Performance evaluation of characteristics of black cotton soils modified with lime kiln dust and sand for subgrade construction
Ambrose, Kibuka Agaba
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Black cotton soil is a problematic expansive soil characterized by high swelling and shrinking nature hence poor subgrade characteristics. This research was aimed at improving the poor subgrade geotechnical properties of black cotton soil such as the index, swelling and strength properties by blending it with a combination of lime kiln dust and sand. The study was carried out on black cotton soil sample obtained from Namalu-Kaiku road in Nakapiripiriti district. The soil sample was blended with sand content alone, and a combination of sand and lime kiln dust (LKD), by varying proportions of the stabilizers. Black cotton soil was blended with sand in concentrations of 0%, 20%, 30%, 40% and 50% by dry weight of the soil. The soil sample was further modified with a combination of lime kiln dust and sand content by keeping 30% sand constant and varying the proportions of lime kiln dust to 3%, 4%, 5% and 6% by dry weight of the soil. The laboratory results were analyzed based on the effect of the additives in comparison with the engineering properties of the native and modified soil. This was achieved by conducting the following tests: Initial Consumption of LKD, sieve analysis, Atterberg limit, compaction, free swell and CBR tests. During laboratory testing, it was observed that black cotton soil had a high plasticity index, high swelling potential and low bearing strength. The addition of sand content to the soil showed a relative increase in strength, reduction in the plasticity index and swelling potential of the neat soil sample. Furthermore, the addition of sand-LKD combination to the soil sample showed a higher strength gain than soil samples treated with sand alone. The use of sand-LKD combination showed a significant improvement in strength, reduction in the plasticity index and swelling properties of the soil. The application of 30% sand and 3% LKD increased the CBR value of the neat sample from 5% to 18%, reduced the PI value of the native soil from 49% to 10.2%. Therefore, a combination of 30% sand and 3% LKD satisfied all the MoWT subgrade specification requirements and was considered as the optimum stabilizer content for subgrade soils.