Effect of bedrock stiffness and thickness of weathered rock on response spectrum in Korea

Abstract

The Standard Penetration Test (SPT) is increasingly used to generate shear wave velocity (VS) profile of a site for seismic hazard assessment. However, it is difficult to perform site investigation using the SPT for weathered rock and bedrock even at shallow depths in most cases. Consequently, the stiffness of bedrock (VSb) is assumed to be 760 m/s, and only partial thickness of weathered rock is used for site response assessment in Korea. The increase in VSb and thickness of weathered rock (Hw) may affect the overall seismic response of a site, even though the site response is mostly governed by the upper soil layers. In this study, the effect of VSb and Hw on response spectrum in Korea was investigated by performing equivalent-linear site response analyses on three representative sites each of SC and SD soil categories as defined in the Korean seismic code based on the average shear wave velocity of top 30 m of a site (VS30). The SC and SD categories are defined similar to that in the International Building Code (IBC). Further, the effect of increase in VSb and Hw on the selection of design response spectrum based on seismic codes was also investigated. An increase in VSb resulted in an increase in spectral amplification, while the effect of increase in Hw was not substantial for the representative sites considered in this study. This means that the VS of bedrock needs to be assessed properly. On the other hand, it is possible to get a reliable seismic response even if the Hw cannot be determined accurately. The incorporation of VSb in VS30 for shallow bedrock sites (bedrock depth < 30 m) fails to consider the effect of increase in seismic amplification at large VSb. Hence, it is preferable to consider the effect of VSb separately on the design response spectrum. In addition, the effect on soil non-linearity due to variation in the VSb and Hw was found to be insignificant.