Deformation behaviour of soft ground treated with a group of bottom ash columns

Abstract

The granular column technique has been used as a soft ground improvement for over 50 years, whereby stone aggregate and sand are added as the granular material. In order to reduce the use of natural resources, this study introduces a waste product from coal-fired power plants – bottom ash. An in-depth study of bottom ash‘s morphological and chemical properties is conducted to ensure its suitability as a replacement material and not harmful to the environment. This study adopted the Constant Rate of Strain (CRS) method for loading systems, whereas the majority of other related research uses the incremental loading system by stress control. A series of physical modelling tests are conducted to study the consolidation behaviours and settlements of bottom ash columns, under one dimensional testing. The advantage of CRS is that the consolidation properties can be obtained within only 24 hours, depending on the rate of strain applied. The influence of area improvement ratios and height of columns on the performance of bottom ash columns is investigated through nine physical model tests. Three different area improvement ratios (Ar) of 15%, 20% and 30%, for different length of columns of 50 mm, 100 mm and 150 mm, are used in the experiments. The bottom ash columns are installed in soft ground models prepared from kaolin, which has an undrained shear strength of 10 kPa. In parallel with the experimental investigation, a numerical simulation using Plaxis 3D is performed. An advanced constitutive model is selected for this study namely the Hardening Soil (HS) model, to simulate the models' actual behaviour. The results reveal that the chemical composition and heavy metal traces in bottom ash are minimal and do not exceed regulation standard levels. Hence bottom ash has great potential to be used as a replacement material for stone columns in soft soil improvement works. The area improvement ratio was found to be a very important parameter, which affects the overall performance of the bottom ash columns. Higher area improvement ratios resulted in lower settlements for the composite ground. The value of the compression index, Cc for 30% area improvement ratio obtained is 0.148, compared to 15% for 0.189 at the same height of a column of 150 mm. The results from the numerical simulation corroborated those of the physical modelling, whereby the difference is less than 20%, which is considered acceptable. A consolidation characteristic chart is proposed for predicting settlements based on area improvement ratios and the height of bottom ash columns.