measuremens of soil strength in simple shaer test

8/13/2019 Measuremens of Soil Strength in Simple Shaer Test

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DISCUSSIONS

Measurement of soil strength in simple shear tests: DiscussionUniversity of Science and Technology Kumasi Ghana

A N DF. TATSUOKA

Institute of Industrial Science University of Tokyo 7-22-1Roppongi Minato-ku Tokyo 106 JapanReceived September 10, 1991Accepted February 17, 1992

Can. Geotech.J. 29 874-877 1992)

The use of the conventional simple shear apparatus formeasuring the shear strength parameters of soils has somemerits over many of the existing laboratory testing appara-tuses. However, one of the drawbacks of the conventionalsimple shear apparatus is its inability to measure all the stresscomponents, thus necessitating the need for various assump-tions to construct the Mohr circle of stress (Wroth 1987) andto evaluate the shear strength in terms of principal stresses.In the above paper, it was assumed that the ratio of thehorizontal stress (a, ,) to the vertical stress (a;) at the criticalstate increases with overconsolidation ratio (OCR)~ ndthat for heavily overconsolidated specimens, failure occurswell in the passive state, i.e., a, , > a;. Based on theseassumptions, it was deduced in the paper that, given thatthe critical state friction angle +,, = arcsin[(oi a;)/a{ + a;)],, is independent of the consolidation history,the simple shear critical state stress ratio angle p, , =arctan(~;/o:),, increases with OCR. In the conventionalsimple shear apparatus, such assumptions and any subse-quent deduction cannot be adequately evaluated.As part of a study into the behaviour of reconstitutedkaolin (liquid limit LL) = 84.2%, plasticity index (PI) =43.6%, and G, = 2.65) in simple shear from very smallstrains to failure and the effect of consolidation history, weused an automated hollow cylinder torsional simple shearapparatus, which in addition to several other capabilities canconsolidate specimens along various stress paths and alsomeasure all stress components accurately during all stagesof testing. In these undrained simple shear tests, all theincremental strains in the axial, circumferential and radialdirections were maintained to be zero. For details of theapparatus, test procedure, and test results see Ampadu(1991) and Ampadu and Tatsuoka (1992). Some relevantextracts from this study are presented to discuss some ofour findings with respect to the above-mentioned assumption.Table 1 summarizes the consolidation and undrained sim-ple shear failure conditions of some of our results of testson hollow cylinder specimens of initial dimensions as fol-

lows: 16 cm height, 10 cm outer diameter, and 6 cm innediameter. The relationship between the effective axial stresa; and the effective circumferential stress a; for these tesis shown in Fig. la. In the hollow cylinder, the stresses aand a; are equivalent to the vertical stress a; and the horzontal stress a, ,, respectively, referred to in the paper. Thspecimens were first KO-normally reconsolidated (teA201-1) and, for overconsolidated tests, later KO-reboundalong KO= OCR'.~ from about the same preconsolidatiopressure a;,. The KO-rebound tress path is based on Maynand Kulhawy (1982) using + = 17". The variation of thstress ratio K = a;/u,' as shearing progresses is shown iFig. 2. This figure and Fig. l a illustrate our observation thadespite their different initial values, the stress ratioK = a;/a; change rapidly, moving towards a commovalue close to unity but always on the active side, i.e., a

a;.Owing to slight pressure changes between the recordeend of consolidation and the beginning of shearing, differevalues of overconsolidation ratios may be obtained depending on whether the definition is based on the stress state the end of consolidation (OCRc = U,',/U;~) r at the begining of shearing (OCR = a;,/a,',). The differences in thvalues obtained with the different definitions, listed iTable 1, may be considered insignificant except for teA098-1 conducted at a relatively low pressure level. Thdetails of this test are shown in Fig. l b as a typical heavioverconsolidated specimen. For this test, the slight changin pressure reduced the stress ratio K at the start of shearinfrom 1.30 to 1.12. Nevertheless, this figure and Fig. 2 clearshow that for a heavily overconsolidated specimen, evethough the stress path starts from a passive stress sta(K > l), it rapidly crosses over from this state to the activstress state (K 1) and remains in this state until failureThe implications of this behaviour in terms of the diretion a of the major principal stress from the vertical ashown in Fig. 3. (In the hollow cylinder apparatus, a cabe determined, since all the stress components are measured.) The figure shows that a changes rapidly from 0" fo

