Author(s): Abtsega Alayu and Dr. Melaku Sisay

Abstract: Several studies have been conducted to minimize or eliminate the danger of damage to structures because of the heave and shrinkage of expansive soil. Road construction industries have attempted mechanisms to improve the ground dominated by weak soils. The methods of soil stabilization using different additives such as red ash and lime alone have been studied so far. However, stabilization using red ash or lime only is not efficient, economical, and eco-friendly. This study was conducted to improve the strength of weak expansive sub-grade soil with locally available materials i.e., Red Ash (RA), and with a suitable proportion of Hydrated Lime (HL) for the required sub-grade strength parameter. Index property (i.e., gradation, Atterberg’s limit test, linear shrinkage (LS), and free swell index (FSI)) and mechanical property tests (compaction, unconfined compressive strength (UCS), and California bearing ratio (CBR)) were conducted for both natural black cotton soil (NBCS) and soil treated with red ash and hydrated lime in per-defined combination. As per the test result, NBCS has a plasticity index (PI) of 61%, and under sieve analysis, more than 35% passes 75µm sieve, and the soil is classified as A-7-5 or A-7-6 and CH as per AASHTO and USCS respectively which is categorized under poor sub-grade materials. Also, the CBR value was 1.51%, which didn’t meet the minimum sub-grade CBR strength value of 5% according to ERA 2013.From the index property test result of treated soil, the LL, PI, LS, and FSI showed a continuous reduction with an increase in percentage red ash, hydrated lime, and a combination of red ash-hydrated lime compared with NBCS. The PI value observed ≤ 20% and LS < 10%, for a higher percentage of red ash-hydrated lime combination. The MDD significantly increased with the addition of red ash and reduced with the addition of hydrated lime. The OMC value decreased for with the addition of red ash and hydrated lime on NBCS. As the curing days increase the value of UCS with the addition of red ash and hydrated lime. The CBR value slightly increased with increasing percent of red ash alone, and a much higher increment was obtained with the addition of hydrated lime. With finite element software Abacus simulation of pavement structure treated with hydrated lime and red ash the deformation of pavement structure reduces by 20% compared to natural untreated sub-grade material. Comparing cost for replacement method of stabilization with soil treatment by red ash and hydrated lime up to 10% saving in construction cost can be achieved.

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