Numerical study of behavior of square footing on geogrid reinforced flyash beds under static loading

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Special Issue: 06 | May-2014 | RRDCE - 2014, Available @ http://www.ijret.org 82
NUMERICAL STUDY OF BEHAVIOR OF SQUARE FOOTING ON
GEOGRID-REINFORCED FLYASH BEDS UNDER STATIC LOADING
Bindiya K1
, S. Gangadhara2
, H. C. Muddaraju3
, Tejaswini B. R4
1
Assistant Professor, Civil Engineering Department, JNNCE, Shivamogga, Karnataka, India
2
Associate Professor, Civil Engineering Department, UVCE, Bangalore, Karnataka, India
3
Assistant Professor, Civil Engineering Department, UVCE, Bangalore, Karnataka, India
4
Assistant Professor, Civil Engineering Department, SVCE, Bangalore, Karnataka, India
Abstract
The technique of ground improvement using geosynthetics has been developed extensively over the last few decades, in particular to
those applied in pavement and foundation engineering. Fly ash is a naturally-cementations coal combustion by-product. Coal-based
thermal power plants all over the world face serious problems of handling and disposal of the ash produced. Hence attempts are
being made to utilize the ash rather than dump it. The main objective of this research study is to investigate the potential benefits of
using the reinforced soil foundations to improve the load carrying capacity and reduce the settlement of shallow foundations on
flyash. Numerical modelling is performed using PLAXIS version 8.2 which is finite element code for soil and rock analysis and is
capable of modelling reinforced soils. A series of finite element analysis were performed to study the effectiveness of the reinforcement
distribution (i.e, number of reinforcement layers and the spacing between the reinforcement layers) on the behaviour of the square
footing on flyash beds. The dimensions of the tank used in analysis are 11m diameter and 3.6m height. The mild steel footing is used
with the dimensions of 1m width and 0.04 m thickness respectively. The loading condition used for the present study is Point load.
The input values of point loads are given in force per unit width (kN/m). The static loads are applied up to 50mm settlement. The
analysis demonstrates that the reinforced flyash beds perform better than the unreinforced flyash beds. The performance of footing
improves with increase in number of reinforcement layers. As the spacing between the reinforcement increases settlement will
reduces.
Keywords: Reinforced fly ash beds, numerical modeling, PLAXIS, Point load.
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1. INTRODUCTION
The technique of ground improvement using geosynthetics has
been developed extensively over the last few decades, in
particular to those applied in pavement and foundation
engineering. The concept of reinforced soil as construction
material is based on the existence of soil-reinforcement
interaction due to tensile strength, frictional and the adhesion
properties of the reinforcement and was first introduced by the
French architect and engineer Henri Vidal in the 1960s. Since
then, this technique has been widely used in geotechnical
engineering practice.
Studies have shown that geogrid reinforced foundations can
increase the ultimate bearing capacity and reduce the
settlement of shallow footings, compared to the conventional
methods, such as to replacing natural soils or increasing
footing dimensions.(e.g. King et al. 1993; Mosallanezhad et al.
2010; Gill et al. 2011; Zidan 2012; Nagy et al. 2012;
Gangadara et al. 2013)
Fly ash is a naturally-cementations coal combustion by-
product. Coal-based thermal power plants all over the world
face serious problems of handling and disposal of the ash
produced. Safe disposal of the ash without adversely affecting
the environment and the large storage area required are major
concerns. Hence attempts are being made to utilize the ash
rather than dump it. This solves the problems associated with
the disposal of fly ash. According to two dimensional finite
element analysis on reinforced fly ash slope carried by Gill et
al.(2011) fly ash can be used successfully as an embankment
fill material.
Analyses have been made to obtain the behavior of square
footing on geogrid-reinforced flyash beds under static loading.
Most of the studies have been done on reinforced sand under
both static and dynamic loads. Hence analysis is made to study
the effectiveness of the reinforcement distribution (number of
reinforcement layers and spacing between the reinforcement
layers) on the behaviour of the square footing on flyash beds.
2. NUMERICAL MODELING
Numerical models in this study were made using the finite
element program called PLAXIS version 8.2 which is finite
element code for soil and rock analysis and is capable of

