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Soft Soil Response and Behaviour of Piles Under a Geotextile Reinforced Embankment

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It is always problematic to design and construct road embankments in soft soils having low shear strength and high compressibility characteristics due to the risk of excessive settlement, bearing failure and, lateral movement. But the inclusion of rigid piles in the soft soil and a geotextile layer at the base of the embankment can efficiently transfer the superstructure load to competent strata beneath. In this paper, a road embankment is first modeled using finite element software and its overall response is studied. Thereafter, piles and a single geotextile layer are incorporated in the system as reinforcement to investigate the change in the response of the foundation soil. The behavior of the rigid piles is also analyzed at different locations under the embankment base. Numerical results indicate that there is considerable reduction in subsoil settlement, lateral displacement of soil, vertical pressure on soft foundation soil and excess pore water pressure due to reinforcement action. The ground reaction modulus is found to decrease with the increase of ground settlement. The frictional resistance of a driven pile in clay soil after a month is found to be considerably higher than its value immediately after installation due to the thixotropic hardening phenomenon.
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Indian Geotechnical Conference IGC2016
15-17 December 2016, IIT Madras, Chennai, India
1
SOFT SOIL RESPONSE AND BEHAVIOUR OF PILES UNDER A
GEOTEXTILE REINFORCED EMBANKMENT
Prasun Halder
1
Baleshwar Singh
2
1
M.Tech. Student,
2
Professor, Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati – 781039.
p.halder@iitg.ernet.in and baleshwar@iitg.ernet.in
ABATRACT: It is always problematic to design and construct road embankments in soft soils having low shear strength
and high compressibility characteristics due to the risk of excessive settlement, bearing failure and, lateral movement. But
the inclusion of rigid piles in the soft soil and a geotextile layer at the base of the embankment can efficiently transfer the
superstructure load to competent strata beneath. In this paper, a road embankment is first modeled using finite element
software and its overall response is studied. Thereafter, piles and a single geotextile layer are incorporated in the system as
reinforcement to investigate the change in the response of the foundation soil. The behavior of the rigid piles is also
analyzed at different locations under the embankment base. Numerical results indicate that there is considerable reduction
in subsoil settlement, lateral displacement of soil, vertical pressure on soft foundation soil and excess pore water pressure
due to reinforcement action. The ground reaction modulus is found to decrease with the increase of ground settlement.
The frictional resistance of a driven pile in clay soil after a month is found to be considerably higher than its value
immediately after installation due to the thixotropic hardening phenomenon.
Keywords: Pile-supported embankment, geotextile reinforcement, settlement, ground reaction modulus
1 INTRODUCTION
It is very risky to construct road embankment in soft
soils owing to its susceptibility to failure due to
excessive settlement, differential settlement,
inadequate bearing capacity and lateral displacement of
soils under the toe of the embankment. To address such
problems embankments can be supported by piles
which appear to be a useful ground improvement
method. In this technique vertical stiff piles are driven
through the soft layers and embedded in a
competent substratum beneath to support the
granular earth embankment above. The partial load
transfer onto the piles takes place due to the arching
effect in the embankment fill. Addition of geosynthetic
layer over piles enhances the load transfer mechanism
resulting stress reduction on the subsoil and consequent
surface settlement reduction.
Many researchers have studied this problem over the
years and come up with solutions either analytically or,
numerically. Terzaghi (1936) described the concept of
soil arching through trap-door experiment. The concept
of arching with a semi-spherical dome of arching shell
was proposed by Hewlett and Randolph (1988). Low et
al. (1994) made some refinements by introducing a
factor α for the unreinforced case to incorporate
the non-uniform vertical stress on subsoil and then
given the formula for stress concentration ratio,
efficiency. Abusharar et al. (2009) came up with some
refinements like introduction of uniform surcharge load
over the embankment and consideration of the skin
friction mechanism to address the soil-geotextile
interaction problem. Through a numerical study Han
and Garb (2002) concluded that GRPS system reduces
settlement and larger stiffness of piles promotes higher
soil arching effect. A 3-D finite element analysis was
performed by Liu et al. (2007) of a road embankment
which was originally constructed in Shanghai, China.
They compared the computed 3-D results with the field
data available and found it to be in close agreement
with the field study. Rowe and Liu (2015) performed a
fully coupled and fully three dimensional finite element
analyses for an embankment to study the behavior of
the same under different ground improvement
techniques. In this paper an embankment is modelled in
PLAXIS 2D and the response of soft soil under the
embankment is evaluated. The behavior of piles is also
studied.
