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Characterization of Singapore, Bangkok, and

Ariake clays

Hiroyuki Tanaka, Jacques Locat, Satoru Shibuya, Tan Thiam Soon,

and Dinesh R. Shiwakoti

Abstract: A soil investigation was carried out at two sites in Singapore and Bangkok, Southeast Asia, and the resultswere compared with those from a site in Ariake, Japan. Soil samples at all the sites were retrieved using the Japanesesampling method to nullify the effect of sampling on sample quality. From the laboratory tests, consolidation character-istics and undrained shear strength were measured. In addition to the mechanical tests, X-ray diffraction and scanningelectron microscope tests were carried out to identify clay minerals and to study their microstructure. Great differencesin physical and mechanical properties of these clays were observed, which may be attributed to the difference in theirclay mineral components and variation in the sedimentation environment.

Key words: site investigation, marine clay, undrained shear strength, anisotropy, consolidation, clay mineral.

Résumé : Les études de sols ont été réalisées à deux sites en Asie du sud-est : Singapour et Bangkok. Les résultatsdes essais obtenus de ces études ont été comparés à ceux obtenus au site de Ariake, Japon. Dans cette étude, tous leséchantillons analysés ont été recueillis à l’aide de techniques d’échantillonnage japonaises afin d’éviter l’effet de la mé-thode d’échantillonnage sur la qualité des résultats. Les propriétés caractéristiques de consolidation et de résistance aucisaillement ont ainsi pu être déterminés par des essais en laboratoire. En plus des essais mécaniques, des analyses pardiffraction-X et des observations en microscopie à balayage ont permis d’identifier les minéraux argileux présents demême que les caractéristiques microstructurales. Des différences importantes ont été observées dans les propriétés phy-sique et mécanique lesquelles peuvent être attribuées à leur différence tant au niveau de la minéralogie des argiles quedes environnements sédimentaires.

Mots clés : étude de site, argiles marines, résistance au cisaillement non-drainée, anisotropie, consolidation, minéralogiedes argiles. Tanaka et al. 400

Introduction

Unlike manufactured products, there is hardly any interna-tional standard on testing and quality control for soils. Thishas made difficult the realistic comparison of geotechnicalproperties of soils evaluated in different regions.Geotechnical engineers have long realized the importance of sample quality in evaluating the geotechnical parametersfrom laboratory tests. Tanaka et al. (1996) have shown thatthe unconfined compressive strength (qu) for a sample of

Ariake clay collected using a Shelby tube is as low as 60%of that of a sample collected using a suitable high-qualitysampling method. In addition, from a site investigation inSingapore using the Shelby tube and the Japanese samplingmethod, Chong et al. (1998) indicated that compressibility isquite different depending on the sample quality. In mostcountries or regions, however, soil sampling is done using asimple method which may induce significant disturbance inthe soil sample. For example, in Southeast Asian countries aborehole is made by wash boring, and the Shelby tube is ex-tensively used to collect soft clay samples.

It has been recognized that the mechanical properties of soil are strongly influenced by the composition of clay min-erals and existing environmental conditions both during andafter sedimentation. Many attempts have been made to deter-mine the various mechanical properties of these soils interms of plasticity index ( I p), and some of these attempts areused in practical design procedures. A typical example is theratio of undrained shear strength (su) to consolidation pres-sure ( p) proposed by Skempton: su / p = 0.11 + 0.0037 I p.These correlations were established based on data obtainedmainly in northern Europe and North America, where thesediments were intensively influenced by glaciers in the IceAge. Tanaka (1994) indicated that some of these well-knowncorrelations cannot be applied to Japanese marine clays. Forexample, internal friction angles (φ) of Japanese soils areconsiderably higher despite their high I p. Recently, dueto the development of physicochemical analyses, some

Can. Geotech. J. 38: 378–400 (2001) © 2001 NRC Canada

378

DOI: 10.1139/cgj-38-2-378

Received August 9, 1999. Accepted October 13, 2000.Published on the NRC Research Press Web site onApril 20, 2001.

H. Tanaka1 and D.R. Shiwakoti. Geotechnical Engineering

Division, Port and Harbour Research Institute, Nagase 3-1-1,Yokosuka, Japan.J. Locat. Department of Geology and GeologicalEngineering, Laval University, Sainte-Foy, QC G1K 7P4,Canada.S. Shibuya2. Division of Geotechnical and TransportationEngineering, Asian Institute of Technology, Bangkok,Thailand.T.T. Soon. Department of Civil Engineering, NationalUniversity of Singapore, Singapore.1Corresponding author (e-mail: [email protected]).2Present address: Department of Civil Engineering, HokkaidoUniversity, Sapporo, Japan.



researchers have tried to explain the mechanical behavior of soil in terms of chemical or microstructural properties, forexample, Ohtsubo et al. (1995), Locat et al. (1996), andTanaka and Locat (1999).

For this study, site investigations in Singapore and Bang-kok were carried out in 1996 and 1997, respectively. Chonget al. (1998) and Shibuya and Tamrakar (1999) have pub-

lished parts of these studies. In these investigations, soilsamples were collected by the Japanese sampling methodand some soil samples were transported to Japan and Can-ada. Laboratory tests for mechanical and physical propertieswere carried out at the Port and Harbour Research Institute(PHRI), Yokosuka, Japan, and the physicochemical tests in-cluding microstructure analyses were done at the Departmentof Geology and Geological Engineering of Laval University,Sainte-Foy, Quebec, Canada. This paper describes propertiesfrom these studies and compares these characteristics withthose of Ariake clay, a typical Japanese clay deposited atAriake in Kyushu Island, Japan. Ariake is a test site forstudying soft clay by the PHRI (Tanaka et al. 1996).

Testing methods

Sampling methodSample quality is important for obtaining reliable me-

chanical properties of soils. Unlike laboratory tests, however,there is no international standard or reference procedureconcerning the sampling method. For example, in Japan thefixed piston sampler with a thin-wall tube is used for retriev-ing soft clay whose N value is less than 4. The specificationsof the sampler and sampling method such as making a bore-hole should strictly follow the standards of the JapaneseGeotechnical Society (JGS). On the other hand, the Shelbytube without a piston is commonly used for soft soil sam-pling in Southeast Asian countries, including Singapore andThailand, whose clays are the subject of this investigation.Tanaka and Tanaka (1999) compared the sample quality us-ing six different samplers at Ariake and indicated that thesample quality was nearly equal for the Sherbrooke, Laval,and Japanese standard samplers. The unconfined compres-sive strength (qu) of the sample collected using the Shelbytube was, on average, only 60% of that for samples collectedusing the high-quality samplers. In this investigation, all sampleswere retrieved using the Japanese standard sampling method.

In the Japanese sampling method, a borehole is made by arotary drilling machine. The sampler is a fixed piston sam-pler whose inside diameter and tube thickness are 75 and2 mm, respectively. The tube is made of stainless steel andthe sampler is 100 cm in length. For more detailed informa-tion on this sampler and the sampling method, refer to JGS(1998).

At the Singapore and Bangkok sites, Japanese samplerswere brought from Japan, but a boring machine and opera-tors for sampling were employed locally. All sampling workwas done under the supervision of the first author. Soil sam-ples were sealed with wax at both ends of sampling tubesand transported to the PHRI by air cargo. Previous studieshave confirmed that no change in sample quality occurs dur-ing transportation of the sample under the above conditions(Lunne et al. 1997; Tanaka and Tanaka 1999). Soil sampleswere extruded from the sampling tubes at the PHRI labora-

tory, cut into suitable pieces, wrapped in thin plastic film,coated with wax, and then stored in a temperature-controlledroom until testing. Some of these samples were sent to LavalUniversity.

