Vane testing in soft claysReport ofBritish Geotechnical Society'sinformal discussion held at the InstitutionofCivil Engineers on 8June 1988

By M Mahmoud', MSc, DIC

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The vane shear test is one of the mostwidely used in situ methods fordetermination of the undrained shearstrength of soft clays. Although historicalrecords date the development of the vanetest back to the Olsson vane borer in1910s,it was not until the late 1940s that thefield vane was developed in its modernform by such people as

Garison(1948)'nd

Skempton (1948) .An extensiveinvestigation of the vane shear test wasGrat carried out by Cadling &Odenstad(1950)3,in which they examined variouspossible factors expected to influence themeasurement of the vane shear strength.Since then, the vane test has been thesubject of interest for numerous authorswho have attempted to gain abetterunderstanding of this apparently simpledevice, and to establish why themeasurement of the vane shear strength isoften different from the undrained soilshear strength measured with othermethods.

The informal discussion was introducedby Dr RJChandler (Imperial College),with contributions from NA Trenter (SirWilliam Halcrow and Partners), Dr CMMerriGeld (University ofManchester), DrMCR Davies (University College, Cardiff),Prof CP Wroth (Oxford University), DFTNash (University ofBristol) and Prof N ESimons (University of Surrey). The

'Formerly Department of CivQ

Engineering, Imperial College, London,

3$ now at Sir William Halcrow Partners.

GROUND ENGINEERING . OCTOBER 19BB

meeting was chaired by Prof SFBrown forBGS.

Dr Chandler's presentation was based onhis January 1987ASTM State of the Artinvited paper, and concentrated on (i)abrief review of the current understandingof the use of the field vane test, and (ii)correlation of the Geld vane strength withthe strength measured with other testmethods. This report is a summary of themeeting, following a layout similar to themain speaker's introduction, withcontributions from the other speakerspresented where appropriate. The abovediscumion is limited to the Geld vane test,unless stated otherwise.

Review ofthe field vane testThe various factors which are consideredto influence the measurement of the vaneshear strength include the dimensions ofthe vane, disturbance due to vaneinsertion, 'rest'eriod following insertion,rate ofvane rotation, the time to failurerequired to ensure that undrainedconditions apply during shear, and the testinterpretation.

1.Vane dimemaionsThe pioneering work ofCadling &Odenstad (1950)'recommended that afour bladed vane with a height to diameter(H/D) ratio of2 should be adopted asstandard. Although thesereccommendations have subsequentlybeen accepted by the internationalcommunity, different vane diameters havebeen used depending on the 'softness'fthe clay; in stiffer materials small diametervanes are used in order to minimise thepossibility ofbreakage of the vane (ProfSimons). Since strain rate effects —whichare a function of the vane tip velocity (see,eg Perlow &Richards, 1977)' areimportant in the vane test, it is reasonableto adopt one particular vane size, and turnthis at a standard rate of rotation. Since a65mm diameter vane is used on aworldwide basis, it is suggested that thisvane diameter, rotated at 6'/min, beadopted as the standard.

2.Vane insertion disturbanceeffectsWhen down the borehole vane tests arecarried out, as opposed to the use of apenetration vane, it is considerednecessary to insert the vane below theborehole base to a depth, z) 4B, where Bis the borehole diameter, thus ensuringthat the vane is in truly undisturbed soil. It

is worthwhile to note, as did Prof Simons,that vane rotation could be non concentricin down the borehole tests.

The extent of fabric disturbance as aconsequence ofvane insertion wasstudied by M Rochelle et al. (1973) .Basedon correlations between the measuredvane strength and perimeter ratio,a (= nt/AD, in which n is the number ofblades on the vane, and t is the bladethickness), it was concluded that the Geldshear strength would be given byextrapolating to a = 0 (which correspondsto t = 0).Table 1 shows that vane insertiondisturbance effects are apparently morepronounced for sensitive days, where theavailable strength in the vicinity of thevane may have been significantly reducedas a consequence of disturbance to theclay fabric. Therefore, in order tominimise disturbance effects due to vaneinsertion, a vane thickness of t = 2mmseems appropriate for standardisation.

