Influence of Stratigraphic Variability and Layering on Liquefiable Soils Near and Away from Structures

Seismic soil-structure interaction (SSI), stratigraphic variability, and layering influence liquefaction triggering, the resulting total surface deformations, and damage to buildings and infrastructure. The existing empirical procedures for evaluating liquefaction triggering are primarily based on case history observations of surface manifestation and treat susceptible granular deposits as uniform and isolated. Meanwhile, most natural granular deposits are spatially variable due to heterogeneities in soil hydraulic conductivity, layer thickness, relative density, and continuity. In this paper, three-dimensional (3D), fully coupled, nonlinear finite-element analyses (FEA) in OpenSees, validated with centrifuge experimental results, are used to systematically evaluate the influence of layering and stratigraphic variations on the system's performance. We evaluate the response of two dissimilar, multi-degree-of-freedom (MDOF), shallow-founded structures. The ejecta potential index (EPI) is used to quantify the potential for formation of soil ejecta in different configurations. EPI is shown to depend strongly on the location of the groundwater table, average thickness and continuity of the liquefiable layer, and properties of and proximity to the structure. The mechanisms of deformation captured by a continuum FEA (volumetric and deviatoric) as well as the extent of softening (quantified with ru) within the critical layer are shown as uncorrelated with EPI, which depends primarily on the nature of hydraulic gradients developed within the profile. The results point to the importance of considering stratigraphic variability and potential variations in groundwater table together with average soil properties and structural characteristics when evaluating the ejecta potential on liquefiable sites.