NOTE / NOTE

Stressstrain behaviour of weathered weak rockin middle-sized triaxial tests

Dean Sun, Tugen Feng, and Hajime Matsuoka

Abstract: A middle-sized triaxial test apparatus for a specimen 20 cm in height and 10 cm in diameter was developedto measure the deformation and strength of weak rock or gravel. High-quality undisturbed samples of a weatheredweak rock were taken from a dam site by a core drilling method. To avoid damage to the structure of the weak rockdue to saturation of specimens as a result of measuring volume change through the water change in a burette, the lat-eral deformation of specimens was directly measured in the unsaturated condition using three rings mounted on thespecimen. Using the developed triaxial test apparatus, isotropic compression tests and consolidateddrained triaxialcompression tests were performed on unsaturated or saturated undisturbed samples under confining pressures of 49, 98,196, 392, 539, and 683 kPa. The test results show that the stressstrain relationship of the weathered weak rock underboth unsaturated and saturated conditions is strongly influenced by the confining pressure when the confining pressureis less than 392 kPa, and the stressstrain behaviour becomes similar to that of normally consolidated clay when theconfining pressure is greater than 392 kPa. Comparison of results of triaxial tests on unsaturated and saturated speci-mens shows that the saturated samples become somewhat weak. The test results also show that the bonding and stresshistory largely influence the stressstrain relationship at small strain levels.

Key words: weathered weak rock, microstructure, undisturbed sample, deformation, strength, triaxial test, unsaturatedsample.

Rsum : On a dvelopp un appareil triaxial de grosseur moyenne pour les spcimens de 20 cm de hauteur et 10 cmde diamtre afin de mesurer la dformation et la rsistance de la roche molle et du gravier. Des chantillons non rema-nis de haute qualit de roche molle altre ont t prlevs sur le site dun barrage par une mthode de forage de ca-rottes. De faon viter dendommager la structure de la roche molle cause de la saturation des spcimens durant lamesure du changement de volume par la variation deau dans la burette, la dformation latrale des spcimens a tmesure directement dans la condition non sature au moyen de trois anneaux fixs sur le spcimen. Utilisantlappareil triaxial dvelopp, des essais de compression isotrope et des essais de compression triaxiale consolids nondrains ont t raliss sur des chantillons non remanis saturs et non saturs sous des pressions de confinement de49, 98, 196, 392, 539, et 683 kPa. Les rsultats des essais montrent que la relation contraintedformation de la rochemolle altre tant sature que non sature est fortement influence par la pression de confinement lorsque la pressionde confinement est infrieure 392 kPa, et le comportement contraintedformation devient similaire celui de largileconsolide lorsque la pression de confinement est plus grande que 392 kPa. La comparaison des rsultats des essaistriaxiaux sur les spcimens saturs et non saturs montre que les chantillons saturs deviennent quelque peu mous.Les rsultats des essais montrent galement que les liens et lhistoire des contraintes influencent fortement la relationcontraintedformation aux faibles dformations.

Mots cls : roche molle altre, microstructure, chantillon non remani, dformation, rsistance, essai triaxial, chan-tillon non satur.

[Traduit par la Rdaction] Sun et al. 1104

Can. Geotech. J. 43: 10961104 (2006) doi:10.1139/T06-057 2006 NRC Canada

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Received 31 August 2005. Accepted 28 April 2006. Published on the NRC Research Press Web site at http://cgj.nrc.ca on11 October 2006.

D.A. Sun.1 Department of Civil Engineering, Shanghai University, 149 Yanchang Road, Shanghai 200072, China.T.G. Feng. Department of Civil Engineering, Hohai University, 1 Xikang Road, Nanjing 210098, China.H. Matsuoka. Department of Civil Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan.1Corresponding author (e-mail: sundean@hotmail.com).