Table l.Vane insertion disturbanceeffects (data from La Rochelle et aL,1973)Sensivity, g712

c„~= ratio of undisturbed'„ to thestrength measured with vane havingt = 2mm.In this light, it is important to note thatpenetration of the vane results in both (a)disturbance to the soil fabric, withaccompanying destruction of theinterparticle bonds built up overthousands ofyears and consequentreduction of the available undrained shearstrength by an unknown amount,particularly in the more sensitive days,and (b) displacement of the soil particles,with corresponding increases in the porepressures around the vane andsubsequent consolidation which typicallyresult in the measurement ofhigher vanestrengths.

Dr Davies presented the results of testsusing a laboratory scale centrifuge vaneapparatus in a normal gravity Geld. Theapparatus was devised for thedetermination of the shear strength pro61eofcentrifuge samples. Radiographictechniques using lead thread showed thatvane insertion had caused littledisturbance to the soil fabric around thevertical periphery of the vane blades,whilst the disturbed soil around the vaneshaft had extended symmetrically beyond

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37 Merritmtd (1980)6- 19 Roy 8 Leblixm )1987)

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o 0 5l 200 20 2 0 2 0.02 0.002 0 0002

Approx rotation role, la, degrees/m nute

0—10 10 10 1 10 10 10 10 10

Time to toilure, tl, minutes

Fiy. 1 Effect of rate ofrotation on iieldvane strength

the shaft periphery by about one shaftradius. Vane inseztion disturbance to theclay fabric was found to result innegligible strength loss. Pore pressuremeasurements taken during vaneinsertion, at locations about 50mm awayfrom the vane blades increased oncepenetration had stopped. Readingscontinued to rise for a few minutes,probably as a consequence of porepressure equilibration and subsequentconsolidation in the vicinity of the vane.Pore pressure response might have beendelayed because the transducers were atsome distance from the vane.

3.Elapsed thne between vaneinsertion and rotationIt is usual to restrict the delay time prior tovane insertion to five minutes, sinceexcess pore pressures generated uponinsertion of the vane will dissipate (see,eg Kimura &Saitoh, 19836,for alaboratory study) resulting in an increasein the lateral effective stresses.

In an investigation of the effect ofconsolidation following vane insertion intwo highly plastic (I = 50% to 90%)clays,Torstensson (1977) found that themeasured vane strengths at Grstincreased exponentially with delay time,but then approached a constant value ofstrength with further increase in the

'rest'eriod.

It was seen that the values ofc„corresponding to rest periods of one hourand seven days were respectively about9% and 19%higher than c„valuescorresponding to a 5 minute delay time. DrDavies'aboratory examination of thisaspect of the vane test showed that thevane strength measured after 20 minutesdelay was on average about 15% greaterthan that measured after 6 minutes;Torstensson's Geld results show a 2%increase in strength over the same delaytimes.

4.Rate ofvane rotationCadling &Odenstad (1950)recommended the rate ofvane rotation of6a/min as a standard, since they found thatthis rate corresponded both to themeasurement of the smallest shear

strength and to the time of loading inlaboratory tests. Since then, the Geld vanetest has almost invariably been carried outregarding rate of6a/min as the standard.

In general two phenomena areconsidered to be associated with strainrate effects in the vane test. On the onehand, pore pressure effects are likely tobe more important at both (a) slow rates ofrotation, where drainage effects will leadto changes in the obsezved strength as aconsequence of changes in porepressures and hence the normal effectivestresses acting across the shear zone (seeeg Kenney &Landva, 1965),and (b) fastrates of shearing, where, as suggested byProf Simons, the generation of lower porepressures will subsequently lead to themobilisation of higher effective stresses atfailure. On the other hand, as the rate ofrotation increases under undrainedconditions the measurement of a highershear strength may well be as a result ofviscous effects related to the clay contentof the soiL

