Briefing: Efficiency of stone-column groups installed in very soft clays

Field measurements from stone-column groups installed in very soft clays show that there is a direct relationship between the stress split between columns and soil and the column spacing/diameter ratio, with the proportion of stress carried by the columns decreasing with decreasing spacing/diameter ratio. This results in the settlement reduction achieved in practice falling increasingly short of theoretical predictions with increasing stone replacement. Comparison of the theoretical variation in column efficiency with stone replacement with that achieved in practice enables the effect of column installation to be quantified, demonstrating that the efficiency of stone columns in very soft clays, and thus the amount of settlement reduction achieved, is primarily dependent on the amount of installation-induced soil disturbance. Theoretical methods are therefore inappropriate for predicting settlement reduction and a practical upper-bound settlement reduction/stone replacement relationship is proposed for design purposes. Corresponding creep reduction factors, to be used in conjunction with the proposed primary settlement reductions, are presented, suggesting that stone columns are unlikely to be effective in soils for which creep is the major component of settlement.