Peat is a mixture of fragmented organic material formed in wetlands under
appropriate climatic and topographic conditions and it is derived from
vegetation that has been chemically changed and fossilized (Dhowian et
al., 1980). Peat is generally found in thick layers in limited areas,
has low shear strength and high compressive deformation which often results
in difficulties when construction work is undertaken on the deposit.
Study conducted by Islam et al. (2008) and Andriesse (1988) revealed
that the bearing capacity of peat soil was very low and was apparently
influenced by the water table and the presence of subsurface woody debris.
Peat poses serious problems in construction industry due to its long-term
consolidation settlements even when subjected to a moderate load. Hence,
peat is considered as unsuitable for supporting foundations in its natural
state. Wong et al. (2008) conducted a test on peatland in Peninsular,
Malaysia and found that the water holding capacity of this peat was very
high and it was found dark brown in colour and the soil was classified
H4 according to Von Post classification system.
According to Jamil et al. (1989), where soil with peat depth of
<1.0, 1.0-1.5, 1.5-3.0 and >3.0 m is classified as shallow, moderate,
deep and very deep peat. Soil fabric, characterized by organic coarse
particles, holds a considerable amount of water because the coarse particles
are generally very loose and the organic particle itself is hollow and
largely full of water. The water content of peat researched in West Malaysia
ranges from 200 to 700% and unit weight of peat is typically lower compared
to inorganic soils. Previous studies suggested that for peat water content
about 500%, the unit weight ranges from 10 to 13 kN m-3. A
range of 8.3-11 kN m-3 is common for unit weight of fibrous
peat in West Malaysia (Huat et al., 2004). The organic content
in the range of 50 to 95% and the liquid limit was in the range of 200
to 500% as reported by Huat et al. (2004).
Duraisamy et al. (2007) reported that tropical fabric causes highest
settlement and followed by hemic and sapric when subjected to a load and
over the time period. The authors also reported that fibric, hemic and
sapric classified as very compressible and they found compression index
(Cc) values for tropical peatland from oedometer test for fibric was 1.453
to 3.211, hemic was 1.29 to 2.78 and sapric was 1.15 to 2.44.
Before starting construction works in this problematic soil a proper
soil investigation require. Objectives of this study are to:
||Observe the profile of soil in peatland by field investigation
and using hand auger
||Estimates bearing capacity of peat layer in terms of N value by
using Mackintosh Probe test
||Determine physical properties of disturbed sample of peat soil collected
from site by performing various laboratory experiments
||Determine unconfined compressive strength of undisturbed sample
of peat soil by performing Unconfined Compressive Test in laboratory
||Observe compressibility and settlement characteristics of the peat
soil sample by doing Oedometer test
MATERIALS AND METHODS
Field investigation: For laboratory investigation, peat soil was
sampled from the Klang, Peninsular, West Malaysia. This research was conducted
from November, 2007 to February, 2008. Trial pits were excavated to a
depth of 1 m below the ground surface to measure ground water table and
to obtain both undisturbed and disturbed soil samples below the ground
water table. Hand Auger test (Fig. 1) was performed
to characterise soil stratigraphy and for sample collection. Mackintosh
Probe Test (Fig. 2) was performed to observe bearing
Laboratory experiment: The physical properties of peat soil which
was collected from peat layer was determined by performing moisture content,
fibre content, organic content, ash content, bulk density, linear shrinkage,
liquid limit, pH, specific gravity test. All these tests were performed
as per standard of BS1377: 1990. Unconfined compression test (Fig.
3) was performed according to the BS 1377:1990: Part 7 to determine
the unconfined compressive strength. Consolidation properties were determined
by using conventional oedometer apparatus (Fig. 4).
|| Hand auger test
|| Mackintosh probe test
|| UCT test
|| Oedometer test
RESULTS AND DISCUSSION
Close examination of each trial pit indicated that the ground water table was
below 0.3 m from the ground surface. This showed that the peat had a very high
water holding capacity. Depth of peat is 1.5 m and according to Jamil et
al. (1989) classification system it can be classified moderate depth peat.