'paperby J.H. Atkinson, W.H.W. Lau, an d J. J.M . Powell. normally and lightly overconsolidated specimens and from1991. Canadian Geotechnical Journal, 28: 255-262. 90" for heavily overconsolidated specimens towards a com20verconsolidation ratio OCR is given as R in the DaDer under mon value slightly less than 45". There is also the tendenc-discussion. for a slight increase of with OCR at large strain levelPrinted in Canada / Imprime au Canada


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DISCUSSIONS 875TABLE . Consolidation and failure parameters

Consolidation FailureaTest Iw d p d d s e ac max a; PFname K~ (070) (kgf/cm2) (kgf/cm2) (kgf/cm2) OCR: O c R h (kgf/cm2) (deg) (deg) (deg)

A201-1 0.70 50.00 3.71 3.71 3.72 1.00 1.00 0.631 39 17.1 15.3A132-1 0.86 51.85 3.75 1.70 1.67 2.25 2.20 0.484 40 18.1 16.3A154-1 1.03 52.57 3.77 0.75 0.74 5.09 5.03 0.342 42 20.8 18.8A098-1 1.30 55.36 3.81 0.39 0.30 12.7 9.80 0.229 43 21.2 19.4

aThe failure parameters are defined at = 20 and at the end of test for test A098 1.b ~ t r e s satio o;/o; at the end of cons olidati on.CWater content a t the end of test.d~reconsolidation ffective axial stress.eEffective axial stress at the beginning of shearing.f~ ff ec tiv e xial stress at the end of consolidation.gOCRc = o ~ , / o ~ , .h~~~ = o;,/o;,.',Direction of o; from the vertical.Jq = arcsin[(oi u;)/(u~+ u ;)]~ .kph = a r c t a n ( ~ / o ; ) ~ .

1.00 b ) 1 - I . f lTest A098-11 4 2 : Undrained simple shear

bD.,.0.60 -

0 2 3 4 0.00 0.20 0.40 0.60 0.80 1.00Effective circumferential stress, us (kgf/cm2) Effective circumferential stress, ao/ (kgf/cm2)

FIG. 1. (a) Relationship between o i and a,' during undrained simple shear. (b) Details of relationship between a; and a,' for a typicalheavily overconsolidated specimen. 1 kgf = 9.806 N. KO oefficient of e arth pressure at rest; K coefficient of active earth pressure;K coefficient of passive earth pressure.

The stress paths on the horizontal and vertical planes areshown in Fig. 4. The top half of the figure shows the stresspath on the horizontal plane, whereas the bottom half showsthe stress path on the vertical plane. The arrows and the cor-responding numerical values show the direction of the majorprincipal stress a{and the corresponding value of the shearstrain.Figure 5 shows the relationships between the stress ratioT/U; and the shear strain for all the tests. It may be seenfrom Figs. 4 and 5 that a t or around a shear strain, y, of

about 20070, at which the tests were terminated, the rate ofincrease of the stress ratio T/U; has slowed to a minimalvalue. The variations of the principal stress ratio c ~ / u ; dur-ing simple shear for these tests are shown in Fig. 6. Here,the strengths defined at y = 20 or at the end of shearing,whichever was attained first, were taken to represent the fail-ure strengths. The variation with OCR of the failure fric-tion angles 4; = arcsin[(ui u;)/(ui T;)IF and p F =arctan(~/u;)~s shown in Fig. 7. The similar trends for thetwo friction angles are obvious. It is clear from this figure


