CHAPTER 6 SHEAR STRENGTH AND STIFFNESS OF SINGAPORE … 6.pdf · Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________

CHAPTER 6

SHEAR STRENGTH AND STIFFNESS OF

SINGAPORE OLD ALLUVIUM ___________________________________________ 6.1 Introduction Since Old Alluvium is not a uniform soil, it exhibits a wide range of index properties.

The engineering properties of OA were found to be highly variable (PWD, 1976; Dames

& Moore, 1983). Shown in Figure 6-1, the shear strength of OA seems to have little

relation with depth. A review of some basic geotechnical properties of the Old Alluvium

has been presented by Tan et al. (1980). They also reported that there was no consistent

relation between strength and sample depths. For engineering analyses, the material is

usually characterised by Standard Penetration Test (SPT) N values (Orihara and Khoo,

1998, Li & Wong, 2001). Orihara and Khoo (1998) suggested a relationship between the

undrained shear strength and SPT N-values. Their experimental data fell between the

lines of Sus = 4N (kPa) and Sus = 12.5N (kPa), a huge variation. Li and Wong (2001) also

reported such a big variation between Su and SPT N-values. However, using the SPT can

not provide the reasons for such great variability in engineering properties. The objective

of this chapter is to explore reasons for the variability in shear strength of OA, and to

propose a framework for characterization.

155

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ It is known that OA is heterogeneous from two aspects: degree of cemention and

particle size distribution (PSD). Cemented OA shows the behaviour of weak rock and is

not covered in this thesis. Majority of uncemented OA is clayey sand and it is the focus

of this thesis. The behaviour of this material is discussed in this chapter and part of this

chapter is published by Géotechnique (Ni et al., 2004).

6.2 Equivalent Granular Void Ratio, ege 6.2.1 Concept of Granular Void Ratio eg Void ratio, e, defined as volume of voids divided by volume of solids, has long been

thought of as the parameter governing the strength, stiffness and dilatancy behaviour of

soil. However, application of void ratio was not successful for soils which have both fine

and coarse materials e.g. clayey sand, silty sand etc. The difficulty mainly comes as no

consistent relation could be established between strength and void ratio for these mixed

materials. At the same void ratio, samples with identical coarse material but different

fine contents show different behaviour. It is then realized that for such soils, the

undrained shear strength Su at a given confining stress is not only related to e, but also to

fc, the fines content (fc).

At a given confining stress and fixed void ratio e, the relationship between Su and

fc is illustrated in Figure 6-2. It can be seen that Su first decrease with the increase of fine

content, but after a critical value the strength increases. The critical value of fines content

is around fc=20% to fc=30%, and fc below this transition zone is regarded as low fc

content (Thevanayagam and Mohan, 2000).

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One key question is why the Su of mixed soils, unlike clean sand, not only related

with e, but also relates with fc. Mitchell (1976) and Kenny (1977) proposed that for such

mixed soils, fine particles at low fine contents, because of their size, nature, or position,

may not participate in the force transfer mechanism and thus the space they occupied

should be considered as void. This led them to introduce another index known as

granular void ratio (eg), which is computed by considering the fines as voids. Thus, eg is

calculated from the void ratio (e) and percentage of fines content(fc) as follows:

materialcoarseofVolumefinesofvolumevoidsactualofvolumeeg

+=

fcfceeg −

+=

1 (6.1)

Many researchers have used this index to provide a more consistent

characterisation of such mixed materials (Georgiannou et al., 1990, Pitman et al., 1994

and Thevanayagam and Mohan, 2000). One particular aspect of interest and practical

importance is whether the concept of granular void ratio, as currently defined, can

provide a consistent means of characterising the undrained strength of a mixed material

comprising the same type of granular material and different fine materials. This is

particularly important for the characterisation of Singapore OA, which is mostly a natural

sandy deposits with variable quantities and varying types of fines. To arrive at an

informed assessment, data from a number of previous works are reviewed first. The

results from these works are then combined with results of tests conducted specifically

for this research to arrive at a better understanding of the contribution of plastic and non-

plastic fines to the shear strength of mixed soils.

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ 6.2.2 Role of Fines

Georgiannou et al. (1990) investigated undrained behaviour of Ham river sand mixed

with medium plastic kaolin clay. Clayey sand specimens were prepared by sedimentation

of the Ham river sand through a suspension of kaolin. Undrained triaxial tests were

carried out. They found that at a given granular void ratio, as the clay content increased

up to a critical value and the corresponding void ratio decreased, the undrained shear

strength decreased, as shown in Figure 6-3. In their view, clay fraction up to 20% did not

reduce the angle of shearing resistance but the presence of the clay reduced the stability

of the fabric of sand, causing reduction in undrained shear strength. Clearly, in this case,

plastic fines are not simply acting as voids, but worse than voids, and the granular void

ratio alone is not enough to provide a consistent relation.

Thevanayagam and Mohan (2000) carried out laboratory triaxial tests on normally

consolidated clean sand mixed with silica or kaolin. Triaxial samples were prepared at

different granular void ratios with same amount of silica or kaolin fine content, using dry

air pluviation method. Data from some of their tests are plotted in Figure 6-4, which

shows the relationship between undrained shear strength and granular void ratio,

calculated by treating kaolin and silica as voids. Clearly, if both types of fines have the

same effect on the shear strength, then the strength versus granular void ratio relation of

both the fines should be the same, and if their effect is merely acting as void, then they

should also be the same as that of HS. But it can be seen that there are 3 different curves

for the host sand alone (HS), host sand with ground silica (GS) and host sand with kaolin

(KS), though to a large degree, the results for the GS case is nearly the same as that for

the host sand, except at low granular void ratio. But for eg less than 1, the strength for the

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ KS case lies below that for the HS case. This means the presence of kaolin, a plastic fine,

has reduced the shear strength, an observation consistent with the data from Georgiannou

et al. (1990).

On the other hand, Thevanayagam and Mohan’s test data show that generally

sand with silica fines (GS), a non-plastic fine, appears to be much stronger than sand with

kaolin fines (KS), and is a little stronger than the clean host sand line, HS. In their study,

the fines content in both cases was kept constant (10%) and both fines have similar

particle size distribution curves. Thus silica fines are able to contribute more to the

strength of a mixture than kaolin fines.

Pitman et al. (1994) studied the influence of fines on the collapse of loose sands.

Kaolin plastic fines or ground silica non-plastic fines (<74µm) were added to Ottawa

sand. Triaxial tests were conducted on soil samples compacted using moist-tamping.

Soil samples with varying amount of fines were consolidated to the same effective stress

(350kPa) and sheared in undrained condition. Undrained shear strength was defined

according to the quasi-steady state proposed by Ishihara (1993). Data from Pitman et al.

(1994) are plotted in Figure 6-5 in terms of undrained shear strength and granular void

ratio. As can be seen, the shear strength with 20% silica is higher compared with 10%

silica, despite a higher granular void ratio with 20% silica. Clearly, in this case, the silt

size particles of silica are also contributing to the strength, and not acting merely as

voids. What is interesting now is that the plastic fines apparently look like also

contributing to strength – but the important point to note is that the granular void ratio is

very low, and thus very tight packing of the fines can be expected. So, it appears that this

159

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ is also a factor to consider. But as there were no data to compare results at the same

granular void ratio, no more detailed discussion can be made.

Zlatović and Ishihara (1995) studied the influence of non-plastic fines using

Toyoura sand. The silt was made by crushing sand. Samples with different percentage

of such silt were subjected to triaxial undrained shearing, and they concluded that the soil

weakened with an increase in the silt content up to 30%, as shown in Figure 6-6(a).

However, it should be noted that such conclusion was based on comparison at the same

void ratio e, which means that the silt did not contribute to the shear strength as much as

sand. It is more meaningful to compare the mean effective stress ssp′ with granular void

ratio, which is shown in Figure 6-6 (b). Now, at the same eg, the soil strength increases

with an increase in silt content. Therefore, though the silt is not contributing as much as

sand in shear strength, it is not as weak as just acting as voids, but is providing some

beneficial effects to the shear strength.