2 MODELING DETAILS
The numerical analysis is performed using the finite-
element software PLAXIS 2D. The finite-element
mesh used in the analysis is shown in Fig. 1. The
embankment is 1.8 m high having a base width of 15.6
m. The groundwater table lies 1 m below the ground
surface. The soft foundation soil is 13 m deep and
consists of three layers. The subsoil profile is modelled
with a rough rigid bottom boundary. To minimize
boundary effects, the lateral boundary of the finite-
element mesh is extended 40 m horizontally either
side of the embankment center. The planes along x= -
Paper title
2
40 and x= +40 are smooth and rigid i.e. Zero
displacement in the x-direction. At the bottom of the
mesh the impervious boundary condition has been
applied also. First only the embankment is modeled to
see its overall performance. Then the combination of
piles and one layer of geotextile (4000 kN/m) is used to
study the improvement in response. The geotextile
layer is placed in between two gravel layers, each
having the thickness of 250 mm. 8 m long and 300 mm
diameter piles are used at 1.3 m spacing center to
center for support.
Fig. 1 Geometry of the embankment model
Mohr-Coulomb and soft soil models have been used to
simulate the behavior of embankment fill and soft
foundation soil respectively. Here the 15-node
triangular soil element is used. The stiff pile is
modelled with 5-node embedded pile element which is
isotropic linear elastic material. The three layered
foundation soil is constructed in one step in the first
phase. Then installation of piles and embankment
construction are done in the next phases one by one.
The whole system is kept for 567 days of monitoring
period after the end of embankment construction. The
properties of embankment fill, gravel and pile are given
in Table 1 and the properties of soft foundation soil are
indicated in Table 2. In this table, λ is slope of virgin
consolidation line, κ is slope of swelling line, C
c
is
compression index, C
r
is recompression index, Φ' is
effective friction angle, c' is effective cohesion, K
v
is
vertical hydraulic conductivity, and K
o
is coefficient of
earth pressure at rest.
Table 1 Properties of Embankment Fill, Gravel and Pile
used in FE Analysis
Material Unit
weigh
t
(kN/
m
3
)
Frictio
n angle
(degre
es)
Cohesi
on
(kPa)
Young
’s
modul
us
(MPa)
Poisso
n’s
ratio
Embankm
ent
Fill
18.60 33.8 11.5 20 0.3
Gravel 20 36 60 70 0.3
Pile 24 - - 20000 0.2
Table 2 Soft Foundation Soil Properties used in FE
Analysis
Layers Sandy silt Soft clay Clayey silt
Thickness (m) 2 6 5
λ 0.092 0.116 0.027
k 0.014 0.017 0.004
C
c
0.212 0.267 0.062
C
r
0.032 0.040 0.009
Φ
(deg) 30.6 27 34
C
(kPa) 4 13 0
K
v
(m/day) 6.91*10
-
5
8.3*10
-
8
8.9*10
-
6
K
0
1.668 0.662 0.52
3 RESULTS AND DISCUSSION
Point ‘A’ (19.80, 0) and ‘K’ (19.73, -0.02) are chosen
on the top of the subsoil layer for the calculation of
vertical settlement of sub soil and vertical stress. For
the calculation of excess pore pressure three points
have been marked on soft soil along the centerline of
embankment as ‘C’ (19.80, -1), ‘D’ (19.80, -5), and ‘E’
(19.80, -10.5).
3.1 Settlement of Foundation Soil
This is almost 67% reduction in maximum settlement
due to the inclusion of piles and geotextile. Figure 2
indicates the variation of vertical settlement at different
depth with elapsed time. The neutral point comes down
as the consolidation process progresses.
Fig. 2 Settlement profile along the centerline
Figure 3 shows the stress acting on the soft soil is 37.5
kPa for the unimproved case and 12.5 kPa for the
improved case which is 33.3% of the previous case.
This is due to the arching action.
2m
6m
5m
C
E
Indian Geotechnical Conference IGC2016
15-17 December 2016, IIT Madras, Chennai, India
3
Fig. 3 Vertical pressure variation on the soft ground
3.3 Ground Reaction Curves
The ground modulus (k) has been calculated by
dividing the vertical stress acting on the natural subsoil
under embankment centerline with the ground surface
settlement measured between two adjacent piles.
Figure 4 shows the plot between the ground modulus
and the settlement. The values of ground modulus
obtained from the improved section (i.e. with piles) are
lower than those for the unimproved section (no pile),
which is a consequence of soil arching. Actually for
such soil it was found that the increase of stress (due to
the absence of piles) is larger than the increase of
settlement, resulting in a larger ground modulus value.
Fig. 4 Variation of ground modulus with ground
settlement for unimproved and improved sections
3.4 Excess Pore Pressure Generation and
Dissipation
Figure 5 shows that at points C and E, the excess pore
water pressure increases during construction and starts
dissipating during the consolidation period. But at
point D, this excess pore pressure increases during
dissipation. Huang et al. (2009) explained that due to
longer drainage path and low permeability of silty clay
such increase in pore pressure takes place. This is
called the Mandel-Cryer effect. Maximum excess pore
pressure parameter B
max
is defined as the ratio of the
maximum excess pore pressure to the change in total
vertical stress of the embankment. Rowe and Soderman
(1985) suggested that this value should be less than
0.34 for the safety of embankment against bearing
failure. Here the estimated ‘B
max
’ is 0.15. It can be well
understood that for all the depths the ratio is less than
the limiting value. Therefore, the embankment is safe
against bearing capacity failure state.
