Suitability of using Iwasaki's PL in characterization of liquefaction damages during the 1999 Chi-Chi Earthquake, Taiwan

摘要

One of currently adopted methods in the Taiwanese Building Codes (ARBI 1998) for liquefaction damage assessment is based on Iwasaki's liquefaction potential index, P_L, which was also suggested by ISSMFE TC4 in 1993. The index was defined and calibrated by Iwasaki et al. (1982) using liquefaction and non-liquefaction incidents that had occurred in Japan. On September 21, 1999, a severe earthquake struck the center of Taiwan and resulted in significant liquefaction damages in sandy deposits of the Tzou-swei River, the largest alluvial fan to the mid-west of the island. An assessment of the liquefaction damages was conducted in this study based on the potential index approach and the results showed the computed damages appear to not completely agree with the observed situations. Original derivations of the liquefaction potential index by Iwasaki et al. were therefore reviewed. The results showed the threshold values of liquefaction potential index of 5 (insignificant potential) and 15 (significant potential), as proposed by Iwasaki et al. (1982), appeared to be sensitive to the database for analysis, and were too high (or unconservative) for use in Taiwan. Possible reasons might attribute to selections of the companion method for analysis of factor of safety against liquefaction, the depth-weighing function, as well as the sample space of liquefaction incidents. Based on liquefaction incidents of the 1999 earthquake, the aforementioned threshold values are suggested to be revised in order to be more consistent with the observations in Taiwan.