g. levefre - undrained shear strength in the surficial weathered crust - 1986

5/26/2018 G. Levefre - Undrained Shear Strength in the Surficial Weathered Crust - 1986

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Undrained shear strength in the surficial weathered crustG U YLEFEBVRE

Department of C ivil Engineering, U niversitt rle Slzerbrooke, Sherbrooke , Qu e., C anada J1K 2R1JEAN-JACQUESARE

Sociktk rl irzergie de la baie James, MontrCal, Qu e., Cana da H2Z 4M 8A N DOSCARDASCAL

Hydro-Qu@bec,M011trkr11,Qrre., Catlacln H2Z l A 4Received June 16, 1986

Accepted October 2 , 1986Most geotechnical engineers feel that the undrained shear strength measured by the field vane in the surficial weathered crustcannot be fully m obilized during the failure of an embankment built of soft clay deposits. Con sequen tly, the vane strength in thecrust is generally reduced by som e arbitrary means before stability calculations are performed. This paper p resents the results ofan experimental program aimed at evaluating the available undrained shear strength in the crust. In situ shear box and plateloading tests as well as triaxial compression and extension tests indicate that the available undrained shear strength in the crust isof the same m agnitude as the vane strength measured in the intact clay immediately below the crust. This paper also proposes amethod to account for the effect of the embankment confinement in the central portion of the foundation. The proposedmethodology is checked by a back analysis of the failure of the Olga test embankment built at Matagami, Quebec.Key worr1.s: vane test, vane test correction, weathered crust, shear box, plate loading test, embankment stability.Le scissomk tre de terrain mon tre habituellem ent des valeurs ClevCes de rCsistance dans la couche alt ert e la surface de s dCpBtsd'argile molle. C ette resistance ClevCe est cependant gtnerale ment percue com me non com plktement mobilisable la ruptured'un remblai construit sur un dCpBt d'argile. Pour fin d'analys e de stabilitC, cette resistance tle vt e mesurke au scissombtre dans lacouche altCrCe est habituellement rCduite par diffkrentes corrections totalement arbitraires. Ce t article pr tsen te les resultats d'unprogramme experimental realis6 pour Cvaluer la rtsistance au cisaillement non drain6 de la couche alt tr te . Des essais a boitecisaillement in situ et des essais de p laque de chargement furent effectuCs sur le terrain et le eomportement fut aussi CtudiC enlaboratoire au moyen d'essais triaxiaux en compression et en extension. Les rksultats obtenus ambnent la conclusion que larCsistance au cisaillement non drain6 de I'argile de la cro0te est peu prks tga le la resistance mesurCe au scissomktre dansl'argile intacte immediatement sous la cro0te. Une mCthodologie est propo sie po ur Cvaluer la resistance au cisaillementdisponible dans la cro0te pour fin d'analy se de stabiliti en considCrant l'effet de confinement du remb lai dans la partie centrale dela fondation. Enfin, la mtthod ologie est vkrifite par I'analyse rebours de la rupture d'une digue d'essai Matagami (QuCbec).Mots clks ssais au scissom tre,correction du scissomktre, crocte altCrCe, boite cisaillement, essai de plaque, stabilitC deremblai.

Can Geotech. J. 24 23-34 1987)

A hardened crust has developed at the surface of manydeposits of soft marine or lacustrine soft clay, as a result ofdesiccation, frost action, and weathering in general . Th e watercon tent has been significantly reduced in this layer, often result-ing in a l iquidity index lower than 0. 5, a s compared with valuesgenerally higher than unity in sensit ive nonweathered clays.One of the most significant characterist ics of this crust is i tsmuch higher undrained shear strength measured by the fieldvan e compared with values measured in the intact clay underly-ing the crust . In eastern Canadian clay deposits, the thickness ofthe weathered crus t generally varies from 1 to 5 m and is often ofthe order of 3 m. T he field vane test is an accepted me thod forevaluating the undrained shear strength of clay foundation mate-rials for embank ment stabil i ty analyses. The strength measuredin the weathered crust m ay ha ve significant effect on the calcu-la ted fac tor of safe ty , s ince i t m a y be severa l t imes a s high a sthat in the underlying intact clav (Graham 1979).

strength appears too high. La Rochelle e t a l . (1974) proposemore fundamental approach by suggesting the use of the called undrained residual strength determined in the laboratby unconsolidated undrained tests. Th is approach leads to dculty since, in the crust material , the undrained compressstrength generally increases with strain as a result of dilatanTh e purpose of this paper is to compare the field vane strenin the crust with the results of other field tests (namely ploading and large in situ shear box tests) an d laboratory triatests. Th e comparison of the undrained strengths determineddifferent tests will give som e insight into the shearing resistatha t can be mobi lized dur ing the fa i lure of an em bankmen twill form the basis of a methodology for evaluating undrained shea r strength in the crust . Finally, the results wiltested by the back analysis of an embankment failure.