Introduction

In the past, weathered weak rock was removed during theconstruction of large important structures such as rock-filldams. There are many recent cases in which, for economicreasons, weathered weak rock has been left as the founda-tions of such structures. It is well known that weatheredweak rock will produce relatively large deformations whensubjected to loading because it has a high void ratio andweaker bonding. The design criteria of structures must sat-isfy a specified safety factor for ultimate stability and thedisplacement within its serviceability limit under workingload conditions. One of the major steps in designing struc-tures constructed on weathered weak rock is the predictionof ground deformation and structural displacement. There-fore, it is necessary to investigate deformation and strengthcharacteristics of weathered weak rock in various stressstates. In this study, a middle-sized triaxial test apparatuswas developed to measure deformation and strength of aweathered weak rock. Using the developed triaxial test appa-ratus, isotropic compression tests and consolidateddrainedtriaxial compression tests were performed on unsaturated orsaturated undisturbed samples.

This paper presents a newly developed middle-sizedtriaxial test apparatus, the test methods for the apparatus,and the results for weathered weak rock tested using the ap-paratus. The stressstrain behaviour of a weathered weakrock is identified from the results of triaxial tests under awide range of confining pressures. In addition, a series oftriaxial test data during isotropic compression and shear aregiven in detail so that the constitutive models for weak rocksor so-called structural soils can be checked or developed bycomparing the model predictions with the data presented inthis paper.

Experimental program

MaterialsThe weak rock used in the triaxial tests is a weathered

porphyrite. The minerals in the weak rock have been partly

decomposed to clays, and the rock is classified as D-classweathering rock according to the Japanese rock mass classi-fication standard, which classifies rocks according to thedegree of weathering (Tanaka 1964). According to the geo-logical strength index (GSI) classification (Hoek et al.1998), the weak rock has a GSI of about 10. Sampling wasundertaken at a dam site in Hyogo Prefecture, Japan. Thehigh-quality undisturbed samples were taken using a so-called core drilling method (Ijiri et al. 1998). The core drill-ing tube consists of two tubes, inner and outer. During theoperation of the outer rotary core tube sampling, the acrylicresin inner sampling tube does not rotate to reduce theamount of damage to the weak rock sample. It is easy tosample weak rocks at shallow depths using this samplingmethod. The tested samples were taken at depths of about0.51.5 m beneath the ground surface. The initial water con-tent, degree of saturation, and void ratio of all specimens aresummarized in Table 1. The samples have a high void ratioof about 1.31.4 and a specific gravity of 2.74.

Figure 1 shows a retrieved sample on the ground surface.The sample is 10 cm in diameter and 100 cm in length. Thesample in the acrylic resin sampling tube was cut into 20 cmlong specimens for triaxial testing, and then the specimenswere pushed out of the acrylic tube in a way that minimizedthe friction force between the tube and the specimen.

Middle-sized triaxial test apparatusFigure 2 is a photograph of the developed middle-sized

triaxial test apparatus, which can be used for testing unsatu-rated or saturated weak rocks and soils. The apparatus candirectly measure the lateral strain of the 10 cm in diameterand 20 cm in height cylindrical specimens. The lateral dis-placement of each specimen was measured using three ringsmade of stainless steel mounted at heights of H/4, H/2, andH from the bottom of the specimen as shown in Figs. 2 and3, where H is the specimen height. This method for measur-ing the lateral displacement of specimens was originally de-veloped for unsaturated soils (Sun et al. 2000). The lateraldeformation shape was assumed to be a third-order polyno-mial for calculating the lateral strain (Sun et al. 2004). Fig-

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Notation

Confiningpressure duringshear (kPa)

Water contentbefore testing(%)

Void ratiobeforetesting

Degree ofsaturation beforetesting (%)

