Shear behaviour of regular and irregular rock joints under cyclic conditions

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1 Pper No. 69 ISMS 2016 Sher ehviour of regulr n irregulr rok joints uner yli onitions S. M. Mhi Niktr, *, K. Seshgiri Ro, Amit Kumr Shrivstv Deprtment of Civil Engineering, Inin Institute of Tehnology Delhi, New Delhi, Ini, Deprtment of Civil Engineering, Delhi Tehnology University, New Delhi, Ini, ABSTRACT Rok msses often hve sets of joints or frtures n lmost ll filures re ue to the presene of these isontinuities. Nturl joints hve irregulr surfes, n the orret evlution of sher strength n eformtion of these irregulr joints is very importnt for the nlysis n engineering esign of rok strutures. These joints re often sujete to ynmi los euse of erthquke n lsting uring mining n rok utting. Hene, it is importnt to orretly evlute the sher ehviour of regulr n irregulr rok joints uner ynmi onitions. In the present stuy, syntheti rok joints re prepre with plster of Pris n regulr joints re replite y keeping regulr sperities with sperity ngles n Irregulr rok joints re prepre y keeping the sperity ngles n The smple size n mplitue of roughness re kept onstnt for oth regulr n irregulr joints, t mm n 5 mm respetively. Sher tests hve een performe on these joints with lrge sle iret sher testing mhine y keeping the frequeny n mplitue of sher los onstnt uner yli lo onitions n vrying the norml stress. The sher strength of rok joints inreses with inrese in the sperity ngle n norml lo uring the first yle of shering. With the inrese in the numer of sher yles, the sher strength reues for ll the sperity ngles, ut the rte of reution is greter in the se of high sperity ngles. Test results inite tht the sher strength of irregulr joints is higher thn regulr joints t ifferent yles of shering t low norml stresses. The mehnism of the shering for regulr n irregulr joints is ifferent uner the yli onitions t low norml stresses. Shering n egrtion of joint sperities on regulr joints etween loing n unloing re the sme, ut for irregulr joints they re ifferent t low norml stresses. Sher strength n joint egrtion re more signifint on the slope of sperity with higher ngles on the irregulr joint, until two ngles of sperities eome equl uring the yle of shering n it strts to ehve like regulr joint. KEYWORDS: Cyli sher ehviour; sher strength; regulr joint; irregulr joint; joint iltion 1. INTRODUCTION Sher stress n eformtion ehviour of rok joints ply n importnt role for esign n nlysis of unergroun strutures, fountion, slope stility n risk ssessment of unergroun isposl. Mny reserhers in the fiel of rok mehnis n rok engineering hve presente the sher ehviour of jointe rok, se on pek stress-strin long the joint uner uniiretionl or monotoni (stti) sher los. However, joints re sujete to ynmi los ue to erthquke, lsting, n virtion, whih n e simulte s sher long the joint uner yli los. In this onition, lo iretion is reverse on the shering plne repetely. In the present work, physil moel is prepre in orer to exmine the sher ehviour of nturl jointe rok mss. In the pst sher ehviour of regulr joints uner yli onitions ws stuie y mny reserhers (Hung et l.,1993; Hutson & Dowing, 1990; Homn et l., 2001; Jfri et l., 2003; Inrrtn et l., 2012; Mirzghornli et l., 2013; Niktr et l., 2015). However, joints in the rok mss re irregulr n hve ifferent roughness. In the present stuy regulr n irregulr joints with sperity ngles n n n re prepre oringly. Joints with irregulr sperities re more representtive n loser to nturl joints. Eh yle is ivie into four stges s esrie y Lee et l. (2001), suh s forwr vne (FA), forwr return (FR), kwr vne (BA), n kwr return (BR); for etter unerstning, the four stges re illustrte in Figure 1. The FA movement t first sher yle is similr to stti or monotoni sher los. To stuy the effet of irregulr sperities on the sher ehviour of rok joints uner yli onitions, series of tests were performe on regulr n irregulr jointe smples. *S. M. Mhi Niktr emil : ez128284@ivil.iit..in

