The Bargate Centre, Southampton: engineering geological and geohydrological aspects of the excavation for basement construction

Abstract The Bargate Centre is a large, four-storey shopping arcade constructed during 1986–1990 within the medieval town walls of Southampton. It incorporates a basement requiring construction to between 6–7 m below ground level through fill, brickearth, and water-bearing river terrace gravels overlying mainly laminated clays of the Bracklesham Group. The case record is of particular interest in that the excavation cut-off wall acted as a barrier to the original (that is, pre-excavation) groundwater seepage in the terrace gravels. Observations of the piezometric levels before and after construction confirm ponding on the upstream side of the basement. However, it appears that the pre-construction seepage pattern on the downstream side was modified by the medieval double moats which are known, at least in part, to have been excavated into the terrace gravels.