An Improved Description of the Seismic Response of Sites with High Plasticity Soils, Organic Clays, and Deep Soft Soil Deposits

Near surface soils can greatly influence the amplitude, duration, and frequency content of ground motions. The amount of their influence depends on many factors, such as the geometry and engineering properties of the soils and underlying bedrock, as well as the earthquake source mechanism and travel path. Building codes such as the 2012 International Building Code (IBC) define six site categories for seismic design of structures, which are based on the sites defined by the National Earthquake Hazards Reduction Program (NEHRP). Site categories A, B, C, D, and E are defined by the time averaged shear wave velocity over the top 30 meters of the soil deposit. Site category F is defined as sites that include liquefiable or sensitive soils, as well as sites with more than 3 meters (10 ft) of peat or highly organic clays, more than 7.5 meters (25 ft) of soil with PI > 75, and more than 37 meters (120 ft) of soft to medium stiff clays. The IBC specifies simplified procedures to calculate design spectra for NEHRP sites A through E, and requires a site specific investigation for NEHRP F sites. However, established procedures for performing the required site specific investigations for NEHRP F sites are limited.;The objective of this research is to develop a simplified procedure to estimate design spectra for non-liquefiable NEHRP F sites, specifically sites with organic soils, highly plastic soils, and deep soft soil deposits. The results from this research will directly affect US practice by developing much needed guidelines in this area.;There is little empirical data on the seismic response of non-liquefiable NEHRP F sites. As a result, this study focused on generating data from site response analyses. To capture the variability of ground motions, this study selected five base case scenarios according to tectonic environments and representative cases encountered in common US practice. Suites of ground motions for each scenario were created by collecting ground motions from online databases. Some of the ground motions were scaled and others were spectrally matched to their respective target response spectra. Fifteen different NEHRP E and F sites were created for the site response analyses. Seven of the sites are based on actual sites from the San Francisco Bay Area, New York City, Ottawa, Canada, Guayaquil, Ecuador, and Hokkaido, Japan. The other eight sites are variations of the seven base case sites. This study conducted a total of 14,541 site response analyses using a well documented site response analysis program.;This study then developed a simplified model to estimate response spectra for non-liquefiable NEHRP F sites. The simplified model was developed in two stages. In the first stage, the results for each site were regressed separately against the ground motion intensity to estimate the effect of the ground motion scenario. In the second stage, the site specific coefficients calculated from the first stage were regressed against site properties to determine their site dependence. These two parts were then combined to form the final model. The simplified model was validated against a separate database than the one used to develop it. This validation database consisted of 24 effective stress nonlinear site response analyses for three sites and eight ground motion scenarios.;The simplified model developed in this study does not replace a site response analysis, but rather augments it. It is hoped that the results of this dissertation will help practicing engineers gain a better understanding of their site before conducting site response analyses.