Information 3

Evaluatian of shear strength in cohesive soils with special reference to Swedish practice and experience

ROLF LARSSON ULF BERGDAHL LEIF ERIKSSON

STATENS GEOTEKNISKA INSTITUT EVALUATlON OF SHEAR STRENGTH IN COHE8­ IVE SOILS WITH SPECIAL REFERENCE TO SWE­ DISH PRACTICE AND EXPERIENCE

In this publication, information is given on how different test results and empirical experience can be weighted and combined to give the best possible estimation of the shear strength proper­ ties in cohesive soils.

The procedure described has evolved during the continuing work at SGI as research results and practical experience from the Institute as weil as others have been obtained and taken into con­ sideration.

The publication describes how the Institute cur­ rently proceeds in the evaluation of shear strength in cohesive soils. The intention has aiso been to bring about a more unified and objective proce­ dure t han that has been used up to now.

Valuable views on this work have been given by colleagues in and outside the Institute.

Linköping November 1984

Rolf Larsson Ulf Bergdahl Leif Eriksson CONTENTS

INTRODUCTION 2

DEFINITIONs AND SYMBOLS 3

UNDRAINED SHEAR STRENGTH IN NORMALLY CONSOLIDATED 4 AND SLIGHTLY OVERCONSOLIDATED SOILS eorrection of values of undrained shear strength 4 obtained by vane shear tests and fall cone tests

Estimatian of shear strength values on the basis 5 of Hansbo's relation

Estimatian of corrected shear strengths on the 6 basis of results from other soundings

Estimatian of corrected shear strengths on the 6 basis of laboratory tests and empirical rela­ tions for undrained shear strength

Selection of undrained shear strength 8

Estimatian of increase in shear strength at 8 consolidation

SHEAR STRENGTH IN DRY CRUST, OVEReONSOLIDATED CLAY AND IN 9 CONNECTION WITH LAYERS WITH HIGH PORE WATER PRESSURES

DRAINED SHEAR STRENGTH 9

CHOICE OF DRAINED OR UNDRAINED SHEAR STRENGTH 10

EXAMPLE 10

REFERENCES 19

APPENDIX: BACKGROUND TO THE NEW RECOMMENDATION FOR CORRECTION OF 21 VALUES OF SHEAR STRENGTH OBTAINED BY VANE SHEAR TESTS AND F ALL CONE TESTS

Introduction 22

Historical development up to 1969 22

SGI correction factors of 1969 23

Bjerrum's correction factors 23

Later studies of correction factors 23

Results from research on vane shear tests 25

Empirical relations for undrained shear strength 27

earrelations between empirical relations and full scale 29 failures in the field

earrelations between empirical relations and shear strength 29 values obtained by vane shear tests and fall cone tests

eompilation of results and comparison with the new 30 recommended correction l INTRODUCTION

In Sweden, calculation of stability in cohesive soil is mainly based on shear strength values determi­ ned by vane shear tests or fall cone tests. These values of shear strength have to be corrected be­ fore they are used in calculations. An engineer­ ing judgement must also be made as to the rel­ evancy of these corrected strengths to the par­ ticular case.

An empirical knowledge of how the shear strength varies with plasticity, preconsolidation pressure and overconsolidation ratio has evolved from re­ sults from other types of tests such as triaxial tests and direct shear tests.

A new sounding method, the combined cone pen­ etration test and pore pressure sounding (CPTV­ test), has also come into use. This method gives a good picture of stratigraphy and variation in shear strength, although it cannot yet be con­ sidered to give good enough numerical values of the shear strength.

In this publication, a recommendation is given for how the shear strength values obtained by vane shear tests and fall cone tests should be cor­ rected. Guidance is also given for how other test results, empirical knowledge and results from soundings can be incorporated in the engineering evaluation of "real" shear strength.

The recommendation for correction of shear strength values obtained by vane shear tests and fall cone tests given here does not involve any larger differences in the end result campared to the old SGI recommendation from 1969. The new procedure, however, is more precise and objec­ tive.

The recommendation and the guidelines are main­ ly based on experience of Scandinavian soils and cannot be applied directly to soils from other geo­ logical regions.

Part of the historical development and research that has led to the present recommendation and the empirical relations that are used is presented in an appendix.

2 DEFINITIONs AND SYMBOLS c' shear strength parameter CRS-test oedometer test with eonstant rate of strain FB calculated factor of safety at failure k eonstant point resistance in cone penetration tests time to failure reference time liquid limit (wL in equations is given in decimal numbers) coefficient !J. a 'c increase in preconsolidation pressure increase in undrained shear strength shear strength parameter correction factor for shear strength values obtained by vane shear tests and fall cone tests existing overburden pressure a' effective normal stress preconsolidation pressure existing effective overburden pressure T shear stress uncorrected value of shear strength obtained by vane shear test uncorrected value of shear strength obtained by fall cone test Tf u undrained sh ear strength Tfd drained shear strength TACTIVE undrained shear strength at "active shear" TDIRECT SHEAR undrained shear strength at "direct shear" TPASSIVE undrained shear strength at "passive shear" TeR mobilized shearstress in passive triaxial tests at a deformation corresponding to the deformation at failure in active triaxial tests TAVERAGE average undrained shear strength (TACTIVE + TDIRECT SHEAR +T PASSIVE)/3 ~VERA GE from laboratory tests value of shear strength obtained by vane shear test with a waiting time of 5 minutes between installation and start of the test value of shear strength obtained by vane shear test with a waiting time of 24 hours between installation and start of the test value of shear strength obtained by vane shear test with standard rate of rotation value of shear strength obtained by vane shear test with a rate of rotation giving the time to failure

3 UNDRAINED SHEAR STRENGTH IN NORMALL Y CONSOLIDATED AND SLIGHTLY OVERCONSOLIDATED SOILS

Correction of values of undrained shear strength obtained by vane shear tests and fall cone tests

The correction that was previously recom­ The correction factor l1 is a function of the mended by SGI dated 1969 and consequently liquid limit w1 and can be calculated from did not include the research results obtained during the last 15 years. 1t implied a pre­ 043 045 treatment of the measured values which JA = (- l - ) l ~o 5 was subjective and also differed from what WL l is customary in other countries. or taken from Fig. l. New building codes, new methods for statis­ tical treatment of measured data and also w1 in the equation is expressed as a deci­ the need for compilation and comparison mal number. of experiences demand a uniform treatment of the measured data. This also calls for Correction factors higher than 1.2 ought an according actjustment of SGPs correction not to be used without supporting evidence factors. 1t is therefore now recommended from complementary investigations. that s hear strength values obtained by vane shear tests and fall cone tests be corrected Each shear strength value ought to be cor­ with respect to the liquid limit of the soil rected using a correction factor calculated in accordance with from the appropriate liquid limit. For cor­ rection of a shear strength value obtained by vane shear test a determination of the TfU= JA • Ty liquid limit at the same point is thus nor­ TfU =JA• Tk mally required. where Tf u= undrained shear strength The corrected shear strengths are then plot­ l1 = correction factor ted as a shear strength profile versus depth. Tv = shear strength value In slopes created by erosion, the strengths obtained by vane shear are normally plotted versus elevation. An test engineering evaluation of the corrected Tk = shear strength value ob­ strengths considering all other known pro­ tained by fall cone test perties of the soil is then made. After even­ 1,3 l l l 1,2 l

::s__ 11 \ 0::: o \ l- u <( 1,O LL z 0,9 \ o \ l­ u w 0,8 ~ 0::: 0::: o ;;IL u 0,7 """ ~ K ~ 0,6 l-- 1---- 0,5 r-­ o 20 40 60 80 100 120 140 160 180 200 LIQUID LIMIT, wl%

Fig 1. Re.c.omme.nde.d c.oJUte mon 6ac;toJL nOfL .6 he.cuz.. .6Vl. e.ng-th vatuu o b-tained b!f van e. .6he.cuz.. -tuu o!L t)at.t c.on e. -tut. 4 tual exclusion, supplementation or modifi­ which are normal for the type of soil in cation of values considered erroneous on question. The typical result in normally con­ the basis of the following guidelines, aver­ solidated and slightly overconsolidated ages of the corrected shear strengths are Scandinavian soils is that the shear strength then used in the caleulations. values obtained by vane shear tests and fall cone tests mainly follow Hansbo's relation. The correction factors are based on arith­ metic averages of measured shear strength Tv,k =oc · 0.45 wL (Hansbo 1957) values and consequently arithmetic averages of corrected shear strengths must be used where when more than one shear strength value Tv= uncorrected shear strength from the same level is taken into consider­ value from vane shear tests ation. Tk = uncorrected shear strength value from fall cone test In cases where only a few determinations = preconsolidation pressure of shear strength have been made, this de­ ac ficiency should be accounted for by the re­ WL = liquid limit quired calculated factor of safety. A compilation of a number of shear strength values obtained by vane shear tests in Scan­ Estimatian of shear strength values on the dinavian soils is shown in relation to precon­ basis of Hansbo's relation solidatian pressure and liquid limit in Fig. 2. The correction factors are mainly derived from comparisons between averages of As can be observed in the figure, the spread measured shear strength values and mobili­ is considerable. In the figure a number of zed shear stress at failure in full scale fail­ cases where the shear strength values ob­ ures in the field. Comparisons of averages tained by the vane shear tests have been of shear strength values obtained by vane unusually high or low have been specially shear tests and fall cone tests in relation marked. In these cases the recommended to averages of shear strength values from correction factors have also proved to be direct shear tests and active and passive too high or too low respectively. triaxial tests have been used as comp­ lements. These comparisons have naturally As a simple estimation in this way can be resulted in a certain spread and the correc­ made of the reasonableness of the shear tion factors represent averages. strength values a comparison with Hansbo's relation ought to be made. If the shear A condition for these correction factors to strength value is unusually high there is a give shear strengths of use is that the vane great risk that it must be reduced more than shear test and the fall cone test give results