Field tests

Vane shear test (VST)The same vane shear apparatus was used at the Singaporeand Bangkok sites. The vane blade was 5 cm in diameterand 10 cm in height. For the Ariake site, another type of vane was used whose blade was 4 cm in diameter and 8 cmin height. The rotation speed for both vanes was maintainedat 0.1°/min.

Piezocone test (CPT)

Piezocones used in this study follow the reference stan-dard defined by the International Society of Soil Mechanicsand Foundation Engineering (ISSMFE 1988): the diameterof the cone is 35.7 mm (the projected area of the cone is10 cm2) and the angle of the cone is 60°. The pore-water

pressure was measured at the shoulder of the cone duringpenetration. The speed of the cone penetration was about1 cm/s. The tip resistance (qt) is calculated taking into ac-count the effective area ratio of the cone.

Dilatometer test (DMT)

The dilatometer used in the investigation was developedby Marchetti (1980). Two pressures were measured, namelythe pressure at the membrane lift off of 0.1 mm ( p0) and thepressure when the membrane expands by 1.1 mm ( p1). Thesemeasurements were taken 15 s after installing the blade atthe testing depth, as recommended by Marchetti. From theDMT, Marchetti proposed three indices for characterizingsoil as follows: (i) material index ( I D)

[1] I p p

p uD

1 0

1 0

= −−

(ii) horizontal stress index (K D)

[2] K p u

D 1 0

vo

= −

′σ

and (iii) dilatometer modulus ( E D)

[3] E D = 34.7( p1 – p0)

where σ vo′ is the effective overburden pressure, and u0 is thehydrostatic pore pressure at the tested depth.

Seismic cone test (SCT)

The in situ shear modulus at small strain was measured bythe SCT. The principle of the SCT is the same as that devel-oped by Campanella et al. (1986): the shear wave is gener-ated at the ground surface by hitting a wooden block. Theshear wave is received by two accelerometers, one at the endof the cone and one 1 m above the cone. The velocity of theshear wave was calculated using the travel time from theupper to the lower accelerometer (Tanaka et al. 1994). Theshear modulus (G) can be calculated from the followingequation and the G obtained from the SCT will be denotedas Gsc to distinguish it from other shear moduli:

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Tanaka et al. 379



[4] Gsc = ρtvs2

where ρt is the unit weight of the soil, and vs is the velocityof the shear wave.

Laboratory tests for physical and chemical properties

Physical testsGrain-size distribution, Atterberg limits, and density of

solid particles (ρs) were measured following the test stan-dards defined by the JGS (JSF T 131-1990, JSF T 141-1990,and JSF T 111-1990, respectively). The testing method de-fined by the JGS for measuring liquid limit (wL) isCasagrande’s test, which uses a cup. The JGS defines themargin grain size between clay and silt as 5 µm; however, inthis paper the margin grain size has been taken as 2 µm, fol-lowing the soil classification system used in most othercountries.

X-ray diffraction and microstructure observations

X-ray diffraction analyses were performed with either a

Philips or a Seimens diffractometer. Some samples were an-alyzed as a dried powder mount but most of the analyseswere carried out on oriented mounts in various states: natu-ral, glycolated, and heated to 550°C. Scanning electron mi-croscope (SEM) investigations were carried out with a JEOL55 with magnification up to 20 000 times and on samplespreviously freeze-dried in nitrogen.

Laboratory tests for mechanical properties

Constant rate of strain (CRS) oedometer test

The specimen for the CRS oedometer test was 60 mm indiameter and 20 mm in initial height. A back pressure of

200 kPa was applied. The pore-water pressure was measuredat the bottom of the specimen, and drainage for the pore-water pressure was allowed from the upper part of the speci-men. Consolidation was done at a strain rate of 0.02%/min(3.3 × 10–6 /s). This rate is low enough to accurately measurethe pore-water pressure at the bottom for calculation of the

coefficients of consolidation or permeability.Unconfined compression (UC) test

The UC test was carried out following the testing standardof the JGS. The diameter and height of the specimen were35 and 80 mm, respectively. Compression was done at anaxial strain rate of 1%/min.

Recompression triaxial (CK 0UC and CK 0UE) test

The size of the specimen used in this test is the same asthat for the UC test. The specimen was consolidated to thein situ effective stresses in the K 0 condition, allowing lateraldrainage. The K 0 condition was maintained by controllinglateral and axial pressures to ensure a proper balance be -

tween a change of volume and an axial deformation. A backpressure of 200 kPa was applied. After the consolidation wascompleted, the specimen was compressed (CK0UC) or ex-tended (CK0UE) under undrained conditions at an axialstrain rate of 0.1%/min.

Constant-volume direct shear (DS) test

The DS test was done using the apparatus developed byMikasa (1960). The size of the specimen is the same as thatfor the CRS oedometer test, that is, 60 mm in diameter and20 mm in initial height. The in situ vertical effective pres-sure was applied for 30 min, which is long enough tocomplete the primary consolidation. At the end of the

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380 Can. Geotech. J. Vol. 38, 2001

Ariake Singapore Bangkok

3–12 m 12–16 m 14–28 m 5–12 m 12–14 m

Geological age Holocene Holocene Pleistocene Holocene Holocene

Main clay mineral Smectite Smectite Kaolinite Smectite Smectite

ρs (g/cm3

) 2.60–2.63 2.60–2.63 2.76–2.78 2.72–2.75 2.72–2.75Clay fraction (%) 50 50 65 50 50

wn (%) 120–150 90–100 50–60 55–80 60

wL (%) 105–130 65–95 65–80 45–85 80–85

I p 60–100 40–50 40–60 30–70 60

I L 1.2–1.5 1.1–1.7 0.6–0.8 0.6–1.1 0.6

OCR 1.2–1.7 1.5 1.1–1.4 1.3 1.6–1.7

cv (cm2 /day) 100 100 30 10 10

φ r′ (°) 65 47 25 37 37

φ p′ (°) 46–57 45 22 37 37

su / σy (qu /2) 0.31 0.30 0.21* 0.21 0.21

su / σy (vane) 0.32 0.28 0.19 0.33 —

su / σy (CK0UC) 0.41 0.41 0.23 0.33 0.23su / σy (CK0UE) 0.41 0.26 0.23 0.26 0.22

su / σy (DS) 0.38 0.35 0.27 0.29 0.26

Gsc / σy 100 140 140 100 75

Note: φ r′ and φ p′ , internal friction angle at residual state and at peak deviator stress, respectively.*From UU test.

Table 1. Characterization of Ariake, Singapore, and Bangkok clays.



consolidation, movement of the shear box in the vertical

direction was fixed so that the constant volume can be keptduring shearing. There is a load cell on the vertical rod tomeasure change of vertical stress during shearing. Thechange in the vertical stress corresponds to the excess pore-water pressure in the triaxial test under the undrained condi-tion. The speed of the shearing was 0.25 mm/min.

Soil profile at tested sites

AriakeThe Ariake site is located at Hizen-Kashima, Saga Prefec-

ture, on Kyushu Island, Japan. This site has long been usedby the PHRI as a test field for studying soft clay. Mechanical

and chemical properties of this site have been extensively

studied by several researchers (Hanzawa et al. 1990;Ohtsubo et al. 1995; Tanaka et al. 1996). Characteristicgeotechnical properties of this site are indicated in Fig. 1and summarized in Table 1. Grain-size composition showsonly a small decrease with a clay fraction from about 55%near the surface to 45% near the bottom. Index propertiessuddenly change at a depth of 12 m. The liquid limit (wL) isgreater than 100% at depths less than 12 m but less than100% at depths greater than 12 m, although the grain-sizecomposition does not change.