From a comparison of the resultsof highplasticity Swedish clays (Wiesel, 19739;Torstensson, 1977)'with those of lowplasticity clays (Roy &Leblanc, 1987)', inFig. 1, it can be seen that the measuredstrengths decrease with increasing timesto failure (ie reducing rates of rotation) inthe case of the high plasticity clays, whilsta different trend is shown by the lowplasticity clays. The trend observed fromthe results ofvane tests camed out by DrMerrifield at different rates of rotation onBrent Knoll alluvial clay (I~ = 37%;OCR = 1.5),which are also presented inFig. 1, shows increasing shear strengthswith decreasing times to failure, similar tothe high plasticity clays of Wiesel andTorstensson. The vane strengths had beennormalised to the strength measured at ti= 8 minutes, as this was taken as thestandard. However, the results ofDrDavies'aboratory tests on kaolin at ratesof rotation in the range >22 = 5'/min to6500'/min showed a trend similar to thatobserved by Roy &Leblanc (1987)'3.Reducing strengths, which wereattributed to consolidation around thevane, were measured in the range02 = 5'/min to'30'/min, with the measuredvane strength increasing by 2.5%per logcycle at rates faster than 30'/min.

In addition, Dr Davies commented that thestrengths measured with vanes ofdifferent diameters (D = 18mm to 36mm;H/D = 0.4to 1.5),rotated at a rate of

Fig.2 Real(Ra)andapparent(Ra)degrees ofanisotropy for the twovane test method

72 /min. The rate was equivalent to the0.15mm/sec which Perlow &Richards(1977)recommended as the standardvane tip velocity for ensuring undrainedconditions in the vane test —showed a30% decrease with a two fold increase indiameter. The observed effect wasattributed to the greater disturbance as aresult ofvane insertion. However, it shouldbe remembered that the vane diameteraffects both the strain rate (by influencingthe vane tip velocity), and the drainagepath length (which is dependent on thediameter of the vane failure surface).Consequently, it is worth noting that thewriter has also observed a similar trendfrom laboratory tests using 12mm and24mm diameter vanes. But he hassuggested that the lower strengthsmeasured with the larger vane areprobably due to the lower degree ofconsolidation in the vicinity of the largervane, since his results show that when a12qun diameter vane is rotated in kaolin,undrained conditions will not apply untilr27> 300'/min.

Although measurement ofhigherstrengths at faster rates of rotation (ieshorter times to failure) maybe explainedin terms of rheological phenomena, itseems probable that considerableconsolidation must have occurred duringthe very slow tests of Torstensson.Therefore it is reasonable to conclude thatthe observed continual decrease instrength with increasing times to failure isa phenomenon associated with highplasticity clays. In this light, it is importantto note that Bjerrum (1973)'ad earlierpostulated that viscous rate effects wouldvary with plasticity index, I .In an attemptto determine the Geld strength, Bjerrumintroduced a correction factor,/2 (= /2a22z,

where/2~ and/2a resPectively corresPondto anisotropic and strain rate effects;/2represents the ratio between the strengthobtained from the back analysis of full

scale field failures and the Geld vanestrength. The importance of the Geldstrength, in contrast to the strengthmeasured with the vane or other methods,was further emphasised by Prof Simons.

Relative magnitude of/2z and itsdependence on I was demonstrated bythe close agreement observed betweenBjerrum's estimated relationship for/2zand values of/2z calculated from the 37

GROUND ENGINEERING . OCTOBER 19BB

1 15'Real'roned stmngth on sotropy: 10

ft.)0IS el

(c„nlm Ilb=

Ic„l

105 1 57 0

I 00 10 10

stra nlsohent grata, Kt I Kl

100 1000(

Reyd-ptoshc

c

IT„'ig.

3 Influence ofstrain softeningratio on ayyarent strength

results presented in Fig. 1,havingassumed the times to failure (ti) in thestandard Geld vane test and in fieldfailures tobe tz = 1 minute and 10 000minutes respectively. A similarcomparison can be made for times tofailure of ti ——100minutes, which aretypical ofmany laboratory triaxial tests.