1,0

e 0,8 • o o o • o • • o o C\) .. .. o o .. .. o o o

0,2 o

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 wl% • Lower correction factor than usual @ Higher correction factor than usual

Fig z. The JL ela;t{_on Tv / o' M a {)u.ncAA_on o{) uqu.id .tbnit {)o !L a numbeJL o{) s~andinavian ~o~ . 5 the general correction factor specifies and method for evaluation yet. The development if it is unusually low supplementary inves­ is promising, though, and some conclusions tigations will probably give higher values can be drawn from the test results. From of the shear strength. the curves it can be seen whether the stiffness of the soil increases, decreases No further conclusions can be drawn and or is eonstant with depth. It can also be seen the empirical relation can never replace whether there are any layers. From the real tests. point resistance it can be observed whether the layers are stiff or loose and from the Estimatian of corrected shear strengths on pore pressure it can often be concluded what the basis of results from other soundings kind of soil the layers contain.

The use of vane shear tests and fall cone If low shear strength values are thus tests invalves certain probleJ;Yls which may obtained from vane shear tests and/or fall effect the measured values of shear con e tests a t a certain level and a lo os e strength. In undisturbed sampling a strati­ layer is detected at the same level by cone graphy with a stiffer layer overlying softer penetration testing, the existence of the soil often entails that the samples in the layer can be considered confirmed. If the upper part of the soft soil become disturbed. cone penetration test does not register any This problem almost always occurs at the loose layer at the level, the existence of lower boundary of the dry crust but can here a layer is more doubtful and if the test be avoided by preboring through the crust. results show that an overlying stiffer layer Varying stiffness at greater depths is more has just been passed disturbance is often difficult to handle, which is often reflected a probable explanation for the low shear in the results from fall cone tests. Sampling strength values. Low values of shear invalves an unloading and redistribution of strength can also be measured in connection with silt layers with high pore water stresses in the samples, which also affects pressures, especially in vane shear tests. the results from fall cone tests. Based on experience, shear strength values measured in samples from greater depths than 10 to Estimation of corrected shear strengths on 15 m e tres are t hus often too low. the basis of laboratory tests and empirical relations for undrained shear strength Disturbance effects at the transition from stiffer to softer soil also occur in vane shear In order to obtain a broader basis for the tests. When a vane apparatus type SGI with estimation of the reasonableness of correc­ a casing for the vane is used, this ted values of undrained shear strength from disturbance is confined to the first test level vane shear tests and fall cone tests, below the stiffer layer. In very layered or earrelations can be made with empirical varved soil a conutinous disturbance may relations between undrained shear strength, occur. When a vane apparatus without casing preconsolidation pressure and liquid lim i t, is used the risk of disturbance is greater, if necessary after supplementary, more as soil from an overlying stiffer layer may qualified laboratory tests. Effects of stick to the vane and all of the underlying anisotropy cause these relations to vary with softer clay profile may then be disturbed loading case. The loading cases are normally at the installation of the vane. divided into active shear, direct shear and passive shear. These cases can be simulated Occurrence of dense layers of silt and other in the laboratory in active triaxial tests, strata may in some cases give too high direct shear tests and passive triaxial tests, values of shear strength. In other cases with Fig. 3. for example layers of loose silt the shear strength values may be too lo w. Compilations of test results from Scandina­ vian inorganic clays have given empirical For estimation of the reasonableness of the relations for the way in which the undrained corrected shear strengths, support may shear strength in the different cases varies therefore have to be sought from other with preconsolidation pressure and liquid observations and empirical relations. limit, Fig. 4. A corresponding empirical relation for arganie clays is not yet at hand. An important complement is the combined cone penetration test and pore pressure The undrained shear strength that should sounding. The equipments for this type of be campared to the corrected shear strength testing used in Sweden today are too coarse values from vane shear tests and fall cone for a direct evaluation of the shear strength. tests is the shear strength at direct shear Besides, there is no generally accepted in a harizontal slip surface.* This relations * see Fig. 19 6 The effects of anisotropy are largest in low plastic clays and in these clays large differences in evaluated undrained shear strength may occur depending on whether anisotropy is considered or not. (Fig. 4).

For estimation of the shear strength the preconsolidatian pressure o c must be known. This value is normally determined by l oedometer tests. --0­ In, for example, investigations of the stability conditions in larger areas, a general Passive zone Zone of d irect Active zone picture of the preconsolidation in the whole Passive s hear Act i ve triaxial test Direct shear test triaxial test area can often be obtained by performing a number of oedometer tests and by linking F;.g 3. v;.v.,U;.on on toac{)_ng c.a!.>e!.> and the evaluated preconsolidation pressures JLUe.vanc.e. on .taboJLa:to.tLIJ ;tuu ;to to the geological history of the area. This c()_ nn eJLen.;t toac()_ng C.a!.> e!.>. is especially valuable in areas with such stratigraphies that vane shear testing as well as fall cone testing invalves problems.

'( ACTIVE '( AVERAGE If no oedometer tests have been performed 0 .3 1------,------,-----r------,~;..,- '( DIRECT SHEAR the empirical relations in Fig. 4 can be used '( PASSIVE tagether with the existing effective overburden pressure. A lower boundary value of the undrained shear strength is hereby obtained. In this procedure, however, the risk of nonhydrostatic pore water pressures must be considered when the existing effective overburden pressure is estimated.

There is, of course, a certain spread in the 0 +---~---4---+---~---- 0 25 50 75 100 % empirical relations for undrained shear LIQUID LIMIT,wl strength and the relations can never substitute for other tests but only be F;.g 4. Emp~c.at JLUation...6 be;twe.e.n complementary to them. A special case p.!Le.c.on...6oudation p.!Le.J.>.OUJLe. and und.!Lune.d where the empirical relations may .ohe.aJL .o;tJLe.ng;th noJL Sc.anc{)_nav;.an ;.n­ overestimate the shear strength is in low OJLgaMc. dalj.O. plastic and highly sensitive clays. In these soils, however, vane shear tests and fall cone tests give low values of shear strength gives shear strength values which are diretly and low values of point resistance are also applicable to calculations of slope stability registered in cone penetration tests. In such where the inclination of the slip surface is cases the low values ought to be used and small and which gives results that are some­ the empirical values should be disregarded. what on the safe side when applied to A lower boundary value of Tfu ~ 0.12oc may steeper slip surfaces. In calculations of be considered, though. Undrained shear stability of embankments on harizontal strengths lower than this value, have not, ground the average shear strength as far as known, been found for Scandinavian (TA VERA GE) should preferably be used but soils in either direct shear tests in campared with the difference the shear laboratory or by back calculation of full strength a t dire c t sh ear is small. scale failures in the field. (Liquefaction as well as thawing of frozen masses is Active triaxial tests generally give the high­ disregarded.) est value of undrained shear strength but these values are applicable only to zones with active shear. Passive triaxial tests give the lowest values and these may be relevant in cases with passive shear.

