3D Seismic Response Analysis of Shallow Foundation Resting on Sandy Soil

1. Introduction

The overall performance of a foundation during earthquake shaking is characterized by the interaction between foundation and the complex geological media. Seismic excitation can cause permanent displacement, distortion and distress of the foundation. The excess pore pressure develops during earthquake loading causes liquefaction and reduces bearing capacity of the saturated soil. A significant cause of damage of the structure during earthquake is displacement and sinking of foundation in saturated sand. There are many ground-damaging activities observed during the great Bihar-Nepal earthquake of 1934, which gives the best examples of widespread liquefaction. Tilting of building into the alluvium soil of Ganga Plains was continued for several days even after the earthquake. The chief criterion adopted in the demarcation of this slump belt was seismic response of the built environment (Auden et al., 1939). The foundation failure during Alaska and Niigata, 1964 earthquake has increased the knowledge of soil liquefaction phenomenon. During these earthquakes approximately 340 reinforced concrete buildings were damaged in Niigata city (Ohsaki 1966, Seed and Idriss, 1967). Settlement of 0.25 m to 2.5 m was noticed in the damaged buildings of Dagupan City area (Adachi et al., 1992). Most of these buildings were founded on shallow foundations supported by uniform fine clean sand. The thickness of liquefiable sand layer was in the range of 6 and 10 m. From the study of 120 damaged reinforced concrete buildings in Dagupan City, Tokimatsu et al., (1991) observed that most of the buildings were two to four stories and rested on shallow footings without any piles. Most significant settlement were observed in corner buildings, in buildings without adjacent structures on one or both sides, in buildings surrounded by lightweight structures, and in those parts of the areas where there was greater separation between the adjacent buildings. All these observations have pointed out the importance of the external confinement effect on reduction of foundation damage. Studies on the influence of boundary condition or confinement and water table on seismic response of shallow foundation are limited. Past researchers have reported very few experimental investigations on confinement effect on behavior of soil-foundation system. Hence, in the present study an attempt has been made to study the effect of different boundary condition/confinement and water table on seismic response of soil-foundation system. A square shaped shallow foundation resting on sandy soil has been adopted for the analysis. Two different cases have been considered here. In first case, effect of boundary condition on seismic response of soil-foundation system has been studied by assigning rigid box and shear beam type boundary conditions to the soil domain. Comparison has been made between ‘Rigid Box’ and ‘Shear Beam’ type boundary conditions. In second case, effect of water table position on seismic response of soil-foundation system has been studied by varying the water table level below the ground surface (0 m, 1 m and 2 m) with shear beam type boundary condition.