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cement caused an increase of about 35% in the bearing capacity of the column (failure
load increased from 7 to 10.5 kN).
7.10 GEOGRID REINFORCEDVIBROCOMPACTED
STONE COLUMN
Ground improvement techniques such as compacted stone have been used increasingly
to reinforce soft soils and increase the bearing capacity of the foundation soil (Al-
Homud and Degen, 2006; Ambily and Gandhi, 2007; Chen
et al
., 2008). This ground
improvement technique has been successfully applied for the foundations of structures
like liquid storage tanks, earth embankments and raft foundations, where a relatively
large settlement can be tolerated by the structure. The stone columns develop their load-
carrying capacity through bulging, and near-passive pressure conditions are developed
in the surrounding soil. In weak deposits, the lateral support is significantly low and
the column fails by bulging. In order to improve the performance of stone columns
when treating weak deposits, it is imperative that the tendency of the columns to
bulge should be resisted/prevented effectively. This will facilitate an increase of load
transfer through the stone column and thus enhance the load-carrying capacity. Such
a condition can be achieved by encasing the stone columns with geosynthetics over the
full or partial height of the column (Alexiew
et al
., 2005; Black
et al
., 2007; Gniel and
Bouazza, 2009; Raithel and Kempfert, 2000; Murugesan and Rajagopal, 2006, 2009,
2010; Raithel
et al
., 2002; Yoo and Kim, 2009). The geosynthetic encasement will
significantly increase the load-carrying capacity of stone columns due to the additional
confinement by the geosynthetic. The geosynthetic encasement will also prevent lateral
squeezing of stones when the stone column is installed in some extremely soft soils,
leading to a minimal loss of stones.
The effectiveness of geogrid encasement on vibrocompacted stone columns was
investigated by Prasad
et al
. (2012) through a parametric study carried out using the
commercially available finite element package PLAXIS. The influence of parameters
such as the stiffness of geogrid encasement, the depth of encasement from ground level,
the diameter of the stone columns, spacing of the stone columns and the shear strength
of the surrounding peat were analyzed.
In order to evaluate the improvement achieved due to the geogrid encasement,
two cases were analyzed: stone columns without geogrid encasement (SC) and stone
columns encased with geogrid (GC). In order to directly assess the influence of the con-
finement effects due to encasement, the analyses were performed by applying uniform
pressure on the stone column portion alone. Analysis was also performed by applying
a load on the entire area of the unit cell, and finally loading was applied to a group of
columns having seven columns arranged in triangular pattern.
All the analyses, for column diameters, 0.6m and 1.0m and group of seven
columns, were carried out by varying
s
/
d
from 2 to 4, geogrid stiffness from 50 to
5,000 kNm
−
1
, length of encasement from 1
d
to 4
d
from the top (where
d
is the diam-
eter and
s
is the centre to centre spacing of the columns). The improved performance
was evaluated based on the reduced settlement and lateral bulging of the stone column.
Figure 7.41 shows the typical deformed mesh, at a prescribed displacement, for
the case of a single column loaded for SC and GC for
s
/
d
=
3 and
c
=
6 kPa. It is
