ASSESSMENT OF FACTORS INFLUENCING IMPROVEMENT OF SOFT CLAYS USING GEOSYNTHETIC ENCASED STONE COLUMNS

Abstract

A common approach for ground improvement of soft clays is installing stone columns to increase the bearing capacity, reduce the settlement, and accelerate the consolidation time. However, stone columns may not have sufficient capacity in very soft formations due to the lack of confinement of the enfolding soft soils. The contamination of column aggregate by the soft clay ingress also inhibits the beneficial drainage function of columns. An ideal solution for the abovementioned shortcomings is to include geosynthetic encasements around columns. The encased stone columns have been recently increasingly used in place of the uncased columns. However, the available geotechnical literature covering this relatively recent improvement technique is still not fully mature to completely assimilate the varied factors influencing this technique. This study uses a three-dimensional finite element analysis to evaluate this technique's performance and parameters for different column configurations and geotechnical conditions. A published well-instrumented case study for a test embankment is utilized to validate the envisaged model for the parametric analyses. The extensive measured data, including vertical and horizontal deformations, excess pore water pressure, and geosynthetic tensile stresses, are used in the validation process. The model was also adjusted to match an established analytical method that covers specific limited cases; the model results agree well with the analytical solution. A parametric study is conducted to address the impacts of the different influential factors, including the column diameter, the stones' modulus of elasticity and friction angle, the geosynthetic encasement length and strength, and the working platform thickness. It was concluded that by increasing the diameter, modulus of elasticity, friction angle, geosynthetic length, and geosynthetic strength, the settlement under the embankment nonlinearly decreases, and the time needed to reach the final settlement decreases with different influences of each parameter. The results are presented in dimensionless graphs to allow the geotechnical practitioner to best exploit this technique by optimizing the configuration and the parameters of the encased columns using the presented results.

Full text

AIN SHAMS UNIVERSITY
F
ACULTY OF
E
NGINEERING
S
TRUCTURAL
E
NGINEERING
D
EPARTMENT
16
TH
I
NTERNATIONAL
C
ONFERENCE ON
S
TRUCTURAL AND
G
EOTECHNICAL
E
NGINEERING
Mega Projects: Bridges, Tunnels & Tall Buildings
1
2
ASSESSMENT OF FACTORS INFLUENCING IMPROVEMENT OF
SOFT CLAYS USING GEOSYNTHETIC ENCASED STONE
COLUMNS
1
2
ENG. YAHYA HOSSAM ABDELFATTAH
Department of Civil Engineering, Ain Shams University
Abbasya, Cairo, Egypt
E-mail: Yahya.hossam@eng.asu.edu.eg
PROF. DR. SAYED MOHAMED EL-ARABY
Department of Civil Engineering, Ain Shams University
Abbasya, Cairo, Egypt
E-mail: Sayed_mohamed@eng.asu.edu.eg
AND DR. MAHMOUD S. HAMMAD
Department of Civil Engineering, Ain Shams University
Abbasya, Cairo, Egypt
E-mail: mahmoud.elshawaf@eng.asu.edu.eg
1
2
ABSTRACT
A common approach for ground improvement of soft clays is installing stone columns to
increase the bearing capacity, reduce the settlement, and accelerate the consolidation
time. However, stone columns may not have sufficient capacity in very soft formations
due to the lack of confinement of the enfolding soft soils [1]. The contamination of
column aggregate by the soft clay ingress also inhibits the beneficial drainage function of
columns. An ideal solution for the abovementioned shortcomings is to include
geosynthetic encasements around columns. The encased stone columns have been
recently increasingly used in place of the uncased columns. However, the available
geotechnical literature covering this relatively recent improvement technique is still not
fully mature to completely assimilate the varied factors influencing this technique. This
study uses a three-dimensional finite element analysis to evaluate this technique's
performance and parameters for different column configurations and geotechnical
conditions. A published well-instrumented case study for a test embankment is utilized to
validate the envisaged model for the parametric analyses. The extensive measured data,
including vertical and horizontal deformations, excess pore water pressure, and
geosynthetic tensile stresses, are used in the validation process. The model was also