From the above cases, it is clear that plastic (kaolin) and non-plastic (crushed

silica) fines contribute differently to the strength. For plastic fines, generally, its

contribution is negative which means that they are acting worse than occupying voids,

whereas non-plastic fines, like silica, contribute positively to the shear strength. Thus

granular void ratio, as defined in Equation. 6.1, by itself is not adequate to provide a

consistent framework of strength variability, as it ignores the different ways different

fines contribute to the mechanical properties of a mixed soil. To better understand the

way different fines (plastic or non-plastic) contribute to the strength, a series of tests was

conducted specifically for this study, which will be discussed next.

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ 6.2.3 Experiments on Sand mixed with Kaolin and Silica

Isotropic Consolidated Undrained (CIU) triaxial tests were performed on reconstituted

samples of clean host sand (HS), sand mixed with kaolin (KS) and sand mixed with silica

(SS). To obtain the host sand, OA field sample was washed on a 63µm sieve to remove

silt and clay sized fines. The particle size distribution (PSD) curve of the host sand,

which is made of quartz is shown in Figure 6-7. About 9% kaolin and crushed quartz silt

were added to the host sand to get the KS and SS soil samples respectively. The triaxial

samples were prepared using the moist tamping method to achieve the desired void ratio.

Eighteen triaxial tests were performed, which can be divided into 2 main groups:

the normally consolidated (NC) mixed soil and the over consolidated (OC) mixed soil.

Each group consists of 9 samples, namely 3 HS samples, 3 KS samples and 3 SS

samples. The samples were first saturated using about 300kPa back pressure at an

effective confining stress of 10kPa. The soil sample was considered saturated when the

pore pressure parameter B was equal or more than 0.95. After saturation, the NC samples

were consolidated to an effective confining stress of 215kPa (the confining stress was

selected according to the depth of the host sand sample) and sheared in undrained

condition. The OC samples were first consolidated to an effective confining pressure of

500kPa, then swelled back to an effective confining pressure of 215kPa. Therefore, the

OC soil samples have an over consolidation ratio of 2.3 before shearing. Details of the

18 CIU tests are given in Table 6-1.

Figure 6-8 shows the stress paths and the stress-strain behaviour of the NC group.

The q and refer to deviatoric and mean effective stress respectively whilep′ aε refers to

the axial strain. Generally, the host sand samples are dense and show dilative behaviour.

161

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ The stress paths of KS soils show an ‘elbow’ which suggest a contractive-dilative

behaviour though there is no drop in the q values. In p′ -q plot, all the three different

types of samples reach the same steady (critical) state line (SSL), suggesting that the

presence of small amount of fines does not change the angle of shearing resistance of the

granular component, consistent with the observation made by Georgiannou et al. (1990).

Table 6-1 Details of CIU tests on Sand mixed with Kaolin and Silica

Before Consolidation After Consolidation Steady State Sample

Fine Faction

(%) e eg e eg p' (kPa) q (kPa)

NC-HS01 0 0.750 0.750 0.721 0.721 352.5 489.0 NC-HS02 0 0.700 0.700 0.682 0.682 420.9 619.0 NC-HS03 0 0.650 0.650 0.625 0.625 565.0 837.4 NC-KS01 9 0.591 0.750 0.535 0.687 257.4 358.3 NC-KS02 9 0.545 0.700 0.500 0.648 312.8 436.6 NC-KS03 9 0.500 0.650 0.444 0.587 404.4 568.7 NC-SS01 9 0.591 0.750 0.590 0.747 430.7 628.2 NC-SS02 9 0.545 0.700 0.532 0.684 487.8 718.1 NC-SS03 9 0.500 0.650 0.485 0.632 601.0 915.0 OC-HS01 0 0.750 0.750 0.702 0.702 418.6 602.6 OC-HS02 0 0.700 0.700 0.643 0.643 530.4 796.3 OC-HS03 0 0.650 0.650 0.595 0.595 577.0 885.0 OC-KS01 9 0.591 0.750 0.531 0.682 514.3 748.0 OC-KS02 9 0.545 0.700 0.490 0.637 584.3 866.4 OC-KS03 9 0.500 0.650 0.445 0.588 725.8 1081.0 OC-SS01 9 0.591 0.750 0.581 0.737 501.5 733.7 OC-SS02 9 0.545 0.700 0.530 0.681 539.6 814.5 OC-SS03 9 0.500 0.650 0.480 0.626 701.9 1051.5

The way kaolin and quartz silt contribute to the strength can be clearly seen in

Figure 6-9, in which the steady state q value versus granular void ratio is plotted. At the

same granular void ratio, KS samples have lower shear strength than HS samples. This

162

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ means that presence of kaolin fines has a negative effect, i.e. worse than just acting as

voids. On the other hand, SS samples have higher shear strength than HS samples, and

thus quartz silt contributes positively to the shear strength, and is better than just being

voids.

Figure 6-10 shows the stress paths and the stress-strain behaviour of the OC group.

All samples show dilative behaviour without the ‘elbow’. Compared to the data for NC

mixed soils, the strength of OC-KS samples has increased considerably. Clearly, the

process of over-consolidation has altered the role of plastic kaolin fines in contributing to

the strength of KS samples. Another important observation is that again, all the samples

reach the same steady state line, that is, the angle of shearing resistance is the same in all

these cases. Figure 6-11 shows the steady (critical) state q value versus granular void

ratio for the OC group. Now the OC-KS samples is behaving nearly the same as that of

OC-HS samples, indicating that plastic kaolin fines in over-consolidated samples no

longer have negative contribution to the shear strength, but instead, are acting like voids.

On the other hand, the trend for SS samples still lies above both HS and KS samples,

indicating that quartz silt is not only filling the void but is contributing to the shear

strength.

Comparison of Figure 6-8, 6-9, 6-10 and 6-11 shows that OCR of 2.3 has no

discernable effect on HS samples and only a little effect on SS samples. This means that

stress-strain behaviour of sand and sand with silt are largely controlled by granular void

ratio rather than stress history. However, for mixtures with plastic fines, namely, KS

soils, the stress-strain behaviour and in particular the shear strength was significantly

163

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ improved by over-consolidation. OC-KS samples become more ductile and yet at the

same time, have higher shear strength than NC-KS samples.

6.2.4 Equivalent Granular Void Ratio, ege

Soil particles have specific shapes and hardness related to their mineralogy. Silica quartz

particle are of rotund shape, while most clay minerals form platy particles. The platy

shape of kaolin particles makes them easier to adjust their positions and get out of the

force-carrying skeleton in clayey sand. There are also big differences between minerals

in hardness. Hardness is defined as the resistance of a mineral to scratching and is

usually measured using Moh’s Hardness (Szymański & Szymański, 1989). The Moh’s

Hardness for several minerals is listed in Table 4-2. Common clay minerals like kaolinite

and chlorite are softer than sand minerals like quartz (silica) and feldspar. According to

Cordua (1997), quartz easily scratches calcite, owing to the large difference in hardness.

Similarly quartz will also scratch kaolinite easily.

Table 6-2 Moh’s Hardness for Several Materials

Material Moh's HardnessDiamond 10

Aluminum oxide 9 Quartz (Silica) 7

Feldspar 6 Calcite 3 Gold 2.5~3

Kaolin 2.5 Kaolinite 2~2.5 Chlorite 2

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The concept of granular void ratio for a mixed soil, as given by Equation 6.1, is

based on the idea that coarse grains carry external force by grain clusters formed by

point-to-point contacts, whereas fines (at low fine content) due to their size, shape,

position, or their ability to adjust, may not participate in the force transmission and thus

should be treated as voids. However, examination of published test results and results

from triaxial tests carried out as part of this study leads to the conclusion that this concept

of granular void ratio, whereby both plastic and non-plastic fines are treated as voids does

not provide a consistent explanation to the variability in shear strength of a mixed soil.