Compared with cohesive soils at other sites in Japan, themost distinguished feature of this soil is that natural watercontent (wn) exceeds wL, resulting in a liquidity index ( I L) of

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Tanaka et al. 381

Fig. 1. Geotechnical properties at the Ariake site.



more than unity for most depths. For cohesive soils incoastal areas of Japan, I L usually is within the range 0.7–0.8(Ogawa and Matsumoto 1978).

The yield consolidation pressure (σy), which was mea-sured by the CRS oedometer test at a strain rate of 0.02%/min (3.3 × 10–6 /s), indicates that the clay is slightlyoverconsolidated for the first 16 m of depth. This slight

overconsolidation is attributed not to the change in geologi-cal setting such as erosion, but to aging effects such as sec-ondary consolidation and chemical bonding. When the depthexceeds 16 m, the overconsolidation ratio (OCR) becomeslarger. The depth of the clay layer considered in the presentinvestigation is restricted to between 4 and 16 m.

The undrained shear strength measured by the field vanetest increases with depth. Commonly the field vane strengthobtained is modified by Bjerrum’s correction factor (µ) toget the design value of undrained shear strength (Bjerrum1973). However, the vane strengths shown in Fig. 1 forAriake and in Figs. 3 and 4 for Singapore and Bangkok, re-spectively, are raw values not corrected by Bjerrum’s correc-tion factor. The uncorrected vane shear strength at Ariake

agrees fairy well with the undrained shear strength obtainedfrom the piezocone test (CPT). In calculating the undrainedshear strength from the CPT, a cone factor ( N kt) of 10 hasbeen assumed in the relationship su = (qt – σvo)/ N kt, whereσvo is the total vertical pressure.

Since the material index ( I D) from the DMT (see eq. [1])is around 0.2, the soil may be classified as clay according toMarchetti’s (1980) soil classification chart. Marchetti sug-gested that the boundary value of I D for clay and silt is 0.6.The value of K D in most parts of the layer is less than 2 ex-cept for in the upper and the lower layers. The Gsc valuemeasured by the seismic cone also increases with an in-crease in depth. Tanaka et al. (1994) have suggested that re-lations between Gsc and E D from the DMT or net point

resistance (qt – σvo) from the CPT for normally or slightlyoverconsolidated clay are as follows:

[5] Gsc = 7.5 E D

[6] Gsc = 50(qt – σvo)

It seems that Gsc derived from E D values somewhat underes-timates the measured Gsc value from the SCT, but this differ-ence is very small. The value of Gsc calculated from eq. [6]from CPT overestimates the measured Gsc, the difference be-ing constant with depth.

Singapore clayThere are two marine clay layers at the Singapore site: up-

per and lower layers. The upper clay was deposited duringthe Holocene era, but the lower one was deposited more that10 000 years ago (Hanzawa and Adachi 1983; Pitts 1984).The test site was located on land reclaimed around 1975.The soil profile obtained from the CPT is shown in Fig. 2.The original seabed is at a depth of 1.5 m. Below this is nat-ural sandy layer which extends to a depth of 5 m. The uppermarine clay layer is very thin at the site, its thickness beingonly 2 m. Below the upper marine clay is another sandylayer about 1 m thick which divides the marine clay layersinto the upper and the lower clay layers. The lower claylayer starts at 8.3 m depth. The upper part of this lower clay

layer is intensively affected by desiccation caused by lower-ing of the sea level during the Ice Age. The color of thelower marine clay is bluish gray but the desiccated part of the clay has turned to brown or yellow due to oxidization.From the soil profile obtained by the CPT, the desiccatedlayer can be identified by its relatively high q t and high fric-tion ratio ( f s / qt, where f s is the friction). To avoid scatterdue to the desiccation effect, the investigated layer in thispaper will be restricted to between 14 and 28 m.

The properties of Singapore clay are shown in Fig. 3 andsummarized in Table 1. The grain-size composition and theindex properties are nearly constant throughout the investi-gated depth. The clay content (< 2 µm) is about 65%, and wLand the plastic limit (wP) are about 75% and 25%, respec-tively, although both values increase slightly until a depth of 20 m. The variation in natural water content follows theseindex properties. The I L value is about 0.6 at all depths.

The yield consolidation pressure is slightly greater thanthe in situ effective burden pressure (σ vo′ ) except at depthsshallower than 18 m. The high OCR at shallower depthsmay be attributed to the desiccation mentioned previously.This site was located in reclaimed land, and whether theconsolidation due to this reclamation work is completed ornot is a very important question in evaluating soil parame-ters measured by field and laboratory tests. Since the mea-sured σy is larger than σ vo′ , it can be assumed that theprimary consolidation is over. That is, the value of σ vo′ canbe calculated assuming the hydrostatic pore-water pressuredistribution.

In addition to the vane shear strength, the undrained shearstrength estimated by the CPT has also been compared inFig. 3, assuming N kt = 10. The vane strength increases withan increase in depth, with values ranging from 30 kPa at theupper depth to 60 kPa at the lower depth. However, thestrength from the CPT is nearly constant at all depths. This

© 2001 NRC Canada


Fig. 2. Soil profile from the CPT at the Singapore site.



difference in strength will be discussed in more detail laterin the paper in terms of the cone factor N kt.

The material index ( I D) for the objective layer lies be-tween 0.25 and 0.3, classifying the soil as clay according tothe chart of Marchetti (1980). However, I D values for Singa-pore clay are slightly greater than those for Ariake clay. Thehorizontal stress index (K D) decreases with depth, and theorder of these values is slightly higher than that of Ariakeclay. The shear modulus measured by the seismic cone isabout 35–40 MPa. Figure 3 also shows a special relationship

between the dilatometer modulus ( E D) and the tip resistance(qt) from the CPT as in Ariake clay.

Bangkok clayThe Norwegian Geotechnical Institute (NGI) may be the

first organization to systematically characterize Bangkokclay. The extensive soil investigation of Bangkok clay wasdone by the NGI for construction of the Bangkok–SirachaHighway embankment (Eide and Holmberg 1972). Slidingtook place at the test embankment, even with the application

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Tanaka et al. 383

Fig. 3. Geotechnical properties at the Singapore site (lower clay).



of calculated factor safety of 1.5 on the field vane strength.This experience might have strongly inspired Bjerrum(1973) in introducing the correction factor for the vanestrength based on the I p value of a soil.

The present investigated site is located in Sutthisan, an ur-ban area of Bangkok. At this site, the field vane shear andpiezocone tests were performed by the Asian Institute of Technology (AIT). Other field tests including sampling weredone either by the PHRI or under the supervision of thePHRI. To avoid the influence of excavation or fill due to

construction of buildings or desiccation, sampling andsoundings were started from a depth of 5 m. The propertiesobtained by the investigation are shown in Fig. 4, and someof the properties are summarized in Table 1. Like the sam-ples at Ariake and Singapore, the grain-size composition of the samples from this site consist of very fine particles: theportion of clay particles exceeds 50% at most depths, andthe content of sand is very small. Index properties are dis-tributed in a relatively complicated way and vary with depth.The order of I L changes at 10 m depth. For the upper parts

© 2001 NRC Canada


Fig. 4. Geotechnical properties at the Bangkok site (Sutthisan).



of this depth, I L is near unity, and below this depth I L re-duces to values as small as 0.6.

The yield consolidation pressure (σy) is slightly largerthan the in situ effective vertical pressure (σ vo′ ). However, atdepths larger than 12 m, σy values are significantly greaterthan σ vo′ . This may be attributed to the over-pumping up of water from the sand layer underlying this clay layer (Yudhbirand Honjo 1991). Indeed, Bangkok has suffered from subsi-dence over considerable areas due to this pumping.

