ISRM-EUROCK-1993-066_Strength Directionality in Cyclic Joint Shear Tests

8/12/2019 ISRM-EUROCK-1993-066_Strength Directionality in Cyclic Joint Shear Tests

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Eurock '93, Ribeiro e Sousa & Grossmann (eds) 1993 Balkema, Rotterdam 90 54103396

Strength directionality in cyclic joint shear testsDirectionalit de la rsistance au cisaillement pendant des essais cicliques sur discontinuits

rocheusesRichtungsabhngigkeit der Festigkeit bei zykJischen Scherversuchen an Klften

Giovanni Crosta Dip. Scienze della Terra, Milano.Italy

ABSTRACT: Joint shear strength dependence on shear direction and sense is of major importanceduring cyclic shear. In fact, cyclic tests are particularly useful to simulate dynamic loads or shear load

reversals so frequently observed in nature. Some simple laboratory tests have been performed atconstant normal load on a massive serpentinitic rock. Results, in accordance with previous authors, are presented with particular regard to shear strength, shear stiffness and asperity degradation.

~SUME': La dependence entre la resistence au cisaillement a une grande importance pendant unCIsaillement ciclique. En effect, les tests cicliques sont particulierment utiles pour simuler les chargesdynamiques et les inversion du charge de cisaillement, qui sont frequentes en nature. On a fait des testsde laboratoire avec des roches serpentiniques massives sous un charge normale et constante. On pre.sente les resultes, qui sont en accord avec les precendes auteurs, avec particuliere attention pour lareSlstence et la rigidit au cisaillement et la degradation des asperites.

ZUSAMMENFASSUNG: Bei wechselnder Scherbeanspruchung h ngt die K1uftscherfestigkeit in hohemMa13e von der Scherrichtung und dem Schersinn ab. Zur Simulation von dynamischen Belastungen undder Scherspannungsumkehr, die hufig in der Natur beobachtet warden, sind Scherversuche mit",:echselnden Beanspruchungen besonders geeignet. Unter Normallast wurden an massivem SerpentiniteInfache Laborversuche durchgef hrt. Die Ergebnisse werden, in bereinstimmung mit anderen~uto~en, unter besonderer Ber cksichtigung der Scherfestigkeit, der Scherungsteifigkeit und der

auhlgkeitsverluste vorgestellt.

I. INTRoDUCTION

The behaviour of rock joints during cyclic or ~eversal shear has been recognised as anisotropic

d~cause of its dependence on both the shear Ire f G c IOn and the shear sense (Celestino &Moodman, 1979; Hutson & Dowding, 1990;th uralha & Pinto da Cunha, 1990). During shear,de s.trength and deformability of rock

ISCOntmuities are in continuous evolution and the;eC~anical properties of the discontinuitiesPamInate the response of the entire rock mass.s~esent and previous normal and shear loads,

r ear displacement, shear direction and shear ate camp . d . ., ressive strength of intact rock an JOIntWalls ., together with some scale-dependent

parameters as joint roughness, waviness andinfling material, are the main factors influencing

joint behaviour under a shearing action. Other parameters influencing joint behaviour are jointasperity degradation and the initial aperture (gapwidth).

The accumulation of shear displacement inlaboratory tests by changing the sense of shearing but not the direction represents one of the bestways to model seismic loads, cyclic shear loads or the unloading of previously sheared discontinuity,reaching amounts of displacement large enough tomobilize the entire range of discontinuity shear

strength, from the peak up to the residual shear strength.As a consequence, displacement accumulation

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ranged between 0.98 and 5.88 MPa. In particular,different tests were conducted at 0.98, 1.96, 2.94,3.92,4.90 and 5.88 MPa. From the reported values(Table f) results that the alae ratio (normal stressto unconfined compressive strength) ranged from0.006 to 0.036. Velocity, even if important

(Crawford & Curran, )98), must be considered asecondary factor for shear strength, excepted for eXtremely wide ranges of variation.

The relative shear velocity adopted between thetwo halves of the shear box ranged between 2 and3 mm/minute.

Finally, it is worth mentioning for its influenceOn experimental results the most common problem with this experimental technique andreported in the literature as "joint seating". SeatingConsists of negative bulking or decreasing dilation

acCOmpanying continued back and forth shearingas a consequence of loss of shearing producedgauge.

