Pore-pressure-dependent performance of rocking foundations

Abstract

This study explores the concept of rocking foundations to mitigate damage during seismic events. By weakening the footing intentionally, the foundation acts as a “fuse” to prevent plastic hinges from forming in columns. Shake table experiments were conducted on a lightweight prototype deck mass-column-footing model founded on a fine, medium-dense sand, in two states of nearly dry and saturated. Kinetic energy dissipation, hysteresis, and decay are examined for various structure masses, for two nominal low and high motion frequencies. Findings suggest that energy dissipation is higher in saturated sands ― as compared to nearly dry and despite the absence of liquefaction ― due to fluctuating pore water pressure and a suction effect that evolves beneath foundations’ edge. Where the substrate retains its original porosity, flow of water and subsequent damping becomes pivotal mechanisms to enable the rocking motion. In dry sands, energy dissipation occurs mainly through rotation, and enhances with motion frequency (from testing 3–5 Hz). In saturated sands, energy dissipation occurs predominantly through plastic settlement, and becomes less effective with motion frequency. Lighter structures experience greater rotational movement, especially in the case of dry sands. That enhanced rotational movement is drove by lower rotational stiffness. Overall, lighter structures facilitate the re-centring of the foundation upon rocking motion.