Dense Granular Columns in Liquefiable Ground. II: Effects on Deformations

Abstract

Dense granular columns can be used to mitigate liquefaction hazards through a combination of densification, increases in lateral stress, reinforcement, and drainage effects. Three-dimensional (3D) nonlinear dynamic finite-element analyses are used to examine the effectiveness of dense granular columns for reducing liquefaction-induced deformations in the event that the triggering of liquefaction in the native soils is not prevented. The finite-element analyses consider unit cells with dense granular columns (improved case) and without granular columns (unimproved case). Parametric analyses are used to isolate aspects related to the different improvement mechanisms. The parametric studies consider a range of area replacement ratios, shear modulus ratios, diameter of granular columns, liquefiable soil depth, hydraulic conductivity, surface pressures, slope angle, penetration resistances in the native soil, and spatial variations in those penetration resistances. A set of 10 acceleration time histories were used as input motions. Dense granular columns were shown to be effective in reducing lateral spreading displacements of sloping ground, even if liquefaction triggering is not prevented. The reductions in lateral spreading displacement are primarily attributable to the reinforcing and strengthening effects of the granular columns, with drainage being a secondary benefit for cleaner sand profiles. The effect of spatially varying penetration resistances as develops around a column are examined and recommendations are developed for selecting an equivalent uniform value for design.