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CHAPTER 5

In-Situ Evaluation of Initial State of Lumpy Fill

5.1 Introduction

When clay lumps are used for reclamation, the fill form will have large inter-

lump voids initially. If these large voids do not close completely, even with surcharge,

this will lead to serious long-term settlement when a structure is constructed on it.

Therefore, it is important to understand the behavior of such a fill at different stages of

construction. Laboratory modeling (small scale and centrifuge) of reclamation process

using small clay lumps could achieve only limited success due to scale effects and also

cannot replicate the heterogeneity of the problem. One key and difficult to overcome

problem is the rapid softening of small clay lumps used in the laboratory experiments,

and also the contact stresses generated on small sized lumps for a given surcharge, are

higher than in the field (Tan, 2002). Therefore, in-situ evaluation is needed to

complement these small scale tests. A major site characterization program was carried

out as part of a research project. Field pilot tests are conducted in an ongoing

reclamation site at Pulau Tekong in Singapore to provide a basic understanding of the

state of lumpy fills at different stages during land reclamation works.

This chapter summarizes the results of an investigation from one such pilot

field test involving the use of dredged clay lumps and sand surcharge as fill materials

at this reclamation site. The Nuclear-Density cone penetration test (ND-CPT) is

employed to measure the continuous changes in density in addition to other usual cone

parameters. The ND-CPTs were conducted in two stages namely just after the lumpy

fill is formed prior to any soil treatment, and then after a sand cap has been placed on

top of the lumpy fill. A third stage is planned, but this will be after this thesis work,

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namely the state of the same fill after completion of consolidation with the aid of

vertical drain. However, for this final state, another site is used and this will be

reported in Chapter 6.

5.2 Site Description

The project site is located close to Changi in the northeastern part of Singapore

Island, as shown in Figure 5.1. The project site is approximately 5 nautical miles in the

longitudinal direction and 6.5 nautical miles in the latitudinal direction. Nautical charts

of the project area indicate wide ranging seabed elevations varying from 1.5m above

Chart Datum (CD) in the tidal flats and coastal shallows, to greater than 20m below the

Chart Datum (CD).

5.3 Geotechnical Characteristics of seabed soils

The economical and technical feasibility of any reclamation scheme will be

largely influenced by the availability of the fill materials and its characteristics. In part

of the project, dredged clay lumps obtained from excavations of seabed soils for the

construction of sand-keys and sand-bunds along the periphery of the land reclamation

project itself will be used back as fill. These subsoil conditions are important to decide

the dredging level for the construction of sand-keys and sand-bunds along the

periphery of the land reclamation projects. At the same time, the geotechnical

characteristics of seabed soils are needed before dredging to understand the nature and

behavior when they are dredged and used as fill. This is because it is difficult to

characterize the fill materials after dredging. Therefore, an extensive site investigation

was carried out as part of this project to investigate the geotechnical characteristics of

seabed soils. Marine boreholes, piezocone soundings, field vane shear tests and

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standard penetration tests were conducted around Pulau Tekong in the initial phase of

this mega reclamation project. However, all the data were collected by commercial

companies. A detailed characterization of Singapore marine clay at this site is also

given in Tan et al. (2002) and Low (2004). Thus as part of this thesis, the results from

the above site investigation will be used to review the geotechnical characteristics of

seabed soils in the following section.

5.3.1 Geological Profile

A desk study was first conducted on existing marine boreholes, which had been

carried out earlier by other parties and the geological profile of the underlying soil was

then derived. The preliminary site investigation and geophysical survey revealed that

the seabed soil at this site consists of four layers locally known as the surface soft

marine clay (SSMC) of recent deposits; upper marine clay (UMC), intermediate layer

(IML) and lower marine clay (LMC) followed by the residual soil or weathered

granite. A typical geological profile of the seabed soils at the project site is shown in

Figure 5.2. The borehole data revealed that the intermediate layer contains fragmented

shells and decomposed wood, which forms a heterogeneous layer. This intermediate

layer is now believed to be the desiccated top of the lower marine clay.

5.3.2 Physical Properties

The grain-size composition and index properties with depth are shown in

Figure 5.3. In the upper 4m of the sediment, the natural water content is greater than

the liquid limit, which results in a liquidity index greater than unity. Thus, the

sediment can be considered as a recent deposit lying on top of the upper marine clay

(UMC) layer and is given the name as surface soft marine clay (SSMC). Typically, the

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thickness of the upper marine clay layer which follows the SSMC varies from 8 m to

10 m. The range of index properties for different clay layers is similar to that of other

near shore areas in the territory of Singapore. The upper marine clay is usually very

soft with its natural water content in the range of 80 - 100 %. The liquid limit and

plastic limit are in the range of 80 - 110% and 30 - 60% respectively. Below this soft

upper marine clay layer is the intermediate heterogeneous layer, which is of medium

stiff-to-stiff clay layer of thickness of about 10-15m. The lower marine clay layer has

natural water content in the range of 50 – 60 %, liquid limit of 65 - 85 % and plastic

limit of 30 – 60 %. The Singapore marine clay also showed a wide range in swelling

potential, as shown in Figure 5.4(d).

5.3.3 Undrained Shear Strength Characteristics

Figure 5.4(a) shows the undrained shear strength profiles measured by

laboratory and field tests. Interpretation of the results indicates that a trend of

increasing undrained shear strength with increase in depth. The surface soft marine

clay (SSMC) is extremely soft and the upper marine clay (UMC) is usually very soft to

soft with its undrained shear strength between 10 to 30 kPa. Below this soft upper

marine clay layer is intermediate layer (IML), which is dense sandy silt or medium

stiff to stiff clay layer. This stiff intermediate clay layer is currently believed to be a

desiccated crust of the lower marine clay subjected to varying degree of weathering.

The other deposit, the LMC, is stiff to hard clay, with undrained shear strengths in the

range of 60 kPa – 100 kPa.

Lunne et al. (1997) suggested that the undrained shear strength can be

calculated from CPTU results from the following equation:

qt =su Nkt + σv (5.1)

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where qt is the corrected cone resistance, su is undrained strength, Nkt is the cone factor

and σv is the in-situ overburden total pressure. The cone factor was established by

comparing the undrained shear strength measured from UU tests and UCC test with

nearby CPTs from this project site and it showed that the range of values of the cone

factor, Nkt for seabed soils was found to vary between 8 and 12, with an average of

around 10, as shown in Figure 5.4(a). Studies carried out by Dobbie (1988) in

Singapore marine clay also showed that the Nkt ranged from 9 to 12 with an average of

about 10.5. Tanaka et al. (2001) also obtained the cone factor Nkt of 10 for Singapore

marine clay.

In addition, the properties of undrained remoulded strength were also

estimated. Wroth & Wood (1978) proposed the following unique relationship between

liquidity index and remoulded strength of soil.

Sur = 170 e-4.6LI (5.2)

where Sur is the remoulded undrained shear strength and LI is the liquidity index. The

remoulded strength represents the lower bound on undrained shear strength of the

material. Terzaghi (1944) has defined the sensitivity of clays as the ratio between the

undisturbed strength to remoulded strength at the same water content and therefore, it

is a measure of strength upon disturbance. The sensitivity of the seabed soils is

estimated and shown in Figure 5.4(b). Based on the sensitivity classification suggested

by Skempton and Northey (1952), the Singapore marine clay can be considered as

medium sensitive to sensitive clay. From the dredging point of view, the remoulding

loss is important and estimated, as shown in Figure 5.4(c). The remoulding loss of the

seabed of soils is about 80-90% at shallow depths and reduces to about 60 – 70% at

deeper depths. The soil structure of the seabed clay that has originally developed at the

seabed may be disturbed in the dredging and transporting processes. The state of the

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seabed soils immediately after dredging needs to be examined and discussed in the

subsequent section.

5.3.4 Consolidation Characteristics

The variations of the preconsolidation pressure and OCR profiles with depth

are shown in Figure 5.5(a) and (b). The value of preconsolidation pressure was

determined from the oedometer test results using Casagrande procedure (Casagrande,

1936). A line representing the present vertical effective stress is also shown in Figure

5.5(a). As can be seen in Figure 5.5(b), the OCR is relatively high near the seabed, and

is seen to decrease with depth approximately from between 2 and 6 just beneath the

seabed to between 2 and 3 at the end of the lower marine clay. The upper marine clay

was moderately to heavily overconsolidated with an overconsolidation ratio (OCR) of

2 to 6, the lower marine clay were lightly overconsolidated, with an OCR of 2 to 3.