adjusted to match an established analytical method that covers specific limited cases; the
model results agree well with the analytical solution. A parametric study is conducted to
address the impacts of the different influential factors, including the column diameter, the
stones' modulus of elasticity and friction angle, the geosynthetic encasement length and
strength, and the working platform thickness. It was concluded that by increasing the
diameter, modulus of elasticity, friction angle, geosynthetic length, and geosynthetic
strength, the settlement under the embankment nonlinearly decreases, and the time needed
to reach the final settlement decreases with different influences of each parameter. The
results are presented in dimensionless graphs to allow the geotechnical practitioner to best
exploit this technique by optimizing the configuration and the parameters of the encased
columns using the presented results.
KEYWORDS
Geosynthetics, Soft Soil, Stone Columns, Ground Improvement, Geosynthetic encased
columns, GEC, Encased Columns.
1 INTRODUCTION
Civil work faces many challenges, including the difficulty of dealing with soft soil
stratum. As the soft soil deposits are mainly characterized by their high void ratio and
their needle-like shaped microscopic particles, they tend to exhibit larger deformation
values and lower shear strength compared to other soil types[1]. Thus, to overcome such
problems when dealing with soft soil deposits, different soil improvement methods can
be used such as Soil improvement using Geosynthetic encased stone columns (GECs),
which is one of the types of improvement[2].
(GECs) are mix of well-graded stones with pre-determined modulus of elasticity and
void ratio, surrounded by geogrids which can’t stop water from flowing but can increase
the confinement of the stones and help it not to mix with surrounding soil[3].
Compacted granular columns are one of the most effective soft ground treatment
solutions[4]. Stone columns are used to reduce the settlement due to its higher stiffness
compared to the soft soil, and to speed up the consolidation process by introducing
radial drainage due to the high permeability of the columns[5]. To achieve the required
settlement reduction, the columns must have high load-carrying capacity which is
achieved by many factors including high confinement of the columns’ body. However,
for very soft soil formations, such confinement is not reached, thus, geosynthetic
encasement can be utilized to provide the required confinement for the stone columns.
Furthermore, geogrids also prevent the column material from mixing with the soft soil
while allowing water to drain through the columns[6].
Embankments may experience many problems when built on soft soils such as
excessive deformations in both vertical and horizontal directions[7]. Geosynthetic
encased stone columns can be used to support such embankments and help reduce the
deformations and local instability.
In the current proposed research, three-dimensional finite element modeling using finite
element software “Plaxis 3D” is used to study the performance of embankments built on
soft soils improved utilizing geosynthetic encased stone columns.

2 CASE STUDY
2.1 General description
Test embankment will be described along with the soft soil deposits and its’
characteristics. Site investigation tests were performed, and results will be discussed to
illustrate the geotechnical parameters of soil layers in this area. In-situ tests were
performed to study the behaviour of the soft soil improved with geosynthetic encased
columns (GECs), And other behaviours like excess pore water pressure, vertical stresses
and geosynthetic tensile forces generated in the GECs. A summary of instruments used
is presented along with the procedures to prepare the test area with the most important
results from the [5
]
. The test embankment was built in Rio de Janeiro, for a steel
company in stockyard. The dimensions of the test area are approximately 0.8 km x 0.6
km. Fig. 1 shows the test area in Brazil Rio de Janeiro general location and close view
of the area [8].
Fig. 1 General view of the TKCSA plant (Hossein & Babaei, n.d.)
In 2008, Thirty-six (36) GECs were installed in the test area by displacement method in
a square form of 6m x 6m with diameter 0.8m and length 11.0m as shown in Fig. 2. [10]
Fig. 2 Overview of GECs layout (Hossein & Babaei, n.d.)
After four years, in 2012, the test embankment was constructed over the GECs. And
extensive site investigation tests were performed to study the main geotechnical
characteristics of the soil below. The height of the embankment is 5.3m with slope
1:1.5, applying total vertical stress of 150 kPa. Vertical deformation, Horizontal

