Ijciet 06 10_008

http://www.iaeme.com/IJCIET/index.asp 97 editor@iaeme.com
International Journal of Civil Engineering and Technology (IJCIET)
Volume 6, Issue 10, Oct 2015, pp. 97-107, Article ID: IJCIET_06_10_008
Available online at
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=6&IType=10
ISSN Print: 0976-6308 and ISSN Online: 0976-6316
© IAEME Publication
ANALYSIS OF EMBANKMENT SUPPORTED
BY STONE COLUMNS ENCASED WITH
GEOSYNTHETIC MATERIAL
Asst. Prof. Dr. Qassun S. Mohammed Shafiqu
Civil Engineering Department
Collage of Engineering, Al-Nahrain University, Iraq
Fadhl Abbas Ahmed Al-Assady
Civil Engineering Department,
Collage of Engineering, Al-Nahrain University, Iraq
ABSTRACT
Stone column is often employed for strengthening of an embankment
seated on deep soft clay. But in very soft clay, stone column may not derive
adequate load carrying capacity and undergo large lateral deformation
because of inadequate lateral confinement. In such cases, reinforcement to
individual stone column by geosynthetics enhances load caring capacity and
reducing lateral deformation.
In this study analysis of an embankment resting on encased stone columns
ESC are investigated using the latest version of finite element program ''Plaxis
3D 2013''. Example of model prediction and accuracy of finite element
formulation were given. Then the transient analysis of three-dimensional
embankment problem supported on geogrid reinforced stone columns has
been carried out taken in to consideration the influence of some parameters on
the long-term behavior of the system, such as stone columns diameter (d),
stone columns length ratio (L/d), encasement stiffness, undrianed shear
strength.
The results also indicated that the percentage of decrease in settlement for
embankment reinforced with ESC relative to untreated embankment for
(L/d=5) were (49.83 %, 43.53 %, 38.7 % and 32.63 %) for S/d (2.5, 3.0, 3.5,
and 4.0); respectively. And for (L/d=8) being (68.86 %, 67.23 %, 55.48 % and
51.36 %) for S/d (2.5, 3.0, 3.5, and 4.0); respectively. And the pattern of ESC
has significant influence on the settlement comparing with the effect of
spacing between them. Also for (L/d = 8) the maximum horizontal deformation
was observed at a depth of about 1.7d.
The results showed significant decreasing in lateral displacement with
increasing the diameter of encased stone columns. Also increasing the
undrianed shear strength, cu by 100 % decreased the vertical settlement by

Asst. Prof. Dr. Qassun S. Mohammed Shafiqu and Fadhl Abbas Ahmed Al-Assady
about 65 % and a noticeable decrease in the lateral displacement was
obtained with increasing cu.
Cite this Article: Asst. Prof. Dr. Qassun S. Mohammed Shafiqu and Fadhl
Abbas Ahmed Al-Assady. Analysis of Embankment Supported by Stone
Columns Encased with Geosynthetic Material. International Journal of Civil
Engineering and Technology, 6(10), 2015, pp. 97-107.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=6&IType=10
INTRODUCTION
Soft soils are fairly widespread all over the world and the places of them are in
important cities. Geotechnical design and execution of civil engineering structures on
soft to very soft soil are usually associated with essential difficulties. Two main
problems are encountered when undertaking construction in soft soil layers, excessive
settlement and low shear strength (Barksdale and Bachus, 1983), (Bergado et al.,
1996).
Embankment seated on soft clay may undergo large displacements both vertically
and horizontally. A lot of soil improvement methods have been used to deal with soft
soil problems. The improvement methods of the soft soil include replacement, sand
drains, vacuum pressure, preloading, dynamic compaction, lime stabilization,
geosynthetic reinforcement and stone or gravel columns (Kemphert and
Gebresealssie, 2006). Stone columns provide an influential improvement process for
soft soils under light structures such as road embankments or rail. The use of stone
columns as a method of soft soil improvement has been successfully implemented
around the world. The stone column technique was developed in Germany about 60
years ago (Elsawy, 2010).
The use of geosynthetic encased columns provides an economic and effective
solution when rapid construction and small deformation are required (Raithel et al.,
2004). Geosynthetic reinforcement reduces the maximum as well as differential
settlement and helps transfer stresses from soft soil to stone columns. The
embankment soil in between stone columns has a tendency to settle more due to
stiffness difference between stone columns and soft soil (Naughton and Kempton,
2003).
Thus in this study analysis of an embankment resting on stone columns will be
investigated using the finite element method. And In order to avoid dispersion of the
stones into the clay and to improve the stone columns as reinforcing elements,
geogrids are used in this study as a geosynthetic material encased the stone columns.
Stone columns provide an influential improvement process for soft soils under light
structures such as road embankments or rail. In order to avoid dispersion of the stones
into the clay and to improve the stone columns as reinforcing elements, geogrids are
used in this study as an encasement of the stone columns.
THEORETICAL WORK
Modeling
Finite element mesh and boundary conditions
The dimensions of soft soil and the stone columns were taken according to the
experimental work by Al-Shammarie (2013) scaled by 20 and as shown in Figures (1