The variation of compression index and swelling index with depth are also shown in

Figure 5.5(c) and (d).

5.4 State of Fill Materials after Dredging

Engineering properties of dredged material vary not only by in-situ geographic

location but also with type of dredger used. The type of dredger used will be dictated

primarily by the properties and quantities of geotechnical materials to be excavated

(Leussen et al. 1984), although other factors such as dredger availability and

production rate, dredging and disposal site locations and conditions also matter.

Poindexter-Rollings (1994) have also reported the sediment type and conditions

associated with dredger type and its variations in the dredged material volume over

time for selected sediments that were hydraulically dredged. Katagiri et al. (2000)

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studied the change of geotechnical characteristics of clay lumps from dredging to

reclamation. They also found that the degree of disturbance of dredged clay lumps

depends on the dredging procedure and original properties of the deposit. Bo et al.

(2001) also compared the geotechnical characteristics of dredged materials at a borrow

area and after placement of dredged materials in a reclamation pond. They reported

that the geotechnical properties of dredged big clay lumps do not change very much

due to dredging, transportation and dumping.

In the present project reported in this thesis, the seabed is dredged using very

large clam-shell grab, with capacity ranging from 4m3 to 25m3. The seabed soils are

disturbed during dredging, but the degree of disturbance depends on the state of

original soil properties and dredging procedures. The geotechnical properties obtained

from the site investigation revealed that the surface soft marine clay (SSMC) layer is a

very soft and recent deposit. Visual observations made during the dredging process of

the SSMC revealed that the weak structure of the soil that had developed over time

was completely destroyed by the dredging procedure and existed in the form of slurry

after dredging. At some places, where the dredging of SSMC produced big soft clay

lumps, these lumps deformed completely during subsequent dumping with other

dredged materials in the barge, as shown in Figure 5.6(a). When these dredged

materials are dumped into a reclamation pond, their water content increases largely due

to mixing with water and some degree of particulate segregation also occur in the

disposal pond.

On the other hand, UMC is generally soft, IML is medium stiff to stiff and

highly heterogeneous and LMC is medium stiff. After dredging, UMC and LMC exist

in the form of clay lumps while for the case of IML, if the sand and silt contents are

high then lumps could not be formed. Typical state of the clay lumps obtained from

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UMC, IML and LMC immediately after dredging is shown in Figures 5.6(b), (c) and

(d). The dredging of these materials produces stiff clay lumps of varying size and

shapes but in general, these lumps are usually quite big (1m3 – 8m3 in volume), as

shown in Figures 5.6 (b), (c) and (d). At the same time, these dredged clay lumps when

placed in the reclamation pond, will undergo three-dimensional swelling due to high

suction developed when dredged and the subsequent availability of seawater all around

(Robinson et al. 2004). However, it is still not understood how the geotechnical

characteristics of these stiff clay lumps will change after dumping in the reclamation

pond. The in-situ state of these dumped dredged materials needs to be evaluated and is

discussed in the subsequent sections.

5.5 Method of Reclamation using big clay lumps

As stated earlier, large dredged clay lumps of up to 8m3 in volume were

excavated along the periphery of the reclamation pond using clam-shell grab. A typical

clay lump dredged from the seabed was shown earlier in Figure 1.5. These dredged

clay lumps were placed in bottom-opening barges, as shown in Figure 1.6 and

transported to a reclaimed site, where the lumps were discharged on to the seabed, as

shown in Figure 5.7. After the lumps had been dumped to a desired depth (-3mCD),

usually limited by the draught of the vessels used for transportation, sand was used to

cap the dredged clay lumpy fills, and also for subsequent filling above the sea level to

provide the surcharge to accelerate settlement. The capping sand was placed in several

lifts up to + 4mCD.

Figure 5.8 shows a schematic section of the reclamation site with dredged clay

lumps and sand surcharge. During initial stages of reclamation, there would be large

voids between the big dredged clay lumps and these inter-lump voids (void between

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lumps) could be partly filled with water / or slurry and small clay lumps. As a result of

consolidation and compression from the sand surcharge, average density and void ratio

of lumpy fills constructed using dredged clay lumps change continuously. If the inter-

lump voids (void between lumps) reduce to the size of intra-lump voids (void within

the lumps), then the lumpy fill is considered to have been “homogenized” that is, no

more inter-lump structure. From a construction perspective, the initial state of the

lumpy fill needs to be established before a contract can be called for the topping up.

Subsequently, the change in the state of the ground in particular the final state also

needs to be ascertained. This is clearly a major challenge and the results presented here

is a first step towards achieving this understanding.

5.6 In-Situ Evaluation of Initial State of Lumpy Fill

As part of the Pulau Tekong project carried out by the Housing and

Development Board (SURBANA), 3 field pilot test areas were created, TA 1, TA 2,

and TA 3 as shown earlier in Figure 5.1. In the following section, the results of an

investigation from one such pilot field test involving the use of dredged clay lumps and

sand surcharge as fill materials will be discussed.

5.6.1 Site Investigation Scope

A ND-CPT was used in a series of site investigations to evaluate the state of

the lumpy fill at two different stages. The first stage is immediately after placing the

dredged materials in the reclamation area up to -3 mCD (Stage 1) and the second stage

is immediately after dumping the capping sand, up to +4 mCD (Stage 2). A third stage

is also planned and this will be after the end of consolidation and is going to be after

the completion of this thesis work.

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In Stage 1, as the surface of the fill is below sea level, ND-CPTs were carried

out using a Marine Cone Penetration System as shown in Figure 5.9. The control unit

and the hydraulic power unit were placed on the barge. A hydraulic penetration

system, which was mounted on seabed rig or steel frame/ tower, was lowered on to the

surface. The reaction force was provided by the dead weight of the steel tower. The

barge is equipped with a four-point anchor mooring system to secure the barge in

position and to maintain its stability during operations. In Stage 2, land is already

formed above the sea level using sand to cap the lumpy fill and a conventional CPT

crawler was used to carry out the ND-CPTs, as shown in Figure 5.10.

5.6.2 Field Investigation Layout

In this study, the results of an investigation from a field pilot test (TA 2) will be

discussed in greater detail. This is one of 3 planned field pilot test areas involving the

use of dredged clay lumps and sand surcharge as fill materials. The pilot test area (TA

2) is 100m long and 100m wide. The clay lumps were placed directly on the seabed

which is about -13 to -14mCD from bottom-opening barges to form an 8m to 9m thick

lumpy fill layer from June to July 2002. A total of 120 numbers of marine ND-CPTs

were conducted from August to November 2002 (Stage 1). Subsequently, 7m of sand

was placed in several lifts up to +4mCD over a period of 10 months. 120 numbers of

ND-CPTs were conducted at the same locations as in Stage 1 from December 2003 to

May 2004 (Stage 2). The Stage 2 ND-CPTs were established in accordance with the

built–up coordinates of Stage 1. However, it is impossible to make sure that the ND-

CPT is carried out in exactly the same locations as in Stage 1, because of all inevitable

surveying setting-out error of about ± 0.5m. While such a small deviation usually does

not pose any problem in a site investigation, in a lumpy fill, this could be a problem.

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The investigation plan carried out at TA 2 for Stage 1 and Stage 2 is shown in

Figure 5.11, which consists of a big grid and a small dense grid. The big grid is 99m x

100m and the small grid is 10m x 11m. The small dense grid is located at the centre of

the big grid, which is highlighted in Figure 5.11 as ‘A’. The big grid consists of 110

ND-CPTs and the small dense grid has 10 ND-CPTs, which are very closely spaced, as

shown in Figure 5.11. The present test arrangement was designed specially to provide

adequate information to characterize a representative volume of lumpy fill and also

sufficient idea of the variation.

