Weak sub-grade soil is one of the most plentiful soils in Ethiopia, which mostly causes significant damage to structures such as roads and bridges due to their weak in strength. Therefore this study assessed the effects of gypsum and brick mix on weak sub-grade soil to use as a road subgrade preparation. Weak sub-grade soil sample was collected from different location of Ethiopia. The weak subgrade soil were mixed with the brick and gypsum by percentage of the weight of soil taken for each samples tests starting from to within differenceand 0 to within difference respectivelyand laboratory tests such as Atterberg limit, Compaction, and CBR are carried out to assess the alteration in its strength characteristics and index properties. Based on the laboratory test results, the subgrade material quality improved from A-7-5 to A-2-4 at combination of crushed waste brick and of gypsum with expansive soil. By the addition of material to weak sub-grade soil the least plasticity index value obtained was and the CBR increased to from initial CBR value at the percentage of brick and gypsum. The MDD was increased to from the initial untreated soil test at percentage of crushed waste brick and gypsum mix with expansive soil. From analysis of results,it shown that the mixture of of crushed waste brick and of gypsum was the optimum combination material for stabilization of weak sub-grade soil to comply with the required technical specification specified in AASHTO. Treating weak sub-grade soil with the mix of crushed waste brick and gypsum respond and exhibited an improvement on its engineering properties including reduction in plasticity, increased strength and compaction characteristics.

Keywords : Crushed Waste Brick, Gypsum; Strength, Weak Sub-grade Soi