What is clear is that for plastic fines, the contribution appears to be a function of

stress history. For example, when it is normally consolidated, the fines are acting worse

than voids, but with over-consolidation and the resulting re-arrangement of clay particles,

the fines no longer affect the strength. This means that clay particles, even if they are in a

force-carrying cluster, cannot adjust their position, due to the fact that both the normal

and shear stresses at the contact points have magnitude much higher than the effective

stress and the very big difference in hardness between sand minerals (quartz and feldspar)

and clay minerals, the clay particles are likely to be scratched and damaged, thus getting

out of the force-carrying skeleton. Because of this, plastic fines generally do not

contribute positively to the mechanical properties. The presence of plastic fines in

between coarse grain may induce some instability and in this case their contribution will

be negative as shown by the data from Georgiannou et al. (1990) and the NC triaxial tests

carried out in this research. On the other hand, non-plastic fines such as silica quartz

generally contribute positively to the strength.

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To be able to provide a coherent way to characterise the strength of a mixed soil,

clearly the way fines affect the strength of the host sand needs to be factored into the

definition of granular void ratio. In Thevanayagam et al. (2002), a new parameter b was

introduced to represent the beneficial secondary cushioning effect of silica silts in silty

sand as follows:

( )( ) fcb

fcbeege −−−+

=111 (6.2)

where b is defined as the portion of fines that contributes to the active inter-grain

contacts. Thevanayagam et al. (2002) proposed that b should be bounded between 0 and

1, that is, . When b is 0, the fines act exactly like voids and when b is 1, the

fines are indistinguishable from the host sand particles.

10 ≤≤ b

The introduction of an additional factor, b, is a recognition that different fines can

contribute differently to the strength, and the proposed void ratio, perhaps best thought of

as an equivalent granular void ratio, is now based on the actual void ratio and the balance

of fines which are acting like voids and not contributing. But as shown earlier, plastic

fines can contribute negatively to the strength. Hence, an important extension to the

proposal by Thevanayagam et al. (2002) is that when used to characterise strength of a

mixed soil with plastic fines, the value of b could also be negative, that is, the range of b

is . On the other hand, for non-plastic fines, the value of b will be within 0≤b 10 ≤≤ b ,

as in Thevanayagam et al. (2002). In this way, the parameter b can be used to account

for the contribution of fines, both plastic and non-plastic.

By assigning different b values, some of the anomalies observed in the previous

discussion can be resolved. If the granular void ratio in Figure 6-4 is re-calculated by

166

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ assigning a b value of -1 for kaolin and 0.2 for ground silica, the results presented in

Figure 6-12 show that almost all the data for HS, KS and GS fall into a very narrow band,

except for 2 points for the case of sand with silica (GS) at large void ratios. These two

points are close to the maximum void ratio (emax=1.000 for this soil) and thus their

structure is unstable and therefore the data may not be reliable. Comparison of Figure 6-

4 and Figure 6-12 reveals the importance of recognizing contributions of kaolin and silica

while using the concept of granular void ratio for shear strength. In Figure 6-4, though

the data of HS and GS agree well, the KS points lie much lower, indicating that the

concept of granular void ratio eg failed to unify sand mixed with different kinds of fines.

In Figure 6-12, by assigning different b values, all three groups are brought together.

For Zlatović & Ishihara’s test data on Toyoura sand, if a b value of 0.25 is

assigned to the silt, all the steady state lines with various percentage of silt, including the

clean sand, now fall within a small band, as shown in Figure 6-13. This is a significant

improvement over the results shown in Figure 6-6 as the data for 30% fines now also fall

within a narrow band together with data for 0%, 5%, 10% and 15%.The same procedure

was also applied to the CIU test data presented in this paper. In the NC group, a value of

-0.8 was assigned to kaolin and 0.7 for quartz silt. In the OC group, the b values for

kaolin and quartz silt were 0 and 0.75 respectively. With this, all the data in Figure 6-9

and Figure 6-11 now fall within a small band as shown in Figure 6-14, implying a

consistent relation between strength and the equivalent granular void ratio given by

Equation 6.2.

Clearly, one issue that needs examination is whether the parameter b is a kind of

“fudge” factor. This would be the case if there is no consistency to the way b behaves. It

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ is clear that for kaolin and other plastic fines, the value of 0≤b , whereas for non-plastic

fines, the value of b is . It is important to note that in every set of data re-

examined, only a single b value was needed for each class of mixed soils, regardless of

the percentage of fines. Coupled with this, if there is a clear rationale as to how b is

varying and reflective of the nature of the mixed soils, then Equation 6.2 can become a

simple and powerful index to describe strength of a mixed material, and also provide a

way to quantify how fines could affect the strength.

10 ≤≤ b

The data examined in this paper suggest that contribution of fines towards the

strength of a mixed-soil depends on two factors, namely how well the fines are confined

in the voids and the relative hardness between the fines and the coarser sand grains. For

quartz silt (silica) particles, the hardness is about the same as the host sand grains. If a

silt particle is confined in a relatively small void, compared to its own size, then the silt

has little freedom to change position and thus will be forced to contribute to the shear

strength. In the extreme, it is effectively a part of the host sand, and b value will be 1.

On the other hand, if the silt particle is in a relatively large void, then that particle has

freedom to change its position and avoid being part of the force-transfer mechanism.

This means that the b parameter is related to the ratio of the void size distribution of the

host sand and the particle size distribution of the silt itself. Both of these are statistical

distributions and may be inconvenient to use to quantify b value. A simpler way is to

look at the mean value of silt size and mean value of voids in host sand. The mean value

of silt size can be represented by d50 of the silt. The mean void size is difficult to obtain

and needs further discussion.

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In a practical sense, it is unrealistic to conduct pore size distribution tests for all

the mixed soils. However, the mean void size of the host sand is certainly related to eg.

The larger the value of eg, the larger is the mean void size. There are also evidences

showing that the mean void size is heavily related to the size of finest particles in a soil

(Åberg, 1992). Therefore, d10 of the host sand may be used as a rough index of the mean

void size. Note that in the determination of permeability for clean sand, such as in

Hazen’s formula, d10 is also used (Craig, 1997). According to this argument, b should be

correlated to the ratio, χ, defined below:

silt50,

hostsand10,

dd

=χ (6.3)

Clearly, the bigger is the value of χ, the more room the fines will have to move

and thus not participate in the force transfer. The expectation in this case is for the b

value to decrease with an increase in χ. For all silty-sands examined in this paper where

such information is available, the value of χ and the corresponding value of b for each

type of mixed soil are summarised in Table 6-3. The results confirms the trend that as χ

increase, b decreases.

Table 6-3 b values with eg and χ

Test b eg χ OA (Author’s test data) 0.7,0.75 0.62~0.75 4.4

Zlatović & Ishihara (1995) 0.25 0.85~1.15 11

Thevanayagam & Mohan, (2000) 0.2 0.69~1.06 15.71

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In the case of a mixed soil involving plastic fines, the issue is a little more

complicated, as in this case, 3 factors may be involved, namely, the relative hardness

between the fines and host sand, the relative size of the fines to that of the void spaces in

the host sand and the potential of platy fines getting in between inter-grain contacts to

reduce the stability and hence the strength. It is clear that because of the large difference

in hardness, plastic fines are unlikely to contribute to the strength. Hence as shown in

this paper, for plastic fines, b value generally does not exceed 0. In the case where the

plastic fines in between host sand particles have not been forced out, plastic fine

contribution is negative as is shown in the tests conducted for this paper where the value

of b is -0.8 when it is NC. However, if either over-consolidation or some other

mechanism changes the structure such that plastic fines are no longer destabilizing the

grain-grain contacts of the host sand, then the b value can be greatly improved, as is the

case for the OC mixed soil. In the case of Thevanayagam & Mohan, (2000) tests,

because of the large eg, and the fact that the soil is not overconsolidated, b value is also -

1.

