Seismic Design Site Classification Using Spectral Analysis of Surface Waves (SASW) and Resistivity Imaging

The past decade has seen increased interest in standardizing seismic design criteria for structures. In many states the IBC 2000 Building Code is replacing previous codes such as the UBC, SBC, and BOCA. The IBC 2000 code requires a site-specific seismic classification based on the stiffness of the soil or rock in the upper 100 feet of the subsurface. The site classification will drive the structural design and thus the construction costs. For large commercial structures, the difference in construction costs between one category and the next can be many hundreds of thousands of dollars. Traditionally, methods for determining soil stiffness have relied on pressure methods such as undrained shear strength, standard penetration resistance, and dilatometer tests. Research has shown that the apparent stiffness of the soil tends to be largely under-predicted by pressure methods because the apparent stiffness is related to the strain of the test. Pressure methods tend to induce large strains, resulting in under-prediction of stiffness with a corresponding lower site class. At small strains, such as those induced by seismic methods, the apparent stiffness is at its maximum. By using the spectral analysis of surface waves (SASW) method, more realistic stiffness estimations are obtained. Incorporating SASW measurements with two-dimensional resistivity imaging provides two independent non-invasive methods to verify the subsurface distribution of soil and bedrock materials. The authors have correlated SASW measurements with resistivity imaging data at several sites in which a site class had already been determined with standard penetration tests. In each case a higher site class was obtained with the SASW-resistivity investigation than with the SPT. The resulting savings to the client in construction costs have ranged from $80,000 to $650,000.