yield strenght and critical state tropical soil

l Sted

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t on asoil p

les weens s

sed toas de

ted satent wto obves fo

ength;

on the K0 line (Mitchell 1970; Tavenas and Leroueil 1977; Gra- influence of the natural structure on the mechanical behavior. For

ham et al. 1983; Smith et al. 1992), but the shape of the limit statecurve in tropical soils is not yet well known, as limited data hasbeen presented. Leroueil and Vaughan (1990) suggested that re-sidual soils and soft rocks exhibit isotropic behavior and theiryield curves are centered on the hydrostatic axis, based on limiteddata (Uriel and Serrano 1973; Sandroni and Maccarini 1981).However, a reassessment of the data used shows that some yieldcurves of tropical soils do not exhibit isotropic behavior. There-

this reason, compacted specimens were tested at water contentsand void ratios close to in situ conditions, and the results werecompared to tests on intact material. The studies reported hereinare just for saturated conditions. However, studies under unsatur-ated conditions have also been carried out by means of suctioncontrolled tests, as well as tests at the natural water content (Futai(2002).

Characterization of the Soil Profile

The tropical profile studied comprises a reddish top (horizon B)layer about 2.0 m thick followed by a saprolitic soil (horizon C)which may reach depths up to 40 m. The water level is at about20 m depth. The present study is concentrated on the top 7.0 mfrom which block samples were taken every meter. Samples werealso tested from the nearby saprolitic soil slope, which had beenexposed due to erosion and associated stability problems. Inves-tigations included index and chemical tests, and studies of the soilmineralogy (x-ray diffraction, thermodifferential and thermo-gravimetric tests) and soil microstructure (electron microscopyand porosimetry tests by mercury intrusion).

Results of index tests are summarized in Fig. 1. The amount ofclay is greater in horizon B and grain size analyses revealed that

1Associate Professor, Polytechnic School of Engineering, Univ. ofSo Paulo, Brazil, Av. Prof. Luciano Gualberto, Travessa 3/380, CEP05508-900, So Paulo, Brazil.

2Professor of Geotecnical Engineering COPPE, Graduate School ofEngineering, Federal Univ. of Rio de Janeiro, CEP 21945-970, P.O. Box68506, Rio de Janeiro, Brazil.

3Professor of Geotecnical Engineering COPPE, Graduate School ofEngineering, Federal Univ. of Rio de Janeiro, CEP 21945-970, P.O. Box68506, Rio de Janeiro, Brazil.

Note. Discussion open until April 1, 2005. Separate discussions mustbe submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possiblepublication on September 5, 2003; approved on March 16, 2004. Thispaper is part of the Journal of Geotechnical and GeoenvironmentalEngineering, Vol. 130, No. 11, November 1, 2004. ASCE, ISSN 1090-0241/2004/11-11691179/$18.00.

JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2004 / 1169Yield, Strength, and CriticaSatura

M. M. Futai1; M. S. S. Almeida, M

Abstract: The paper reports laboratory investigations carried oustate and limit state conditions, and their variation with depth. Thea saprolitic soil (horizon C) from which a number of block sampand drained and undrained triaxial tests, were conducted on specimsaprolitic soil. Special triaxial tests, with the pore pressure increastress levels. On this basis a tensile cutoff on the failure envelope wdrained and undrained triaxial tests were carried out on compacparameters were measured for the horizon C soil, which is consisin q : p8 stress space was identified, but linear regression was usedhorizon B are centered on the hydrostatic axis, but limit state cur

DOI: 10.1061/(ASCE)1090-0241(2004)130:11(1169)

CE Database subject headings: Soil compression; Shear str

Introduction

Tropical soils appear in large regions of the world and have beenless studied than soils from temperate climates, particularly withrespect to critical state and limit state conditions. Most geomate-rials are structured in nature and this natural structure affects thebehavior of tropical soils. Structural features affecting soil behav-ior include soil cementation and soil fabric (Vaughan 1985;Vaughan et al. 1988; Vaughan 1992).

