Coal Bottom Ash (CBA) which is one of coal combustion product (CCPs) obtained from thermal power plant will negatively affect the ecological system which human health ultimately depends on. The increase in demands for landfill requirements and the limitations natural aggregates is quite disturbing unless the new alternative to cut the usage of natural aggregates is found. Therefore, this paper present the laboratory investigation work to improve the strength of subgrade by the replacement of CBA with different percentage varies (0%, 15%, 30% & 40%) by weight. California Bearing Ratio (CBR) test is carried out to obtain the optimum percentage replacement CBA and then some physical which are particle size distribution, Atterberg limit test, specific gravity test and compaction test also carried out to observe the mechanical properties of optimum sample. The optimum percentage of CBA replacement obtained is 40.65% at 30% CBA replacement compared to 17.99% of subgrade soil alone. The moisture dry-density of unmodified soil sample is higher (1.99Mg/m3, 15.34%) than optimum sample (1.95Mg/m3, 10.2%). Moreover, 30% CBA replacement resulted in intermediate plasticity than the subgrade soil which is highly plastic. In conclusion, strength of subgrade soil is highest at 20-30% of CBA replacement.