5.6.3 Need of ND-CPT in Characterization of a Lumpy Fill

As discussed previously, when the dredged clay lumps were used for land

reclamation, initially, there would be large voids between the big dredged clay lumps

and these inter-lump voids could partly be filled with small lumps and / or water. Now,

when the cone penetrates into this complicated soil, a number of probable soil profiles

can result. Typical cone resistance, pore pressure and wet density profiles obtained

using ND-CPTs at locations RI 111B, RI 104, RI 13 and 101B are shown in Figures

5.12 to 5.15. The changes in wet density, cone resistance and pore pressure together

can be employed to identify the various sizes of clay lumps and inter-lump voids, as

shown with horizontal lines in Figures 5.12 to 5.15. These figures show that there are

appreciable changes in properties, confirming the heterogeneity of a lumpy fill. From

the density profile in Figure 5.12, two significant spikes could be observed at -7mCD

and -11mCD. However, cone resistance and pore pressure profiles shows that there is

only one spike at -7mCD. As concluded previously in Chapter 4, in a ND-CPT, the

maximum radius of the influence zone or measuring sphere about the radioactive

source is about 23.6 cm radius, and thus the density measured reflects the average

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around the central point of the radioactive source and detector configuration, in

contrast with the essentially point-wise measurement in cone resistance and pore

pressure. Thus Figure 5.12 suggests that this particular ND-CPT penetrates near to the

edge of a clay lump from -8mCD to -12mCD, this is distinguished by the high average

wet density and very low cone resistance. However, from -6mCD to -7mCD, it is

penetrating into the interior of a clay lump, thereby measuring a high wet density and

very high cone resistance at this depth. This vividly shows that using the cone

resistance and pore pressure alone would have not picked up this detail.

At the same time, if there is no density measurement, then it is possible to infer

from the usual cone parameter profiles that at this depth, the cone has penetrated only

soft slurry in the void and this will mislead the interpretation of the actual ground.

Similar observation could also be made in Figure 5.13 and 5.14. The wet density, cone

resistance and pore pressure profiles measured by ND-CPTs at these locations are

quite similar to the profile described earlier. Figure 5.15 shows typical results obtained

by the ND-CPT at location RI 101B. This figure shows that there are significant spikes

indicating the presence of lumps, as reflected by the combination of density and usual

cone parameters. Therefore, for a highly heterogeneous lumpy fill site, the addition of

one more parameter, the wet density, helps tremendously in the interpretation of

results.

5.7 Problems related in marine based investigation (Stage 1)

Problems in marine based ND-CPT investigation comes mainly from the fact

that the current design of the ND-CPT requires two probings for each measuring point,

to obtain first the background count due to naturally occurring gamma photons, and

then to measure the actual nuclear density (RI) count, so as to obtain the gamma-ray

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emitting from the gamma-source and scattered in the soil medium. The background

count is measured using a dummy cone, in which only the detector is placed to

measure the naturally occurring gamma photons. As already discussed in Chapter 4,

the intensity of background count measured at a point varies greatly depending on soil

type, mineral makeup, density of materials and its geological history. In a rough way,

these profiles could possibly be used for a very preliminary classification of the soil

type when other more decisive information is not available.

Figure 5.16 shows typical background count profiles obtained for

homogeneous soils in Japan. It can be seen in these background count profiles that

there are little fluctuations with depth. Hence, if the soil is homogeneous and the

variation in the background count profile is negligible within a test site, then double

probing is not required for every single measuring point. Instead, only the RI count

needs to be measured at each point and an average background count within the test

site is then used to determine the wet density of soil. This is especially useful in marine

measurement where every probing is time consuming, and there will be great saving in

cost if the total number of probings can be reduced.

Figure 5.17 shows typical RI and background count profiles obtained for the

lumpy fill site at locations RI 45, RI 111, RI 115, RI 116, RI 118 and RI 120, which

are in the small dense grid, shown in Figure 5.11. Although all the six ND-CPTs are

very closely spaced within a 5.0m x 5.5m square grid, significant variation in RI and

background count profiles can still be observed, suggesting that the lumpy fill is highly

heterogeneous. With this degree of variation in the background count, the use of an

average background count profile within the test site needs to be examined. Figure

5.17(b) also shows an average background count profile obtained for the above six

profiles. If an average background count profile is used to determine the wet density,

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then, an error of about 10% is observed in the estimated wet density as shown in

Figure 5.18 for locations RI 45 and RI 115. Therefore, for highly heterogeneous lumpy

fill sites, two probings are required at every single measuring point.

But in a marine based investigation (Stage 1), it is very difficult to ensure that

the two probes are pushed through exactly the same hole due to operational difficulties

unless divers are used, which is economically not viable. The variation in built-in

coordinates for the two different probings is about 0.15m to 1.5m. The non-

correspondence of data obtained with two different holes will affect the accuracy of the

measured density for the highly heterogeneous lumpy fill site. To avoid this

discrepancy in the measured density profile, it is desirable to measure both the RI and

background count with one probing. The detailed description of the development of a

single probe Nuclear-Density Cone penetrometer will be discussed in detail in the

following section.

5.7 Improvements in Nuclear-Density Cone Penetrometer for marine

based investigation

To support the ongoing engineering works, the above issues need to be

addressed urgently. Towards this purpose, a new single probe Nuclear-Density Cone

Penetrometer is developed by modifying the double probe where the gamma-ray

section is extended so as to insert an additional detector that is outside of the gamma-

ray zone emitting from the source. With this, the cone is able to measure both the

background and actual RI count during the same probing, thus removing the need to do

two probings.

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5.8.1 Description of Single Probe ND-CPT

Figure 5.19 shows major components of the Single Probe Nuclear-Density

Cone Penetrometer (ND-CPT). The lower part of the new cone is similar to the old

ND-CPT where it houses various sensors to measure the usual cone parameters,

namely, cone resistance (qc), pore pressure (u2), and the sleeve friction (fs). The upper

part of the cone section is extended so as to insert an additional detector to measure the

naturally occurring gamma photons. This upper part houses the Radioisotope source,

and two detectors. The detector-1 is used to measure the actual nuclear density (RI)

count and the detector-2 is used to obtain the background count of naturally occurring

gamma photons.

Depth Corrections

As already mentioned in Chapter 3, the original measured cone data needs to be

corrected for depth corrections to determine the actual wet density of material.

Therefore, depth adjustment needs to be carried out on measured data from the Single

Probe ND-CPT. If the measurement center for the cone is considered at the cone

resistance sensor, then the other sensors readings are adjusted accordingly. The depth

corrections for pore pressure (0.04m), sleeve friction (0.11m) and RI Count (0.60m)

remain the same as in the old ND-CPT while the BG Count data needs to be shifted up

by about 1.201m.

Detector Efficiency Ratio

As described earlier, in the single probe ND-CPT, two detectors were housed to

measure both the actual nuclear density (RI) count and the background (BG) count of

naturally occurring gamma photons, whereas, in the double probe ND-CPT, the RI

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count and BG count are measured by the same detector but in two separate probing.

The material used in the construction of the new additional detector (detector-2) in this

new cone is not the same as that of the old detector (Detector-1). Hence, the counting

efficiency (numbers of counts recorded per unit time) of these two detectors are not

similar and it was found that the detector efficiency ratio between Detector-1 and

Detector-2 is 1.0353. Therefore, it is necessary to employ a detector efficiency ratio

while calculating the effective gamma count for the single probe ND-CPT. The

effective gamma count is calculated by multiplying the BG count by the detector

efficiency ratio (1.0353) and this value is then subtracted from RI count.

5.8.2 Comparison of Single and Double Probe ND-CPT

As discussed previously, in a marine based investigation, it is impractical to

ensure that the two probes can be pushed into an identical hole. Therefore, the

comparison of the double and single probe ND-CPT was carried out when land is

formed above the sea level. Out of a total of 120 tests in Stage 2, 10 tests were used

for this purpose. The locations are 19C, 28C, 33C 36C, 40C, 65C, 71C, 91C, 109C and

121C in Figure 5.11. 10 numbers of double probe ND-CPTs were conducted from 4th

December to 12th December 2003. Unfortunately, the single probe ND-CPT could not

perform immediately after completing the double probe due to some practical

problems encountered. The problems arose mainly because of an increase in the length

of the single probe ND-CPT due to the need to insert an additional detector. When this

new long cone was initially used, the cone was damaged as a result of bending due to

the presence of a dense sandy layer above the ground-water table and it takes about a

month to modify the new cone structure. To avoid damages to the equipment, the pilot

test hole was drilled about 6m from the ground level for the remaining ND-CPT

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locations in Stage 2. The loss of information for the first 6m of the sandy layer is not

critical to this study. The single probe ND-CPTs using a new cone were conducted in

the same hole from 5th January 2004 to 15th January 2004. As this was carried out one

month after the double probe test, during this time, some additional settlement was

observed in the newly reclaimed land. The settlement was measured and adjusted in

the measured wet density profiles accordingly.