deformation and excess pore water pressure were measured through the consolidation
period using the instruments shown in Fig. 3. [11]
Fig. 3 Embankment side view, column layout and location of the instruments [8]
2.2 Subsoil properties
Fig. 4 shows the layers of soil, which consist of a working platform with thickness of
1.8m to ensure stable construction activities of the embankment. Followed by two layers
of soft clay, The first layer is 6m of soft clay with shear strength below 20kPa. The
second layer is approximately 2.5m of soft clay with shear strength 50 kPa. Between
these two layers, there is a thin layer of sand with thickness of 0.5m. The ground water
was located at the top of the first clay surface [12]
Fig. 4 Geotechnical profile of the test area and location of the in-situ tests [13]
2.3 Geosynthetic stone columns
The stone columns were made using aggregate size from 10mm to 35mm in diameter.
The diameter of stone columns was 0.8m with height 11m.Elastic modulus E was
calculated numerically and experimental to be 80 MPa, Friction angel φ equals 40o. The

spacing between the stone columns was 2m in each direction [9]
The geosynthetics used has allowable tension and stiffness moduli of 95 kN/m and 1750
kN/m, respectively. There was also a biaxial geogrid reinforcement placed under the
embankment with an axial tensile stiffness of 2200 kN/m and a maximum tension of 82
kN/m. [14]
3 BACK ANALYSIS OF THE CASE STUDY
A finite element model using “Plaxis 3D” was used to simulate the test embankment,
subsoil layers and GECs. The modelling steps are presented in this section, then the
results were used to verify the model.
3.1 Model geometry and boundary conditions
In the 3D model shown in Fig. 5, we have simulated three rows of GECs in the center of
the test embankment. Also, only half of the embankment 6m width and half of the
GECs were simulated due to symmetry in both longitudinal and transverse directions.
The model has a lateral dimension 50m to avoid any influence of outer boundaries. Due
to symmetry, no displacement was allowed in the direction perpendicular to symmetry
plans. But the model was free to move in the vertical direction at the lateral borders. The
finite element mesh was set to Medium. Mohr-column model was chosen for granular
materials like embankment and granular fill of the column as well as sand layers. On the
other hand, soft soil model was chosen for clay layers. Water level was set to -1.5m
below surface as reported in in-situ measurements. The soil volume in PLAXIS 3D is
modeled by means of 10-node tetrahedral elements to represent soft clay layers,
granular columns, and embankment fill material. The 10-node tetrahedral elements are
created in the 3D mesh generation procedure.
Table 1 Layers thickness
Layer name
Thickness (m)
Working platform
1.5
Soft clay 1
5.3
Sand lens
0.6
Soft clay 2
2.6
Sand lens
1.4
Stiff clay
2.8
Dense sand
5.8

Fig. 5 finite element model
3.2 Material Properties
Mohr-column failure criteria was used to model granular materials. As for clay layers,
soft soil model was used to simulate clay I and clay II. The following table shows the
properties of materials modeled in the “Plaxis 3D” model and these parameters were
obtained from detailed site investigation and laboratory tests.
Table 2 Mohr-Coulomb parameters used for the granular material type. [15]
Table 3 Soft soil model properties used for soft clay layers. [15]
Material Saturated
unit weight
Drained angel of
friction
Drained
cohesion
Drained
elastic
modulus E
(MPa)
Coefficient
Of horizontal
permeability
Coefficient
Of vertical
permeability
Embankment
28
38
0
50
1
1
Granular column
20
40
0
80
10
10
Working platform
19.5
33
0
15
0.6
0.6
Sand lens
18.5
30
0
22
0.5
0.5
Dense sand
20
38
0
30
1
1
Soil type
Saturated
unit
weight
Drained
angel of
friction
Drained
cohesion
Coefficient
of compression
Coefficient
of swelling
Coefficient
Of horizontal
permeability
Coefficient
Of vertical
permeability
Initial
voids
ratio
OCR
value
Soft clay
I
14.4
26
2
1.14
0.096 3 x 10-5 2 x 10-5 3 1
Soft clay
II
17.2 28 3 0.21 0.068 8.8 x 10-5 5.6 x 10-5 2.12 1.04
Stiff clay
17.8
30
12
0.09
0.0081
3.3 x
10
-
2
1.6 x 10
-
2
0.87
1