State duringtesting

State of samplebefore testing

Figures showingtest data

SU49 49 41.2 1.26 88.8 Saturated Undisturbed 10, 13SU98 98 48.9 1.40 94.4 Saturated Undisturbed 10, 11, 14SU196 196 45.4 1.27 96.9 Saturated Undisturbed 10, 11, 14SU392 392 36.3 1.29 82.6 Saturated Undisturbed 7, 1012, 14SU539 539 41.9 1.32 97.5 Saturated Undisturbed 68, 12SU683 683 45.0 1.34 90.9 Saturated Undisturbed 7, 8, 12UU98 98 41.2 1.26 88.1 Unsaturated Undisturbed 9, 11, 14, 15, 16UU196 196 50.8 1.42 96.4 Unsaturated Undisturbed 9, 11, 14UU392 392 46.7 1.42 88.9 Unsaturated Undisturbed 7, 9, 11, 14UC98 98 30.0 1.34 60.4 Unsaturated Compacted 16SR392 392 39.8 1.08 100.0 Saturated Remoulded 8, 12SR686 686a 39.3 1.07 100.0 Saturated Remoulded 8UU98L 98 40.5 1.34 81.6 Unsaturated Undisturbed, from

1 m tube15

aIsotropic compression test only.

Table 1. Summary of initial states, confining pressures, and saturation states for the triaxial tests.

ure 3 also shows that the cell pressure is controlled by thepressurized air, and the axial force is exerted on the speci-men by raising the base pedestal while keeping the top capstill. Compared to the conventional triaxial cell, the new ap-paratus is capable of testing soils or weak rocks under an un-saturated condition and at a small strain level.

Figure 4 shows the results from calibration of a mountedlateral displacement transducer, which is a 145 mm diameterring with two strain gauges on two opposite sides. A goodlinear relationship between the change in diameter and theoutput voltage is observed. Hence, it is easy to calculate thechange in diameter using the transducers.

In most conventional triaxial tests on soils, the axial dis-placements are measured externally from a rigid boundary ofthe specimen such as the loading piston and the top cap. Forweak rocks, the effects of the test system compliance andbedding error are not negligible and are caused by the non-smooth and aligned contacts between the specimen ends andthe top and base of the pedestal, because the weak rock isharder than most soils and it is difficult to trim both ends of

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Fig. 1. Photograph of weak rock sample in acrylic tube.

Fig. 2. Photograph of triaxial test cell with three lateral displace-ment rings and two LDTs.

Fig. 4. Calibration of 145 mm diameter lateral displacementtransducer. d, lateral displacement.

Fig. 3. Locations of LDTs and lateral displacement transducers.

the specimen perfectly flat and parallel to each other. Theeffects become especially noticeable in tests to measure de-formation at small strain levels such as during isotropiccompression and the early stages of shear testing at smallconfining pressures. To avoid these measuring errors, theaxial strain was measured using a local deformation trans-ducer (LDT) (Goto et al. 1991) as shown in Figs. 2 and 3.Some researchers showed that the axial strain should bemeasured locally in triaxial compression tests on weakrocks, as large bedding errors could be involved in exter-nally measured axial strain (e.g., Jardine et al. 1985; Claytonet al. 1994; Kim et al. 1994). In this study, two 160 mm longpieces of LDT were installed on opposite sides of the speci-men diameter. The LDT detects changes in the distance be-tween the two ends of a slightly bent phosphor bronze stripwith electrical resistance gauges attached to the center of thestrip. The axial strain of the specimen is the average of thoseobtained from the change rates in the distances between twopair of hinges attached to the specimen surface.

Figure 5 shows the calibration of a 160 mm long LDT.The relative displacement (L, where L is the length of theLDT) was measured using a micrometer with a resolution of10 m. L0 is the length of the LDT at the state of no gaugestrain, and V0 is the output voltage corresponding to L0. Asshown in Fig. 5, the measured correlation between (V V0),where V is the output voltage, and L can be assumed as asecond-order polynomial for measuring the change in LDTlength.

Figure 6 shows the relationship between axial strain andmean effective stress, p, for the weak rock during isotropiccompression loading. The axial strain was measured usingexternal displacement transducers and LDTs represented inFig. 6 by the open triangles and circles, respectively. It canbe seen that the measured external axial strain is greater thanthe locally measured axial strain because there are beddingerrors at the top and bottom ends of the specimen and thetesting apparatus compliance. The discrepancy between theexternal and local axial displacement increments becomessmall, however, when the isotropic stress is relatively large.This is because the newly occurring bedding error is verysmall at relatively high stresses.

