Seismic settlement and bearing capacity of shallow footings on cohesionless soil.

Abstract

Seismic loading reduces the bearing capacity of soils and large settlement can occur. These effects have not been considered adequately in design codes. In this dissertation, the seismic bearing capacity and settlement of soils have been investigated theoretically and experimentally. The theoretical analysis was developed for a dry c-φ soil, considering the effect of the cohesion, and the vertical and the horizontal acceleration components. The seismic bearing capacity was examined by using the concept of shear fluidization of soil, while the seismic settlement was examined using the sliding block model technique. The theory of the shear fluidization of soil was developed for c-φ soils and extended the original application which was limited to cohesionless soils. The experiments were conducted on dry and saturated cohesionless soil using a shake box designed and constructed during this research. The shake box was designed to subject the soil to simple shear conditions during shaking. Model footings, constructed from lead, were used to study the seismic bearing capacity and settlement of shallow footings. The parameters investigated include the horizontal acceleration, the frequency, the safety factor, the footing width, the footing shape and size, the depth of embedment, and the relative density of the soil. The theoretical and the experimental results showed good agreement. Significant reduction in the bearing capacity of the soil, even at low acceleration (e.g. < 0.3 g) and excessive settlement can occur if the seismic bearing capacity becomes lower than the allowable static bearing capacity. Seismic design procedures are proposed and illustrative examples are used to demonstrate the design procedures.