Impacts of Fixed-End and Flexible Boundary Conditions on Seismic Response of Shallow Foundations on Saturated Sand in 1-g Shaking Table Tests

A key consideration in physical modeling of seismic problems in geotechnical engineering is the impact of the model container boundaries on the soil layer response. The container boundaries may alter the stress-strain behavior from free-field conditions through the possible reflection of incident shear waves and generation of P-waves within the soil layers. In this study, 1-g shaking table experiments were performed to evaluate the impacts of container boundary conditions on the response of saturated loose sand layers subjected to harmonic base motions (1) in a free-field condition, (2) with a shallow foundation, and (3) with a shallow foundation supporting a single degree of freedom superstructure. The sand layers were formed in a newly fabricated laminar shear container that can be converted to a rigid box by adding elements to the end walls. Acceleration, excess pore water pressure, and settlement measurements demonstrate that the rigidity of the container boundaries can have a major impact on seismic behavior of the models. In particular, the observed permanent settlement of the foundations increased by 58–115 % in the soil models with fixed-end (or rigid) boundaries compared with those in soil models with flexible conditions. This was attributed to nonuniformity of strains near the fixed-end container boundaries and a higher level of energy trapped inside the model. Furthermore, higher spectral accelerations were captured in tests with fixed-end boundary conditions compared with those with flexible boundary conditions. Scaling issues associated with 1-g shaking table testing were also discussed.