Geoscience Reference
In-Depth Information
which dictates the size of the inflow surface (Abuel-Naga and Bouazza, 2009). PVDs
are rectangular in cross-section and not round. In order to design for geotechnical or
remediation purposes or some laboratory work, an equivalent circular drain diameter
of PVD must be used. There are many methods and procedures for determining the
equivalent diameter of a PVD with a rectangular cross-section (Welker
et al
., 2000).
Long and Covo (1994) used an electric analogue approach to develop their equation
(Eqn. 6.1). Abuel-Naga and Bouazza (2009) conducted a numerical study and stated
that the diameter of a PVD well under equal flow conditions shows that for such a
rectangular section (0
.
033
0.0875), the equivalent diameter is a function of
PVD width only and is in agreement with Long and Covo's (1994) findings:
≤
t/w
≤
d
eq
=
0
.
5
w
+
0
.
7
t
(6.1)
where
d
eq
=
equivalent diameter,
w
=
width of PVD and
t
=
thickness of PVD.
6.4 DEEP STABILIZATION
In Sweden and Finland, deep stabilization techniques are quite popularly used for sta-
bilization of soft soil (Åhnberg
et al.
, 1995b). Chemicals such as lime may be used to
improve the strength and settlement characteristics of the soft soils. But the effective-
ness of the method varies. Unslaked lime has been replaced with cement/lime mixes,
usually in the ratio of 50:50, while pure cement has also been used. The strength of
silt and clay can be improved up to 30-fold. In peat, however, the strength gain may
not be that high. The high water content and low strength of peat require a significant
gain in strength, which is inhibited by organic matter (Figure 6.11).
However, by adding enough stabilizers, such as cement, the strength gain may be
adequate. Pure cement is found to be more effective in peat than cement/lime mix-
tures, and certain additives, such as gypsum, improve the cement's reactivity. The
strength gain is mainly due to hydration products formed by cementitious reactions. It
depends on the type of soil, dosage of binder, water content and curing conditions. The
mechanisms whereby organic matter interferes with strength gain are not completely
understood but are thought to include the following (Janz and Johansson, 2002):
•
Organic matter can alter the composition and structure of calcium silica hydrate
(C-S-H) gel, a cementing compound that forms bonds between particles, and also
the type and amount of other hydration products, e.g. ettringite.
•
Organic matter holds 10 or more times its dry weight in water and may limit the
water available for hydration.
•
Organic matter forms complexes with aluminosilicates and metal ions, interfering
with hydration.
Figure 6.12 illustrates the application of lime/cement columns as deep stabilizers.
Huttunen
et al
. (1996) report the unconfined compressive strength of peat with
different degrees of humification. They found that strength increases with increas-
ing dosage of cement and decreases as humification increases, and the chemical and
