Soil improvement and construction methods in peat - Geotechnics of Organic Soils and Peat - page 130

remain in place. Geosynthetic vertical drains are commonly 100mm wide and 5mm
thick and are composed of a plastic drainage core wrapped in a non-woven geotex-
tile sock. They come in long rolls and are commonly punched into the ground with a
mandrel in a triangular or square configuration with a spacing of 1m to 2m to what-
ever depth is required. This punching process causes a problem for this technology,
namely the displacement and disturbance of the soil by the mandrel and the smear-
ing of the geotextile sock by the disturbed soil. This can cause a radical decrease in
permeability at the drain entrance and prevent the free drainage expected. There has
been considerable discussion concerning this topic and whether the drains do in fact
work. An example is presented by Othman et al. (1994). Specifically, the high initial
permeability of peat may render the drains useless. Later in the consolidation process,
permeability might decrease to levels making the drains effective, but buckling and
filter contamination might render them less effective before then (den Haan, 1997).
Koda and Wolski (1994) reported successful applications of this technique for
construction in peat in Poland. Figure 6.8 shows the peat soil profile, construction
schedule and settlement performance of their field trial.
However, as mentioned above, there is some controversy regarding the effective-
ness of vertical drains in peat. Surficial peats often exhibit very high permeability until
they are compressed, and a large part of their total compression takes place under
constant effective stress. Vertical drains may be effective in accelerating strength gain
(Kurihara et al. , 1994) but not total settlement. The effectiveness of the strip drains
may additionally be limited by deterioration and buckling of the drain and the con-
sequent decline in discharge capacity. However, the general consensus is that vertical
drains are effective tools for construction over peat (Edil, 1994).

6.3.3 Vacuum preloading
Vacuum preloading techniques have also been developed, in which an imperme-
able cover is placed over the surface and a vacuum pressure is created under it to
speed consolidation (Figure 6.9). This technique is often used in conjunction with
vertical drains. One interesting aspect of this method is the possibility of achieving
preloading or precompression without the stability problem often associated with high
embankments.
The vacuum is applied by means of a tight sheet or membrane and vacuum pump.
This method enables the equivalent construction of a very high embankment on very
soft ground to be made over a relatively short period of time by reducing the develop-
ment of shear strain in the soil (Mitachi et al ., 2003). An interesting observation about
this technique when applied to peat was made by Hayashi et al. (2003): the ( C u

σ v )
of the peat layer showed a higher value than that of the untreated peat layer (Fig-
ure 6.10). The ( C u

σ v ) of the peat layer to which only the prefabricated vertical drain
method is applied demonstrated a higher value than that of the untreated peat layer.
The difference, however, is not as significant as in the vacuum preloading method.
Hayashi et al. (2003) also suggested that the spacing of vertical drains in peat must
be less than 90 cm.
Further, vacuum dewatering (vacuum consolidation method), with or without
preloading, is a soft soil improvement methods and it has been applied in a number of
countries (Shang et al ., 1998; Chu et al ., 2000; Mohamedelhassan and Shang, 2002;