Site description and testing programMo st geotechr;icai engineeri feel that the high undrained Th e stud y presented in this paper was conducted at the Oshear strength measured in the crust by the field vane cannot be si te, near Mat agam i, som e 70 0 km northwest of Montrea l . fully mobilized in the failure of an embankment on a clay lacustrine clay d eposits formed in the ancient glacial lake Bfoundation because of the existence of fissuring, stress-strain low-Ojib way overlay the region. A test embankm ent wa s bcompatibil ity, as well as other reasons (La Rochelle e t a l . 1974; at this si te in 1972 by H ydro-QuCbec. One of the test emba~ e f e b v r e l al. 1974). The different corrections used to reduce ments (dyke A) was brought to failure and the results wthe vane strength in the crust varies from on e engineer to the published by Dascal e t a / . (1972) . The dy ke fa i led a t a he ighother and are purely arbitrary, based simply on the fact that the m with a calculated factor of safety of 1.6. Th e factor of sa

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CAN. GEOTECH. J VOL. 24 9x7UNDRAINED SH EAR STRENGT H kPa)ATER CONTENT ( )

0 30 40 5 0 6 0 70 8 00 2

f P L A T E T E S T3C SHEAR BOX TE ST

FIG 1 Geotechnical properties of the weathered c rust, east trench, Olga site.was calculated using the geo metry of the dyke just before fa ilureand an average undrained shear strength profile measured withthe field vane. The vane strengths were not reduced in theweathered crust, which was about 4.0 m thick. Undrained shearstengths of the order of 45 kPa were noted in the crust ascompared with 15 kPa in the underlying intact clay. The Olgalacustrine clay has a plasticity index of about 40. A pplication ofthe widely accep ted Bjerrum correction of the field vane , whichis based on the plasticity index , results in a redu ction of 15 inthe calculated factor of safety at failure.

D ur in g a r ecen t r e a s s e ~ s ~ en tf the Olga test dyke results,part of the overestimation of the factor of safety was thought tobe related to the difference between the strength measured in thecrust by the field vane and the one actually mobilized at failure.A testing program was carried out in 1981 by the SociCtCd'knergie de la baie James and the UniversitC de Sherb rooke toobtain insight into the actual shear strength mobilized in thecrust during shear.Field work was carried out at Olga for two locations 400 mapart and within 350 m of the 1972 failed test dyke. At bothlocations, the clay deposit is covered by about 0. 3 m of peat. Itshould be noted that in this paper, depths for the 198 testingprogram refer to the surface of the clay deposit. T he clay in theweathered crust is light brown in colour, contains many m icro-fissures, and could be easily broken into small cubes. Someopen subvertical fissures were noted. The clav-size fraction(< 2 pm ) was of the order of 80 in the crust as well as in theintact underlying clay.At each location (east trench, north trench), two large shearbox tests and six plate loading tests were performed in additionto eight Nilcon vane profiles. Block samples were obtained ateach location from surficial trenches for laboratory triaxialtesting . It should be noted that, except in the case of the vane, allof the tests were done in the upper 1.1 m. Th e index propertiesand the vane test results in the crust are presented for eachlocation in Figs. 1 and 2. The wa ter content and plasticity datashown in Figs. 1 and 2 between the 2.3 and 3.2 m depths wereobtained in 1977 from a neighbouring borehole. At the two

locations, the vane profiles identify a crust thickness of ab2 m. Even if there exists a gap in the water content data betwethe 1 1 and 2.3 m depths, the thickness of the crust appereasonably confirmed by the variation of water content wdepth.Testing procedures