3.3 Construction phases
3.3.1 Initial phase
As the embankment was built after the installation of GECs with four years, The initial
phase had to have the GECs installed and activated, and the basal geogrid activated. Then
in the following phases, the embankment was activated according to the construction
schedule illustrated below.
3.3.2 Construction phases
Embankment was constructed over 65 days. Each stage consumed different intervals and
had different constructions heights to reach the total height of 5.3m eventually. The stages
vary between construction stages in which the load is applied, and consolidation stages
in which the loads results in consolidation for clay layers. The following table shows the
construction stages and their time intervals that were used in modelled the test
embankment. Taking into consideration the fact that the construction has begun four years
after the installation of GECs, initial displacements were set to zero and the stone column
installation effect was not taken into consideration.
Table 4 Details of construction steps
Fig. 6 Measure settlement readings on the column and the soil bed with respect to construction
height [6]
Calculation
step
Type of analysis Layer
thickness
Total embankment
height
Consolidation
interval days
Event
Step 1
Consolidation
1.5
1.5
3
Construction
Step 2
Consolidation
1.5
1.5
10
Consolidation
Step 3
Consolidation
1.5
3
2
Construction
Step 4
Consolidation
1.5
3
32
Consolidation
Step 5
Consolidation
1.3
4.3
2
Construction
Step 6
Consolidation
1.3
4.3
14
Consolidation
Step 7
Consolidation
1
5.3
2
Construction
Step 8
Consolidation
1
5.3
180
Consolidation

Table 5 Details of calculation phases on “Plaxis 3D”
3.4 Boundary Conditions
The water level was set to -1.5m, and the boundaries in X and Y directions were closed
for water flow, while they were open in both Z directions.
The deformation conditions were set to be Normally Fixed in both X and Y directions
due to symmetry, however it was set to Fully fixed in Zmin direction and Free in Zmax
direction.
3.5 Numerical model outputs
The results from the 3D model were compared with the field provided measurements.
Instruments were installed to measure vertical settlement, horizontal deformation, excess
pore water pressure and vertical stresses under test embankment.
3.5.1 Settlement
Fig. 6 below shows the measured settlement in two locations, (S1) on the top of GEC,
and (S2) at the mid-point between two GECs, these measurements are compared with the
results from the numerical analysis. The settlement values increased along with
construction stages along the 65 construction days to reach approximately 350mm. then
the settlement continued till the end of monitoring period 180 days, to reach
approximately 500mm.
As shown in figures, the settlement in the in-situ and in numerical analysis were very
similar to each other with small differences due to approximations.
Calculation
Phase
Calculation type
Loading type Pore
pressure
calculation
type
Layer
thickness
Total
embankment
height
Consolidation
interval days
Phase 1
Consolidation
Staged construction
Phreatic
1.5
1.5
3
Phase 2
Consolidation
Staged construction
Phreatic
1.5
1.5
10
Phase 3
Consolidation
Staged
construction
Phreatic
1.5
3
2
Phase 4
Consolidation
Staged construction
Phreatic
1.5
3
32
Phase 5
Consolidation
Staged construction
Phreatic
1.3
4.3
2
Phase 6
Consolidation
Staged construction
Phreatic
1.3
4.3
14
Phase 7
Consolidation
Staged
construction
Phreatic
1
5.3
2
Phase 8
Consolidation
Staged construction
Phreatic
1
5.3
180

Fig. 7 Settlement below the embankment at the top of GECs in
mms
Fig. 8 Settlement below the embankment mid-point
between GECs in mms
3.5.2 Excess pore water pressure
Fig. 8 and Fig. 9 below represent the variation of excess pore water pressure at variable
depths and different construction stages, and also along the monitoring period, two
piezometers were installed at depth 6m and 8m. it can be noticed that water pore pressure
increased during construction stages to reach the highest value at depth 6m in the soft
clay layer. Then the excess pore water pressure dissipated during consolidation over 180
days. On the other hand, the minimum excess pore water pressure was measured using
piezometer installed 3m below surface near the sand layer.
Fig. 9 Excess pore water pressure 6m below ground surface
Fig. 10 Excess pore water pressure 8m below ground surface
4 PARAMETRIC STUDY
After model validation, a parametric study was performed to study the influence of
working platform thickness, tensile stiffness of the geosynthetic encasement, length of
geosynthetic encasement and other parameters on settlement below the embankment. The
parameters considered for the sensitivity analyses are listed in table 8, while the other
material parameters were kept constant. A simplified model was used for this parametric
study. The model consisted of three layers. Working platform, soft clay and dense sand.
The default thicknesses of working platform and soft clay are 1.5m and 12m respectively.
Their parameters are shown in the following table 6.
0
100
200
300
400
500
0 50 100 150 200 250 300
Settlement (mm)
Time (Days)
Measured in site
Finite model
0
100
200
300
400
500
600
0 100 200 300
Settlement (mm)
Time (Days)
Measured in
site
0
10
20
30
40
50
60
0 100 200 300
Pore Water Pressure (kPa)
Time (Days)
Measured in
site
0
5
10
15
20
0 100 200 300
Pore Water Pressure (kPa)
Time (Days)
Measured in
site

