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Use of field vane tests under earth-structures Utilisation des essais in-situ au scissomètre sous des ouvrages en terre

K. T. LAW, Research Officer, National Research Council of Canada, Division of Building Research, Ottawa, Canada

SYNOPSIS This paper reviews the results of field va ne tests conducted through exi sti ng embankments constructed over a vari ety of soft clay s. The shear strengths determi ned from the vane tests, when compared wi th the preconstructi on value s, i ncreased or remai ned unchanged. Strength i ncreases are associ ated wi th large loading areas o r rel ati vel y thin compressi bl e layers while little or no strength increase is associated with s mall loading areas or rel atively thi n compressible layers. In all cases the strength to e ffective vertical stress ratio under the existing earth-structure is less than that before c onstructi on.

Laboratory tests on soils from some of the studi ed cases have been carri ed out to hel p understand field observations. Practical implications are disc ussed and gui del i nes are drawn for usi ng fi el d vane tests to esti mate strength changes.

I NTRODUCTI ON CASE RECORDS

The field vane test has been used for 30 years Publ i shed resul ts of shear strength changes to measure the strength of soft clays. It was determi ned by vane tests under exi sti ng i ntroduced to avoid the soil di sturbance that earth-structures fall into two separate occurs when taki ng sampl es for l aboratory categories. The first group consists of testing. Initial experience with the vane test defi ni te vane strength gains whi l e the second was sati sfactory (Eden and Hami l ton, 1956). shows zero strength change. The maj ori ty of Bj errum (1972), however, showed that the vane the earth-structures were founded on l i ghtl y strength was general l y hi gher than the overconsol i dated soi l s wi th l oads exceedi ng the available strength in the field. He proposed a preconsol i dati on pressure throughout or in part correcti on curve expressed in terms of of the subsoils. The soil information pl asti ci ty index, based on actual embankment pertai ni ng to the layers stressed to the failure records. These were first-time normal l y consol i dated state and the failures in which the natural soil had not been earth-structure geometry are summari zed in subj ected to any previ ous man-made loading. In Tabl e I and Tabl e II. The following symbols spi te of some cri t i ci sm (Schmertmann, 1975), are used in the tables: Bj errum' s met hod of correcti on has gai ned wi de acceptance for desi gn and anal ysi s. T = time after compl eti on of the earth-structure through whi ch the There are a growi ng number of si tuati ons in vane shear test was conducted; whi ch strength measurements are requi red after P^ = preconsol i dat i on pressure; a man-made earth-structure is built. This a' , o' , = vertical effective pressure before i ncl udes stage const ruct i on on weak ground and vo vf design revi ew of existing dams. In each constructi on and at ti me T, situation, a uni que loading condi ti on is respecti vel y; i mposed so that the soil will no longer respond S , S ^ = vane strength before constructi on uo uf to further load accordi ng to first-time loading and at ti me T, respectively; behaviour. Whether Bjerrum' s correction is B, H = total wi dth and hei ght of the still appl icable is a question that needs to be earth-structure, respectively; st udi ed. D = thi ckness of soft clay.

This paper summari zes field experi ence wi th the (a) Cases wi th vane strength i ncrease vane to measure strength under exi sti ng earth-structures. This is followed by a The fi l l s at Rang St-George and Rang du l aboratory study on the factors i nfl uenci ng the Fleuve, Quebec (Tavenas et al, 1978) were vane strength. Based on the field and founded on a l ayer of 12 m thi ck l acustri ne l aboratory studi es, some gui del i nes are drawn deposits, above a thi ck l ayer of Champl ai n Sea for using the vane shear test to detect clay. Berms, 2.5 m hi gh and 20 m wide, were st rengt h changes. placed on both sides of the fills.

