Enhancement in Effectiveness of Cemented Stone Columns for Soft Clay Ground Improvement by Providing Underreamed Bulbs

Abstract

A straight-shafted cemented stone column technique for effective improvement of soft clay ground was reported in 217. The method involved transforming the conventionally used uncemented stone or granular mix fill material from a bulging type to a cemented porous fill material of a semirigid nonbulging type. This paper addresses further improvement in the performance of cemented stone columns by incorporating an underreamed enlarged bulb in the column at its bottom or at a suitable intermediate level. Analytical and experimental studies were conducted on such underreamed cemented stone columns in soft clay. Considering an idealized load-transfer mechanism and failure mode, expressions were then developed for predicting the load-carrying capacity of soft clay ground treated with underreamed cemented stone columns. Two cases were considered in the present study: one for an enlarged bulb at the bottom of the stone column and another with a bulb at an intermediate level (at a depth of 5 times the column diameter). The laboratory investigations involved load tests of six unit cells of bulbed cemented stone columns in soft clayey soil, corresponding to area replacement ratios of 4% and 9% and column length ratios of 1, 11.67, and 13.33. The measured values of load-carrying capacity agreed very closely with theoretically computed values. Additionally, straight-shafted conventional uncemented stone columns with four other unit cells were also load tested. The test data of these 1 unit cells in the present study along with the six-unit-cell test data reported earlier were combined and analyzed to evaluate the performance of underreamed cemented stone columns in comparison to other systems of stone columns in soft clays. It was found that the cemented stone columns with an underreamed enlarged bulb at the bottom or at a depth 5 times the shaft diameter were highly effective in improving the soft clay ground condition with respect to both the load-carrying capacity and ground stiffness. The provision of a bulb at the intermediate level was found to be better than a bulb at the bottom of the cemented stone columns.