Ground improvement work is crucial in enhancing the characteristics of weak soils commonly encountered in Civil Engineering, and one such technique commonly used is vibro-stone columns. An assessment of the effectiveness of such an approach is critical to determine whether the quality of the works meets the prescribed requirements. Conventional quality testing suffers limitations including: limited coverage (both area and depth) and problems with sampling quality. Traditionally quality assurance measurements use laboratory and in-situ invasive and destructive tests. However geophysical approaches, which are typically non-invasive and non-destructive, offer a method by which improvement profiles can be measured in a cost effective way. Of these seismic surface waves have proved the most useful to assess vibro-stone columns, however, to date much of the previous work conducted has focussed on field based observations making detailed evaluation of this approach difficult. This study evaluates the application of surface waves in characterizing the properties of laterally heterogeneous soil, specifically for using in the quality control of vibro-stone column. Three models were employed which began with a simple model and extended finally to complex model: (1) concrete mortar was used to establish the method, equipment and its system, (2) pilot test on a small scale soft kaolin to adopt a model vibro-stone column and (3) main test contained a configuration of vibro-stone column in soft Oxford clay. A generic scaled-down model of vibro-stone column(s) was constructed. Measurements were conducted using different arrays of column configuration, using sand to simulate stone material. This idealized set of laboratory conditions were used to provide guidelines for the interpretation of field measurements. The phase velocity obtained from the controlled tests showed close agreement to those reported in literature and with those generated through empirical correlations with vane shear test. The dispersive curve 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 non-homogeneities, the phase velocity versus wavelength relationship varies on stone columns of different diameters and densities. This illustrated that the shear modulus profiles are influenced by the effective region that spans both the lateral and depth axes, and also demonstrated how the results can be influenced by the positioning of sensors with respect to the survey target. This research demonstrates how Rayleigh waves can be used for quality assurance when constructing vibro-stone columns.
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