The wet density profiles obtained from the double probe ND-CPT are

compared with the single probe ND-CPT and the results using the same calibration

chart show a very good agreement, as shown in Figure 5.20. The results from only four

out of the 10 ND-CPTs are shown in Figure 5.20 for illustration. This also means that

the separation distance between the two detectors is sufficient, so that Detector-2 is

able to measure the background count accurately. It can be also seen that there is a

very good agreement in the measured wet density profiles for both the sand fill at the

top and the soft clayey seabed at the bottom. However, there are some small

differences in the spikes between the single probe and double probe for the highly

heterogeneous lumpy fill in between the sand and seabed. The small variation in wet

density spikes may be attributed to the average measurement of ND-CPT within the

measuring volume. As explained earlier, the cone measures density over a radius of

23.6 cm, therefore results from the cone tend to be average values rather than spot

values. In addition, these measurements are averaged over a span of 10cm depth to

minimize the statistical fluctuations. In spite of these small differences, it is clear that

the single probe ND-CPT has performed very well in measuring the wet density

profiles.

One of the main advantages of this newly developed Single Probe ND-CPT is

that it eliminates the need for double probing and the uncertainties that are involved in

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the investigation of a highly heterogeneous profile as result of the inability to carry out

both probings in an identical hole. The use of a single probe device in a marine

investigation is a significant improvement in the design.

5.9 Characterization of Initial State of a Lumpy Fill

As shown schematically in Figure 5.8, a lumpy fill layer of about 8m thickness

was constructed using big clay lumps. On top of this, 8m thick sand layer, which acts

as a surcharge to accelerate the consolidation of the clay lumps, was placed. The ND-

CPT was extensively used to evaluate the in-situ initial state of the fill formed by the

dredged clay lumps. The test has been conducted on a recently formed lumpy fill prior

to placement of the sand fill (Stage 1) and after a sand cap has been provided (Stage 2).

The initial state of the reclaimed land will be described in the following paragraphs

using two typical profiles obtained using the ND-CPT.

Figures 5.21 and 5.22 shows typical cone resistance, pore pressure and wet

density profiles obtained at pilot test area (TA 2) in Stage 1 and Stage 2 at locations RI

91 and RI 71. The changes in wet density, cone resistance and pore pressure together

can be employed to identify the various interfaces between soils layers, as shown by

the horizontal lines in Figure 5.21 and 5.22. The pore pressure response suggests that

the water table was about 2.5m below the ground level. The cone resistance was quite

high near the ground-water table as a result of compaction brought about by flow of

water in the fill carrying away finer material from the sand mixture and the possible

over-consolidation effect due to the downward seepage flow. The sand density above

the ground water table was also increased due to mechanical compaction as a result of

leveling operations by bulldozers. By comparing the cone resistance, pore pressure

and wet density profiles in Stage 2, it is clear that the sand fill which is above -7.0

136

mCD and the seabed clay which is below -14 mCD are homogeneous, but the lumpy

fill in between these two is heterogeneous. This is also clearly observed in the Qt

versus Bq chart (Robertson, 1990), as shown in Figure 5.23.

It is also observed that there is no clean sand pocket found in the lumpy fill, an

indication that the sand used to cap the lumpy fill could not replace the materials in the

large inter-lump voids. This is attributed to the construction method used to place very

soft clay materials on top of the dredged clay lumpy fills prior to placement of the sand

capping. This was to facilitate the trimming of the clay fill to exactly -3 mCD as

stipulated in the contract. This very soft clay material is believed to have filled into the

large initial inter-lump voids present in the lumpy fill layer. Hence, the sand used to

cap the lumpy fill could no longer penetrate the slurry in the inter-lump voids. This

argument is supported by the pore pressure response within the lumpy fill layer at

Stage 2 (Figure 5.21 and 5.22). The average pore pressure increase over the entire

thickness of lumpy fill is almost identical with that measured in homogeneous seabed

and consistent with the initial increase in pore pressure, as expected in one-

dimensional consolidation from 8m of sand surcharge. The initial state of lumpy fill

layer will be discussed in detail in the following section.

5.10 Initial State of Lumpy Fill Layer

There are two important issues on the initial state of lumpy fill layer that need

to be evaluated: (i) whether the inter-lump voids are filled with slurry or water and (ii)

the state of dredged clay lumps at various stages. These issues are explored with the

help of the ND-CPT results.

Figure 5.24 shows enlarged wet density profiles within the lumpy fill layer at

RI 91, RI 71 and RI 36 during Stage 1 and Stage 2. As demonstrated previously in

137

Chapter 4, from the back analysis of the field measurement using the theoretical

models, the size of clay lumps and inter-lump voids can be identified. In Figure 5.24

(a), two significant density spikes could be observed, and the depth between these

peaks is an indirect indicator of the size of the inter-lump voids. It can be also deduced

that the clay lump sizes are about 1m to 2m, consistent with that shown in Figure 1.6.

Also, in the inter-lump voids, the density is relatively high, generally about 14 kN/m3.

This confirms that the voids have not been filled with sea-water, but with slurry. This

is consistent with the observation that the seabed is covered with a very soft marine

clay, with wet density of about 14 kN/m3 or water content of 110%, as shown in Figure

5.3. While comparing the wet density profiles in Stage 1 and Stage 2 in Figure 5.24

(a), upon the application of sand surcharge, there are large vertical and horizontal

movements taking place in the lumpy fill layer. This is due to the nature of the fill

which consists of large clay lumps and inter-lump voids filled with clay slurry.

Similar observations could be also made in Figure 5.24 (b) and (c). In Figure 5.24 (b),

during Stage 2, there is a decrease in wet density of the lumpy fill layer from -7mCD

to -10mCD and this may be attributed to the softening of clay lumps, which is

explained in a subsequent section.

5.11 Intrinsic and Intact Compression Curves

The compressibility characteristics of reconstituted clays are used as a valuable

frame of reference for assessing the properties of an intact natural material (Burland

1990). The properties of the reconstituted clay are termed as ‘intrinsic’ since they are

inherent to the soil and independent of the natural state. The intrinsic compression line

(ICL) is a valuable reference line for studying the compression characteristics of

natural clays. Burland (1990) introduced a normalizing parameter called the ‘void

138

index’ to aid in correlating the compression characteristics of various undisturbed soil

samples. The void index (Iv) is defined as

Iv = *

*100

*1000

*100

*100

cCee

eeee −

=−

− (5.3)

where e is in-situ void ratio; e*100 and e*1000 are the intrinsic void ratios on the ICL

corresponding to σv′ = 100 kPa and σv′=1000 kPa respectively. C*c is the intrinsic

compression index which is defined as (e*100 – e*1000). Burland (1990) showed that the

intrinsic compression curve from soils within a wide range of liquid limits, when

plotted in terms of void index, could be represented with sufficient accuracy by the

following equation:

Iv = 2.45 – 1.285 X + 0.015 X3 (5.4)

where X = log σv′and σv′ is in kPa. The values of e*100 and C*c are measured by

means of an oedometer test on the reconstituted soil. Burland (1990) also showed that

e*100 and C*c could be reasonably related to the void ratio at the liquid limit, eL, as

follows:

e*100 = 0.109 + 0.679eL - 0.089eL

2 + 0.016eL3 (5.5)

C*c = 0.256eL – 0.04 (5.6) Low (2004) found that for reconstituted Singapore marine clay, Eq. (5.4) to Eq. (5.6)

were also applicable. Therefore, this framework will be used to study the compression

characteristics of the lumpy fill layer. As part of this thesis, this author has already

demonstrated that the ICL and void index from oedometer tests can be used in the

study of the compression characteristics of the ultimate state of a reclaimed land

(Karthikeyan et al. 2004).

139

As the ND-CPT provides a continuous profile of wet density, it can be used to

calculate the in-situ void ratio of the soil. The effective vertical pressure can be

estimated from total vertical stress and the pore water pressure. However, the pore

water pressure is assumed to be hydrostatic. Hence, the in-situ void ratio versus

effective vertical pressure relation can be obtained from the ND-CPT results. Burland

(1990) intrinsic framework is then used to interpret this relation. As a first step, the

result from pilot test area TA 1 shown earlier in Figure 5.1 is discussed. In this pilot

test area, limited numbers of ND-CPTs were carried out in the seabed clay prior to the

dumping of dredged materials. Figure 5.25 shows the typical cone resistance, pore

pressure, and wet density profiles obtained from ND-CPTs in the seabed clay. The wet

density profiles show here are consistent with the soil layers shown in Figure 5.3. The

results obtained from the ND-CPT, as shown in Figure 5.25 are re-plotted in terms

void ratio (e) and effective vertical pressure (σv′) and shown in Figure 5.26.

