Geoscience Reference
In-Depth Information
mixing blades, and (3) injecting part of the binder during penetration and the rest dur-
ing withdrawal. In the case of peaty ground, to which a large amount of the binder is
applied, the viscosity of soil may increase if the entire amount of binder is injected dur-
ing penetration, thus decreasing its efficiency (Hayashi
et al
., 2003). Al-Tabbaa
et al.
(1999) and Hayashi and Nishikawa (2003) reported that the number of drilling and
mixing cycles in DMM is 400-450 times m
−
1
to achieve high efficiency and adequate
mixing. Hayashi and Nishikawa (1999) developed Equation (7.12) to find the number
of mixings per metre.
T
=
N
×
(
R
P
/S
P
×
W
i
/W
+
R
W
/S
W
)
(7.12)
where:
T
=
number of mixings per metre in depth (times m
−
1
),
N
total number of mixing blades,
R
P
&
R
W
=
=
penetration and withdrawal speeds (mmin
−
1
),
S
P
&
S
W
=
rotation speeds of mixing blades during penetration and withdrawal (rpm),
W
i
=
binder injection amount during penetration, and
Total amount of binder (kgm
−
3
).
W
=
Grouting is generally used to fill voids in the ground (fissures and porous struc-
tures) with the aim of increasing resistance to deformation; augmenting cohesion,
shear-strength and uni-axial compressive strength; and finally (even more frequently)
reducing conductivity and interconnected porosity in an aquifer (Moseley and Kirsch,
2004). The vacuum dewatering method (vacuum consolidation method), with or with-
out preloading, is one of the methods for improving soft soils and has been applied
in a number of countries (Hayashi
et al.
, 2003; Chai
et al
., 2006). Kjellmann (1952)
stated that when a vacuum is applied to a soil mass, it generates a negative pore-water
pressure. If the total stress remains constant (unchanged), the negative pore pressure
results in an increase in the effective stress in the soil, which leads to consolidation.
In the vacuum dewatering method, a typical on-land system consists of an airtight
membrane over the ground surface, anchored along the periphery of the site. Slotted
collection pipes are embedded into a sand blanket beneath the membrane. The outlet
is a solid pipe connected to a vacuum pump and discharge system. Typically, for an
existing soft ground site, vertical wick drains (prefabricated vertical drains - PVDs)
are installed beneath the sand blanket up to a depth of about 1m above the bottom
of soft soil deposit (Thevanayagam
et al
., 1994; Hayashi
et al.
, 2003). This technique
is often used in conjunction with vertical drains as well as preloading methods. It
enables the construction of a very high embankment on very soft ground to be made
over a relatively short period of time by reducing the development of shear strain in
the soil. In addition, the amount of surcharge fill may be reduced by several metres if
a vacuum pressure of at least 70% of atmospheric pressure is applied and sustained
(Rujikiatkamjorn
et al
., 2008).
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) consisting of a PVD, a drainage hose and a cap connecting the
PVD and the hose, known as 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