Testing proceduresUsing the developed middle-sized triaxial test apparatus,

isotropic compression tests and consolidateddrained triaxialcompression tests were performed on unsaturated and satu-rated undisturbed specimens under confining pressures of49, 98, 196, 392, 539, and 683 kPa. For unsaturated speci-mens, the tests were directly performed on samples takenfrom the construction site. The initial degree of saturationfor unsaturated samples is from about 80% to 95%. For satu-rated specimens, carbon dioxide gas and deaired water werecirculated through the specimens, and then a backpressure of196 kPa was applied to the specimens to ensure a high de-gree of saturation with a B value greater than 0.90, where Bis the ratio of the pore-water pressure increment to the con-fining pressure increment under undrained conditions.

To reduce the friction between the polished surfaces of thetop cap and bottom pedestal and the surfaces of the speci-men, the surfaces of the top cap and bottom pedestal werelubricated with Dow high vacuum silicone grease and then

two sheets of thin rubber membrane with a small hole atcenter were placed on the surfaces of the cap and pedestal(Tatsuoka et al. 1984). To speed up the drainage of the spec-imen during testing, two sheets of filter paper were placedon each membrane to gather water into the porous stones in-stalled in the centers of the cap and pedestal. The axial com-pression rate in isotropic compression tests and triaxial sheartests was about 3% per day under drained conditions. Drain-age was permitted at the upper and lower ends of the speci-men. Five days to a week is needed to complete one test,including the isotropic compression test and subsequenttriaxial shear test. In the isotropic compression tests, the ra-dial stress was adjusted to be equal to the measured axialstress; in the triaxial shear tests, the radial stress was keptconstant.

The initial states, confining pressures, and saturationstates during the shear tests are summarized in Table 1.

Test results and analysis

Deformation characteristics in isotropic compressionFigure 7 shows the results obtained from the four isotro-

pic compression tests on unsaturated and saturated speci-mens in the ep and e log p planes, where e is the void

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Fig. 5. Calibration of 160 mm long LDT.

Fig. 6. Axial strains measured by external displacement trans-ducer and LDT during isotropic compression.

ratio and p is the mean effective stress. An unsaturated spec-imen was tested in its in situ state and a saturated specimenwas tested after saturating an unsaturated sample. It can beseen that the initial void ratio (e0) of the specimen is ratherhigh (about 1.281.42). The arrows in Fig. 7a indicate theinitial yield stresses of the weak rocks, which were deter-mined at the stress above which the compression curve devi-ates from its initial linear behaviour in the ep plane. Thisdefinition is consistent with that used in other works (e.g.,Cuccovillo and Coop 1997; Rotta et al. 2003). The initialyield stress for the tested weak rock is about 200 kPa, atwhich it is considered that the bonding breakage of weakrock initiates. The arrows in Fig. 7b indicate the breakage-ending stress above which the bonding breakage of weakrock ceases, i.e., no new bonding breakage takes places. Thebreakage-ending stress is obtained from the stressstraincurve in the e log p plane, i.e., from which the e log prelation becomes linear. The breakage-ending stress is about400 kPa for the tested weak rock. The change in void ratiobefore the initial yield stress is very small and becomeslarge after the initial yield stress. This is because the bond-ing between particles begins to break after the initial yieldstress. The compression index Cc (i.e., the slope of the com-

pression line in the e log p diagram) does not remain con-stant but varies with p at isotropic stresses greater than theinitial yield stress, which increases with increasing p andthen becomes a constant value from the breakage-endingstress. These findings indicate that during the isotropic load-ing the bonding of the tested weak rock breaks gradually af-ter the initial yield stress and then the tested specimensbehave as destructured materials.