7 Selection of undrained shear strength This increase in shear strength can be utilized in stage construction where the load When the undrained shear strength to be is applied in steps with intervals for used in calculation is selected an estimate consolidatian as well as in old constructions, has, as described above, to be made as to where the increase in strength beneath the the reasonableness and the relevancy of the construction can be utilized when these are measured shear strengths and the applicabil­ to be added or enlarged. ity of the empirical relations, whereupon obviously erroneous or irrelevant values can Direct measurement of this increase of be eliminated. Thereafter, the remaining shear strength often involves difficulties values can be weighted and combined into as well as expense. Holes have to be made an undrained shear strength t hat can be used through the construction and the shear in the calculations. strengths vary laterally if the construction has a limited width. A simplified way to In this evaluation the corrected values from estimate the increase of shear strength vane shear tests are very weighty uniess below, for example, an old road embankment they significantly deviate from Hansbo's is to make the assumption that the shear relation. Corrected values from fall cone strength increase is confined to the extent tests are also often considered relevant with of the embankment and that the increase the same limitation. In samples taken at follows the empirical relations. Fig. 5. In greater depths than 10 to 15 metres the this context, it should be considered that shear strength values from fall cone tests the change in preconsolidation pressure is often are too low. Results from qualified different in different parts below the laboratory tests are of very great weight embankment. The increase in vertical stress provided that they are relevant to the .6..acxz should be calculated according to particular loading case. Empirical relations theory of elasticity. Purthermare it must between shear strength and preconsolidation be confirmed that the loading really has pressure can be used to support and brought a change in preconsolidation supplement determinations of the shear pressure, that is, that the soil has strength. Results from cone penetration Consolidated for the load increase. tests can so far only be used to study the trend for shear strength evolution with As criterion of consolidatian of the soil, depth and to evaluate the occurrence of observations showing that the settlements layers. have stopped can be used. Alternatively, pore pressure measurements can be made. Estimatian of increase in shear strength at In this case the pore pressures should be consolidatian measured in the middle of the layers where an increase of the preconsolidation pressure When normally consolidated clay is loaded is expected. and consolidates the preconsolidation press­ ure and thereby the undrained shear strength The empirical relations are valid only for increase. The increase in shear strength can inorganic soils and if the same procedure be assumed to follow the increase in is to be used in organic soils, the relations between undrained shear strength and preconsolidation pressure in accordance with preconsolidation pressure must be relations determined by tests or by determined by laboratory tests for the empirical relations: L'.Tfu =k·L'.o c where k particular case. depends on the loading case. (See Fig. 4).

'( _ 1. DIRECT SHEAR . a' 11 fu- • 11 Cxz oc

Fig 5. Simpunied utima.Uon. on in.CJLealle in. .oheCUL .obl.en.g;th du.e ;to c.on..oouda.Uon. betow an. emban.kmen.:t.

8 SHEAR STRENGTH IN DRY CRUST, OVERCONSOLIDATED CLAY AND IN CONNECTION WITH LAYERS WITH HIGH PORE WATER PRESSURES

The undrained shear strength is often regarded as a eonstant although tests have shown that it decreases when the effective pressure decreases and the overconsolidation ratio increases (e.g. Ladd et al, 1977). 0,4 Undroined silear stre th '!,u However, when the shear strength is

selected, the drained shear strength also // has to be considered. When the latter 0,2 becomes lower than the undrained shear / //~Drained shear strength '(td / strength, it also becomes the strength that 0~---,----,---~r----r----r------should be used in caleulations, except in a 0 0.2 0,4 o.6 o.e 1.0 el' ;o~ very short time aspect. Drained shear l 1 l l l l l Overconsali­ oo 5,0 2.5 2.0 1,5 1.25 1,0 dation Ratio strength becomes lower than the undrained shear strength at an overconsolidation ratio of about 2. At this overconsolidation ratio Fig 6. Va.!Ua.;tion in .ohe.a.Jt .o.:tfl..e..ng.th wdh the decrease in undrained shear strength ovVLc.on.oouda.;tion tr..a.;tio a.t ditr..e.c..t .ohe.a.Jt. in relation to the normally Consolidated state is small. The decrease in shear strength with increasing overconsolidation ratio thus normally becomes of limited interest, Fig. 6.

DRAINED SHEAR STRENGTH

The drained shear strength fd is expressed TABLE 7. Apptr..oxima.te. vatue..o 6otr.. angte. by the shear strength parameters c 1 and Sand Grave l San dy Gra ve l- Maea-Rock 1 1 parameters C = O and

The angle of frietian in coarser soils is selected from test results or on the basis of particle size distribution and relative density from Table l.

9 CHOICE OF DRAINED OR UNDRAINED In Connection with the erosion the ground SHEAR STRENGTH water level which was originally close to the ground surface has been lowered so that Consideration to the overconsolidation ratio at the time of the measurements it was is automatically taken at the choice of shear situated just above the lower boundary of strength if a combined analysis is made (see the silt and sand layer. The lowest seasonal SGI Report No. 19). In other cases the ground water level will probably be at this nundrained shear strengthn in the undrained boundary. This general picture is supported analysis must be modified by a manual by the results from the oedometer tests. comparison between drained and undrained Figs. 7 and 8. The general picture is further shear strength in the different parts along supported by the results from the other the shear surface and selection of the lowest sections in the investigation. strength for each part. In this comparison the effective stress state existing after a Fig. 7 shows the test results from point change in the leading situation should be 0/061.5 in Section IX. The preconsolidation considered. This nundrainedn analysis should pressures from the oedometer tests show also be controHed by a completely drained a certain spread which can be expected analysis. considering the difficulties in taking undisturbed samples in such a stratigraphy. The general trend supports the assumption EXAMPLE of a soil normally consolidated for a ground water level at the lower boundary of the As an example, a section in a larger silt and sand layer. In the figure the investigation of slope stability along the corrected values of undrained shear strength river Norsälven has been chosen, Section obtained by vane shear tests and fall cone IX drawing Nos. 8 and 9. The soil consists tests are shown together with empirical of a 3 to 5 m thick layer of silt and sand shear strengths. The latter are based on the which covers a varved clay with frequent curve for the variation of preconsolidation silt layers. The layer of varved clay extends pressure with depth and measured liquid to great depths. limits. The relation for the case of direct shear has been used. The trend for shear In the section the following have been strength variation obtained by cone performed: undisturbed sampling with penetration testing is also shown. The shear standard piston sampler, sampling with strengths from vane shear tests and fall auger vane shear tests, weight sounding cone tests are equal down to a depth of 13 static penetration testing, cone penetration metres from where the fall cone values testing, pore pressure sounding and pore consistently become lower than the values pressure measurements. The investigations from vane shear tests. The shear strength are thus fairly comprehensive. As it was values indicate a weaker layer at the +34 known already that the stability of the area m to +35 m level but a comparison with the was poor and that the investigations due cone penetration test, the static penetration to the stratigraphy of the soil would involve test and the pore pressure sounding shows some difficulties, the field investigations that this is probably an effect of passing have been performed with special care and a stiffer silt layer at the +35 m level so that most of the different ways of estimating the +35 m to +34 m layer has become the shear strength have been used. In disturbed. There is nothing in the other soil addition to the routine tests a large number parameters, such as sensitivity or liquidity of oedometer tests have been performed index, that indicates anything special at this in order to establish the preconsolidation level. Like the trend obtained from cone in various parts of the section. penetration testing the shear strengths obtained by vane shear tests increase from Geologically, the ground surface in the the +38 m level and downwards. The results section was probably originally mainly show a certain spread at greater depths but horizontal. The river has then eroded when the empirical shear strength values downwards so that the bottom of the river are added a considerably broader basis is is now 13 metres below the surrounding obtained for the evaluation of the undrained ground surface. shear strength.

lO The evaluated undrained shear strength is to the overconsolidation ratio and at great thus mainly based on the corrected shear depths it is estimated to be higher than the strength values from vane shear tests. The corrected values from fall cone tests, due empirical shear strengths and the results to the effect of stress relief and from the cone penetration test are used to redistribution at sampling at greater depths. support the trend in shear strength That an estimated shear strength higher development versus depth and to even out than the measured values from fall cone the spread in the corrected shear strength tests may be used at this point without values from the vane shear tests. supporting investigations is due to the comprehensive investigations in the In Fig. 8 the corresponding results from surrounding areas. Without this support the point -0/036 are shown. This point is presented evaluation could not have been situated close to the middle of the river. made. At this point only weight sounding and standard piston sampling have been performed. The results from the oedometer tests have been compared to maximum previous overburden pressure with the assumption that the soilprofile originally was identical to that of the surrounding ground. The comparison shows that the soil is normally consolidated for a ground surface level with the surrounding ground surface and a ground water level about 2 m below this ground surface. This is in good agreement with the results from the rest of the area.

The figure shows the corrected shear strength values from fall cone tests and the empirical shear strength for direct shear from preconsolidation pressures and liquid limits. The two sets of shear strength values are close down to a depth of l O m below the river bottom. Below this level the values from fall cone tests consistently become lower. As there is nothig special in terms of sensitivity or other properties or comparisons with the results from vane shear tests at point 0/009 indicates that any weaker layers the empirical shear strengths are considered as most relevant.

Due to the erosion the upper layers of the clay are heavily overconsolidated. A comparison between the undrained shear strength and the empirically estimated drained shear strength shows that the drained shear strength is lower than the undrained down to a depth of about 6 metres below the river bottom.