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modelling reinforced soils. The analysis is carried out by
considering square shaped footing on flyash beds. In the
analysis unreinforced and reinforced flyash beds are
considered in order to check their effectiveness.
Plane strain model is used in the present case since footing are
square in shape and in this model, deformation and stress state
are assumed to be zero in Z directions. The 15-node triangle is
considered as very accurate element as it produces high
quality stress results for difficult problems. Hence 15 node
triangular elements were used for the analysis. The geometry
model of specified dimensions i.e. a tank of 11 m diameter and
3.6m height is created using Line option in the PLAXIS
window. Figure 1 shows the Finite element mesh of geometry
around footing with geogrid.
Fig -1: Finite element mesh.
The Mohr-Coulomb model is used for assigning flyash
properties. The type of material used for footing is mild steel.
Table 1 and Table 2 presents the properties of flyash and
footing used in analysis. The biaxial geogrid are used as
reinforcing material. The distance of geogrid is 10m with
0.5m clearence between tank and geogrid on both sides. The
properties of geogrid used for analysis are shown in Table 3.
Geogrids can be activated or de-activated in calculation phases
using staged construction as loading input. The boundary
condition used for present analysis is Standard fixities.
PLAXIS contains the options to introduce distributed loads,
line loads or point loads and prescribed displacements in the
geometry model. The loading condition used for the present
study is Point load. The input values of point loads are given
in force per unit width (kN/m). The static loads are applied up
to 50mm settlement. Figure 2 shows the deformed mesh after
analysis.
Fig -2: Deformed mesh after analysis.
Table 1: Properties of flyash
Table 2: Properties of footing
Table 3: Properties of geogrids
PARAMETERS VALUES
1. Axial Stiffness (EA) 14.38*106
kN/m
3. RESULTS AND DISCUSSION
A series of finite element analysis were performed to study the
effectiveness of the reinforcement distribution on the
behaviour of the square footing on flyash beds. Zidan (2012)
conducted a series of axi-symmetry models using finite
element analyses (PLAXIS 2-DV8.2). The influence of
geogrid becomes negligible when the ratio of depth of first
layer to the footing diameter is equal to 0.5. Hence a minimum
spacing between the reinforcement layers is 0.3 times the
width of footing is used in the analysis as this minimum
thickness of the flyash bed is necessary for the reinforcement
PARAMETERS VALUES
1. Unsaturated Unit Weight 14.37 kN/m3
2. Saturated Unit Weight 15.28 kN/m3
3. Permeability 0
4. Reference Young’s Modulus 1202 kN/m2
5. Poisson’s Ratio 0.2
6. Cohesion 40 kN/m2
7. Friction Angle 27◦
8. Dilatancy Angle 0◦
9. Interface Strength 0.8
PARAMETERS VALUES
1. Axial Stiffness (EA) 8.4*106
kN/m
2. Flexural Rigidity (EI) 1120 kN-m2
/m
3.Equivalent Plate Thickness 0.04 m
4. Poisson’s Ratio 0.3
5. Specific Weight 3.14 kN/m/m
Footing
Geogrid
Flyash
11m
3.6m

