Impact of Ground Densification on the Response of Urban Liquefiable Sites and Structures

Abstract

Ground densification is a common countermeasure against soil liquefaction. The state-of-practice for designing ground densification is typically based on empirical estimates of liquefaction triggering and ground settlement in the free field, ignoring seismic soil-structure interaction (SSI) near building structures. The existing guidelines for the geometry of ground densification also do not take into account constraints introduced by the presence of a neighboring foundation, or the impact of densification on seismic structure-soil-structure interaction (SSSI) and performance of adjacent buildings in urban settings. In this paper, three-dimensional (3D), fully-coupled, nonlinear finite-element analyses, validated with centrifuge experimental results, are used to evaluate the influence of ground densification on the seismic performance of isolated and adjacent, similar and dissimilar, inelastic structures on liquefiable soils. The response is evaluated for treatment of varying dimensions (depth [dDS] and width [WDS]), location (e.g., below one or both neighboring buildings), and symmetry configurations. Ground densification is shown to effectively reduce the permanent settlement of isolated structures when covering the full depth of the critical layer, while potentially amplifying column strains and structural deflections. Depending on the properties of the structure and symmetry of treatment, densification could also adversely impact a foundation’s permanent tilt. The influence of densification on SSSI is shown to depend strongly on the geometry of densification, dynamic properties of the neighboring structures, and building spacing. For the spacings (S) considered, ground densification effectively reduced the permanent settlement of two adjacent mitigated structure(s). However, the combination of SSSI and ground densification typically notably amplified asymmetrical deformations below the foundations (hence, permanent tilt) as well as column strains, particularly for an unmitigated neighbor. This effect was strongest when closely spaced and when WDS=S. The results indicate that ground densification location and geometry must be designed with extreme care in urban settings, particularly when near a taller and weaker neighboring structure.