Numerical analysis of consolidation of soft soils fully-penetrated by deep-mixed columns

Abstract

Deep mixing is a common ground improvement technology that can be used to increase bearing capacity and stability and reduce total and differential settlements of foundations constructed on soft soils. Field data have shown that deep mixed column foundations had a higher consolidation rate than untreated foundations even though deep mixed columns had similar or lower permeability than untreated soils. The consolidation mechanisms and behavior of deep mixed column foundations have not been well understood. In this study, the consolidation of soft soils fully penetrated by deep mixed columns was investigated using a mechanically and hydraulically coupled three-dimensional finite element method. One quarter of a unit cell was used considering its symmetry and the column and the surrounding soil were modeled as elastic materials. A case study for a stone column foundation in the literature was first used to verify the numerical model and then this model was adopted to analyze the stress transfer, settlement, and consolidation of the soft soil fully penetrated by a deep mixed column. A parametric study was conducted to evaluate the influence of four key factors, soft soil thickness, area replacement ratio, column modulus, and column permeability, on the stress concentration ratio (the ratio of the average vertical stress on the column to that on the soil), settlement, and average degree of consolidation of the deep mixed column foundation. The numerical results show that the average degree of consolidation calculated based on the settlement or the excess pore water pressure was identical. The stress concentration ratio increased with the column modulus and time, but the effect of the soft soil thickness, area replacement ratio, and column permeability was not significant. The simplified method based on a composite foundation concept could conservatively estimate the consolidation settlement. An increase of the column modulus, area replacement ratio, and/or column permeability increased the rate of consolidation.