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876 CAN. GEOTECH. J. VOL. 29, 1992

0.60 0 5 10 15 2 0Shear strain, y( )

FIG.2 Variation of stress ratio K a aL during undrainedsimple shear

90.Start of t e s t AO98 1

00 5 10 15 20 25S h e a r s t r a i n y( )

FIG. 3 Variatio n of principal stress direction durinundrained simple shear

FIG.4. Effective stress paths on the horizontal and vertical planes during undrained simple shearthat, as far as our test results in torsional simple shear areconcerned, the increase in pb with overconsolidation ratiois due probably almost entirely to the increase in ; withOCR. The contribution, if any, of any increase inu; u; at failure with OCR will at best be very small.The undrained simple shear tests presented in this discus-sion may not have attained a clearly defined critical stateunlike the constant-volume simple shear tests performed bythe authors and shown in Fig. 9b of their paper. This not-withstanding, Fig. 2 clearly shows the tendency that, as the

shear strain increases, the stress ratio a; a; approacha common value irrespective of OCR. A similar tendencas described above was observed in drained torsional simpshear tests on normally and overconsolidated sand specimeTeachervorasinskun1989).It therefore seems that the streratio at failure is almost independent of the OCR, irrespetive of the drainage condition.A common value of the stress ratio may also be arriveat by noting that, in a drained simple shear test at the criticstate where there is no volumetric strain and also throughou


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1O5 10 5 20 5Shear strain r(%)

FIG.6. Variation of principal stress ratio @;/a; duringundrained simple shear.

DISCUSSIONS 877

undrained simple shear, the direction of the major principalstrain increment del is 45' from the vertical. Since strainincrements are predominantly plastic at the critical state, thedirection of a; neglecting any remaining inherent aniso-tropy even at large strain levels, can be said to be close to

0.40 - 0m0

WJ-..

0.30 , 2 5 -E:

\b -4,.

0.20 2 0 -Y WYalz C.-+

e : 1 5 -0.10 b--k10

that of del (i.e., 45 ), irrespective of OCR. A tendencytowards co-axiality between qi,and d q at large strains hasalso been observed in simple shear tests on sand by Stroud(1971).In summary, our results of torsional simple shear testsdo not support the assumption used in the paper that thestress ratio K = ai/a; at the critical state increases appreci-ably with overconsolidation ratio. It is unlikely, therefore,that the increase in the friction anglep with OCR observedin the results shown in the paper arises from the increasein Kat the critical state with OCR, as suggested in the paper.

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Ampadu, S.K. 1991. The behaviour of kaolin in undrained tor-sional simple shear. Ph.D. thesis, University of Tokyo, Tokyo,Japan.Ampadu, S.K., and Tatsuoka, F. 1992. A hollow cylinder torsionalsimple shear apparatus capable of a wide range of shear strainmeasurement. Geotechnical Testing Journal , ASTM, acceptedfor publication.Mayne, P.W. and Kulhawy, F.H. 1982. KO-OCR relationshipsin soils. ASCE Journal of the Geotechnical Engineering Divi-sion, 108(GT6): 851-866.Stroud, M.A. 1971. The behaviour of sand at low stress levels inthe simple shear apparatus. Ph.D. thesis, University ofCambridge, Cambridge, U.K.Teachervorasinskun. S. 1989. Stress-strain and strength charac-teristics of granular materials in simple shear. M.Eng. thesis,University of Tokyo, Tokyo, Japan.Wroth, C.P. 1987. The behaviour of normally consolidated clayas observed in undrained direct shear tests. Geotechnique, 37:37-43.

0.00 1 2 5 10 20 55 Overconsolidation Ratio, OCRShear strain 7( )FIG.7. Variation of failure strengths f and pf with OCR. sinFIG.5. Variation of stress ratio r/a; during undrained simple 4; = [(a; a;)/(a,' a;)lF; tan p; = (T/ushear.

measuremens of soil strength in simple shaer test

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