Soil improvement and construction methods in peat - Geotechnics of Organic Soils and Peat - page 149

Figure 6.25 Raft and pile method (isometric view of raft (made of reapers), piles, side reapers and tie
members) ( after Poungchompu, 2009).

to the ground surface and partly to piles. The piles would then transfer the load to a
deeper and stronger soil layer.
Piled supports of structures are a fundamental mean of construction over all soft
soils. By carrying the structural forces to a competent layer, the problem of settlement
of the structure can be largely avoided. However, one major problem with this method
of construction is the relative movement between the supported and unsupported sec-
tions of the structure/embankment. In the past, the concept of fully piling an earth
embankment with the provision of an extensive pile cap to support the earth fill was
viewed as uneconomic. Some recent developments, however, have led to a review of
the use of pile supports as a means of fully or partially supporting lengths of earth
embankments on soft compressible soils. These methods may be used to provide tran-
sition zones between the completely piled structures and settling earth embankments
or to support earth embankments with only limited settlements.
Where settlement is to be kept to minimum, such as adjacent to a road culvert
or overhead bridge, the piles are usually driven to set. A transition zone is where the
piles are driven to length (not set) in order to grade the settlements as uniformly as
possible. The lengths of each row of floating piles are steadily reduced over a distance
to achieve the specified differential settlement tolerance. A typical section is shown in
Figure 6.26.
Large differential settlements between the piled and un-piled section of an embank-
ment have caused a number of failures to road structures built using this technique
(Huat et al. , 1994).
An example in which this method was employed specifically for soft organic
deposits is in a section of the Bakun access road project in Sarawak Malaysia, where
the soft deposits are rather extensive, 20m or so in thickness. 200

200mm RC piles
at a spacing of 1.5m to 2.0m were used, capped with a 375mm thick continuous RC
slab (Figure 6.27).
Alternatives to a continuous RC slab above the piles are to rely on the principle of
arching to distribute the weight of the earth fill to the piles with individual caps (Huat
et al ., 1994), and the possibility of reinforcing the embankment base with geosynthetic
materials such as geogrids. This technique was used for the construction of the Tungku
Link road, Brunei, over peaty soil (Younger et al ., 1997). Individual piles with pile caps
of 2.0m diameter were used, above which was placed a geogrid mattress in a 200mm
crushed stone layer.

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