International Conference on Structural and Geotechnical Engineering, Ain Shams University ICSGE 14 NUMERICAL MODELING OF SOIL-STRUCTURE INTERACTION WITH APPLICATIONS TO GEOSYNTHETICS Mahmoud G. Hussein 1 and Mohamed A. Meguid 2 1 Ph.D. Candidate, 2 Associate Professor of Geotechnical Engineering Department of Civil Engineering and Applied Mechanics, McGill University, Montreal, Quebec, Canada H3A 0C3 ABSTRACT Finite element method has proven to be a powerful tool in modelling boundary value problems, particularly those involving soil-structure interaction. Incorporating geosynthetics in civil engineering projects is rapidly growing, especially in the design of earth supported structures. Applications include reinforced earth fills, retaining walls, embankments, buried structures and shallow foundations. In this study, 2D and 3D finite element analyses are conducted using ABAQUS software to investigate two different soil-structure interaction problems: 1) three-dimensional analysis of unconfined and soil-confined geogrid with an example of a square footing over geogrid- reinforced soil, 2) two-dimensional plane strain analysis of a box culvert overlain by EPS geofoam inclusion to reduce earth pressure on the walls of the structure. Validation is performed by comparing the FE results with experimental data. Conclusions are made regarding the effectiveness of using the finite element method to solve these classes of geotechnical engineering problems. Keywords: soil-geogrid interaction, finite element, reinforced soil, EPS geofoam INTRODUCTION Numerical modeling of soil-structure interaction problems involving flexible or soft geosynthetic inclusions is known to be challenging, especially in the presence of nearby rigid structures. This is attributed to the complicated nature of the created soil- geosynthetic-structure system with different material models and interaction behavior. Analyzing the problem using continuum approaches (e.g. finite element method) consists of finding a unique system of displacements for each component that satisfies both force equilibrium and material continuity. The objective of this study is to present a numerical approach that has been successfully used to model two different soil- structure interaction problems with geosynthetics inclusion. The steps taken in modelling the response of each involved material and interaction details are summarized. The results of this numerical investigation allowed for the merits of using geosynthetic material in two practical applications to be investigated.
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International Conference on Structural and Geotechnical Engineering, Ain Shams University ICSGE 14
NUMERICAL MODELING OF SOIL-STRUCTURE INTERACTION WITH
APPLICATIONS TO GEOSYNTHETICS
Mahmoud G. Hussein1 and Mohamed A. Meguid2 1 Ph.D. Candidate, 2 Associate Professor of Geotechnical Engineering
Department of Civil Engineering and Applied Mechanics,
International Conference on Structural and Geotechnical Engineering, Ain Shams University ICSGE 14
(a)
(b)
(c)
Figure 10. Effect of EPS density on the change of earth pressure on the culvert walls For comparison purposes, the calculated pressure for each case is also compared
with the benchmark analysis (no geofoam). The vertical axes in Figure 10 represent
the contact pressure ratio normalized with respect to the benchmark case. For the
upper wall (Figure 10a), the EPS density was found to have a significant impact on the
earth pressure acting on the wall. Compared to the benchmark, the lowest contact
pressure is calculated for the case of EPS15. The pressure reduction at the upper wall
for different applied surface pressures (up to 1% deformation) were found to be 65%,
54% and 23% for EPS15, EPS22 and EPS39, respectively. On the lower wall, these
ratios (Figure 10b) were found to be 28%, 25% and 14% for EPS15, EPS22 and
EPS39, respectively. These effects are considered to be significantly smaller as
compared to that calculated for the upper wall. Similar trends were found for the
contact pressures on the side wall (Figure 10c) with pressure reduction ratios of 34%,
28% and 15%.
SUMMARY AND CONCLUSIONS
In this study, the finite element method is used to simulate two different classes of
soil-structure interaction problems involving two types of geosynthetics.
First, a procedure for the 3D FE modeling of unconfined and soil-confined geogrid is
developed using ABAQUS software. A numerical model that is capable of simulating
the response of unconfined biaxial geogrid under tensile loading is introduced and
validated using index test results. In developing this model, the details of the geogrid
0.0
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Applied surface pressure (kPa)
Upper wall
EPS15: 113 kPa @ 1% strain
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ct p
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Applied surface pressure (kPa)
Lower wall
EPS15: 113 kPa @ 1%
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Con
tact pre
ssure
ratio
Applied surface pressure (kPa)
Side wall
EPS15: 113 kPa @ 1%
International Conference on Structural and Geotechnical Engineering, Ain Shams University ICSGE 14
geometry is explicitly simulated. The geogrid material is represented using elasto-
plastic constitutive model. Tensile load applied to a geogrid specimen is carried mostly
by the longitudinal ribs in the direction of the applied load and the portion carried by
the junctions and transvers bars are insignificant. The displacement is distributed
linearly with distance from the loaded boundary.
To confirm the validity of the unconfined geogrid model, a 3D analysis is performed to
examine the geogrid performance as it interacts with the backfill material. A case study
involving a square footing supported by a geogrid-reinforced crushed limestone is
investigated. The 3D geometry of the geogrid, its deformation, and stress distribution
were presented. The model was able to capture the 3D response of multiple geogrid
layers installed under the footing. Increasing the number of geogrid layers resulted in
an increase in the ultimate bearing capacity of the supporting soil. The geogrid
deformations and tensile stresses for the case of N = 1 were found to be generally
larger than those calculated for N = 2.
Based on the results of the numerical analyses, it can be concluded that the proposed
FE approach is efficient in capturing the 3D responses of both unconfined and soil-
confined geogrid and allowed for the details of the interaction between the geogrid and
the surrounding backfill material to be simulated.
Another class of soil-geosynthetic interaction problems is investigated using 2D
plane strain analysis to study the role of EPS inclusion above a buried box culvert in
reducing the earth pressure on the walls of the structure. The developed model was
used to investigate a case study of an instrumented HSS section (with and without
EPS) that was placed within a rigid steel container backfilled with sandy gravel
material and loaded incrementally with a vertical pressure using an air bag. The effect
of the EPS density on the earth pressure acting on the HSS section was examined and
found to have a significant impact on the changes in earth pressure. This study
suggests that placing light weight EPS block above a rigid subsurface structure can
result in a significant reduction in vertical earth pressure resulting in economic design.
ACKNOWLEDGEMENTS
This research is supported by a research grant from the Natural Science and Engineering
Research Council of Canada (NSERC). The support of Plasti-Fab Ltd. throughout this
study is greatly appreciated.
REFERENCES
1. Rowe RK, Mylleville BLJ. Analysis and design of reinforced embankments on soft or