2 n 4, respetively. The unixil ompressive strength of moel mteril t 0.60 wter ement (POP) rtio n fter 14 ys of ir uring ws 6 MP. 3. LARGE-SCALE DIRECT SHEAR APPARATUES A servo ontrolle lrge-sle iret sher system esigne y Shrivstv n Ro (2012) ws moifie to e le to rry out tests uner yli sher onitions. This system is illustrte in Figure 5, whih onsists of three min units, nmely the 1) Figure 1: Lo iretion n joint movement uner sher yli onition. 2. SAMPLE PREPARATION Similr smple preprtion methoology is opte for prepring oth regulr n irregulr joints, exept for the use of ifferent sperity pltes to rete ifferent sperity ngles. A moel mteril is foun in suh wy tht it n esily e hnle n reprouiility of the smple n e ensure. To hieve this, ifferent rns of plster of Pris n entl plsters t ifferent moisture ontent n uring perio in isoltion or omintions were ssesse. Finlly, plster of Pris (POP) ws selete euse of its universl vilility n its ility to moul into ny shpe when mixe with wter to proue the esire joints, s well s its long term strength eing inepenent of time one the hemil hyrtion is omplete. The presrie perentge of wter ws eie so s to hieve proper workility of the pste n require strength to simulte the soft rok. Different wter POP rtios were trie in orer to otin esire strength n workility. The rtio whih ws finlly selete ws Size of smples n mplitue of sperities re mm n 5 mm respetively for ll of the joints se on mouls n sperity pltes. The sperity pltes of ifferent ngles like n n n were esigne n frite to proue esire sperities in the smple. The POP with 60% of the moisture ws mixe in the mixing tnk for 2 minutes n then the mteril ws poure in the sting moul whih ws ple on the virting tle s presente in Figure 2. Virtions were given to the smple for perio of 1 minute to remove ny entrppe ir. The smple ws emoule from the moul fter 45 minutes n kept for ir uring for 14 ys efore testing. Pre-test regulr n irregulr joints re shown in Figures sperity pltes Figure 2: Set up for smple preprtion. Figure 3: Regulr joint (15-15 sperity ngle). Figure 4: Irregulr joint (15-30 sperity ngles). loing unit, 2) t quisition with ontrolling unit n, 3) hyruli power pk. Mximum pity of the norml n horizontl lo ell ws 500 KN n 1000 KN, respetively. The size of eh sher ox ws 300 mm 300 mm 448 mm. This system n work uner oth stti n yli onition. 194

3 4. EXPERIMENTATION The yli sher tests were onute on regulr n irregulr joints uner ifferent norml stresses. Norml stresses re 0.1 MP n 1 MP s low n high, respetively. The frequeny n mplitue of sher los re set to onstnt vlues of 0.01 HZ n +-8 mm, respetively for ll yli sher tests. Eh type of speimen is teste on thirty sher yles uner onstnt norml lo onitions. The orienttion of smples with irregulr joints is importnt when loe euse of the ifferent sperity ngles. Experimentl results re plotte, first n lst sher yles re shown in lk n re olors in the grphs n from 2-29 sher yles re presente in lue olor for lrity. Figure 5: Lrge sle iret sher testing mhine. 4.1 Low norml stress (P=0.1 MP) Sher stress versus horizontl isplement on the joints with sperity ngles () 15-15, () 30-30, () n () t P=0.1MP re presente in Figure 6. Figure 6 () inites tht no signifint hnge is oserve on the pek sher stress from first to lst (30) sher yles on the joint with sperity ngle, wheres it ereses with inresing numer of sher yles for sperity ngle, s shown in Figure 6 (). The pek sher stress t initil yles for the joint with sperity ngle is more thn for the sperity ngle, ut it ereses for sperity ngle n the pek sher stress is less thn for the sperity ngle fter severl sher yles, i.e. pek sher stress on joint with sperity ngle is less thn sperity ngle t 30 sher yles. Norml isplement versus horizontl isplement on the joints with sperity ngles () 15-15, () 30-30, () n () t P=0.1 MP is presente in Figure 7. The iltion ngle for sperity ngle is onstnt uring 30 sher yles s shown in Figure 7(), ut it ereses y inresing the numer of sher yles for sperity ngle n iltion on the joint fter severl sher yles is onverte to ompression (positive norml isplement) s inite in Figure 7 (). Mehnism of the sher is hnge from sliing on the low sperity ngle (15-15 ) to shering on the joint with the high sperity ngle (30-30 ) t the sme norml stress uner yli sher los. Irregulr joint sher ehviour is signifintly ifferent from tht of the regulr joint. Sher strength of the irregulr joint s inite in Figure 6() is more thn tht of the regulr joint s presente in Figure 6() on the sme slope of sperities t ifferent yles of shering. With inresing irregulrities (the ifferene of ngles etween two slopes of sperities is inrese) or the joint with sperity ngle, the pek sher stress is inrese. However, it rpily ereses with inresing numer of sher yles, s shown in Figure 6(). No signifint hnge is oserve on pek sher stress on the lower slope of sperities or kwr movements. Figures 7() n () reflet tht iltion n iltion ngle is more on the higher slope of sperities n ereses grully until it is equl with the lower slope of sperities. However, iltion n iltion ngle of the regulr joints oth re onstnt or erese grully n eqully on forwr n kwr movements from first to lst sher yles, s inite in Figures 7() n (). Irregulr joints hve teneny to onvert to regulr joints t low norml stress uner yli sher los. Degrtion of irregulr joints is more preomint on the slopes t higher ngles rther thn t lower ngles on the sperities. 4.2 High norml stress (P=1 MP) Sher stress versus horizontl isplement on the joints with sperity ngles () 15-15, () 30-30, () n () t P=1MP is presente in Figure 8. Norml isplement versus horizontl isplement of the sme joints t P=1MP is inite in Figure 9. The pek sher stress inreses with the inresing norml stress for ll types of joints. In ition, the pek sher stress inreses with the inresing sperity ngle n irregulrity of the joint t first sher yle (in forwr vne). After two or three sher yles, ll types of joints reh reltively the sme sher stress level n ehviour of the joints hnges from non plnr to plnr, ue to the omplete shering of sperities of the joint t high norml stress. This joint ehviour or trnsition from nonplnr to plnr eomes more ovious s the norml stress on the joint uner yli los inreses. With inresing norml stress, ompression is rete on the joints. Smll iltion is oserve on regulr joints only t first yle t high norml stress, s presente in Figures 9() n (). 195