Table 6 Material properties
Fig. 11 Finite element model
Table 7 Details of calculation phases on “Plaxis 3D”
Soil type
Saturated
unit
weight
Drained
angel of
friction
Drained
cohesion
Coefficient
of compression
Coefficient
of swelling
Coefficient
Of horizontal
permeability
Coefficient
Of vertical
permeability
Initial
voids
ratio
OCR
value
Soft clay I
14.4
26
2
1.14
0.096
3 x 10
-
5
2 x 10
-
5
3
1
Material Saturated
unit weight
Drained angel of
friction
Drained
cohesion
Drained elastic
modulus E
(MPa)
Coefficient
Of horizontal
permeability
Coefficient
Of vertical
permeability
Embankment
28
38
0
50
1
1
Granular column
20
40
0
80
10
10
Working platform
19.5
33
0
15
0.6
0.6
Dense sand
20
38
0
30
1
1
Calculation
Phase
Calculation
type
Loading type Pore
pressure
calculation
type
Layer
thickness
Total embankment
height
Consolidation
interval days
Phase 1
Consolidation
Staged construction
Phreatic
1.5
1.5
3
Phase 2
Consolidation
Staged construction
Phreatic
1.5
1.5
10
Phase 3
Consolidation
Staged construction
Phreatic
1.5
3
2
Phase 4
Consolidation
Staged construction
Phreatic
1.5
3
32
Phase 5
Consolidation
Staged construction
Phreatic
1.3
4.3
2
Phase 6
Consolidation
Staged construction
Phreatic
1.3
4.3
14
Phase 7
Consolidation
Staged construction
Phreatic
1
5.3
2
Phase 8
Consolidation
90% Degree of Consolidation
Phreatic
1
5.3
-

Table 8 Detailed of the parameters taken into consideration.
4.1 Boundary Conditions
The water level was set to -1.5m, and the boundaries in X and Y directions were closed
for water flow, while they were open in both Z directions. The deformation conditions
were set to be Normally Fixed in both X and Y directions due to symmetry, however it
was set to Fully fixed in Zmin direction and Free in Zmax direction. The loading phases
and sequence were set exactly as the case study model.
Table 9 Details of calculation phases on “Plaxis 3D”
Table 10 default parameters
Parameter Value
GEC Diameter
0.5m to 1.2m
Granular Column E
50MPA to 80MPA
GEC Length
4m to 15m
GEC Friction Angel
25
O
to 40
O
Geosynthetic Length
0m to 12m
Geosynthetic strength
0 to 3000
Working platform thickness
1.5m to 2.5m
Calculation
Phase
Calculation type
Loading type Pore
pressure
calculation
type
Layer
thickness
Total
embankment
height
Consolidation
interval days
Phase 1
Consolidation
Staged construction
Phreatic
1.5
1.5
3
Phase 2
Consolidation
Staged construction
Phreatic
1.5
1.5
10
Phase 3
Consolidation
Staged construction
Phreatic
1.5
3
2
Phase 4
Consolidation
Staged construction
Phreatic
1.5
3
32
Phase 5
Consolidation
Staged construction
Phreatic
1.3
4.3
2
Phase 6
Consolidation
Staged construction
Phreatic
1.3
4.3
14
Phase 7
Consolidation
Staged construction
Phreatic
1
5.3
2
Phase 8
Consolidation
Staged construction
Phreatic
1
5.3
180
Parameter
Default Value
GEC Diameter
0.8m
Granular Column E
80MPA
GEC Length
11m
GEC Friction Angel
40
O
Geosynthetic Length
11m
Geosynthetic strength
1750
Working platform thickness
1.5m

