Influence of Dense Granular Columns on the Performance of Level and Gently Sloping Liquefiable Sites

Dense granular columns are often used as a liquefaction mitigation measure to (1) enhance drainage; (2) provide shear reinforcement; and (3) densify and increase lateral stresses in the surrounding soil during installation. However, the independent influence and contribution of these mitigation mechanisms on the excess pore pressures, accelerations (or shear stresses), and lateral and vertical deformations are not sufficiently understood to facilitate a reliable design. This paper presents the results of a series of dynamic centrifuge tests to fundamentally evaluate the influence of dense granular columns on the seismic performance of level and gently sloped sites, including a liquefiable layer of clean sand. Specific consideration was given to the relative importance of enhanced drainage and shear reinforcement. Granular columns with greater area replacement ratios (${\mathrm{A}}_{\mathrm{r}}$), for example ${\mathrm{A}}_{\mathrm{r}}$ greater than about 20%, were shown to be highly effective in reducing the seismic settlement and lateral deformations in gentle slopes, owing primarily to the expedited dissipation of excess pore water pressures. The influence of granular columns on accelerations (and therefore, the shear stress demand) in the surrounding soil depended on the column’s ${\mathrm{A}}_{\mathrm{r}}$ and drainage capacity. Increasing ${\mathrm{A}}_{\mathrm{r}}$ from 0 to 10% was shown to reduce the accelerations across a range of frequencies in the surrounding soil due to the shear reinforcement effect alone. However, enhanced drainage simultaneously increased the rate of excess pore pressure dissipation, helping the surrounding soil regain more quickly its shear strength and stiffness. At short drainage distances or higher ${\mathrm{A}}_{\mathrm{r}}$ values (for example, 20%), this could notably amplify the acceleration and shear stress demand on soil, particularly at greater frequencies that influence PGA. The experimental insight presented in this paper aims to improve our understanding of the mechanics of liquefaction and lateral spreading mitigation with granular columns, and it may be used to validate the numerical models used in their design.