Performance of Stone Columns in Soft Clay - A Comparative Study of Bearing Capacity of Soil Estimated using IS Code Method, Actual Field Load Tests and Observations from Numerical Model

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The study evaluates vibro-stone columns for improving soft clay beneath hospital buildings at Jorhat, Assam, a region in seismic Zone V. Site investigations found firm silty clay extending to about 13.5 m, with a high groundwater table. The untreated soil’s safe bearing capacity was approximately 70 kPa, below the required 140 kPa, and predicted raft settlement exceeded the allowable 75 mm.<br /><br />The ground improvement scheme used 900 mm diameter stone columns, spaced 2 m apart in a square pattern, installed by the dry-bottom-feed vibro-displacement method. Columns extended 9 m below the foundation level, with a 500 mm compacted sand blanket above. The design followed IS 15284 guidelines.<br /><br />Performance was assessed through initial single- and group-column load tests, three routine tests, and PLAXIS 2D finite-element analysis. At the design pressure of 140 kPa, measured settlements were about 7–8 mm in the tests; the group test recorded 12 mm at 210 kPa. These results met the applicable test acceptance criteria and indicated that the treated ground could support the proposed loads. The numerical model predicted a raft settlement of 55 mm, below the 75 mm allowable limit. Although the model predicted greater settlement than the field plate tests, the authors note that the tests represented individual and group columns, while the model considered raft–soil interaction and group effects.<br /><br />The authors conclude that stone columns successfully increased bearing capacity and controlled settlement. The 16% replacement ratio used at the site was effective, though an 18–20% ratio may be preferable where soil fines vary. Overall, field tests, code-based design, and numerical analysis supported the suitability of the improvement scheme.

Keywords

vibro-stone columns

soft clay improvement

bearing capacity

raft settlement

seismic Zone V

Jorhat Assam

stone column design

PLAXIS 2D

ground improvement

IS 15284