Marine Clay Data

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CHAPTER 3

REVIEW ON PROPERTIES OF NATURAL

SINGAPORE MARINE CLAY

3.1 Introduction

The understanding of Singapore Marine Clay is very limited. The majority of the

available soil data in Singapore are from site investigation carried out during major

infrastructure projects such as Mass Rapid Transit and only the index properties,

compressibility and undrained shear strength were determined (Tan, 1983, Dames &

Moore 1983). The first effort on the detailed characterisation of Singapore Marine

Clay was undertaken by National University of Singapore and Nanyang Technological

University in the late nineties. This chapter will first give a brief description of

geological history of Singapore marine clay, followed by a review on the current

understanding on the properties of natural Singapore marine clay.

3.2 Brief Description of Geological History of Singapore

Marine Clay

The most frequently cited sources on the geology of Singapore are Pitts (1983) and

Pitts (1992). The ensuing brief description is a summary of the existing state of



CHAPTER 3 REVIEW ON PROPERTIES OF NATURAL SINGAPORE MARINE CLAY 43

knowledge, though the Building and Construction Authority is currently attempting to

update the geological map of Singapore.

The soil on Singapore Island is conventionally classified into six major

formations known locally as the Kallang formation, Old Alluvium, Jurong Formation,

Bukit Timah Granite, Gombak Norite and Sahajat Formation (PWD 1976). Marine

clay is the main constituent of the Kallang Formation which is estimated covers one

quarter of Singapore Island (Pitts, 1992). The marine clay formation varies in

thickness; it is usually between 10 m to 15 m near the estuaries, but in some instances,

it can be thicker than 40 m. What is also interesting is the presence of this formation

up the deeply incised river valleys, which penetrates to the centre of the Singapore

Island, and in some cases, this clay is above the present sea level (Tan et al. 2002b).

The marine clay generally consists of two members typically referred as Upper Marine

Clay (UMC) and Lower Marine Clay (LMC). These two layers are separated by a

stiffer intermediate layer, widely considered to be the desiccated crust of the LMC.

This stratification is typical in areas where the deposit is thick.

According to Pitts (1992), the LMC was deposited some time between 12,000

to 18,000 years ago, at the end of the Pleistocene epoch. Between 10,000 to 12,000

years, the sea level dropped as a result of the Small Ice Age (Figure 3.1) and it was

hypothesized that the top part of this clay was exposed and became desiccated and

weathered. The UMC is a Holocene Deposit that arrived after the last Ice Age and is

usually thought to be younger than 10,000 years. The earlier paper by Tan (1983)

reckoned that the LMC was deposited during the Riss-Wurm interglacial period some

120,000 years ago when sea level stood +2 to +10 m above present level. The drop in

sea level was attributed to the Wurm glaciation.




Based on the current understanding of global Quaternary environment and

particularly in the magnitude, timing and forcing of glacial-interglacial cycle in global

climate and sea-level over the last two decades and some recent radiocarbon dating of

Singapore marine clay (Chang, 1995, Hesp et al., 1998, Tan et al., 2002b), Bird et al.

(2003) revised the chronologies proposed by PWD (1976) for the depositional of

Singapore marine clay. They proposed that LMC was deposited after the end of the

penultimate glacial period, about 140,000 years ago (Figure 3.2). Thereafter, the sea

retreated from the Singapore region to a low of -120 to -130 m below present sea level

during the Last Glacial Maximum – 20,000 years ago. As a result the LMC was

exposed to terrestrial pedogenetic processes, producing the ‘stiff’ mottled clay at the

top of the LMC. At the end of the Last Glacial Maximum, global sea-level began to

rise rapidly from -120 m. Once sea-level rose to about -25 m about 10,000-11,000

years ago, the sills to east and west of the Singapore Straits were breached virtually

simultaneously and the Singapore area was inundated in a matter of years or less. As

sea-level continued to rise, UMC was deposited.

3.3 Previous Characterisation Studies on Natural

Singapore Marine Clay

Generally, characterisation studies on Singapore marine clay could be broadly divided

into two major stages. The first stage was carried out in the early of 1980s when a

number of major infrastructure projects were implemented. One of the major studies

was carried out by Tan (1983). He summarized data on the UMC and LMC which was

collected by Public Work Department. At about the same time, a detailed geotechnicalstudy was also conducted by Dames and Moore (1983) for the Mass Rapid Transit




Authority, in preparation for the construction of the first Mass Rapid Transit (MRT)

line in Singapore. In these studies, only engineering parameters such as

compressibility and undrained shear strength were determined because they are of

immediate relevance to the application at hand.