The evaluated 11 real11 shear strength to be used in the stability calculations for this point thus differs somewhat from the results of the only direct determinations of shear strength that have been made here. In the upper layers the shear strength is estimated to be lower than the undrained shear strength determined by fall cone tests due

11

Kv St n /+45 Vim v v t.t. .t.O Skr Vi m Vim ' 81-07-30 ~ ~,, ~ ~~ s ..Y/-/,1,1 - 0.50 0,750.50~- -..,...... _...... o/!0" l 0.15 0.25 l 0.25 0.15 0.25 0.50 0.750,75 0.75 o - -­ l +40 - 0,50 l l 0.75 0.15 "' ~" l Vattenkvot •;. 'tf kPo l ore d.) o · : 1,00 o 10 20 30 t.O~S /,, ------­2\1------!'o- ~~o L Il 0.15 l , L 0.25 .. Il 1 - \ ~ - - ~-75 + +35 ~. so ~~ .. _ - - V- li!Jj(rrJ) Il 35 :;:./ l l> l -­ 0.50 'o vmj~lml 0.25 l ~ l. :• Il \ : ' vmjLQ)jiQ)f Il l • lol ~l 0.75 ' ' l .; ~ vm]U!!!l Il 0.50 ' \\ vmjU[ll +30 - - -­ l 30 0.50 : l ~ Vffijl(p) \ ' 0,75 vmjli'!IKI!l] \ ~ lt' r-­ vmjlil!l) ~ l Vmjl(f!l) 075 0.50 \ r-­ vmjl l '!l l +25 0,75 f - -­ + 25 l r­ f-­ ' vmj l l f!:! l : l ·~n - 6 r--­ vmjlll!ll 10 20 30 ~ hv/020m 0.5 1.0 1,5 2,0 h o "o sa-so : w~_ .,,, 5 Sk r ymd ens i ~ e t 9 tlm Se nsitivitet St 102030 hv/0.20 m > +20 1---100 + 20 10 20 hv/0.20 m

+15 15 -0/076 - 01070 -0/060 -0/050 -0/040 - 0/030 -0/020 -0/010 01000

KAR LSTAD KOMMUN STATENS GEOTEKNISKA * NORSÄLVEN- STABILITETSUTREDNING \!J INSTITUT FACK 58101 UNKOPIN G 1 TEL 013·1! 51 00 VÅLBERG SEKTION IX 13 GEOTEKNISK UNDERSÖKNING 1100 2:2·04/81~1"~' " ";:;;-'8-~, .~,, - L______------~~------~------C------~1 --~ Q~·51.01 RtRIRf ök•51.09 Vb Kv S t I Vatt enkvot •;, Te P Sk e Te - +50 o 20 LO 60 80 Te - +50 -· ... - --=:..._ 1 ,(:b -:Mjvx .. 0 l W : 1!. /, j mJM ·­-­ l ·-­ - mJM s Fb GW .. L.8,11 ..__, mJM _;:== -Skrf.60 ~81-03- 28 81-08-28 mJ M .. --­- --­ .. ~ " MAX IM ALT mjM .. !? "" SKRIVARVÄROE GW • L6.8L ~81- 08 -28 \r­ ... GW· L6 57 81-0 -2 .. l h ' 81 ­ 08-28 mj M .. l 1'---.: SPE TSTRYCK ... ·-­ mjM ~ - ~~ '(, k Pa ( ore dJ ' 10 2 30­ ' "T - - · PORTR YC K +45 -----­ o t --~------l\ v mj LJ!lj '' ' / ' l ~ ' vmjLrni \\ ·-----­ "\ - ---­ ' '' ' ··(" ' ' \\ \\ ' vmjl(l!ljl b ' MAX . UTSLAG ' : \il. 1.1. mvp \ vmj LI[!j) i\ ~ ~ ' r-­ ' ' ~ l~ ' ' / l/ vmjlll!llf!,lj l ·---­ ' : ' u : -­ l ( 40 ---­ - vmj l (J!lj) l\ ·-­ -­ l ' '' ~ i :' vmj L l \ ., l ! l " l ' l l ! ! -­- vmjL !mil [\ ' l '' ~ 1 L__~~: ' " vmjl ' ' ' ' f~. l ! ' l/ 'lmj l ( ~JJ j) l --·· t:= i== ' l :J - '' l ,_,.-' l _ 35 L miUI!lXJDil ~ l y P • {\ ' l ( ··­ -· ______, -­ - vLI!!'jlf!\1 ~ ,l !l l l \ vlt rni l \ ------'----:-·'"'"! 1\ l l l l MAX .UTSLAG ' ' vLII!ljl - .. -----+---_j 51. mvp -­- l ' '-'' .,, ' vll mil : \ -­ l -+-l --­.;- · '' l l - ~ ! ______..~...-.- - +30 ' ..)),! vLf!!( fD j) 1­ FEL R ' ' i ; l ' PAPPER S­ ' \ .,.. vLII!!il ~ l o o Lo 60 0.0 05 ID l.S 2.0 ·- -~- M(TIINF l 3 l F===-0.-.t------·--···-·---···- ··-·····- ­ sensitivitet s, Skrymdensitiet 9 t/m ' } ' l -· -~· - -­ VR ~-~-t------·-·-- 05 1 1.5 2 25 3 35 L L.5 5 55 6 65 7 MPa g 1 2 3 L s 6 7 8 10MPa L 8 12 16 20 '21. 28 32 35 40 tJ. 1.8 52 56 mvp 0

+ 25

P===--..(­--­-·­ ---­ 0 N I V~•35.00:UTJÄ M NAT PORTRYCK 12 TIM 1~13,L1m'p ·-­ N IVA+ 27.60: l 81-08-25 ) UTJAMNAT PORTRYCK l 14 T IM) = 20.94 mvp -·­

· vR g 10 MPa

+20

+15 +15 ~------.------. 01070 01000 0/010 0/020 01030 0/040 0/050 0/060

l l .., KARL5TA D KO MMUN STATENS GEOTEKNISKA * NORSÄLVEN ­ 5 TABILIT ETSUT REDNING ~ '· INSTITUT

15 KG. J G. HULT GEOTEKNISK UNDERSÖKNING l 1•100 Section IX

0/061,5 SHEAR STRENGTH l PRECONSOLIDATION PRESSURE o 50 100 150 200 kPa

lowest ground water leve l

• • +40 • • E • ....J- Oense layers LU of silt > l LU ....J • • • +30 •

v 'ttu fall cone corrected } u= ( Ow. 4L3 ) 0.45 X '(fu vane shear test corrected r

o t f u empirical direct shear \ 'L tu con e penetration - porepressure sound i ng test ( evaluated according to Lunne & Kleven. 1982 as '( = Q.c- Oo +20 tu 19 \ Evaluated shear strength • 0~ from CRS- test

F.{_g 7. Te6t Jte6uLt6 and e.valuate.d .ohe.aJt .ot.Jte.ngth at po.{_nt 0/ 067.5.

17 Section IX

-0/036 SHEAR STRENGTH l PRECONSOLIDATION PRESSURE

o 50 100 150 200 250 kPa

+50+-,~------~------~------~------~------L--

' ' ' \ \ \

\ \ ' ' \ \ +40 \ \ \ \

..=:' :t'/ E"/.'/..::::;/// -=:///.=;;;:-/. / .:::;;::-//~/// .=-//' ~///.=-/.'/-' /.:::::.=//. ~///...:::-/// ..::=/// ...=-7"//-=.~// =---;.-//~/// -==///..:::Y// ~ E \ \ _J- . UJ ..'\ > UJ _J "'\ '\ . \ ' +30 • "" • ". ."" " • v n-' j'\" 0 45 ~ v Lfu fall cone corrected jl-=(~43) ' +20 o Lfu empirical direct shear L •OC from CRS-test "' Evaluated shear strength

U .g 8. Trut !L e!.l uLt6 and e..va.J..uate..d .o he..a!L .ot!Le..n.gth at point 0/ 036.

