Geoscience Reference
In-Depth Information
the following classifications: (i) mechanical modifications, (ii) hydraulic modifica-
tions, (iii) physical and chemical modifications and (iv) modifications by inclusions
and confinement (Hausman, 1990). This classification is explained in detail in
Table 6.1. The methods that are related to peat will be discussed later.
•
Stage construction and preloading:
used to overcome problems of instability in
fills constructed over weak deposits. It takes time but can be accelerated by the
use of vertical drains (wick drains) and stability can be enhanced by geosynthetic
reinforcement. Loading can be achieved by the placement of loads on the surface
or vacuum consolidation.
•
Deep
in situ
mixing (lime-cement columns):
forced mixing of lime, cement or both
with soft deposits to form stabilized soil columns (application to peat is still under
development).
•
Stone columns:
filling water jetted holes in soft ground with compacted gravel.
•
Piles:
expensive, but reliable for building foundations if used with suspended
floors. Also used for embankment support in some countries.
Thermal precompression:
moderate ground heating (15-25
◦
C) is used to accelerate
settlement and reduce long-term compression upon cooling (field tested but no
commercial applications yet).
•
•
Preload piers:
preload piers or geopiers involve densely packing stone in layers
in a hole; i.e. an intermediate foundation system. They are intended for radial
precompression of the ground and are currently under development.
•
Reduce driving forces by lightweight fill:
use of woodchips, sawdust, tire chips,
geofoam or expanded shale. Lighter but sufficiently strong and stiff fill materials.
The methods for improving the engineering performance of peat are described in detail
in the following sections.
6.2 EXCAVATION - DISPLACEMENT AND REPLACEMENT
One solution is to replace the poor soil by excavation or by dumping suitable imported
fill materials if the soils are of very high liquid type, as illustrated in Figure 6.4. This
is naturally very expensive on materials. It is also difficult to control the underground
movement of the material. In addition, there must be an environmentally accept-
able location to waste the excavated soil within an economically acceptable haul
distance and there must be a source of adequate fill, again within an economically
acceptable haul distance (Jarrett, 1995). Furthermore, this method can only be effective
for up to a depth of 5-6m. It is normally used for surface peat in France to avoid main-
tenance work related to long-term settlement and horizontal movements (Magnan,
1994).
An example case history is the 1.4 km Samariang ring road project in the division
of Kuching, Sarawak, Malaysia (Figure 6.5). The site was reported to be overlain by
soft peat and organic soils 5.0m to 7.0m deep, with very high moisture content (1000-
1700%), low specific gravity (1.38-1.54) and low bulk density (1.015-1.025Mgm
−
3
).
The method adopted in this construction is to excavate the soft deposits to a depth of
4.5m to 5.5m below the existing ground surface with a side slope of 1:3. Extensive
dewatering had to be employed because of the high water table. Sand was used as the
