STATIC AND SEISMIC BEHAVIOUR OF CANTILEVER SHEET PILE WALLS UNDER SURCHARGE LOAD

Abstract

Deep excavation is the need of present time for rapid urbanization due to lack of space in urban congested areas. This has placed high pressure on the geotechnical engineers for proper planning and design of the retaining walls during deep excavations. The cantilever sheet pile walls are generally used to retain the height of excavation below 6m. Conventionally, cantilever sheet pile wall is designed assuming rectilinear earth pressure distribution. Also, on the field, a surcharge in the form of vehicle, building and storage area may be present on the backfill of the cantilever sheet pile wall which can produce additional pressure on the wall. Studies related to the surcharge present on the backfill of the sheet pile wall are scarce in literature. Therefore, an attempt is made to analyze the behavior of cantilever sheet pile wall having surcharge present on the backfill for static as well as seismic conditions. In the present study, the influence of infinite uniform surcharge and strip load on the cantilever sheet pile wall at varying distances from the top of the wall under static as well as seismic conditions are analyzed using finite difference based computer program. The numerical analysis has been conducted at different magnitudes of horizontal and vertical seismic acceleration coefficient and by varying the magnitude and position of surcharge from the top of the wall for different embedded depths and types of soil. The results indicate that the presence of the surcharge load at the top of the wall influences the cantilever sheet pile wall significantly and as the distance of surcharge load increases, the influence gets reduced and beyond 2 times excavation depth, the influence is insignificant. Mobilization of earth pressure takes place up to a greater depth for excavation depth greater than 60% of the total length of the wall and causes more settlement and deflection. It is observed that the maximum bending moment increases and more mobilization of earth pressure takes place with increase in horizontal seismic acceleration coefficients, magnitude of surcharge load, embedded depth and decrease in the distance of surcharge load from the top of the wall in loose sand. The optimum behavior of the cantilever sheet pile wall is observed for strip load having width 2m to 3m on the ground surface A theoretical method to design the cantilever sheet pile wall having infinite uniform surcharge and strip load on the backfill is developed using simplified and partially mobilized method. In simplified method, coefficients of earth pressure are constant while in partially mobilized method, coefficients of earth pressure depend on the displacement of the cantilever sheet pile wall. The sheet pile wall is assumed to be a rigid body to rotate about a pivot point near the toe of the wall and generates active and passive earth pressures. Coefficients of earth pressure due to surcharge loading are calculated using Coulomb earth pressure theory. The influence of surcharge load is calculated using 45˚ distribution approach considering the soil shear strength parameters. Pivot point and depth of penetration are calculated by considering horizontal force equilibrium and moment equilibrium simultaneously. Closed form expressions for the bending moment and shear force at various depths of the sheet pile wall are also proposed. Dynamic analysis of cantilever sheet pile wall having surcharge load is executed by using finite difference based computer program subjected to 1994 Northridge, 1989 Loma Gilroy and 2001 Bhuj motion. The liquefiable and non-liquefiable soil layers are simulated by properties of Pohang sand for UBCSAND and Mohr-Coulomb constitutive models respectively. The ground water table is assumed at the ground surface. The maximum bending moments are observed to occur at any instant of time not necessarily the instant of maximum acceleration. In liquefiable layer, the soil stiffness and shear strength reduces which results the loss in acceleration amplification factor and maximum amplification factor is obtained for motion having lowest bed rock level acceleration. The maximum bending moment of the cantilever sheet pile wall depends on the bed rock level acceleration while maximum displacement is observed for 1994 Northridge motion and 2001 Bhuj motion of duration of time history 75s gives more displacement than 1989 Loma Gilroy motion of duration of time history 40s. The bending moment, displacement and settlement decreases with increasing the distance of the surcharge load from the cantilever sheet pile wall and decreasing the width of the load. The research work carried out and presented in this thesis will be helpful to introduce the design guidelines for cantilever sheet pile wall. The design charts presented is of practical significance for geotechnical engineers working in the field of designing the cantilever sheet pile wall having surcharge load on the backfill under static as well as seismic conditions.