Vane testingStandard Nilcon vane tests were performed every 30 down the soil profile.late loading testsPlate loading tests were performed in the bottom of trenchlarge enough to avoid the depth surcharge effect using 30 a45 cm diameter plates. The last 10- 15 cm was carefully hadug and a thin sand cushion a few millimetres thick was placbetween the plates and the clay to make a better plate-to-socontact. The vertical deformation during loading was measurwith three deflectometers installed at the periphery of the platLoads were applied with a hydraulic jack and monitored frothe manometer readings. The hydraulic flow in the jack wcontrolled to have a constant rate of deformation of 1.25 mmmin. The tests were terminated at a vertical deformation about 70 mm. The plate loading tests were performed at depof about 0.2 0,0 .70 , and 1.10 m. Two tests were performedeach depth, using the 30 and 45 cm diameter plates.

In situ shear testsA steel shear box 60 x 60 cm and 4 0 cm high w as used for in situ shear tests. T he digging of the trenches was stopped so40 cm above the desired depth of the shear plane. Afterpositioing the shear box, the digging of the trench was continuoutside the shear box and the latter was gradually pushed dowith the lowering of the bottom of the trench. At the fiposition, a vertical clearance of about 15 mm w as kept betwethe lower end of the shear box and the bottom of the trenchallow a free horizontal displacement of the shear box. A stcove r fitting inside the shea r box was placed on top of the soibe sheared. The normal stress was applied by me ans of a de


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L E FE BVRE ET AL.WATER CONTENT ( ) UNDRAINED SHEAR ST RE NG TH ~P ~)

P L AT E T E S TSHEAR BOX TEST

FIG Geotechnical properties of the weathered crust, north trench, Olga site.

APPLIED PRESSURE q k ~ a )FIG 3 Pressure-deformation curves from plate loading tests, north trench

weight placed on the steel cover. The horizontal displacement of was controlled to provide a horizontal displacement of athe shear box and the vertical displacement of the steel cover 1.25 m m/ mi n. The shear tests were stopped at an horizwere monitored by deflectometers. The horizontal force was displacement of about 8 mm. The shear planes were locapplied by two parallel hydraulic jacks connected to a hydraulic between 0 4 and 1 1 depths. Thre e of the tests were shepump and evaluated with a manometer. The flow of the pump under a normal stress of 25 kPa; the other, under 45 kPa.

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26 C A N . GEOTECH. I VOL. 24. 987

PPLIED PRESSURE q k ~ a )FIG.4. Pressure-deformation curves from plate loading tests, east trench

aboratory triaxial testsSpecimens 35.5 mm in diameter and 71 mm in height werecut from block samples and sheared undrained in both triaxialcompression and extension. T hey were anisotropically consoli-dated to a vertical pressure of 20 kPa using a u, /u , ratio of 0.55.A back pressure of 100 kPa was used in the tests. Extension wasachieved by reducing the vertical pressure and keeping the cellpressure constant. Vertical compression and extension wasstrain controlled at a rate of 0.006 m m/m in.Results o field tests

Plate loading testsThe curves expressing vertical deformation as a function ofapplied pressure are presented in Figs. 3 and 4 for the twolocations. Th e diameter of the plate used in the tests is identifiedon each figure; plate diameter does not appear to have a signifi-cant effect on the results. None of the curves shows a well-identified yield point and for interpretation purpose s, the failurehas been defined at the point of maximum curvature. Th e resultsare summarized in Table 1. The undrained shear strength hasbeen estimated using the applied pressure at failure, q,, and abearing capacity formula where C qs/5.5. The mobilizedundrained shear strength does not appear to vary with depth forthe interval 0.15-1.10 m, except at the east trench where ahigher undrained shear strength was noted at the I .O m depth.For all of the 12 plate loading tests, the estimated undrainedshear strength varies from 15.5 to 30.9 kPa with an average of20.8 kPa.