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action. Also the first layer of reinforcement is placed at a
minimum depth of 0.3times the width of footing, as this
minimum thickness becomes essential for confinement effect.
3.1 Effect of Number of Reinforcement Layers
Figure 3, 4 and 5 presents the load v/s settlement curves for
square footing resting on unreinforced and reinforced flyash
beds with two, three and four layers of reinforcement at
S=0.3B, 0.4B and 0.5B respectively. It is observed from the
figures that as the number of reinforcement layers increases
the settlement reduces at any loading magnitude. For example,
from the figure 3, the footing on unreinforced flyash bed
experiences a settlement of 31.15mm at a loading magnitude
of 50kN/m, whereas footing on reinforced flyash beds with
two, three and four layers experiences the settlement of
19.54mm, 18.94mm and 16.5mm at the same loading
magnitude of 50kN/m respectively. This indicates that the
performance of reinforced flyash bed is better than
unreinforced flyash bed. In reinforced flyash bed, as the
number of reinforcement layers increases settlement will be
less for the same loading compared to its counterparts.
Fig- 3: Effect of number of reinforcement layers on the
performance of square footing resting on flyash beds under
static load and S=0.3B.
performance of square footing resting on flyash beds under a
performance of square footing resting on flyash beds under a
3.2 Effect of Reinforcement Spacing
Figure 6, 7 and 8 presents the load v/s settlement curves for
square footing resting on two, three and four layer reinforced
flyash beds with spacing of 0.3B, 0.4B and 0.5B respectively.
It is observed from the figures that as the spacing between the
reinforcement layers increases the settlement reduces at any
loading magnitude. For example, from the figure 6, the footing
on flyash bed having reinforcement at S=0.5B experiences a
settlement of 16.41mm at a loading magnitude of 40kN/m,
whereas footing on flyash bed having reinforcement at S=0.4B
and S=0.3B experiences the settlement of 16.94mm and
17.57mm at the same loading magnitude of 40kN/m
respectively. This indicates as the spacing between the
reinforcement increases settlement will be less for same
loading magnitude.
Fig- 6: Effect of reinforcement spacing on the performance of
square footing resting on two layer reinforced flyash beds
under a static load.

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square footing resting on three layer reinforced flyash beds
square footing resting on four layer reinforced flyash beds
4. CONCLUSIONS
The following conclusions are drawn from the numerical
analysis
1. Square footing resting on reinforced flyash bed
perform better than its counterpart resting on
unreinforced flyash beds.
2. As the number of reinforcement layer increases the
performance of footing increases by taking more load
and undergoing less settlement than its counterpart
resting on reinforced flyash beds.
3. As the spacing between the reinforcement layer
increases the performance of footing increases by
taking more load and undergoing less settlement than
its counterpart resting on reinforced flyash beds.
REFERENCES
[1]. Gangadara S. and Muddaraju H.C (2013) Effect of
reinforcement Spacing on the Performance of
Embedded Circular Footing in Reinforced Fly Ash,
International Conference on Innovations In Civil
Engineering.
[2]. Gill, K.S, Choudhary, A.K, Jha, J.N, and Shukla, S.K
(2011) Load Bearing Capacity of Footing Resting on a
Multilayer Reinforced Fly Ash Slope. Indian
Geotechnical Conference 2011: pp. 4262-4271.
[3]. Khing, K.H., Das, B.M., Puri, V.K., Cook, E.E. and
Yen, S.C. (1993) The bearing capacity of a strip
foundation on geogrid-reinforced sand. Geotextiles and
Geomembranes, 12(4), 351-361.
[4]. Mosallanezhad M, Hataf N. and Ghahramani A, 2010.
Three Dimensional Bearing Capacity Analysis Of
Granular Soil, Reinforced With Innovative Grid-
Anchore System. Iranian Journal of Science and
Technology 34:419-431.
[5]. Nagy A. El Mahallawy and Ahmad S. Rashed. (2012)
Experimental Study for the Behaviour of Footings on
Reinforced Sand Beds Overlying Soft Clay Zone,
Journal of American Science, 8(1).
[6]. Plaxies Version 8.2,(2002) Materials models manual.
[7]. Zidan A. F. (2012) Numerical Study of Behavior of
Circular Footing on Geogrid-Reinforced Sand Under
Static and Dynamic Loading, Geotech Geol Eng
30:499–510.
BIOGRAPHIES
Assistant Professor in JNNCE, Shimoga. ME
(Geo Technical Engineering), UVCE,
Bangalore. BE (Civil Engineering), JNNCE,
Shimoga
Associate Professor Dept of Civil
Engineering UVCE, Bangalore University,
Bangalore.56
Assistant Professor Dept of Civil Engineering
UVCE, Bangalore University,Bangalore.56,
Assistant Professor in Sri Venkateshwara
College of Engineering, Bangalore. ME (Geo
Technical Engineering), UVCE, Bangalore.
BE (Civil Engineering), Dr. AIT, Bangalore

Numerical study of behavior of square footing on geogrid reinforced flyash beds under static loading

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Numerical study of behavior of square footing on geogrid reinforced flyash beds under static loading