Soft clays have been reported to have a yield curve centeredDownloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject totate Behavior of a TropicalSoil

E2; and W. A. Lacerda, M.ASCE3

tropical soil profile to study its compressibility, strength, criticalrofile comprises a reddish lateritic layer (horizon B) underlain byre taken. A series of isotropic and anisotropic compression tests,ampled at depths between 1.0 and 7.0 m, and also in the exposedinduce failure, were performed to investigate the failure at low

fined. In order to assess the influence of the natural soil structure,mples obtained from depths of 1.0 and 5.0 m. Higher strengthith its lower clay content. A nonlinearity in the critical state line

tain critical state parameters. The limit state curves for soils fromr horizon C suggested anisotropic behavior.

Triaxial tests; Tropical soils; Saturated soils; Yield.

fore limit state and yield conditions of tropical soils deserve fur-ther study, which is one of the objectives of this paper.

This paper presents the laboratory behavior of a tropical soilbased on the studies carried out by (Futai (2002) on blocksamples collected from various depths. The tropical soil studied isa residual soil from gneissic rock located close to the city of OuroPreto, in the State of Minas Gerais, Brazil. Isotropic and aniso-tropic compression tests as well as shearing tests were conductedfor determination of strength, critical state, and limit state condi-tions. A particular aim of the test program was to determine the ASCE license or copyright. Visit http://www.ascelibrary.org

te charthe clay is in the flocculated state. The water content at the time ofsampling ranged between 28 and 46%, which is within the rangeenclosed by liquid and plastic limits. Atterberg limits and voidratio decrease with depth. The degree of saturation, S, duringsampling ranged from 80 to 96%, but in a drier season S valueswere at a lower range (4070%). The average value of the specificgravity of soil grains, Gs, was 2.63 and 2.68, in horizons B and C,respectively, which may be attributed to the different mineralogi-cal composition of the two horizons.

Kaolinite is the main clay mineral, which is consistent with acation exchange capacity lower than 8 meq/100 g. The amount ofquartz is approximately constant with depth, which is in accor-dance with the relatively constant proportion of sand with depth.Scanning electron microscopy studies indicated a metastablestructure for the horizon B soil. Porosimetry measurements, usingthe mercury intrusion technique, confirm that horizon B possessessmaller pores and, accordingly, higher clay content than horizonC.

Compression Tests on Intact Specimens

Compression tests performed included one-dimensional tests car-ried out in oedometer cells and isotropic and anisotropic compres-

Table 1. Compression Parameters

Horizon Depth (m) CrB 1 0.03

2 0.05C 3 0.05

4 0.045 0.056 0.037 0.04

Exposed saprolite 0.03

FIG. 1. Si1170 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject tosion tests carried out in triaxial cells. Data from oedometer com-pression tests on intact flooded specimens are summarized inTable 1. Values of the compression index Cc for horizon B aregreater than those for horizon C, which may result from the po-rous cemented structure of horizon B. As shown in Table 1, Ccwithin horizon C does not vary much with depth. The Cc data ofthe 7.0 m depth is, however, an exception but the reason for thisis not clear. Also the yield (preconsolidated) stresses svm8 increasewith depth. Values of the recompression ratio Cr are in the range0.03 to 0.04 (Futai 2002), thus there is little variation within therange of depths studied.

Isotropic and anisotropic (K=s38 /s18=0.5 and 0.75) compres-sion tests on intact flooded specimens were performed. Data fromthe test with K=0.5 are shown in Fig. 2. Anisotropic compressiontests were strain controlled, with measurement of the major prin-cipal stress s1 and continuous adjustment of the minor principalstress s3 by a feedback control program. Isotropic yield stressespc8 are shown in Table 1 together with oedometer test data. It isobserved that the yield stress pc8 is greater than svm8 for horizon B,while the opposite is observed in horizon C, which suggests thatthe coefficient of earth pressure K0 is greater than unity for hori-zon B. Data of compression tests (isotropic tests and constantanisotropic s38 /s18 tests) will be subsequently used for definitionand interpretation of the yield curves.

