Scale Model Shake Table Testing of Seismic Earth Pressures in Soft Clay

Abstract

This research consists of scale model shake table tests to investigate the development of seismic earth pressures in soft clay. The soft clay was modeled after prototype San Francisco Bay Mud consisting of a mixture of kaolinite, bentonite, class C fly ash and water. A flexible walled testing container founded on a 1g shake table was used to house the model soil and mimic 1D site response. An array of accelerometers embedded in the model soil measure during an input earthquake motion. A scale model wall is equipped with pressure sensors to measure the seismic earth pressures over the duration of an input earthquake motion. A total of 14 time histories were run through this test set up. A single degree of freedom oscillator was added to the scale model wall and used to mimic the period of a structure. Test results show that for retaining walls with clay soils seismic earth pressures develop triangularly over the face of the wall with an amplitude of about 3.8 times the static pressures. For small building structures, the development of seismic earth pressures depends on height above the base of the wall. Although the pressure distribution is not well defined, localized peaks in pressure are observed at depths of 1/3H and 2/3H below the ground surface. Arias intensity and cumulative absolute velocity correlate linearly with the measured dynamic pressures. Differences between arias intensity and cumulative absolute velocity for each scale model configuration are not pronounced. The simplified Monnobe-Okabe method was also evaluated in this study. Although the Mononobe-Okabe method may be inappropriate for cohesive soils, a seismic coefficient of about 1/10 the PGA was back-calculated from empirically measured earth pressures. The results of this investigation provides an empirical basis to the behavior of walls in clay.