The second stage was carried out at the late of 1990s by National University of

Singapore (NUS) and Nanyang Technological University (NTU). NUS carried out the

characterisation study during the construction of Singapore Art Centre (presently

called Esplanade and denoted as SAC from hereon) and the reclamation project at

Pulau Tekong (denoted as PT from hereon) and most of the data were reported in Tan

et al. (2002a and 2002b). Tan et al. (2002a and 2002b) paid attention specifically to

the effect of sample disturbance on the mechanical properties of LMC. They also

studied the small strain behaviour of LMC. The finding of these studies has provided a

reasonable database to arrive at soil models that commensurate with the increasing

sophistication involved in numerical analysis, which has become almost a standard

requirement for the geotechnical design in densely built-up Singapore. This study also

highlighted the need to obtain high quality samples to ensure reliable interpretation of

the behaviour of Singapore marine clay. On the other hand, NTU carried out some

characterisation study of UMC and LMC during the Changi Airport reclamation. A

number of conference and journal papers (e.g. Chang et al. 1997; Chu et al. 2002) were

published and mainly focused on application of in-situ tests (e.g. piezocone,

dilatometer) in the estimation of overconsolidation ratio, undrained shear strength and

consolidation properties (e.g. coefficient of consolidation). They showed that in-situ

tests provide a reasonable and cost-effective estimation of overconsolidation ratio,

undrained shear strength and consolidation properties. Some minor characterisation




studies of UMC and LMC were also carried out in between these two stages of major

studies (at the late of 1980s and early 1990s) such as Chang (1991) on the prediction of

overconsolidation ratio from piezocone and Todo et al. (1993) on the compressibility

of UMC and LMC.

This study only focuses on the effect of soil structure on the compressibility,

undrained behaviour of natural Singapore marine clay. Therefore, only findings of the

relevant studies will be summarized and reviewed in this chapter.

3.3.1 Index Properties

Index properties of the UMC and LMC have been extensively reported by Tan (1983)

and are summarised in Table 3.1. According to Tan (1983), Singapore marine clay is

essentially weakly flocculated, kaolinite-rich clay with moderate contents of

monmorillonite and illite. Similarly, through X-ray diffraction tests, Tan et al. (2002b)

found that the predominant mineral in LMC from Singapore Art Centre site and Pulau

Tekong site is kaolinite with traces of illite and smectite. They also found the LMC

from PT site contains more smectite than LMC from SAC site. As a result, liquid limit

of LMC from PT site (ranges from 80% to 100%) is higher than the liquid limit of

LMC from SAC site (ranges from 60% to 80%) and those reported in Table 3.1. The

clay content is generally more than 50% and the activity is around 1.25-0.5 for both

UMC and LMC (Tan, 1983; Tan et al., 2002b). Typically, the specific gravity is

between 2.58 to 2.72 for both members (Tan, 1983). A recent study on UMC and

LMC from PT site, Locat et al. (2002) found that UMC contains the highest organic

content among the marine clay members (i.e. LMC and intermediate layer) that

ranging from 5-8% in upper PT clay while intermediate layer contains the lowest




organic content that ranging from 0.2-3%. The organic content in LMC is fairly

uniform with about 3.5% throughout the depth.

3.3.2 Yield stress and Yield Stress Ratio

Tan (1983) reported YSR values between 1.0 and 1.5 for Singapore marine clays. A

graphical plot provided by Dames & Moore (1983) indicated an average YSR less than

1.5 and a maximum YSR of 2.0. Tan (1983) attribute the overconsolidated state in

Singapore marine clay to secondary compression. Tan et al. (2002b) found that YSR

of LMC from SAC site is very sensitive to sample disturbance. Figure 3.3 shows that

low quality samples (retrieved with Shelby samplers – BH3) indicate YSR in the range

between 1.0 and 1.2 while high quality samples (retrieved with Japanese samplers –

BH1) indicate higher YSR with value ranging from 1.4 to 1.5. They also noticed that

low quality samples are also sensitive to test type where standard oedometer give

lower YSR (ranging from 1 to 1.2) than constant rate of strain test (CRS) (ranging

from 1.2 to 1.3). On the other hand, when both tests were performed on high quality

samples, no marked difference between measured YSR values was observed. Overall,

the YSR values for LMC from SAC site fall within the range reported by Tan (1983)

with YSR averaging 1.2 from oedometer test and 1.5 from CRS test. YSR values

higher than 1.5 for depths less than 18 m in Figure 3.3 belong to the intermediate layer.