c svm8 skPad e0 pc8skPad

.44 60 1.34 100

.41 100 1.02 120

.27 200 0.88 140

.29 250 0.93 .3 400 0.88 260

.28 450 0.9

.42 500 1.05 400

.33 400 1.25 340

acteristicsC

00000

000EERING ASCE / NOVEMBER 2004

ASCE license or copyright. Visit http://www.ascelibrary.org

formed on each block, thus 16 sets of tests (strength envelopes)

s for 1Triaxial Tests on Intact Specimens

Test Program and ProceduresShear strength was evaluated by drained and undrained isotropicconsolidated triaxial tests. Tests were performed on intact soilsfrom block samples of the entire profile (depths 1.07.0 m) andon exposed saprolitic soil, with the aim to understand the stress-strain-strength behavior. Undrained and drained tests were per-

FIG. 2. Anisotropy compression curves for K=0.5

FIG. 3. Triaxial tests resultJOURNAL OF GEOTECHNICAL AND GEOE

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject towere performed in total. Particular attention was given to thosetests performed on samples from 1.0 m (lateritic soil) and 5.0 m(saprolitic soil) depths. In these tests, at least six different confin-ing stresses were applied to obtain a detailed impression of thesoil behavior. For the remaining depths, typically 4 to 5 confiningstresses were used to obtain the strength envelope.

Saturation of each specimen (diameter 50 mm; height 100mm) was ensured by water flow followed by application of back-pressure. 95% consolidation was achieved in less than 2 min viaradial and base drainage. The adopted rates of shearing were 0.05and 0.013 mm/min for undrained and drained tests, respectively,which are about 10 times slower than calculated rates (Bishop andHenkel 1974). The deviatoric piston force was measured by aninternal load cell and the change in volume by an automatedvolume change device. Pore pressures during undrained testswere measured at the triaxial cell base.

Tests in Horizon B

Results of drained and undrained tests on samples from 1.0 mdepth are shown in Fig. 3 for different confining stresses sc8. Thepeak deviator stress is not well defined in drained tests [Figs. 3(aand b) and is observed just for the test carried out at a low con-fining stress of 25 kPa. As confining stresses sc8 increase, maxi-

m depth soil (horizon B)NVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2004 / 1171


mum deviator stresses are achieved at axial strains a of the orderof 15%, or even greater. Some tests were stopped at large strainswithout reaching a well defined maximum deviator stress. Typicalnormally consolidated compressive behavior is observed for thesoil in horizon B.

FIG. 4. Triaxial tests result

FIG. 5. Undrained stress1172 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject toResults of undrained tests on samples from 1.0 m depth, Figs.3(c and d), also show a normally consolidated behavior, withpositive excess pore pressures being generated [Fig. 3(d)]. Appar-ent peak strength is observed in undrained tests at axial strains upto 5% and the critical state condition is achieved at larger strains.

m depth soil (horizon C)

or 1 and 5 m depth soilss for 5

paths fEERING ASCE / NOVEMBER 2004


Tests in Horizon CTriaxial tests were also conducted on samples retrieved fromdepths of 3.0, 4.0, 5.0, 6.0, and 7.0 m within horizon C. Datacorresponding to the soil at 5.0 m depth were chosen as represen-tative for this horizon. Results of drained tests performed onsamples from 5.0 m depth are shown in Figs. 4(a and b). Theincrease of sc8 in the drained tests is accompanied by a gradualchange in soil behavior [Fig. 4(b)] and up to sc8=100 kPa a dila-tant behavior is observed. Well-defined failure planes were visu-ally observed in all drained tests up to sc8=200 kPa and may beattributed to the heterogeneities common in residual soils. Failureplanes were sometimes associated with shear stresses developedat the extremities of the triaxial cell specimens, but failure planeswere also noticed (Futai 2002) in an undrained test performedwith free-end specimens for which lubricated and enlarged ends(Rowe and Barden 1964) were used.

Stress-strain curves from undrained tests performed onsamples from 5.0 m depth, Fig. 4(c), display peak deviatorstresses up to sc8=690 kPa, accompanied by a decrease in porepressure. For sc8,100 kPa negative excess pore pressures aregenerated [Fig. 4(d)].

