Influence of Stratigraphic Variability and Ground Motion Properties on the Effectiveness of Dense Granular Columns as Liquefaction Mitigation for Structures

Abstract

The current practice of designing dense granular columns (DGCs) to mitigate liquefaction is based on assumptions of uniformly layered deposits of relatively shallow, saturated, continuous, clean sand with no structure. As a result, these procedures fail to account for the complexities of natural deposits, mitigation mechanisms, and the foundation-structure system. In this paper, we employ fully-coupled, three-dimensional (3D), effective stress, nonlinear, dynamic finite element analyses, validated with centrifuge experiments, to investigate fundamentally how DGCs with different properties influence the seismic performance of sites and structures with realistic stratigraphy. The results show that DGCs’ enhanced drainage, particularly when combined with ground densification, notably reduce triggering, strength loss, ejecta severity, and permanent foundation settlement. However, the results highlight that deep critical layers, silt interlayers, and nonuniform layer thickness can notably reduce DGC effectiveness in terms of foundation tilt, accelerations, and flexural drifts within the structure, depending on the characteristics of the foundation-structure system. These effects were more pronounced in the presence of clogged drains and silt interlayers, pointing to the importance of construction procedures that minimize the likelihood of clogging. Ground motion directivity and cumulative intensity are shown to be influential in determining DGC effectiveness, challenging conventional peak ground acceleration (PGA)-based design methods. The findings highlight the need for a more nuanced approach to understanding the response and interactions among soil layers, mitigation mechanisms, foundation, superstructure, and motion characteristics in evaluating and improving system performance holistically.