Field Investigation and Laboratory Soil Tests on Heterogeneous Nature of Alluvial Deposits

Abstract

This text is concerned with the basic principles underlying current practice in the earthquake response analysis of soft alluvial deposits. Its special interest is focused on the stratification of alluvium that has been modeled in analyses by layers in which soil properties are uniform. Since the age and the effective stress are continuous in the vertical direction, it is possible that the soil-dynamic properties such as shear modulus and damping ratio are not uniform but varying continuously in the vertical direction. From this viewpoint, the conventional idea of uniform layers was examined in detail by conducting detailed investigations in-situ, as well as laboratory tests on continuously collected undisturbed soil samples. The test results show that soil properties continuously vary in the vertical direction within an alluvium. This holds true even when the soil type changes from clayey to sandy. In contrast, a discontinuity of properties was detected at an unconformity of Pleistocene layers where the age and sedimentation process are discontinuous. With this knowledge, an attempt was made to propose a recommendable modelling of soil stratification for practical earthquake response analysis on alluvium.