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Table I Soil information and fill geometry for case s wi th vane strength i ncreases

Soil Properti es Fi l l geometry B H D T WP W1 wn ( m) ( m) ( m) yrs No. Locat i on ( %) ( %) ( %) P q / Ov o ®Uo/ ^C Suf/avf

1 Rang St. George, Que. 20 70 80 a 1. 2 0. 35 0.26 75 7.4 66-86 3 35 60 — b l . 3 0. 27 0. 24 2 Rang du Fl euve, Que. 20 70 80 al . 2 0. 34 0.25 75 5.8 66-86 2.3 3 Mat agami , Que. 30 65 100 1. 8 0. 25 0.16 103 6.5 11.2 6 1. 8 0.28 e0. 26 103 6. 5 11.2 6 4 Rupert, Que. 18 28 30 2. 2 0.20 0.21 140 9.1 15.2 5 5 New Li skeard, Ont. 25 60 50 1. 5 0. 23 0. 21 50 2. 7 -40 10 0. 23 42 10.6 4.3 2 6 Laguni l l as, Ven. 23 73 65 — suo/ : >o Ska Edeby, Sweden: 0. 29 7 • Area I 20 60 70 1.0 0. 28 0. 22 c70 1.5 10.0 14 8 • Area II 20 60 70 1. 0 0. 27 0.21 c3 5 1.5 12.0 14 9 • Area III 20 60 70 1. 0 0. 26 0.24 c55 1.5 12.2 14 10 • Area IV 20 60 70 1.0 0. 28 0. 23 c3 5 1.5 12.6 14

Not e : a - lacustrine clay; b - mari ne clay; c - circular fill; d - under centre; e - under ber m

Table II Soil information and fill geometry for cas es wi th no vane strength i ncrease

Soil Properti es Fi l l geometry B H D T WP W1 Wn ( m) ( m) ( m) (yrs) Locat i on (*)(*) ( %) Pc/° VO ®uo/pc Suf / avf

Gl oucester, Ont. 20 50 70 1.4 0.37 0.27 20.1 3.65 20.1 8 Boundary Road, Ont. 20 50 70 1.6 0. 35 0.28 24.4 4.27 21. 3 5 Kars, Ont. 20 40 60 2.3 0.24 0.19 48. 2 7.92 16.8 16 Ska- Edeby, Sweden 20 60 70 1.0 0. 23 0.15 8. 5 1.50 15. 2 10

The t est embankment near Mat agami , Quebec was subsoil consi sted of 5.3 m of silt underl ai n by built to study the behavi our of the underlying 4.3 m soft clay. The full preload was applied l acustri ne deposi t upon whi ch numerous dykes in three stages over a peri od of al most one and dams woul d be construct ed in connect i on year. Vane strength gains were detected under wi t h the James Bay hydroel ectri c power proj ect. the preload at di fferent times after the The st rength measurement t hrough and out si de commencement of const ruct i on. the fill were reported by Eden and Law (1980). The test fi el d at Ska-Edeby, Sweden, was The test embankment at Rupert, Quebec, was compri sed of four ci rcul ar l oaded areas and one bui l t on a soft mari ne deposi t of low test embankment (Hol tz and Li ndskog, 1972; plasticity. The purpose was also to provide Hol tz and Broms, 1972). Sand drai ns of desi gn i nformati on for the James Bay di fferent spacings were installed through three hydroel ectri c power project. Two berms were of the circular fills. In 1957 the circular pl aced on each si de of the embankment whi ch fills were built to a load of 27 kPa except for reached a maxi mum hei ght of 9.1 m. A part of Area III whi ch had the same l oad under the berm the embankment was del i berat el y fai l ed and but 39 kPa under the centre. The test document ed by Dascal and Tourni er (1975). embankment was const ruct ed four years l ater using materi al s from unl oadi ng the Area III The approach fill in New Li skeard (Lo and fill to the berm level. In 1971, vane tests Stermac, 1965; Stermac et al, 1967) was bui l t were conducted through the fills and the on a deep deposit of varved clay. The original embankment. St rengt h gai ns were measured under pl an cal l ed for a maxi mum fill hei ght of 10 m. all the circular fills but no strength change Stage constructi on was adopted but part of the was det ect ed under the embankment al t hough the fill failed at 5.5 m before reaching the excess pore pressures under it were di ssi pated. intended first stage height of 6.1 m. The excess pore wat er pressure measured under the rest of the fill remained high and di ssi pated (b) Cases wi t h no vane strength i ncrease very slowly over a peri od of two years. The The Gl oucester test fill (Bozozuk and fill was subsequentl y lowered to 2.7 m; from Leonards, 1972; Law et al, 1977) was bui l t over thi s the resul ts shown in Tabl e I were a soft, hi ghl y sensi ti ve Champl ai n Sea cl ay obt ai ned. deposi ted in several stages. The top 6 m was stressed to about 40% beyond the The Laguni l l as prel oad, Venezuel a (Lambe, 1962 preconsol i dati on pressure. Vane tests through and 1973) was used to i mprove the ground the centre of the fill eight years after condi ti on to support heavy process tanks. The constructi on showed no vane strength i ncrease.