3. PEAK AND RESIDUAL SHEAR STRENGTH

Peak shear strength of rock joints has been aSubject of interest for many years and differentequations have been suggested to define shear

strength envelopes and their change as aConsequence of scale effects. Tn fact, it is wellestablished that shear strength increases withnOrmal compressive stress, resulting in slightlycurved failure envelope. To evaluate the quality of the experimental data, peak shear strength values;er e Compared with the power law model and the

arton-Choubey's model (1977; fig. I). Bothrno?els offers a good fit but the last one adoptseasily evaluable data to characterize joint~~ometry and material properties. Figure I shows

e extremely good correlation found from thecomparison. We see the flattening of the failureenvelopes for higher normal load (a) levels, more pronounced in correspondence of (a) 4.90 MPa,and the shear strength dependence from the IRCvalue Fdi . rom the shear stress versus shear

Isplacement curves (fig 2a b) we see the smallrel . . , ,2 " h atlve shear displacement (0.7 to 2 mm, 0.7 toO O h of sample size) to reach peak resistance.t er c. .eatures are the shear strength peak VanIsh' .e 109 WIth the increase of normal load level,r xcept for samples of small surface area and highcoughness coefficients, with increasing number of lCI~s and ultimately the cumulation of shear

ISpacement in the same direction. Finally, a

6000

5000 JRC-23

1RC- 6-S

~ 4000

~~ 3000

; 2000

1RC-SIO

o 1Re 10-12

- - - .- 1RC- 2

--1RC-6

----- 1RC- 10

1000 --0-- 1Re 12

2000 4000 6000

NORMAL STRESS (kPa)

Fig. 1. Shear stress/normalfailure envelopes accordingChoubey's criterion.

stress plot andto the Barton-

hardening behaviour and a peak migration is oftenrecognized within the tests though the residualshear strength zone of the various curves tend tocorrespond from one cycle to another.

The residual shear strength has been defined for each shear in both directions. Figure 3a shows astrikingly good and regular relationship between

averaged values of the residual friction angle andthe shear direction and normal load. The residualshear strength slightly increases going throughsuccessive shears, both on direct (forward) andreverse (backward) shear while it shows a moreregular trend in backward shears. Eventually, onewould judge as impressive the extremely lowvalues found for some backward shears, lower than the base friction angle (Ih = 230)determined by means of tilt and direct shear tests,on perfectly smooth surfaces.

4. PEAK SHEAR STIFFNESS

Joint stress-deformation behaviour, under certainnormal and shear stresses, is described by:

kM kIfS( dv)

du

where Jc.n and k ss are the normal and shear stiffnesses, respectively. Stiffness parameters arestress (e.g.: log Ksi log an plot subdivided in four zones using friction angle and peak shear displacement; Infanti & Kanji, 1990) and scale

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12

' "E E

~ .0-

z , ; -20 20 . "

E-

-10

dh (mm)

[ o lst Backward Sh. . - 1st Forward Sh.Fig. 2a. Shear strength/shear displacement plot.Displacement is reported as for a cycle even if

joints were simply sheared and then backwardsheared during the test up to a completerecomposition.

dependent (Barton, 1990; Bandis, 1990).Furthermore, stiffness is dependent on roughness,

joint aperture, infilling properties, test apparatusand is dependent by the initial degree of

interlocking which is logically a function of the previous normal and shear stress history and,consequently, of previous shear cycles number andmodality (fig. 2a). For example, matched andunmatched joints will result in different behaviorsin function also of the experimental stage in whichthese different conditions are realized. Anunmatched joint after forward shear displacementhas been completely recovered, will inducedilation and an increase in shear strength duringthe next forward shear only after the occurrence

of some horizontal displacement (fig. 2a, b) inorder to engage the most important andcontrolling (influencing) asperities (Sun et al.,1985; Hutson & Dowding, 1990), unmatched or separated by some just mechanically formed gaugeinfilling. This behaviour seems typical of jointswith increasing infilling thickness, as recentlyreported by Papaliangas et al. (1990). In fact,during laboratory tests shear stiffness resultedgenerally decreasing through the cycles in theforward shear direction (from category I to 1IJaccording to Sun et al., 1985). Furthermore, it wasfound that shear stiffness for the first forwardcycle is about 1.1 -i- 2.0 times the stiffness of thetwo following cycles. The average trend followed

2.5

-20 -10 10

-0.5

dh (mm)

2nd Forward Sil . o 2nd Backward Sh. -,

Fig. 2b. Dilation plot for the same sample as infig. 2a. Closure is negative in sign, dilation is positive.

by the shear stiffness from the first forwardshearto the third one are summarized in figure 3bAn opposite trend was found for stiffness in the backward (reversed) shear direction. The stiffness

values for the first backward shear always result ~lower values with respect to the forward shears, 10the ratio of about I: 1 up to I :6. A slight increaseis always observable for the successive backwardshears. From fig. 3b the increase of peak shear stiffness isevident as the convergence of thef averaged recorded values with the increase 0applied normal loads. Test results show that shearstiffness of rough competent joints and weaker smooth joints converges with the increase of theapplied stress level (fig. 3b and especially fig. 4)

in accordance with Barton (1990) who speaks alsoabout the block size effect on shear stiffness.

