ANALYTICAL AND NUMERICAL MODELING OF BACKFILL SOILTO ESTIMATE STATIC AND SEISMIC EARTH PRESSURES

Abstract

The rise in failure of retaining walls subjected to dynamic loads is an increasing concern in earthquake-prone areas. Limit equilibrium method is adopted to estimate critical seismic active earth pressure on vertical and inclined back-face retaining wall supporting cohesion-less soil with surcharge. Planar potential failure surface is considered to estimate seismic active earth pressure. It is assumed that the soil-wall system is overlying on a rigid bedrock. The backfill soil in the study is assumed as a visco-elastic material, specifically Kelvin-Voigt material, that resists shear deformation due to dynamic loading by elastic component and viscous component. The proposed methodology satisfies the boundary condition at the surface of the backfill and at the bottom of the backfill. The variation of acceleration is neither constant (as that of pseudo-static method) nor linear (as that of Pseudo-dynamic method) throughout the height of the retaining wall. It depends on the dynamic soil properties and frequency of the seismic excitation. The obtained acceleration distribution is observed to be non-linear and importantly, it is not in-phase throughout the height of the backfill. An attempt is made to study the effect of strain-dependent dynamic soil properties on seismic lateral coefficient for varying surcharge magnitudes. An algorithm is proposed to estimate the strain-dependent dynamic soil properties, which are further used to estimate seismic active coefficient. The effect of Primary wave on the lateral earth pressure distribution is studied in detail. It is observed that the effect of vertical acceleration is considerable in estimating vertical inertial force due to critical soil wedge and surcharge. Thereby affecting seismic lateral earth pressure acting on retaining wall due to surcharged backfill. A closed-form solution is proposed to estimate seismic active thrust acting on the retaining wall with submerged backfill. The excess pore water pressure is also considered in the analysis. The horizontal inertial forces are estimated by considering dry unit weight, assuming cohesion-less backfill soil as highly permeable soil. Results obtained from limit equilibrium analysis are compared with those obtained by performing numerical analysis using finite-element modeling. It is observed that the frequency of the seismic wave plays an important role in estimating the seismic coefficient of earth pressure for different surcharge magnitudes. The study reveals that though the amplification of the wave is not much affected by the magnitude of the surcharge, consideration of the amplified wave when estimating the inertial force due to surcharge influences the total seismic i active thrust. It is also observed that the effect of shear wave propagation on earth pressure is relatively dominant as compared to that of primary wave propagation. Obtained results are in good agreement with the well-established pseudo-static, pseudo-dynamic methods. A comparison of results with existing laboratory investigations also yielded a good agreement. Apart from seismic analysis of conventional retaining walls, a numerical study of narrow backfill retaining walls for static case is also proposed. The failure surfaces, lateral thrust acting on the retaining for different backfill thicknesses are presented. It is observed that the soil-wall interface friction plays an important role in estimating lateral earth pressure acting on the retaining wall with narrow backfill width.