Comparison of Tests in Horizons B and C

The effective stress paths of undrained tests performed on speci-mens from 1.0 and 5.0 m depths are shown in Fig. 5. Specimensfrom 1.0 m depth (horizon B) display a typical normally consoli-dated behavior with the stress paths turning to the left [Fig. 5(a)].The failure envelope is curved, a feature commonly found intropical residual soils (Vargas 1953; Massey et al. 1989; Gan andFredlund 1995). Three distinct patterns are observed from stresspaths for horizon C soils, as shown in Fig. 5: (1) for sc8 up to 200kPa, stress paths turn to the right; (2) tests with sc8 of 300400kPa are close to vertical; and (3) tests conducted with the twohighest confining stresses initially bend slightly to the right butthen to the left, particularly after the peak deviator stress has beenreached.

Data (Futai 2002) from the exposed saprolitic soil (horizon C)displayed behavior similar to the horizon B soil: drained testsdisplayed compressive behavior, while in undrained tests, positivepore pressures were measured. This behavior is in strong contrastto the nonexposed intact saprolitic soil for which a dilatant be-havior was shown in drained tests [Fig. 4(b)] and negative porepressure was measured at lower stress levels [Fig. 4(d)].

Special Triaxial Tests

A series of special triaxial tests were performed in order to inves-tigate the failure envelope at low stress levels under saturatedconditions. They were carried out in three steps: (1) specimenswere initially isotropically consolidated up to sc8=75 kPa; (2) adeviator stress q was then applied in a stress controlled drainedmanner; and (3) keeping q constant, the average effective stressp8= ss18+2s38d /3 was decreased by increasing the excess porepressure Du until failure was reached. Similar tests have beenperformed by others for different purposes (e.g., Bressani andVaughan 1989). Tests to assess the failure envelope at low stresslevels were carried out just for the specimens from a depth of 5.0m. Three special tests were performed with q=25, 35, and 68 kPa,as shown in Fig. 6. As Du increased q remained approximatelyconstant [Figs. 6(a and b)], p8 decreased, and the no-tension cut-off failure line inclination 3:1was reached (Schofield 1980).JOURNAL OF GEOTECHNICAL AND GEOE

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject toFollowing the failure, q decreased until the stress paths ap-proached the critical state line, as shown in Fig. 6(a). During thedecrease of p8, the saprolitic soil experienced continuous expan-sion, as shown in Fig. 6(c).

Triaxial Tests on Compacted Specimens

In order to assess the influence of the natural soil structure on soilbehavior, a number of undrained and drained triaxial tests were

FIG. 6. Special triaxial tests results for 5 m depth soilNVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2004 / 1173


carried out on compacted specimens of soils from depths 1.0 and5.0 m and the results were compared with those from intact soil.

those for the 1.0 m specimens, with the exception of the volumet-

ted drFor each depth, five levels of sc8 were applied, varying from 20 to400 kPa, thus a total of 20 saturated triaxial tests were carried outon compacted specimens. Compacted specimens were prepared ina strain controlled press by static compaction inside a cylindricalmold at water content and void ratio e0 close to in situ conditions.In order to evaluate the influence of the void ratio on soil behav-ior, for the 1.0 m deep soil additional undrained tests were per-formed at e0=1.05, which is lower than in-situ conditionsse0=1.28d. Yield curves for the tests at e0=1.05 are shown sub-sequently.

Fig. 7 shows results from drained tests, at small and large sc8,on specimens of 1.0 m se0=1.28d and 5.0 m se0=1.28d depth.Intact specimens from 1.0 m depth [Figs. 7(a and b)] show greaterpeak strength than the compacted soil at sc8=25 kPa, although atlarger strains the same deviator stresses are reached for both intactand compacted conditions. The intact soil is less compressive thanthe compacted soil [Fig. 7(b)]. The natural structure, therefore,appears to be stiffer as the compaction destroys the original natu-ral structure. At sc8=400 kPa (larger sc8), differences in deviatorstresses q and in volumetric strains v are smaller [Figs. 7(a andb)], hence the effect of the structure appears to be less significant.