Fig. 5 Excess pore pressure variations at different depths
within the subsoil
3.5 Lateral Displacement of Soil below the Toe
of the Embankment
Figure 6 shows the lateral displacement profile of soft
soil calculated at 1 m distance from the toe of the
embankment for both unimproved and improved cases.
It can be seen that the maximum displacement occurs
at depth of 3.5 m below the ground level due to higher
compressibility.
Fig. 6 Lateral displacement of subsoil below the toe of the
embankment
The ratio of lateral displacement– maximum settlement
is a good indicator of embankment stability. Chai et al.
Paper title
4
(2002) showed that for the embankment safety, this
ratio must be less than 0.5. In this case the ratio is 0.07
which is on the safer side in terms of the stability of the
road embankment considered.
3.6 Axial Load and Skin Friction Distribution
in Piles
Figure 7 and 8 show the axial load distribution and skin
friction variation along the length of the piles. The
values are directly obtained from the numerical
analysis. The maximum load at the pile head is 37
kN/m and 18 kN/m for the central and corner pile
respectively.
Fig. 7 Comparison of axial load distribution between the
central and corner piles
Fig. 8 Comparison of skin friction resistances between the
central and corner piles
Due to the high compressibility characteristics of the
clay soils, they move more than the piles in the
downward direction resulting negative skin friction
upto some depth in the soil as shown in Figure 8.
4 CONCLUSIONS
From the work presented in this study the following
conclusions can be drawn:
1. The subsoil settlement and the vertical
pressure acting on the soft foundation soil get
reduced by 67% and 65% respectively.
2. The excess pore water pressure increases
during dissipation in clayey soil having very
low permeability away from the drainage
boundary.
3. For the considered embankment, the ratio
‘B
max
is less than the limiting value of 0.34.
Therefore, the embankment is safe against
bearing capacity failure state.
4. The ratio of the maximum lateral
displacement to maximum central settlement
has been calculated as 0.07 which the stability
of the road embankment considered.
References
Abusharar, S.W., Zheng, J.J., Chen, B.G., Yin, J.H. (2009)
‘A simplified method for analysis of a piled
embankment reinforced with geosynthetics’,
Geotextiles and Geomembranes, 27, pp 39-52.
Chai, J.C., Miura, N., and Shen, S.L. (2002) ‘Performance
of embankments with and without reinforcement on
soft subsoil’, Canadian Geotechnical Journal, 39 (4),
pp 838-848.
Han, J., Gabr, M.A.(2002) ‘Numerical analysis of
geosynthetic-reinforced and pilesupported earth
platforms over soft soil’, Journal of Geotechnical and
Geoenvironmental Engineering, 128 (1), pp 44–53.
Liu, H. L., Ng, C. W. W., and Fei, K. (2007)
‘Performance of a geogrid reinforced and pile-
supported highway embankment over soft clay:
Case study’, Journal of Geotechnical and
Geoenvironmental Engineering, 133(12), pp 1483–
1493.
Low, B.K., Tang, S.K., Choa, V. (1994) ‘Arching in piled
embankments’, Journal of Geotechnical Engineering,
120 (11), pp 1917–1938.
Rowe, R.K., and Liu, K.W. (2015) ‘Three-dimensional
finite element modeling of a full-scale geosynthetic-
reinforced, pile-supported embankment’, Canadian
geotechnical Journal, 52, pp 1-14.
Terzaghi, K. (1936) ‘Stress distribution in dry and in
saturated sand above a yielding trap door’, Proc., 1st
Int. Conf. on Soil Mechanics and Foundation
Engineering, Harvard Univ., Cambridge, Mass., pp
307–311.
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A case history of both reinforced and unreinforced embankments on soft subsoil built-to-failure is described and analyzed. The effect of geotextile reinforcements on embankment behavior is discussed by comparing the field and numerical analysis results of cases with and without reinforcement. The results of a laboratory model test on the behavior of embankments on soft subsoil are discussed. Both field and laboratory tests, as well as analysis results, indicate that the reinforcement had a positive effect on embankment stability. However, at a working state (for a factor of safety of FS = 1.2∼1.3) the reinforcement did not have an obvious effect on the subsoil response. The effect of reinforcement on subsoil deformation could be noticed only when the unreinforced embankment was close to failure. The laboratory model test results indicated that if the reinforcement is stiff and strong enough, the effect of reinforcement is considerable. It is suggested that although the geotextile has a beneficial effect on embankment over soft subsoil due to its relative lower stiffness, to achieve a substantial improvement on embankment behavior, the stiffer and stronger reinforcements should be used. This case history also demonstrated that the rate of lateral displacement and excess pore pressure development are sensitive indicators of the stability of embankment on soft subsoil.
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