How do granular columns affect the seismic performance of non-uniform liquefiable sites and their overlying structures?

tThis paper presents results of a series of three dynamic centrifuge experiments to evaluate how granular columns affect the seismic performance of level and gently sloped sites that include a non-uniform liquefiable deposit and an overlying, 3-story structure. A slight variation in the thickness of the liquefiable layer is shown to produce unacceptable permanent lateral ground deformations, even in the absence of a surface slope or a structure. The presence of a gentle slope further amplified lateral displacements but did not influence settlements notably. Granular columns (with an area replacement ratio of 20%) were successful in reducing the extent and duration of large excess pore pressures, void redistribution, and shear strain localization in such layered deposits. Hence, they generally reduced settlements, rotations, and lateral displacements both near and away from structures, but not necessarily to acceptable design limits. Granular columns could increase the acceleration demand transferred to the foundation and superstructure by increasing soil stiffness and reducing damping. The impact of this mitigation on roof accelerations and beam/column bending strains was shown to depend on the intensity and frequency content of the input motion in relation to the modal frequencies of the structure. The presented experimental results highlight the importance of considering even slight variations in liquefiable layer thickness as well as soil-foundation-structure interaction when designing liquefaction mitigation strategies in the context of system performance.