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Tri axi al undrai ned tests, however, on sampl es and the l aboratory vane shear machine. It taken from under the fill at the time of the enabl ed di fferent consol i dati on pressures to be vane testi ng showed a defi ni te strength appl i ed on a soil sampl e in whi ch a vane test increase. Settl ement observati ons and could be conducted at different shear rates. A measurement of the change of moi st ure cont ent detai l ed descri pti on of the devi ce was gi ven by supported the triaxial test results. Law (1979).

The Boundary Road approach fill (Law et al, The first test series was performed on soils 1977) was founded on materi al si mi l ar to that from Gl oucest er and Matagami . Two of the Gloucester test fill. Vane tests were consol i dati on pressures were appl i ed to the conducted through the shoul der of the fill soils, one at the in situ pressure and the under whi ch the top 8 m was stressed other beyond the preconsolidation. For each beyond Pc- consol i dati on pressure, tests were run at different rates to study the time effect. The Kars bri dge approach fill (Eden and Poorooshasb, 1968; Law et al, 1977) was The second test seri es was conducted on soi l s constructed over 17 m of soft Champl ai n Sea from Boundary Road and Matagami . Varyi ng clay, the top 6 m of whi ch consi sted of a isotropic consolidation pressures ( ) were weathered crust. Again, no vane strength appl i ed and the vane test was conducted at a i ncrease was det ect ed 16 years after constant rate that led to failure in about constructi on even though the triaxial tests and 15 mi nut es. moi st ure change measurement s i ndi cat ed a defi ni te strength increase. The thi rd series was carri ed out on the Gl oucester soi l s under i sotropi c and The test embankment at the Ska-Edeby test fi el d anisotropic consolidation. The isotropic was descri bed earl i er. consol i dated tests were similar to the first test series. In the first part of the ani sotropi c tests, the vertical consolidation (c) General observat i ons pressure (° ^c) was kept constant whi l e the The fol l owi ng general observati ons may be hori zontal consol i dati on pressure (ai ) was made from Tabl es I and II. he varied. In the second part, a^c was kept 1) For soil stressed to the normal l y constant whi l e a^ c pressure was changed. consol i dated state under fills, the vane shear strength i ncrease does not depend on pl asti ci ty The results of the first test series on rate index, overconsol i dati on ratio, or length of effect are shown in Figs. 1 and 2 where the time (less than 16 years) after compl eti on of measured strength is pl otted agai nst the time constructi on. to reach failure. The strength is normalized by the value correspondi ng to a failure time of 2) Vane shear strength i ncreases are found 10 minutes. There is a general decrease of under fills wi th a wi de base (B/H>10) or in strength with increase of time to failure or thin compressible layers. The final strength decrease of shear rate. The rate of strength ratio, ®uf/° yf, i n these cases are general l y decrease at the in situ pressure and at the smal l er than the correspondi ng val ues, suo/Pc normal l y consol i dat ed state are 11% and 5.5% per log cycle of time, respectively. This or suc/ ay0 for normal l y consol i dated soil, difference in rate effect is related to the bef ore constructi on. rel ati ve magni tude of the cohesi ve component of the undrained strength. At the in situ 3) No vane shear strength i ncrease is found pressure or under first-time loading, the under fills wi th a narrow base or in thick cohesi ve component is hi gh because of compressi bl e layers even though the tri axi al overconsol i dat i on and agi ng processes' si nce the tests may i ndi cate the exi stence of a strength soil was deposited. At the normally gai n. consol i dated state or under man-made l oadi ng

It appears therefore that the vane shear strength i ncrease is rel ated to the geometry of the fill. In general for a given fill height, the base wi dth has less effect on the vertical stress increase than on the hori zontal stress increase. Hence the vane shear strength i ncrease may be strongl y affected by the change of horizontal stress. In addition, the relative degree of rate effect for first-time loading and for post first-time loadi ng need to be understood. These aspects were studi ed in the l aboratory and are reported in the next secti on.