Figure 3.4 shows a YSR profile of the Singapore marine clay presented by

Hanzawa and Adachi (1983) based on a comprehensive oedometer test program. The

profile is representative of the natural Singapore marine clay below seabed (but the

origin of samples was not known), which is traditionally believed to be normally

consolidated based on the depositional history (Hanzawa & Adachi, 1983). It could be




noticed that YSR decrease with depth from about 4 just below the seabed to between

1.5 and 2 at the bottom of the UMC. For LMC, Figure 3.4 shows that the YSR is

about constant throughout the depth and ranging from 1.3 to 1.6. The YSR values in

UMC are consistently higher than the values reported in previous paragraph while the

YSR values in LMC are within the range reported in previous paragraph. Based on

this yield stress and YSR profile, Hanzawa and Adachi (1983) postulated that high

YSR values in UMC might be due to chemical bonding while the overconsolidation

state in LMC is probably due to secondary compression. Figure 3.4 also indicates that,

intermediate layer is moderately overconsolidated (YSR = 2.5 to 4). They attributed

this overconsolidation behaviour to dessication during the exposure of LMC during the

Last Glacial Maximum (see Section 3.2).

Tan et al. (2002b) also found that a similar YSR profiles of natural Singapore

marine clay below the seabed near Pulau Tekong. Figure 3.5a shows the results

carried out by two independent commercial laboratories while Figure 3.5b shows the

results from CRS tests carried out by Port and Airport Research Institute, Japan

(PARI). Both figures include YSR values for UMC (0 to10 m), which can exceed 8 at

the top and reduce to about 4 at bottom of UMC which is higher than the data reported

by Hanzawa and Adachi (1983). Clay deposit at the depth between 10 to 14 m belongs

to intermediate layer where high sand content was found and their YSR values were

found ranging from 2 to 4. However, the reliability of these YSR values are doubtful

because the high sand content in the samples from this layer complicate the

determination of yield stress from e-log σv’ curve. For LMC, Figure 3.5b shows that

the YSR increases very slightly with depth from 2.7 to 3.1 while Figure 3.5a indicates

a nearly constant YSR of about 2.5 in LMC. Surprisingly, the YSR in LMC remains




3.3.2.1 Estimation of Yield Stress or Yield Stress Ratio Profile from In-situ Tests

Profile of yield stress and parameters derived from yield stress such as YSR, YSD and

YSG provide a useful first indication of the causes of overconsolidation in clay (Perret

et al., 1995 and see also Section 2.8). The interpretation of yield stress profiles from

in-situ tests provide an economical alternative to oedometer test for the study of the

origin of structuration of a clay. Chang (1991) had demonstrated the determination of

YSR in Singapore marine clay by field vane test, piezocone test and dilatometer test.

According to Chang (1991), YSR could be estimated from the undrained shear

strength measured by field vane shear test, (s u)FV, that normalised by the corresponding

in-situ effective overburden stress, σvo’ based on the following two correlations:

FV'vo

u48.0 s)PI(22YSR

σ= − (3.1)

05.1

NC'vou

FV'vou

) / s(

) / s(YSR

σσ

= (3.2)

where PI is plasticity index and (s u / σvo’)NC is the undrained shear ratio for normally

consolidated clay. Eq. 3.1 was proposed by Mayne & Mitchell (1988) while Eq 3.2

was proposed by Chandler (1987) on the basis of a large collection of field vane and

oedometer tests results. Chang (1991) suggested that both equations appeared to

provide reasonable to slightly conservative estimates of YSR for the Singapore marine

clay.

In addition to field vane test, Chang (1991) also showed that YSR for

Singapore marine clay could be estimated from piezocone tests. He related the YSR to

the pore pressure ratio, B q (=(u 2-uo)/(q t-σvo)) , where u 2 and u o are the penetration pore




pressure measured behind the cone and the initial hydrostatic pore pressure,

respectively, q t is the cone resistance (corrected for the unequal end area effect) and

σvo is the in-situ total overburden stress. He proposed the following correlation for

non-sensitive to highly sensitive clay (strength sensitivity < 8 with YSR less than 8):

)1B7.3(

B3.2YSR

q

q

−= (3.3)

This correlation was found to produce generally conservative estimates of YSR for the

Singapore marine clay (Chang, 1991).

Dilatometer (DMT) results were also used for estimating the YSR for clays.

The commonly use correlation between the YSR and horizontal stress index (K D) is:

λ= )K5.0(YSR D (3.4)

where K D = (p o-uo)/ σvo’ with p o is the contact pressure after corrected for membrane

stiffness, u o is the initial hydrostatic pore pressure and σvo’ is in-situ effective

overburden stress. Marchetti (1980) recommended λ=1.56 for uncemented natural

clays. However, Chang (1991) proposed λ value of 0.84 for the Singapore marine

clay.

