Improvement of Expansive Soil using Volcanic Ash and Lime Additives in the Southern Parts of Addis Ababa City, Ethiopia

Abstract The presence of expansive subgrade soil in road construction causes a series of damage events because of its susceptibility to differential settlement, low load bearing capacity, high compressibility, cracking and low strength. In this study, the use of volcanic ash alone (with a concentration ranging from 10%-40%) or as an enhancement to lime (5%) was investigated. To evaluate the effects of these additives on expansive subgrade soil stabilization, laboratory tests, namely, Atterberg’s limits, compaction, the California bearing ratio (CBR), the CBR-swell and unconfined compressive strength (UCS) tests, were conducted on natural and stabilized soils. Chemical tests to characterize the stabilizer, volcanic ash, were also carried out. Laboratory test results revealed that the addition of volcanic ash alone has an insignificant effect on the geotechnical properties of expansive subgrade soil. However, a significant improvement in the index and engineering properties of expansive subgrade soil was observed when the treatment was performed with a combination of lime (5%) and volcanic ash (10–40%). The test results revealed that the liquid limit, plasticity index, OMC, linear shrinkage and CBR swell values decreased, whereas the CBR increased as the percentage of volcanic ash increased from 0–40%. The maximum dry density and unconfined compressive strength increased up to a certain percentage; however, a further increase in volcanic ash content resulted in a decrease in these parameters. This research showed that these two additives can be used together to enhance expansive subgrade soil properties and that the treated soil can meet the minimum requirement of the ERA pavement manual specification for use as a subgrade material in road construction.