THE LIQUEFACTION POTENTIAL INDEX

The liquefaction potential index (LPI) was used to prepare quantitative maps of liquefaction hazard in the greater Oakland, California, area. LPI, which was developed in Japan to assess liquefaction severity at a specific location, combines depth, cumulative thickness of liquefiable intervals, and factor of safety of liquefiable intervals into a single parameter. Values range from 0 to 100. Although the “simplified procedure” is used to compute the factor of safety in LPI, the advantage of LPI over the “simplified procedure” is that it predicts the performance of the whole soil column as opposed to a single soil element. We calibrated LPI by computing values from 243 cone penetration test (CPT) soundings at soil sites in California that had been strongly shaken historically and correlating these values with locally observed surface manifestations of liquefaction. The correlations indicate that the severity of liquefaction increases with increasing LPI. In general, sand boils appear at LPI=5; lateral spreads occur at LPI=12. Liquefaction hazard maps for the greater Oakland area were prepared by conducting 210 CPT soundings in the study area and computing values of LPI for a magnitude 7.1 earthquake on the nearby Hayward Fault. Cumulative distributions of LPI were then compiled for each geologic unit. The cumulative percentage of each unit with LPI>5 indicates the proportion of the area underlain by the unit that will exhibit surface manifestations of liquefaction. For example, we estimate that the M7.1 earthquake on the Hayward Fault will cause 73% of the area underlain by artificial fill to have surface manifestations of liquefaction.