4.2 The effect of geometric and properties configuration: -
We have performed parametric study to analyze the behaviour of geometric configuration
including GEC diameter, GEC spacing, GEC length, GEC slenderness ratio, geogrid
length and working platform thickness and its effect on settlement and time required for
90% consolidation.
4.3 The effect of GEC diameter
Figures below show the influence of changing GEC diameter from 0.5m to 1.2m on the
settlement of embankment and on time needed to reach 90% consolidation while keeping
the rest of parameters as their default parameters.
The increase in diameter has a positive effect on reducing the settlement at construction
and at 90% consolidation. It is also noticed that increasing the diameter had a positive
effect on reducing the time to reach 90% consolidation. This is due to the larger of area
replacement ratio which affects the load transfer from the embankment to underlain soil.
Also, the larger diameter, the larger area for drainage for the water which results in less
consolidation time. This is also showed when investigating the effect of area replacement
ration on settlement. This agrees with Alexiew and Almedia in their book (geosynthetic
encased columns for soft soil improvement) when it stated that increasing the area
replacement ratio reduces settlement.
Two different spacings were investigated to show the behavior of soil under different
arrangement conditions. Changing the diameter showed the same behavior for different
spacings. The settlement was reduced by approximately 50% by increasing the diameter
by 100%
By increasing the area replacement ratio, the soil showed the same behavior of decreasing
the settlement. Which is normal considering the increase in the stress concentration factor
hence more stresses is transferred to the GECs rather than the surrounding soil which
results in less settlement.

Fig. 12 GEC Diameter Vs Settlement
Fig. 13 Area Placement Ratio Vs Settlement
4.4 The effect of spacing
Fig. 14 S/D Vs Settlement
Fig. 15 Spacing Vs Settlement
As for the time to reach 90% consolidation, increasing the diameter and area replacement
ratio had a positive effect which was indicated by reducing the time for consolidation
from approximately 5000 days to 3000 days which means 40% decrease in time.
200
400
600
800
1000
1200
1400
200 700 1200 1700
Settlement (mm)
GEC diameter (mm)
S=1.5m
S=2m
0
200
400
600
800
1000
1200
1400
0% 10% 20% 30% 40% 50% 60%
Settlement (mm)
Area replacement ratio
S=1.5m
S=2m
0
200
400
600
800
1000
1200
1400
12345
Settlement (mm)
S/D
S=1.5m
S=2m 870
880
890
900
910
920
930
940
950
0 0.5 1 1.5 2 2.5
Settlement (mm)
Spacing (m)
D=0.8m

Fig. 16 GEC Diameter Vs Time for 90% consolidation
Fig. 17 Area placement ratio Vs. Time for 90%
consolidation
Time for 90% consolidation reduces as a result for the increase in radial drainage which
increases the consolidation rate.
4.5 The effect of GEC length
Increasing GEC length resulted in a significant reduction in settlement. The behavior of
the GECs affects the settlement’s decrease, as the reduction in settlement became
insignificant after the GECs became end-bearing piles.
Also, increasing the length had a positive effect on time needed to reach 90%
consolidation, then increasing the length had a negative effect by increasing the time
needed.
It can be noticed that by reaching GEC length/Soil thickness to 100% which indicates the
start of bearing piles behavior, the settlement becomes constant, and the time needed to
reach 90% consolidation increases slightly and becomes constant.
Fig. 18 GEC length/soil thickness Vs settlement
2500
3000
3500
4000
4500
5000
5500
0 500 1000 1500
Time for 90% consolidation
(Days)
GEC diameter (mm)
S=1.5m
S=2m 2500
3000
3500
4000
4500
5000
5500
0% 20% 40% 60%
Time for 90% consolidation
(Days)
Area replacement ratio
S=1.5m
S=2m
0
200
400
600
800
1000
1200
1400
1600
1800
40% 60% 80% 100% 120% 140%
Settlement
GEC length/Soil thickness
Soil thickness=12m
Soil thickness=15m