A recent theoretical study carried out by Cao et al. (1996) and Cao (1997)

based on the modified Cam clay concept, the following equations were proposed for

the estimation of YSR from field vane test:

Λ

φσ=

/ 1

'ps

'vo

FVu

cosM

)s(32YSR (3.5)




from piezocone test:

Λ

ε +σα−σ+

= / 1

'vo

2'vot

)M66.01(u134.0q866.0

2YSR for B q < 0.75 (3.6)

Λ

ε +σα−

= / 1

'vo

2t

)M67.01(

uq2YSR for 0.75 < B q < 0.85 (3.7)

from dilatometer test:

Λ

+σασ−

= / 1

r'vod

voo

)1I(lnM)p(3

2YSR (3.8)

where Λ is the plastic volumetric strain ratio with typical value of 0.75 (Cao et al.,

1996), M is the slope of critical state line, φps’ is the effective friction angle in the plane

strain compression condition, I r is the rigidity index, α ε is the strain rate correction

factor for piezocone which can be taken as 1.64 (Cao et al. 1996) and α d is the strain

rate correction factor for flat dilatometer which can be taken as 1.57 (Cao, 1997).

Chang et al. (2001) showed that these equations provided a reasonable estimation of

YSR of Singapore marine clay from Changi site. However, these equations involve

parameters such as M and φps’ that are not readily known without conducting detailed

experimental tests. Hence, these equations are difficult to use without a detailed

knowledge about a soil.

According to the comprehensive analytical study carried out Aubeny (1992)

using an advanced elastoplastic anisotropic soil model MIT-E3 (Whittle 1987, Whittle

and Kavvadas 1994), normalized cone tip resistance (Q t) is one of the piezocone

parameters exhibiting a higher sensitivity to the YSR. In addition, the correlation




between YSR and Q t is one of the most common proposals found in the literature (e.g.,

Mayne & Holtz 1988, Chen & Mayne 1996, Demers & Leroueil 2002):

t'vo

vot kQq

k YSR =σ

σ−= (3.9)

where k is a constant, q t is the corrected cone tip resistance, and σvo and σ vo’ are the in-

situ total and effective overburben stresses, respectively. Powell et al. (1988) found

that the k value for some clays from United Kingdom varies between 0.2 and 0.5 for

nonfissured clays and between 0.9 and 2.2 for heavily overconsolidated fissured clays.

Lutenegger & Kabir (1988) also obtained scattered results for marine clays in New

York with a mean k value of 0.3. Based on a large worldwide database, Chen &

Mayne (1996) obtained a k value of 0.32 with a relatively low coefficient of

determination (r 2 = 0.67). The degree of variability increases further when fissured

clays (r 2 = 0.47) are included. For sensitive clays in Quebec, Demers & Leroueil

(2002) found a k value of 0.29 with a high coefficient of determination (r 2 = 0.99).

This degree of accuracy is exceptional, given the spatial scope of the calibration.

Nevertheless, Demers & Leroueil (2002) concluded that their correlations and others

found in literature are not accurate enough for final designs, which concurred with

sentiments expressed by Lunne et al. (2002).

Due to the availability of data in this study, the YSR profile will only be

estimated from piezocone test with Eq. 3.9. The wide range of reported k value in the

literature led to a need to develop a site specific correlation that is suitable for

Singapore marine clay. A site specific calibration will be carried out to examine how

accurate are the YSR-Q t correlation for the estimation of yield stress ratio profiles for

the clay deposit below the seabed near Pulau Tekong in Chapter 6. The yield stress




profile will be obtained by multiplying Eq. 3.9 with the corresponding in-situ effective

overburden stress, σvo’

3.3.3 Compressibility

Tan (1983) reported that Singapore marine clay has a compression index C c of 0.7 to

1.3 for upper marine clay and 0.45 to 0.95 for the lower marine clay, whereas the

recompression index ranges from 0.083C c to 0.3C c with an average of 0.18 for both the

UMC and LMC. Dames and Moore (1983) also reported the similar average values

for both UMC and LMC. Tan (1983) proposed a correlation between C c and initial

void ratio (measured at the start of oedometer test), e o for Singapore marine clay (both

UMC and LMC) based on 233 oedometer tests:

Cc = 0.344 (e o + 0.51) (3.10)

Another correlation was proposed by Dames & Moore (1983) for design purposes in

Singapore:

Cc = 0.54 (e o - 0.15) (3.11)

Todo et al. (1993) observed that the e-log σ v’ curve of the Singapore marine

clay after yield stress is non-linear. They attributed this behaviour to the postulation

that Singapore marine clay is cemented. However, the cementation agent was not

revealed. Tan et al. (2002b) also observed similar behaviour in soil from SAC and PT

site (Figure 3.6). However, no clear evidence of cementation was observed through

the investigation of microfabric using SEM. In addition, they also found that this




behaviour is only noticeable when sampling disturbance is minimized and the more

refined CRS test is used.

Tan et al. (2002b) described the non-linear behaviour in compression curve

after σvy’ with two compressibility indices which are C c1 (termed as C cmax from hereon)

and C c2. The parameter C cmax is the compression index immediately after σvy’ and C c2

is the usual value along the normally consolidated line well after σvy’ (>1000 kPa). In

Chapter 5, it will be shown that C c2 reported by Tan et al. (2002b) is comparable with

compression index for reconstituted clay, C c* as defined by Burland (1990).

