Soil-structure-interaction (SSI) plays an important role in seismically-induced ground failure; however, the contribution is poorly understood and rarely considered in geotechnical engineering practice. Semi-empirical methodologies used in practice to evaluate ground failure solely consider demand from vertically propagating shear waves in the free field. Mounting evidence suggests this approach may significantly underpredict induced demand in soils below shallow foundations by neglecting SSI-induced stress contributions. Observations made during post-earthquake reconnaissance suggest that soils below low- to mid-rise structures supported on shallow foundations may be more susceptible to ground failure than soils in the corresponding free field. Recent physical and numerical modeling studies substantiate this evidence by demonstrating that SSI-induced stresses contribute to ground failure potential and may exacerbate the consequences of ground failure. Studies to date have centered on liquefaction-based (e.g. sand-like) ground failure; complementary studies on cyclic softening (e.g. clay-like) ground failure are not available.

This research seeks to fill that knowledge gap by evaluating the SSI-induced seismic loading increment imparted by shallow foundations on fine-grained soils within the context of cyclic softening ground failure. The research is based on an analysis framework that utilizes elastic solutions to define SSI-induced stress demands beneath shallow foundations. The framework proposes a new demand parameter called the deviatoric strength ratio to quantify incremental stress demands under complex stress paths below footings. To substantiate the framework and provide empirical case history data on cyclic softening, the author performed a geotechnical centrifuge testing program at the Center for Geotechnical Modeling at the University of California Davis. The program consisted of two dynamic centrifuge tests incorporating structures founded on strip footings and bearing on fine grained soil. The structures incorporated a range of mass and stiffness properties at approximately the same bearing pressure to evaluate the influence of structure response on cyclic softening. Analysis results indicate a strong relationship between permanent foundation settlement or rotation and indices that represent spatially averaged cyclic strains in foundation soils when subjected to deviatoric stress demands from ground response and SSI.