UC San Diego

This study aims to advance the use of a lightweight recycled backfill formed from shredded waste tires with large particle sizes, referred to as Type B tire derived aggregate (TDA), for geotechnical seismic isolation (GSI). A shake table was used to perform dynamic testing of single-degree-of-freedom (SDOF) superstructures on a shallow footing embedded in layers of Type B TDA with different thicknesses across a broad set of base excitations. Results indicate nonlinearity in the moment-rocking response and the hysteretic damping of TDA confirmed its effectiveness as a low-modulus seismic isolation layer. TDA exhibited excellent re-centering behavior, outperforming soil-based seismic protection systems for the conditions studied. While TDA thickness had little direct effect on the moment-rocking response, specimen mass strongly influenced stability and hysteresis. Residual settlements remained small and independent of layer thickness, with larger settlements occurring only when the predominant period of input motions matched the rocking system's natural period. While more energy dissipation occurred through rocking compared to shearing, greater shear displacements were observed for more heavily loaded footings on thicker TDA layers. Residual shear displacements were negligible for all cases. System identification analyses revealed that TDA led to a period lengthening of 2.15 compared to a fixed-base condition. Although TDA amplifies ground motion as expected, the layer thickness plays complex effects due to multiple modes and the TDA first-mode shearing mechanism remained approximately constant even when the specimen was subjected to high-amplitude ground motions.