Characterization and quality control of stone columns using surface wave testing

Seismic surface waves are well-suited for the study of the elastic profile of soils. This study evaluates the application of surface waves in characterizing the properties of laterally heterogeneous soil, specifically for use in the quality control of stone columns used for ground improvement. Here, laterally heterogeneous soil refers to the gravelly sand material being formed into cylindrical columns and inserted into a homogenous clay bed. A scaled-down model of typical stone columns in a clay matrix was constructed. Measurements were made on stone columns of different dimensions, as well as on a defective column, under controlled conditions so that the properties of the materials used in the model under test were within limited ranges. Shear moduli obtained from the phase velocity determined from the controlled tests showed close agreement with those measured indirectly with the vane shear test. The dispersive curve produced in this study demonstrated an increased phase velocity with increasing wavelength for the measurements on the clay (between columns), and decreased phase velocity with increasing wavelength for the measurements on the column. More interestingly, the results showed that in the characterization of lateral nonhomogeneities, the phase velocity versus wavelength relationship varies for stone columns of different diameters and densities. These results point to the potential for estimating the phase velocity and, thus, the shear modulus of an effective region that spans both the lateral and depth axes, and also demonstrate that the results can be influenced by the positioning of sensors with respect to the survey target