5. DILATION AND JOINT ASPERrr Y

DEGRADA TIaN

Dilation plots for forward shears are usuallYcharacterised by a first compressive zone, up tothe peak shear stress, for a 0.7 -i- 2 mm she~displacement (see fig. 2b), a second long anregularly steep dilational zone and eventually athird zone, difficult to reach for low normal loadswithin this range of shear displacements, wheredeformation continues at constant volume. It haS

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e

~

~ 30

~8 20

~f a

~ 10

-c

I SI forw ar d I st b ac kw ar d 2 nd forward 2nd backward Jrd forward

SHEAR STAGE

l-a: -.- 9S1 kl'lI 11)61 kP.1 2~2kP.1

L

Fig. 3a. Variation in the measured residualfriction angle (averaged from different tests) asa function of the shearing sense and the appliednormal load.

been observed that with the increase of thenormal load level the compressive zone becomesmore important while the third or constant onev~lume zone becomes flatter and well developed.Dilation Curves are slowly shifted downward from~ycle to another and increasing normal load. TheIn~reased importance of the compressive phase iseVident running through the cycles.B~ckward shear dilation curves are always slightlyshifted upward with respect to the forward~orr


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1.6E-!J3

~ 10,7 1Re VALUES

1961 aVALUF.S(kP.)

~ 1.2E-!J3

9.58.59811961

~.10,7

981 6.9

~i 6.5 39ZJ 11.68.0E-!J4 3.3 1961 29421941 8.9~5 8.6

'. 2 .39ZJ1961 . 1 H 1 . i 39ZJ 4,Q 8.3

0 2941- 1941 2.5 8.5u 4,OE-04 58lU 58lU 12.2~ 8.9 ' 904~

. - .12.8 11.6" 102

39ZJ .904 !I8lu

O .O E +O O

1000 2000 3000 4000 5000

SHEAR STRESS (kPa)

Fig. 4. Shear compliance component (inverse of the shear stiffness)/shear stress plot. IRC ando values, increasing towards the right hand sideof the plot, are reported for each point.

(Celestino & Goodman, 1979; Johnston & Lam,1989; Xu & Freitas, 1990), or geometriccomponent of the friction, and the asperitydegradation component, expressed by an angle,corresponding to the frictional resistance induced by wearing, breaking off and grinding of theasperities (w; expressed as R = IRC * Log[JCS/onl by Barton & Bandis, 1982) at differentscales (primary and secondary asperities):

lforward = Ir + d mob + w.

As a consequence of reversing the shear loaddirection, according to Celestino and Goodman(1979), a "downhill sliding" occurs and a negativeor subtractive dilation takes place (some oppositesituation could be imposed by differently inclinedshearing planes through the asperities, Johnston &Lam, 1989; Xu & Freitas, 1990), without breakingoff minor order asperities, up to values lower thenthe base friction value, so we write:

lbackward = Ir - dmob + w.

It is easy to understand how the geometriccomponent of the frictional resistance will reacha maximum for rough joint surfaces, low normalstress and former forward shears, while it will

suffer a decrease for higher normal stress levelsand decreasing surface roughness. Roughnessdependence is connected with sample size

dependence because of the continuous and progressive change observable in the role of steepand small asperities and of joint waviness with the

joint size increase. On the other hand, thecontribution given by the asperity degradation will be minimal for smooth surfaces, very low%e

ratios and backward shears, in consequence of thedirectional dependence of asperity failure and of the damages previously tolerated by asperities.What has been observed during the tests, asalready reported by Krahn & Morgenstern (1971)and Hutson & Dowding (1990), is a backwardresidual friction angle constantly lower than theforward one. Dilation curves, for backward shears,are always slightly shifted upward with respect tothe forward corresponding curve. Dilation curves, both for forward and backward shears, are slowlyshifted downward from a cycle to another andincreasing the normal load level. This phenomenon could be explained by a gradualseating of the joint, as mentioned above, or sometimes by a continuous degradation,homogeneization and grain size decrease to givea better compaction of the gauge infilling. Somegrain size analysis performed on the gaugematerial revealed a grain size distribution rangingfrom sands to silts.

