International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Effect of Changing of Dilation Angle and the
Cohesion of Clayey Soil on Stone Column
BahadorReihani #1, Alireza Eskandarinejad *2, Masouddehghani #3
1- Department of Civil Engineering, pardis of gheshm, Iran
2- PhD Candidate, Department of Civil Engineering, Shiraz University, Iran
3- Assistant Professor, Department of Civil Engineering, University of Hormozgan, Iran
Abstract— Stone columns have been used as an effective
technique for improving the engineering behaviour of soft clayey
grounds and loose silt deposits. The soil improvement via stone
columns are achieved from accelerating the consolidation of
weak soil due to shortened drainage path, increasing the load
carrying capacity and/ or settlement reduction due to inclusion of
stronger granular material. The research describes the results of
numerical simulations investigating the installation effects of
stone columns in natural soft clay. The geometry of the problem
is simplified to axial symmetry, considering the installation of
one column only. A parametric study is carried out to investigate
the behavior of stone columns in different conditions. Different
parameters were studied to show their effect on the bearing
improvement and settlement reduction of the stone column. In
this modeling, the effects of dilation angle and cohesion of the soil
on the load settlement curve and the settlement ratio (SR) have
been paid.
Keywords— Stone columns, Settlement ratio, Finite element
method, ground improvement.
I. INTRODUCTION
Stone columns are a ground improvement
technique, which not only increases the overall
strength and stiffness of the foundation system, but
also modifies the properties of the soil surrounding
the columns. Design of stone columns is usually
based on their performance as rigid inclusions
(Balaam and Booker 1981; Barksdale and Bachus
1983; Priebe 1995; Castro and Sagaseta 2009) and
the alteration caused in the surrounding soil by
column installation is commonly not considered.
However, the installation effects, whether they are
positive, negative or negligible, are one of the
major concerns for an accurate design (Egan et al.
2008).
Field measurements (Watts et al. 2000; Watts et
al. 2001; Kirsch 2004; Gäb et al. 2007; Castro 2008)
have shown some of the effects of column
ISSN: 2231-5381
installation, like the increase of pore pressures and
horizontal stresses, and the remolding of the
surrounding soil caused by the vibrator penetration.
However, based on these measurements it is
difficult to achieve conclusions that can be used in
stone column design, because they relate to a
specific case and hence cannot be generalized in a
straightforward manner. There have also been
attempts to investigate these effects through
physical modeling of the process by means of
centrifuge testing (Lee et al. 2004; Weber et al.
2006), but the soils used are reconstituted and hence
not representative of natural clays.
Numerical modelling is a useful tool that may
well help to derive some conclusions or
recommendations of installation effects for column
design, if the assumptions made in the model are
validated by experimental measurements. Few
attempts (Kirsch 2006; Guetif et al. 2007) have
been made in this field. In both cases, the soil
model used was very simplistic, and not
representative of real soil behaviour: elasticperfectly plastic with a non-associate Mohr
Coulomb failure criterion. Hence, for example, it
was not possible to account for any hardening of the
soil due to installation.
Abusharar et al. (2011) were investigate on Twodimensional deep-seated slope stability analysis of
embankments over stone column-improved soft
clay. Based on the numerical results, a reduction
factor was proposed to account for the difference in
the FS when the individual column model is
converted to the equivalent area model. The effects
of the influence factors on the reduction factor were
also investigated. The comparative study shows that
the FS values obtained by the equivalent area model
are higher than those by the individual column
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
model. The results of these analyses are
summarized into a series of design charts, which
can be used in engineering practice.
Castro et al. (2011) survey on Deformation and
consolidation around encased stone columns. The
solution is presented in a closed form and is directly
usable in a spreadsheet. Parametric studies of the
settlement reduction, stress concentration and
consolidation time show the efficiency of column
encasement, which is mainly ruled by the
encasement stiffness compared to that of the soil.
Column encasement is equally useful for common
area replacement ratios but columns of smaller
diameters are better confined. Furthermore, the
applied load should be limited to prevent the
encasement from reaching its tensile strength limit.
A simplified formulation of the solution is
developed assuming drained condition. The results
are in agreement with numerical analyses.
Hassen et al. (2010) were investigate on Finite
element implementation of a homogenized
constitutive law for stone column-reinforced
foundation soils, with application to the design of
structures. The closed-form expressions derived for
such a constitutive law allow for its implementation
into af.e.m-based numerical procedure. The
computational code so obtained is then applied to
simulating the response of a foundation soil
reinforced by a group of floating columns,
expressed in terms of load–settlement curves drawn
up to the ultimate bearing capacity.
Hassen et al. (2013) studied a homogenization
approach for assessing the yield strength properties
of stone column reinforced soils. The paper
concludes with a first illustrative implementation of
the method on the derivation of an upper bound
estimate for the ultimate bearing capacity of a stone
column reinforced foundation.
Vekli et al. (2012) examined the experimental
and numerical investigation of slope stabilization
by stone columns. From the test results of the
laboratory model, and the results obtained from the
numerical analyses, it was observed that the bearing
capacity of the footing constructed on the top of the
slope in soft soil was increased; settlements were
decreased after the improvement with stone
columns (SCs).