Figure 5.26 shows typical e-log σv′ curves obtained from interpretation of the

ND-CPT results. The results show that marked scatter in the e-log σv′ curves which is

due to variations in depositional conditions as the different layers of soil profiles were

being formed. The depositional environments differ significantly as a result of sea

level changes and changes in the course of the river. There is no reason to anticipate a

set of smooth compression curves. Rates and modes of deposition are likely to vary

considerably during the formation of a sedimentary soil layers and in these

circumstances a wavy curve must be expected (Edge and Sills, 1989). The e-log σv′

curves obtained from ND-CPT results will be examined using Burland (1990) intrinsic

framework to capture the compressibility characteristics of natural deposits in the

following paragraphs.

140

Figure 5.27 shows ve σ ′− log compression and swelling curves obtained from

oedometer tests on reconstituted soil samples from this area. The reconstituted soil

samples were prepared with the water content of 1.5 times the liquid limit. The

compression data in Figure 5.27 are re-plotted in Figure 5.28 by replacing void ratio

(e) with the parameter void index (Iv). When plotted in terms of void index (Iv), the

data indeed fall on to a unique ICL curve, as reported by Burland (1990). The ICL

proposed by Burland (1990) as given by Eq. (5.4) is also plotted in Figure 5.28 and it

can be seen that this indeed is appropriate for the soils tested here.

Intact compression curves obtained from the ND-CPT, as shown in Figure 5.26

are plotted in terms of Iv and compared with the ICL and shown in Figure 5.29. The

two mechanical properties (e*100 and C*c) were determined from one-dimensional

compression tests on reconstituted soil samples. These parameters were used to

compute void index using Eq. (5.3). Burland (1990) found that many low to medium

sensitivity natural clays that are deposited in still water environment lie close to a well

defined line to the right of the ICL which is defined as the Sedimentation Compression

Line (SCL) and this is also shown in Figure 5.29. The location of the intact

compression curves relative to the ICL and SCL reflects the depositional and post-

depositional history of the deposit. This figure shows that the intact compression

curves for the upper part of the seabed clay lies on or close to SCL while the lower part

lies below the ICL. If an intact compression curve lies on or close to ICL, the in-situ

state of the sample is normally consolidated (NC). Similarly, if an intact compression

curve lies below the ICL, the state of the sample is overconsolidated (OC). The data

obtained from ND-CPT clearly indicate that the upper 4m of the sediment is a recent

deposit of surface soft marine clay and is normally consolidated. However, the other

deposits are over consolidated clays. These observations are consistent with the

141

consolidation characteristics of seabed soils given in Tan et al. (2002) which had

indicated that the upper marine clay below the surface soft marine clay is

overconsolidated. The results above show that the ND-CPT is very useful to provide

the kind of results needed to use the intrinsic frameworks proposed by Burland (1990)

to capture the compressibility characteristics of the natural deposit. Hence, this

framework will now be used to describe the qualitative nature of the initial state of a

lumpy fill.

The measurements from ND-CPTs for the lumpy fill layer are now used to

obtain the intact compression curves, which are plotted in terms of Iv and compared

with ICL and SCL, as shown in Figure 5.30 and 5.31. These figures clearly show that

certain zones of the compression curves lie above ICL and close to SCL, which is

similar to the recent natural deposit of the surface soft marine clay, as shown in Figure

5.29 and other zones that are overconsolidated. The intact compression curves

resulting from the placement of sand surcharge are very much steeper than the ICL and

it is clear that these curves will drop below the SCL at higher values of effective

overburden pressure. These data clearly indicate that the lumpy fill layer has both

consolidating and overconsolidated zones. The formation of this lumpy fill layer will

be described in the subsequent section.

5.12 Radioisotope (RI) Cone based assessment

The void index (Iv) may be used as a measure of intrinsic compactness of soil

(Burland 1990). When the void index (Iv) is less than zero the soil is “compact” and

when it is greater than zero the soil is “loose”. The term “loose” was used by Burland

(1990); however the term “loose” may lead to confusion when used for clayey soils.

The term “loose” hitherto will be referred as “less compact”. The void indices versus

142

depth profiles at three typical locations are shown in Figure 5.32. The density

measured by the ND-CPT was used to calculate the in-situ void ratio. These values

were used to compute void index, as given by Eq. (5.3). As can be seen, compact and

less compact zones exist within the lumpy fill layer.

5.13 Softening of Clay Lumps and Remoulded Shear Strength

Because of the removal of overburden stresses during dredging operations,

suction equivalent to the mean overburden pressure would develop in the clay lumps.

When these clay lumps are dumped in seawater during the land reclamation process,

they will absorb water because of the suction present in the soil. Subsequently, the

suction reduces with time and the water content increases resulting in a decrease in the

strength of the clay lumps. (Robinson et al. 2004). However, the degree of softening of

these stiff clay lumps after dumping in the reclamation pond has not been established

to date.

If the clay lumps swell and soften in the reclamation pond, the state of clay

lumps may collapse and remoulding/ or re-consolidation took place after the placement

of a sand surcharge. Therefore, the remoulded shear strength of these dumped dredged

materials needs to be evaluated. Houston and Mitchell (1969) suggested the existence

of a unique relation between liquidity index and remoulded undrained shear strength of

clays. Wroth and Wood (1978) proposed the relationship between liquidity index and

remoulded shear strength of clays, as given by Eq. (5.2). Kulhawy and Mayne (1990)

reported that the Wroth and Wood (1978) equation shows good agreement with

measured data in the range of 10 kPa to 300 kPa. For soil samples with natural water

contents close to the liquid limit, the estimate of the (very low) remoulded strength

may be misleading, but these will be the soil deposits that tend to be sensitive, and for

143

which the estimates of the remoulded shear strength would be inappropriate anyway.

However, for overconsolidated clays, having natural water contents closer to the

plastic limit, there is usually little sensitivity and the remoulded strength should form a

lower bound to the in-situ strength. To examine the in-situ state of clay lumps, the

remoulded shear strength of the lumpy fill layer was estimated.

The specific gravity for clay particles in Singapore marine clays varies from 2.6

to 2.7, but in the present calculation, an average specific gravity of 2.65 was assumed.

The wet density measured by ND-CPT is then used to calculate the in-situ water

content. As discussed previously, the liquid limit and plastic limit of Singapore marine

clays varies from 80 – 100 % and 30 - 40% respectively (Tan et al. 2002), as shown in

Figure 5.2. In the present calculation of the liquidity index, however, an average liquid

limit of 90% and plastic limit of 35% were used. These parameters were used to

compute the remoulded undrained shear strength, as given by Eq. (5.2). The variation

of the estimated profile of remoulded undrained shear strength of lumpy fill layer with

depth is shown in Figure 5.33. In Stage 2, there is a slight increase / or decrease in the

shear strength of the material in the inter-lump voids and this may be due to the re-

consolidation/ or remoulding of the clay lumps after the placement of sand surcharge.

As confirmed earlier, the inter-lump voids have not been filled with seawater, but with

slurry of a significantly higher density than seawater. Hence, the strength is relatively

low in the inter-lump voids and this water content is greater than the liquid limit of the

in-situ seabed marine clay. But the key issue here is that the material in the inter-lump

voids is soil-like, exhibiting some shearing resistance, rather than behaving like a fluid.

As can be seen in Figure 5.33, the remoulded undrained shear strength of the clay

lumps is almost close to the lower bound of the in-situ undrained shear strength

developed at the original seabed clays and it is clearly indicating that the big clay

144

lumps have not softened completely. The initial pore pressure in all the clay lumps

excavated is negative, and thus this will swell once in contact with seawater after being

dumped.