Therefore, to ensure the consistency of symbol for the discussions in subsequent

chapters, C c2 used by Tan et al. (2002b) will be replaced by C c*. Results on C cmax and

Cc* from the complete series of one-dimensional compression tests carried out by Tan

et al. (2002b) on LMC from SAC site are summarized in Table 3.2 and Figure 3.7. As

shown in Figure 3.7, the effect of sample quality on compressibility is most obvious in

CRS tests. It could be noticed that for samples retrieved using a local sampler (BH3),

the measured ratio of C cmax /C c* are significantly lower than C cmax /C c

* measured from

samples retrieved using the Japanese sampler. It is also shown in Figure 3.7, for a

given sampling method, CRS tests also tend to produce higher ratios of C cmax /C c* than

standard oedometer tests.

In addition, Tan et al. (2002b) also compared the measured C cmax and C c* from

CRS tests on LMC from SAC and PT site retrieved using Japanese sampler with Eq.

3.10 and Eq. 3.11. They found that these equations act as a lower bound for the C cmax

data in Figure 3.8, but is an upper bound for the C c* data in Figure 3.9. Hence, they

concluded that these equations could be potentially unconservative for compressibility




calculations when C cmax is operative, but is acceptable when the compression is well

beyond σvy’.

From the previous paragraphs, the effect of sample quality and test type on

compressibility was well-studied. Sample quality and test type have a pronounced

effect on the measured compressibility curve immediately after σvy’. The reduction of

measured compressibility caused by sample disturbance is believed due to the partial

destructuration resulting from strain induced on soil samples during sampling (Hight et

al., 2002). On the hand, the non-linear compression curve after σvy’ might be

attributed to the progressive destructuration of soil structure due to one-dimensional

loading (Leroueil & Vaughan 1990). Therefore, it is reasonable to believe that

Singapore marine clay is structured and some in-depth study is needed to further verify

this postulation and to investigate the origin of this soil structure. In addition, if this

progressive destructuration of soil structure could be quantified, the non-linear

compression curve after σvy’ could be predicted (Liu & Carter, 2000).

3.3.4 Undrained Shear Strength

The undrained shear strength (s u) for Singapore marine clay is often determined by

unconfined compression test (UCT), the unconsolidated undrained triaxial test (UU)

and the laboratory (LVT) and field vane shear tests (FVT) (Tan, 1983; Dames &

Moore, 1983). Since s u is highly dependent on the effective stress prior to undrained

shearing, its value commonly is normalized by the vertical effective overburden stress,

σvo’ at the depth where s u is measured. This undrained strength ratio, s u / σvo

’ is

generally influenced by test type, strain rate and overconsolidation ratio (Kulhawy &

Mayne, 1990). Based on 500 tests data from UU and UCT tests, Tan (1983) reported




that s u / σvo’ for UMC ranging from 0.18 to 0.41 while s u / σvo

’ for LMC ranging from

0.25 to 0.41 (Table 3.1). Dames & Moore (1983) also reported a similar range

between 0.22 and 0.40 for both UMC and LMC from another independent set of data

obtained from UU tests. They attributed this wide range of s u / σvo’ to error in the

estimation of σvo’, sample disturbance and overconsolidation state of Singapore marine

clay.

Tan et al. (2002b) showed that s u from UCT tests on LMC from SAC site is

sensitive to sampling quality. For samples retrieved using the Japanese sampler, s u is

generally higher than those determined from samples retrieved using local sampler

(Figure 3.10a). The average difference is roughly 30%. However, with isotropic

(CIU) or anisotropic (CK oU) reconsolidation to in-situ stresses, the difference in shear

strengths arising from samples of differing quality and the variability with depth

reduced very significantly (Figure 3.10b). In particular, the results show little

difference between CIU and CK oU tests (Figure 3.10b). They attributed these

observations to the nearly strength isotropy of LMC from SAC site. Tanaka et al.

(2000) reported that the ratio between the compression undrained shear strength and

extension undrained shear strength for LMC from SAC is ranging from 0.9 to 1.1. In

addition, it could be noticed in Figure 3.10b that the shear strength profile obtained

from UCT tests is highly variable as compared to that obtained from CIU and CK oU.

Nevertheless, when compared the data from UCT tests to the range of s u / σvo’ reported

by Tan (1983) and Dames & Moore (1983), it can be seen that using an s u / σvo’ of 0.40

is a gross overestimation, whereas the lower bound value of 0.22 seems more

appropriate (Tan et al. 200b).




Based on 303 FVT test results, Tan (1983) found that the s u / σvo’ ranges from

0.19 to 0.42 for both UMC and LMC. Dames & Moore (1983) also reported the

almost similar range (from 0.22 to 0.42) for s u / σvo’ obtained from LVT and FVT tests.