ISSN: 2231-5381
Six et al. (2012) were investigate on Numerical
Analysis of Elastoplastic Behavior of Stone
Column Foundation. Their paper deals with the
influence of the earth pressure at rest coefficient K0
on stone columns behavior. This parameter is
related to the initial stresses caused by the
compression of in situ soil and by the installation
(expansion of column material).
II. ANALYSIS
The initial vertical stress due to gravity load has
been considered in the analysis. The stress caused
by column installation depends on the method of
construction and type of soil. In this investigation
for considering the stress due to column installation,
initial horizontal stress (K0) is increased.
Groundwater was supposed to be more than 10 m
below the ground surface. Hence, there was no need
to enter groundwater condition. In this analysis, the
improvement of the stiffness (reduction of
settlement) of the treated ground was evaluated.
Improvement of a soft soil by stone columns is due
to three factors. The first factor is inclusion of a
stiffer column material (such as crushed stones,
gravel, and so on...) in the soft soil. The second
factor is the densification of the soft soil
surrounding the stone columns during the
installation of stone column. The third factor is the
vertical drainage provided by stone columns (Guetif
et al. 2007). Therefore, the insertion of stone
columns into weak soils is not just a replacement
operation and stone column can change both the
material properties and the state of stresses in the
treated soil mass. In this analysis, the effect of
stiffness of column material and the densification of
the surrounding soft soil during the installation of
stone column were considered. A uniform vertical
displacement (ε=2%) was prescribed to the model.
The average settlement (Se) can be calculated by
the following equation (Christian and Carrier 1978):
(1)
Where q is the applied footing load, E is elastic
Modulus of the soil, and µ0 and µ1 values depend
on the depth of the footing and the thickness
between the footing base and hard strata,
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
respectively. Assuming the whole soil medium to
be homogeneous, the equivalent secant modulus
values (Eeq) have been calculated as
(2)
TABLE I: Properties of materials
Rinter
γ(kN/m3)
c(kPa)
ψ(˚)
φ(˚)
υ
E(kPa)
0.7
17
5
0
21
0.35
4000
clay
0.9
19
0
10
43
0.3
55000
stone
column
-
16
0
4
30
0.3
20000
sand
Where
The basic axisymmetric finite-element mesh and
boundary conditions used to represent the stone
column and the surrounding clay and the typical
deformed mesh for single column is shown in
Where, σ is the average applied stress, ε is the Figure1.
average strain, S is the settlement of the footing,
and L is the thickness of the clay bed (=10 m).
Figure 3 shows typical axial stress versus settlement
behavior for improved ground based on finite
element analysis at different friction angle of stone
columns material. The vertical stress versus
settlement relation is almost linear. The equivalent
Young’s modulus of the composite ground can be
obtained from average slope of the plot. Settlement
ratio (SR), settlement of the composite ground
divided by settlement of ground without stone
column at the same stress level, was calculated.
Using Eq. 1, SR can be expressed as
(3)
(4)
Where, E0 is Young’s modulus of ground without
stone column.
Fig.1 Finite-element discretization for clay with single column, Typical
deformed mesh
III. NUMERICAL MODEL
After applying settlement of the soil, plastic
The finite element program Plaxis v8 was used to
Points are shown in Figure 2. It can be concluded
develop a numerical model of a reference problem
that in this region, further strain and deformations
to study installation effects of stone columns. The
occur.
installation of only one stone column was
considered to simplify the problem to an
axisymmetric two dimensional geometry. In order
to consider a realistic situation, properties of clay
were used for the soft soil. Properties of different
materials are shown in Table 1. An axisymmetric
analysis was carried out using Mohr-Coulomb
criterion. A drained behaviour was assumed for all
materials.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
Fig. 4 SR changes with changes in the dilation angle of clayey soil
B. Effect of changing the cohesion of clayey soil
In this section, the effect of changing the
cohesion of clayey soil has been studied. According
Fig. 2 plasticPointsin theunit cellmodel
to the conducted modeling in figure 5, all diagrams
In this modeling, the effect of dilation angle and have been put together with little difference. As a
the effect of cohesion of the stone column and the result, it can be said that this changes on the
surrounding soil material on the load-settlement corrective performance of stone columns, are
curve and the settlement ratio (SR) have been paid. almost ineffective. Thus by increasing the cohesion
of clayey soil, the bearing capacity of stone
columns does not change, thatshowninFigure5.
A. Effect of dilation angle
In this section, the effects of changing the dilation
angle of clayey soil have been investigated.
According to the conducted modeling, with
increasing the dilation angle of stone columns
materials, the bearing capacity of stone columns
increases, thatshowninFigure3.
Fig3- Stress settlement behavior under loading
for different cohesion of clayey soil
IV. CONCLUSIONS
From the finite element analysis, the following
conclusions can be drawn:
Fig. 3 Stress settlement behavior under loading for
Different dilation angle of clayey soil
For quantify the results, the SR parameter that
calculation described above is used. As can be seen
in Figure 4, with increasing the dilation angle of
clayey soil from 5 to 40, the SR is reduced
approximately 17 percent.
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1- The bearing improvement ratio and the
settlement reduction ratio are increased with
decrease in undrained shear strength of the
surrounding soil for all end bearing soil
undrained shear strengths.
2- With increasing the dilation angle of clayey soil,
the bearing capacity of stone columns increases.
3- With increasing the cohesion of clayey soil, the
bearing capacity of stone columns does not
change.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 16 Number 6 – Oct 2014
[12]
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