GUIDE FOR ESTIMATING THE DYNAMIC PROPERTIES OF SOUTH CAROLINA SOILS FOR GROUND RESPONSE ANALYSIS

South Carolina is the second most seismically active region in the eastern U.S. The 1886 Charleston earthquake caused about 60 deaths and an estimated $23 million (1886 dollars) in damage. An important step in the engineering design of new and the retrofit of existing structures in earthquake-prone regions is the prediction of strong ground motions. Required inputs for ground response analysis include the small-strain shear wave velocity, the variation of normalized shear modulus with shear strain, and the variation of material damping ratio with shear strain for each soil layer beneath the site in question. Collectively, these inputs are known as the dynamic soil properties. This report presents guidelines for estimating the dynamic properties of South Carolina soils for ground response analysis. Regression equations for estimating small-strain shear-wave velocity from cone penetration test (CPT) and standard penetration test (SPT) data are presented in this guide. The regression equations are based on findings in previous studies and 123 penetration-velocity data pairs from South Carolina. Variables in the CPT-velocity equations are: cone tip resistance, soil behavior type index, depth, and geology. In the SPT-velocity equations, variables are: corrected blow count, fines content, depth, and geology. Shear-wave velocity measurements in Pleistocene soils are 20% to 30% greater than velocity measurements in Holocene soils with the same penetration resistance. In Tertiary soils, shear-wave velocity measurements are 40% to 130% greater than velocity measurements in Holocene soils with the same penetration resistance, and appear to depend on the amount of carbonate in the soils. Predictive equations for estimating normalized shear modulus and material damping ratio are also presented. They are based on a modified hyperbolic model and test results from Resonant column and Torsional Shear tests on 122 samples. Input variables in the predictive equation for normalized shear modulus are: strain amplitude, confining stress, plasticity index (PI), and geology. In general, the recommended normalized shear modulus curve for Holocene soils with PI=0 follows the Seed et al. upper range curve for sand, the Idriss curve for sand, and the Stokoe et al. curve for sand. On the other hand, the recommended normalized shear modulus curves for the older soils with PI=0 generally follow the Seed et al. mean or lower range curves for sand and the Vucetic and Dobry curve for PI=0 soil. The material damping ratio curves are expressed in terms of normalized shear modulus and minimum material damping ratio. Relationships between minimum damping and PI are developed based only on Torsional Shear test data. In general, the recommended damping curve for Holocene soils with PI=0 follows the Seed et al. lower range curve for sand and the Idriss curve for sand and clay. The recommended damping curves for the older soils with PI=0 generally follow the Seed et al. mean curve for sand and the Vucetic and Dobry curve for PI=0 soils.