Numerical Evaluation of Consolidation of Soft Foundations Improved by Sand–Deep-Mixed Composite Columns

Abstract

With the development of column technology, a new concept of composite columns has been proposed. Composite columns integrate two columns into one single column, which has a small inner column installed in the middle of a large column. The purpose of the composite column is to combine favorable features of these two constituent columns. The inner and outer columns can have different stiffness, strength, and permeability values and different combinations. In this study, a sand–deep-mixed column was investigated, which included a deep-mixed (DM) column in the middle of a sand column. Because two component columns work together in the composite column, the load-transfer mechanism of the foundation becomes complicated. The bearing capacity of the soft foundation improved by composite columns has been studied. However, the consolidation of the soil foundation improved by composite columns has not been investigated. In this study, a finite-element method was used to model the soft foundation improved by the sand–DM composite (SDMC) column subjected to rigid loading. The SDMC column was fully penetrated through the soft soil and keyed into a firm soil layer. The concept of a unit cell was adopted, and one-quarter of the unit cell was utilized due to the symmetry of the model. The SDMC column was modeled as a linearly elastic perfectly plastic material, and the surrounding soil was modeled as an elastic material. Two additional soft foundations improved by sand columns and DM columns were modeled as well for comparison purposes. The stress concentration ratio, excess pore-water pressure, column bulging, and consolidation settlement of the three soft foundations were analyzed and are discussed. The numerical results show that the vertical effective stress on the top of the SDMC column increased to the maximum value and then decreased to the steady-state value, whereas the vertical effective stress on the top of the soil decreased first and then slightly increased to the steady-state value. The maximum stress values of the columns and the steady-state stress values of the soil indicated the yielding of the columns and the completion of consolidation, respectively. The process of stress transfer in the foundation improved by the SDMC column can be divided into the following three phases: (1) the stress was transferred from the soil to the inner DM column and the outer sand column before the inner DM column yielded, (2) the stress was transferred from the inner DM column to the outer sand column after the inner DM column yielded, and (3) the stress was transferred from the outer sand column to the soil after the outer sand column yielded. The SDMC column had a larger stress concentration ratio than the sand column but a smaller ratio than did the DM column. In addition, the SDMC column resulted in larger bulging and settlement than the DM column but smaller ones than the sand column. The foundations improved by the SDMC column and the sand column had higher degrees of consolidation than did the DM column.