Field vane shear (FVS) tests were performed at three

points. The undrained shear strength measured by the FVStest is plotted in Fig. 4, together with that measured by theCPT assuming N kt = 10. Strengths measured by the FVS testare approximately constant at about 30 kPa until a depth of 9 m. The strength estimated by the CPT seems to be rela-tively small compared with that from the FVS test. Thistrend of strength underestimation from the CPT will be dis-cussed in more detail later in the paper together withstrengths at Ariake and Singapore in terms of the cone factor N kt.

The DMT was also conducted at this site. The test resultsfrom this site are shown in Fig. 4. The values of I D arenearly constant until 11 m depth, ranging from 0.2 to 0.3.The I D value increases below a depth of 11 m, although the

grain composition does not change. However, the indexproperties and OCR obtained by the CRS oedometer testchange at this depth. Thus, it is implied that the I D value isnot a unique function of soil grain size alone and may alsobe influenced by other soil properties, such as I p or OCR.The K D value falls in the range of 3.0–3.5 at most depths,which is the largest value of all sites investigated in thisstudy.

The shear modulus measured by the seismic cone (Gsc)fluctuates slightly with changes in depth, the average valuebeing approximately 15 MPa. The Gsc values are in goodagreement with the modulus obtained by the DMT or CPT

using eqs. [5] and [6]. In the case of CPT, the results agreewell at all depths. However, the shear modulus estimated by7.5 E D from the DMT overestimates the measured value atdepths greater than 11 m. It is interesting to note that thisdepth coincides with that of changes in I D.

Comparison of physical properties

Density of soil particles (ρs )The values of ρs versus depth at all three sites are com-

pared in Fig. 5. At each site, the values of ρs are nearly con-stant with depth but differ considerably from site to site. Theρs value of Singapore clay is the largest among these clays,and its value ranges between 2.76 and 2.78 except at a depthof 28 m. The ρs values of Bangkok clay also exceed 2.7, andits values lie between 2.72 and 2.75 except at depths of 5.6and 16.7 m. The ρs value of Ariake clay is noticeablysmaller, its average value being less than 2.65. The histo-grams of ρs for the Tokyo Bay and Osaka Bay areas areshown in Fig. 6 (Tanaka and Sakakibara 1991), which indi-cates that Ariake clay is not a special case among Japanesemarine clays, although the value of ρs at the Ariake site isrelatively small compared with those from Tokyo Bay andOsaka Bay. Tanaka and Locat (1999) have shown that Japa-

nese marine clays contain a large proportion of diatoms.Shiwakoti et al. (1998) conducted a series of tests on a mix-ture of diatomite and kaolinite which indicated that the in-clusion of diatoms reduces the value ρs of a soil.

Grain-size composition and plasticity indicesFigure 7 shows typical grain-size distribution curves for

the clays from Bangkok, Singapore, and Ariake. The compo-sition of grain particles is nearly the same at each site. How-ever, as already shown in the previous sections, indexproperties are considerably different at each site. The activ-ity of each of these clays is examined in Fig. 8, where clay

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Tanaka et al. 385

Fig. 5. Comparison of density of solid particles ρs at the Ariake, Singapore, and Bangkok sites.



content (grain size < 2 µm) is plotted on the horizontal axisand the I p value on the vertical axis. The relations for someother clays investigated by the authors (Tanaka and Tanaka1997, 1999) are also shown in Fig. 8. The activity of Singa-pore clay is relatively small and is slightly larger than that of the Drammen clay, which is a famous lean clay. The reasonfor the relatively small activity of the Drammen clay is thepresence of a large proportion of so-called “rock flour.” Therock flours have clay-sized grain particles, however, most of these particles do not contain clay minerals. The rock flourof the Drammen clay was produced by the abrasive action of

glacial ice during the Ice Age. On the other hand, it is cer-tain that the Singapore clay investigated in this paper has not

experienced glaciation. Therefore, it is surprising that the ac-tivity of the Singapore clay is close to that of the Drammenclay despite the difference in nature of the deposits. TheBangkok clay is more active than the Singapore clay and hasan activity of about unity. Although there is a wide scatter inactivity of the Ariake clay, in general its activity is the high-est among those from all three sites. Careful observation of the soil profile of the Ariake clay in Fig. 1 reveals that thereduction of I p values takes place at deeper depths, despitealmost constant grain-size composition.

The plasticity chart for the clays from the three sites in-vestigated is shown in Fig. 9, where wL and I p are plotted onthe horizontal and the vertical axes, respectively. The datafor the Ariake clay fall along the A line. It is well known

that not only the Ariake clay but also most Japanese marineclays have a close correlation with the A line. Contrary tothe Japanese clays, the relationships for both the Bangkokand Singapore clays are located considerably above the Aline, although their data are parallel to the A line. Thismeans that, compared with the Ariake clay, I p for these claysis larger by about 10 at a given wL.

Mineralogy and microstructure analysis

Ariake clayClay mineralogy of the Ariake site has been investigated

in detail by various researchers (e.g., Egashira and Ohtsubo

© 2001 NRC Canada


Fig. 6. Histograms of ρs for marine clays at Tokyo Bay andOsaka Bay areas.

Fig. 7. Comparison of grain-size distribution at the three sitesfor samples taken from the depths specified.

Fig. 8. Comparison of the activities of the three clays investi-gated in this paper with those of three clays investigated byother researchers.



1982; Ohtsubo et al. 1995). Previous studies have recog-nized the dominance of smectite (low swelling type) in theclay and the presence of other clay minerals such askaolinite, illite, and vermiculite.

In addition, the present study using X-ray diffraction onall depths investigated (3, 8, 14, and 16 m) indicates also thepresence of detectable amounts of chlorite in the Ariakeclay, as implied by the noncollapsible peaks of 14 Å (a re-flection angle 2θ = 6.3° in Fig. 10) even when heated to550°C. The glycolated spectra reveal a strong portion of smectite, which can be distinguished by its much more in-tense peaks compared with those for kaolinite or illite. It isinteresting to compare the X-ray spectra at 8 and 14 m

depths. Although the pattern and intensity of each peak arevery similar at both depths, the index properties change re-markably at the 12 m depth.

Microstructural studies of the Ariake clay (3 and 16 m)reveal the presence of a well-developed flocculated structurecombined with abundant fossil remains, mostly derived fromdiatoms (Figs. 11a, 11b). Aggregates are made of agglomer-ated fine particles (mostly smectite) while the coarse compo-nent is composed of quartz grains or fossil debris. Theresulting pore space families are intra-aggregate, inter-aggregate, and skeletal (Tanaka and Locat 1999; Shiwakotiet al. 1999). Since most fossil remains have been found bro-ken, very little intra-skeletal pore family is visible here.

Singapore clay

The lower layer of Singapore clay contains a large propor-tion of kaolinite, as shown by very strong reflection of thekaolinite peaks (Fig. 12). The other main minerals present inthe Singapore clay are illite and a small proportion of smectite or mixed-layer minerals. The relative increase inthe intensity of the illite peak (10 Å, 2θ = 8.8° in Fig. 12)could also indicate the presence of vermiculite in the mix-ture.

The overall structure is flocculated, with aggregatesformed mostly by lumps of fine clay minerals such askaolinite and illite (Fig. 13c). The layer contains a small

quantity of organic matter and is composed of large quanti-ties of kaolinite minerals with a typical kaolinite group asshown in Figs. 13a and 13b. The clay contains only a lim-

ited amount of fossiliferous remains, which consist mostlyof foraminifera (Fig. 13c) and occasional diatom debris. Inaddition, pellets can be frequently observed (Fig. 13d ).

Bangkok clayThe mineralogy of the clay fraction of the Bangkok clay

is shown in Fig. 14. The types of minerals in order of de-creasing abundance are smectite, illite, kaolinite, chlorite,and some mixed-layer minerals. The overall soil also con-tains primary minerals such as quartz. If we consider thevariation of specific surface area with depth, the lower por-tion of the profile from a depth of about 10 m contains al-most twice as much smectite as the layer above, with the

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Tanaka et al. 387

Fig. 9. Plasticity chart.

