Stability Analysis of Fly Ash over Burden Backfill Structure Using Physical Modeling Techniques

Due to rapidly growing energy needs and the resultant rise in the use of coal to meet them, exploration and exploitation of coal resources has seen exponential growth. Global concerns about the environmental impact of coal exploitation have led to an increasing shift to open-cast mining of coal, in place of the traditional deep-pit mining. This has led to search for a suitable method of handling the large quantity of soil – termed overburden – that is a by-product of open-cast coal mining. The most practical method of disposal of overburden has been to dump it in nearby open areas, which causes concern about the stability and mechanical behavior of the resultant hillocks. This thesis attempts a study of the operations of the open-cast mines of Singareni Collieries Company Limited (SCCL) in the Godavari Valley coalfield, which are spread over four districts of Andhra Pradesh, viz. Adilabad, Karimnagar, Khammam and Warangal. In pursuance of the objective of this study, a comprehensive field data-collection exercise was undertaken to collect details of mining practices, geology, rainfall pattern, ground water conditions and other relevant factors. This was followed by a field study, conducted to ascertain the foundation characteristics of the soil at the external dumps of the open-cast mines. Samples of soil were procured from the open-cast mines and subjected to laboratory study for evaluating the physico-mechanical and geo-technical characteristics of soil for determining various attributes such as specific gravity, bulk density, porosity, permeability, granulometric distribution of different size fractions, Atterburg’s limit, cohesion, angle of internal friction, California bearing ratio etc. Numerical modeling was then conducted to determine the structural stability of dump configuration on the basis of data obtained from laboratory studies. Lastly, field trials were carried out using physical modeling, to evaluate dump stability when backfilling using an admixture of pond ash and overburden. This study establishes the suitability of mixing overburden with pond ash due to its various advantages keeping in mind the civil engineering aspects viz. bearing capacity, geotechnical parameters etc of the mix and existing ground conditions. Additionally, since substantial quantity of pond ash gets used up in creation of admixture dumps, the extent of an ecological and health hazard is reduced. Thus, mixing pond ash to overburden prior to backfilling provides dual benefit; increase in the stability of dumps and reduction in the free quantity of an environmental pollutant.