The seismic analysis of sandy sites

Abstract

This paper presents an approach for performing one dimensional effective stress site response analyses for sandy sites, including the evaluation of liquefaction potential. This type of analysis differs from the more common total stress response analyses in that induced pore pressures in saturated sandy soils are accounted for, including the resulting influence on soil properties. This analytical method has been refined to the point where the need for complex and expensive laboratory soil testing is no longer required, a factor which has traditionally held back developments in the effective stress area. The effective stress analysis requires the determination of five soil specific parameters. A trial and error backfitting procedure was developed to successfully determine these parameters from traditional site investigation data rather than detailed laboratory testing. This procedure was investigated using two case studies, the Edgecumbe earthquake of 1987 and the Loma Prieta event of 1989, which both exhibited significant liquefaction damage. The Edgecumbe analysis produced useful results. The predicted ground acceleration required to initiate liquefaction was 2.8 m/s2 (0.29g) which is close to the estimated value of 3 m/s2 (0.31g). This was a good result as a reasonable amount of estimated and correlated data had to be used due to a lack of specific site data. The case study of Treasure Island, in the San Francisco Bay area, also produced encouraging results with both the prediction of liquefaction and surface response spectra in good agreement with recorded data. Both case studies used liquefaction resistance curves determined empirically from SPT blow count data. While this data proved acceptable it was discovered that care must be taken in the use of such overseas derived empirical data, particularly if no corroborating site specific information is available.