Fig. 10. X-ray diffraction charts for Ariake clay (from 14 m) invarious states: natural (N), glycolated (G), and heated to 550°C(H). C, chlorite; I, illite; K, kaolinite; S, smectite; V, vermiculite.



maximum proportion of smectite recorded at depths of 9.5and 16 m. The existence of a large proportion of smectitecould explain the reason for the higher plasticity index atlower depths. Another interesting observation is the occur-rence of new peaks upon heating at angles of 14° and 17°(as indicated by a and b in Fig. 14). This has also been ob-served mostly in the lower part of the profile and could bethe result of a significant amount of mixed-layer minerals.At this time, we have not been able to link these peaks to aparticular mineral suite.

The microstructure of the Bangkok clay is characterizedby the frequent presence of pellets filled with pyrite(Fig. 15b). The pore space consists primarily of the inter-aggregate pore family (Fig. 15a). The aggregates themselvesare quite compact and consist of an assemblage of clay-sizeparticles providing a flocculated structure. Microfossils such

as diatoms or foraminifera are rare. Near the surface, somediatom fossils were found partly dissolved with apparentreprecipitation on surrounding aggregates, which might havecontributed to the structural bonding of the soil.

Consolidation characteristics

Sampling qualityAs mentioned in the section on Testing methods, sample

quality is a very important consideration in evaluating thebehavior of a soil from laboratory tests. Figure 16 shows atypical example indicating the importance of sample quality.Results have been compared for soil samples taken by theJapanese samplers and the Shelby tube. A preboring methodwas used in the case of the Japanese samplers, which is thestandard practice in Japan. In the Shelby tube sampling,however, a wash-boring method was employed, which is themethod of sampling usually followed in Southeast Asia.

The void ratio (e) – log p curves in Fig. 16 were measuredby the CRS oedometer tests at a strain rate of 0.02%/min. Aclear bending point corresponding to yield consolidationpressure (σy) can be observed on the e – log p curve for thespecimen collected by the Japanese sampling method. Whenthe consolidation pressure ( p) in the specimen is less thanσy, the change in e is very small. But when p is greater thanσy, suddenly a large volume change occurs. The compres-sion index (C c), which is the slope of the e – log p curve, isnot constant even at the normally consolidated stage; thelargest C c is attained immediately after σy and gradually de-creases with increasing consolidation pressure.

In Fig. 16 the e – log p curve for the sample at the samedepth but collected by the Shelby tube is compared. Theshape of the e – log p curve for the specimen collected bythis method is markedly different from that of the Japanesemethod; a considerable shift in the void ratio can be ob-served at σ vo′ . The yield consolidation pressure (σy) is notclearly identified, and C c becomes practically constant in thenormally consolidated stage. The change in the e – log p re-lationship becomes more pronounced if C c is plotted against p, as shown in the lower plot of Fig. 16.

© 2001 NRC Canada


Fig. 11. Scanning electron micrographs of Ariake clay at (a) 3 m depth, and (b) 16 m depth. Scale bars = 10 µm.

Fig. 12. X-ray diffraction charts for Singapore clay (from 20 m)in various states: natural (N), glycolated (G), and heated to550°C (H). C, chlorite; I, illite; K, kaolinite; S, smectite; V, ver-miculite.



Similar differences in the consolidation characteristicswere also observed for the Bangkok clay. Discussion of thesampling disturbance is beyond the scope of the present pa-per. Interested readers may refer elsewhere for details of thevarious sampling methods and their influences on laboratorytest results (Chong et al. 1998; Tanaka and Tanaka 1999). In

this paper, only the test results of soil samples obtained fromthe Japanese sampling method will be examined.

Void ratio (e) – log p curves and compression indexTypical e – log p curves for the Ariake clay, Singapore

clay, and Bangkok clay are shown in Figs. 17, 18, and 19,respectively. The C c indices at each of the three sites are notconstant, even at the normally consolidated stage; instead,they are considerably nonlinear. To make the comparison of these test results easy, change in C c has been plotted against p in normalized form in Fig. 20. In Fig. 20, C c at a particularpressure is normalized by C c1, where C c1 is C c at a pressure

large enough to become constant. Consolidation pressure pis also normalized by σy, as p / σy. If these treatments aredone on both axes of C c and p, the compressibility charac-teristics of all three clays can be compared easily. The maxi-mum value of the compressibility ratio of C c / C c1 variesbetween 1.5 and 4.0 for all three clays. It is very interesting

to find that C c is nearly constant when the consolidationpressure becomes about twice as large as σy.

The C c value has been correlated with index properties byvarious researchers in numerous ways since Terzaghi pro-posed the well-known relationship C c = 0.009(wL – 10). InFig. 21, C c1 and C c2 are plotted against wL, where C c2 is themaximum of C c, that is, C c at the pressure just after the σyvalue. There is a strong correlation between wL and C c1. It iswell recognized that C c for a remolded specimen or at alarge consolidation pressure for an intact specimen can berelated to wL for most soils. It has been confirmed againfrom the present study that Bangkok and Singapore clays

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Tanaka et al. 389

Fig. 13. Scanning electron micrographs of Singapore clay at (a, b) 16 m depth, and (c, d ) 20 m depth. Scale bars = 10 µm.



and Ariake clay follow this relationship. However, Fig. 21shows that there is no relationship between C c2 and wL. Theindex C c2 may be a value strongly related to soil structure,which is a function of parent materials, sedimentationenvironment, and post-depositional factors such as second-ary consolidation or chemical bonding. Therefore, C c2 andwL may vary with depth even for the same clay deposit.

It should also be noted that in most previous attempts tocorrelate C c and wL from conventional oedometer consolida-tion tests, C c values have been taken at some middle pointbetween C

c2 and C

c1. This is because from the e – log p

curves of conventional oedometer tests, the measurement of variation in C c cannot be made continuously, as the speci-men is subjected to a constant consolidation pressure at eachconsolidation stage, and this consolidation pressure is dou-ble at each sub-sequence. In addition, it should be kept in

mind that if the sample is heavily disturbed, the measured C cis the same as C c1. In other words, C c is significantly af-fected by the sample quality.

Coefficient of consolidationThe distributions of the coefficient of consolidation (cv)

with depth at the three sites are given in Fig. 22. Figure 17

shows that the cv values for Ariake clay are not constant,even at the normally consolidated stage, unlike either Bang-kok clay or Singapore clay (see Figs. 18, 19). Thus, cv forAriake clay is taken at a consolidation pressure of 1 MPa.The cv values for other clays are taken at the normally con-solidated stage because the cv values are constant at thisstage. For all three sites, the values of cv are nearly constantwith depth. However, the values at each site are considerablydifferent from one other. The cv value for Bangkok clay isthe lowest of those for all three sites, its value being about10 cm2 /day, except at a depth of 7.5 m, where the soil con-tains a lot of sand, so the cv value is much larger than thoseat other depths. On the other hand, the cv value for Ariakeclay is about 100 cm2 /day, which is the largest value of those

for all the clays in the present study. The cv value for Singa-pore clay is about 30 cm2 /day. The cv value at a normallyconsolidated state is not sensitive to sample quality. The cvvalues for samples collected by the Japanese samplingmethod and the wash boring method using the Shelby tubeyield nearly the same result, as seen in Fig. 22.