TABLE. Summary of the plate loading testsPlateTest diameter Depth qr AHrnumber Location (cm) (m) (kPa) (cm) (k

PL-0PL-02PL-03PL-04PL-05PL-06PL-07PL-08PL-09PL- 10PL- IPL- 12

NorthNorthNorthNorthNorthNorthEastEastEastEastEastEastFigs. 5 and 6 Both tests were performed under a normal strof 26 .4 kPa in the east trenc h. well-defined yield point wobse rved in each of the tests (Fig . 5) , defining shear strength14 kPa at 0.8 1 m depth and 20 kPa at 1.14 m depth. The testhe 0.41 m depth in the north trench was performed undenormal stress of 25.9 kPa, resulting in a shear strength 19.5 kPa (Fig. 6). For the test at .O m, a higher normal strwas used (45.1 kPa) and a different behaviour was observedthe shearing resistance continued to increase with horizondisplacement. A shear strength of 39 kPa was mobilized at end of the test with a horizontal dis~ lac em en t f 7.5 c m. InIn situ shear tests four shea r box tests, the vertical deflectometer indicatedThe results of the large in situ shea r box tests are presented in downward displacement of the top cover, possibly due

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LEFEBVRE ET AL .

_r 2 0VVW

t;;WrV

0 0 2 4 6 8 10 12HORIZONTAL DISPLACEMENT cm)0 2 4 6 8 10 12

TEST no. DEPTH =N

O

0 . 5

W

1.0I

n

C D - O I 0 . 4 1 m 2 5 . 9 k PaC D - 0 2 1 . 0 0 m 4 5 .1 kP a

> \ '\ \ CO-04\ ,

00 2 4 6 8 10 1HORIZONTAL DISPLACEMENT cm)

FIG.5 Stress-displacement curves from in s tu shear box tests, easttrench. FIG . Stress-displacement curves from in s tu shear box tests, ntrench.closure of the fissures during shear and or) compression of thesoil against the sides of the shea r box.Discussion of the in situ test resultsClose agreement in the observed values of undrained shearstrength was obtained from the in situ shear box tests using anormal vertical stress of 26 kPa. Th e measured shear strengthsfor those three tests varied from 14 to 20 kPa with an avera gevalue of 18 kPa. The test performed under a normal confiningstress of 45 kPa yielded a significantly higher shear strength,which indicates some effect of the confining pressure. Theeffect of the confin ing pressure is possibly related to som e rapidconsolidation in this fissured crust material or to some increasein the dilatancy resulting from the closing of the fissures.If the result of the shear box test at the higher confinementstress is neglected, the average shear strength of 18 kPa mea-

sured in the shear box test is comparable to the value of 2 l kobtained from the plate loading tests. The results of the shbox and the plate loading tests are compared with the vprofiles for each location in Figs. and 2. For the plate loadtests, an average shear strength has been calculated for edepth from results obtained with 3 0 and 45 cm diameter platAt both locations, the plate loading and shear box tests hdefined undrained shear strengths of about 20 kPa. This is mulower than the shear strength measured by the field vane, whis of the order of 80 kPa, but is fairly similar to the vane shstrength measured in the underlying intact clay.

esults of laboratory testingOne set each of triaxial compression CAU C) and triax

extension CAU E) tests was performed for each of the t


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CAN GEOTECH. J VOL. 21 1987

DEPTH iNORTH TRENCH 0 .54 m 3 2 OO EAST TRENCH 0.69 m 3 4INTACT CLAY 3.76 m 9 0 Oo

AXIAL DEFORMATION O/o)

FIG.7 Results of triaxial compression tests consolidated at a ,, = 20 kPa.locations. The specimens were taken from depths of 0.54-0.6 9 m. Triaxial specimens were consolidated anisotropically toa vertical pressure of 20 kPa and to a horizontal pressure of11 kPa. The results of the laboratory tests are presented inTable 2.Th e pore pressure and stress-strain curves are presented inFig. 7 for the two triaxial compression tests. Th e curve obtainedin 1977 from a similar test on an intact clay specime n cut from ablock sample retrieved from a depth of 3.76 m in the same areais presented for compa rison in Fig. 7 . Note that the three tests inFig. 7 were consolidated to a vertical pressure of 20 kPa. Thespeciment of intact clay was however isotropically consoli-dated.While the intact clay specimen showed a very sharp peak inthe stress-strain curve followed by a rapid loss of strength, the

strength of the specimens of weathered clay continueincrease with deformation, owing to dilatancy. The weathclay spec imens tested in triaxial com pression mobilized a stress of 38 kPa (east) or 30 kPa (north) at an axial deformof 10 . However. at an axial deformation of 1%. whicomparable to the'deformation at failure for intact claymaximum mobilized shear stress was only 15-20 kPa. Tfore, because of high dilatancy, the weathered crust is caof mobilizing a relatively large shear strength with corresping large deformations; how ever, if one assumes a criteristrain compatibility with the underlying intact clay and siders the possibility of progressive fa ilure, the available strength in compression is significantly reduced. The weatclay specim ens have failed on a shear band inclined at 50 to the horizontal.


