Reduction Of Lateral Pressure On Retaining Walls Backfilling With Expansive Soil Using Expanded Polystyrene(EPS) Geofoam

Expansive soils swell or shrinkage due to an increase or a decrease in water content.Swelling pressure occurs when the swelling deformation of the expansive soils is restrained.Retaining structures such as retaining walls often suffer from the adverse influence of lateral swelling pressure of expansive soils when the expansive soils are retained.Introducing a layer of expanded polystyrene（EPS）geofoam inclusion between retaining walls and expansive soils can significantly reduce the lateral pressure acting on retaining walls.However,the use of EPS geofoam in expansive soil is limited,as the system“expansive soil-EPS inclusion-retaining wall”is not well studied.To investigate the interaction mechanisms between expansive soil,EPS inclusion and retaining wall and to propose the design method of the“expansive soil-EPS inclusion-retaining wall”system,including the choose requirements and the calculating model of the lateral pressure on the retaining wall,experiments,numerical simulations and theoretical analysis were conducted in this paper.These research results will provide a scientific basis for the reduced pressure applications of EPS in expansive soil areas.To accomplish the goal,expansive soil from Nanyang and commercial EPS were used to carried out the laboratory tests,finite element method was adopted in the numerical simulation and mechanical analysis was conducted based on experimental and numerical results and assumptions.The following conclusions can be obtained.（1）K₀ consolidometers were modified for the measurement of lateral pressure of expansive soil after swelling.Factors that may affect the lateral pressure of expansive soil after swelling were investigated by using the modifed K₀ consolidometer.The results indicated that the lateral pressure of expansive soil after swelling is mainly affected by the vertical load,dry density,initial water content and increased water content;wetting-drying cycles and compaction induced lateral pressure do not influence the lateral pressure of expansive soil after swelling;both swelling deformation and elastic modulus of expansive soil affect the lateral swelling pressure of expansive soil and a peak value of the lateral swelling pressure may occur at a certain time or water content during swelling;and the current theoretical method of estimating the lateral swelling pressure of expansive soil is inaccurate and inconvenient and the test method using the K₀ consolidometer is simpler and more reliable.（2）The stress-strain curves,creep and stress relaxation properties of EPS with stress history were investigated by laboratory tests,since EPS inclusions may be subjected to cyclic compression of expansive soils when the EPS inclusion is put into service in expansive soils enriched areas.The test results showed that stress histories increase the elastic limit of EPS and decrease the elastic modulus,creep and stress relaxation of EPS and cyclic compression of expansive soils does not decrease the efficiency of EPS in lateral pressure reduction.（3）Element tests were conducted to study the“expansive soil-EPS inclusion-retaining wall”system.The results showed that the pressure reduction efficiency of EPS inclusion is better than that of cohesive non-swelling soil（CNS）and sand inclusions;the lower the EPS density,the higher efficiency of the EPS inclusion;and the vertical load on expansive soil influences the efficiency of EPS inclusion and when the vertical load is equal to the pressure,under which,the vertical swelling deformation is zero,the efficiency reach the maximum value.The pressure reduction efficiency of the EPS inclusion levels off as the wetting-drying cycle of expansive soils increases.（4）The ABAQUS package was employed to conduct numerical simulation.Thermal fields were used to simulate moisture fields and thermal expansion was used to simulate swelling due to wetting.Validation of the simulation of the“expansive soil-EPS inclusion-retaining wall”system was performed by comparing the numerical and experimental results.Factors affecting the efficiency of EPS inclusion were investigated by numerical simulation.Parametric analysis indicated that a lower density and higher thickness of EPS inclusion,a lower width of expansive soil and hight of retaining wall cause a higher lateral pressure reduction efficiency of EPS inclusion;EPS inclusion should be placed along the whole back of the retaining wall with a inverted ladder pattern（i.e.the EPS inclusion is thicker at the top of the retaining wall and thinner at the bottom of the retaining wall）or uniform thickness pattern;rainfall intensity and the duration of the rainfall influence the efficiency of EPS inclusion by affecting the elastic modulus of the expansive soil.（5）The interaction mechanisms between expansive soil,EPS inclusion and retaining wall were explored based on the experimental and numerical results:1)the EPS inclusion is compressed during the swelling of the expansive soil,thereby,more space is provided for the expansive soil and lateral pressure is reduced;2)shearing strength of the expansive soil was mobilized as the lateral deformation occurs;3)friction between the EPS inclusion,retaining wall and expansive soil changes the distribution of the lateral pressure on the retaining wall,which increases the moment on the wall but the horizontal force on the retaining wall is unchanged.A calculating model of lateral pressure on the“expansive soil-EPS inclusion-retaining wall”was proposed based on some assumptions and mechanic analysis.Design method of the“expansive soil-EPS inclusion-retaining wall”was illustrated by taking the gravity retaining wall for example.