This research aims to improve the geotechnical properties of a clay soil with the addition of sewage sludge ash and lime and to determine appropriate amounts of additives for natural soil at construction sites. Usually natural soils are not suitable to bear the desired structure. Clay soils especially expansive or disperssive ones have poor engineering properties. Different treatments with the addition of sewage sludge ash in different levels (0, 5, 10 and 15 percent) and limes in different levels (0, 1, 3 and 5 percent) were tested. In order to improve the geotechnical properties of caly soil by addition of sewage sludge ash and lime, laboratory tests including compaction and unconfined compressive strength (soaked and unsoaked condition) were performed. Statistic analysis was conducted using software. Unconfined compressive strength test specimens were cured for 7, 14, 28 and 90 days. Compaction test results showed that by addition of sewage sludge ash and lime to clay soil, maximum dry bulk density decreased and optimum moisture content increased. Also by addition of sewage sludge ash and lime, unconfined compressive strength increased in comparison with natural soil. By addition of sewage sludge ash and lime to clay soil, unconfined compressive strength values increased significantly in comparison with natural soil and curing time had considerable role in increasing of unconfined compressive strength. Based on the results, it is concluded that the clay soil could be successfully stabilized by sewage sludge ash-lime mixture. Keywords: Clay soil, Lime, Sewage sludge ash, Harvard miniature compaction apparatus, Unconfined compressive strength.
