Observations on the Seismic Loading of Rigid Inclusions based on 3D Numerical Simulations

Rigid inclusions are increasingly being specified in seismic regions to transmit foundation loads to competent strata at depth due to their ability to provide excellent static performance and potential for cost-savings over other forms of ground improvement and/or deep foundations. Despite their widespread application in seismic regions, the seismic kinematic interaction of rigid inclusions is not well understood. This study presents a series of parametric numerical simulations specifically conducted to capture the kinematic soil-rigid inclusion interaction under seismic loading to identify key mechanisms that contribute to seismic performance. The effect of ground motion variability, area replacement ratio, surface crust thickness, embedment into a competent layer, liquefiable layer thickness, and steel reinforcement is systematically investigated. Although the rigid inclusion may not prevent liquefaction triggering, severe amplification associated with dilation spiking of transiently liquefied soil is mitigated and the onset of liquefaction delayed due to the soil-rigid inclusion interaction. The role of static arching to alter the geostatic stress state prior to and during shaking is shown to be responsible for significant and heretofore unknown coupled fluid-mechanical interaction that drives the performance of the rigid inclusion within the reinforced soil system. The kinematic flexural demand following the triggering of liquefaction and phase transformation associated with cyclic mobility is met through transient increases in axial load, which increases the confinement of the grout comprising the rigid inclusion, and therefore its moment capacity. The complex mechanisms identified in this work should be used to help identify which subsurface conditions may lead to responses, guide design decisions regarding selection of reinforcement and embedment, and provide a basis for assessments of the anticipated kinematic demands in view of the beneficial axial load-moment capacity interaction following soil liquefaction.