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LEFEBVRE ET LTABLE. Characteristics and results of the triaxial compression and extension tests

Identification Consolidation fd/ u4) md xTest Depth w u:, a; A H / H A V / V (01-03) Aunumber m) Location ( ) (kPa) (kPa) ( ) ( ) ( ) (kPa) (kPa)

CAUC-01 0.69 East 32.2 20.1 11.0 0.33 -0.14 11.21 77.4 15.4 -0.23CAUC-02 0.54 North 34.4 20.0 11.0 0.49 0.35 12.37 62.8 11.5 -0.21CAUE-01 0.69 East 32.1 20.1 11.0 0.22 0.21 -6.12 -52.2 -42.2 0.31CAUE-02 0.54 North 32.7 20.0 11.0 0.41 0.31 -7.96 -38.1 -28.0 0.41

*End of the test.

AXIAL DEFORMATION (/o)5 . 0 2 . 0 4 .0 6 .0 8 .0 10 0 12 .0 14 0

I I I I I I I I I *DEPTH

NORTH TRENCH 0.5 4 mEAST TRENCH 0.6 9 m

AXIAL DEFORMATION ( 10)

FIG.8. Results of triaxial extension tests consolidated at or 20 kPa.The pore pressure and stress-strain curves are presented inFig. 8 for the triaxial extension tests. A maximum shear stresswas obtained at an axial deformation of about 6 . Failureplanes inclined at about 25 to the horizontal were observed inthe upper parts together with a necking of the specimens.Stress paths for the compression and extension triaxial testsare presented in Fig. 9 . It should be noted that in the extensiontests, the o 0 condition was reached fairly early in the tests.

Discussion of laboratory test resultsFor the extension tests, the shear strength mobilization hbeen possibly limited by the a 0 condition. The anisotroratio (strength in extension divided by that in compression)difficult to evaluate because of a no-failure condition in tcompression tests. At an axial deformation of 5 , the ratiomobilized extension to compression strength is of the order 0.75. According to Ladd et al. (1977), this anisotropy ra


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CAN. GEOTECH. J . VOL. 24 987

05~

E ST TRENCH------ NORTH TRENCH

FIG.9 . Stress path for compression and extension triaxial tests.should b e on the order of 0. 6 for a clay with a plasticity index of40 . On this basis, the shear strength in extension does not appearto have been underestimated in the present program.In a simplified man ner, the s tate of stress on a failure circle inan embankm ent foundation can be associated with compressionfor the foundation zone under the central part of the embank-ment, with extension for the foundation zone under the toe ofembankment, and with simple shear in the zone in between(Fig. 10). Since the zone associated with simple shear isexpected to generally lie in the intact clay below the crust, onecan assu me that the loading in the crust will be compression andextension as illustrated in Fig. 10. As an approximation, theaverage shear strength mobilized on the failure surface in thecrust can be taken as the mean shear strength obtained fromcompression and extension triaxial tests. The meanstress-strain curv es from compression and extension triaxialtests are presented in Fig. 1 1 for both locations (north and easttrenches). Since failure planes have developed in triaxial tests atan angl e of about 2.5 to the direction of the major principalstress, the half deviator stress has been multiplied by cos 25 toarrive at the undrained shear strength mobilized on the failureplane presented in Fig. 1 1As noticed in the individual triaxial tests, the weathered crustappears stronger in the east trench. The mean curves do notshow a well-defined yield point and the shear strength keepsincreasing with deformation. The mobilized shear strengthincreases from 18 to 26 kPa when the axial deformation is

increased from 2 to 5% fo r the east trench and from 3to for the north trench. Considering the brittle response underlying intact clay, it is doubtful that a deformation alent to an axial deformation higher than a few percentdeve lop before comp lete failure of the foundation. In any assuming an axial deformation of 2-5% , it can be concthat according to the results of the triaxial tests, the undshear strength in the crust is of the order of 20 kPa. Thisconfirms the data from the n s tu shear box and plate lotests.pplication to embankment stability analysis

