Physical and numerical modelling of bottom ash columns installed in soft soil

Abstract

Stone column technique is one of the most widely used ground improvement techniques over the past 50 years. The technique includes the replacement of the soft soil with granular materials in order to increase the bearing capacity and reduce the settlement. This research investigated the role of a group of bottom ash columns in improving the bearing capacity of soft reconstituted kaolin clay. A series of physical modelling test was conducted to study the behaviour of clay reinforced with bottom ash columns under a rigid footing. The influence of important parameters, including area replacement ratio, length of the columns and the geotextile encasement on the performance of reinforced ground was investigated through a total of 13 model tests. Three (3) different area replacement ratios of 13%, 20 % and 26% and two (2) different lengths of 100 mm (floating) and 200 mm (end bearing) of the columns were investigated in this study. In addition, bottom ash columns were installed in two different methods, which was with geotextile encasement and without encasement. In parallel with physical modelling, finite element analyses were performed using Plaxis 3D Foundation software. The results clearly show that the ultimate bearing capacity of kaolin clay was significantly enhanced by the installation of bottom ash columns. This bottom ash has a great potential to be used as a replacement material for stone column in soft soil improvement work. The area replacement ratio was found to be an extremely important parameter controlling the overall performance of the reinforced foundation in the way that increasing the area replacement ratio resulted with up to 30% increase in the ultimate bearing capacity of composite ground. Increasing the length of the column also enhanced the bearing capacity of the reinforced ground of more than 15%. Floating columns were punched into the clay below the column base, but punching behaviour was eliminated by increasing the length of the column to become end bearing which resulted in the improvement of bearing capacity. Encasing the bottom ash columns with geotextile also resulted in an increase of the ultimate bearing capacity significantly from 25% for end bearing and up to 45% for floating columns. Finally, a design chart was established on the parameters affecting the ultimate bearing capacity of soft clay improved with bottom ash columns.