POND ASH AS A SUSTAINABLE BACKFILL FOR REINFORCED EARTH (RE) WALL

Abstract

The rapid expansion of highway infrastructure in India has intensified the demand for granular backfill in reinforced soil wall construction, placing sustained pressure on natural borrow sources and renewing interest in industrial by-products such as pond ash as an alternative fill material. This dissertation investigates whether pond ash can serve as a technically sound, code-compliant and economically viable backfill for reinforced soil walls at a live highway project site on the NH-46 Guna-Biaora section (chainage 118+400), where the native fill is a plastic, SC-classified sandy clayey soil. The investigation proceeds through a comparative geotechnical characterisation of the native soil and pond ash, an assessment of the mechanical and reinforcement-interaction behaviour of pond ash under site-specific loading, an examination of hydraulic and environmental response under seasonal groundwater fluctuation, the identification of field performance monitoring requirements, and an economic and sustainability appraisal of pond ash against conventional borrow material within an Engineering, Procurement and Construction (EPC) project framework. The native soil exhibits a dry density of 1.821–1.829 g/cc, cohesion of 0.03–0.04 kg/cm² and an internal friction angle of 25.3°–25.9°, with a shallow groundwater table at approximately 3.50 m governing drainage and pore-pressure behaviour. By comparison, the pond ash is non-plastic, contains 6.75% fines, and returns a markedly higher friction angle of 31.50° with negligible cohesion, a maximum dry density of 1.344 g/cc at an optimum moisture content of 25.30%, and full compliance with the gradation and strength criteria of IRC:SP:102-2014. Limit-equilibrium stability checks for the reinforced soil wall section return factors of safety of 1.82 against sliding, 2.45 against overturning and 3.10 against bearing failure, with a global factor of safety of 1.68, all exceeding their respective minimum requirements. These results indicate that pond ash offers genuine geotechnical and sustainability advantages — reduced dependence on natural borrow material, beneficial reuse of an industrial residue, and favourable frictional behaviour — but also reveal material properties that differ substantially from the native soil baseline, particularly in density and cohesion. The economic case for pond ash is accordingly found to be conditional rather than assured: within an EPC-mode project framework, any cost or schedule advantage depends on verified field compaction, drainage performance and reinforcement interaction, since unvalidated deviations from design assumptions could erode the anticipated savings. The dissertation concludes that pond ash is a technically promising and potentially sustainable backfill alternative for reinforced soil wall construction, whose safe and economical adoption should be confirmed through calibrated field-scale trials and continued instrumented monitoring prior to full-scale implementation. This thesis addresses the technical, structural, economic, field and sustainability feasibility of using source-specific pond ash as an alternative sustainable backfill material for Reinforced Earth walls, through comparison with conventional soil and compliance with IRC:SP:102-2014.