Advantages of using stone columns in soft clay are recognized as an applied method to improve bearing capacity of shallow foundations. In order to study behavior of stone columns in soft clay a foundation simulating setup manufactured included a large test box (1.20×1.20×0.90 m) and hydraulic loading system. Twelve experimental tests were carried out to investigate effects of three different diameters of stone columns (63, 80 and 92 mm) on bearing capacity of them. Also, geotextile with both full-length and ringed forms were applied for encasing columns. Using ringed form of geotextile for encasement give the opportunity to investigate the possibility of use of similar encasing materials such as worn out tires. Diameters of 63, 80 and 92 mm with a ratio of length to diameter of 5 for both ordinary and encased columns were chosen. Results are shown to compare effectiveness of diameter variations and encasement.