A typical relationship between void ratio (e) and perme-ability (k ) for Bangkok clay is given in Fig. 23. When thestress level reaches the normally consolidated stage, k de-creases with a decrease in e, the relationship being linearwhen k is plotted on a logarithmic scale. If this line in thenormally consolidated stage is extended to meet the horizon-tal line drawn from the initial void ratio eo, the k value at theintercept point may be considered as the in situ permeability(k o), as described by Tavenas et al. (1983). The k o valuesthus obtained are plotted against eo in Fig. 24. It is very in-teresting to find a clear correlation between eo and k o. Thetrend of a decrease in k o with a decrease in eo is similar forall three clays. It may be noted, however, that a data point

© 2001 NRC Canada


Fig. 14. X-ray diffraction charts for Bangkok clay (from 12 m)in various states: natural (N), glycolated (G), and heated to550°C (H). C, chlorite; I, illite; K, kaolinite; S, smectite; V,vermiculite. New peaks upon heating are indicated at a and b

(see text).

Fig. 15. Scanning electron micrographs of Bangkok clay at (a) 12 m depth, and (b) 13 m depth. Scale bars = 10 µm.



for Bangkok clay at the 7.5 m depth does not follow thetrend, which may be attributed to the presence of a sandlayer at that depth. Figure 24 shows that the relativelysmaller k o for Bangkok clay and Singapore clay is related totheir smaller void ratio. This is the reason why the cv valuesfor Singapore clay and Bangkok clay are relatively small

compared with that for Ariake clay.

Strength and deformation characteristics

General tendency of strengths measured by varioustests

Figures 25–27 show undrained shear strength (su) profilesmeasured by laboratory and field tests. The strengths fromthese tests are summarized in Table 1 in normalized form(normalized by σy) for ease of comparison. The σ y valuewas measured by the CRS oedometer test and using theCasagrande interpretation method. At all three sites investi-gated, there exists a certain order of variation in strength

measured in the laboratory depending on the test type. Withfew exceptions, the largest su was obtained from the CK0UCtriaxial test. The second largest strength was recorded fromthe constant-volume direct shear (DS) test, where the speci-men was consolidated to the in situ vertical stress beforeshearing it at a constant-volume condition. The CK0UEtriaxial test yielded nearly the same or slightly smaller un-drained shear strength as that of the DS test. The su valuesfrom the UC test or the UU triaxial test are considerably

smaller than those from the above three tests.Contrary to the strengths measured from the laboratorytests, the position of the field vane shear strength is differentfrom site to site, and there is no definite trend. For example,at Ariake the vane shear strength has the same order as thatof the UC test (qu /2). This fact can be applied to most of theJapanese marine clays, provided that the soil sample is col-lected properly to ensure high quality (Tanaka 1994). AtBangkok, however, the field vane test definitely yields largerstrength than the UC test, the order being nearly the same assu from the CK0UC triaxial test. On the other hand, the fieldvane shear strength for Singapore clay is even smaller thanthat of the UC test.

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Tanaka et al. 391

Fig. 16. Oedometer test results on Singapore clay to illustratedifferences in sampling quality.

Fig. 17. Consolidation characteristics of Ariake clay.



These strength characteristics will be discussed in moredetail in the following sections.

Cone factorThe difference between the strengths measured by the

CPT (assuming N kt = 10) and those from other methods var-ies from site to site. This fact may imply that the N kt factoris not constant, but is different for different clays. A ques-tion as to which test provides the suitable strength for stabil-ity analysis is still a very difficult issue among geotechnicalengineers, since many factors such as anisotropy, rate effect,and failure pattern may govern the design strength. However,the strength from the FVS test has been taken as the basicstrength for obtaining the N kt factor in most previous studies.

Considering the su value from the FVS test as the basicstrength, the resulting N kt factors for the three investigatedsites are plotted in Fig. 28 along with the results from otherwell-documented sites (data for Drammen from T. Lunne,personal communication, 1995; for Bothkennar from Nash etal. 1992; and for Louiseville from La Rochelle et al. 1988).Some researchers have shown that the N kt factor is related to I p in such a way that N kt decreases with an increase in I p (forexample, Jamiolkowski et al. 1988). However, Fig. 28 showsno clear correlation between N kt and I p.

The range of values of the N kt factor for various Japanesemarine clays, including Ariake clay, is between 7 and 15,with an average of around 10 (Tanaka 1996). However, N ktfor Drammen clay is very large compared with those forBothkennar clay and Ariake clay.

The N kt factor for Singapore clay exceeds 20 at somedepths. These points were measured at depths of 17 and21 m. The measured vane shear strength at 21 m is verysmall compared with the strengths measured by other labo-ratory tests, as shown in Fig. 26. The measured vane shearstrength at this depth might have been too small due to someunknown reasons. However, the vane strengths measured atother depths above 17 m have the same order of strength asthat from the UU test. Also, the vane shear strengths ob-served at two other depths have nearly the same value.Therefore, it is likely that the large N kt factor at these depthswas not caused by an error in the measurement but due toother reasons such as desiccation. For all other depths, the N kt factors for Singapore clay fall around the upper bound of those for Japanese clays and are similar to that forLouiseville clay, which is one of the Champlain Sea clays inQuebec, Canada.

© 2001 NRC Canada


Fig. 18. Consolidation characteristics of Singapore clay. Fig. 19. Consolidation characteristics of Bangkok clay.



On the other hand, the N kt factors for Bangkok clay arerelatively small and are mostly scattered around the lowerboundary of the Japanese clays. These observations indicatethat N kt is not easily related to I p but is a complicated param-eter influenced by many factors, such as soil type anddepositional environment.

Strength anisotropyFigures 25–27 show that the undrained shear strengthmeasured by the CK0UE triaxial test (sue) is, in general,smaller than that measured by the CK0UC triaxial test (suc).Unlike the compression test, the extension test hardly gives apeak strength. As such, there is more than one definition indetermining the extension strength, for example, the strengthat the same strain level as the peak compression strength, orthe strength at 15% of strain. The second definition is usedextensively in practical design, whenever a clear peak instrength is not observed. Since the peak strength was not ob-served in all the extension tests in the present investigation,the strength from the extension test has been defined as thestrength at an axial strain of 15%. Also, the strength aniso-

tropy has been defined as the ratio of the extension strengthto the compression strength (sue / suc).Figure 29 shows the plot of strength anisotropy ratio

(sue / suc) for the three clays investigated in the present study.Also included in the figure are the data for other Japanesemarine clays previously investigated by Tanaka and Tanaka(1997). It can be seen that for the Japanese clays, includingAriake clay, the ratio sue / suc clearly increases with an in-crease in I p. This tendency is in fairly good agreement withBjerrum’s observation, which was based on the correctionfactor to the vane shear strength (Bjerrum 1973).

The sue / suc ratio for Bangkok clay and Singapore clay,however, is very large despite the relatively low I p values forthese clays. This is particularly true for the sue / suc ratio for

Singapore clay, which exceeds unity at a depth of 28 m, im-plying that the extension strength is greater than the com-pression strength. Tanaka and Tanaka (1997) reported thatthe sue / suc ratio for Bothkennar clay, whose I p is about 40, isas low as 0.45. Thus, it can be concluded that compared tothe European or Japanese marine clays, the Southeast Asianclays behave more isotropically despite their moderate I pvalues.

Stress path and effective parametersFigure 30 shows typical stress–strain relations measured

by compression triaxial tests. Since the sample depths aredifferent at each site, the deviator stresses are normalized bythe corresponding σ vo′ to make the curves comparable. The

highest deviator strength ratio is observed for Bangkok clayand the lowest for Singapore clay. The strength ratio[(σ1′ – σ 3′ )/2]/ σ vo′ for Bangkok clay is more than 0.5, andthat for Singapore clay is less than 0.3.

