Geotechnical properties of Holocene and latest Pleistocene sediments with special reference to shear wave velocities

摘要

Geotechnical properties of Holocene and latest Pleistocene unconsolidated sediments were statistically investigated in correlation with shear wave velocities accurately measured in boreholes using the PS suspension logging tool. The geotechnical data, collected from two regions, more than 200 boreholes, and 9,000 depth points, comprise N-values obtained by in situ Standard Penetration Test (SPT), bulk densities, solidities, and median grain sizes measured by the standard laboratory test, and the elastic constants determined by tri-axial dynamic loading tests. Recent advances in sedimentological research have revealed detailed lithofacies of the Holocene and latest Pleistocene sediments of the southern part of the Kanto Plain and the coastal region of the Nobi Plain based on comprehensive core investigations. The revealed lithofacies were taken into account in the correlation analysis. The results of the correlation analysis are as follows. First, natural water content ratio and void ratio are inappropriate as an indicator of lithofacies or geotechnical properties because the parameters are denned as the relative ratio to the solid weight or to the solid volume. Instead, bulk density and solidity (the complement of porosity) are practical as such indicators. Second, ambiguity still remained in the statistical relationship between N-values and S-wave velocities even when we used high precision S-wave velocity data obtained using the suspension logging tool. This means the ambiguity is mainly attributed to the poor quality of N-values. So it is possible to estimate Nvalues from S-wave velocity data using an empirically synthesized equation, but it is very hard to estimate S-wave velocities from revalue data. Third, a clear relationship was recognized between S-wave velocities and solidities or bulk densities, and it seemed to vary with lithofacies and depositional stage. This indicates that S-wave velocity itself is helpful as a good indicator to evaluate other geotechnical properties of Holocene sediments and to estimate the sedimentological environments.