18 REFERENCES

Aas, G., 1976. Totalspänningsanalyser, pnns1pp, Flodin, N. and Broms, B., 1981. Historical development grunnlag. Norske Sivilingenimers Forening. Kurs of civil engineering in soft clay. Soft Clay Engng, i Jordartsegenskaper - bestemmelse og anven­ Elsevier, Amsterdam, Oxford, New York. delse. Gol 20-22/5. Hansbo, S., 1957. A new approach to the determina­ Aas, G., Lacasse, S., Lunne, T, och Madshus, C., 1984. tion of the shear strength of clay by the fall-cone In situ testing - new developments, Nordiska test. statens geotekniska institut, Proc. No 14. Geoteknikermötet i Linköping 1984. Helenelund, K.V., 1977. Methods for reducing un­ Andreasson, L., 1974. Förslag till ändrade reduktions­ drained shear strength of soft clay. statens geotek­ faktorer vid reduktion av vingborrbestämd skjuv­ niska institut. Rapport No 3. hållfasthet med ledning av flytgränsvärdet Intern rapport. Chalmers Tekniska Högskola, lnst för Hultin, S., 1916. Grusfyllningar för kajbyggnader. geoteknik. Bidrag till frågan om deras stabilitet. Teknisk Tid­ skrift. V. o V. 46(36):292-294. Bergdahl, U. 1984. Geotekniska undersökningar i fält. Statens geotekniska institut. Information nr 2. Hultin, T., 1937. Försök till bestämning av Göteborgs­ lerans hållfasthet. Tekn. Samfundets Handlingar, Bjerrum, L., 1972. Embankments on soft ground. No. 2. Proceedings, ASCE Speciaity Conference on Performance of Earth and Earth-supported Struc­ Jacobson, B., 1946. Kortfattat kompendium i geotek­ tures, Purdue University, Lafayette, IN, Vol. 2, nik. Statens geotekniska institut, Meddelande PP 1-54. No. 1. Bjerrum, L., 1973. Problems of soil mechanics and Kallstenius, T., 1963. studies on clay samplestaken construction on soft clays and structurally unstable with standard piston sampler. statens geotek­ soils. State-of-the-art Report, Session 4. Proceed­ niska institut, Proc. No. 21. ings, 8th International Conference on Soil Me­ Karlsson, R., and Vi berg, L., 1967. Ratio e/p' in relation chanics and Foundation Engineering, Moscow, to liquid limit and plasticity index, with special Vol. 3, pp 111-159. reference to Swedish clays. Proceedings, Geo­ Cadling, L. och Odenstad, S. 1950. The vane borer. technical Conference, Oslo, Vol. 1, pp 43-47. An apparatus for determining the shear strength Kjellman, W., 1942. Väg- och Vattenbyggnadsstyrel­ of clay soils directly in ground . statens geotek­ sens geotekniska laboratorium. Teknisk Tidskrift, niska institut, Proc. No. 2. v. o v. 72(8):105-116. Caldenius, C., 1938. Några rön från grundundersök­ Kjellman, W., 1950. Testing the shear strength of clay ningar i Göteborg rörande fasthetens variation in Sweden. Geotechnique 2:225-232. inom lerorna. Teknisk Tidskrift , V. o V. 68(12):137-142. Ladd, C.C. et al1977. stress-deformation and strength Dascal, 0., and Tournier, J.P., 1975. Embankments on characteristics. state-of-the-art report. Proc. IX soft and sensitive ciay foundations. ASCE Journal ISSFME, Tokyo. Vol. 2. of the Geotechnical Engineering Division, 101(GT3A Ladd, C.C. and Foott, R. 1974. New design procedure pp 297-314. for stability of soft clays. ASCE Journal of the Fellenius, W., 1918. Kaj- och jordrasen i Göteborg. Geotechnical Engineering Division, 100(GT7), Teknisk Tidskrift, V. o V. 48:17-19. pp 763-786. Fellenius, W., 1919. Utredning beträffande kajbygg­ Larsson, R., 1977. Basic behaviour of Scandinavian nader för Trondhjems hamn, Teknisk Tidskrift, soft clays. statens geotekniska institut, Rapport v. o v. 49:1-19. No. 4, Linköping. Fellenius, W., 1926. Jordstatiska beräkningar med Larsson, R., 1980. Undrained s hear strength in stability friktion och kohesion för cirkulärcylindriska glid­ calculation of embankments and foundations on ytor. Kungl. Väg- och Vattenbyggarkårens 75-års­ soft clays. Ganadian Geotechnical Journal. Vol. 17. skrift. No.4. Fellenius, W., 1927. Erdstatische Berechnung mit Larsson, R., 1982 a. Jords egenskaper. Statens geo­ Reibung und Kohäsion (Adhäsion) und unter An­ tekniska institut. Information No. 1. nahme krieszylindrischerGieitfläcken . Ernst, Berlin. Larsson, R., 1982 b. Några praktiska konsekvenser av Fellenius, W., 1929. Jordstatiska beräkningar förverti­ resultaten från forskning i vingsondering . Geotek­ kal belastning på horisontal markyta under anta­ niknytt No. 3, 1982. gande av cirkulärcylindriska glidytor. Teknisk Tidskrift, V. o V. 59:57-63 och 75-80. Larsson, R., 1983. Släntstabilitetsberäkningar i lera. Fellenius, B., 1936. Discussion on soil properties. Statens geotekniska institut, Rapport No. 19, Proc. 1 st lnt. Conf, on Soil Mech. and Found. Linköping. Engng., Cambridge, Mass., Vol. III. Olsson, J., 1925. Kolvborr. Ny borrtyp för upptagning Fellenius, B., 1938. Provbelastning av i lera nedpres­ av lerprov. Teknisk Tidskrift, V. o V. 52(2):13-16. sade järnrör. Teknisk Tidskrift, 68(1938):10. Olsson, J., 1936/37. Angående (om) inre friktion (och kohesion) i lera. Teknisk Tidskrift , V. o V. 66(5):60 och 67(2):21 -23.

19 Petterson, K. E., 1916. Kajraset i Göteborg den 5 mars 1916. Teknisk Tidskrift, V. o V. 46(52):471-478. Petterson, K.E., 1933. Förbättrad apparat för upptag­ ning av lerprov uordprov~ Teknisk Tidskrift, V. o V. 63:85-87. Pilot, G., 1972. Study of five embankment failures on soft soils. Proceedings, ASCE Speciaity Con­ ferenes on Performance of Earth and Earth-support­ ed Structures, Purdue University, Lafayette, IN, Vol.1, (Part 1), pp 81-99. Skaven-Haug, S., 1931. Skjaerfasthetsfors0k med leire. Norges Statsbaner, Meddelande 6(6), pp 101-105. Skempton, A.W., 1948. Vane tests in the alluvial plain of the river Forth. Geotechnique, 1(2), pp 111-124. Skempton, A.W., 1954. Discussion of the structure of inorganic soil. ASCE, Proceedings, 80, Separate No. 478. pp 19-22. Slunga, E., 1983. On the influence of arganie content ontheshear strength of fine grained soil. Väg- och Vattenbyggaren. No. 7/8 1983. statens Geotekniska Institut (SGI). 1970. Reducering av skjuvhållfasthet med avseende på finlekstal och sulfidhalt Sammandrag av teknisk träff 1969-12-11. Statens Järnvägars Geotekniska Kommission. 1914-1922. slutbetänkande. statens Järnvägar, stockholm. Geotekn iska Meddelanden Nr 2, 1922. Svenska Geotekniska Föreningens (SGF) Kolvborr­ kommitte. 1962. Tolkning av fallkonprov på prov­ kroppar av lera tagna med standardkolvborr. Torstensson, B.A., 1977. Time dependent effects in the field vane test. Proceedings lnt. Symp. on Soft Clay, Bangkok. ~ndel, E., 1900. Om profbelastning på pålar med tillämpning deraf på grundläggningsförhållanden i Göteborg. Tekn. Samfundets Handlingar, No. 7.

20 APPENDIX

BACKGROUND TO THE NEW RECOMMENDATION FOR CORRECTION OF VALUES OF SHEAR STRENGTH OBTAINED BY VANE SHEAR TESTS AND FALL CONE TESTS TOGETHER AND TO EMPIRIGAL RELATIONS FOR UNDRAINED SHEAR STRENGTH

21 Introduction These led to the method of calculating stab­ ility with the Swedish Slip Circle Method (Hultin 1916, Pettersson 1916 and Fellenius In Sweden, undrained shear strength is mainly 1918, 1919, 1926, 1927, 1929) and to the fall estimated by fall cone tests and vane shear cone test which was developed by John Olsson tests. From the values obtained the geotech­ (The Commission of the Swedish State nician makes a judgement of what can be re­ Railways. Final Report 1922). garded as "safe" measured values and these values are then corrected with the SGI cor­ The fall cone test was calibrated by tests rection factors from 1969. These correction on samples taken with a newly constructed factors have now been used for 15 years. sampler for "undisturbed" samples, which were compared to results from a series of Provided that the corrected shear strength load tests on piles made by Wendel in 1900. is used in the normal way, together the The strength values from fall cone tests were appropriate safety factors, the procedure also compared to landslides which were back­ works fairly well. However, the user must calculated with the use of undrained shear be aware of the development that has led strength and slip circles and this testing pro­ to the SGI correction factors and how these cedure was recommended in the final report are intended to be used. from the commission.