Analysis of Embankment Supported by Stone Columns Encased with Geosynthetic Material
and 2). Due to symmetry, only half of the embankment was modeled to save
computation time. Soft soil dimensions were 30 m in x-direction, 16 m in y-direction
and 11.2 m downward in z-direction. The boundary effect was investigated indicating
that it would be sufficient to eliminate its effect if the boundary was set at 18 m from
the toe. The stone columns length is taken equal to L=5d and L=8d where d is the
stone columns diameter, in some cases a geogrid material has been added as
encasement for stone columns. As seen in the figures, a 12 m wide embankment
having side slope 1V: 1.2H and height H=2 m resting on 11.2 m soft clay underlaying
by rigid layer is constructed in two layers, each layer constructed in 30 days, the first
layer 1 m depth was consolidated for further 30 days while the consolidation of the
second layer was continued till excess pore water pressure reached near 1 kPa, The
phreatic level was set at the top surface of the soft clay layer to generate a hydrostatic
pore water pressure profile in the domain. In this study, settlement and excess pore
water pressure are observed for reference points A and B, respectively, as shown in
Figure (2). Also, lateral displacement was computed for the extreme left stone
column.
Due to the large model’s dimensions,
medium mesh was created and refining at the stone columns and the surrounding area
to allow for the use of small time step (i.e. 0.001 day onward) to achieve greater
accuracy in the beginning of consolidation progress. The mesh consists of nearly
33,250 elements (10-node tetrahedrons) with over 46,000 nodes Figure (3).
Figure 3 Elements volume in three-Dimensional finite element mesh
Figure 1 Cross section and
measurement points for embankment
supported on stone columns
Figure 2 Three-Dimensional finite
element modeling of embankment
supported on stone columns