Early work by Skempton (1957) suggested that the undrained shear strength ratio of

normally consolidated clays (s u / σvo’) is a linear function of PI for vane shear test

(Figure 3.11). Bjerrum (1973) subsequently generalised Skempton’s linear

relationship for overconsolidated clays by replacing the effective overburden stress in

the denominator of the undrained shear strength ratio with the σvy’. Tan et al. (2002b)

showed that the s u / σvy’ for vane shear test (FVT) for LMC from SAC and PT site is

about 0.21 and insensitive to PI (Figure 3.11) despite their significant difference in

YSR as shown in Section 3.3.2. More interestingly, they found that this s u / σvy’ ratio is

equal to the s u / σvy’ obtained from direct shear test (DST) (Figure 3.12). This

observation for DST is consistent with that reported by Mesri (1989) (s u / σvy’ = 0.22),

but very low for FVT in comparison with Bjerrum (1973). They attributed the equality

of s u / σvy’ for FVT and DST to the strength isotropy as discussed above.

It is important to recognize that the sampling process causes two different

disturbances, namely, the loss of residual effective stress and soil structure (Tanaka,

2000; Tan et al. 2002a). Reconsolidation to in-situ stresses are used to negate the

effect of loss of residual effective stress. Since it was shown that reconsolidation to in-

situ stresses can recover significant portion of strength loss due sample disturbance,

this implied that the lower s u measured by UCT or UU test is primarily due to the loss

of residual effective stress and sample disturbance induced during sampling is likely to

have resulted in small scale of destructuration (Tan et al. 2002a). However, the degree

of destructuration induced by that the sampling disturbance could not be assessed




because comparison with s u from ‘ideal’ sample that is free from sample disturbance is

not possible.

3.3.4.1 Strength Sensitivity

Based on 303 test data obtained from FVT, Tan (1983) reported that the strength

sensitivity, S t of UMC and LMC lies between 1.5 to 6 and 3 to 5, respectively. Dames

& Moore (1983) also reported the almost similar range (between 2.5 to 7.5) from both

UMC and LMC determined by FVT. According to the classification given by

Skempton & Northey (1952), Singapore marine clay can be classified as medium

sensitive to sensitive clay. As mentioned in Section 2.5.1, S t gives an indication of the

effect of soil structure on the enhanced resistance to undrained shearing of intact soil

as compared to the remoulded soil at the same water content. Therefore, these data

clearly reveal that natural Singapore marine clay is structured. However, systematic

study on the origin of soil structure and its effect of soil structure on mechanical

behaviour of natural Singapore marine clay is still lacking.

3.3.5 Undrained Young’s Modulus

For undrained loading, the modulus of cohesive soils can be described by either the

undrained Young’s modulus (E u) or the shear modulus (G). For undrained loading, E u

is equal to three times G. E u data for Singapore marine clay were obtained from the

stress-strain curve from UU test with cell pressure equal to the overburden pressure.

An unloading and reloading cycle is carried out at about one-third the peak deviator

stress and E u, is taken to be the secant modulus at the unloading and reloading cycle

(Tan, 1983). More commonly, E u is normalized directly by the s u obtained from the




same test to give E u /su. Based on 13 test data obtained in conjunction with the

laboratory testing works for the MRT Detailed Geotechnical Study, Tan (1983) found

that most of the E u /su data lie between 170 to 300. This range falls within the right

zone given in the chart for E u /su as a function of plasticity index and yield stress ratio

by Duncan & Buchignani (1976) (Figure 3.13). However, it should be reminded that

Figure 3.13 is derived based on data from direct simple shear test reported by Ladd et

al. (1977) following the SHANSEP testing procedure which is generally known to

eliminate the contribution of soil structure to the stiffness of soil (Burland, 1990).

It is now recognised that laboratory measurement of soil stiffness using

external LVDT is typically far smaller than local strain measurement with Hall’s effect

strain transducer (Tan et al., 2002a). In addition, it is also recognised that E u is also

strain dependent where E u decreases with increasing shear strain (Hight & Leroueil,

2002). Therefore, E u obtained from the unload-reload cycle at about one-third the

peak deviator stress might experience a higher strain level than those induced by

engineering structures such as foundation of building and hence the modulus may to be

underestimated. This highly likely be one of the main reasons contributing to the

discrepancy between the stiffness determined using conventional triaxial apparatus and

those back analysed stiffness from field data. However, due to the availability of data,

Eu was estimated as secant modulus at stress level of about one-third the peak deviator

stress with external strain measurement in UCT and UU will used in this study as a

qualitative indication of the effect of soil structure on the stiffness of natural Singapore

marine clay.