The direct shear stress dependence fromdilation and joint matching has been proven by acouple of experiments where compressioncharacterized the entire forward shearing instea.dof the reversed one. This situation resulted 10lower forward and higher backward resistanCes;Finally, this seems to be in agreement with Sun s(Sun et al., 1985) observation about the behaviour of joints with dissimilar or mismatched surfaces.From the experimental results, the main reasonfor such a behaviour is supposed to be the chan~eof dilational direction, negative (closure). Inforward shears and positive (dilation) dunng backward shears, due to initial joint unmatchingand to the anisotropy originated by the impo~dshear direction and the consequent asymmetnC

joint asperity degradation (Celestino & Goodman,1979).

Furthermore, joints wearing results proba?ly

~~r~r~~tlf~:~~n s~:~~c~ r:~lt d~~t~~'m~;;~f asperities shearing and breakage, functIon 0inhomogeneities presence and applied normalload level (Johnston & Lam, 1989; Xu & d~

Freitas, 1990), while its effect becomes 0secondary importance upon shear stress reversal.In fact, the role that asperity recomposition playS,

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at least during the first backward shear, could bea function of the geometry of the shear planesoriginated through the asperities.

Joint asperity degradation and the formation of filling gauge are between the causes of the shear strength peak vanishing and the decrease in shear stiffness during successive cyclic forward shears. Infact, with an increased infilling thickness andOCcasional rock-to-rock contacts, the peak shear stiffness will tend to be less stress dependent, because of lower shear strength, and almost no?Jore size dependent. On the contrary, an increaseIn shear stiffness was commonly observed inlaboratory through cyclic backward shears. Causeof Such a behaviour could be the increasedmismatching of opposite joint surfaces, recovered

through a backward horizontal displacement, andthe increased degree of indentation of the grains,forming the infilling gauge, on the joint surface. Infact, after each 20 mm of forward displacement,under normal stress action and shear load release,grains constituting the gauge infilling can settle alIttle bit and penetrate the joint surfaces (as could be thought from the vertical versus horizontaldisplacement plots; fig. 2b).

The slow and gradual increase of the residual

reSiStance (hardening) during successive forwardshears, in spite of the increased asperity?egradation, could be explained by theInterposition of grains between opposite jointWalls that could induce, before of a completeComminution of the grains, a greater mutualCOmpenetration like an artificially inducedrOughness. The final effect is the one that we alle~perienced walking and scratching a smooth floor ;Hh sand grains under the shoes. About scaleBependence of shear stiffness and of friction angle

p,arton

(1980) and more recently Muralha andI~to da Cunha (1990) found that no scale effectseXISt when joint area reduction is made parallelytOfthe shear stress direction as evidenced in shear Sb fness . . ,I SIze correction by adoption of referenceengths (Lo and t.; Barton, 1990; Bandis, 1990)

meaSured parallel to the applied shear stress.

UI'f bmately, the stress-deformation dependence

Cr om the stress-path and the shear sense historyan

d' represent a mean to understand past

o~eCti?ns ?f movement and the geological historythe main . ~Iscontinuity systems, besides to assessstru StabilIty of different natural or man-made

Ctures submitted to cyclic shear loads.

7. APPLICATIONS

The directional behaviour of joint shear strengthcould be of major interest for slope stabilityanalyses in seismic areas, anchor dynamic andfoundation design or displacement estimates along previously overloaded discontinuities. In dynamicstability problems the amount of tolerabledeformations is considered more important thanthe istantaneous overload under the action of seismic loads. As a consequence of suchdeformational dependence and importance, thefrictional strength mobilized during each loadingand deformational cycle is of major interest for rock slope analysis and design.

As already mentioned, it is not difficult toimagine other cyclic load situations and the effectsinduced upon load reversal, especially consideringsurficial or underground artificial excavations inrocks with discontinuities already undergone tosome shear stress and displacement by artificial or natural (tectonic) activity. The simple release of the confining pressure can trigger displacementshigher than expected in the direction opposite tothe previous loading direction, even if rock massstiffness could playa controlling role.

An earthquake could represent the source for a

compressive or an extensive initial impulse. If weimagine an unconstrained rigid body laying on aslope, it could be pushed uphill or downhill byexceptional accelerations inducing a first shear displacement of the mass in a specific direction.The hypothesis of an uphill movement iscommonly excluded in classical slope stabilityanalyses, even for the occurrence of strong uphillacceleration, but it is still possible and it can cover an important role. More simply and realistically,examples of landslide main scarp and sliding

surface superimposed on pre-existing faults arenot difficult to be retrieved in the field and case-history literature. At this point, it could beinteresting to evaluate the relative influence of directionality on shear strength with respect to theinfluence of joint size changes.