The physical meaning of negative contribution factor (b<0) for plastic fines is

difficult to understand. One feasible explanation is that the presence of plastic fines in the

mixture made some very unstable packing of sand possible, while the packing is not

possible for clean sands alone. When the mixture is subject to shearing, the packing

collapses and results in a lower shear strength. However, without thorough investigation

and comparison on the structure of normally consolidated sand and clayey sand, this

explanation is at best speculative. Further research is needed to explore this area.

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ 6.3 Characterization of Singapore Old Alluvium Using the

Concept of Equivalent Granular Void ratio, ege

6.3.1 Calculation of ege Based on Composition of OA

Gupta & Pitts (1992) stated that the sand size particles in OA consist of mainly quartz

and feldspar and the matrix is silt and clay, often segregated. The silt fraction is almost

entirely quartz. The clay minerals comprise mainly kaolinite, smectite and others. Thus,

the difference between sand and silt is mainly in size and the shape and hardness is nearly

the same. On the other hand, the difference between silt and clay is in size, shape and

hardness. It seems for OA, it is appropriate to establish a force carrying hierarchy of 3

levels: sand being the highest, silt the middle, and clay the lowest. Such a hierarchy need

to be reflected in the granular void ratio. Taken into consideration of the 3-level hierarchy

and the secondary cushioning effect of silt, Equation (6.2) is further moderated to:

SCbCCaSCbCCaeege )1()1(1

)1()1(−−−−−+−+

= (6.4)

CC: clay content

SC: silt content

a: contribution factor of clay

b: contribution factor of silt

Based on the test results of overconsolidated silty sand mixed with silt and kaolin

(OC-SS and OC-KS group in section 4.2.3) and equation 4.3, parameter a value was

taken to be 0 and b value as 0.75.

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ 6.3.2 Intact Soil Samples

In Chapter 4, it was found that due to sampling disturbance, thick-wall OA samples do

not reflect the true properties. To study the shear strength of OA, undisturbed Mazier soil

samples of Singapore were obtained from Tanah Merah and Kim Chuan test sites, see

Figure 1-1. In Tanah Merah, the samples were collected from boreholes, spanned over

30m distance, along a proposed tunnel route. Along this route, OA is overlain by a recent

deposit called Kallang formation whose thickness varied from 10-20m. At Kim Chuan

test site, soil samples were obtained from 1 borehole specifically carried out for this

research. All the Mazier soil samples were obtained using the triple-tube rotary Mazier

sampler (Appendix A). The sample quality was controlled by limiting protrusion of inner

barrel rotary shoe. At Kim Chuan site, block samples were also taken at 2 meter distance

from the Mazier Borehole. All the soil samples were below the existing water table.

Although the material contains predominantly sand, presence of a small amount of fines

helps to develop suction sufficient to hold a soil sample together. When extracted from

the inner barrel, the soil samples appeared very strong indicating that sufficient suction

was present. Before any detailed laboratory testing, undisturbed samples were classified

using the framework presented in Chapter 3 i.e. they were separated as cemented and

uncemented. PSD curves of these undisturbed samples are shown in Figure 6-15. Since

OA contains silt and clay, the wet sieving method was used. The soil was first washed on

a 0.063mm sieve to remove the fine particles and the fines in water were collected to do

hydrometer tests. PSD of a sample was determined by combining the results of wet

sieving and hydrometer tests.

172


Triaxial isotropic consolidated isotropic (CIU) tests were carried out on these

samples of OA. Details of the triaxial testing program are shown in Table 6-4. TM

Samples are from Tanah Merah are referred to as TM and TM05 contains clay content

higher than 30% and therefore excluded from the analysis. The other OA samples are

from Kim Chuan site, and Mazier samples are referred to as KCHM and Block samples

KCHB. All the soil samples were first saturated at an effective stress of 10kPa, and then

consolidated to an estimated in-situ horizontal stress. The effective in-situ horizontal

stress was taken as 10ds kPa, where ds is the sample depth in metre. In this research

project, OA samples from Tanah Merah were obtained first but no in-situ stress state data

were available at that time. It was then decided to adopt a confining stress which is

isotropic and increases with the sample depth. Later Ko values from Kim Chuan site were

obtained and it was realized the horizontal stress may be greater than the vertical stress.

However, it was decided to continue to do the CIU tests to be consistent with the Tanah

Merah triaxial tests. A difference of 10kPa was maintained during this saturation stage

and the B value was checked. The sample was considered to be saturated when B>0.95.

Soil samples were sheared in strain-controlled mode and the radial pressure was

kept constant during undrained shearing. Fourteen samples contain clay content between

10 ~ 20% and can be classified by the Triangular classification chart as Clayey Sand. The

particle density is measured using the density bottle method. Results vary from 2.62 to

2.67, with an average of 2.65.

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ Table 6-4 Details of CIU Compression Test on Intact OA Samples

Depth Clay Silt Before Consolidation σr’

After Consolidation Steady State

Sample (m) (%) (%) e ege (kPa) e ege

p'us (kPa)

qus (kPa)

ηmax

TM01 16.5~17.5 16.2 5.9 0.500 0.822 170 0.482 0.800 563.9 997.0 2.06 TM02 15.0~16.0 18.0 5.0 0.579 0.955 155 0.563 0.936 237.7 329.7 1.54 TM03 16.5~17.5 16.1 9.2 0.521 0.864 170 0.512 0.853 482.8 738.3 1.77 TM04 18.0~19.0 25.4 5.3 0.477 1.015 185 0.454 0.984 157.9 202.8 1.31 TM06 21.0~21.5 14.5 4.1 0.575 0.864 208 0.550 0.835 293.5 394.4 1.36 TM07 21.5~22.5 16.5 6.1 0.528 0.864 220 0.507 0.838 418.3 608.3 1.50 TM08 18.0~19.0 18.4 5.5 0.537 0.916 185 0.512 0.885 396.0 566.2 1.57 TM09 19.5~20.5 15.3 5.8 0.513 0.817 200 0.495 0.796 581.0 916.6 1.83 TM10 19.5~20.5 16.1 9.6 0.520 0.865 200 0.506 0.848 516.0 745.0 1.66 TM11 25.5~26.5 19.9 4.8 0.535 0.946 260 0.496 0.896 364.4 401.5 1.16 TM12 27.5~27.8 20.4 15.4 0.448 0.912 275 0.420 0.875 481.3 662.5 1.38 TM13 25.5~26.0 18.7 6.6 0.451 0.822 258 0.420 0.783 732.6 1259.8 1.96 TM14 30.0~31.0 10.1 14.0 0.485 0.719 300 0.465 0.696 725.0 1356.8 1.88 TM15 27.0~28.0 22.6 4.3 0.488 0.950 275 0.438 0.884 225.7 252.6 1.15 TM16 27.0~28.0 10.0 13.6 0.548 0.788 275 0.528 0.764 617.0 928.0 1.59

