Characterization of shear wave velocity profiles for South Carolina Coastal Plain

Abstract

The Atlantic Coastal Plain is a geological formation along the east coast of the United States that consists of unconsolidated sediments as thick as 1000 m underlain by very hard rock with shear wave velocity, V s of over 2600 m/s. In South Carolina, this very hard rock layer is located close to or at the ground surface in the central part of the state, and increases in depth towards the coast, and from north to south. The deep sediments are mostly Cretaceous and younger in age and consist of unlithified sediments with weakly lithified units. The deep sediments are unique geological and geotechnical conditions that pose significant challenges to seismic hazard analyses. Having an accurate V s profile for the deep sediment is critical for predicting the level of ground shaking at a particular site. Geotechnical borings were drilled at two locations to depths of approximately 150 m and 190 m for borehole geophysical testing, undisturbed sampling, and soil/rock coring. Geophysical testing was conducted independently by groups of experts at both locations and included P-S suspension logging, a combined multi-channel and spectral analysis of surface waves, and a combined multi-channel analysis of surface waves and microtremor array measurement methods. This paper presents a comparison between field testing results from the different methods and visual classification of soil and rock samples to evaluate uncertainties of deep soil profiles obtained by different methods. In addition, soil samples collected at both sites were tested using the resonant column method to evaluate dynamic properties. The shear wave velocities measured in the lab for soil and rock samples were compared with results from the field measurements. Cementation was found to be one of the important factors affecting the shear wave velocity measurements.