As indicated by many authors (e.g., Leroueil and Vaughan1990), weak rocks are microstructured (bonded), i.e., at agiven void ratio, they can sustain stresses higher than thoseof the same material without microstructure. In the isotropiccompression, microstructure can be shown by comparison ofthe compression curves obtained on the undisturbed weakrock and on remoulded and reconstituted sample of the samematerial. Figure 8 shows the results of isotropic compressiontests on undisturbed and remoulded samples. The remouldedsample was reconstituted by submerging an undisturbedsample in water and thoroughly disturbing it using a mixerto form a slurry. The remoulded specimen was made bypouring the slurry into a 10 cm in height and 5 cm in diame-ter cylinder and then immediately freezing it in a freezer.After being frozen, the specimen was set up on a conven-tional triaxial apparatus for isotropic compression testingand subsequent triaxial shear testing. The solid and open cir-cles in Fig. 8 are the results of isotropic compression testson two remoulded specimens. Line ICL1 is a normal consol-idation line for remoulded samples. The open triangles inFig. 8 are the test results from two undisturbed samples thatwere shown in Fig. 7. Line ICL2 is the compression line forundisturbed weak rock in the high stress ranges in which thebonding has vanished. It can be seen that ICL2 is higherthan ICL1 in the plane, which means that the undisturbedsample with no bonding has a higher void ratio than theremoulded sample. This is due to the difference in fabric be-tween the two samples. It can also be seen that the slope ofICL2 is greater than that of ICL1, which is considered theresult of a higher initial void ratio in the undisturbed sample.It can be inferred that the two lines ICL1 and ICL2 tend toconverge at a high stress. These deformation characteristicsof weak rock are similar to those of natural clay (Burland1990).

Deformation characteristics in triaxial compressionFigure 9 shows the results obtained from the isotropically

consolidated drained triaxial compression tests on unsatu-rated specimens under confining pressures r of 98, 196,and 392 kPa in terms of a r/ , a, r , and v relations, wherea and r are the axial and radial stresses, respectively, inthe triaxial test; a r/ is a principal stress ratio; and a, r ,and v are the axial, radial, and volumetric strains, respec-tively. It can be seen that there are differences in the defor-mation and strength due to differences in the confiningpressures, i.e., the peak stress ratio decreases with an in-crease in the confining pressure, the stress ratio versus straincurve slopes down with an increase in confining pressure,and the contraction of specimens during shear becomes largewhen the confining pressure increases.

Figure 10 shows the results of isotropically consolidated drained triaxial compression tests on saturated specimensunder confining pressures r of 49, 98, 196, and 392 kPa. It

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Fig. 7. Isotropic compression results: (a) linear scale; (b) loga-rithmic scale.

can be seen that the strength and deformation characteristicsare similar to those of the unsaturated specimens, i.e., thestress ratio versus strain curve slopes down with an increasein confining pressure, and the contraction of specimens dur-ing shear becomes large when the confining pressure in-creases. In detail, at a low confining pressure of 49 kPa, theweak rock shows a peak strength followed by strain soften-ing, as in a dense noncohesive soil; at higher confining pres-sures of 98, 196, and 392 kPa, yield occurs well beforefailure, which is only approached after a large strain accom-panied by significant contraction.

Figure 11 shows the comparisons of deviator stress( ) a r versus axial strain relation and volumetric strainversus axial strain relation obtained from the isotropicallyconsolidated drained triaxial compression tests on satu-rated and unsaturated specimens under confining pressuresr of 98, 196, and 392 kPa. Figure 11 is the result of rear-ranging Figs. 9 and 10 for comparing the stressstrainrelationship of unsaturated and saturated specimens. It can

be seen that two groups of stressstrain curves are similarand the deviator stress ( ) a r versus axial strain curves ofunsaturated specimens are slightly higher than those of thecorresponding saturated specimens. In particular, the initialYoungs moduli of unsaturated specimens are larger thanthose of corresponding saturated specimens. It can also beseen that two groups of the dilatancy curves are similar andthe amount of volume contraction of saturated specimens isslightly greater than that of corresponding unsaturated speci-

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Fig. 8. Isotropic compression curves (ICL1, ICL2) for undis-turbed and remoulded samples.

Fig. 9. Results of consolidateddrained triaxial compression testson unsaturated specimens.

Fig. 10. Results of consolidateddrained triaxial compressiontests on saturated specimens.

