Cyclic Softening of Low-plasticity Clay and its Effect on Seismic Foundation Performance

Significant occurrences of ground failure in the form of liquefaction, ground softening, and lateral spreading occurred in Wufeng, Taiwan during the 1999 Chi-Chi earthquake (Mw = 7.6). The Wufeng region is located adjacent to the ruptured fault on the foot wall, and experienced peak accelerations ∼0.7 g. We describe the results of field investigations and analyses of a small region within Wufeng that exhibited a range of ground performance. The investigated region consists of an E-W trending line of 350 m length. The east end of the line is a residential area with single-story structures for which there was no surface evidence of ground failure. The west end of the line had 3-6 story reinforced concrete structures that were heavily damaged structurally, but which also underwent extensive foundation failures including differential settlement, foundation punching failures, and full foundation bearing failures. No ground failure was observed in the free-field at the west end of the line. Surficial soils consist of low-plasticity silty clays that extended to 8-12 m depth in the damaged area (west side), and 3-5 m depth in the undamaged area (east side). Analyses were performed of the potential for cyclic softening of the clays, which effectively investigates the potential for large cyclic shear strains to develop. The resistance of the clay to cyclic softening is evaluated using the results o