S.Time to failure to entureundrained conditionsRecognising that a single rate ofvanerotation cannot be valid for all soils, Blight(1968)"developed a simple theory ofconsolidation which maybe used fordetermining the rate ofvane rotation(and/or the time to failure) required toensure undrained cohditions at failure.This approximate theory assumes that thevolume of soil which vrill be inQuenced bypore pressure effects associated withvane rotation is a sphere ofradius a,centred on themid height of the vane, withhydrostatic conditions prevailing at thesurface of this sphere of infhence. Byconsidering the dissipation of excess porepressures at a radius, r, within the sphere,and assuming that the maximum torque,M, is related to the average degree ofconsolidation, U, Blight derivedrelationships between U and the timefactor, T (nm c,t~', in which c,is thecoeEcient of consolidation of the soil andD = diameter of the vane failure surface),suchthat:

M-MoU= =f(T)

Mi—Mo Eq.l,where Mo and Mi are the torquescorresponding to fully undrained and fully

drained conditions respectively.

For vanes having H/D = 2m Blight foundgood agreement between experimentalresults and the theorectical relationshipwith a = D and r = D/2; Roy &Leblanc's(1987)'~vane test data also support Blight'sapproximate theory. According to theabove theoretical relationship, undrainedconditions (ie U~ 10%)apply at T < 0.05;Dr Davies had also used a simQarapproach for determining the rate ofrotation corresponding to undrainedconditions in his laboratory tests. Hence toachieve undrained conditions in the fieldc„must be approximately ~ 100m~/yr,

since the standard field vane has D =38 65mm, and the time to failure is typically I

GROUND ENGINEERING OCTOBER 1988

10- Remoulded kaoln. 0 * 8mm, HI 0.- 2

ts 08-

a 06

e Occa

02a8

z 0

ai ma)or consoltdohonslress*'lookpa(exceplforK=ldlan .. mmor consol datron stress0, mean ccnsokdahon stress = 1(am '01,),

05 10 15In nal stress rotto, K = at,/am

Fig. 4 Effect of initial stress ratio onthe measured vane strength

ti ——1 minute. Since most unifozm softclays show values of c„less than 100m3/yr, it seems highly probable that anundrained soil strength is measuredwhen the field vane is rotated at thestandard rate of6'/min.

6.Test interyretationIn conventional interpretation of the vanetest it is assumedthat (a) the undrainedsoil strength (~is isotropic, (b) themeasured peak torque (M) represents thefull and instantaneous mobilisation of c„onboth the veztical cylindrical surface andthe horizontal end surfaces of the vane,and (c)the shear stress distributionaround the entire surface circumsczibingthe vane is uniform. These assumptionsimply that the vane failure surface isdefmed by the cylindrical circuznscriptionof the vane blades, such that for a vanewith H/D = 2 the interpretation of the testmaybe expressed by the relationship:

xMEq. 2

mD3

where x = 0.86if the conventionalinterpretation is adopted.

Based on Menzies 88 Memfield's(1980)'xperimentalresults, Wroth's

(1984)'odified

intezpretation presumes aa nonuniform (almost parabolic) shear stressdistribution on the two horizontal ends ofthe vane, thus giving an xvalue of 0.94.Ifsoil strength anisotropy is also taken intoconsideration the value ofx will be lowerthan 0.94.x = 0.91when the ratio of thestrength ofveztical to horizontal planes~,is assumed to be about 0.6.Hence, itappears that when the standard (H/D = 2)vane is used, the conventionalinterpretation of the vane test onlyunderestimates the soil strength by about6% to 9%.In connection with the implication thatthe vane failure surface is closely

0.2

0 20 I 0 60 80 100

Plot(ratty Irides, I s,

ZZZ0X+

OCR

I1.0Low

1-7 51-1.5

1 6-1.7

Reference

Skempton (1957)Bjerrum (1973)Leroueh etaL (1983)Chandler (1987)TN'ntefNosh

I OCR), Oslo (rom Osatdler 11987) ond Nash

Curves correspondmg lo OCR ~ 1.5.2 0and 3 0 were const noted

by Nash using ga.3

Fig. Sa Relati~~~t< between fieMvane strength ratio, cue~, andylasticity index

defmedby the vane perimeter, DrDavies presented a radiograph taken atthe end of a laboratory vane test,supporting the conventional assumption.In this regard, Prof Simons askedtherhetorical questions Can we beabsolutely certain that at failure the fourbladed vane will result in a cylindricalvane sized surface? Ifnot, how manyblades are required in order to achieve acylinder of revolution at failuzey

