Soft clay stabilisation using lightweight aggregate for raft and column matrices

Abstract

The stone column technique has been used as a soft ground stabilisation method over the past 30 years by using partial replacement of the compressible soil with a more competent granular material such as stone aggregate and sand. The column filler material in current practice normally consist of stone aggregates of 20 mm to 75 mm in diameter. It is compacted into a vertical hole generally of 0.6 meter to 1.0 meter diameters and depths of more than 6 time diameter. The major part of this research is to present the behaviour of stone column group beneath large loaded area through three-dimensional finite element (FE) analysis. Instead of normal aggregate for stone columns, Lightweight Expanded Clay Aggregate (LECA) has the potential to be used as replacement material. By using PLAXIS 3D, LECA columns-raft were modelled as Mohr-Coulomb material and the nonlinear behaviour of soft soil is modelled with Hardening Soil constitutive model. To permit timely analysis in this research and to simulate long term settlement behaviour, drained analysis was adopted to allow for a greater number of sensitivity and parametric analysis to be performed. Parametric study was performed to evaluate the effect of LECA raft thickness (Hr), depth ratio (ß), and area replacement ratio (a) on the settlement performance. The obtained results proved that increasing the thickness of replaced soft clay with compacted LECA layer evidently decreases the settlement, where the performance of settlement improvement up to 40%, 60% and 80% for 1.5 m, 2.5 m and 3.5 m depth of LECA replacement, respectively. The settlement ratio of LECA column was found to be reduced as the column length increases until end bearing condition, where the highest settlement ratio of 4 for a = 0.4 can be achieved. Higher area replacement ratio results in a higher settlement ratio, which indicating that closer spacing provide better improvement. In addition, the replacement of LECA raft on LECA column (LECA columns-raft) significantly decreases the settlement of the soft ground. For higher a with longer columns and thicker rafts provide better settlement improvement. However, LECA columns-raft performance chart can be referred for economic design. Meanwhile, the bearing capacity of LECA raft was found to be increased with increasing of raft thickness. The research indicates that the most controlling parameter in the prediction of LECA columns-raft bearing capacity (qu) is the area replacement ratio, where qu increases considerably with increase of a. The physical modelling was established in laboratory with 1:20 scaling factor to validate numerical analysis. The method can provide even more accurate verification, economical and require less time to perform compare to field testing and full-scale model. The results from physical modelling agree well with numerical prediction where the difference is less than 20% and is considered acceptable. The dimensionless relationship between settlement ratio (S/Suc) against ß was plotted for every a and Hr. Five design charts are proposed for practical engineer to predict the settlement of LECA columns-raft and LECA columns under large loaded area. Two design charts are also developed for LECA raft settlement prediction. It can be referred to as early estimation of settlement ratio according to the thickness of replacement. The reliability of design charts is successfully validated using PLAXIS 3D.