KCM01 2.5~3.5 19.1 10.1 0.470 0.875 30 0.630 1.080 41.0 58.4 1.80 KCM03 6.5~7.5 16.4 6.5 0.442 0.760 70 0.588 0.937 120.9 157.3 1.40 KCM04 8.5~9.5 16.3 12.2 0.390 0.724 90 0.499 0.859 308.6 460.9 1.82 KCM05 10.5~11.5 19.5 10.6 0.360 0.746 110 0.496 0.921 262.2 380.0 1.98 KCM06 12.5~13.5 12.5 7.7 0.476 0.723 130 0.520 0.775 492.2 732.6 1.89 KCM07 14.5~15.5 18.3 4.6 0.359 0.686 150 0.434 0.779 511.1 795.4 1.82 KCM08 18.5~19.5 12.4 9.1 0.383 0.620 190 0.535 0.798 547.7 911.1 1.92 KCB01 11.0~11.3 19.0 7.8 0.549 0.960 110 0.451 0.835 467.3 735.3 1.99 KCB02 11.0~11.3 19.0 7.8 0.516 0.917 110 0.465 0.853 447.4 714.3 1.97 KCB03 15.0~15.3 18.0 5.9 0.548 0.921 150 0.477 0.833 637.2 1013.2 2.21 KCB04 15.0~15.3 18.0 5.9 0.558 0.934 150 0.469 0.823 668.1 1100.2 2.32 KCB05 19.0~19.3 17.2 10.1 0.458 0.816 190 0.393 0.735 828.1 1289.1 2.04 KCB06 19.0~19.3 17.2 10.1 0.446 0.802 190 0.378 0.716 633.4 1045.9 2.07 KCB07 21.0~21.3 15.5 7.9 0.476 0.787 210 0.415 0.713 847.5 1433.5 2.06 KCB08 21.0~21.3 15.5 7.9 0.460 0.768 210 0.421 0.721 706.5 1328.5 2.05 KCB09 21.0~21.3 14.4 6.7 0.538 0.832 210 0.504 0.792 655.5 1078.2 1.87 KCB10 21.0~21.3 14.4 6.7 0.527 0.819 210 0.510 0.799 643.6 1001.7 1.82

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ 6.3.3 Strength of Undisturbed OA

The distinctive feature of this formation is that there is a great scatter in the value of

steady state deviatoric stresses. The lowest value is about 50kPa and the highest is about

1400kPa. The undrained steady state deviatoric stress qus versus both depth and void ratio

e is plotted in Figure 6-16. There is no consistent trend such as increase with

depth/consolidation pressure. Samples from the same depth, hence the same

consolidation pressure, showed significantly different strengths. On the other hand, there

is an approximate relationship between qus and e though the scatter is still very large. At

some depth or e, the difference of qus can be up to 1200kPa. This is because OA is not a

uniform soil, as it was deposited by a braided river system, and the composition of

individual samples need to be considered in understanding the behaviour. For such a

heterogeneous soil with a significant amount of both silt and clay, the modified

equivalent granular void ratio ege (Equation 6.4) must be used.

6.3.4 Stress-strain behaviour of OA It will not be clear to plot all the stress-strain curves of the 32 undisturbed samples.

Therefore, only some typical samples are plotted in Figure 6-17. The undrained

behaviour of undisturbed OA falls into 3 patterns: pattern (1) is contraction and is only

shown by 1 sample (TM04). However, there is no reduction in q value with axial strain,

so no strain-softening behaviour is found. Pattern (2) is contraction followed by dilation,

which is found in 5 samples (TM02, TM11, KCM01, KCM03, KCM05). Again there is

no reduction in q value with axial strain. A close examination found samples which show

pattern (1) and (2) all have equivalent granular void ratio ege values greater than 0.8. The

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ remaining 26 samples, which have ege values lower than 0.8, all show dilative behaviour,

as shown in pattern (3).

Comparison of Figure 2-5 and Figure 6-17 reveals that there is a big difference in

the stress-strain behaviour of clean sand and OA. In clean sand, loose sand is quite brittle

and dense samples reach steady state at higher axial strain than loose sand. In undisturbed

OA, loose samples are quite ductile and dense samples reach steady state at lower axial

strain. This is probably due to the clay in OA samples. If an OA sample has a high ege

value, the clay lies between sands and it probably requires some axial strain to drive the

clay minerals into the voids to establish the sand to sand contacts. If an OA samples has a

low ege value, the clay is already in the voids of sand skeleton, and it requires less axial

strain to establish the sand to sand contacts.

There are great differences of behaviour before steady state among the samples.

Some samples show strong dilation, reaching a η value above 2, while some show

contractive behaviour. However, all the stress ratio values converge as axial strain

increase and it seems that the all the samples will reach the same stress ratio at the steady

state, as will be discussed later. The maximum stress ratio ηmax can be used as an

indication for dilation. Figure 6-18 shows the relationship between ηmax and ege and it

seems ηmax decrease with the increase in equivalent granular void ratio ege defined in

Equation 6.4. The data is scattering considerably because ege cannot account for other

factors such as grading and angularity which will also influence the maximum stress ratio.

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ 6.3.5 Steady state of OA

Figure 6-19(a) shows that all the soil samples reach a steady state line in p’-q plain. It

seems that the all the samples will reach the same stress ratio at the steady state, which

means all the samples share the same friction angle regardless of their different

composition and structure. The result is reasonable because it is obvious that at the steady

state, the initial structure has been destroyed. According to Bolton (1986), the angle of

shearing resistance at critical state is primarily a function of mineralogy. Georgiannou et

al. (1990) found that for clay fraction up to 20%, the clay does not significantly reduce

the angle of shearing resistance of the granular component.

The friction angle of OA at steady state is 39o, which is much higher than clean

sand made of quartz. However, it is similar to the value of 40o measured by Chu et al.

(2003) of OA in triaxial tests. This number also equals to the friction angle of

decomposed granite (39o , Atkinson, 1993). The reason is that the provenance of OA is

the decomposed granite from the north (Tai, 1972) and since the transportation distance

was not far and buried rapidly, the material still contains a significant amount of fresh

feldspar (Gupta et al., 1987), which has a critical state friction angle of 40o.

Figure 6-19(b) shows the relationship between deviatoric stress value and

equivalent granular void ratio ege at steady state. A clear trend can be seen that the qus

decreases with increasing ege and all the data falls into a band. Comparing Figure 6-16(b)

and Figure 6-19(b), it is clear that ege is a better index to use in uncemented OA than void

ratio e. From Figure 6-19, it seems that at the critical state, the OA soil samples will reach

a band in p’-q-ege space, though a unique critical state line doesn’t exist for OA because

of the heterogeneity of OA soil.

177


Some amount of scatter still exists in Figure 6-19(b), which is caused both by

natural property (intrinsic) and experimental errors. For the material itself, it is unlikely

that a unique steady state line exists for OA samples in p’-q-ege space, because the soil is

heterogeneous in PSD. Though the concept of ege can take the influence of fines contents

into account, OA samples with different graded grains (host sand and silt) and different

angularity will still differ in behavour. The clay in natural OA is also more complex than

the kaolin clay used in reconstituted samples; it is a mixture of kaolinite, illite and

smectite. All these contributed to the scattering in Figure 6-19(b). However, the trend in

Figure 6-19 indicates that the ege is the controlling factor and can provide a sensible

relation.

There are other experimental errors that caused the scattering of the data. The

quantity of hard sand minerals (quartz and feldspar) and soft clay-making minerals is not

measured directly; it is still calculated from the PSD curves, assuming that sand-size and

silt-size particles are made of quartz and feldspar, while clay-size particles are made of

clay minerals. Though XRD tests demonstrated this is generally correct (Pitts and Gupta,

1992), some deviation is still expected, which may cause the calculated ege value to

deviate from the truth. The influence of strain rate on shear strength cannot be neglected

either. Increasing the strain rate applied to a saturated soil means larger effective stresses

and consequently greater shear resistance (Richardson and Whitman, 1963; Leroueil. and

Maria Esther Soares, 1996). Therefore, it is important to find the appropriate strain rates

for soil samples. In this research, the strain rates are determined using Head’s method

(1992), which requires consolidation time (t100) and strain to failure (εf). Though t100 for

each sample was measured, the accurate prediction of εf proved to be quite difficult,

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Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ because each OA sample seems to have a different εf due to its heterogeneous nature. The

result is some soil samples are sheared either faster or slower than the appropriate strain

rate, therefore causing some further deviations in the measured shear strength. These

difficulties will continue to arise when characterizing natural heterogeneous soils and

further research is needed to tackle them. Nevertheless, comparing Figure 6-16 and

Figure 6-19 the superiority of using ege is clearly seen. The concept of ege can be used to

predict the shear strength of uncemented OA, a natural heterogeneous sand mixture.