TRI AXI AL- VANE TESTS TI ME TO FAI LURE, min

The study was carri ed out using a tri axi al -vane Fig. 1 Vane shear strength variation with time machi ne bui l t by combi ni ng the triaxial cell at in situ consol i dati on pressure

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"i------1------r

x BOTH o -^ & a^c VARYING H O z w 30 |— cr’hc VARYING cc h- C/5 cCt

z #- < > I 20 - S' crhc CONSTANT < ¿VARYING X < cc TI ME TO FAI LURE, mi n h- Fig. 2 Vane shear strength variation with time 10 _L _i _ _i_ _L at normal l y consol i dated state 20 40 60 80 100 CONSOLI DATI ON PRESSURE, kPa

Fig. 4 Variation of triaxial-vane strength wi th ani sotropi c consol i dati on pressur e

I H a cc shear strength i ncreases wi th o'. The same is H c V) al so true wi th ° hc when a^c is Kept constant. LU Z < Under const ant a¿ , however, the vane shear > he I strength hardl y i ncreases wi th vc*

X Whi l e it is expected that the vane shear < strength should increase wi th o' or o' , the cc c he 1- lack of increase with a' can be explained by

the fact that the bulk of the resistance to a standard vane comes from the verti cal face of the cyl i nder ci rcumscri bed by the vane CONSOLI DATI ON PRESSURE, kPa rotation. The strength on this vertical face is dependent on a^c . When a^c is kept Fig. 3 Tri axial-vane strength versus constant, the overal l vane shear strength consol i dat i on pressure therefore hardl y changes despi te an i ncrease in ayc - This probably explains the lack of a vane

shear strength i ncrease noted in the above records in whi ch the base wi dth is rel atively narrow, a condi ti on leading to only a small i ncrease of hori zontal stress. for a limited time, the effects of overconsol i dat i on and agi ng are removed hence the cohesi ve component di mi ni shes. Consequentl y the rate effect, whi ch is mai nl y associ ated wi th the cohesi ve component, wi l l be DI SCUSSI ON reduced at the hi gher pressure. (a) Ani sotropy of vane shear strength The resul ts of the second test series are shown Many soft cl ays di spl ay undrai ned strength i n Fig. 3 where the measured strength is anisotropy. If and Sv are the strengths on plotted against the isotropic consol i dati on pressuré. Near the in situ pressure, the the hori zontal and verti cal failure surfaces, strength ei ther remai ns constant or i ncreases respectively, the standard vane, wi th a slightly with consolidation pressure. At a hei ght-to-di ameter rati o of 2, measures an pressure sli ghtl y less than the overall strength S given by: preconsol i dati on pressure P , the strength starts to increase steadily. This change of S =0.06 S +0.14 S, rate of strength increase is caused by the u v h destructi on of bond or cementati on in the cl ay structure. S /S = 0.06 + 0.14 S,_/S (1) u v h' v The results of the third test series is shown in Fig. 4. As in the second series, the vane where S^/Sv is an ani sotropy ratio.

