Evaluation of Ground Failure Potential Due to Soil–Structure Interaction and Vertically Propagating Shear Waves

Evaluation of ground failure potential in geotechnical practice is typically based on demand parameters that solely consider vertically propagating shear waves in the free-field. Soil–structure interaction (SSI) modifies demands beneath foundations, and observations from recent earthquakes, physical modeling studies, and numerical modeling simulations indicate that SSI contributes significantly to ground failure. We present a methodology that utilizes elastic solutions to define SSI-induced stresses imposed on the soil beneath shallow foundations during earthquake shaking. Input parameters include a free-field ground surface motion as well as static and dynamic base shear, moment, and axial stresses imposed on the soil by a shallow foundation. The resulting stresses in the soil are analyzed in terms of the deviatoric stress invariant and the mean effective stress, which represents the states of stress leading to shear failure more accurately than the traditional use of stresses on horizontal planes. The invariant-based cyclic stress ratio (${\mathit{CSR}}_q$) is introduced to quantify demands, which is equivalent to the conventional CSR in the free-field. The ratio of the corresponding cyclic resistance parameter, ${\mathit{CRR}}_q$, to ${\mathit{CSR}}_q$ is the factor of safety against ground failure at a point. Application of this methodology to results of centrifuge modeling of shallow foundations resting on low-plasticity fine-grained soils shows that the factor of safety computed from the proposed methodology at a location in the soil below the edge of the foundation correlates strongly to measured permanent settlements and rotations, whereas the free-field factor of safety underpredicts ground failure potential.