Case studies of soil liquefaction of sands and cyclic softening of clays induced by the 1999 Taiwan Chi-Chi earthquake.

摘要

This study is concerned with occurrences of ground failure and non-ground failure in alluvial soils located in near-fault areas strongly shaken by the Mw=7.6 1999 Chi-Chi, Taiwan earthquake. Sites identified during reconnaissance work as being of significant geotechnical interest from a ground failure perspective were subsequently investigated to evaluate site conditions. Detailed data analysis was performed for two specific problems. One was lateral spreading of liquefiable soils in near fault soils. The other is foundation failures and nonfailure of buildings supported on clayey soils.; Liquefaction-induced lateral ground deformation are documented and analyzed for at five sites located in the near-fault region of the Chi-Chi earthquake. Each of the lateral spreads involved cyclic mobility of young alluvial soils towards a free face at creek channels. The field displacements are back-analyzed using recent SPT-based empirical and CPT-based semi-empirical models, and it is found that the models generally overestimate the observed ground displacements. Possible causes of the models' overprediction bias include partial drainage of the liquefied soils during shaking, low but measurable plasticity of some of the soils' fines fraction, and the absence of non spread sites in the empirical database.; The second problem analyzed as part of this research consists of case histories of foundation failure for mid-rise (4- to 6-story) reinforced concrete buildings along with non-failures for single-story buildings and free-field ground sites over an area approximately 350 m in length. Detailed site characterization work across the area was performed including borings with standard penetration testing and in situ vane shear testing, CPT soundings, and test pits with in situ vane shear testing. Soil tests performed included index testing, consolidation testing, and consolidated-undrained monotonic and cyclic triaxial tests. The site was found to be underlain by low- to medium-plasticity clays roughly 8 to 12 m in thickness overlying silty sand.; Analyses were performed of the potential for cyclic softening of the clays.... The resistance of the clay to cyclic softening is evaluated using the results of material-specific strength testing (both monotonic and cyclic). The seismic demand applied to the soil is evaluated through an extensive sequence of ground response analyses as well as soil-structure interaction analyses. Results of the analysis indicate low factors of safety in foundation soils below the six-story building during earthquake shaking, which contributes to bearing capacity failures at the edges of the foundation due to rocking effects. Similar analyses indicate high factors of safety (above unity) in foundation soils below one-story buildings as well in the free field. Accordingly, analysis of the site within a framework that systematically accounts for the clayey nature of the foundation soils successfully predicted the field performance.