Seismic performance of mat-founded building clusters on liquefiable soils treated with ground densification

Abstract

Current guidelines for evaluating the performance of ground densification as a liquefaction countermeasure near buildings are based on free-field conditions or, at best, consider one structure experiencing soil-structure interaction (SSI) in isolation. However, in urban areas, where structures are constructed in close vicinity of each other, structure-soil-structure interaction in liquefiable deposits near two (SSSI2) or multiple (≥3) buildings in a cluster (SSSI3+) has been shown as consequential on key engineering demand parameters (EDPs), particularly differential settlement. Furthermore, the potential tradeoffs associated with ground improvement in urban settings, considering SSSI2 and SSSI3+, are currently not well understood or defined. In this paper, three-dimensional (3D), fully-coupled, nonlinear, dynamic finite element analyses are first validated with centrifuge models of SSI and SSSI2, including ground densification. These models are subsequently used to explore the influence of building arrangement (two adjacent structures and four structures in a square block) and spacing on key EDPs for mitigated structures undergoing SSSI2 and SSSI3+ compared to that under isolated SSI. For the conditions evaluated, it is shown that both SSSI2 and SSSI3+ could reduce the average settlement of mitigated structures compared to SSI at building spacings (S) > 0.5Wfnd (where Wfnd is the foundation width), particularly in larger clusters experiencing SSSI3+. On the other hand, both SSSI2 and SSSI3+ amplified the permanent tilt of the mitigated structures compared to SSI at S < 0.5Wfnd. The impact of these interactions on tilt reduced at larger spacings. A limited, subsequent numerical sensitivity study showed that pulse-like input motions together with the stress and flow-path bias introduced by SSSI2 and SSSI3+ can increase the uneven accumulation of soil strains below the mitigated structures compared to cases experiencing SSI or the same building clusters subject to non-pulse-like motions. This led to a greater amplification in tilt of mitigated structures experiencing SSSI2 and SSSI3+ at shorter spacings under the selected pulse-like motions. Overall, the results point to the importance of considering the impact of building cluster arrangement, spacing, soil and structural properties, and ground motion characteristics in the design of ground improvement in urban settings.