The overall patterns of the q : and v :a curves observed forthe 5.0 m specimens [Figs. 7(c and d)] are more accentuated than

FIG. 7. Intact and compac1174 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject toric strains for the test at higher sc8. Greater differences are ob-served between peak strengths at small and large confining pres-sures. The axial strain a=23%, reached at sc8=400 kPa, was notenough to bring intact and compacted specimens to similar ulti-mate strengths at large stress levels. This is expected since atthese stress levels the intact soil is unlikely to retain its originalstructure.

The effect of the soil structure is clearly evident from stresspaths of undrained tests shown in Fig. 8. The different pore pres-sures measured in intact and compacted specimens resulted indifferent stress paths at the latter stages of the tests. At 1.0 mdepth, for horizon B soils [Fig. 8(a)] the behavior of intact andcompacted specimens are fairly similar for sc8=400 kPa but, forsc8=25 kPa, the intact specimens exhibit greater strengths. Thislatter trend is observed at all values of sc8 for the specimens from5.0 m depth [Fig. 8(b)] and is understandable, since the deeperspecimens are less weathered, thus intact specimens exhibit no-ticeably greater strengths than compacted specimens.

Strength and Critical State Parameters

Triaxial test data of tropical soils at large confining stress ssc8dformed curved failure envelopes, as discussed previously, but a

ained triaxial tests resultsEERING ASCE / NOVEMBER 2004


FIG. 8. Intact and compacted undrained triaxial tests results

FIG. 9. Strength envelopes for 1 and 5 m soils specimensTable 2. Shear Strength and Critical State ParametersHorizon Depth (m) c8skPad f8 (deg) M G l

B 1 7 28.1 1.14 2.89 0.1762 5 29.1 1.03 2.93 0.182

C 3 8 29.6 1.20 2.51 0.151

4 3 33.9 1.36 2.27 0.1045 15 31.0 1.01 2.50 0.1506 10 30.5 1.25 2.70 0.1837 20 25.4 1.08 2.80 0.182

Exposed saprolite 2 31.6 1.12 2.78 0.177JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2004 / 1175

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org

linear failure envelope can be defined for a lower range of sc8.Thus, with the purpose of defining a cohesion intercept c8 and afriction angle f8 for each depth, data of drained and undrainedtests up to stress levels of 400 kPa are presented in Fig. 9. Table2 shows data of c8 and f8 for the maximum deviator stress con-dition ss18s38dmax. Friction angles are in the range 28.129.1 forhorizon B. For horizon C no clear pattern of variation of f8 withdepth was obtained. Disregarding the minimum (25.4) and maxi-mum (33.9) values, the range of friction angle is narrower,29.631.0. The average value of the friction angle in horizon Cis very close to 30, regardless of the range adopted. The lowervalue of friction angle for horizon B sf8ave=28.6 d is consistentwith its greater clay content.

There is a slight decrease in c8 with depth, but for depthsgreater than 4 m a trend of increasing c8 is observed. The value ofc8 for the exposed saprolitic soil is much smaller than c8 for theintact saprolitic soils (horizon C), which reflects the leaching pro-cess of the exposed soil. Values of c8 about one order of magni-tude higher were obtained from triaxial tests with controlled suc-tion (Futai 2002).

Critical state parameters were determined for each depth usingdata of drained and undrained tests close to critical state condi-tions. Sometimes critical state conditions could not be achieved,thus large strain data close to critical state conditions was used.Fig. 10 shows critical state lines in the Cambridge q : p8 and e : p8spaces at a chosen depth of 3.0 m. The linear regression is con-

FIG. 10. Critical state lines for 1 and 5 m depth soils1176 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject tovenient, but is not necessarily the best fit. Table 2 shows data ofthe critical state parameters M ,G, and l (Atkinson and Bransby1978; Wood 1990) for the 8 block samples tested. There is noclear trend of the variation of M (or other critical state parameter)with depth. Data from all depths are presented in Fig. 11(a) andthe nonlinearity of the critical state line in q : p8 stress space isrecognizable, particularly for less weathered soils. The influenceof the weathering is not clear in the e : p8 plot [Fig. 11(b)], but itis clearer in the De /e0 : p8 plot shown in Fig. 11(c), whereDe=accumulated change in void ratio during both consolidationand shearing, and e0=initial void ratio at the start of the test. From

FIG. 11. Critical state condition in (a) p8 :q plane, (b) lnsp8d :e plane,and (c) lnsp8d :De /e0 planeEERING ASCE / NOVEMBER 2004


act anthe De /e0 : p8 plot it is possible to establish two ranges of valuesfor the critical state line, one for horizon B and another for hori-zon C, as seen in Fig. 11(c). A tendency of convergence at highstress levels can also be noticed.