The stress paths of these clays are also indicated in theright part of Fig. 30. The internal friction angle (φ′ ) at a re-sidual stress condition for Singapore clay is considerablysmaller than that for Ariake clay. The φ ′ values are summa-rized in Table 1 and are defined both at the maximum devia-tor stress state (φp′ ) and at the residual stress state (φ r′ ). Theφ′ value even at the maximum deviator stress for Ariake clayis more than 45°, and the φ′ value for Singapore clay is as

low as 25°. It is well known that the φ ′ value for Japaneseclays is very high despite their high I p values (for example,Lambe and Whitman 1978). The large φ ′ value for Japanesemarine clays can be explained by the component of clayminerals or the existence of angular materials includingmicrofossils. Tanaka and Locat (1999) reported that

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Tanaka et al. 393

Fig. 20. The relationship between normalized compression indexand consolidation pressure for the three clays.



Japanese marine clays contain a lot of microfossils, espe-cially diatoms, and their existence in Japanese clays is likelyto contribute to high φ′ values. Indeed, Shiwakoti et al.(1999) showed a considerable increase in the φ ′ value in soilcontaining diatoms by mixing diatomite with pure kaolinand Singapore clays.

It is interesting to note the difference in nature of thestress path among soil groups. The shapes of the stress pathsfor the Ariake and Singapore clays are very similar, althoughthe deviator stress for Singapore clay is much smaller due toits small φ′. However, the shape of the stress path for theBangkok clay is markedly different from those of the Bang-kok and Singapore clays. The stress path for the Bangkokclay goes almost straight up and reaches the failure enve-

lope. After attaining the peak strength, the deviator stresssuddenly decreases with a decrease in effective stress. Thisbehavior is consistent with microstructural observationsmade using the SEM, as described earlier.

One may argue that these differences in the pattern of thestress path among clays are induced by variation in the sam-ple quality. Indeed, various researchers (Hight et al. 1992;Lunne et al. 1997; Tanaka and Tanaka 1999) have indicatedthat if the sample quality is not good, the sharp bendingpoint at the peak strength in the stress path disappears andthe shape of the stress path becomes similar to that of theAriake or Singapore clays as shown in Fig. 30. However, in

this study, all soil samples were retrieved using the Japanesesampler. Although Fig. 30 shows only a test result at a cer-tain depth for each clay type, the same trend has been ob-tained at other depths as well. As already mentioned, theshape of the e – log p curve for the sample collected usingthe Japanese sampler is completely different from that col-lected using the Shelby tube. This indicates the superiorityof the Japanese sampler in terms of sample quality. Tanakaet al. (1996) showed that the sample quality obtained usingthe Japanese sampler has the same order as the Laval or theSherbrooke samplers, the latter two being renowned world-wide for their superiority in obtaining high-quality samples.Therefore, it can be concluded that the difference in thestress path indicated in Fig. 30 is caused by the inherent

properties of soil, and not by the variation in sample quality.These differences in the pattern of the stress path may becaused by the variation in arrangement of soil particles orthe presence of fragments of microfossils.

Shear modulus

As indicated in the section titled Soil profile at testedsites, shear modulus measured by the seismic cone (Gsc) canbe correlated with either CPT or DMT values, and its rela-tionship is the same for normally or slightly overconsolidatedJapanese clays (Tanaka et al. 1994). These relations areshown in Figs. 31 and 32. It may be considered that the CPT

© 2001 NRC Canada


Fig. 21. The relationship between compression index and liquid limit.



or DMT behavior is governed by deformation characteristicsat relatively large strain. In contrast, Gsc from the seismiccone is the shear modulus at very small strain. The existenceof a relationship between Gsc and either CPT or DMT im-plies that the G and strain (γ ) relationship is nearly the samefor most of the soft cohesive soils.

The Gsc values for the three clays normalized by σy arelisted in the last row of Table 1. The value of Gsc is gov-erned by many factors such as the OCR, void ratio, lateraleffective stress, and vertical stress. However, it is very inter-

esting to know that Gsc for Singapore clay is the same orderor even larger than that of the Ariake or Bangkok clays,which have considerably larger strength than Singapore clay.This suggests that Gsc is not governed by the φ ′ value or theundrained shear strength.

The undrained shear strength as a design

value

Until now, several methods of obtaining undrained shearstrength for design, sometimes called the mobilized strength(sumob), have been proposed by various researchers. Amongthem, the one method which uses Bjerrum’s correction fac-tor (µ) is well known, where the anisotropy and the rate ef-fect on strength are taken into account in the vane shear

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Tanaka et al. 395

Fig. 22. Profiles of the coefficient of consolidation for the three sites.

Fig. 23. A typical example of the relationship between void ratioand permeability (measured at the Bangkok site). eo, in situ voidratio; k o, in situ permeability.

Fig. 24. The relationship between the in situ void ratio and per-meability for the three clays.



strength by correlating it with I p. Mesri (1975) also pro-posed, based on the Bjerrum correction factor and ageingeffect on py, the ratio of mobilized strength to consolidationpressure (sumob / pc) as 0.22, regardless of I p.

On the other hand, the qu /2 value from the unconfinedcompression test has traditionally been used in Japan, with-out correcting for factors such as I p. The qu /2 value is wellbalanced between factors which underestimate the truestrength, such as soil disturbance caused by sampling, andfactors which overestimate the true strength, such as the ani-sotropy or the rate effects (Tsuchida and Mizukami 1991;Tsuchida and Tanaka 1995; Tanaka and Tanaka 1997).

The sumob values for all three test sites as proposed by var-ious researchers are given in Table 2 in the form normalized

by σy. From the triaxial test, s umob is the average strength of the compression (CK

0UC) and extension (CK

0UE)

recompression triaxial tests, considering the strength aniso-tropy. In addition, the strength is further reduced to accountfor the rate effect. According to Hanzawa and Tanaka(1992), the rate effect of the Japanese clays is about 7% perthe logarithmic scale of strain rate regardless of I p. As-suming that this rate effect can be applied to both the Singa-pore and Bangkok clays and the strain rate at failure in thefield is 10–3%/min, the correction factor for the rate effect is0.86, as the strain rate in the triaxial test is 0.1%/min, as de-scribed previously. Hanzawa (1992) proposed the sumobvalue using the constant-volume direct shear (DS) test,where the measured value is multiplied by 0.85 to take into

© 2001 NRC Canada


Fig. 25. The undrained shear strength of Ariake clay measuredby various tests.

Fig. 26. The undrained shear strength of Singapore clay mea-sured by various tests.

Fig. 27. The undrained shear strength of Bangkok clay measuredby various tests.

Fig. 28. The cone factors N kt based on the field vane test. Datafor Drammen, Bothkennar, and Louiseville are from T. Lunne(personal communication, 1995), Nash et al. (1992), and La Ro-chelle et al. (1988), respectively.



account the strain rate effect. The sumob values are also ob-tained from the DMT following the proposal of Marchetti(1980). In his proposal, sumob is expressed by the followingequation:

[7] s s mu

vo

u

vo nc

OCRσ σ′ ′

=

where (su / σ vo′ )nc is the strength ratio at the normally consol-idated stage. Marchetti adopted 0.22 as (su / σ vo′ )nc based onthe proposal of Mesri (1975) and m = 0.8 from previous ex-perimental data.

The OCR is estimated from the DMT using the followingequation:

[8] OCR = (0.5K D)1.56

Thus to determine su from the DMT, eq. [7] becomes

[9] sK u

vo

D 1.250.22(0.5 )

σ ′=

It is important to remember that the strength estimated byeq. [9] is the mobilized strength because 0.22 of (su / σ vo′ )ncis a design value taking into account the anisotropy and rateeffect, according to Mesri (1975). In addition, for reviewing

these strength ratios in Table 2, it is useful to know the rela-tionship between (su / σ vo′ )nc and su / σy. Using eq. [7], su / σycan be expressed by the following equation:

[10] s s mu

y

u

vo nc

OCRσ σ

=

′

−1

If OCR is 1.5 and m is assumed to be 0.8, then su / σy be-comes 0.92(su / σ vo′ )nc.