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Based on Bj errum' s work (1973) on fi rst-ti me horizontal stress are both operative. This is loading, this ratio varies from 1.0 to 2.0 given by S ,/o‘ >0. 9 S /P where the uf vf uo c depending on plasticity. It will be higher factor 0.9 denotes that the effecti ve after the first-time loading because the ratio horizontal stress increase under fills with a of vertical effective stress increase to wi de base is about 90% of that under hori zontal effective stress increase is greater one-di mensi onal consol i dati on. (hence hi gher S^/Sv) than before. It can also be deduced f rom the same work (Bjerrum, The range for suf/° yf appl i cabl e for desi gni ng 1973), that hi gher S^/Sv leads to larger stage constructi ons is therefore gi ven by underesti mati on of the strength avai l abl e in the field. „ i s 2 ( 2 I c vf c (b) Bj errum' s (1972) reducti on factor

Bj errum' s reducti on factor p was based on If S _/o' falls within the limits, no uf vo three processes involved in first-time loading correcti on is required. A comparison of failures: rate effect, strength anisotropy, S^f/o^f and 0.9 u SuQ/Pc based on data from and progressive failure. The effects of these processes are changed substanti al l y after Tabl e I is shown in Fig. 6. For most cases. first-time loading. As shown earlier, the rate effect will be reduced; ani sotropy leads to a greater strength underesti mati on and progressive failure wil l be less significant as more pl asti c behavi our prevai l s at the normal l y consol i dated state. All these processes together tend to bri ng the vane shear strength closer to the field strength. The p factor, therefore, will be too severe to apply directly to the vane shear st rengt h measured some ti me after constructi on.

Cal cul ati ons were carri ed out usi ng the elastic anal ysi s of Poulos and Davis (1974) for the post first-time loading condition. The resul ts were expressed in terms of R where R is the ratio of the hori zontal stress increase under a fill to that under a truly one-dimensional state. For fills wi th a wide base (B/H>10) on a thi n compressi bl e l ayer (D/B<0.25), R was about 0.9. Decreasi ng D/B VERTI CAL EFFECTI VE STRESS, cr'v did not si gni fi cantl y change this value. Increasi ng D/B decreased the val ue of R, but at Fig. 5 Ideal i zed strength vari ati on wi th depths between the surface to two ti mes the verti cal effecti ve stress under one- fill height, R remained closed to 0.9. For a dimensiona,! consol i dat i on condi t i on narrow base (B/H<6) on a thi ck compressi bl e layer, R was l ess than 0.6.

(d) Limits of strength changes under fills with a wi de base

Upper and l ower l i mi ts can now be establ i shed for S - l a1 . for desi gn with post first-time uf vf loading conditions. Figure 5 illustrates the change of vane st rengt h under one-di mensi onal consolidation. The initial vane strength S r - uo is assumed constant from o' to P . Beyond P vo c c the effects of overconsol i dati on and agi ng are obliterated and the strength starts to rise with o' at a rate S ,/a', = S /P . In the v uf vf uo c field situation, however, S ,/o', is always uf uf J smaller than S /P because the one-di mensi onal uo c condi ti on is sel dom compl etel y reached, hence S ./o', < S /P . This is substantiated by the uf vf uo c PLASTI CITY I NDEX, data in Tabl e I. Fig. 6 Measured strength ratio, S under The lowest possible limit for S ,/a‘, 'uf/° vf ' ^ uf vf fill compared wi th theoreti cal lower corresponds to the condi ti on where Bj errum' s limit. Note: numbers refer to cases reduction factor and the influence of listed in Table I

