Geoscience Reference
In-Depth Information
The excavation and replacement method is suitable for peat with a depth of less
than about six metres. In this process the peat is excavated and replaced with stable
fill, such as sand.
The surface reinforcement and preloading methods are essentially for road con-
struction. They use geotextile, geogrids, timber or bamboo mattresses that separate
the granular fill from the soft peat on which they are placed. The geotextile also serves
to prevent the fill from becoming contaminated by the weak subsoil material, thus
preserving the better load-bearing properties of the fill. As the fill becomes thicker, the
need for separation decreases and the geotextile then becomes more of a reinforcement.
The vertical drain technique uses geosynthetic vertical drains or sand drains, and
even stone columns, in order to speed up the dissipation of pore water pressures and
hence speed up the settlement process.
Piled supports are a fundamental means of construction in all soft soils. By carrying
the structural forces to a competent layer, the problem of settlement of the structure
can largely be avoided. But this method is generally very costly. A newer alterna-
tive is geomaterials or lightweight fill, an example of the latter being EPS (expanded
polystyrene). This technique has been used for a number of road constructions in
countries like Japan and Norway, and has been successfully applied for high bridge
approach embankments in Malaysia. Because they are lightweight (about 20 times
lighter than conventional fill materials) very little pressure is actually exerted on the
existing ground, hence minimising the stability and settlement problem.
There are also several other alternatives, some of which have been applied while
others are still very much at an experimental stage. Examples of these are geocells,
geo mattress rafts, vibrated concrete columns, deep stabilization techniques with
cement and lime or chemical additives using injection or vacuum preloading, thermal
compression, electrokinetics and biogrout.
Having said all the above, whatever techniques that we may employ, unless the
structure's load is exclusively transferred to a hard stratum, the stability of any structure
built on peat will inevitably depend on the overall change of the peat with time. For
example, over-drainage of a peat land will result in general land subsidence, and this in
turnmay result in the instability of structures such as roads and buildings built there. As
another illustration, there is a conflict between the need to drain peat for traffic, such
as farm machinery, to gain access and the moisture supply required for crop growth.
The water table has to be lowered sufficiently to attain the required bearing capacity
for mechanization, which takes water from the crops. Thus an integrated approach is
vital for peat land development. It cannot be done in isolation.