To support these evaluations, as part of this reclamation project, co-researchers

Robinson et al. (2004) conducted a series of three-dimensional swelling tests using big

clay lumps when different diameters of clay lump specimens were fully submerged

underwater. Here, 105mm and 205mm diameter clay lumps were used. The

dissipations of suction at the centre of these samples were also measured. More details

about these experimental investigations can be found in Robinson et al. (2004). Figure

5.34 shows the photographs of 205 mm diameter Singapore marine clay lump before

and after three-dimensional swelling. It was found that the state of the clay lumps were

stable at the end of the dissipation of suction, as can be seen in Figure 5.34. The

variations of water content with depth of the sample at three different locations were

also measured, as shown in Figure 5.35. This figure shows that the measured water

content at the end of the test is not uniform along the diameter of the specimen. The

water content at the edge of the clay lump is higher than that at the centre, a fact of

some practical relevance. During the deformation process of the clay lumps in a lumpy

fill under sand surcharge, the edges of the clay lumps experience very high contact

stress and will cause partial disintegration of edges of the bigger clay lumps while the

centre of these lumps remains close to its original consolidated state.

5.14 Formation of initial state of lumpy fill layer

The measured engineering properties of the lumpy fill layer are now used to

reconstruct the formation of the layer. An explanation for the existence of

consolidating and overconsolidated zones is as follows. The significant variation in

145

soil properties within the lumpy fill layer indicates the complex interaction between

clay lumps that have not softened completely and the new infillings comprising mainly

of the slurry clay and also smaller lumps from the lumpy clays disposed at the site. The

initial pore pressure in all the lumps excavated is negative, and thus it will swell once

in contact with seawater after being dumped. This swelling as well as the high contact

forces between lumps will cause disintegration of the smaller lumps and partial

disintegration of edges of bigger clay lumps while the centre of these lumps remains

close to its original consolidated state. These small breakaway lumps will also fill the

inter-lumps voids. The lumpy fill layer, after the placement of a sand surcharge, has

consolidating and overconsolidated zones, in which the overconsolidated zone is

consistent with that of the original clay lump while the consolidating zone is due to the

infilling of inter- lump voids with very soft clay slurry and also breakaway little lumps.

This observation has important practical ramifications because it implies that the inter-

lump voids are filled with pockets of very soft clay and therefore unlikely to have such

voids still opened to cause long term settlement. It also means that these pockets of

very soft clay can be treated using vertical drains and sand surcharge, a soil

improvement that is needed anyway to treat the thick deposits of soft seabed clay

below the reclaimed area.

Figure 5.36 shows a histogram of frequency of in-situ void ratio measured by

the ND-CPT for lumpy fill layer in Stage 1 and Stage 2. All ND-CPT measurements

are captured in Figure 5.36 to provide sufficient data for more meaningful statistical

analysis. The average in-situ void ratio within the lumpy fill layer in Stage 1 is about

2.18. As expected, it has been reduced to 2.09 upon the application of sand surcharge.

With this, it can be calculated that about 5% of inter-lump voids have been closed after

the placement of sand surcharge.

146

Due to subsequent consolidation, the excess pore pressure in the lumpy fill

layer will dissipate and it results in further reduction of both inter-lump and intra-lump

voids (voids within lumps). At the ultimate state, if the size of the inter-lump voids

reduces to size of the intra-lump voids, then it is considered that the lumpy fill has

been “homogenized”. However, it is not clear whether the lumpy fills constructed

using such clay lumps indeed “homogenize” after fully consolidated. This issue needs

to be examined so that the large dredged clay lumps can be used as fill materials for

land reclamation with increased confidence. Therefore, a case study on the ultimate

state of a reclaimed land using big clay lumps has been carried out. For this ultimate

state of a reclaimed land, another site is used and this will be discussed in next chapter.

5.15 Concluding Remarks

A comprehensive site investigation was carried out at a reclamation site where

dredged clay lumps are used as fill materials. The investigations have been conducted

in two stages, one just after the lumpy fill is formed prior to placement of capping

layer any, and the second after a sand cap has been placed on top of the lumpy fill. The

ND-CPT was extensively used to evaluate the in-situ initial state of the fill formed by

these dredged clay lumps. Results from these tests show that usual cone parameters

coupled with the additional density measurement will provide a much more complete

picture of the fill formed. Problems faced while performing ND-CPTs in marine based

investigation were also highlighted and this has led to the development of a new single

probe ND-CPT. The comparison of the wet density measured by the double probing

ND-CPT with the single probe ND-CPT measurements showed very good agreement.

The density measured by ND-CPTs revealed that the lumpy fill layer is highly

heterogeneous, consisting of high density zones, with large voids that are filled with

147

very soft clay that are found at this site. The lowest density measured in the lumpy fill

layer is about 14 kN/m3. The lumpy fill layer after the placement of a sand surcharge

has consolidating and over-consolidated zones. The over-consolidated zone is

consistent with that of the original lump while the consolidating zone is due to the

infilling of inter-lump voids with very soft clay and also breakaway little lumps.

In addition, it is necessary to gain the understanding on the transition from the

initial state of a lumpy fill to the ultimate state of lumpy fill layer so that the large

dredged clay lumps can be used for land reclamation as fill materials. A case study will

be carried out to evaluate the ultimate state of a reclaimed land constructed using large

clay lumps and this will be discussed in next chapter.

148

Figure 5.1 Geological locations of project site and pilot test areas (TA 1, TA 2 and TA 3) in the reclamation site.

Figure 5.2. Typical Geological Profiles for seabed soils prior to reclamation.

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Dep

th (

mC

D)

Lower Marine Clay

Intermediate ClaySand

Upper Marine Clay

Surface Soft Marine Clay

Weathered granite

Seawater

C182 C183 A39/C184

?

149

Figure 5.3. Typical soil profiles and index properties at the project site.

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120

Water Content (%)

Dept

h be

low

Sea

bed

(m)

Wp

Wn

WL

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100

Soil Composition (%)

Dept

h be

low

Sea

bed

(m)

Clay

Silt

0

5

10

15

20

25

30

35

40

0.0 0.4 0.8 1.2 1.6Liquidity Index

Dept

h be

low

Sea

bed

(m)

0

5

10

15

20

25

30

35

40

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Activity

Dept

h be

low

Sea

bed

(m)

Lower Marine Clay

Intermediate Layer

Upper Marine Clay

SSMC

150

(a) (b) (c) (d) Figure 5.4 Geotechnical Characteristics of Seabed Soils prior to reclamation (a) Undrained Shear Strength (b) Sensitivity (c) Remoulding loss (d) Swelling Potential.

0

5

10

15

20

25

30

35

40

0 40 80 120 160 200

Undrained Shear Strength (kPa)

Dept

h be

low

Sea

bed

(m)

UU Test

UC Test

Nkt = 8

Nkt = 10

Nkt = 12

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10Sensitivity of Clays, St

Dept

h be

low

Sea

bed

(m)

2 31 4

1 - Low Sensitivity2 - Medium Sensitivity3 - Sensitive4 - Extra-sensitive

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100Remoulding loss (%)

Dept

h be

low

Sea

bed

(m)

0

5

10

15

20

25

30

35

40

0 10 20 30 40Sw elling Potential (%)

Dept

h be

low

Sea

bed

(m)

Very High High

SSMC

Upper Marine Clay

Intermediate Layer

Lower Marine Clay

151

Figure 5.5 Geotechnical Characteristics of Seabed Soils prior to reclamation (a) Preconsolidation pressure (b) OCR (c) Compression index (d) Swelling index

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500 600 700 800

Preconsoildation Pressure (kPa)

Dep

th b

elow

Sea

bed

(m)

σv'

0

5

10

15

20

25

30

35

40

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

OCR

Dep

th b

elow

Sea

bed

(m)

0

5

10

15

20

25

30

35

40

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Compression Index, Cc

Dep

th b

elow

Sea

bed

(m)

0

5

10

15

20

25

30

35

40

0.0 0.1 0.2 0.3 0.4

Swelling index, Cr

Dep

th b

elow

Sea

bed

(m)Intermediate

Layer

Lower Marine Clay

Upper Marine Clay

SSMC

152

Figure 5.6 Typical dredged materials obtained from (a) Surface soft marine clay

(b) upper marine clay.

(a)

(b)

~ 2 m

153

Figure 5.6 Typical dredged materials obtained from (c) Intermediate layer

(d) Lower marine clay.

(c)

(d)

1m

154

Figure 5.7 Dumping of dredged clay lumps using bottom opening barge.

Figure 5.8 Schematic profile of reclamation with dredged clay lumps and sand Surcharge.

