ng a c or r ec t i on f or t he c ompac t i onc aus ed by s oi l bei ng s qeezed out , t he s hear r e s ul t s c l ear l y s how t hat t he mi ni mum s t r engt h ob-t a i nabl e i s t hat of s oi l and t hat t he s t r engt h ac t ual l y dec r eas es wi t h i nc r eas i ng f i l l er t h i c k-ness.

RESUME:L e but de c e t r a vai l pr e sent e et ai t d' anal ys er l ' i nf l uenc e des r u gos i t es , de l ' epai s s eur et del ' es pc e du r empl i s s age s ur l a r e s i s t a nc e r es i duel l e d' une f i s s ur e r empl i e. Pour c el a on a f ai tdes es s ai s de c i s ai l l ement avec des f i s s ur es en gr es des degr es var i abl es de r ugos i t e et der empl i ssages d' argi l e, de sabl e ou d' une mel ange. Un pr emi er examen des r esul t ats a i ndi que uneaugment at i on de l a r es i s t anc e avec l as epar a t i on de l a f i s s ur e y une di mi nut i on avec l a r u gos i t edi mi nuant e; pour une sur f ace l i sse I e r esul t at et ai t m~me moi ns que l a r esi st ance du r empl i ssage.L' i nf l uence du t assement el i mi nee pr odui t e par I e mat er i au pr essur e l es essai s avai ent demont r ess ans dout e que l a pl us pet i t e r es i s t anc e au c i s ai l l ement pos s i bl e c ' es t c et t e du r e mpl i s s age et

que' l a r esi st ance di mi nue avec l a separ at i on des f i ssur es augment ant e.

ZUSAMMENFASSUNG:Es war das Zi el di eser Ar bei t , den Ei nf l uB von Kl uf t r auhi gkei t , St ~r ke und Ar t des Zwi schenmi t -t el s auf di e Res t s c her f es t i gkei t ei ner gef Ul l t en Kl uf t zu u nt er s uc hen. Es wur den dazu di r ekt eScherver suche an Sandst ei nkl Uf t en mi t ver schi edenen Rauhi gkei t en und mi t ver schi eden st arkenZwi schenschi cht en aus Ton, Sand oder ei nem Gemi sch dur chgef Uhr t . Ei ne erst e Auswert ung der Ver-suchsergebni sse zei gt e ei ne Zunahme der Scherf esti gkei t mi t der Kl uf t wei t e und ei ne Abnahme mi tabnehmender Rauhi gkei t , f Ur ei ne gl at t e Ober f l ~che sogar bi s unt er hal b der Fest i gkei t des Zwi -schenmi t t el s. NachDur chf Uhr ung ei ner Kor r ektur f Ur di e dur ch das Auspr essen des wei chen Bodenser zeugt e Kompakti on zei gt e si ch bei den Ver suchsdat en ei ndeut i g, daB di e ni edr i gst e m5gl i cheSc her f es t i gkei t di e des FUl l mi t t el s i s t und daB di e Sc her f es t i gkei t mi t wac hs ender Kl uf t 5f f nungabni mmt .

I NTRODUCTI ONThe usual ' pr ocedur e of desi gni ng r ock

s t r uc t ur e s and anal yz i ng t he s t a bi l i t y of ar ock mass st ar t s wi t h l ocat i ng and mappi ng t hemost pr eval ent di scont i nui t i es and assessi ngt hei r st r engt h and def or mat i onal behavi our .

They r epr esent t he weakest el ement s of t her ock mass and consequent l y st r ongl y cont r oli t s st abi l i t y and mechani cal behavi our andf r e quent l y f or m t he l oc at i on of i ni t i al and/ orpr o gr e ss i ve f ai l ur e . I t i s not a pr o bl em of anal ys i s but a mat t er of exper i enc e, of geo-t echni cal t est i ng and of engi neer i ng deci si ont o assi gn r epr esent ati ve mechani cal par ameter st o r o ck j oi nt s . Thei r s t r engt h may var y be-t ween t hat of t he r o ck s ubs t a nc e and t hat of

t he l oos e or c ohes i ve s oi l f i l l er .Dependi ng on t he extent of shear di s-

pl acement t hat has al r eady t aken pl ace al ongt h e j oi nt , t h e s t r engt h may c hange f r o m i t s

peak val ue at no pr evi ous shear di spl acement t oi t s r esi dual val ue af t er l ar ger shear movement &Modern comput er anal yses per mi t t he eval uat i onof s uc h a br oad r ange of s t r e ngt h par amet er s ,wher eby t he r esi dual st r engt h par amet er s l eadt owar ds a r at her c ons er vat i ve des i gn. But evenwhen i t has been dec i ded t o s t a y on t h e s af es i de and us e t he r e s i dual s t r engt h par a met er s ,one i s s t i l l c onf r o nt e d wi t h t h e pr o bl em of c hoos i ng t he c or r e c t s hear par amet er s . For i thas been f ound t hat even t he r e s i dual s t r engt hpar amet er s of a r oc k j oi nt ar e not nec es s ar i l yconst ant s whi ch onl y depend on t he r ock mat er i albut al s o on t he degr e e of i ni t i al r o ughnes s of t he j oi nt s ur f ac e ( KUTTER, 1974) , as wel l as on

t he s t r es s and s t r ai n hi s t or y of t he j oi nt .( WEI SSBACH and KUTTER, 1978) . Vari ati ons i n t heef f ec t i ve angl e of f r i c t i on of up t o 40 % andmore were observed experi ment al l y. The choi ce of