Constitutive models and material parameters
An elasto-plastic analysis was conducted to simulate an embankment resting on
ordinary stone columns (OSC) and encased stone columns (ESC). Hardening Soil
model (HS) was used to simulate the soft soil and embankment fill material, while the
stone columns volumes were modeled by Mohr-Coulomb model (MC). These
constitutive models were verified with experimental results of Al-Shammarie (2013).
Parameters for the bench mark case are mentioned in Table (1), these parameters
adopted in this numerical analyses are taken from Al-Shammarie (2013)
experimental work.
Reinforce stone column by geosynthetics enhance load carrying capacity and
reduces lateral deformation. The geogrid is selected as geosynthetic encasement and is
modeled as a linear elastic. Two interfaces were product for each encasement, the first
outside the geogrid (positive interface) between geogrid and soft soil carrying the soft
soil properties, and the second one inside the geogrid (negative interface) between
geogrid and stone column carrying the stone column properties. Interfaces are
composed of 12-node elements consisting of six pairs of nodes, which are compatible
with the 6-noded triangular sides of the soil (Brinkgreve et al, 2013).
Table 1 Parameters for the benchmark case adopted in the numerical analyses.
Properties Soft clay Embankment fill Stone column
Unsaturated unit weight, γunsat (kN/m3
) 13.24 21.83 14.4
Saturated unit weight ,γsat (kN/m3
) 18.85 24.22 15.7
Material model Hardening soil Hardening soil Mohr-Coulomb
Initial void ratio, e˳ 0.92 0.16 0.5
Drainage type Undrained Drained Drained
E (kPa) - - 150000
E50
ref
(kPa) 600 10000 -
Eoed
ref
(kPa) 1425 10000 -
Eur
ref
(kPa) 1800 30000 -
Power (m) 1 1 -
Cohesion ,C (kPa) 10 1 1
Friction angle ,Ø (deg) - 40 41.5
Poisson’s ratio, ν 0.35 0.3 0.3
Permeability kx=ky=kz (m/day) 8.640 x 10-5
0.6 0.6
Geosynthetic stiffness, (kN/m) - - 1000
NUMERICAL ANALYSIS PROCEDURE
The reinforced soft soil with encased stone columns has been investigated by using an
elastic perfectly plastic finite element analysis which involves a several of steps or
phases.
The numerical modeling consisted of three steps: initial geostatic equilibrium,
inserting the ordinary stone columns or encased stone columns, construction of
embankment and long term time depending phase.
First step is the generation of initial stress and pore water pressure, the initial
stresses had been generated using the K˳ procedure calculation type. The K˳
procedure is used in case with a horizontal surface and with all soil layers and
phreatic levels parallel to the surface, the phreatic level was set at the top surface of
the soft clay layer.

The second step is modeling stone column by replacing soft soil elements by stone
column, in some cases, the geosynthetic reinforcement and interface elements was
added as wished in place. Plastic calculation type was used in this step.
The third step is modeling embankment’s two layers, the first layer with 1 m
depth is constructed in 30 days and consolidated for further 30 days while the second
layer with 1 m depth will be constructed in 30 day and the consolidation continued till
excess pore water pressure reached near 1 kPa. Consolidation calculation type was
used in this step which is time-dependent analysis of deformation and excess pore
water pressures. Input of soil permeability required. Also, non-zero time interval will
be used.
PARAMETRIC STUDY
1. Effect of diameter of stone column
In order to investigate the effect of diameter of encased stone column on long-term
behavior, the stone column center-to-center spacing of 3.5 m, the stone column
length L=7 m, stone columns diameter (d = 0.8 m, 1 m, 1.1 m, 1.2 m, 1.3 m and 1.4
m) given in Figure (4).
Figure 4 Pattern of encased stone columns at different spacing (L/d=5).
2. Effect of L/d ratio of encased stone columns
To understand the long-term behavior for encased stone column supported
embankments, the effect of change in stone columns length has been investigated, the
stone columns diameter (d = 1.4 m), the stone column spacing to diameter S/d =2.5,
the stone column length to diameter (L/d =5 and 8) shown in Figures (4, 5, and 6).
Figure 5 Cross section for embankment
supported on encased stone columns
(L/d=5).
Figure 6 Cross section for embankment
supported on encased stone columns
(L/d=8).

