Evaluation of ground improvement on soft clay deposit using shear wave velocity

Abstract

The evaluation of field degree of consolidation on soft clays has been an important problem in the areas of geotechnical engineering. Monitoring either settlements or pore water pressures has been widely adopted to evaluate the degree of consolidation in the filed, but occasionally they have some problems in practical application. The objective of this study is the development and feasibility study of a procedure to evaluate the degree of consolidation of soft clays based on the shear wave velocity ($V_s$). As a first step, the testing procedure and accuracy of the bender element test, which has been widely used to measure the shear wave velocity of soils in the laboratory, were verified. The bender element (BE), resonant column (RC), and torsional shear (TS) tests were performed on the same specimens. Two clean sands for which the frequency effect on the small-strain shear modulus ($G_{max}$) is negligible were tested at various densities and mean effective stresses under dry and saturated conditions. Based on the test results, methods of determining travel time in BE tests were evaluated by comparing the results of RC, TS, and BE tests. Also, methods to evaluate $G_{max}$ of saturated sands from the $V_s$ obtained by RC and BE tests were investigated by comparing the three sets of test results; Biot’s theory on frequency dependence of $V_s$ was adopted in order to consider dispersion of a shear wave in saturated conditions. A procedure to evaluate the degree of consolidation of soft clay during ground improvement was proposed on the basis of collected experimental data from the existing studies on the relation between $V_s$ and effective stress of clayey soil, and the applicability of the proposed evaluation procedure was investigated based on a case study performed in field. A research site where soft clay layers were consolidated by surcharging loads was chosen. Laboratory tests were performed to determine the relation between $V_s$ and effective stress...