The soil profile of the peatland by visual inspection of soil sample collected
using hand auger is shown in Fig. 5.
|| Soil profile found from field experiment
Visual observation on the peat soil indicated that the soil was dark
brown in colour. When the soil was extruded on squeezing (passing between
fingers), it was observed that the soil was somewhat pasty with muddy
water squeezed out and the plant structure was not easily identified.
Based on the visual observation, the soil can be classified as fibrous
peat mixed with vegetal fibre, wooden chips inside and roots appear top
layer. According to Von Post classification system based on its degree
of humification the peat can be classified as H4 which same
as Wong et al. (2008) findings.
From the result of Mackintosh Probe test it was found that N value varies
from 0 to 2 in peat and organic layer. Then N value gradually increases
upto 10 m depth and reaches at 40 after 10 m depth. This means bearing
capacity of peat soil is very low which was indicated Islam et al.
(2008) and Andriesse (1988). Figure 6 shows the mackintosh
probe result for three different points.
All the engineering properties such as specific gravity and bulk density
of the samples were within the range as reported by Huat et al.
(2004). Physical properties of the peat soil are shown in Table
From UCT test it was observed that unconfined compressive strength of
peat soil was very low. Result showed that it varies from 4.7 to 6.9 kPa.
Figure 7 shows the stress vs. strain curve.
From oedometer test it was observed that settlement under 1600 kPa load
is reached at 2.5 mm after 3 days. Figure 8 shows time
(square root of time in minute) vs. Settlement curve for various loading condition.
|| Result of Mackintosh probe test (N-value vs. depth)
|| Physical properties of peat soil
|| Stress vs. strain graph
|| Time vs. Settlement curve
|| Void ratio and effective stress (e-logp) curve
From void ratio and effective stress (e-logp) curve it was found that compression index (Cc) varies from 2.43 to 2.84 and swelling index varies from 0.014 to 0.016. This result is matching results which have been found by Duraisamy et al. (2007). Figure 9 shows e-logp graph for three different samples.
Based on field investigation and laboratory experimentation on peat soil
the following conclusions are made:
||Ground water table is below 0.3 m from top surface and
pure peat layer is upto 1.5 m and below peat layer there is a slightly
organic clay layer upto 6 m depth
||From Mackintosh Probe it was observed that N value varies from 0
to 2 upto 4 m depth. Then it was increased and after 10 m depth N
value exceeds 40 which mean that there is a hard layer after that
||One unique characteristics of peat soil is high water content. Moisture
content was calculated in this soil was 555.55%. Other significant
properties of peat soil its organic content, fibre content, specific
gravity, bulk density and pH which were 96.45%, 90.39%, 1.24, 1037.73
kg m-3 and 3.51, respectively.
||Unconfined compressive strength of peat soil is very low and from
UCT test it was found that the strength varies from 4.7 to 6.9 kPa
||High compressibility is the other property which makes the peat
soil difficult to construct any engineering structure over this type
of soil. From oedometer test it was observed that under 1600 kPa load
the settlement occur upto 2.5 mm after 3 days. Compression index (Cc)
varied from 2.43 to 2.84 and swelling index varies from 0.014 to 0.016
Peat soil has unique characteristics and there is a tendency in construction
industry to avoid this type of problematic soils. So, proper soil stabilisation
method which is economical and consume less time can overcome this type
of problem. This experimental data can be used to find out a proper ground
improvement method. Now a day`s deep mixing method for ground improvement
especially by columnar technique in which soil-cement columns are made
by using mixing auger and using some admixture, is becoming more popular.
Binder can be provided in dry form as peat soil has high water content.
The authors are wish to gratefully acknowledge for financial support
provided by the Postgraduate Research Fund (PPP) under University of Malaya
Research University Grant PS012-2007C and Science Fund under the Ministry
of Science and Technology, Malaysia, Project No. 13-02-03-3003.