The paper aims to access and evaluate the index as well as engineering properties of clay soil when subjected to controlled thermal treatment. To achieve this, clay soil was collected from a specific location in Amassoma, Southern Nigeria, and heated up at 200oC, 400oC, 600oC and 800oC. To adequately inform the engineering society on the potentials of thermally treated clay soil in the construction industry, properties such as specific gravity, Atterberg limits, California bearing ratio test (CBR), and compactibility were determined. Specific gravity increased by 60% between the control at room temperature and the specimen produced at 800oC. The increment was direct showing flattening tendencies between 600oC and 800oC. A direct relationship was found to exist between calcination and compactibility for all specimens between 27oC and 800oC. A similar trend was observed for CBR values of which at 800oC, the soil specimen having a CBR of 13.51 had improved in CBR by 655% when compared to the control’s CBR of 1.79. An inverse relationship was observed between calcination and Plasticity Index. At 800oC, the plasticity index having a value of 6.9 had reduced by 63% when compared to the control specimen having a PI of 18.4. By the process of calcination, the soil was successfully transformed from a clayey, high plastic, cohesive soil with very high affinity for water and unsuitable subgrade for pavement purposes, into a silty, low plastic, partly cohesive soil with reduced affinity for water having a fair CBR and applicable for subbase utilization in pavement construction.