4.6 The effect of slenderness ratio
The chart below shows that the slenderness ratio affects the settlement as the increase in
GEC length/GEC diameter results in reduce in settlement to a limit. But at the same GEC
length/GEC diameter ratio, the less diameter results in more settlement.
The chart below shows the effect GEC length/soil thickness has a huge effect on time
needed for 90% consolidation. The increase in GEC length causes a huge decrease in time
needed for consolidation, till the GEC length equals the soil thickness then the increase
has no effect.
Fig. 19 GEC length/GEC diameter Vs settlement
Fig. 20 GEC length/soil thickness Vs time for 90%
consolidation
4.7 The effect of geogrid length
The influence of the geosynthetic length increase shows the main reason of using
geosynthetic encased columns. By increasing the encasement length, the confinement of
the columns increases, hence the settlement decreases tremendously. Also, by increasing
the length of encasement, the time needed to reach maximum settlement decreases as
shown in figure.
Fig. 21 geogrid length Vs settlement
Fig. 22 encasement length/column length vs settlement
0
200
400
600
800
1000
1200
1400
1600
0 5 10 15 20 25
Settlement
…
GEC length/GEC diamater
D=0.8
m0
2000
4000
6000
8000
10000
12000
14000
40% 60% 80% 100% 120% 140%
Time for 90% consolidation
(Days)
GEC length/Soil thickness
Soil
thickness=12m
0
500
1000
1500
2000
0 5 10 15
Settlement (mm)
L geogrid (m)
Lcol=8m
Lcol=12m
0.8
500.8
1000.8
1500.8
2000.8
0% 50% 100% 150%
Settlement
L encasement/ L column
Lcol=8m
Lcol=12m

Fig. 23 Encasement % vs time for 90% consolidation
4.8 The effect of working platform thickness
Building a working platform is essential for the construction of stone columns. And, it
also has a great effect on reducing the settlement of soil. On the other hand, it increases
the time needed to reach the final settlement as shown in figures below.
Fig. 24 working platform thickness vs settlement
Fig. 25 working platform thickness vs time for 90%
consolidation
4.9 The effect of GEC modulus of elasticity
Figures below show the influence of changing GEC modulus of elasticity from 50 MPA
to 80 MPA on the settlement of embankment and time of settlement while keeping the
other parameters as their default values.
The graphs show a reduction in the settlement at the end of the construction stage and at
90% consolidation. That is a result of increasing the stiffness of GEC with respect to the
surrounding soil which increases the stress concentration ration accordingly and leads to
transfer more stresses to the GECs rather than the surrounding soil.
3000
4000
5000
6000
7000
8000
9000
0% 20% 40% 60% 80% 100% 120%
Time for 90% consolidation (Days)
Encasement %
GEC length=8m
GEC length=12m
750
800
850
900
950
1000
1.5 1.7 1.9 2.1 2.3 2.5 2.7
Settlement (mm)
Working platform thickness (m)
3500
3600
3700
3800
3900
4000
1.5 2 2.5 3
Time for 90%
consolidation (mm)
Working platform thickness (m)

The Increase in the modulus of elasticity had an insignificant effect on consolidation time
because it didn’t have any effect on drainage path or permeability of soil.
Fig. 26 GEC E Vs Settlement
Fig. 27 GEC E Vs time for 90% consolidation
4.10 The effect of geogrid strength
The figures show the effect of increasing the geosynthetic encasement strength on the
settlement at construction and at 90% consolidation. By increasing the geosynthetic
strength, we can reduce the construction and final settlement. Also, increasing the strength
had a positive effect on reducing the time needed to reach final settlement.
Fig. 28 Geogrid E Vs. settlement
Fig. 29 Geogrid E Vs. time for 90% consolidation
4.11 The effect of GEC friction angel
Increasing the friction angel of the aggregate material inside GECs has a small effect of
reducing the settlement of the embankment, which indicates that it is not the governing
factor of reducing the settlement.
0
200
400
600
800
1000
1200
40 50 60 70 80 90
Settlement (mm)
GEC E (MPA)
2000
2500
3000
3500
4000
4500
40 50 60 70 80 90
Time for 90% consolidation
(Days)
GEC E (MPA)
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 1000 2000 3000 4000
Settlement (mm)
Geogrid E (MPA)
2500
3000
3500
4000
4500
5000
5500
6000
6500
7000
0 1000 2000 3000 4000
Time for 90% onsolidation
(Days)
Geogrid E (MPA)