Barge Size Width : ~ 10m Length : ~20m Height : ~ 5m Volume : ~ 900 – 1000m3

Seabed

Sand Fill

~9m

~8m

Dredged Clay Lumps

Inter-lump voids filled with small clay blocks

and water

155

Figure 5.9 Photographic View of the Marine Cone Penetration system used in field investigation

Figure 5.10 Photographic View of the CPT crawler used in field investigation on land.

Marine Cone Penetration System Dimensions of Platform: 60 ft x 26 ftCounter weight = 10 ton

Diameter of Counter weight = 3.4m

Reaction frame or Tower and

Hydraulic Penetration system

Control Unit

156

Details at Location ‘A’

Figure 5.11 Layout of ND-CPT in site investigation at TA 2 (Stage 1 and Stage 2).

Legend

ND-CPT

99m

100m

11m

10m

110B

109C

11m

10m108107

98 10099

106105104103102

91C 92 93 97969594

20

30

60B

8681

71C

82 83 84 85

72 73 74

61 62

52 53

63 64

11

40C

2 3 4 5

12 13 14 15

2524

121C

2321 22

31

41

32

42

33C 34 35

4543 44

54 55

65C

76

66

77

67 68 70

80

69

79

90898887

47

59

49 5048

5857

101B

1B

26B

10B

78B

75B

51B

56

46

36C 37 38 39

2928C27

19C

181716

987

A

10m

11m

119 56

112111B

4645 118

115

55

113

117116

114

120

11m

10m

157

Figure 5.12 Typical cone resistance, pore pressure and wet density profiles at location RI 111B in TA 2 (Stage 1). Figure 5.13 Typical cone resistance, pore pressure and wet density profiles at location RI 104 in TA 2 (Stage 1).

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8

Pore water pressure, u2 (MPa)

Dept

h(m

CD)

RI 111B

Hydrostatic

Lumpy Fill Layer

Seawater

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8 1.0 1.2

Corrected cone resistance, qt, (MPa)

Dept

h(m

CD)

RI 111B

Clay Lump

Seabed Soil

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 13 14 15 16 17 18 19 20

Wet Density (kN/m3)

Dept

h(m

CD)

RI 111B

Inter-lump voids filled withwith slurry

Clay Lump

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8 1.0 1.2Corrected cone resistance, qt, (MPa)

Dep

th(m

CD

)

RI 104

Seabed Soil

Seawater

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8

Pore water pressure, u2 (MPa)

Dep

th(m

CD

)

RI 104

Hydrostatic

Lumpy Fill Layer

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 13 14 15 16 17 18 19 20

Wet Density (kN/m 3)

Dep

th(m

CD

)

RI 104

Clay Lump

Inter-lump voids filled w ith slurry

158

Figure 5.14 Typical cone resistance, pore pressure and wet density profiles at location RI 13 in TA 2 (Stage 1).

Figure 5.15 Typical cone resistance, pore pressure and wet density profiles at location RI 101B in TA 2 (Stage 1).

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8

Pore water pressure, u2 (MPa)

Dept

h (m

CD)

RI 13

Hydrostatic

Clay Lump

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Corrected cone resistance, qt, (MPa)

Dept

h(m

CD)

RI 13

Seawater

Seabed Soil

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 13 14 15 16 17 18 19 20

Wet Density (kN/m3)

Dept

h (m

CD)

RI 13

Clay Lump

Inter-lump voids filled with soft clay slurry

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.4 0.8 1.2 1.6 2.0

Corrected cone resistance, qt, (MPa)

Dept

h(m

CD)

RI 101B

Seabed Soil

Seawater

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0.0 0.2 0.4 0.6 0.8Pore water pressure, u2 (MPa)

Dept

h (m

CD)

RI 101BHydrostatic

Lumpy fill layer

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 13 14 15 16 17 18 19 20

Wet Density (kN/m3)

Dept

h (m

CD)

RI 101B

Inter-lump voids filled with slurry

Clay Lump

Clay Lump

Clay Lump

159

Figure 5. 16 Typical background count profiles obtained for homogenous soils Clay deposits (b) Sandy deposits (Nobuyama, 2000).

Figure 5.17 Typical RI count and Background count profiles obtained for highly heterogeneous lumpy fill site.

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350 400Background count (CPS)

Dept

h (m

)

Drammen Clay (Norw ay)Kinkai BayHachirogata (Akita)

(a)

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350 400Background count (CPS)

Dept

h (m

)

Holmen Sand (Norw ay)Vancouver Sand (Canada)Kamigaw a Sand (Chiba)Higashi Ohgishima Sand (Kanagaw a)

(b)

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0600 700 800 900 1000 1100 1200

RI count (CPS)

Dept

h(m

CD)

RI 115 RI 116

RI 118 RI 120

RI 45 RI 111

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

050 100 150 200 250 300

Background count (CPS)

Dept

h(m

CD)

RI 115 RI 116RI 118 RI 120RI 45 RI 111Average BG

(a)

(b)

160

Figure 5.18 Comparison of estimated wet density using actual measured background count and average background count.

Figure 5.19 Diagram of Single Probe Nuclear-Density Cone Penetrometer (ND-CPT)

(All dimensions in millimeters).

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 13 14 15 16 17 18 19 20

Wet Density (kN/m3)

Dept

h(m

CD)

RI 45Average BG

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 13 14 15 16 17 18 19 20

Wet Density (kN/m3)

Dept

h(m

CD)

RI 115Average BG

Detector-1 Detector-2 Gamma source

1538.83

48.6 35.6

161

Figure 5.20. Comparison of wet density profiles measured by double and single probe ND-CPTs.

0

2

4

6

8

10

12

14

16

18

20

22

12 14 16 18 20 22Wet Density (kN/m3)

Dept

h(m

)

RI 91-Single Probe(07/01/2004)RI 91-Double Probe(09/12/2003)

Sand Fill

Lumpy Fill

0

2

4

6

8

10

12

14

16

18

20

22

12 14 16 18 20 22Wet Density (kN/m3)

Dept

h(m

)

RI 121-Single Probe(15.01.2004)RI 121-Double Probe(12.12.2003)

Sand Fill

Lumpy FIll

Seabed

0

2

4

6

8

10

12

14

16

18

20

22

12 14 16 18 20 22Wet Density (kN/m3)

Dept

h(m

)

RI 71-Single Probe(09/01/2004)RI 71-Double Probe(09/12/2003)

Sand Fill

Lumpy FIll

0

2

4

6

8

10

12

14

16

18

20

22

12 14 16 18 20 22Wet Density (kN/m3)

Dept

h(m

)

RI 28-Single(09/01/2004)RI 28-Double(10/12/2003)

Sand Fill

Lumpy Fill

Seabed

162

Figure 5.21 Typical Cone resistance, Pore pressure and wet density profiles obtained at location RI 91

Figure 5.22 Typical Cone resistance, Pore pressure and wet density profiles obtained at location RI 71.

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

40 2 4 6 8 10 12 14 16

Corrected cone resistance, qt, (MPa)

Dept

h(m

CD)

RI 91 (Stage 2)(09/12/2003)

RI 91 (Stage 1)(30/08/2002)

Lumpy Fill Layer

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

40.0 0.2 0.4 0.6 0.8

Pore water pressure, u2 (MPa)

Dept

h(m

CD)

RI 91 (Stage 2) (09/12/2003)Hydrostatic RI 91 (Stage 1) (30/08/2002)

Sand FIll

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

410 12 14 16 18 20 22

Wet Density (kN/m 3)

Dept

h(m

CD)

RI 91 (Stage 2)(09/12/2003)RI 91 (Stage 1)(30/08/2002)

Seabed

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

40.0 2.0 4.0 6.0 8.0 10.0

Corrected cone resistance, qt, (MPa)

Dept

h(m

CD)

RI 71 (Stage 2)(09/12/2003)

RI 71 (Stage 1)(28/08/2002)

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

40.0 0.2 0.4 0.6 0.8

Pore water pressure, u2 (MPa)

Dept

h(m

CD)

RI 71 (Stage 2) (09/12/2003)

Hydrostatic

RI 71 (Stage 1) (28/08/2002)

Lumpy Fill Layer

Sand Fill

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

410 12 14 16 18 20 22

Wet Density (kN/m 3)

Dept

h(m

CD)

RI 71 (Stage 2)(09/12/2003)RI 71 (Stage 1)(28/08/2002)

Seabed

163

1

10

100

1000

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Pore Pressure Ratio Bq

Nor

mal

ized

Con

e R

esis

tanc

e Q

t

SeabedLumpy FillSand Fill

1 2

3

Increasing sensitivity

4

IncreasingOCR 5

6

7

RI- 91 (Stage 2)

1

10

100

1000

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Pore Pressure Ratio Bq

Nor

mal

ized

Con

e R

esis

tanc

e Q

t

SeabedLumpy FillSand Fill

12

3

Increasing sensitivity

4

IncreasingOCR 5

6

7

RI-71 (Stage 2)

Zone Soil Zone Soil

1 Sensitive, Fine Grained 5 Sand mixtures; silty sand to sand silty

2 Organic soils-peats 6 Sands; clean sands to silty sands

3 Clays-clay to silty clay 7 Gravelly Sand to sand

4 Silt mixtures clayey silt to silty clay

Figure 5.23 Robertson (1990) charts for ND-CPT data at locations RI 91 and RI 71.

