Effects of granular columns in compacted fills

Compacted fills are frequently used in engineering construction where site levels need to be raised. Increased environmental restrictions, coupled with the growing cost of virgin aggregate, have prompted more widespread reuse of site-won fill from earthworks. It is well established that the subsequent behaviour of the fill is highly related to the nature of its placement, which, if not correct, can lead to significant future difficulties. When placed, fills are generally unsaturated; however, over time they may experience an increase in water content owing to infiltration arising from rainwater, surface runoff or groundwater recharge. This saturation process can be accompanied by substantial changes in volume or reduction in shear strength; these can result in large and often non-uniform ground movements, which typically cause loss of serviceability. Previous research has indicated that such effects may be reduced through the use of ground improvement techniques such as granular columns. The work reported in this paper examines the performance of both engineered and unengineered compacted fills, prepared at four different initial moisture contents within 2% of optimum prior to saturation. Specimens were prepared by mixing kaolin and well-graded medium sand at a ratio of 1:9. Two loading conditions were considered: (a) monotonic loading and (b) static loading, during which saturation occurred. The performance of the ground improvement was examined by introducing a single 32 mm diameter granular column. In addition, the possibility of column strengthening was also considered by encasing the column in geogrid reinforcement and cement stabilisation. The results have shown that the inclusion of granular columns in unengineered cohesive fill improved the overall bearing capacity and settlement performance; however, a marginal reduction in performance was observed in the engineered fill specimens.