Large size rigid reinforced concrete plate-load test data is evaluated to determine the performance of stone columns proposed for the foundation of a power plant. Soil profile consists of a top soil of sands, silts and transition soils up to a depth of 5.5 m - 6.5 m, and below coral limestone exists. Soil investigations indicated that the soil in the upper horizon has low shear strength and relatively high compressibility. Also, a large size zone load test, at an elevation approximately 1 m above the proposed foundation level was conducted using a 5 m(2) plate and it was observed that shear failure and excessive settlements occurred before reaching the design load. Hence, ground improvement technique by stone columns was selected to upgrade the foundation soil. The stiffnesses of improved and unimproved soils were compared. Settlement reduction and stress concentration ratios were also evaluated as a function of applied normal load.