A laboratory investigation of shear wave velocity in stabilised soft soils
The stabilisation of soft clay soils is intended to increase their shear strength and to reduce their compressibility. The possibility exists of using geophysical methods to monitor changes in these properties. Laboratory experiments were carried out on stabilised clays to study the relationships between shear wave velocity, and hence small strain shear stiffness, and shear strength or one-dimensional compressibility. One artificial clay, Speswhite kaolin, and two natural clays, from Malaysia and Sweden, were used as the base clays. Either ordinary Portland cement or a 1: 1 mix of the cement with unslaked lime was added to the base clays in order to stabilize them. In the first part of the investigation, samples of stabilised clay were initially subjected to a non-destructive bender element test to obtain the shear wave velocity and then to an unconfined compressive strength test or vane shear strength test. It was evident that small stabiliser amounts (less than 10 % of the dry weight of the base clay) could significantly improve both the strength and stiffness of the originally soft material. In addition, good correlations between the shear strength and the shear wave velocity (or small strain shear stiffness) of the stabilised clays were established. In the second part of the investigation, an instrumented oedometer was used to simultaneously monitor shear wave velocity and one-dimensional compression during tests on samples cured for a set period. Lateral stresses were also measured. Complementary tests were conducted in standard oedometers, to study the effect of the curing period. In these tests yield stresses were identified and corresponded to the onset of changes in shear wave velocity. After yield, the constrained moduli could be correlated with shear wave velocity. Tests were also carried out on samples of clay in which a central stabilised column had been created. Equal strain predictions of the compression of these samples, based on the results of separate tests on the two components, were relatively successful. The results of the research suggest that shear wave velocity measurements could be useful in practice to enable the shear strength and post-yield compressibility of stabilised clay soil to be estimated.