In the upper weathered crust at the Olga site, the n s laboratory triaxial tests both indicate that the mobundrained shear strength is much lower than the strengthsured by the field vane and is in fact very close to the stmeasured by the field vane in the underlying intact clay (Fand 2). Th e results indicate that the field vane strength m ein the crust should be discarded for stability analysis and bonly to define the crust thickness. For truly undrained tions, the shear strength in the crust should be assumedequal to the strength measured in the intact clay immedbelow the crust. However, owing to a relatively highmeability related to fissuring and or incom plete saturationcrust material, it is probable that a portion of the confinpressure applied by the embankment becomes effective construction and results in an increase in undrained


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LEFEBVRE ET AL

CENTER OF THE EMBANKMENT TOE OF THE EMBANKMENT MEAN PROFILECONFINED ZONE A) UNCONFINED ZONE 0 )

FIG 10. Estimation of the undrained shear strength in the weathered crust.strength. The effect of the confinement pressure has not beenspecifically studied in this testing program, but one in situ shearbox test conducted under a larger normal confinement stressseems to confirm the influence of confinement pressure.In the area where the failure surface is near the groundsurface, at the toe of the embankment, the confinement is notsignificantly affected by the embankment load and theundrained shear strength in the crust should be assumed to beequal to the strength measured in the intact clay immediatelybelow the embankment. This assumption could, however, betoo conservative under the central portion of the embankmentwhere the confinement is significantly increased especially forhigh embankment. Under the central portion of the embank-men t, the undrained shear strength in the crustal material shouldbe related to embankment confinement. The ratio C,,/u:obtained from triaxial compression tests for normally con-solidated clay is of the order of 0 .3 (Ladd et ul 1977). Untilstudies become available on the confinement effect, it wouldappear reasonable to express the shear strength in the crust be-neath the central portion of the embankment as the larger ofu 0.25Au: (Au: being the embankment load), or the mea-sured u n the intact clay.For stability analysis, an average undrained shear strengthprofile can be compiled in the crust for the toe zone and thecentral zone as illustrated in Fig. 10.Reanalysis of Olga dyke ATh e site conditions, construction, and failure of the Olga Atest emban kment have been described by Dascal et al (1972).At the location of the dyke the clay deposit was covered by apeat layer close to 1.8 m thick, and extended to a depth of about17 m. T he surficial weathered c rust had a thickness of the orderof 4 m. According to the design, the peat was supposed to be

exca vated inside strips 15 m wide and oriented alon g the tranverse and longitudinal directions of the em bankment. Problewere however encountered in the excavation and some peat hpossibly been left in place at the bottom of the excavated stripAccording to Dascal et a l (1972), there were s ome uncertaties on the unit weight of the material of the dyke (nonplastill); three unit weight measurements during construction hgiven values varying between 18.9 and 21kN /m 3. T he dyke is reported to have failed at a height of 4and with slope of about 3.5:l. The working pad put in plabefore construction was also playing the role of a small berm ,shown in Fig. 12. From field observations and the breakageinstruments, the failure was reported to have taken place aloncircular arc extending to depths between 9 and 13 m in the clfound ation. A nalysis of the failure using the field vane profilemeasured yielded a calculated factor of safety of the orde r of 1(Dascal et a / 1972) and the authors recommended firstly thamodified plasticity index I,, w w,) be calcu lated to appthe Bjerrum correction based on plasticity index and that seondly a further reduction factor be applied to account for pgressive failure.In the reanalysis of the test embankment failure, the invetigation and construction data were reexamined and no evidenwas found to change the geometry at failure reported by Dascet a l (1972 ). Th e unit weight of the fill material was taken as taverage of the values obtained during construction, resultinga unit weight of 20.8 kN/m3. A friction angle of 30 wassumed for the fill material.The stability analysis was carried out using the modifiBishop method assuming a circular failure surface and usingcom puter program that allows the undrained sh ear strength toentered as a continuous profile. The geome try of the dyke befofailure is show n in Fig. 12 with the vane profiles in the found


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CAN. GEOTECH. J. VOL. 24 1987

NORTH TRENCHE ST TRENCH

XI L DEFORM TION O/o)FIG 1 Undrained shear streng th on the failure plane averaged for compression and extension triaxial tests.