For a compressive uphill impulse, the first shear displacement for unconstrained body will bedirected uphill, mobilizing in that direction themaximum shear resistance after small

displacements. Upon seismic load reversal, alower friction angle will be mobilized inducing, for the same time interval of load application, alarger relative displacement in this direction with

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respect to the uphill one. Continuing, the masscould become unstable through few load cycles,supported by a favourable downhill lower frictional resistance, slightly increasing, throughthe cycles, up to the ultimate frictional resistance.Under an initial extensional seismic load the slope

will be sheared along the discontinuity in adownslope direction mobilizing the higher shear resistance. Opposite to the previous case, wecould observe a more stable slope unable, in fewcycles, to reach the instability threshold of cumulated displacement or being in need tocumulate higher displacement to mobilize theresidual friction angle. Furthermore, not to beforgotten it is the increasing shear stiffnessthrough the backward shears and the regular decrease in forward shears and eventually the

computation of the total work done during thetests compared with the dilational trend toevaluate the degree of damage of the shearedsurfaces.

ACKNOWLEDGEMENTS

This research has been funded by the ResearchProject 1.3.2 of the CNR-GNDT.

REFERENCES

Bandis, S.C. 1990. Scale effects in the strength anddeformability of rocks and rock joints. In: Scaleeffects in rock masses, Pinto de Cunha (ed.), 59-76.

Barton, N. & Y. Choubey 1977. The shear strength of rock joints in theory and practice.Rock Mechanics, 10, 57.

Barton, N. & S.c. Bandis 1982. Effects of block-size on the shear behaviour of jointed rock.Keynote lecture. Proc. 23rd U.S. Symp. on Rock Mechanics, Berkeley, Ca, 739-760.

Barton, N. 1990. Scale effects or sampling bias?In: Scale effects in rock masses, Pinto de Cunha(ed.), 31-55.

Celestino T.B. & R.E. Goodman, RE. 1979. Pathdependency of rough joints in bi-directionalshearing. Proc. Iyth Int. Congo Rock Mech.,Montreux, Switzerland, I: 91-98.

Crawford, A.M. & J.H. Curran 1981. Theinfluence of shear velocity on the frictional

resistance of rock discontinuities. Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 18: 505-'SIS.

Goodman, R.E., L.L. Taylor & T.L. Brekke 1968.A model for the mechanics of jointed rock. 1.Soil Mech. Found. Div., ASCE, 94: 637-659.

Hutson, R.W. & C.H. Dowding 1990. Jointasperity degradation during cyclic shear. Int. 1.Rock Mech. Min. Sci. & Geomech. Abstr., 27:

109-119.Infanti, N.Jr. & M.A. Kanji 1990. Estimating theshear stiffness of rock joints. In: Rock Joints,Barton & Stephansson (eds.), Balkema, 799-804.

Johnston, LW. & T.S.K. Lam 1989. Shear behaviour of regular triangular concrete/Rock joints. Analysis. J. Geotech. Engng. Div., ASCE.115,5: 711-727.

Krahn, J. & N.R Morgenstern 1979. The ultimatefrictional resistance of rock discontinuities. Int.J. Rock Mech. Min. Sci. & Geomech. Abstr., 16:

127-133.Muralha, J. & A. Pinto da Cunha 1990. Analysisof scale effects in joint mechanical behaviour.In: Scale effects in rock masses, Pinto de Cunha(ed.), 191-200.

Papaliangas, T., A.C. Lumsden, S.R. Hencher & S.Manolopoulou 1990. Shear strength of modelledfilled rock joints. In: Rock Joints, Barton &Stephansson (eds.), Balkema, 275-282.

Sun, Z., C. Gerrard & O. Stephansson 1985. Rock joint compliance tests for compression and shear loads. Int. J. Rock Mech. Min. Sci. & Geomech.Abstr., 22: 197- 213.

Tse, R & D.M. Cruden 1979. Estimating jointroughness coefficients. Int. J. Rock Mech. Min.Sci. & Geomech. Abstr., 16: 303-307.

Xu, S. & M.H. de Freitas 1990. Kinematicmechanisms of shear deformation and thevalidity of Barton's shear models. In: Rock Joints, Barton & Stephansson (eds.), BaIkema767-774.

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ISRM-EUROCK-1993-066_Strength Directionality in Cyclic Joint Shear Tests