What is particularly satisfying is that comparing Equation 6.4 to Equation 6.2, only 1

additional modification factor is needed for the very different OA samples!

Contribution factor b=0.75 is assigned to silt in OA, which means that silt is

having beneficial effect to the steady state shear strength. This is because the silt in OA is

mainly quartz and feldspar, similar to the mineralogy of sand in OA. Another reason is

the low χ values in OA: the silt particles have little room to move in the pores of hostsand,

and form the force-carrying skeleton together with sand particles.

Contribution factor a=0 is assigned to clay in OA, which means clay in OA is

acting as voids. As mentioned before in this chapter, for a normally consolidated clayey

sand, clay can even have negative contribution factors in some cases (Georgiannou et al.,

1990; Thevanayagam and Mohan, 2000). It seems that the over-consolidation history of

OA improved the behaviour of clay in it.

Various evidences support that Singapore OA has an over-consolidation stress

history (Pitts, 1986; Gupta et al., 1987). This stress history has important effects to OA.

As shown in section 6.2.3, though over-consolidation has almost no effect on sand and

silt, it greatly improved the behaviour of clay in uncemented OA. Without the over-

179

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ consolidation, clay in OA would introduce instability, making OA a very problematic

material.

Since over-consolidation can improve the behaviour of clay in OA, it is very

possible that the contribution factor a in OA increases with OCR. As shown in Chapter 5,

OCR in OA generally decreases with depth. That is to say theoretically for OA samples

from different depths, parameter a should decrease with depth. However, this approach is

not adopted in Figure 6-19(b) for several reasons. First, the relationship of a value with

OCR was not established. Second, the samples shown in Table 6-4 were from different

locations and boreholes so a unique OCR-depth relationship does not exist. Third, if a

changes with individual samples, it would make Equation (6.4) very complicated.

Therefore, a=0 is assigned to all the OA samples regardless of depth. This approach has

the merit of being simple, but may added some scattering to the data in Figure 6-19(b).

The shear strength of OA shows no relationship with depth and for uncemented

OA, the reason lies deeply in the deposition of OA, which is similar to that of sand. Sand

can be deposited at various initial e values at zero stress and in normal stress ranges, it

does not have a unique consolidation line. Like sand, for OA at a certain depth (related to

a certain confining stress state), a range of ege values can exist. It is also possible for a

shallow OA layer to have a lower ege value than a deeper OA layer. The result is the high

variability of OA shear strength with depth.

6.3.6 Stiffness of OA

The undrained, secant Young’s modulus Eus, which is calculated using q over axial strain,

is presented in Table 6-5. The stiffness increases with an increase in confining stress;

180

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ Therefore, Eus obtained at different confining stresses must be normalized for comparison.

Kohata et al. (1997) studied the pressure-level-dependence of elastic Young’s modulus

and concluded that E0 for each type of geomaterial can be expressed as

E0 = Cσ’vm (6.5)

which means that the Young’s modulus E0 is dependant to the power m of the pressure

level. It is also stated that the values of the power m for the granular materials are similar

to each other and nearly equal to 0.5. Hicher (1996) also reported a m value of 0.5 for dry

Hostun sand. In this study, the Eus is normalized using the square root of the confining

stress, as following:

normalized E’us=

Pa

E

r

us

σ (6.6)

in which

σ’r: confining stress

Pa: atmosphere pressure

The normalized Young’s modulus is shown in Figure 6-20. From axial strain 0.2% to 2%,

it is found that the normalized E’us generally decreases with an increase in ege, though

considerable scattering in the data exists. It is known that clayey sand carries load mainly

by the sand skeleton in it. When a load is applied to a soil sample, some axial strain is

necessary to mobilize the skeleton. Judging from the stress-strain behaviour of OA, it

needs some axial strain to establish fully the sand-to-sand contacts and form the skeleton.

Thus, the larger the axial strain, the more dominant is the influence of ege. When the

sample is at the steady state (critical state), the initial fabric is destroyed and the sand-to-

sand contacts are fully established, so the strength is mainly controlled by ege. However,

181


Table 6-5 Stiffness of OA

Depth σr’ After Consolidation E’us,0.2% E’us,0.5% E’us,1.0% E’us,2.0%Sample

(m) (kPa) e ege (MPa) (MPa) (MPa) (MPa) TM01 16.5~17.5 170 0.482 0.800 33.1 37.3 37.1 31.0 TM02 15.0~16.0 155 0.563 0.936 45.5 28.2 16.5 10.5 TM03 16.5~17.5 170 0.512 0.853 41.2 31.2 25.7 20.1 TM04 18.0~19.0 185 0.454 0.984 12.2 11.7 8.3 5.4 TM06 21.0~21.5 208 0.550 0.835 30.0 24.9 18.6 13.5 TM07 21.5~22.5 220 0.507 0.838 51.5 33.2 24.8 18.2 TM08 18.0~19.0 185 0.512 0.885 23.1 23.7 19.7 16.1 TM09 19.5~20.5 200 0.495 0.796 58.1 54.1 50.0 37.7 TM10 19.5~20.5 200 0.506 0.848 50.9 38.8 30.2 25.1 TM11 25.5~26.5 260 0.496 0.896 31.0 21.6 15.9 8.4 TM12 27.5~27.8 275 0.420 0.875 38.0 34.8 28.7 21.4 TM13 25.5~26.0 258 0.420 0.783 94.8 52.7 46.8 35.5 TM14 30.0~31.0 300 0.465 0.696 103.9 88.9 72.6 42.1 TM15 27.0~28.0 275 0.438 0.884 22.9 20.7 14.0 8.4 TM16 27.0~28.0 275 0.528 0.764 77.9 56.2 33.8 21.1

KCM01 2.5~3.5 30 0.630 1.080 3.8 3.1 2.4 1.6 KCM03 6.5~7.5 70 0.588 0.937 13.7 10.3 6.9 4.3 KCM04 8.5~9.5 90 0.499 0.859 11.8 9.0 6.1 3.8 KCM05 10.5~11.5 110 0.496 0.921 45.3 29.7 19.6 11.6 KCM06 12.5~13.5 130 0.520 0.775 48.9 40.4 37.3 30.9 KCM07 14.5~15.5 150 0.434 0.779 14.1 15.4 14.3 11.6 KCM09 18.5~19.5 190 0.535 0.798 29.7 21.6 13.7 8.70 KCB01 11.0~11.3 110 0.451 0.835 66.1 62.7 53.2 36.0 KCB02 11.0~11.3 110 0.465 0.853 52.1 43.6 32.6 25.1 KCB03 15.0~15.3 150 0.477 0.833 51.6 50.7 43.8 36.2 KCB04 15.0~15.3 150 0.469 0.823 70.5 63.0 49.0 40.6 KCB05 19.0~19.3 190 0.393 0.735 78.1 71.0 63.2 50.6 KCB06 19.0~19.3 190 0.378 0.716 73.6 66.3 64.2 50.4 KCB07 21.0~21.3 210 0.415 0.713 158.1 150.7 116.4 69.8 KCB08 21.0~21.3 210 0.421 0.721 97.4 93.7 82.8 61.4 KCB09 21.0~21.3 210 0.504 0.792 40.2 54.5 54.9 45.4 KCB10 21.0~21.3 210 0.510 0.799 63.7 62.0 56.9 45.3

182

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ in Figure 6-20, the stiffness values are measured at axial strains between 0.2% and 2%.

