An Empirical Geotechnical Seismic Site Response Procedure

TLDR

This geotechnical site classification scheme offers an alternative to geologic‑based and shear‑wave‑velocity‑based classification methods. The authors introduce a simplified empirically based seismic site‑response evaluation procedure that uses dynamic stiffness of surficial materials and bedrock depth as primary parameters. The procedure is applied to ground‑motion data from the 1989 Loma Prieta and 1994 Northridge earthquakes. The scheme yields period‑ and intensity‑dependent spectral‑acceleration amplification factors, reduces standard error versus a simple rock‑vs‑soil classification, distinguishes competent rock from weathered soft rock/shallow stiff soil, highlights soil depth as a key parameter, and achieves error levels comparable to code‑based shear‑wave‑velocity classifications.

Abstract

A simplified empirically based seismic site response evaluation procedure that includes measures of the dynamic stiffness of the surficial materials and the depth to bedrock as primary parameters is introduced. This geotechnical site classification scheme provides an alternative to geologic-based and shear wave velocity-based site classification schemes. The proposed scheme is used to analyze the ground motion data from the 1989 Loma Prieta and 1994 Northridge earthquakes. Period-dependent and intensity-dependent spectral acceleration amplification factors for different site conditions are presented. The proposed scheme results in a significant reduction in standard error when compared with a simpler “rock vs. soil” classification system. Moreover, results show that sites previously grouped as “rock” should be subdivided as competent rock sites and weathered soft rock/shallow stiff soil sites to reduce uncertainty in defining site-dependent ground motions. Results also show that soil depth is an important parameter in estimating seismic site response. The standard errors resulting from the proposed site classification system are comparable with those obtained using the more elaborate code-based average shear-wave velocity classification system.