Fig. 11. Comparison of stress versus strain relations for unsatu-rated and saturated specimens.

mens. These results indicate that the saturated specimens be-come somewhat weak due to saturation.

Figure 12 shows the results of isotropically consolidated drained triaxial compression tests on saturated specimensunder confining pressures r of 392, 539, and 683 kPa andon a remoulded specimen under a confining pressure r of392 kPa. It can be seen that the strength and deformationcharacteristics of the weak rock in the stress range greaterthan 392 kPa are similar to those of normally consolidatedclay, i.e., the stress ratio versus axial strain curves duringshear at different confining pressures are almost the same,and the volumetric strain versus axial strain curves duringshear are also almost the same at different confining pres-sures. These results are consistent with those obtained in theisotropic compression test at the stress range greater than392 kPa, as shown in Fig. 8, which indicated that the testedweak rocks behave as destructured material in the stressrange greater than 392 kPa, i.e., the e log p relation is lin-ear in that stress range. Comparing the stressstrain curvesof remoulded and weak rock samples along the same stresspath, i.e., r = 392 kPa, we can see that the stress ratio ver-sus axial strain curve of the remoulded sample is steeperthan that of the weak rock samples and the volume contrac-tion of the remoulded sample is smaller than that of theweak rock samples. The difference in these two types ofsamples is consistent with that from the results of isotropiccompression tests, i.e., the slope of ICL2 is greater than thatof ICL1. The reason for this difference is that the remouldedsample is stiffer than the weak rock because of the smallvoid ratio.

Figure 13 shows Mohr stress circles at peak or at a =15% as obtained from the results of isotropically consoli-dated drained triaxial compression tests on saturated speci-mens. The broken line is drawn to be tangent to the Mohrstress circle of r = 683 kPa through the origin. It can beseen that the broken line is almost tangent to the Mohr stress

circles of r = 392 and 539 kPa and cuts across the Mohrstress circles of r = 49, 98, and 196 kPa, which is due tothe disappearance of the bonding of undisturbed weak rockat the shear failure of r = 392, 539, and 683 kPa.

Figure 14 shows Mohr stress circles at peak or at a =15% and their failure envelopes obtained from theisotropically consolidated drained triaxial compressiontests on unsaturated and saturated specimens in the range ofr = 98392 kPa. Although there are many excellent criteriasuch as the HoekBrown criterion for rock (Hoek andBrown 1980), the MohrCoulomb criterion is chosen herefor simplicity, and different conditions can be compared eas-ily by the corresponding parameters. It can be seen that thevalue of the apparent friction angle, , for both unsaturatedand saturated specimens is 30, whereas the value of appar-

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Fig. 12. Results of consolidateddrained triaxial compressiontests on saturated samples under confining pressures of 392, 539,and 683 kPa.

Fig. 13. Mohr stress circles at failure for saturated specimens inthe range of r = 49683 kPa.

Fig. 14. Mohr stress circles at failure under r = 98, 196, and392 kPa: (a) unsaturated state; (b) saturated state.

ent cohesion, c, for unsaturated specimens is larger than thatfor saturated specimens in that stress range. As the degree ofsaturation for unsaturated samples is at 80%95%, the suc-tion effect can be ignored. Hence, it can be concluded thatthe damage of bonding due to saturation only induces a de-crease in the value of apparent cohesion and does not changethe value of the apparent friction angle.

Influence of pushing pressureIt is well known that the stress history has a strong influ-

ence on the subsequent stressstrain relationship andstrength. Figure 15 shows the results of isotropic compres-sion tests and isotropically consolidated drained triaxialtests on two undisturbed samples obtained using differentmethods to push the sample out from the thin-walled sam-pling tube. As described earlier, the length of the thin-walledsampling tube is 100 cm. The solid curves shown in Fig. 15are the results of isotropic compression tests and triaxialtests on the sample pushed from the 100 cm long tube. Thebroken curves shown in Fig. 15 are the results of isotropiccompression tests and triaxial tests on the sample pushedfrom the 20 cm long tube, which was obtained by cutting the100 cm long sampling tube before the samples were re-moved. Since pushing the sample out needs to overcome thefriction between the sample and the tube, the maximumpressure of the former is about five times that of the latter. Itcan be seen from Fig. 15a that the axial strain of the formerspecimen is much smaller than that of the latter during iso-tropic loading, and the axial strain measured using the LDT