At those strain levels, it is highly possible that the initial fabric is not fully destroyed yet

and a large part of clay between sand particles has not been driven out. Thus, both ege and

the fabric play important parts in the soil stiffness. The clay bridges are not as strong as

sand-to-sand contacts. When the soil is loaded, there are two opposite trends taking place:

one is the loss of strength, due to the large deformation of clay between sand grains;

another is the gain of strength, due to the new sand-to-sand contacts formed. So in terms

of strength, the mobilized shear strength of OA generally increases with axial strain till

failure; however, since axial strain is needed to mobilize the strength, the secant Young’s

modulus decreases with axial strain. Due to the natural heterogeneity of OA, the fabrics

of soil samples are highly variable, thus causing the scattering in the plot of stiffness

versus equivalent granular void ratio ege.

6.3.7 Small Strain Stiffness of OA

In the previous section, test results of stiffness (Secant Young’s modulus) at axial strain

εa greater than 0.2% were presented and it was found at a given εa value in that range, the

stiffness generally increases with the decrease of ege. However, through extensive

geotechnical research, it was now well established that stiffness of soil is non-linear. The

stiffness of soil is relatively large at very small strains (<0.001%), and degrades to a

relatively small value at strains greater than 0.1%. This is one of the important advances

of geotechnical engineering research in the last 20 years (Atkinson, 2000).

As reviewed by Baldi et al. (1988), in case of triaxial tests, at small strains errors

are caused by external measurement of soil samples. The errors are due to seating,

183

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ alignment, bedding, system compliance and nonuniform strains along the specimen.

Local gauges should be used to measure the small strain stiffness of soils. In the present

research, RDP submersible LVDTs similar to the ones used by Cuccovillo & Coop (1997)

are adopted. Details of the LVDTs were given in chapter 3. Two LVDTs (RDP D5/200)

were set up to measure the axial strain and one to measure the radial strain (RDP D5/100).

Since OA contains coarse grains, pins could not be used to fix the LVDTs to the soil

sample; therefore the LVDTs were simply glued to the membrane using instant glue.

Undrained small strain stiffness tests were performed on some Mazier OA

samples and to help comparison, the undrained secant Young’s modulus results in small

strains were also normalized using Equation 6.6.

To understand how sensitive the in-situ OA small strain stiffness is, several

undrained cyclic loading tests were carried out. Samples used were TM07 and TM09 and

details of the samples can be found in Table 6-4. Samples were first loaded to a given

axial strain, then unloaded back to εa=0. The sample was then rested for 2 hours to avoid

the effect of sudden change of loading direction and overestimate the stiffness. The next

loading-unloading cycle began and the sample was loaded to a larger axial strain.

The stiffness curves of these tests are shown in Figure 6-21. It can be seen from

the figure that for both TM07 and TM09, after the first cyclic loading to axial strain

0.07% and 0.05%, there is significant reduction in the small strain stiffness in the next

loading. The E’us value in the small strain range continue to reduce with further cyclic

loading to larger axial strains and the stiffness curves became almost flat after loading to

around 0.2%. It can be seen that the small strain stiffness of intact OA is very sensitive:

loading to axial strain around 0.05% can greatly reduce the E’us value and the original

184

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ shape of the curve can be completely lost after axial strain of around 0.2%. Therefore,

extreme care has to be taken when handling samples for small strain stiffness

measurement and only high quality Mazier and Block samples should be used if such

properties are needed.

Table 6-6 Normalized Small Strain Stiffness Values of OA

Depth σr’ After

Consolidation Normalized Young’s Secant

Stiffness (MPa) Sample (m) (kPa) e ege E’us,0.001% E’us,0.01% E’us,0.1%

KCM01 2.5~3.5 30 0.630 1.08 109.5 109.5 11.0 KCM03 6.5~7.5 70 0.588 0.937 66.9 37.6 29.3 KCM04 8.5~9.5 90 0.499 0.859 379.5 104.4 49.3 KCM05 10.5~11.5 110 0.496 0.921 335.6 99.6 60.8 KCM06 12.5~13.5 130 0.520 0.775 239.4 136.8 79.8 KCM07 14.5~15.5 150 0.434 0.779 489.9 196.0 49.0

Compression

KCM09 18.5~19.5 190 0.535 0.798 689.2 151.6 34.5 KCM03 6.5~7.5 70 0.588 0.937 585.7 83.7 8.4 KCM04 8.5~9.5 90 0.499 0.859 474.3 94.9 9.5 KCM05 10.5~11.5 110 0.496 0.921 157.3 52.4 15.7 KCM06 12.5~13.5 130 0.52 0.775 570.1 125.4 28.5 KCM07 14.5~15.5 150 0.434 0.779 244.9 61.2 24.5

Extension

KCM09 18.5~19.5 190 0.535 0.798 620.3 137.8 30.3

The normalized small strain stiffness values of several Mazier samples in

compression and extension are shown in Table 6-6. The E’us values are plotted versus

equivalent granular void ratio ege in Figure 6-22 and no evidence was found on whether

ege governs the stiffness of OA in small strains or not.

6.3.8 Structure of uncemented OA and its influences

According to Leroueil and Vaughan (1990), structure is common in a wide range of

natural soils. The effect of structure is as important in the determination of engineering

behaviour as are the effects of initial porosity and stress history.

185


Research on the structure of uncemented OA was carried out with the help from

Professor Jacque Locat, Laval University, using SEM and thin section pictures. At the in-

situ state, as shown in Figure 6-23, clay exists between sand particles and forms bridges

between them. Thin section picture confirmed this (Figure 6-24) and it can be seen there

is iron oxide (red in the picture) in the clay matrix. The iron oxide may provide some

weak bonding between clay matrix and sand particles.

In the uncemented OA type, structure is mainly in the form of packing, which is

the particular way how the sand, silt, clay bridges and pores are arranged to form the soil.

Structure in uncemented OA may also include some weak bonding between clay and

sand particles, through the iron oxide shown in Figure 6-24. The soil can be seen as a

very complex system made of sand, silt, clay bridges, weak bonding and pores, as shown

in Figure 6-25(a). When force is applied, since the clay bridges are not strong enough to

bear such load (due to small particle size and lower hardness in mineralogy), they start to

give way and move into the pores. Thus, new sand-to-sand contacts are formed, as shown

in Figure 6-25(b). It needs relatively large deformation to drive all the clay into the pores

and fully establish the force-carrying skeleton. Therefore, uncemented OA is generally a

ductile material, reaching critical state at large axial strains.

At the steady state (critical state) large axial strain has occurred and it is unlikely

that the initial structure (packing) of the soil still persists after soil particles has moved,

rotated and reorganized. The in-situ weak bonding between clay matrix and sand particles

probably has also been destroyed. This is reflected by the fact that though the shear

strength of samples varies widely, the effective critical state friction angle is practically

constant. Thus, the shear strength is governed primarily by how ‘dense’ the force-

186

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ carrying skeleton is, which can be quantified by ege and the initial structure seems to have

no influence on the shear strength of uncemented OA.

The undrained shear strength of OA correlates strongly with ege and seems to be

insensitive to the consolidation pressure, which is consistent with the shearing behaviour

of sand. If sand is sheared in a drained condition, then the effective confining stress is

constant and the void ratio e of sand sample will change, and the critical shear strength is

governed by the effective confining stress. However, if sand is sheared in an undrained

condition, then the void ratio e will not change. The pore pressure in the sample will

increase or decrease, resulting in a change in effective confining stress. As shown in

Figure 2-6, Ishihara (1993) tested sand consolidated at different stress levels but nearly

the same void ratio in undrained condition. The conclusion was the undrained shear

strength at steady state (Sus), is determined by the void ratio e alone. For sand subjected to

undrained shearing, the pore water pressure increases or decreases, depending on the

initial confining stress, so as to bring the effective confining stress to a unique value

inherent to that void ratio.

The granular void ratio equivalent, ege, which is given by Equation 6.4 and takes

into account differing ways fines to contribute to shear strength, is found to be a better

index than void ratio e or the classic granular void ratio eg (Mitchell, 1976; Kenny, 1977)

to characterize this skeleton and determines the undrained shear strength at critical state.