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t he c or r ec t s hear par a met e r s i s par t i c ul ar l ypr o bl e~t i c i n t he c as e of j oi nt s i n r e l at i vel yhar d r ock f i l l ed wi t h weak and l oose mat er i al ,var y i ng f r o m c oar s e gouge t o s and t o c l ay andconst i t ut i ng ei t her shear debr i s, decomposi t i onand weat her i ng pr oducts of t he r ock mat er i al ,or deposi t ed er osi on pr oducts. Dependi ng on t hechar act er i s t i cs of s uch a f i l l ed j oi nt , i t sst r engt h parameter s may l i e anywhere bet weent hos e of t h e unf i l l ed j oi nt and t h os e of t hef i l l i ng mat er i al , or - as f r equent l y s ugges t ed-even bel ow t hos e. The ai m of t he s t u dy r e por t ed

her e has been t o i nves t i gat e exper i ment al l y t her es i dual s hear s t r e ngt h of f i l l ed j oi nt s and t opr ovi de cri t er i a f or est i mat i ng wher e bet weent he l i mi t s of f i l l and r o ck mat er i al t he ac t ualr e si dual s t r engt h i s .

A r at her compl et e and extensi ve r evi ew oft he s hear s t r engt h of f i l l ed di s cont i nui t i es i nr ock has been pr epar ed by BARTON ( 1974) . Themai n at t ent i on t her e i s pai d t o t he pr i nc i pl esc onc er n i ng t he peak s hear s t r e ngt h and t o r el e-vant t es t met hods . I t i s pr o pos ed t hat t he s hears t r e ngt h of j oi nt s f i l l ed wi t h a t hi ck l ayer i sbas i c al l y t hat of t he s oi l , wher e as es t i mat i ngt he s t r e ngt h of f i l l i ngs wi t h a t h i c knes s l es st han t he r oughnes s ampl i t ude of t he wal l r o c kmay be pr obl emat i c si nce t her e a l i mi t ed sheardi spl acement wi l l r esul t i n a mar ked st i f f eni ng

due t o i nc r eas i ng r o ck t o r o ck c ont ac t . I ni t i al -l y t hi s may appear r easonabl e but t he actualsi t uati on seems t o be more compl i cat ed. LAMA( 197~) , f or i ns t a nc e, has f ound t hat a k aol i nl ayer i n a j oi nt dec r e as es t he peak s hears t r engt h even when t h e f i l l er t hi c knes s i s as mal l f r a ct i on of t he as per i t y hei ght . Ot heraut hor s ( COULSON, 1970; GOODMAN, 1970; J AEGER,1959; ROMERO, 1968; TULI NOV and MOLOKOV, 1971)demons t r at ed i n l abor a t or y and f i el d t e s t s t h att he s t r e ngt h and s t i f f nes s of i nf i l l ed j oi nt sc hange gr adual l y and not abr upt l y wi t h t he r e-l at i ve f i l l er t hi c knes s . An i nf l uenc e of t hes ur f ac e r o ughnes s mus t exi s t even f or t hi c kerf i l l s as l ong as an appr e ci abl e par t of t heshear movement t akes pl ace al ong t he soi l - r ock

cont act .Es t i mat i ng wi t h s uf f i c i ent ac cur ac y t her es i dual s t r engt h of a f i l l ed j oi nt bec omeseven more compl i cat ed, si nce here pur el y geo-met r i c al c ons i der a t i ons bas ed on t he r el at i onbet ween t hi c knes s of f i l l er and hei ght of as pe-r i t y cannot al one pr ovi de t he answer . The mul t i -t u de of i nf l uenc i ng f ac t or s and i nt er a ct i ons c anbest be det er mi ned and possi bl y wei ght ed i ns i mpl e di r ec t s hear t es t s on s i mpl i f i ed, par t l yar t i f i c i al l y pr o duc ed f i l l ed j oi nt s . I n a de-t ai l ed s t u dy by KANJ I ( 1974) s oi l - r oc k c ont ac t sand s andwi c h- l i ke c onf i gur at i ons - wi t h a s oi ll ayer bet ween r oc k pl at ens of di f f er e nt degr eesof r oughnes s - wer e expos ed t o r el at i vel y l ar geshear di spl acement s. The most st r i ki ng r esul tof t hi s t e st s er i es was t hat t h e r e si dual s hear