The use of primarily the vane shear test has The sampling technique was improved and spread all over the world. Ne w research re­ piston samplers came into use (Olsson 1925, sults have been obtained and a number of dif­ Pettersson 1933, Caldenius 1938). The fall ferent correction factors have been sugges­ cone test was recalibrated for these improved ted. To enable a separation of real differen­ samples, versus various tests for determin­ ces in results and soil properties from differ­ ation of strength in the field, load tests in ences in treatment of the test results, the full scale, full scale failures and landslides background and prerequisites for correction and later on also direct shear tests in the factors must be taken into account. In recent laboratory. These calibrations also led to the years, other types of estimations of the un­ first correction factors for undrained shear drained shear strength than fall cone tests strength measured by fall cone tests in and vane shear tests have come into use, for organic and high-plastic clays (Skaven-Haug example cone penetration, pore pressure 1931, Olsson 1936-37, Fellenius 1938, sounding and pressorneter tests in the field Caldenius 1938). In the calibration of the fall and triaxial tests and direct shear tests in cone test versus landslides also other types the laboratory. This is primarily the case of failure surfaces than slip circles were abroad, but is valid also in Sweden. used. It was also made clear that the strength was dependent on the preconsolidation Statistical methods of estimating the aver­ pressure (Fellenius 1936). ages of results are used more and more and partial safety factors are now planned to be The corrected undrained shear strength from introduced in geotechnics. If statistical aver­ fall cone tests was used in all types of stabi­ ages and partial safety factors are to be used, lity problems such as excavations, earth correction factors which themselves imply pressures and stability of road berms with another treatment of the test results cannot and without loading berms (Jakobson 1946). be used. In these cases, new correction fac­ tors based on averages of measured strength A new type of shear apparatus was construc­ values must be used. ted in 1936 by Kjellman (1942, 1950) for un­ drained as well as drained tests. Triaxial tests In view of this, there is reason for noting the have also been used in Sweden since 1940. historical development which has led to the SGI correction factors from 1969 and the way The vane apparatus for determination of un­ in which they were supposed to be used as drained shear strength in the field was intro­ well as for providing recommendations on duced in its present shape by Cadling and the correction factors to be used befor cal- Odenstad in 1950. This apparatus was cali­ brated versus a number of slides that had oc­ Historical development up to 1969 curred and versus on e load test. A t first, the values from the vane shear tests were used Estimation of shear strength and stability without any correction. At the end of the started in Sweden in connection with the 1950s new and improved samplers were work of the Geotechnical Commission of the developed which led to the new standard Swedish State Railways and the Committee piston sampler (Kallstenius 1963). Hansbo on stability Problems in Gothenburg Harbour. (1957) recalibrated the fall cone test on the

22 new samples versus the vane shear test. This This means that the correction factors must new evaluation differed sernewhat from the be lower than those given in Fig. 9 when the previous method but not very significantly. strength is based on statistical averages of Earlier experience had shown that strength evaluated shear strength. values measured by the fall cone test must be corrected with regard to liquid limit. The The corrected strength was intended to be realization that undrained shear strength used in all types of calculations and strength from vane tests had to be corrected in the anisotropy was not considered. If strength same way as the fall cone values also spread anisotropy has been considered a t all in with time. Sweden, the required safety factor has been modified for different leading cases. The correction factors have, like the evalua­ tion of the undrained shear strength, varied with the times according to the kind of sam­ Bjerrum's correction factors pling equipment used. To sum up experience after the introduction of the standard piston Bjerrum (1972 and 1973) made a comprehen­ sampler in 1963 and to obtain uniform correc­ sive compilation of failures and landslides tion factors for modern sampling technique based on what had been reported in the a technical meeting was held at the Swedish literature and on the experience of the Nor­ Geotechnical Institute in Stockholm in 1969 wegian Geotechnical Institute from Norway (SGI 1970). and Bangkok. After analysis of this material Bjerrum suggested new correction factors. These were based on averages of the shear SGI correction factors of 1969 strength values measured by the vane shear tests and related to the plasticity index lp· At the meeting at SGI a recommendation was made as to the correction factors that should In Bjerrum's studies a great number of low be used for undrained shear strength values plastic clays from Norway and clays with estimated by fall cone tests and vane shear varying plasticity from mainly Europe and tests. The correction factors were based on North America were included. No high plas­ the liquid limit of the soil. Fig. 9. tic or organic soil from Seandinavia was in­ cluded in the study. The correction factors LIQUID LIMIT . wl% for this type of soil were mainly based on investigations of organic Bangkok clay. How­ o 50 100 150 200 ever, comparisons with Swedish soils with ::L 1,0 corresponding plasticity show that the pro­ 0:: o perties of the Bangkok clay do not earrespond t­ (_) to those normal for Swedish conditions.

23 1975. In 1976 Aas made a suggestion for how By inserting the Hansbo relation T v = oc·0.45 the vane shear values could be corrected in w1 Helenelund's suggested correction can Norwegian clays with respect to the relation als o be wri tten between the measured strength value and the existing overburden pressure. 1.45 l +2.22 Tv jo b An extensive review of the different correc­ tion factors, which also included some cases Recently an extensive investigation of the from Sweden not earlier taken into account, properties of organic soil has been completed was made in 1977 by Helenelund, who at that in Finland (Slunga 198 3) and a si m ilar investi­ time was visiting research worker at SGI. gation is in progress at SGI. Based on this material a correction factor in the order of Purther results from Norway have also been presented by Aas et al 1984. ll "' 1.45 l+w1 All the compilations of test results involve a spread and suggested correction factors was suggested. become an adaption to the curves for aver­ ages. This correction essentially earresponds to the Bjerrum correction but the correction of the measured strength becomes greater for organic high plastic soils.

1 2,0 • l _,:./ l. 1,8 • x o ...... : ? 1,6 o ~ Q,. p~ o 1,4 ix) ..-­~1.4 A Il v. ~ 1,2 lLID 1,2 ~o Il 1.0 1,o ~ LLm o vj' o / 0,8 o. 8 / , o t:" ) 0,6 o. v 20 50 100 150 200 o 0,2 0,4 0,6 0.8 wl 'lv/O~ aU.'(vfO~ o Data from Andreasson 1974 x '(v l 'LLAB Slunga 1983 o Data from Helenelund 1977 l'( LAB organic so il (SGI) • 'Lv x 1v l 1LAB Slunga 1983 --Andreassons suggestion for correction 1974 • 1v l 1LAB organic so il ( SGI ) 1 45 ---fl= · ( Helenelund 1977) 1 45 1+Wl --- ~ = i( lO ( Helenelund 1977) 1+ 2 l 22 v c --fl-= r-~~3) 0,45 -~ = [0,193·] 0,45 1vl0c

Fig. 7O. Evalu.M:.e.d .oa{je.:ty {jac.toM M:. {ja.{_,f_UJLe. Fig. 77. Evalu.M:.e.d .oa{je.:ty {jac.toM M:. {ja.{_,f_UJLe. FB = 1/ll a-6 a {ju.~c.tio~ o{j liqu.id FB = 1/ll a.o a {ju.~c.tio~ o{j :the. tc.e.la­ LUrlit c.ompCVLe.d :to .ou.gge..o:te.d c.otc.­tio~ be;twe.e.~ :the. .o:ttc.e.~g:th valu.e. tc.e.c.tio~ {jac.toM. me.a.oWLe.d by va~ e. -6 he.CVL :te..o:t-6 a~d :the. ptc.e.c.oMotidatio~ ptc.e.-6-0WLe. (al­ :te.tc.~ative.ty e. {j {j e. dive. O ve.tc.bWLde.~ ptc.e.-6-0WLe.) c.ompCVLe.d :to -6 u.g g e..o:te.d c.oMe.etio~ {jac.toM.

24 Fig. l O shows the values evaluated by At the Nordie Geotechnical Meeting (NGM) Andreasson 197 4 supplemented with relations in Linköping 1984 Aas et al presented a com­ between results from vane shear tests and pilation of comparisons for the relation be­ qualified laboratory tests (averages from di­ tween FB and •v/o 0, Fig. 12. The compilation rect shear tests and active and passive tri­ included comparisons with full scale failures axial tests) from Slunga 1983 and from re­ in the field as well as comparisons with quali­ search at SGI. In the figure the curves ear­ fied laboratory investigations. The points in respanding to the corrections suggested by the figure originate from normally Consoli­ Andreasson 197 4 and Helenelund 1977 are dated and slightly overconsolidated clays and inserted tagether with the correction now a certain displacement of the points to the recommended by SGI. left in the figure would occur if all points could be calculated versus ob. To complete

2,5 the picture, the points for organic soils from Slunga 1983 and SGI have been inserted.