Fig. 30 GEC friction angel Vs. settlement
Fig. 31 GEC friction angel Vs time for 90% consolidation
5 COMPARISON WITH ANALYTICAL METHOD
Manual analysis methods and charts were presented in “Geosynthetic encased columns
for soft soil improvement” book by Marcio Almeida, Mario Riccio, Iman Hosseinpour
and Dimiter Alexiew. The book presented charts to calculate geosynthetic tensile stress,
normalized settlement, vertical stress on columns with respect to area displacement
ratio, applied external stress and geosynthetic strength. A simplified model was used for
this comparison. The model consisted of two layers, soft clay, and dense sand. The
thicknesses of soft clay were 10m followed by 20m of dense sand. The GECs’ length
was 11m. Area replacement ratio was set to be 15% by GEC diameter of 0.873m and
spacing 2m. The Geosynthetic strength was set to 3500 kN/m for the comparison with
the charts presented in figure 25. The parameters are shown in the following table 11.
Table 11 Material properties
0
200
400
600
800
1000
1200
1400
20 25 30 35 40 45
Settlement (mm)
GEC friction angel (Degrees)
3000
3500
4000
4500
5000
20 25 30 35 40 45
Time for 90% consolidation
(Days)
GEC friction angel (Degrees)
Days for 90% consolidation
Soil type
Saturated
unit
weight
Drained
angel of
friction
Drained
cohesion
Coefficient
of
compression
Coefficient
of swelling
Coefficient
Of horizontal
permeability
Coefficient
Of vertical
permeability
Initial
voids
ratio
OCR
value
Soft clay
14.4
28
5
0.6133
0.184
3 x 10
-
5
2 x 10
-
5
3
1
Material Saturated
unit weight
Drained angel of
friction
Drained
cohesion
Drained elastic
modulus E
(MPa)
Coefficient
Of horizontal
permeability
Coefficient
Of vertical
permeability
Embankment
18
38
0
50
1
1
Granular column
20
30
0
80
10
10
Dense sand
20
38
0
30
1
1

Fig. 32 finite element model
Table 12 Details of calculation phases on “Plaxis 3D”
For this study, charts in Annex II, Group D charts were used to compare between the
numerical method and manual method. Group D charts has soft clay strength properties:
C=5 kPa and  = 28.The numerical model showed a settlement under the embankment
of 0.327m at 90% consolidation, and settlement 0.387m at the minimum pore water
pressure. While the manual charts indicated a settlement of 0.3m at 15% area
replacement ratio.
Calculation
Phase
Calculation
type
Loading type Pore
pressure
calculation
type
Layer
thickness
Total
embankment
height
Consolidation
interval days
Phase 1
Consolidation
Staged construction
Phreatic
1
1
3
Phase 2
Consolidation
Staged construction
Phreatic
1
1
10
Phase 3
Consolidation
Staged construction
Phreatic
1
2
2
Phase 4
Consolidation
Staged construction
Phreatic
1
2
32
Phase 5
Consolidation
Staged construction
Phreatic
1
3
10
Phase 6
Consolidation
Staged construction
Phreatic
1
3
14
Phase 7
Consolidation
Staged construction
Phreatic
1
4
2
Phase 8
Consolidation
Minimum excess pore water
pressure/ 90% Consolidation
Phreatic 1
4
-

Fig. 33 applied stress vs S/Hs
There are many factors that could affect the difference in settlement in both manual and
numerical method. One of the major factors is the actual length of GEC as it is not taken
into consideration in analytical analysis method. Other factor is the modulus of elasticity
of the columns’ aggregate material, this factor has a minor effect as was shown in the
parametric analysis. Also, the properties of the sub soil were not taken into consideration
in the analytical analysis method. Ground water table plays a huge role in results but the
analytical analysis method considers in to be at the soil surface only. The analytical
method also comes short when it comes to considering soft soil properties such as OCR,
void ratio or unit weight.
Another major factor is the GEC spacing and arrangement as the analytical method charts
doesn’t provide a variety for arrangements.
6
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