s t r e ngt h of s moot h r o ck- s oi l c ont a ct s may bel ower t han t h at of t h e s oi l al one. Si nc e t hi ss t r e ngt h r e duc t i on i s mor e pr onounc ed t hes moot her t he r oc k s ur f ac e and t he s mal l er t het h i c knes s of t h e f i l l er ar e, i t s c aus e i s s oughti n t he f or c ed al i gnment of t he c l ay par t i c l esal ong t he nar r ow f ai l ur e zone. Si mi l ar r es ul twer e obt ai ned by EURENI US and FAGERSTR~M ( 1969)i n f i el d t e s t s wher e t he c ont ac t z one bet ween abent oni t i c c l ay s eam and c hal k mar l was f ound t obe weak er t han t he i nt e r nal par t s of t he c l ayseam.

I t i s evi dent f r o m t hes e and ot her i nves t i -gat i ons t hat s hear ed zones and f i l l ed j oi nt s mayhave st r engt h and def or mabi l i t y pr oper t i es whi chdi f f er appr e ci abl y f r o m t hos e of t he f i l l i ngmat er i al i t sel f . The pr esent i nf or mat i on, however ,i s not yet s uf f i c i ent t o enabl e t he pr a ct i t i onert o es t i mat e r e l i abl y t he s hear s t r engt h of f i l l ed . j oi nt s si mpl y f r om t hei r geomet r y andf r o m t he bas i c mat er i al pr oper t i es . No c l eardi f f er e nt i at i on exi s t s bet ween t he f ac t or s whi c h

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i nf l uenc e t he peak and t he r es i dual s hear

s t r engt h r es pec t i vel y and t hei r quant i t at i vewei ght i s har dl y known. A l abor a t o r y t es t i ngpr ogr am has t her ef or e been i ni t i at ed t o i nves t i -gat e t he i nf l uence of t he f undament al var i abl esof i nf i l l ed j oi nt s , i . e. j oi nt openi ng, j oi ntr o ughnes s and t h e pr o per t i es of t he f i l l i ngmat e r i al , on t he r e si dual s hear s t r engt h, and t obr i ng i nt o r e l at i on t he s hear pr o per t i es of t h eunf i l l ed j oi nt , of t he f i l l i ng mat e r i al al oneand of a c ont ac t s ur f ac e bet ween r oc k and s oi l .

TEST PROGRAH, MATERI ALS AND EQUI PMENT

The t est ser i es consi st ed i n pr i nci pl e ofdi r e ct s hear t es t s on ar t i f i c i al l y pr o duc ed r o ckj oi nt s of sever al degr ees of r oughness , wi t hvar i ous f i l l er mat er i al s and f i l l er t hi c knes s( RAUTENBERG, 1977) . Wi t h t he mai n emphasi s on

r es i dual s t r e ngt h par t i c ul ar at t ent i on was pai dt o t h e l oc at i on of t h e ac t ual f ai l ur e pl ane andt o t he s hear di s pl ac ement r equi r ed f or and t henormal di spl acement associ at ed wi t h r eachi ng t her e s i dual s t r engt h val ue. I n or der t o keep t he numberof t e s t s wi t hi n r e as onabl e l i mi t s and yet t oal l ow a f ul l i nves t i gat i on of t he pr obl em onl yone r oc k t ype, i . e. Ruhr s ands t o ne ( hi ghl y i nt er -l oc ked quar t z s ands t one of r e l at i vel y l ow por o -s i t y and wi t h an aver age gr ai n s i ze of 0. 5 r om) ,was used. But t hr ee ot her most f undament al par a-met ers wer e var i ed:- j oi nt r oughness- f i l l er mat er i al- f i l l er t h i c knes s .

Thr ee t ypes of sur f ace r oughness wer e i n-vest i gat ed: a smoot h and pl ane sur f ace pr oducedby a di amond s aw c ut ; a pl anar but r ough s ur f ac e~r epar ed by sandbl ast i ng a saw cut ; and a wavyand r o ugh s ur f ac e gener a t ed by t e ns i l e f r a ct ur ei n a Br aZi l i an- t ype t est . The aver age ampl i t udeof t he as per i t i es was f or t he s aw c ut < 0. 1 r om,f or t he s andbl as t e d s ur f ac e about 0. 5 r omandf or t he t ens i l e f r a ct u r e about 2 r om.

Thr ee t ypes of f i l l er ~at er i al s wer e used:f i ne quar t z s and wi t h medi um gr ai n s i ze of 0. 25 r omand 99 % of t he gr ai ns bet ween 0. 1 and0. 4 r om; a s i l t y c l ay wi t h 24 % Kaol i ni t e, 46 %I l l i t e and 30 % quar t z , wi t h a gr a i n s i z e di s t r i -but i on of 25 % > 20~, 39 % 2 - 20~ and ~5 %