Cyclic Softening of Low-Plasticity Clay and Its Effect on Seismic Foundation Performance

During the 1999 Chi-Chi Earthquake $(M_w=7.6)$ , significant incidents of ground failure occurred in Wufeng, Taiwan, which experienced peak accelerations $\sim 0.7\mathrm{g}$ . This paper describes the results of field investigations and analyses of a small region within Wufeng along an E–W trending line $350\mathrm{m}$ long. The east end of the line has single-story structures for which there was no evidence of ground failure. The west end of the line had three to six-story reinforced concrete structures that underwent differential settlement and foundation bearing failures. No ground failure was observed in the free field. Surficial soils consist of low-plasticity silty clays that extend to $8–12\mathrm{m}$ depth in the damaged area (west side), and $3–10\mathrm{m}$ depth in the undamaged area (east side). A significant fraction of the foundation soils at the site are liquefaction susceptible based on several recently proposed criteria, but the site performance cannot be explained by analysis in existing liquefaction frameworks. Accordingly, an alternative approach is used that accounts for the clayey nature of the foundation soils. Field and laboratory tests are used to evaluate the monotonic and cyclic shear resistance of the soil, which is compared to the cyclic demand placed on the soil by ground response and soil–structure interaction. Results of the analysis indicate a potential for cyclic softening and associated strength loss in foundation soils below the six-story buildings, which contributes to bearing capacity failures at the edges of the foundation. Similar analyses indicate high factors of safety in foundation soils below one-story buildings as well in the free field, which is consistent with the observed field performance.