The vane shear strength corrected by Bjerrum’s factor µ isabout 0.22 for Ariake clay, which is apparently the same asthe strength ratio at the normally consolidated stage pro-posed by Mesri (1975). However, this strength value is con-

siderably smaller for Singapore clay and much larger forBangkok clay. Tanaka (1994) has shown by reviewing sev-eral case records that the vane strength corrected byBjerrum’s factor considerably underestimates sumob for Japa-nese clays. Similarly, the value 0.22 proposed by Mesri for(su / σ vo′ )nc is too small in light of the fact that the incremen-tal strength ratio is adopted as 0.3 or 0.33 at step loadingconstruction in Japan. Indeed, for Ariake clay, the order of strength ratios from the average recompression triaxial testand the corrected direct shear test, as well as qu /2, is about0.3–0.33, although the average recompression strength forupper Ariake clay seems too large. For both Singapore andBangkok clays, these strength ratios are of nearly the sameorder except for the average recompression strength ratio of

the upper Bangkok clay. However, these strength ratios areconsiderably smaller than that of Ariake clay.There seems to be no consistency in the strength esti-

mated by the DMT. For example, su / σy for the upper andthe lower Ariake clays estimated by the DMT is 0.15 and0.13, respectively. These ratios are too small, based on the

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Tanaka et al. 397

Fig. 29. The anisotropy ratio sue / suc for clays at three sites andfor various Japanese marine clays.

Fig. 30. The stress and strain relationship and stress path measured by the recompression triaxial test (compression test).



experience in this region. As already indicated, the OCR is akey parameter in estimating s

umob by this method. The calcu-

lated OCRs from the DMT are given in Table 2. For Ariakeclay the OCR is less than 1.0, which implies a state of underconsolidation. On the other hand, for Bangkok clay,the OCR value calculated by the DMT is considerably largerthan those measured by CRS oedometer tests.

It is very difficult to point out which method is suitablefor providing the design strength, because the applicabilityof the method should be verified by drawing on many expe-riences in the region concerned. Running the risk of drawingconclusions with few data from this investigation, the fol-lowing method may be able to evaluate sumob for each of theregions.

(1) For Ariake clay, the average of compression and ex-tension strengths from the recompression triaxial test andstrength from the constant-volume direct shear test are suit-able considering the strain rate effect. It seems that qu /2,which is traditionally used in Japan, is also applicable, pro-vided that sample quality is good. Both the vane strengthcorrected by Bjerrum’s factor and the strength estimated bythe DMT considerably underestimated the undrained shearstrength of Ariake clay.

(2) Singapore clay has the same tendency as Ariake clay,although the DMT apparently provides a good value of su.

(3) For Bangkok clay, the value of qu /2 is very smallcompared with the average values obtained from triaxial ordirect shear tests. The values 0.33 and 0.27 estimated from

© 2001 NRC Canada


Fig. 31. Correlation of Gsc from seismic cone with qt – σvo fromthe CPT for Japanese clays (after Tanaka and Tanaka 1998).

Fig. 32. Correlation of Gsc from seismic cone with E D from theDMT for Japanese clays (after Tanaka and Tanaka 1998).

Ariake

Singapore

Bangkok

Upper Lower Upper Lower

I p 70 50 50 50 —

Bjerrum’s correction factor µ 0.72 0.82 0.82 0.82 —µ su / σy (vane) 0.23 0.22 0.16 0.27 —

Avg. (CK0UE and CK0UC) × 0.86 0.35 0.29 0.19 0.32 0.19

DS × 0.85 (after Hanzawa 1992) 0.32 0.30 0.23 0.25 0.22

(qu /2)/ σy 0.31 0.30 0.21* 0.21 0.21

K D from DMT 1.8 1.8 2.8 3.4 3.4

su / σy from DMT 0.15 0.13 0.24 0.33 0.27

OCR from DMT 0.84 0.84 1.69 2.29 2.29

*From UU test.

Table 2. Comparison of proposed mobilized undrained shear strengths for Ariake, Singapore, and Bangkok clays.



the DMT for the upper and the lower Bangkok clay seem tobe too large. Thus, the method using the DMT cannot beused to obtain sumob for Bangkok clay. The applicability of Bjerrum’s correction factor to Bangkok clay is not clear inthis investigation because there is too much scatter in thestrength data measured by the FVS test.

Conclusions

The properties of two Southeast Asian clays, Singaporeclay and Bangkok clay, are compared with those of Ariakeclay, which is a typical Japanese marine clay. The samplingwas done using the same sampling method and samplers atall sites, thus all the samples can be considered to have thesame level of sample quality. Various tests were performedin the laboratory and in the field. Microstructure analysis us-ing the SEM and X-ray diffraction tests were conducted toexplain the differences in behavior of each clay in terms of clay mineral or soil structure. The clay mineral component isremarkably different among these clays. The use of clay

mineralogy and SEM observations, although qualitative, pro-vides some insight to the complex role of soil formation andevolution (e.g., weathering) in their resulting mechanicalproperties.

The main conclusions from the present study are as fol-lows:

(1) The main clay mineral is smectite in Bangkok clayand Ariake clay and kaolinite in Singapore clay.

(2) Activity, which is defined as the ratio of plasticity toclay content, is considerably different among these threesites: 1.0–2.0 for Ariake clay, 0.5–0.8 for Singapore clay,and 0.9–1.4 for Bangkok clay. These differences can be at-tributed to the difference in composition of clay mineralsand microfossils.

(3) The yield stress (σy) obtained by constant rate of strainoedometer tests is slightly greater than the in situ effectiveburden pressure (σ vo′ ) at all the sites. The ratio of σy to σ vo′(i.e., OCR) is about 1.1–1.7 for the objective layers consid-ered in this investigation.

(4) The shape of the e – log p curve shows nonlinearity,even at the normal consolidation stage. However, when theconsolidation pressure exceeds two times the value of σy, thenonlinearity disappears.

(5) The compression index (C c) at a consolidation pres-sure large enough to make C c constant can be correlatedwith liquid limit wL. The relationship can be expressed byC c = 0.009(wL – 10), as proposed by Terzaghi.

(6) There is a good correlation between the initial void ra-tio (eo) and the in situ permeability (k o) for the three types of clay investigated. Since the eo value for Bangkok clay is thesmallest of all the three clays, the coefficient of consolida-tion (cv) is as low as 10 cm2 /day. This value is considerablysmall compared with cv of 100 cm2 /day for Ariake clay.

(7) Strength anisotropy was defined as the undrained shearstrength ratio of the extension and compression strengthsfrom the recompression triaxial test (sue / suc). For Japanesemarine clays including Ariake clay, the ratio sue / suc in-creases with an increase in I p. For the Singapore and Bang-kok clays, the ratio sue / suc is relatively high despite themoderate value of I p.

(8) The internal friction angle ( φ′ ) for Singapore clay isconsiderably smaller than those for the Bangkok and Ariakeclays. The normalized undrained strength by the in situ ef-fective stress (su / σ vo′ ) is highest for Bangkok clay. Thislarge value for the ratio su / σ vo′ is not due to a high φ ′ valuebut is caused by the difference in structures, as observed bySEM pictures.

(9) The shear modulus measured by the seismic cone (Gsc)can be correlated with the net resistance (qt – σvo) from theCPT and the dilatometer modulus ( E D). The correlationsare the same as those established for other clays: Gsc =50(qt – σvo) or 7.5 E D.

(10) The undrained shear strengths for design were evalu-ated using several methods. The vane strength corrected us-ing Bjerrum’s correction factor is considerably smaller thanthose evaluated by other methods. It is also revealed thatthere is no consistency in the strength estimated bydilatometer following the proposal of Marchetti (1980).

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