Geoscience Reference
In-Depth Information
Recently, a new technique of applying vacuum pressure to soft clayey subsoil has
been developed in which the vacuum pressure is combined with a special prefabricated
vertical drain (PVD). This consists of a PVD, a drainage hose and a cap connecting
the PVD and the hose, and is known as a cap-drain (CPVD) (Fujii
et al
., 2002). Chai
et al
. (2008) explained that the method uses a surface or subsurface soil layer as a
sealing layer and there is no need to place an airtight sheet on the ground surface;
consequently there is no need to 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 variations in this 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).
Vacuum consolidation is an effective method for stabilization of soft soils in par-
ticular. Soils like peat have a high ground water level horizontal permeability (
k
x
),
which is much greater than its vertical permeability (
k
z
) (Mesri and Ajlouni, 2007).
This increases the rate of consolidation. By applying the vacuum below the water
table, so that it is used in combination with dewatering, the equivalent preload can be
increased significantly (Thevanayagam
et al.
, 1994). In comparison with pre-loading
with vertical drains, the vacuum consolidation method has more advantages, such as
(Cognon
et al
., 1994; Jacob
et al
., 1994; Shang
et al
., 1998; Chai
et al
., 2006):
(i)
no/less fill material is required;
(ii)
construction periods are generally shorter;
(iii)
there is no need for heavy machinery;
(iv)
a vacuum pressure up to 600 mm Hg (80 kPa) can be achieved in practice using
the vacuum equipment available, which is equivalent to a fill 4.5m in height;
(v)
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 soil.
However, vacuum consolidation also has some shortcomings. For example, 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 from 60 to 80 kPa (Bergado
et al
., 1998; Tang and Shang, 2000; Qiu
et al
., 2007). A system with 75% efficiency
shows results with only 4.5m of equivalent surcharge and to stabilize very soft soil
and peaty soils, which need more negative pressure, pre-loading is necessary.
The area affected by vacuum consolidation has been investigated by many
researchers (Noto, 1990; Thevanayagam
et al
., 1994; Leong
et al
., 2000; Hayashi
et al
., 2003; Chu and Yan, 2005) and it has been reported that a distance of 0.7-
1.5m between the drains is the zone of effective area. However, Gabr
et al
. (1996a,b)
emphasized that the zone of effective area is approximately 10 times the effective
diameter of the PVDs. One of the key parameters usually addressed in PVD design is
the determination of the equivalent diameter of band-shaped drains to radial drains,
