Effects of Sedimentary Depositional Environment on the Dynamic Mechanical Properties of Holocene Clay in the Southern Kanto Plain, Central Japan

Abstract

 The effects of earthquakes in the Kanto district are strongly related to the three-dimensional distribution of the soil mechanical properties of Chuseki-so, which is the name given to incised-valley fills from the Late Pleistocene to the Holocene. To characterize the effects of the sedimentary depositional environment and sedimentary process of Chuseki-so on soil mechanical properties, PS logging test, physical-chemical test, consolidation test, cyclic tri-axial test, and bender element test were conducted on an undisturbed core sample. Based on the test results, we propose a method to estimate the elastic shear modulus of clay soil from conventional soil properties, and discuss the relationship between the sedimentary process of Chuseki-so and the three-dimensional distribution of soil mechanical properties. Through our investigation, we derived the following conclusions: (1) The delta front and inner bay deposits of the Kasukabe core in the Nakagawa Lowland comprise very soft clay and have a high water content and liquidity index; these characteristics are related to the sedimentary depositional environment such as water depth and salinity (ratio of marine diatoms in the sediment). (2) The proposed method can be used to estimate elastic shear moduli on the basis of water content, depth, and locality parameter; in particular, it can be used to evaluate very soft clay, the N value of which is less than 1, on the basis of water content, an easily measurable property. (3) Both bender element and PS logging tests show similar S-wave velocities, and elastic shear moduli measured using the respective tests, G₀^BE and G₀^PS, were also similar; therefore, elastic shear moduli can be determined through measurements using these two tests or estimations based on water content. (4) The method to determine the elastic shear modulus, G₀^TR, which is obtained using a cyclic test, is proposed based on the H-D model; G₀^TR btained correlates well with G₀^BE, even if though it is obtained by a different method. The proposed method can be used to determine elastic shear moduli with great accurac.