4 Figure 6 : Sher stress versus horizontl isplement on the joints with sperity ngles, () 15-15, () () n () t P=0.1 MP. Figure 7: Norml isplement versus horizontl isplement on the joints with sperity ngles, () 15-15, () 30-30, () n () 15-45t P=0.1 MP. 196

5 Figure 8 : Sher stress versus horizontl isplement on the joints with sperity ngles, () 15-15, () 30-30, () n () t P=1 MP. Figure 9 : Norml isplement versus horizontl isplement on the joints with sperity ngles, () 15-15, () 30-30, () n () t P=1 MP. 197

6 In the se of irregulr joints, iltion n iltion ngles strt to inrese in the kwr movement with inresing irregulrities s shown in Figure 9 (). Diltion n iltion ngles on the slope of sperities with lower ngles inreses in kwr movement with respet to the other joints, n it is either ue to inrese in slope ngle euse of eformtion on nother prt of slope in first movement, or some eris getting eposite on this slope ue to first shering n joint egrtion on the slope of sperity with higher ngle. Horizontl isplement orresponing to pek sher stress ereses with inrese in sperity ngle, wheres it inreses with the inresing irregulrity of the joint. 5. CONCLUSION The test results inite tht the sher strength of rok joints inreses with inrese in sperity ngles n norml los t first yle of shering. With the inrese in the numer of sher yles, the sher strength reues for ll the sperity ngles (exept joint with low sperity ngle t low norml stress). However, the rte of reution is greter in the se of sperities with higher ngles. Nturl rok joints re irregulr, ut most prior stuies hve een onute on regulr joints. In the present stuy, yli sher tests were performe on oth regulr n irregulr joints for omprison. Sher strength of irregulr joints ws foun to e higher thn tht of regulr joints t ifferent yles of shering t low norml stress. The nlysis of esign of struture on roks y using the ove test results mkes the esign sfer n more eonomil. The mehnism of shering for regulr n irregulr joints is ifferent uner yli onitions. Sher strength n egrtion of sperities on the regulr joints etween loing n unloing re the sme t low norml stress, wheres for irregulr joints they re ifferent. Sher strength n joint egrtion re more on the slope of sperity with higher ngles on the irregulr joints, untill two ngles of sperities eome equl uner sher yles t low norml stress when it strts ehving like the regulr joints. joints. Interntionl Journl of Rok Mehnis n Mining Sienes. Volume 30, No. 3, pp Hutson, R. W., & Dowing, C. H. (1990). Joint sperity egrtion uring yli sher. Interntionl Journl of Rok Mehnis n Mining Sienes. Volume 27, No. 2, pp Inrrtn, B., Mirzghornli, A, Olivier,D, A, F. Prems, w (2012). Sher ehviour of rok joints uner yli loing. Proeeings of the 11 th, Austrli - New Zeln onferene on Geomehnis : Groun Engineering in Chnging Worl, In G. A. Nrsilio, A. Arulrjh & J. Koikr (es.), pp Jfri, M. K., Amini Hosseini, K., Pellet, F., Boulon, M., & Buzzi, O. (2003). Evlution of sher strength of rok joints sujete to yli loing. Journl of Soil Dynmis n Erthquke Engineering. Volume 23, No. 7, pp Lee, H. S., Prk, Y. J., Cho, T. F., & You, K. H. (2001). Influene of sperity egrtion on the mehnil ehvior of rough rok joints uner yli sher loing. Interntionl Journl of Rok Mehnis n Mining Sienes Volume 38, No. 7, pp Mirzghornli, A Nemik, j. & Aziz, N. (2013). Effets of Sher Rte on Cyli Loing Sher Behviour of Rok Joints Uner Constnt Norml Stiffness Conitions. Interntionl Journl of Rok Mehnis n rok engineering. Volume 47, No. 5, pp Niktr. S. M. M. Ro, K.S, & Shrivstv, A.K. (2015) Sher Behviour of rok joints uner yli onition. Speil Pulition, Journl of Engineering Geology, Otoer pp Shrivstv, A. K, & Ro, K.S. (2013), Development lrge sle iret sher testing mhine for unfille n infille rok joint uner onstnt norml stiffness onition, Amerin Soiety for Testing n Mterils, Volume 36, No. 5, pp ACKNOWLEDGEMENT The uthors wish to thnk for the finnil support of this reserh from Inin Institute of Thehnology Delhi. We re thnkful to Mr. Jttiner Singh, Mnging Diretor, of Hyruli Engineering n Instruments Compny, New Delhi, for extening help uring the moifition of sher testing mhine. 7. REFERENCES Hung X, Himson BC, Plesh ME & Qiu X. (1993). An investigtion of the mehnis of rok 198