MODEL EXPERIMENTS ON THE EARTHQUAKE RESPONSE OF SOIL-FILLED BASINS

Abstract

The motion of soil filling a model bedrock basin was observed during continuous vibration on a laboratory shaking table. The following factors were varied in different experiments: consistency of the soil; thickness of the soil fill; length-to-width ratio of the basin; and frequency and intensity of “bedrock” motion. The shear-wave velocity was measured in place, and an empirical relationship was found between velocity and soil-water content. The response spectrum—a graph of magnification factor of “bedrock” displacement versus forcing frequency—was measured in and on the basin fill as the soil was forced into vibration by various frequencies and amplitudes of bedrock displacement. Displacements and accelerations decreased for a given set of conditions with depth below the ground surface, and the amplitude of soil displacement increased as the fill deepened. Surface motion increased as the soil consistency was softened by addition of water; this agreed with reports of many geologists who have examined damage patterns resulting from actual earthquakes. When the soil consistency became very soft, however, further addition of water caused the reverse effect, i.e., the softer the soil, the lower the magnification of bedrock motion. Soil displacements increased at a declining rate as the amplitude of displacement was increased in the “bedrock.”