LOTUNG DOWNHOLE ARRAY .2. EVALUATION OF SOIL NONLINEAR PROPERTIES

The characteristics of soil response during earthquake excitations, at a site in Lotung, Taiwan are identified using the Lotung large scale seismic test (LSST) data. A technique is developed to evaluate soil shear stress-strain histories directly from the free-field downhole accelerations. These histories are used to estimate variation of soil shear moduli and material damping characteristics with shear strain amplitude, and to assess the effects of pore pressure buildup. Soil stiffness properties are found to compare satisfactorily with those obtained through laboratory tests conducted by the University of California, Davis; the National Taiwan University; and the University of Texas at Austin. Pore pressure buildup appears to be accompanied by a reduction in soil stiffness. The information obtained in this study demonstrates that downhole accelerometer and pore pressure arrays offer a direct effective means of evaluating seismic soil properties.