x 2,0 x • / - - •• ....------; 1,5 ---­ ,-;x x ~ o Results from research on vane shear tests 1,0 l o During the 196Os and the beginning of the ~~ ~ 1970s extensive research was carried out to ('), 5 o gm determine which factors influence the results o from vane shear tests. The factors investi­ o o 0,1 0,2 0,3 0,4 0.5 0,6 0,7 0,8 0,9 1 .o gated were shape and construction of the t J O~ ('( / 0~ l vane, waiting time after installation before strength testing starts and tagether speed • Full scale failure. of rotation during the test. o earrelation with qualified loborotory investigation x earrelation with qualified loborotory investigation for arganie so il ( Slunga 1983 . SGl ) The shape and construction of the vane proved to have great importance for the re­ _ P.= [o· 193 ·) o.45 'L V/ OC sults. In Sweden, however, vanes with the relationship 2:1 between height and width of the biades are alwa ys used, and the empir­ Fi g . 72. Eva.tw :åe_d .o a !J e;ty nacA:oM ical experience is based on this type of vane. a;t !J a..UU!Le_ (Aa.o e;t a.t va.tue_.o ) In Sweden two types of vane shear equipment c.omp..te;te_d w-U h va.tue_.o fJ !Lom are used today: vane shear equipment of the M gan.ic. .6 oili c.ompMe_d .to SGI type where the vane is withdrawn in a c.oMe_c.tion fJacA:oM !L e.c.omme.nd e.d protective casing during most of the driving by SGI. and vane shear equipment of the Nilcon/Geo­ tech type,where the vane is driven directly Fig. 11 shows the values evaluated by Helene­ into the soil. As the latter vane is unprotec­ lund 1977 supplemented in the same way as ted during driving the design must be made the values evaluated by Andreasson in Fig. samewhat more robust than for the SGI l O w i t h values from organic soils. Helenelund equipment. In the SGI vane shear equipment presented his values as calculated safety fac­ the vane rod is also encased. Torstensson tors at failure FB = 1/ l.l versus •v/o0. As •v (1977) has found that at rotation speeds re­ is dependent on ob this presentation gives commended by the manufacturer failure an unnecessarily large spread since the values occurs approximately three times faster with from normally consolidated and overconsoli­ the Nilcon/Geotech apparatus than with the dated clays are mixed. To the extent that SGI apparatus. The results often seem to be information has been available the values compatible. have therefore been converted and existing effective overburden pressure o 0 has been The SGI apparatus is a relatively heavy piece replaced by the preconsolidation pressure ob. of equipment while the Nilcon/Geotech ap­ Some cases, where investigation has shown paratus is very easy to handle. On the other that the drained strength has been lower than hand, the SGI apparatus would normally the undrained strength or complementary disturb the soil less and in addition the vane information is missing, have been excluded. is scraped clean after each test. With the Curves corresponding to the correction Nilcon/Geotech apparatus there is a risk that suggested by Helenelund 1977 and the correc­ clay from a firmer layer can stick to the vane tion now recommended by SGI (both rewritten and accompany the vane into looser layers with the Hansbo relation •v = ob·0.45 w1) a re with disturbance of the soil as a result. Pre­ inserted in Fig. 11. boring through the dry crust is therefore al­ 25 ways recommended when using the Nilcon/ Geotech apparatus. Even then, problems can occur in layered or varved so il sensiti ve to

disturbance. 2.4

2.3 Through studies of the influence on the wait­ ing time between installation of the vane and 22 the shear tests, i t has appeared that the 2.1 longer the waiting time the higher the measured shear strength is up to a certain 2. waiting time (< l day), after w hi ch the value 1.9 becomes constant. The increase in strength with waiting time could be due to reconsoli­ 1.8 l dation after the disturbance occurring at in­ \ stallation of the vane. Recommended proce­ 1.9 dure is that the strength test be started with­ 1,6 in 5 minutes after installation. Within this 1,5 l, period no significant increase of the strength \ value occurs. 1.4 . \ \ \ 1.3 • Compilation of the relations between ~ measured strength after one day of waiting, 1.2 ...... T lD' and measured strength according to the • 1,1 • standard procedure T 0 , shows that the rela­ tion TlDI T 0 increases considerably at de­ 10+-----~.------,------,------,------creasing liquid limit, which means that the 0 50 100 150 200 % disturbance at installation of the vane is LIQUID L l MIT. wl normally greatest in low plastic clays. The e Average D Spread at ene site spread also increases at decreasing liquid limit, Fig. 13.

Fig . 73. Comp~on b~een ~~ength In studies of the influence of speed of rota­ valuu mealJUJLed by vane ~heM tion on measured strength values, a connec­ tuu a{JtVL one day o{J tion between measured strength values and waiting, T JV, and valuu meUJLed speed of rotation has in most cases been ac.c.O!Ld.tng to the ~tandMd found. This can be written p!Lo c.edUJLe, T 0 •

where Fig. 14 shows measured values of the coef­ Tt = measured strength value at a rota­ ficient from vane shear tests supplemented tion speed which gives the time by &-values from other types of tests on or­ t to failure ganic soil, collected from Slunga 1983.

Tl =reference strength value measured As is shown in Fig. 14 the rate effect at a rotation speed which gives decreases somewhat at decreasing plasticity. time t1 to failure t1 = reference time. Usually 1-3 min­ In a summary of the results from research utes corresponding to recommend­ on vane shear tests, Torstensson (1977) ed procedure showed that if both disturbance and rate ef­ fects are taken into account for time to fail­ 13 = coefficient ure of about one week, strength values are obtained that are about the same as the strength values measured by the standard procedure and corrected according to Bjer­ rum's recommendation of 1973.

26 If the correction for disturbance at installa­ tion obtained in Fig. 13 is corrected for rate o effects so that, instead of applying to a time o to failure of 1- 3 minutes, it earresponds to ~ 0,06 • more normal times to failure of l week to l month, a total correction for the measured zf- 0.05 • • • UJ strength according to the standard procedure 0.04 loJ u • is obtained, Fig. 15. The correcti.9~ factor LL. 0,03 LL. • ll has been caleulat ed as ll = l OO O O · Tl DiT 0 UJo 0,02 and has been assumed to follow the relation u 0.01 in Fig. 14. o o 50 100 150 200 % As the disturbance has been found to be LIQUID LIMIT wL greatest in low plastic clays where the rate • Data from Larsson 1982 b effects are smallest, the final result is that o Data from Slunga 1983 the strength values measured by the normal procedure in low plastic clays are not be re­ duced and that the correction factor may F~g. 74. Th~ ~o~t)t)~ci~nt ~ ~ a t)un~on even be greater than l. The higher the plasti­ at) liq~d LUnd. city is, the smaller is the disturbance and the greater the rate effects become. Conse­ quently, the correction factor decreases with increasing plasticity.

1,7 \ \ Empirical relations for undrained shear 1.6 l \ strength l 1,5 l

\ 110 ; 1 0 1,4 \~ Different values of the undrained shear \ \ strength are obtained in di f f e rent tests de­ 1.3 ' '\ pending on the loading case. The shear ' '-. 1,2 ' strength is thus anisotropic. A hypothesis for ' the variation of the shear strength with the 1,1 loading case in inorganic clays was presented 100 15 0 200 1,0 by Larsson in 1977 .

0,9 The undrained shear strength is usually divi­ 0.8 ded into shear strengths in three main types

0,7 of loading; active shear when the vertical stress is the largest principal stress; passive 0,6 shear when the harizontal stress is the largest

0,5 principal stress and direct shear where the normal stress towards the harizontal slip sur­ 0,4 face is constant, but the shear stresses in­ crease. In the laboratory the three loading cases can be simulated by active and passive F~g. 75. CoM~~on t)actofL ll t)ofL triaxial tests and direct shear tests. Fig. 16. J.JtfL~ngth value...o m~~ufL~d a~~ofL~ng to J.JtandatLd p!Lo­ ~~dUfL~ by van~ .c, h~atL te...ot!.J Results from active and passive triaxial tests ~n~u~ng ~tltl~~ at) ~­ and direct shear tests on Seandinavian clays tUtLban~~ at ~n.c,tal.t~on at) reported in the literature tagether with con­ th~ van~ and fLat~ ~tlb~~ sistency limits and preconsolidation pressures t)ofL ~~ to t)~UtL~ at) 7 were compiled tagether with unpublished data w~~k to 7 month. from investigations at SGI in 1980. (Larsson, 1980). Since then additional data have beem obtained and is included in Fig. 17.

There are no corresponding empirical rela­ tions for organic clays as yet, but research on the properties of these soils is going on. The results from direct shear tests are close to the average of active and passive triaxial tests and direct shear tests.

27 l l ~o- \c=)-

Pass i ve zon e Zene of d irect Active zone Passive s hear Act i ve triaxial test Direct sheor test triaxial test

Fig. 76. Main :typv.o ofi .toading c.a~.>eA wi:th JteApec.;t :to .Mil arU/.,o:tJtopy and c.oJtJteAponding .taboJta:toJty ;tv.o;U.

x x ------X------*'"-----­ x x x x 0,3 x x x x

x x Ltu ACTIVE • ffu DIRECT SHEAR o o 'ffu PASSIVE 0,2 D fCR PASSIVE - ftu AVERAGE

D 0,1 D D D

D

o o 20 40 60 80 100

Fig. 77. UndJtained .oheaJt .o:tJteng:th in Sc.andinavian inOJtganic. c..tay.o all a fiunc.Uon ofi :the pJtec.on.ooUdation pJteA.OUJte and :the Uquid LUnd, TCR• c.oJtJteApond.o :to :the .oheaJt .o:tJteng:th mobi.tized a;t a defioJtmation c.oJtJteAponding :to defioJtmation a:t fiailUJte .{.n ac.Uve ;tv.o;U. The.JtefioJte :the .o:tJteng:th ofi fiailUJte TPASSIVE i.o .oomewha:t highe.Jt :than TCR· Va:ta main-ty fiJtom LaJtMon 7980.

