But in this study, the addition of BA 30% showed a very low compressive strength at the age of 90 days. It may be due to the high replacement of cement and low pozzolanic reaction of BA. Based on the test results mentioned above, 20 % of the BA replacement to cement is the optimal limit in soft soil improvement. It could be that only 20 % BA acted as pozzolanic materials and acted as fillers. The strength development for BA and cement stabilized soft clay soils in this study is consistent with that reported used BA and hydrated lime stabilized expansive soil by Liet et al, 2015 [22]. The strength growth at early stage is attributable to the flocculation and agglomeration of the soil particles [23] while the long-term strength development is determined by the pozzolanic reactions [24]. The proposed equations for strength prediction with curing time had high coefficients of determination and are presented in Table 2.