Improved Liquefaction Hazard Mapping Procedures for Urban Areas

Brigham Young University, Provo UT

Investigators

Abstract

Liquefaction has generated widespread damage and disruption to urban areas during many large US and Japanese earthquakes. This destruction has created public demand for state and local governments, public utilities, and other responsible agencies and corporations to mitigate liquefaction hazard in their respective jurisdictions. An initial step in hazard mitigation is definition of vulnerable areas and potential deleterious effects through liquefaction hazard maps. Most US liquefaction hazard maps are based on geologic units with generalized criteria leading to qualitative hazard ratings of high, moderate, low, etc. Users, including city planners, building officials, engineers, utility companies, and loss estimators, need maps with more detail and accuracy than those based on generalized geologic criteria. The purpose of this study is to develop improved mapping techniques for delineation of liquefaction hazard to meet this need. The compilation of such maps will require evaluations based on detailed subsurface stratigraphy and soil properties. To develop and test improved techniques, a test area is needed where an extensive array of borehole or CPT logs and accompanying soil property data has been compiled and where liquefaction and consequent deleterious effects from a past earthquake have been documented. A unique opportunity to develop and test mapping techniques has occurred with the compilation of a large set of geotechnical data for the Kobe, Japan metropolitan area by Prof. Yasuo Tanaka and his colleagues at Kobe, Japan, University. This data set consists of over 5,000 borehole logs scattered over Kobe and two adjacent communities, with concentrations of logs in areas heavily damaged by the 1995 Hyogoken-Nanbu (Kobe) earthquake. Similarly, an extensive compilation of observed liquefaction effects, including lateral spread displacements, has been assembled by Japanese investigators, including Prof. Masanori Hamada and his colleagues at Waseda University, Tokyo. The objective of this project is to work jointly with Prof. Tanaka and his colleagues to develop improved susceptibility and ground failure hazard mapping procedures. These techniques will be developed by analyzing subsurface sediment stratigraphy, soil properties, local topography, and past liquefaction effects in the Kobe area. From these analyses, several possible procedures will be developed and evaluated for mapping liquefiable layers and consequent hazards. The most viable technique or techniques will then be used to prepare demonstration liquefaction hazard maps for selected parts of the Kobe metropolitan area and an urban area in the US. A likely urban area for the US demonstration map is Oakland, California, urban area where Dr. Thomas L. Holzer, USGS, is in the process of installing and collecting data from a large array of CPT soundings and SPT boreholes. The work of Dr. Holzer specifically directed toward evaluation of liquefaction hazard.