tion clay . Th e mean in situ vane profile represents the averag e of17 profiles obtained before construction with the Nilcon vaneborer. Stability analyses run using the mean in situ vane profilewith no correction yielded a factor of safety of 1.62, which issimilar to the one reported by Dascal et al 1972). Figure 12also shows the undrained shear strength in the crust determinedfollowing the method proposed in this study. The undrainedshear strength at the toe of embankment is assumed to beconstant in the crust and equal to the v alue measured in the intactclay immediately below. Under the central portion of theemba nkme nt the shear strength at the original ground surface istaken to be equal to one-quarter of the weight of the embank-ment and decreases linearly with depth to the intact clay shea rstrength. Profiles from the toe and central portion are combin edto give the undrained shear strength profile used in the analysisFig. 12). A factor of safety of 1.10 was obtained using thismean profile.

iscussionThere is no doubt that assumptions regarding the strength in the wea thered cru st can significantly alter the prtion of emba nkm ent performance Graham 197 9). For examthe failure of the Olga A test embankment was predictoccur at a height of m, but actually happened at a height oDascal et al 1972).The problem is even more acute in back analyzingembankment failures in order to assess the accuracy oundrained sh ear strength estimates for the intact clay foundmaterials, thus verifying the methodology of the shear streevaluation. The knowledge of soft clay behavior obtaineconstructing a test emba nkme nt can be detrimentally affectthe uncertainty of arbitrary corrections made to the strength profile in the weathered crust. The Olga A casegood example. Another good example is reported bRochelle et al 1974 ), where the calculated factor of safe


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LEFEBVRE ET AL.

RANULAR FILL 3 02 0 .8 k ~ / r n ~I 0 2 0 3 0 4 0OUTSIDE ZONECENTRAL PORTIONAVERAGE PRO FILE CLAY 15.0 ~ N / I T I ~

MEAN IN SlTU VANE PROFILE

1 m v DI L LFIG.12. Properties and geometry of the Olga test embankment at failure.

failure of a test embankment at St. A lban varied from 0 .85 to1.2, depending on the shear strength assumptions made for thecrust.In the present stud y, both the plate loading tests and the largen s tu shear box tests yielded an undrained s hear strength in thecrust of the orde r of 20 kPa. This value is confirmed by triaxialcompression and extension tests. It has been concluded on thisbasis that the undrained shear strength in the crust that can bemobilized under initial tz s tu stress conditions is much lowerthan the values measured by the field vane. It was not thepurpose of this paper to explain why the field vane overesti-

mates the undrained shear strength in the crust. Considering,how ever , the large degree of dilatancy observed in triaxial testson the crustal material, which results in a shear strength thatcontinues to increase with deformation, one might conclu de thatthe large strengths measured by the field vane may arise as aresult of the very large shear strains imposed on the soil. Thefact that the available undrained shear strength in the crust isequal to the field vane strength measured in the intact clayimmediately below the crust cannot be fully explained at themoment and may be coincidental. The confinement effect underthe central portion of the embankment requires further study.Bjerrum (1972) proposed an empirical relation based on theplasticity index for correcting the factor of safety calculatedfrom data obtained using the field vane (Fig. 13). The proposedrelation was derived from observ ations of embankmen ts built invarious parts of the world on soft clay that had all failed atcalculated factors of safety ranging from 0. 85 to 1.65. In mostof the case histories studied by B jerrum, the surficial weatheredcrust did not have a significant effect on stability. When apply-ing Bjerru m s empirical corre ction to stability analyses forembankments built on clay deposits with a relatively thickweathered crust, the present study concludes that a correctevaluation of the undrained shear strength in the crust shouldfirst be made . Using the field vane m easurements from the crustas well as the intact clay, a factor of safety of 1.6 2 was obtainedfor the failed Olga A test emba nkment. This is much higher thanthe predicted factor of safety from the Bjerrum relation (1972),which indicates a factor of safety at failure of 1.15 for a

0.8 2 0 4 0 6 0 8 0 100 PLASTICITY INDEX

I SCOTTSDALE 8 PORNIC2 BANGKOK A 9 NEW LISKEARD

BANGKOK B 1 KING S LYNN4 SCRAPSGATE I I PALAVAS5 LANESTER 12 NARBONNE6 SAINT-ANDRE DE CUBZAC 13 PORTSMOUTH N.H.7 MATAGAMI,OLGA 14 FAIR HAVE N