Dr Merrifleld cziticised the conventionalinterpretation by highlighting that it doesnot take account of either strengthanisotropy or Main softening, thusinfemng that the calculation of theundrained strength in the vertical andhorizontal planes using vanes ofdifferent H/D ratios (Aas, 1965)'ightbe seriously in error. Using a simplestrain softening model, a clay exhibitingstrength isotropy maybe shown to havean apparent degree ofshear strengthanisotropy which exceeds the realdegree of anisotropy (of 1))by up to 20%when vanes with H/D ratios of land 2 areused in the two vane test method (seeFig. 2).Application of these shearstrengths in a limit equilibrium analysisofa non circular slip surface ofan earthembankment stabiTity would thereforeresult in calculation ofan enoneousfactor of safety. Moreover, for a clay witha real degree ofanisotropy of 1.0,themeasured strength maybe up to about10% less than the real soil strength (seeFig. 3).

By suggesting that the dominant stresses'n the vane test are those which acthorizontally, Prof Wroth showed that thevane test might underestimate the'ncrease ofundrained strength caused'y the consolidation of clay when thereis an increase in the vertical effectivestresses but little change in the

0 0

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08-

0.6-

O.C. Cr 00rn

0

0 00

0 000 0

n x

cI m': 031 (OCR c 1 50)

horizontal effective stresses, eg in thestage construction of embankments onsoft ground.

Nash presented some results oflaboratory triaxial vane tests (see, egLaw, 1979)'arried out some years agoat the Norwegian Geotechnical Institute.Samples ofday (D = 80mm; H/D = 2)were normally consolidated to differentstress ratios, prior to penetration ofasmall vane (D = Smm to 20mm; H/D = 2).Fig. 4 shows that the initial stress ratio (K)has little or no effect on the observedstrengths normalised to the mean stressprior to vane insertion, whilstnormalisation with respect to either thehorizontal or vertical effective stressesillustrates that the measured vanestrength is dependent on K.The resultsof triaxial vane tests on undisturbedsamples ofDrammen day, initiallyreconsolidated to the in situ stresses andthen allowed to swell back to stresseswithin the yield envelope, showed thatthe measured strengths were largelyindependent of the overconsolidationratio (OCR).

Correlations ofvanestrength with other teststrengthsRelationships between the Geld vanestrength and other measurements ofundrained strength with such factors asplasticity index (I )and OCR are widelyused for comparison of strength datafrom different sites and from differentclay types. From a database consistingof 19case records which give both Geldvane and Ko-consolidated undrainedtriaxial compression (CKOUC) strengthmeasurements, mainly related to marineclays in Scandinavia and elsewhere,various undrained strength relationshipscan be examined after investingating thebasic trends of the database to checktheir conformity with previouslyestablished relationships. As can beseen in Figs. Saand b, plots of (~a',)Fvand(Cu/o'v)cnecagainstI bothcompare well with previouscompilations ofSkempton (1957'7,Bjerrum(1973)', LeroueiletaL (1983)',and Jamiolkowski et al. (1985)'9.For OCR(1.5,(~o',)cx,>c ——0.31,independentofI,whilst Jamiolkowski et al. (1985)obtained a similar value of(~0'„)cz,uc= 0.32,also independent ofI, from adifferent set ofdata

It is of interest to see how the results of

0 20 CO 60 80 100Plaxticrrr index, Ie 66

OCR Retererce

x 1-1.5 TrenterI

Fig. Sb ego'or CKDU tziaxialcomyressicm tests

vane tests carried out recently underNash's supervision at the SERG soft daytest bed site at Bothkennar Gt the ~e,'-I relationships in Fig.Sa. The clay atBothkennar is a postglacial estuarinedeposit (OCR = 1.6to 1.7;I = 40% to60%),giving ~a,'alues which areconsistent with the relationshipspresented in Fig. Sa. In this connection,Prof. Simons commented that it isimportant to distinguish between marineclays and freshwater clays whencorrelating the soQ strength withplasticity index; freshwater clays mayhave a different fabric compared tomarine days.

