Geoscience Reference
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developed is the construction of the Kuching ring road, north of Kuching city (Toh
et al ., 1994). Bamboo was used to aid in the rolling out of the geotextile over the soft
ground. Wide stability berms were constructed on both sides of the main embankment
section to maintain stability. The subsoil was reported to comprise 2-4m of very soft
organic soil and peat, underlain by a 3-5m thick stratum of firm to stiff clayey silts.
The soft organic soil was reported to have the following geotechnical parameters: very
high natural water content of 280-1000%, high compressibility, with compression
index, C c , in the range of 5.6-11.0, and extremely soft, with undrained shear strength
less than 10 kPa.
In a number of instances, only the upper few meters of the very soft deposits were
excavated and replaced with stable fill. This was one of the methods employed in the
construction of some sections of the Bakun access road project in Sarawak Malaysia,
where the nominal thickness of the soft subsoil is quite shallow and the embankments
are of moderate height (2-5m). Some 3m of the very soft subsoil was excavated and
replaced with sand fill. A single layer of woven geotextile (Stabilenka 200) was used
as both separator and reinforcement layer.
6.3.2 Preloading
The preloading principle was also adopted in the above-mentioned example to
minimize post-construction settlement.
It is interesting to note that preloading is not a recent development, but it does
still represent the method of construction with the best economic and engineering
returns (Carlsten, 1988). It is, however, underused because of the necessity of prior
planning (Jarrett, 1987). The basis of preloading is to place a temporary fill over the
construction site that is thicker than the final design fill. This causes settlement to occur
more rapidly than would have occurred under the final fill design height. The preload
is ideally left in place until it has settled more than the total amount that the design fill
is expected to settle in its design life. Then the thickness of the preload fill is reduced
to the final design thickness, with the expectation that most of the settlement has now
finished. Calculations for rates of settlement, however, are notoriously difficult for
all peat and organic soils as they commonly have very large secondary compressions,
and the time-settlement relationship is often not of the characteristic form making it
difficult to separate primary from secondary settlement (den Haan and El Amir, 1994).
In fact it is common to monitor the rate of settlement of the fill after construction and
to make projections of the time required for settlement based on initial field settlement
(Edil and Simon-Gilles, 1986; Garga and Medeiros, 1995; Huat, 1996, 2002a).
An experience with peat preloading has been reported by Kirov (1994) for a Varna
West Harbor project in Bulgaria. The subsoil profile of the site comprises some 17m
thick soft soil deposits, and alternates layers of peat and organic clays overlying marls.
The site was first excavated to 3.75m, then filled with a layer of coarse sand 0.8m
thick, preloaded with hydraulically filled sand to another 6m, and maintained for
one year.
In France, for cases of deep peat ( > 6m), preloading or surcharge and stage
loading are used to control the deformation and stability of structures built on peat
(Magnan, 1994).
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