Simplified Method for Calculating Consolidation Degree of Deep Mixed Column–Improved Soft Soils

Abstract

Deep mixing method has been widely used to improve soft soils by increasing bearing capacity, reducing total and differential settlements, and enhancing stability. Experimental and numerical studies have demonstrated that the inclusion of deep-mixed (DM) columns in soft soils (e.g., clays and silts) could accelerate the consolidation of soft soils. Existing theoretical solutions for calculating the consolidation degree of DM column-improved soft soils are mostly complicated and not easy to use for practical applications. Therefore, a simple analytical solution is needed to calculate the consolidation degree of DM column-improved soft soils. In this study, a unit cell concept was used to develop a simplified method for calculating the consolidation degree of DM column-improved soft soils under a rigid footing. The simplified method considers the stress transfer between columns and their surrounding soil under elastic conditions and the vertical drainage of columns and their surrounding soils. The proposed simplified method is verified after compared with the numerical analysis considering four key influence factors on the consolidation degree of DM column-improved soft soils. The key influence factors include the soil thickness to unit-cell diameter ratio, the area replacement ratio, the modulus ratio of DM columns to soil, and the permeability ratio of DM columns to soil. The proposed simplified solution also captures the characteristics of the consolidation behavior of DM column-improved soft soils by considering stress transfer between columns and soil during consolidation and is easy to use in practice. Three existing analytical solutions are used for the comparisons with the proposed method. Among these methods, the proposed method has the best estimation of the consolidation degree for the DM column-improved soft soils under most conditions. It is recommended that the proposed method should be used for the condition where column permeability is close to soil permeability.