(a)

(b)

164

Figure 5.24 Variation of wet density profiles within lumpy fill layer (a) at location RI 91 (b) at location RI 71 (c) at location RI 36.

-14

-13

-12

-11

-10

-9

-8

-7

-6

-510 12 14 16 18 20 22

Wet Density (kN/m 3)

Dept

h(m

CD)

RI 71 (Stage 2)(09/12/2003)RI 71 (Stage 1)(28/08/2002)

(b)

Softening of edges of the clay lumps

-14

-13

-12

-11

-10

-9

-8

-7

-6

-510 12 14 16 18 20 22

Wet Density (kN/m3)

Dept

h(m

CD)

RI 91 (Stage 2)(09/12/2003)

RI 91 (Stage 1)(30/08/2002)

Inter-lump voids filled w ith slurry

Clay lump

Clay lump

Clay Lump

Inter-lump voids filled with slurry

(a)

-14

-13

-12

-11

-10

-9

-8

-7

-6

-510 12 14 16 18 20 22

Wet Density (kN/m3)

Dept

h(m

CD)

RI 36 (Stage 2)(12/12/2003)

RI 36 (Stage 1)(03/09/2002)

(c)

165

Figure 5.25 Typical cone resistance, pore pressure and wet density profiles obtained

from ND-CPT for seabed clays.

Figure 5.26 Typical e-logσv′ curves obtained from ND-CPT results for seabed soils.

-28

-26

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.5 1.0 1.5 2.0 2.5

Corrected cone resistance, qt, (MPa)

Dept

h(m

CD)

RI 04

Surface Soft Marine clay

Lower marine clay

-28

-26

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

00.0 0.2 0.4 0.6 0.8 1.0

Pore water pressure, u2 (MPa)

Dept

h(m

CD)

RI 04Hydrostatic

Seawater

-28

-26

-24

-22

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

012 14 16 18 20 22

Wet Density (kN/m 3)

Dept

h(m

CD)

RI 04

Intermediate layer

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 10 100 1000

Effective vertical Pressure, σv' (kPa)

Voi

d ra

tio, (

e)

RI 04RI 02

166

Figure 5.27 One-dimensional compression and swelling curves for reconstituted soil samples obtained from the Singapore marine clay at Pulau Tekong Island.

Figure 5.28 One-dimensional compression and swelling curves for reconstituted soil

samples in terms of void index.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1 10 100 1000 10000

Effective vertical pressure, σv' (kPa)

Void

ratio

, (e)

ICL

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000 10000

Effective vertical pressure, σv' (kPa)

Void

Inde

x, (I

v)

ICL

ICL Proposed by Burland (1990)

167

Figure 5.29 Comparison of intact compression curves obtained from ND-CPT results with Intrinsic Compression Line (ICL) and Sedimentation Compression Line (SCL) for seabed clays.

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000

Effective vertical Pressure, σv' (kPa)

Voi

d in

dex,

(Iv)

RI 04

ICL

SCL

OC

NC

(a)

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000

Effective vertical Pressure, σv' (kPa)

Void

inde

x,(I

v)

RI 02

ICL

SCL

OC

NC

168

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000Effective vertical pressure, σv' (kPa)

Void

Inde

x, (I

v)

RI 36-Stage 1 (03/09/2002)

RI 36-Stage 2 (12/12/2003)

SCL

ICL

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000Effective vertical pressure, σv' (kPa)

Voi

d In

dex,

(Iv)

RI 91-Stage 1 (30/08/2002)

RI 91-Stage 2 (09/12/2003)

SCL

ICL

Figure 5.30 Comparison of intact compression curves obtained from ND-CPT results with ICL and SCL for lumpy fill layer.

169

Figure 5.31 Comparison of intact compression curves obtained from ND-CPT results with ICL and SCL for lumpy fill layer.

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000Effective vertical pressure, σv' (kPa)

Void

Inde

x, (I

v)

RI 109-Stage 1 (12/08/2002)

RI 109-Stage 2 (09/12/2003)

SCL

ICL

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

1 10 100 1000Effective vertical pressure, σv' (kPa)

Voi

d In

dex,

(Iv)

RI 71-Stage 1 (28/08/2002)

RI 71-Stage 2 (09/12/2003)

SCL

ICL

170

Figure 5.32 Typical void index versus depth profiles used for identifying the compact and less compact zones within the lumpy fill layer.

-14

-13

-12

-11

-10

-9

-8

-7

-6-4 -3 -2 -1 0 1 2 3 4

Void IndexD

epth

(mC

D)

RI 91 (Stage 2)(09/12/2003)RI 91 (Stage 1)(30/08/2002)

Less Compact

Compact

-14

-13

-12

-11

-10

-9

-8

-7

-6-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Void Index

Dep

th(m

CD

)

RI 71 (Stage 2)(09/12/2003)

RI 71 (Stage 1)(28/08/2002)

Less Compact

Compact

-14

-13

-12

-11

-10

-9

-8

-7

-6-4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0

Void Index

Dep

th(m

CD

)

RI 36 (Stage 2)(12/12/2003)

RI 36 (Stage 1)(03/09/2002)

Compact

Less Compact

171

Figure 5.33 Variation of estimated remoulded undrained shear strength profiles within lumpy fill layer.

-14

-13

-12

-11

-10

-9

-8

-7

-6

-50 40 80 120 160 200

Remoulded shear strength, Sur (kPa)

Dept

h(m

CD)

RI 91 (Stage 2)(09/12/2003)

RI 91 (Stage 1)(30/08/2002)0.25∗σ v '

-14

-13

-12

-11

-10

-9

-8

-7

-6

-50 40 80 120 160 200

Remoulded Shear strength, Sur (kPa)

Dept

h(m

CD)

RI 40 (Stage 2)(10/12/2003)

RI 40 (Stage 1)(09/09/2002)

0.25∗ σ v '

-14

-13

-12

-11

-10

-9

-8

-7

-6

-50 40 80 120 160 200

Remoulded shear strength, Sur (kPa)

Dept

h(m

CD)

RI 33 (Stage 2)(12/12/2003)

RI 33 (Stage 1)(07/08/2002)

0.25∗ σ v '

-14

-13

-12

-11

-10

-9

-8

-7

-6

-50 40 80 120 160 200

Remoulded shear strength Sur (kPa)

Dept

h(m

CD)

RI 71 (Stage 2)(09/12/2003)

RI 71 (Stage 1)(28/08/2002)

0.25∗σv'

172

Figure 5.34. Photographic view of Singapore marine clay lump of 205 mm diameter before and after three-dimensional swelling.

Figure 5.35. Variation of water content at end of three-dimensional swelling clay lumps of diameter, D= 205mm (after Robinson et al. 2004).

0

2

4

6

8

10

12

14

16

18

20

42 44 46 48 50 52 54Water content (%)

Dep

th (c

m)

D/2 D/4 D/4

173

0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 5.2 5.6In-situ void ratio

0

100

200

300

400

500

600

700

800

Freq

uenc

y

Stage 1 (-3mCD)-TA 2

Average void ratio = 2.18

No of data's = 11643

Stage 2 (+4mCD)-TA2

Average void ratio = 2.09

No of data's = 11181Stage 1Stage 2Normal Fit- Stage 1Normal Fit- Stage 2

Figure 5.36 Histogram of frequency of in-situ void ratio measured by the ND-CPT for

the lumpy fill layer in Stage 1 and Stage 2.