Trenter's re+Its regarding a casehistory which involved the constructionof900m ofpile supported quay atBelawan Port, Indonesia, founded in a50m thick silty day desposit (I = 60%;OCR = 1-1.5,Sb = 2) are also presentedin Figs Sa and b.Both unconsolidatedundrained (UU) triaxial tests and downthe borehole vane tests were carried out,yielding linear ~o,'relationships withvalues of 0.37and 0.33respectively; thelatter comparing well with bothSkempton's relationship for normallyconsolidated days and Leroueil et al.'srelationship for lightly overconsolidateddays.

In order to examine the relationshipbetween the measurement of theundrained strength using the Geld vaneand CKOUC tests, V, is plotted in Fig. 6against I~, where V, denotes (pvdivided by (~uc.Despite somescatter, it can be seen that the V,-Irelationship can be represented in termsof a straight line equation. Thecomparatively low Geld vane strengthsfrequently observed at values of I (30% may be attributed to vane insertiondisturbance effects, since low values ofI are typically associated with sensitiveclays in which the Geld vane strengthmaybe inferred to be as much as 25%below the undisturbed value.

Fig. 6 also shows both (a) DrDavies'esults

obtained from vane andisotropically consolidated triaxialcompression (CIUC) tests on kaolin(1 =34%)and (b) Trenter's results; for OCR =

1 to 10.Dr Davies observed goodagreement between the triaxial and vanetest relationships plotted semilogarithmically in terms of~a,'ersusOCIL The ratio of Geld vane to the UU

strength measurements of0.89(obtainedby Trenter) is lower than the value ofV,of 1.08,whilst the value of/6(~o~ of0.21compares well with both Mesri's(1975)~value of 0.24and Chandler's(1987) value of0.22 (see Fig. 7).

It is also of interest to note that V, isrelated in some manner to OCR in Fig.6.To investigate this further, both vane testresults from the database andJamiolkowski et al.'s (1985)vane testresults were plotted on a log-log basis asthe ratio ~0,'against OCR, such that:

~a', = S~ (OCR) Eq. 3,

where S> is the undrained strength ratiofor normally consolidated (OCR = 1)clay.

Table 2 shows that the average values ofSq and m obtained from Chandler's(1987)database are consistent with therespective values obtained fromJamiolkowski et al.'s results.Consequently, in the absence of otherdata, Eq. 3might be used for theestimation of OCR, taking Bjerrum's(1973)relationship in Fig. Sa, with m =0.95(for normal clays). The accuracy ofthis procedure will be about+25%.

Table 2 Values of St and m

Reference Sg m

Jamiolkowski et aL (1985) 0.22 0.97Chandler (1987) 0.25 0.95

It is not possible to make suggestions as tothe likely values of m or St for stronglystructured highly organic, or otherunusual clays.

Conclusionsl. Essential elements of the Geld vane

test which are recommended forstandardisation are presented in Fig.8.

2. The vane rod diameter is important,since vane penetrationtypicallycauses considerable disturbancearound the vane shaft with littledisturbance in the vicinity of the vaneblades. Vane insertion disturbanceeffects can reduce c„by 1S% to 25% insensitive days.

3. In order to ensure a conservativeestimate of the in situ undrained

GROUND ENGINEERING . OCTOBER . 1988

39

10

06

y,:055 ~ 0 008)p(ormtt ne ponts A 5 0)

5, < 5 5.15 >15

Borehotey)M

Torqu e,, 'M

B

)7

7r 77 ~ s,depth below borehole

Oi, ~

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it U UU lestr

(seetextl)Trenterltcoves)

i

tChandler,1987)

0 20 raPlasticity Index. Ip .%

60 80 ca

840

GROUND ENGINEERING OCTOBER . 1988

Fig. 6 Relationship between the ratiooffield vane and CKeUCstrengths, and plasticity index

strength, the rest period prior to vanerotation should not exceed 5 minutes.

