MAPPING LIQUEFACTION POTENTIAL IN THE ST. LOUIS METROPOLITAN AREA USING A GIS-GEODATABASE

This study sought to map liquefaction potential over a 2500 sq km metropolitan area in St. Louis and St. Charles Counties using subsurface information contained in over 500 exploratory boreholes. Liquefaction is a failure mechanism which occurs when the pore water pressure equals or exceeds the effective stress acting on a low cohesion materials, such as sand, silt, or granule gravel. It is commonly triggered by rapid straining, caused by up-flowing seepage, sudden movements, or seismic shaking. For this regional study we performed two screening analyses for liquefaction: 1) estimation of the critical Peak Ground Acceleration (PGA) required to initiate liquefaction; 2) calculation the Liquefaction Potential Index (LPI). Synthetic time histories were generated using Boore's SMSIM code and Atkinson and Boore's attenuation relationship. The model quake was Magnitude 7.5 quake at distances between 200 and 300 km. Peak ground accelerations (PGA) were estimated for each stratigraphic horizon in each borehole using the 1-D program DEEPSOIL. Borehole geotechnical data and the appropriate PGA values were applied to calculate the factor of safety (FS) for liquefaction of each stratigraphic layer using the simplified SPT procedure of Seed and Idriss. LPI was calculated by integrating the FS for each soil layer within 20 m of ground surface. LPI results are typically classified with five levels of severity: 1) Non-liquefied, 2) Low, 3) Moderate, 4) High, and 5) Very High. Preliminary results indicate that the critical threshold acceleration to trigger liquefaction in the study area mainly ranged from 0.15g~0.25g. Using GIS, the results of earthquake-induced liquefaction potential will be plotted as an information layer, which will integrate the severity of the liquefaction hazard with depth in a one-dimensional spatial information layer, overlying the ground surface. These hazard maps should provide a more qualitative assessment of the relative liquefaction susceptibility suitable for disaster planning and management, in lieu of existing hazard maps that tend to paint Quaternary alluvium as having “liquefaction hazard,” without any further division of relative risk.