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i ncl udi ng under the berm of Matagami fill, Dascal, 0. and Tourni er, J.-P. (1975) Expressi on (2) appears to appl y except under Embankment on soft and sensi ti ve cl ay the centerline of Matagami fill. There, soil foundati on. ASCE J. Geo. Eng. Div. , vari abi l i ty may be the explanation. (101), GT3, March, 297-314. Eden, W.J. and Hami l ton, J.J. (1956) The use of a field vane apparatus in sensitive clay. Sym. on Vane Shear Testi ng of Soi l s. ASTM STP No. 1933, 41-53. SUMMARY AND CONCLUSI ONS Eden, W.J. and Law, K.T. (1980) Compari son of undrai ned shear strength resul ts obtai ned The use of the field vane test under exi sti ng by different test methods in soft clays. earth-structures has been revi ewed in thi s Can. Geo. J., (17-3), 369-381. paper. The case records show that the geometry Eden, W.J. and Poorooshasb, H.B. (1968) of the structure is an important factor Settl ement observati ons at Kars Bridge. affecting the i nterpretation of the standard Can. Geo. J. , (5-1), 28-45. vane wi th a hei ght-to-di ameter rati o of 2. Holtz, R.D. and Broms, B. (1972) Long-term Cases wi th a narrow base (B/H < 6) founded over l oadi ng tests at Ska-Edeby, Sweden. ASCE a thi ck compressi bl e l ayer (H/B > 1) show no Speci al ty Conf. on Performance of Earth vane shear strength i ncrease even if other and Earth-Supported Structures, (1-1), evi dence may i ndi cate an increase. Cases wi th 435-464, Lafayette, Indiana. a wi de base (B/H > 10) show apparent vane shear Holtz, R.D. and Lindskog, G. (1972) Soil strength i ncreases wi th a strength ratio, movement s bel ow a test embankment. ASCE S c/ a' a less than the initial value, S /P . uf vf uo c Speci al ty Conf. on Performance of Earth The geometry factor suggests that the and Earth-Supported Structures, (1-1), hori zontal stress i ncrease i mposed under the 273-284, Lafayette, Indiana. structure is of great importance and thi s is Lambe, T.W. (1962) Pore pressures in a confirmed by the triaxial-vane test results. foundati on clay. ASCE J. Soil Mech. Found. Eng., (88), SM2, April, 19-48. The analysis of the field and laboratory data Lambe, T.W. (1973) Predi cti ons in soil leads to the fol l owi ng suggesti ons for usi ng engineering. Geotechni que, (23), the field vane test beneath structures: 149- 202. Law, K.T., Bozozuk, M. and Eden, W.J. (1977) 1) The field vane test is not reliable for Measured strengths under fills on cases wi th a narrow base at least for a sensitive clay. Proc. 9th Int. Conf. Soil peri od of 16 years after construction. Mech. Found. Eng., (1), 187-192, Tokyo. Law, K.T. (1979) Tri axi al vane test on a soft 2) The field vane test may be used for cases marine clay. Can. Geo. J., 9, 313-319. wi th a wi de base. The measured strength Lo, K.Y. and Stermac, A.G. (1965) Fai l ure of an need not be reduced by Bj errum' s (1972) embankment founded on varved clay. Can. reducti on factor \i and the strength ratio Geo. J. , (2-3), 234-253. S ./o' should fall within the limits of Poulos, H.G. and Davis, E.H. (1974) El asti c uf vf Sol uti ons for Soi l and Rock Mechani cs. Expressi on (2). John Wi l ey & Sons Inc., New York. Schmertmann, Z.H. (1975) Measurement of in si tu shear strength. ASCE Speci al ty Conf. on In Si tu Measurement of Soi l Properti es, ACKNOWL EDGEMENTS (2), 57-148, North Carol i na. Stermac, A.G., Lo, K.Y. and Bamvary, A.K. The l aboratory testi ng was conducted by (1967) The perf ormance of an embankment on B. Bordeleau, technical officer. Divisi on of a deep deposi t of varved clay. Can. Bui l di ng Research, Nati onal Research Counci l of Geo. J., (4.1), 45-61. Canada. Tavenas, F.A., Leroueil, S., Blancket, R. , and Garneau, R. (1978) The stabi l i ty of stage This paper is a contri buti on from the Di vi si on const ruct ed embankment s on soft cl ays. of Bui l di ng Research, Nati onal Research Counci l Can. Geo. J., (15), 283-305. of Canada and is publ i shed wi th the permi ssi on of the Di rector of the Division.

REFERENCES

Bj errum, L. (1972) Embankment s on soft ground. ASCE Speci al t y Conf. on Perf ormances of Earth and Earth-Supported Structures, (2), 1-54, Lafayette, Indiana. Bjerrum, L. (1973) Probl ems of soil mechani cs and construction of soft clays. Proc. 8th Int. Conf. Soil Mech. Found. Eng., (3), 111-159, Moscow. Bozozuk, M. and Leonards, G.A. (1972) The Gloucester test fill. ASCE Specialty Conf. on Performance of Earth and Earth-Supported Structures, (1.1), 299-318, Lafayette, Indiana.

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