. (a) Continually reducing vanestrengths with increasing times tofailure are associated with highplasticity clays.

(b) Consolidation around the vaneduring slow rates of rotation resultsin the measurement of increasingstrengths.

(c) In order to establish whether themeasurement ofhigher strengths atfaster rates ofvane rotation is due tarheological or pore pressureeffects, it is necessary to employ aninstrumented vane equipped withpore pressure transducers.

. (a) Thevanetipvelocityof0.15mm/sec (Perlow &Richards) isconsidered much too general to beadopted as a standard, particularlyfor soils with high values ofc„.

(b) Blight's theory may be used toestablish the rates of rotationrequired for undrained conditionsto apply during vane shear.

(c) The standard rate of rotation of6 '/min will result in themeasurement of an undrainedstrength inmost uniform soft clays,providing they have a coefGcient ofconsolidation no greater than about100ms/yr.

. A more accurate measurement of thestandard (H/D = 2) vane strength canbe obtained by increasing the vanestrength determined using theconventional interpretation by (a)between 5% and 10%,depending onthe degree of strength anisotropy ofthe soil, and/or (b) by up to about 10%for soils which are isotropic withrespect to undrained strength, butstrain soften.The shear zone around the vaneperiphery is cylindrical in shape, withthe vane failure surface closelydefined by the vane circumference.Thus, the discrepancies between themeasured vane strength and themeasurement ofundrained strengthusing other test methods cannot beattributed to the assumption of a vanesized failure surface.The two vane test method (Aas, 1965)will overestimate the ratio of the

strength ofvertical to horizontal planesby up to 20% in isotropic, strainsoftening materials. However, thisapproach is questionable, since it isbased on the doubtful assumption onthe top and bottom horizontal surfacesof the vane is uniform.

9. Radial stresses in the soil act normal tothe vertical vane periphery and henceare important in controlling themeasurement of the vane strength.Consequently, vane strengths shouldbe determined on the basis ofchangesin the radial effective stresses ratherthan changes in the vertical effectivestresses.

oc-5 etx

pl'ii

U

is

O.rMesri (')975)

cCC~22 c222VKW&oc72'+C~~Chandler l1987)

Trenter

0 20 40 60 80 100

Plasticity index, Ip .

Fig. Z Strength ratios versus plasticityindex

10.The considerably lower values of thefield vane strength measured in soilswith Ip (30% probably result fromvane insertion disturbance effects.

11.Despite the sparsity of data at values ofIn )40%, there is considerablecorrelation between the ratio of theGeld vane strength to CKDUC triaxialstrength measurement, V„andplasticity index, Im given by

V, = 0.55 + 0.008Ip Eq. 4.Eq. 4 is based mainly on data frommarine clays.

12. Byusing Bjerrum's correlationbetween Si and I,Eq. 3, which is ageneralisation of the Geld vanestrength, may be used as a method forchecking (~or estimating OCR of'normal'lays.

13.Although some may consider the vanetest unfashionable, it provides a directmeasurement of the field shearstrength and is therefore ofconsiderable value to the geotechnicalengineer.

AcImowledgementGrateful acknowledgement is made to DrR 1 Chandler for his comments on thedraft version of this report.

References1.Garison, L. (1948).Detenninationin situ of the shearstrength ofundisturbed day by means of a rotatingauger. Proc. 2nd Int. Cont.S.M. lli F.E.,Vol. 1,pp.265-270.

H/D =2D 65mmt 2mm2 4B tit applicoblel5 min rest'tter inserhonRotation rote = 6 /mmcu = 0.86M/ITD

Fig. 8 'Standard'ield vane test tomeasure undrained strength insoft clays.

2. Skempton, AW. {1948).Vane tests in the aBuvial

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