Suitability of using mixes of heated black cotton soil, molasses and lime in stabilization of expansive clay soil

Abstract

Expansive clays, in moisture content fluctuation situations, cause deformations which result in heavy damage to pavements and structures built on them causing losses which run into billions of dollars worldwide. Most improvement methods use lime or limeactivated materials to treat black cotton soil, or cut to spoil followed by replacement with suitable materials. The former methods have challenges in regions of either high moisture and or in flood-prone areas. Though lime-activated Ground Granulated Blast Furnace Slag (GGBS) stabilized expansive clays have been found to withstand severe flooding conditions, GGBS is not locally available in Kenya. On the other hand, cut to spoil followed by replacement with suitable materials has high cost and environmental implications. This study undertook to stabilize black cotton soil in-place to withstand flooding conditions or high moisture situations using heated black soil in-place and mixes of lime, molasses and heated black cotton soil including evaluating their economic viability. Swelling characteristics and strength properties were conducted on the soil heated at various temperatures. Unconfined Compressive Strength (UCS) tests on various mixes of heated soil, lime and molasses established an optimal mix of 14% heated black cotton soil at 400°C, 6% lime and 0% molasses as the binders with 80% black cotton soil. Properties of this mix subjected to moist curing periods of 7, 14, 28, 56 and 90 days followed by soaking them in water for 4 and 10 days after each curing period to simulate flash and severe flooding respectively were investigated. The durability reduction indices for the each flooding scenario were computed. A model for a 2000x2000 mm building was constructed with a ground floor slab on a 1000 mm fill comprising the optimal mix, from which the heave was measured for moist curing periods of 7, 14, 28, 56 and 90 days and after 10 days side flooding at the end of 90 days. Costs were then determined for filling road embarkments and under ground floor slabs with the heated soil and the optimal mix. The heated soil yielded a material suitable for fill at a temperature of 700°C when the soil became non-plastic with a soaked California Bearing Ratio (CBR) value of 16%. The optimal stabilized mix had a UCS value of 394 kN/m2, and soaked CBR value of 39.2% with CBR swell of 0.22%; it was found durable and able to withstand flooding vi conditions. The ground floor slab attained a maximum settlement of 5 mm, with no heave. The cost per m3 for filling with black cotton soil heated to 400°C activated with lime was Kshs 3900 and Kshs 3850 for roads and ground floor slabs respectively whereas the respective costs for the heated soil was Kshs 11,360 and Kshs 10,340. The heated soil and lime activated heated soil are therefore effective stabilizers for both roads and ground floors slabs. However, since the cost of using heated soil is very high it is recommended that lime-activated heated black cotton soil be used to stabilize black cotton soils in Kenya in areas subject to flooding.