3.4 Concluding Remarks

Strength sensitivity (Tan, 1983 and Dames & Moore, 1983), non-linear virgin

compression curves (Todo et al., 1993; Tan et al. 2002b) and the effect of sample

disturbance on the mechanical properties of Singapore marine clay (Tan et al., 2002a

and 2002b), are the indication of the existence of soil structure in Singapore marine

clay. However, at present, no special attention was paid on the degree of soil structure

and the significance of its effect on the mechanical behaviour of natural Singapore

marine clay. Similarly, no systematic study was carried out on the investigation of the

possible causes of overconsolidation in natural Singapore marine clay.

In order to provided a better understanding of the behaviour of natural

Singapore marine clay, the degree of soil structure and the significance of its effect on

the mechanical behaviour of natural Singapore marine clay will be characterised using

the frameworks discussed in Chapter 2. Besides, the effect of progressive

destructuration on the compressibility of natural Singapore marine clay will also be

studied and a method to estimate the non-linear virgin compression curve will be

proposed. Attempts will also be undertaken to explore the possible causes of

overconsolidation in Singapore marine clay through in-situ and laboratory tests.




Table 3.1 Properties for Singapore Marine Clay (after Tan 1983)

Geotechnical Properties Upper Marine Clay Lower Marine Clay

Liquid Limit (%) 87±9 75 ±7

Plastic Limit (%) 31±4 29 ±3

Plasticity Index (%) 56±8 46 ±6

Liquidity Index 0.65-1 0.6

Activity 1 1

Su / σvo’

0.18-0.41 0.25-0.41Sensitivity 1.5-6 3-5

Effective shear strength parameterc’ (kN/m 2)φ’

321.5 °

721.5 °

Yield stress ratio (YSR) 1-1.5 1-1.5

Compression Index, C c 0.7-1.3 0.45-0.95

Recompression Index, C r 0.083-0.3C c 0.083-0.3C c

Coefficient of consolidation forvertical flow, c v (m 2 /year) 0.5-5.0 0.5-2.0

Coefficient of consolidation forhorizontal flow, c h (m 2 /year)

2-3 c v 2-3 c v

Coefficient of permeability, k v x10 -8 (cm/sec)

1-20

Eu /su 170-300 170-300

Ko 0.52-0.72 0.52-0.72




Table 3.2 Difference in C cmax and C c* for different samplers and test methods.

(after Tan et al., 2002b)

(a) BH1 (retrieved using Japanese Sampler) using CRS

Sample no. C cmax Cc* # C cmax /C c

*

BH1-UDP22 0.56 0.44 1.27

BH1-UDP32 0.96 0.57 1.68

BH1-UPD45 1.3 0.53 2.45

BH1-UDP52 1.42 0.57 2.49

BH1-UDP62 1.80 0.67 2.69

BH1-UDP72 1.07 0.58 1.85

BH1-UDP82 1.14 0.57 2.00

BH1-UDP92 1.42 0.55 2.58

(b) BH1 (retrieved using Japanese Sampler) using oedometer

Sample no. C cmax Cc* # C cmax /C c

*

BH1-UDP46 0.90 0.66 1.36

BH1-UDP64 1.23 0.90 1.37

BH1-UDP84 0.86 0.73 1.17

(c) BH3 (retrieved using Local Sampler) using CRS

Sample no. C cmax C c*# C cmax /C c

*

BH3-UDP22 0.40 0.40 1.00

BH3-UDP32 0.62 0.52 1.19

BH3-UDP44 0.66 0.58 1.13

BH3-UDP52 1.01 0.57 1.77

BH3-UDP64 0.81 0.57 1.42

BH3-UDP73 0.97 0.68 1.42

BH3-UDP83 0.74 0.60 1.23

BH3-UDP92 0.94 0.58 1.62

BH3-UDP102 0.74 0.61 1.21




Table 3.2 Difference in C cmax and C c* for different samplers and test methods.

(Tan et al., 2002b) (continued)

(d) BH3 (retrieved using Local Sampler) using oedometer

Sample no. C cmax C c*# C cmax /C c

*

BH3-UDP25 0.70 0.56 1.25

BH3-UDP35 0.70 0.67 1.04

BH3-UDP45 0.66 0.66 1.00

BH3-UDP55 0.70 0.65 1.08

BH3-UDP65 0.92 0.71 1.30

BH3-UDP75 1.01 0.81 1.25

BH3-UDP85 0.88 0.78 1.13

BH3-UDP95 0.74 0.62 1.19

BH3-UDP105 0.69 0.69 1.00#Cc

* defined by Tan et al. (2002) as C c at large stress (>1000 kPa)




-100

-75

-50

-25

0

25

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 Time (years before present)

D e p

t h b e

l o w

M o

d e r n

S e a

L e v e

l ( m )

Maximum thickness ofmarine clays in Singapore

Deposition of Upper member

Minimum sea level ≈ -130 m at 20,000 yrs. B.P.

Kenney (1964)

Non-deposition

Deposition of Lower member

Figure 3.1 Present understanding of the age of the Kallang Formation (after Tan et al.,2002b).

