Impact of Spatial Variations in Permeability of Liquefiable Deposits on Seismic Performance of Structures and Effectiveness of Drains

Abstract

Sand deposits are often stratified with thin layers of low-permeability silt. Previous studies have shown that the presence of sharp variations in permeability could slow down the dissipation of earthquake-induced excess pore pressures and cause void redistribution and shear strain localization. However, the relative importance and influence of these phenomena on seismic site response, soil–structure interaction, response of foundation and superstructure, and effectiveness of liquefaction countermeasures is not well understood. In this study, we present the results of dynamic centrifuge tests that evaluate the response of 3- and 9-story inelastic steel structures (A and B) founded on layered liquefiable deposits with and without a silt cap. The thin silt layer is also evaluated in terms of its influence on the effectiveness of prefabricated vertical drains (PVDs) as mitigation. The results indicate that a thin silt cap may have beneficial or detrimental effects on a structure’s performance, particularly when evaluated in terms of foundation’s permanent rotation (or tilt). Under the lighter, stronger, and stiffer Structure A, concentration of shear strains in the relatively thin loose zone below the silt layer reduced permanent rotation by 60%–100% compared with the same structure on the soil profile without silt. However, the greater inertial moment and shear demand on the foundation and loose zone below silt from the heavier, weaker, and more flexible Structure B initiated larger shear deformations and rotations, leading to larger dilation tendencies and a momentary reduction in excess pore pressures in the soil below. This amplified accelerations on the foundation, flexural deformations in the superstructure, and P-Δ effects that further exacerbated rotation and damage to the superstructure. The effect of PVDs was similar on both profiles, reducing the foundation’s permanent settlement (by up to 57%) and tilt (by up to 49%), but the influence of silt on performance was similar to that of unmitigated structures. These results point to the importance of identifying and characterizing thin interlayers in the soil profile, together with the key properties of structure, foundation, and ground motion, when assessing and mitigating the consequences of liquefaction.