Stone column support for an embankment on deep alluvial soils

The south roundabout embankment to the River Avon bridge of the St Philips Causeway in Bristol is up to 7 m high and stands on thick alluvial deposits including very soft clays and peat. Stone columns have been used to ensure short-term stability, to accelerate and control consolidation settlements, and to reduce secondary compression effects to meet stringent settlement criteria within a short period. This paper describes the design of the stone column layout and gives details of the monitoring carried out to determine its effectiveness. The results obtained confirmed the design assumptions and indicate that the ground improvement was successful. The paper highlights the importance of the correct estimation of stress levels in assessing shear strength characteristics for the column/host soil combination, and points out the reduced efficiency of the system at its margins with unimproved ground.