28 Correlations between empirical relations and Correlations between empirical relations and full scale failures in the field shear strength values obtained by vane shear tests and fall cone tests The relevance for undrained shear strength determined by qualified laboratory investi­ Direct shear tests usually give shear gation has been continuously shown in the strengths directly compatible to strength investigations primarily made by the Norwe­ values from vane shear tests and fall cone gian Geotechnical Institute (NGI). NGI has tests corrected according to this recommen­ developed the ADP-analysis (active-direct dation. This is as expected if the empirical shear-passive) and this analysis is used in all relation for shear strength and Hanbo's rela­ qualified investigations, Aas 1976. Normally, tion Tv =o c· 0.45 w1 is valid. Fig. 19. no earrelation with the preconsolidation pressure is made.

Corresponding analysis has also been devel­ oped at the Massachusetts Institute of Tech­ nology (MIT) but here all shear strengths are 0,4 -- normalized according to the SHANSEP proce­ Empirical r•lation for _ -­ --- dure (Ladd & Foott, 1974). 0,3 ~- 0,2 / A compilation of full scale failure reported / in literature tagether with information on 0,1 Corrected wh..ll's from vane shear tests ond fall con~ tests plasticity, mobilized shear strength and pre­ O, O consolidatian pressure was made in 1980 20 40 60 80 100 120 140 160 180 (Larsson, 1980). Fig. 18. LIOUID LIMIT. wl%

F;g. 79. CoMelation.-6 baween. emp.UUc.at -6 heaJL J.J:tJLen.g.th cvt cU.Jcec..t -6 heaJL 'rtutoc • EMBANKMENTS an.d c.oMec..ted van. e -6 hecUL an.d o FOUNDATIONS {JaLf c.on.e vatuu c.atc.ul.cvted a LOADI NG TESTS T 0,5 x DATA FROM SKEMPTON 1948 by Han.-6bo' -6 JLelation. v = o ~ • LONG SLOPES · O. 45 wL an.d n.ew JLec.ommen.ded 0,4 c.oMec;tion.. o •

0,3 ~~tu AVERAGE 0,2 _...-;;;-_ • o This means that campared to the corrected l.-. strength determined by vane shear tests and 0,1 fall cone tests, active triaxial tests normally give higher shear strengths and passive tri­ o axial tests normally give lower undrained o 50 100 150 shear strengths. Averages of the undrained shear strength from active and passive tri­ F;g. 78. Mob~zed un.c!AcU_n.ed -6 heaJL axial tests and direct shear tests are some­ J.J:tJLen.g.th a;t {J ~LULe a-6 a what higher than the corrected strength from {Jun.c;tion. o{J .the uqu;d -Um;.t vane shear tests and fall cone tests. The dif­ ( L(.()L6J.J on., 7980). ference increases with decreasing plasticity. Fig. 20 shows correction factors for strength values measured by vane shear tests and fall The compilation includes failures of road em­ cone tests, which would normally have to be bankments, failures under silos and oil stor­ used to obtain a complete correspondence age tanks, results from test embankments both to results from direct shear tests and and other load tests and some slope failures. also to averages of active and passive triaxial Fig. 18 shows that the results spread some­ tests and direct shear tests. what but apart from some landslides in low plastic sensitive clay the empirical value for the average shear strength from labratory tests correlated weil to the strength that could be mobilized in the field.

29 Swedish experience is that in soils with high liquid limit considerably lower correction factors must be used than those in Andreas­ son's suggestion. According to the SGI recommendations from 1969 a correction fac­ tor of 0.5 should be used for all soils with a liquid limit over 180% and according to re­ commendations from the Swedish Road Ad­ ministation correction should be made for organic content according to lJ= 0.8 /wL. The Swedish National Road Administration also uses 0.5 as the lowest correction factor but on the other hand it considers that a higher calculated factor of safety against failure is required in very high plastic soils.

The numerical base for correction factors for Scandinavian organic soils with very high liquid limits is, however, very deficient and is mainly limited to the earrelations with 0.6 qualified laboratory investigations reported

0,5 by Slunga 1983 and findings from current re­ search at SGI.

Fig. ZO. Co~ection fiaeto~ fio~ ~tnength The mathematical expression for the correc­ vatuu detvunined by vane ~heM tion factor best corresponding to the correc­ tuu and na.Le. c.one tuu tion factors which have emerged from differ­ ~equined to c.o~upond to ent investigations is shown in Fig. 22. a. ~eet ~heM tuu = (0,43) 0,45 b. avvw.,qu n~om active and paM-i.ve. tn,{.ax.J...at tuu and /Å WL ~eet ~heM tuu. This relation gives an upper limit of /Åmax::::: 1.3 when applied to Scandinavian clays and organic soils. Clays with a lower Compilation of results and comparison with liquid limit than about 2496 hardly occur in the new recommended correction this region. Owing to the great sensitivity of the correction factor for small changes The results from all the above mentioned in­ in the liquid limit and the great spread in vestigations since 1969 are fairly unanimous. shear strength values often found in low Different correction factors suggested or plastic soils, no correction factors higher indirectly derived are compiled in Fig. 21. than 1.2 should be used without support from complementary investigations. The correction factor p= 1.45/(1 +wL) sugges­ ted by Helenelund in 1977, gives a relatively Norwegian experience (Aas et al, 1984) and good correspondence to all other results. the curve for average shear strength from However, it gives a flatter curve than the Larsson (1977, 1980) indicate that for stab­ other relations in Fig. 21 and therefore a ility calculations of embankments on sernewhat low correction factor for very low harizontal ground the strength could be plastic clays, a sernewhat high correction increased further for very low plastic clays. factor for medium plastic clays and a some­ On the other hand, anisotropy is neither in what low correction for very high plastic or­ Sweden nor abroad determined from this type ganic soils. Andreasson's suggestion from of strength test but a value intended to be 1974 agrees well with other results for low used generally is produced. It can hardly be and medium plastic soils but gives relatively regarded as acceptable to use a test high correction factors for high plastic and procedure where the obtained strength value organic soils. As mentioned earlier, no Sean­ should be increased by more than 20 % before dinavia n very high plastic clays or organic it is used. In these extreme cases it can be soils were included in the basic data for observed that neither vane shear tests nor Andreasson's suggestion. cone tests work sufficiently well and that if higher strengths are to be utilized other test procedures such as direct shear tests and triaxial tests should be used. 30 )..l

1.7 • • • • Andreasson 1974 )l 11 u ~< Corredion for disturbance ettects and rate etteds 1.6 at vane shear tests --- Helenelund 1977 1.5 Correction according to correlation with ~ualitied loborotory 1.4 investigotions Correction for high liquid limits 1,3 according to SGI 1969 1.2 1.1 100 150 200 0/o 1,01+------.t-+-'.-----+-----+-----+------=--­WL

0,9

0,8 0.7 0.6 0.5 -"~'--tf- ---)(-­ 0,4 --~

F,i.g. 21. CoJUte..ta.:ttoYL-6 be;twe_e_n cU_-66eAen:t c.oJUtec;tion i\acA:oM i\oJt J.dJten.gth valu..e!.J dueAm,i.ne_d by vane_ !.>heM te!.Jtl.> and ilo.Lt c.on.e_ te!.Jtl.>.

31 The following is therefore recommended as As was concluded from earrelations between a general correction factor for vane shear Bjerrum's correction factors and SGI's test values and cone test values a.pplied to recommendation from 1969 (Andreasson 1974) averages of the strength values obtained: this new procedure does not imply any greater difference in evaluated undrained shear strength campared with the earlier 043 0,45 1-1 = (.:V L) ;;,:Q,5 procedure with cautiously selected strength values and SGI's correction from 1969. The corrected strength for very low plastic clays with the reservation that correction factors becomes, however, samewhat higher higher than 1.2 should not be used without according to the new suggestion. complementary investigations. The intention with the new procedure is primarily an adaption to a different and more uniform evaluation of the measured data.

-p= [~~3] 0,45

--Other correction factors from 1,3 Fig.20

1,2

1.1 100 150 200 W 0/o 1,0-+------'lll-+-'\------+----+------+------­

0.9

O,

0,7

0,6 0,5

0.4

F-i.g. 22. CoJULUa;tio/U be.Xween. -the JLUa;tion. JJ (O. 43) O. 45 and o-theJL .ougge.o-ted c.oMeruon. 6ac.-toM. WL

32

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T~yck-Cente~ AB, Linköping 1985