I

(AFTER BJERRUM(1972)FIG.13 Empirical field vane correction factor derived from embament failure (Bjerrum 1972).plasticity index of 40 (Fig. 13 . When the undrained shstrength in the crust is evaluated as proposed in this paper, calculated factor of safety is reduced to 1.10 and is in gagreeme nt with the Bjerrum empirical relation.The reanalysis of the failure of the Olga A test embankmindicates that, if the undrained shear strength in the weathecrust is correctly evaluated, the vane tests and the Bjerrcorrection lead to a correct estimation of the stability, withthe need of either a modified plasticity index or of a furtreduction of the shear strength to account for progressfailure.



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12/12

34 CA N . GEOTECH. I VOL. 24, 1987Conclusions

The comparison of the field vane profile with the undrainedshear strength evaluated in the field by plate loading and largeshea r box tests and in the laboratory by triaxial compression andextension triaxial tests, in addition to the reanalysis of thefailure of a test emban kme nt, has permitted certain conclusionsand recomm endations co ncerning the available shear strength inthe weathered crust. The study, however, has limited scope inthat only one site located in lacustrine clay in northwest Queb echas been investigated.1. As generally perceived by soil engineers, the field vanegrossly overestimates the undrained shear strength in the sur-ficial weathered crust for embankment stability purpose.2. In the weathered crust, the field vane measurement shouldbe used mainly to define the thickness of the weathered crustrather than to determine the actual undrained shear strength.

3. At in s tu stresses, the available undrained shea r strength inthe crust is about equal to the field vane strength measured in theintact clay imm ediately below the crust for the site investigated.4. In the central portion of the foun dation, the embankmentconfinement should result in an increase of undrained shear

strength during construction. In the present stud y, the undrainedshear strength has been made equal to one-quarter of the weightof the emban kme nt and linearly decreases to the intact clay vanestrength.5 The crustal material appeared to be highly dilatant intriaxial compression whereas the intact clay was contractive.The dilatancy is probably responsible for the overestimation ofthe undrained shear strength in the crust by the field vane.

cknowledgements

out jointly by the SociCtC d7Cnergie de la baie J am es anUniversitC de Sherbrooke personnel. The laboratory teprogram was executed at the UniversitC de Sherbrooke.study was carried out under the general guidance of a Socd Cnergie d e la baie Jam es experts committee task force posed of 0 . Dascal, (Hydro-QuCbec), C. C . Ladd ( M.IK . T . Law (National Research Council of Canada), G. Lef(UniversitC de Sherbrooke), R . PichC, J. G LavallCeJ. J. Par6 (SociCtC d Cnergie de la baie Ja mes) , G . M(University of Illinois), and F. Tavenas (UniversitC LavaBI ERRUM ,. 1972. Embankm ent on soft ground, state of the art rProceedings, ASCE Specialty Conference on Performance of and E arth Supported Stru ctures. Purdue University, LafeayettVol. 2, pp. 1-54.DASCAL, . , T O U R N I E R ,. P . , TAVCNAS,., and LA ROC I I EL1972. Failure of test embankment on sensitive clay. ProceedASCE Specialty Conference on Performance of Earth and supported structures. Purdue University, Lafayette, IN, Vol. 129- 158.G RA H A M ,. 1979. Embankment stability on anisotropic soft cCanadian Geotechnical Journal, 16 pp. 295-308.LADD, . C . , Foorr R., I SH I I IA IZA . , SCHLOSSER,., and P oH . G. 1977. Stress-deformation and strength characteristics.of the art report, Proceedings, 9th International Conference oMechanics and Foundation En gineering, Vol. 2 , pp. 421 -494LA ROCHELLE,. , TRAK,B., TAVENAS,., and Roy, M .Failure of a test embankm ent on a sensitive Champlain clay deCanadian Geotechnical Journal, 11 pp. 142- 164.LEFEBVRE,. , LEFEBVRE,. M., and ROSENBERG,. 1974. Behof a cemented plastic clay as an embankm ent foundation. CanGeotechnical Journal, 11 pp. 46-58.

The study presented in this paper was supported by theSocittC d Cnergie de la baie James. The field work was carried



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g. levefre - undrained shear strength in the surficial weathered crust - 1986

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