903
Evaluation of Seismic Earth Pressure Reduction using EPS Geofoam
Evaluation de la réduction de la poussée sismique en utilisant du Polystyrène
Expansé
Dave T.N., Dasaka S.M., Khan N.
Indian Institute of Technology Bombay, Mumbai, India
Murali Krishna A.
Indian Institute of Technology Guwahati, India
ABSTRACT: Retaining structures are designed to withstand lateral pressures due to backfill, surcharge load from adjacent structures
and traffic and earthquake loads. The cost of these structures is directly proportional to the earth pressures they are subjected to.
Several techniques have been tried in the literature to minimize the earth pressure exerted on retaining walls. Among them, use of
geofoam as a compressible inclusion placed at the wall-backfill interface, is found to be a simple and effective solution, based on
preliminary studies. However, behaviour of EPS geofoam and its influence on the earth pressure reduction under seismic loading
conditions are not well understood, and need to be investigated further. In the present study, small scale physical model tests were
performed on an instrumented retaining wall subjected to 1-D shaking, to evaluate earth pressures on the wall and to assess
effectiveness of EPS geofoam to reduce seismic earth pressures. Firstly, static surcharge loading was applied in order to evaluate
magnitude and distribution of earth pressure. Further, under maintained surcharge, a seismic load in the form of a stepped sinusoidal
wave from 0 to 0.7 g was applied in increments of 0.045 g, each increment being applied for 5 seconds at 3 Hz frequency. The
experimental results indicate that the earth pressures under the influence of a seismic load show an increase of the order of 23%.
Moreover, by using the geofoam as a seismic buffer, it was observed that the total seismic force on the retaining wall reduced by
about 23% with a corresponding reduction in maximum lateral thrust by 27%.
RÉSUMÉ : Les structures de soutènement sont conçues pour résister à des pressions latérales dues au remblai, à la surcharge de
structures adjacentes, au traffic et aux charges sismiques. Plusieurs études ont été réalisées dans la littérature minimiser la pression des
terres sur des murs de soutènement. Dans la présente étude, des expérimentations ont été exécutées sur un mur de soutènement
instrumenté pour évaluer la pression des terres et l'efficacité du Polystyrène, sous sollicitation sismique générée par une table vibrante
1D. Premièrement, une surcharge statique était appliquée afin d'évaluer la distribution de la pression des terres. Puis, sous la surcharge
maintenue, une charge sismique sous forme de vague sinusoïdale de 0 à 0,7 g était appliquée par paliers de 0,045 g, chaque
augmentation étant appliquée pendant 5 secondes à 3 hertz de fréquence. Les résultats expérimentaux indiquent que les pressions des
terres, sous l'influence d'une charge sismique montrent une augmentation de l'ordre de 23%. De plus, avec le polystyrène comme
amortisseur sismique, on a observé que la force sismique totale sur le mur de soutènement diminue d'environ 23% avec une réduction
de la poussée latérale maximum de 27%.
KEYWORDS: seismic load, earth pressure reduction, geofoam, shake table
1 INTRODUCTION
Earth-retaining structures are integral part of many
infrastructure projects, and underground urban construction to
retain soil on one of its sides. Rigid retaining walls are
commonly found in basements, bridge abutments, box culverts
etc. and they cannot be entirely replaced by reinforced soil
walls. Lateral pressure acting on rigid retaining walls due to
backfill, surcharge load from adjacent structures and loads due
to traffic and natural calamities like earthquake etc. decides
their sectional dimensions. Intensive earthquake loading, which
impose larger forces compared to that of static active or at-rest
conditions. The geotechnical profession has been constantly
working for a viable solution to reduce the earth pressures
exerted on retaining walls, which would eventually reduce the
construction cost of the wall, and post-construction maintenance
cost. A technique of placing a compressible inclusion at the
soil-wall interface has come into existence to minimize earth
pressures on retaining walls. Previous research studies indicate
that provision of a compressible inclusion behind a rigid non-
yielding/limited yielding or yielding wall would contribute to
the economical design of the wall by imparting controlled
yielding in the backfill material. Deformations in a retained soil
mass mobilize a greater portion of the available shear strength
of the material and decrease the unbalanced lateral forces acting
on the retaining structure.
2 REVIEW OF LITERATURE
Among all the methods, provision of a compressible inclusion
in the form of Expanded Polystyrene (EPS) geofoam at the
wall-backfill interface proved successful because of ease in
construction and predictable stress-strain characteristics of the
inclusion. In the past, studies were conducted with materials
such as glass-fiber insulation (Rehnman & Broms, 1972) and
cardboard (Edgar et al., 1989) for similar applications.
However, they were not successful, as their stress-strain
behavior was unpredictable and uncontrollable. On the other
hand, Expanded Polystyrene (EPS) geofoam is considered as a
suitable material as it fulfills the required stress-strain behavior
and has smaller stiffness than any other geofoam materials.
Additionally, Horvath (1997) documented 30 years of proven
durability of EPS geofoam in several geotechnical applications.
A field study on reduction in lateral earth pressure behind
rigid wall by using compressible geo-inclusion has been
reported by Partos and Kazaniwsky (1987). Using instrumented
model studies, McGown et al. (1988) demonstrated significant
reduction in lateral earth pressure even below active earth
pressure, when soil was allowed to yield in a controlled manner.