The strength data for drained and undrained tests on intact andcompacted specimens from 1.0 and 5.0 m depth are compared inFig. 12. Coincident peak strength envelopes for intact and com-pacted samples were obtained for the specimens from 1.0 m depth[Fig. 12(a)]. However, the intact specimens from 5.0 m depthshowed strength envelopes above the compacted specimens [Fig.12(b)], although critical state envelopes (not shown here) werefound to be coincident.

Limit State Curves

Limit state curves were determined by a number of tests chosen inorder to explore different regions of the q : p8 space. Data fromcompression tests (isotropic tests and constant s38 /s18 anisotropictests) shown earlier were used to define yield conditions; like-wise, data from triaxial drained and undrained triaxial tests wereused to define failure states, as exemplified in Fig. 13. Casa-grandes criterion was used to define yield for isotropic and an-isotropic compression conditions. For triaxial tests, the assess-ment proposed by Graham et al. (1988) was adopted to define theinflection point in arithmetic, semilog, and bilog scales. All ap-proaches suggested similar yield states.

Results shown in Fig. 13 for the soil at 5.0 m depth allowedthe definition of three limit state conditions: Region I, where yieldis achieved below the critical state line; Region II, the Hvorslev-type failure envelope with well-defined plane and dilatant behav-ior; and Region III, defined by the tensile cutoff obtained from thespecial triaxial tests.

Limit state curves obtained at all depths using the procedureshown in Fig. 13 are presented in Fig. 14, with the exception ofRegion III that was obtained only for the specimens from 5.0 mdepth. The expansion of the limit state curves with the increase ofdepth is quite clear in Fig. 14(a). It is shown that limit state curvesfor soils from depths of 1.0 and 2.0 m in horizon B are centeredon the hydrostatic axis. Limit state curves of soils from horizon C(depths 35 m) are not centered on the hydrostatic axis, whichmay be due to the remaining mother rock anisotropy.

The limit state curves presented in Fig. 14(a) were normalized

FIG. 12. Shear strength envelope of intJOURNAL OF GEOTECHNICAL AND GEOE

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject tofor each depth with respect to isotropic yield stresses pc8, as shownin Fig. 14(a). Two patterns of limit state curves emerge, one forhorizon B (depths 1 to 2 m) and another for horizon C (depths35 m). The limit state curve of the soil at 5 m depth does not fitinto the general pattern observed for horizon C; visual analysis ofthis soil in the laboratory indicated that the specimens were not

d compacted soil, for 1 and 5 m depth

FIG. 13. Limit state curve for (a) 5 m depth and (b) all depthsNVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2004 / 1177


with different sizes [Figs. 15(a and b)], indicating that the natural

urvestypical of horizon C, exhibiting less weathering than soils at 6.0and 7.0 m depths.

Limit state curves of intact and compacted specimens are com-pared in Fig. 15 for soils from 1.0 and 5.0 m depths. Intact andcompacted specimens with the same void ratio show yield curves

FIG. 14. Normalized limit state curves

FIG. 15. Limit state c1178 / JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGIN

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject tosoil structure has a strong effect on the size of the yield curve.The limit state curve for the soil extracted from 1.0 m depth,compacted to a void ratio of e0=1.05 (lower than in situ condi-tions) is also included in Figs. 15(a and b). Normalized limit statecurves [Fig. 15(b)] for compacted specimens suggest a stress in-duced anisotropy, since the curves are not centered on the hydro-static axis. It is also noted that the stress induced anisotropy ismore pronounced at greater levels of compaction, as shown by thelimit state curve for the lower void ratio condition. Limit statecurves for the soil at 5.0 m depth [Figs. 15(c and d)] have similarshape for intact and compacted specimens, both displaying aniso-tropic behavior.

