Geoscience Reference
In-Depth Information
and there is no need to place an airtight sheet on the ground surface, and therefore no
worry about air leakage caused by damage to the sheet. The thickness of the surface
sealing layer can be determined according to the field conditions, and generally the
variation in thickness will not cause additional cost. In this method, the thickness of
the surface sealing layer is estimated as suggested by Chai
et al
. (2008).
In comparison with pre-loading with vertical drains, the vacuum consolidation
method has more advantages: (1) no/less fill material is required; (2) construction
periods are generally shorter; (3) there is no need for heavy machinery; (4) a vacuum
pressure up to 600mmHg (80 kPa) can be achieved in practice using the vacuum
equipment available, which is equivalent to a fill 4.5m in height; and (5) there is no need
to control the rate of vacuum application to prevent bearing capacity failure because
applying a vacuum pressure leads to an immediate increase in the effective stress in the
soil (Cognon
et al
., 1994; Jacob
et al
., 1994; Shang
et al
., 1998; Chai
et al.
, 2006).
However, this method also has certain disadvantages. A poor skeleton of soft clays
and organic soils needs a large number of columns for injection to get high strength,
which increases the cost as well as the construction time of the project, and applying
a very powerful injection pump is not applicable in all projects because of the large
weight of these instruments, requiring especial techniques to access the soft soils in
the field to avoid sinking. Further, the poor skeleton of these soils may not be able
to tolerate the high pressure of a powerful injection pump. Injection and grouting
methods may be less successful when the voids consist of many fine interstices that are
not always interconnecting (complete filling being very difficult to achieve). When it
is difficult to confine the grouting, it may need either more injection points or more
powerful injection instruments, thus increasing the cost.
Vacuum consolidation also has some shortcomings. The applied vacuum is limited
by atmospheric pressure and it may cause cracks in the surrounding surface area due
to consolidation-induced inward lateral displacement of the ground (Thevanayagam
et al
., 1994; Tang and Shang, 2000; Chai
et al
., 2006). Furthermore, due to the
complication of air-water separation and badly sealed
in situ
boundary conditions, the
efficiency of the system decreases. Theoretically, the maximum vacuum pressure that
may be applied is one atmosphere (about 100 kPa), but practically achievable values are
normally in the range 60-80 kPa (Tang and Shang, 2000; Qiu
et al
., 2007). A system
with an efficiency of 75% shows results with only 4.5m of equivalent surcharge, and
for the stabilization of very soft and organic soils (which need more negative pressure),
pre-loading is necessary. In addition, in very soft soils, with the passage of time and
with increasing settlement, the efficiency of the system will reduce. This is because the
PVDs will crumple and be unable to discharge water properly.
A new deep mixing method (DMM) for stabilization of peat has been developed
by Kazemian (2011), keeping in mind the concepts of injection grouting and vacuum
dewatering, as discussed earlier in this chapter. A detailed procedure and its advantages
are presented below.
Procedure of the new DMM technique (combination of injection and
vacuum technology)
1.
The vacuum well is installed and the vacuum is started to remove (any) water from
the voids within the soil by using its auger at predefined positions and depths.