3. Effect of encasement stiffness of stone columns
To understand the long-term behavior for encased stone column supported
embankments, the influence of geosynthetic stiffness has been investigated, the
encased stone columns diameter (d = 1.4 m), the stone column length to diameter (L/d
=5), J (kN/m) =1000, 1500, 3000, 5000, 7000, 8000, and 10000.
4. Effect of the undrianed shear strength (cu)
The influence of undrained shear strength has been investigated on ordinary stone
columns, the ordinary stone columns diameter (d = 1.4 m), the stone column length to
diameter (L/d =5), the stone column spacing with respect to diameter S/d =2.5, the
shear strength (kPa) =7.5,10,12.5 and 15.
RESULT AND CONCLUSION
1. Effect of diameter of stone column
The percentage of decrease in settlement for embankment reinforced with ESC
relative to untreated embankment were (29.43 %, 30.47 %, 37.96 %, 41.15 %, 44.84
% and 49.83 %) for diameter (8.0, 1.0, 1.1, 1.2, 1.3 and 1.4 m); respectively. Also the
results showed significant decreasing in lateral displacement with increasing the
diameter of encased stone columns. The maximum lateral displacementoccur at the tip
of columns for L/d=5, and that 1.4 m diameter gives the lowest lateral displacement.
The encasement of stone columns decrease the lateral displacement on corner
columns as the encasement prevent the stone column from side moving. Also
increasing diameter lead to decrease the excess pore water pressure at the construction
stages. As shown in Figures (7, 8, and 9).
Figure 7 Settlement of embankment with different diameter of encased stone
columns.
0.35
0.37
0.39
0.41
0.43
0.45
0.47
0.49
0.51
0.53
0.7 0.9 1.1 1.3 1.5
surfacesettlement(m)
Diameter (m)

Figure 8 Lateral displacement of ordinary and encased stone column with different
diameter.
Figure 9 Excess pore water pressure versus time with different diameter for (L/d=5)
2. Effect of L/d ratio of encased stone columns
There are high decreasing in vertical settlement when increasing the stone columns
length (L/d). The end bearing for ESC with (L/d = 8) exhibits a vertical settlement
(1.45 to 1.7) times that of floating column with (L/d = 5). The maximum lateral
displacement occur at the tip of columns for L/d=5, and the percentage of decrease in
lateral displacement for embankment reinforced with ESC with L/d=8 relative to that
with L/d=5 (75.30 %, 68.67 %, 67.68 % and 62.34 %) for S/d (2.5, 3.0, 3.5, and 4.0);
respectively. And increase L/d decrease excess pore water pressure at end of
construction. The decreasing in pore water pressure percentage for embankment
reinforced with ESC with L/d= 8 relative to that with L/d = 5 at the end of
construction were (21.77 %, 21.33 %, 12.52 % and 13.99 %) for S/d (2.5, 3.0, 3.5, and
4.0); respectively. Figures (10, 11, and 12)
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.15 0.2 0.25 0.3 0.35 0.4
Depth(m)
Lateral displacement (m)
ESC 0.8 m
ESC 1.0 m
ESC 1.1 m
ESC 1.2 m
ESC 1.3 m
ESC 1.4 m

Figure 10 Settlement of embankment with different S/d for embankment resting on
encased stone columns in different lengths.
Figure 11 Excess pore water pressure versus time with different L/d and S/d ratio for
embankment resting on encased stone columns.
Figure 12 Lateral displacement of encased stone column with different L/d and S/d
ratios.
0.1
0.2
0.3
0.4
0.5
0.6
2 2.5 3 3.5 4 4.5
surfacesettlement(m)
S/D
l/d=5
l/d=8
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.05 0.1 0.15 0.2 0.25 0.3
Depth(m)
L/d=5,S/d=4
L/d=5,S/d=3.5
L/d=5,S/d=3
L/d=5,S/d=2.5
L/d=8,S/d=2.5
L/d=8,S/d=3.0
L/d=8,S/d=3.5
L/d=8,S/d=4.0