is much smaller than that measured using the external dis-placement transducer for the same isotropic compressiontests on both samples. It can be seen from Fig. 15b that theformer specimen has an initial yield stress greater than thatof the latter, and the strength of the two specimens is almostthe same. Therefore, the initial yield stress and Youngsmodulus of weak rocks or soils will be overestimated usingthe results of tests on the sample pushed out from a longthin-walled sampling tube.

Influence of microstructureFigure 16 shows a comparison of the stress ratio versus

axial strain relationship obtained from the isotropically con-solidated drained triaxial compression tests on undisturbedand compacted unsaturated specimens under a confiningpressure r of 98 kPa. The results for the undisturbed sam-ple are shown in Fig. 9. The compacted specimen was madeby compacting the same crushed weak rock to reach a drydensity similar to that of the undisturbed sample. It can beseen that the two groups of stressstrain curves are similarfor specimens with almost the same initial density. The ini-tial Youngs modulus of the undisturbed specimen is greaterthan that of the compacted specimen, however. This resultshows that the microstructure affects largely the initial de-formation coefficient, i.e., the bonding increases the initialYoungs modulus. It can be seen that the microstructure ofthe tested weathered weak rocks has small effects on thestrength and deformation characteristics except for the initialshear stage, which is due to the loss of bonding at largestrains.

Concluding remarks

Using a newly developed middle-sized triaxial test appara-tus, isotropic compression tests and isotropically consoli-dated drained triaxial compression tests were performedon unsaturated and saturated undisturbed weak rock samplesunder a wide range of confining pressures. The test resultsshow that the stressstrain relationships of the weathered

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Fig. 15. Effect of pushing pressure on stress versus strain rela-tion: (a) results of isotropic compression test; (b) results ofconsolidateddrained triaxial compression test.

Fig. 16. Comparison of stress versus strain relation for undis-turbed and compacted specimens (r = 98 kPa).

weak rock under both unsaturated and saturated conditionsare influenced by the confining pressure. When the confin-ing pressure is large, the specimens exhibit only negativedilatancy, i.e., volume contraction, although the specimensexhibit negative and positive dilatancy at low confiningpressures. The smaller the confining pressure, the steeper isthe stress ratio versus axial strain relationship obtained fromtriaxial tests on unsaturated and saturated specimens. The re-sults of both isotropic compression tests and isotropicallyconsolidated drained triaxial compression tests indicatethat the weak rock behaves as a destructured material in thestress range greater than 392 kPa, and undisturbed sampleshave higher void ratios than remoulded samples at the samestress. Comparison of the results of triaxial tests on unsatu-rated and saturated specimens shows that the samples be-come somewhat weakened due to saturation for measuringthe volume change.

Acknowledgements

The experimental work in this paper was carried out atNagoya Institute of Technology when the first and secondauthors were working there. The authors wish to expresstheir gratitude to the Kansai Electric Power Co., Inc., andNewjec Inc., Japan, for providing the testing materials, un-disturbed weak rock samples. The authors would also like tothank Drs. C.F. Chiu and A. Deng at Hohai University,China, for their help in the revision of this paper.

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List of symbols

B pore pressure parameterc apparent cohesion

Cc compression index, i.e., slope of the compression line ine log p diagram

e void ratioe0 initial void ratioH specimen heightL length of LDT

L0 length of LDT at state of no gauge strainLDT local deformation transducer

p mean effective stressV output voltage

V0 output voltage corresponding to L0d lateral displacementL relative displacement of LDT lengtha axial strainr radial strainv volumetric straina axial effective stressr radial effective stress frictional angle

2006 NRC Canada

1104 Can. Geotech. J. Vol. 43, 2006