Consolidation of Soft Foundations Treated with Composite Columns

Foundations constructed on soft soil often require columns to increase bearing capacity and reduce total and differential settlements. Columns can be: (1) flexible (e.g., stone columns and sand compaction columns); (2) semi-rigid [e.g., deep mixed (DM) columns and jet-grouted columns]; and (3) rigid (e.g., vibro-concrete columns). Research has demonstrated that columns can also accelerate the consolidation of soft foundations by column drainage and stress transfer. In this paper, a three-dimensional, coupled, numerical analysis was adopted to study the consolidation behavior of soft foundations improved with two types of composite columns. For example, a DM column or jet-grouted column can be installed in the middle of the stone column or sand column. In this composite column, the stone column or sand column provides drainage while the DM column or jet-grouted column provides stiffness. Columns and soil in this study were modeled as elastic materials and one quarter of the unit cell was used due to the symmetry. To demonstrate the benefit of the composite columns in accelerating consolidation, two soft foundations improved with stone columns and DM columns were analyzed for comparison purposes. The numerical results show that both column drainage and stress transfer contributed to the acceleration of the consolidation of the soft foundations improved with composite columns. In addition, a parametric study was conducted to evaluate the influence of the stiffness of DM columns on the rate of the consolidation of the soft foundation improved with composite columns.