When foundations are designed on weak clay layers, it is a common practice to modify the subgrade by installing stone columns. Currently used methods for determining the level of ground modification, represented by the percentage of soil replaced (replacement ratio), assume a rigid foundation. These analytical methods provide the designer with the potential settlement reduction based on the compressibility parameters of the subgrade and the replacement ratio. The deficiencies of these methods are the assumption of rigidity of the foundation and the consideration of the settlement reduction as the only design criterion. Furthermore, they do not consider the effects that ground modification has on differential settlement, moments, and shear forces within the slab. In order to determine the effects of ground modification on the overall performance of a flexible foundation, a computer program was formulated which compares a multitude of design parameters of the modified subgrade to those of the unmodified subgrade to determine the impact of ground modification. By performing this investigation, correlations were found between the replacement ratio and the settlement reduction factors. Similarly, correlations were also found between the ratio of the length of the foundation to the radius of relative stiffness, and the moments and shear forces generated within the slab. The use of the findings of this thesis would allow the design to make more informed decisions when designing foundations on modified subgrade resulting in safer and more economical designs.