Conclusive Remarks

This paper reported results of strength and compression tests per-formed on saturated specimens of a tropical soil composed of aclayey reddish (horizon B) layer about 2.0 m thick underlain by asaprolitic gneissic residual soil (horizon C). The main observa-tions were1. Soils from horizon B exhibited volumetric contraction in

drained triaxial tests and positive excess pore pressures weregenerated in undrained triaxial tests. Soils from horizon C

intact and compactedEERING ASCE / NOVEMBER 2004


exhibited dilatant behavior in drained triaxial tests for con-fining stresses sc8 up to 100 kPa. The stress paths in theundrained tests also show an overconsolidated soil behav-ior for sc8 up to 400 kPa; i.e., stress paths turn to the right atlow sc8 values and at intermediate sc8 values are close tovertical.

2. Triaxial tests on intact specimens produced a curved failureenvelope. However, for sc8 up to 400 kPa a linear failureenvelope was found to be a good approximation.

3. Values of the cohesion intercept were in the range 57 kPafor the horizon B soil and in the range 820 kPa for the

during triaxial testing. 12th Int. Conf. Soil Mechanic and FoundationEngineering, Rio de Janeiro, Vol. 1, 1720.

Futai, M. M. (2002). Theoretical and experimental study of unsaturatedtropical soil behavior: Applied a gully case. PhD thesis, COPPE-Federal University of Rio de Janeiro (in Portuguese).

Gan, J. K. M., and Fredlund, D. G. (1995). Shear strength behavior oftwo saprolitic soils, Proc., 1st Int. Conf. on Unsaturated Soils,Alonso and Delage, eds., Balkema, Rotterdam, The Netherlands, Vol.1, 7176.

Graham, J., Crooks, J. H. A., and Lau, S. L. K. (1988). Yield envelope:identification and geometric properties. Geotechnique, 38(1), 125134.horizon C soil, which is consistent with the lower plasticityand smaller void ratio of the horizon C soil. A low value ofc8=2 kPa was obtained for the exposed saprolitic soil, sug-gesting the influence of the leaching process promoted bydirect rainfall.

4. Average values of friction angles for horizons B and C arefave8 =28.6 and 30.1, respectively, which is consistent withthe greater amount of clay in horizon B.

5. Compaction to the same void ratio and water content hadnegligible effect on the strength parameters of horizon B.The results of drained and undrained triaxial tests on com-pacted specimens showed that failure envelopes for the soilfrom 1.0 m depth are similar to those of intact specimens; butfor the soil from 5.0 m depth lower strength parameters weredetermined for the compacted specimens.

6. A nonlinearity in the critical state line in q : p8 stress spacewas noticed. From the plot of De /e0 : p8 two ranges for thecritical state line were observed, one for horizon B and theother for horizon C.

7. Limit state curves for soils from horizon B are centered onthe hydrostatic axis, but limit state curves for horizon C ex-hibited anisotropic behavior. Compacted specimens showedanisotropic limit state curves for horizons B and C as a resultof the stress induced anisotropy caused by compaction.

Acknowledgments

The present study had the financial support of PRONEX andCNPq (Ministry of Science and Technology), FAPERJ (Rio deJaneiro Secretary of Science and Technology), and FUJB (FederalUniversity of Rio de Janeiro). The writers are indebted to EstherMarques and Paul Dimmock for their careful review of this paper.

References

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Bishop, A. W., and Henkel, D. J. (1974). The measurement of soil prop-erties in the triaxial test, 2nd Ed., E. Arnold, ed., London.

Bressani, L. A., and Vaughan, P. R. (1989). Damage to soil structureJOURNAL OF GEOTECHNICAL AND GEOE

Downloaded 06 Nov 2011 to 139.82.115.33. Redistribution subject toGraham, J., Crooks, J. H. A., and Lew, K. V. (1983). Yield state andstress relationships in a natural plastic clay. Can. Geotech. J., 20,502516.

Leroueil, S., and Vaughan, P. R. (1990). The general and congruenteffects of structure in natural clays and weak rocks. Geotechnique,40(3), 467488.

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yield strenght and critical state tropical soil

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