However, from Figure 6-25 it is obvious that when the soil is first loaded in the

small strain range (εa<0.1%), the uncemented OA withstand the load through the sand

contacts, clay bridges, and weak bonding between sand and clay matrix formed in-situ.

Thus, the small strain stiffness is strongly affected by the in-situ structure and not

187

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ governed by ege alone. Due to the fact that it is very difficult to quantify the effect of in-

situ structure, characterization of OA small strain stiffness remains to be further

investigated.

In the cyclic loading tests shown in Figure 6-21, it is known that even loading to a

small εa value around 0.05% followed by unloading will result in great loss in small

strain stiffness of OA. For a soil sample 100mm high, 0.05% axial strain equals to

0.05mm displacement. Since in such a small εa cycle the relative position (packing) of the

sand particles and clay bridges remains unchanged, the most possible reason of

disturbance is the loss of weak bonding between clay matrix and sand particles.

6.3.9 Structure of cemented OA and its influences

Though the main focus of this thesis is on uncemented OA, some research was also done

on cemented OA, which includes triaxial compression and SEM tests. Structure of

cemented OA is mainly in the form of cementing and the effect of structure governs the

shearing behaviour.

Figure 6-26 shows an SEM of a strongly cemented OA sample. In this sample,

clay coats the entire sand grain and the clay fabric is well ordered, lie along the sand-to-

sand direction. Thus, when the sample is sheared, the cemented clay carries the force

together with the sands and the sample appears to be stiff. The failure mode of the sample

is by rupture failure along a surface across such cemented clay fabric. Thus, the sample

shows the behaviour pattern in Figure 6-27: the stiffness is high and the sample reaches

peak shear strength at small axial strain, followed by rupture failure and a rapid loss of

188

Engineering Properties of Singapore Old Alluvium Chapter 6 – Shear Strength and Stiffness of Singapore Old Alluvium ______________________________________________________________________________________ strength. The shear strength of such samples is primarily governed by the cementing and

the concept of granular void ratio equivalent does not apply to such soils.

6.4 Summary

In this chapter, the shear strength of intact Singapore OA was investigated and found to

be highly variable with no consistent relationship with depth or void ratio e. The reason is

due to the heterogeneous nature of the soil. For uncemented OA, the heterogeneity comes

from highly different PSD curves. A new concept of equivalent granular void ratio, ege,

was successfully applied to unify the shear behaviour of uncemented OA. Undrained CIU

tests were performed on both reconstituted and intact samples. The stiffness and small

strain stiffness of OA was also examined. It is concluded that:

1) For sand mixed with fines, void ratio e is no longer the governing factor of

shearing behaviour.

2) The concept of granular void ratio eg is better than e but eg does not differentiate

contribution of plastic and non-plastic fines, and of the relative size of fines to the

pore size.

3) The equivalent granular void ratio ege, allows for different contribution factors to

be assigned to account for the different fines and relative size. For normal

consolidated sand mixture, the contribution of plastic fines is generally negative

and at best, acts like voids. Forr a clayey sand with plastic fines, over-

consolidation will force the clay minerals out of sand-to-sand contact and

generally prevent the fines to destabilize the structure and to cause the negative

contribution.

189


4) For non-plastic fines, the contribution is generally positive and at worst act like

voids. The contribution factor, b, has a physical basis as it reflects the relative size

of the pore size to the non-plastic fines. The higher the ratio, that is big pores with

small fines particles, the more likely the fines will not contribute to the strength or

the smaller is the contribution factor. This is reflected by Equation 6.3 and the

trend is tentatively observed in Table 6-3. Over-consolidation has little effect on

non-plastic fines.

5) The concept of ege was successfully applied to intact Singapore OA. Comparing to

void ratio e and granular void ratio eg, equivalent granular void ratio ege is

superior in reducing the data scattering. OA samples fall in a narrow band in p’-q-

ege space.

6) Normalized Young’s modulus of OA generally decreases with the increase of ege.

7) Structure (packing and fabric) of uncemented OA has strong influence on the

small strain stiffness of OA. The relationship of small strain stiffness and the

equivalent granular void ratio ege is not clearly understood.

8) The shearing behaviour of cemented OA is different from uncemented OA and

the concept of ege does not apply to cemented OA.

190


Figure 6-1 Data of early study on OA shear strength and depth (after Dames & Moore,

1983)

Su

Transition Zone

fc

Figure 6-2 Relationship of shear strength Su and fine content fc of sand with fines, at a

given confining pressure and a certain void ratio e

191


Figure 6-3 Shear strength at quasi-steady state of clayey sand, at nearly the same eg but varying clay contents (data from Georgiannou et al., 1990)

Figure 6-4 Sus vs. eg for silty sands with different fines (data from Thevanayagam & Mohan, 2000)

192


Figure 6-5 Shear strength at the quasi-steady state (Su, qss) vs granular void ratio (eg) for various percentage of kaolinite and crushed silica fines (data from Pitman et al., 1994)

Figure 6-6 Steady state lines of Toyoura sand with silt (a) pss’-e plot (b) pss’-eg plot (data adopted from Zlatović and Ishihara, 1995)

193


Figure 6-7 PSD curve of the host sand

Figure 6-8 Stress path and Stress-Strain behaviour of NC samples (a) p’-q plot (b) εa-q plot

194


Figure 6-9 Steady state qus vs. eg plot of NC samples

Figure 6-10 Stress path and Stress-Strain behaviour of OC samples (a) p’-q plot (b) εa-q plot

195


Figure 6-11 Steady state qus vs. eg plot of OC samples

Figure 6-12 Shear strength versus granular void ratio for silty sands with different

fines, b=-1 for kaolin and b=0.2 for silica (data from Thevanayagam and Mohan, 2000)

196


Figure 6-13 Steady state lines of Toyoura sand with silt pss’-eg plot with eg calculated assigning b=0.25

Figure 6-14 Strength versus granular void ratio equivalent, b=-0.8 for NC-KS, b=0 for OC-KS, b=0.7 for NC-SS, b=0.75 for OC-SS

197


Figure 6-15 PSD curves of Singapore OA samples (a) Tanah Merah (b) Kim Chuan

Figure 6-16 Deviator stress at steady sate of TM undisturbed OA samples (a) qus vs. depth (b) qus vs. e after Consolidation

198


Figure 6-17 Typical undrained behaviour of undisturbed OA samples (a) p’-q plot (b)

εa-q plot

Figure 6-18 Maximum stress ratio ηmax vs. equivalent granular void ratio ege

199


Figure 6-19 Steady state band in p’-q-eg space (a) qus vs. p’us (b) qus vs. ege

200


Figure 6-20 Stiffness of OA (a) at 0.2% axial strain (b) at 0.5% axial strain (c) at 1% axial strain (d) at 2% axial strain

201


Figure 6-21 Small strain stiffness of OA in cyclic loading (a) TM07 (b) TM09

Figure 6-22 Small strain stiffness of OA (a) in compression (b) in extension

202


Sand Sand

Clay Bridge

Figure 6-23 SEM picture of a uncemented OA (picture provided by Professor Jacque

Locat, Laval University)

Quartz

Feldspar

Clay

Iron Oxide

Quartz

Quartz

Figure 6-24 Thin section picture of an uncemented OA sample from Changi (Area:

1.325mmx72 mm, picture provided by Professor Jacque Locat, Laval University)

203


force

sand grain

clay bridge

pore sand to sand contact

silt

(a) (b) Figure 6-25 Sketch of particle movement in uncemented OA: (a) initial state (b) upon

shearing

Sand

Cemented Clay

Sand

Figure 6-26 SEM picture of a strongly cemented OA

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0 10 20 3εa (%)

00

200

400

600

800

1000

q' (k

Pa)

Cemented OA

Uncemented dense OA

Uncemented loose OA

Figure 6-27 Typical undrained shearing behaviour of cemented and uncemented OA

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