Determination of in situ Vs and Gmax using surface wave measurements in cased and uncased boreholes




Kalinski, Michael E. (Michael Edward), 1963-

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The Spectral-Analysis-of-Surface-Waves (SASW) method is a technique in which surface waves are used to determine variations in small-strain shear modulus (G [subscript max]) with depth in layered systems. The SASW method has traditionally been applied to flat systems where the surface is readily accessible, such as soil profiles and pavement systems. However, the research presented herein shows the applicability of the SASW method to characterize materials around cased and uncased boreholes in soil and rock by propagating surface waves along the borehole wall. In cased boreholes, surface wave measurements can be used to determine G [subscript max] of the casing. In many instances, surface wave measurements can be used to determine G [subscript max] of the surrounding material and the quality of the bond between the material and the casing. In these instances, success is limited by the thickness of the casing and the stiffness contrast between the casing and the surrounding material. In uncased boreholes, surface wave measurements were used to identify and delineate zones of disturbance and cracking around cylindrical drilled shafts in rock. In addition, surface wave measurements were used to delineate the extent of affected soil in a large-diameter (42-in. (1.1-m)) borehole treated with a lime slurry. In both cased and uncased boreholes, it was found experimentally that the cylindrical geometry of the borehole significantly affected the dispersive characteristics of the surface wave energy. Furthermore, this geometry-induced dispersion was completely different when propagating in the axial and circumferential directions. Appropriate numerical formulations developed on companion projects were incorporated into the results presented herein to accurately model the experimental surface wave data. The focus of this research was the development of a borehole SASW tool to determine the in situ relationship between G [subscript max] and state of stress in uncased pressurized boreholes in soil. In situ relationships were successfully determined using the borehole SASW tool in unsaturated cohesive and cohesionless soils. These relationships compared favorably to relationships determined from laboratory resonant column testing. At this time, minimization of soil disturbance around the borehole is the single most important issue in advancing this technology, although many other opportunities exist for future research