3. Effect of encasement stiffness of stone columns
The vertical settlement decrease with increasing the geogrid stiffness (J), when J
equal to (1000, 1500, 3000, 5000, 7000, 8000, and 10000) kN/m the vertical
settlement were (0.392, 0.392, 0.380, 0.378, 0.375, 0.374, and 0.372) m; respectively
as shown in Figure (13). Also there is improvement in lateral displacement with
increasing J, Figure (14).
Figure 13 Settlement of embankment with different geogrid stiffness
Figure 14 Lateral displacement for different encasement stiffness J, L/d=5.
4. Effect of the undrianed shear strength (cu)
Increasing the undrianed shear strength cu by 100 % decrease the vertical settlement
by about 65 % and a noticeable decrease in the lateral displacement is obtained with
increasing cu, when the value of cu is equal and greater than 10 kPa, a significant
improvement of lateral displacement is clearly noticed. Also the excess pore water
pressure increase with decreasing the undrianed shear strength. Figures (15, 16, and
17)
0.37
0.375
0.38
0.385
0.39
0.395
0 2000 4000 6000 8000 10000 12000
surfacesettlement
(m)
Geogrid stiffnes (kN/m)
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.15 0.17 0.19 0.21 0.23 0.25
Depth(m)
J=1000 kN/m
J=1500 kN/m
J=3000 kN/m
J=5000 kN/m
J=7000 kN/m
J=8000 kN/m
J=10000 kN/m

Figure 15 Settlement of embankment with different undrianed shear strength
Figure 16 Lateral displacement for different undrianed shear strength.
Figure 17 Excess pore water pressure versus time with different shear strength
0
0.2
0.4
0.6
0.8
1
6 8 10 12 14 16
Surfacesettlement(m)
Undrianed shear strength cu (kPa)
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70
Depth(m)
Cu=7.5 kPa
Cu=10 kPa
Cu=12.5 kPa
Cu=15 kPa

REFERENCES
[1] Al-Shammarie H. A. (2013), An Experimental and Theoretical Study on Ordinary
and Encased Stone Columns Underneath Embankment", Ph.D. Thesis, Civil
Engineering Department, University of Baghdad, Iraq.
[2] Bergado, D.T., Anderson, L.R., Miura, N. and Balasubramaniam, A.S., (1996),
"Soft ground improvement in lowland and other environments, ASCE press, New
York, USA: pp 427 pp.
[3] Barksdale, R.D. and Bachus, H.G., (1983), "Design and construction of stone
columns", Dept. of Transpiration, Federal Highway Administration Report, USA
report No. FHWA/RD 83/026.
[4] Brinkgreve, R. B. J., Engin, E., Swolfs, W. M., editors. (2013). Plaxis 3D 2013
tutorial manual.
[5] Elsawy, M. B. (2010), " Highway Embankment Constructed on Soft Soil
Improved by Stone Columns with Geosynthetic Materials", Ph.D. Thesis,
Institute of Geotechnical Engineering, Department of Civil Engineering, Faculty
of Engineering, Duisburg-Essen University.
[6] Kempfert, H-G, Gebreselassie, B. (2006), "Excavations and
Foundations in Soft Soils", Universitat Kassel, FG Geotechnik.
Monchebergstr. 7, 34125 Kassel, Germany.
[7] Naughton, P. and Kempton, G. (2003), "A New Design Procedure for Piled
Embankments", Proceedings of the 56th Canadian Geotechnical Conference and
the NAGS Conference, Winnipeg, MB, pp. 858-865.
[8] Raithel M, Küster V, Lindmark A (2004), "Geotextile encased columns – a
foundation system for earth structures", Illustrated by a dyke project for a works
extension in Hamburg. Nordic Geotechnical Meeting 2004, Sweden.
[9] V.K.Chakravarthi K.Ramu M.R.Madhav, Slope Stability Analysis of Basal
Reinforced Embankment under Oblique Pull. International Journal of Civil
Engineering and Technology, 1(1), 2010, pp. 1 - 14.
[10] Esraa A. Mandhour, Saad N. Al-Saadi, Saad F. Ibrahim, Study of the Efficiency
of Stone Columns In Soft Clay: Considering The Effect Of Clay Minerals In Soil.
International Journal of Civil Engineering and Technology, 5(9), 2014, pp. 241 -
251.

Ijciet 06 10_008

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Ijciet 06 10_008

Similar to Ijciet 06 10_008 (20)

More from IAEME Publication

More from IAEME Publication (20)

Recently uploaded

Recently uploaded (20)

Ijciet 06 10_008