Stabilisation Of Black Cotton Subgrades Using Landfill Wastes

The Queensland economy is dependent on the maintenance and continued connectivity of its 226,000 km long road network. More than 70,000 km of this network is situated on expansive clay soils, in particular, black cotton soil which is found in rural areas of Queensland, including the Darling Downs, Maranoa and Central Highlands. These expansive soils are problematic for road construction and can cause premature failure and increased maintenance costs, particularly in areas prone to flooding. Black cotton clays are high in both organic matter and iron content which limits the effectiveness of traditional lime stabilisation techniques. This project examined the use of waste materials including low quality brown fly ash and waste glass as innovative subgrade improvement techniques for black cotton clays. Three soil materials were investigated in this study; a black cotton clay, a red clay, (both from the Toowoomba region) and a bentonite clay. Chemical testing was undertaken on soils sample and characterisation using Atterberg limits, optimum moisture content for compaction and particle size distribution.  Between 5% and 15% of waste glass powder or low quality fly ash was then added to soil specimens as a stabiliser.  After curing for a period of 28 days, the samples were tested for unconfined compressive strength and water absorption.  The efficacy of stabilisation was compared to traditional stabilisation with hydrated lime.  Life cycle analysis comparing the economic and environmental benefits of using these materials for stabilisation of black cotton subgrades is also implemented.