Investigation on the applicability of bottom ash for restoration of clay mines in Sri Lanka

Abstract

The higher cost associated with the restoration of clay mines and shortage of suitable filling materials have created many abandoned clay mines in Sri Lanka, leading to various environmental and health issues. Bottom ash (BA) generated during the coal combustion process as a by-product is one such potential fill material. Therefore, utilizing BA as a full or partial replacement of soil during clay mine restoration is the main objective of this study which was fulfilled under four sub-objectives: 1) potential as a backfill material, 2) ability to prevent soil erosion, 3) potential as a soil amendment material and 4) environmental impact. BA was collected from the Lakvijaya power plant in Norochcholai, Sri Lanka. In addition, the suitability of BA as a full or partial replacement for soil was tested with six different fractions of BA (0%, 50%, 60%, 75%, 90% &100%) using a gravelly lateritic soil. The basic properties, shear strength parameters and compressibility characteristics of BA-soil mixtures were investigated. Annual soil loss due to rainfall was measured by conducting artificial rainfall tests for BA-soil mixtures. In addition, chemical composition, pH, electrical conductivity, water holding capacity and micro-structural morphology through Scanning Electron Microscope of BA-soil mixtures were determined to evaluate the potential of soil amendment capabilities. Furthermore, column leaching method was used to evaluate the trace metal concentrations of BA-soil mixtures. According to results, BA behaves as a poorly graded sand, a free draining material with low compressibility and cohesion. BA has a lower maximum dry density (975 kg/m ) and higher optimum moisture content (37%). The annual soil loss is “very low” when the fine fraction of the BA-soil mixture is lesser than 20%. BA has the potential to improve agronomic characteristics of soil due to the better water holding capacity and electrical conductivity. BA can adjust soil pH to a desirable plant growth range. Interestingly, the micropore structure of BA has a positive influence on compressibility characteristics, soil erodibility and plant growth while compaction characteristics are influenced negatively. According to column leaching test results, the leachability potential of trace metals in BA does not exceed the allowable limits. Finally, an empirical relationship for the compaction characteristics was developed using multiple regression analysis with a prediction accuracy less than ±3%. Further, the specific gravity of BA was predicted using the chemical composition by soft computing techniques with an accuracy around 96%. The proposed models could be used for the preliminary assessment of the suitability of BA prior to a project. By considering the results and economic benefits, it can be concluded that, utilizing BA as a partial replacement of a traditional fill material (up to 75%) is the most suitable combination for restoration of abandoned clay mines and similar applications according to the present study.