Figure 3.2 Changes in the volume of water in the oceans of the world, expressed asmetres of equivalent sea-level change from the last interglacial (LIG) to the present.(after Bird et al., 2003)




14

16

18

20

22

24

26

28

30100 200 300 400

Effective Pressure (kPa)

D e p

t h ( m )

BH1-CRS

BH1-OED

BH3-CRS

BH3-OED

σ'vo

14

16

18

20

22

24

26

28

300.8 1.2 1.6 2.0 2.4

Overconsolidation ratio (OCR)

Figure 3.3 Yield stress and YSR for SAC Site (after Tan et al., 2002b)(Samples from BH1 were retrieved with Japanese sampler while samples from BH3were retrieved with local sampler)




Figure 3.4 Yield stress and yield stress ratio profile (after Hanzawa & Adachi, 1983)

Figure 3.5 (a) YSR data from two independent commercial testing facilities for PT(b) Measured YSR from PARI and hypothetical addition of 80 kPa overburden to re-

create YSR values before erosion. (after Tan et al., 2002b)

Yield stress, σvy’ (x10kN/m 2) Yield Stress Ratio, YSR

(a) (b)




0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

10 100 1000 10000

Effective stress (kPa)

V o

i d R a

t i o

BH1(Jap)-CRS

BH1(Jap)-OED

BH3 (Local)-CRS

BH3(Local)-OED

Figure 3.6 Typical compressibility behaviour of samples retrieved using two differentsamplers and subjected to CRS and oedometer tests (sampling depth between 21.5 mand 22.5 m) at SAC site. (after Tan et al., 2002b)

14

16

18

20

22

24

26

28

300.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

CC1 /C C2

D e p

t h ( m )

BH1-CRS

BH1-OED

BH3-CRS

BH3-OED

Figure 3.7 Variation of C cmax /C c

* with depth for different samples subjected tooedometer and CRS tests. (after Tan et al., 2002b)

Ccmax /C c




0.0

0.4

0.8

1.2

1.6

2.0

0.0 0.5 1.0 1.5 2.0 2.5

In-situ void ratio (e 0)

C C 1

( i m m e

d i a t e l y a

f t e r p

' c )

PT-Upper

PT-Intermediate

PT-Lower

SAC

Dames & Moore (1983)

Tan (1983)

Figure 3.8 C cmax versus in-situ void ratio for clays from the two test sites (after Tan etal., 2002b)

0.0

0.4

0.8

1.2

1.6

0.0 0.5 1.0 1.5 2.0 2.5In-situ void ratio (e 0)

C C 2

PT-Upper

PT-Intermediate

PT-Lower

SAC

Dames & Moore (1983)

Tan (1983)

Figure 3.9 C c

* versus in-situ void ratio for clays from the two test sites (after Tan etal., 2002b)

C c m a x

C c

*




14

16

18

20

22

24

26

28

300 20 40 60 80 100 120

Undrained shear strength (kN/m 2)

D e p

t h ( m )

UCT (Japan)

UCT (Local)

0.22 σ 'vo

0.40 σ'vo

14

16

18

20

22

24

26

28

300 20 40 60 80 100 120

Undrained shear strength (kN/m 2)

D e p

t h ( m )

UCT (Japan)

UCT (Local)

CIU (Japan)

CIU (Local)

CKoU (Japan)

CKoU (Local)

0.30 σ'vo

Figure 3.10 (a) Effect of sampling method on undrained shear strength for SAC clay(b) Comparison of undrained shear strength using different recompression methods.(after Tan et al., 2002b)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60 80 100Plasticity Index (PI)

s u ( f i e l d v a n e

) / p ' c

Singapore (SAC)

Singapore (PT)

Drammen

Ariake

Bangkok

Louiseville

Skempton (1957)

Bjerrum (1973)

Figure 3.11 Undrained shear strength ratio (s u / σvy’) versus plasticity index for different

clays. (after Tan et al. 2002b)

(a) (b)

s u ( f i e l d v a n e

) / σ v y

’




14

16

18

20

22

24

26

28

300 40 80 120 160

Undrained Shear Strength (kN/m 2)

D e p

t h ( m )

UCT (Japan-SAC)

UCT (Local-SAC)

Field Vane (SAC)

UCT (PT)

DST (PT)

Field Vane (PT)

0.21 p' c (SAC)

0.21 p' c (PT)

Figure 3.12 Undrained shear strengths from PT and SAC Sites versus prediction basedon σvy’ (after Tan et al. 2002b)

Figure 3.13 Generallized undrained modulus ratio versus yield stress ratio andplasticity index (after Duncan & Buchignani, 1976)

E u / s u

Yield Stress Ratio, YSR

0.21 σvy’ (PT)

0.21 σvy’ (SAC)

Marine Clay Data

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