The A331 is a new dual carriageway link between two major east–west routes; the A31 Winchester–Guildford road and the M3 London–Southampton motorway. Its route follows the valley of the River Blackwater on the Surrey–Hampshire border to the east of Aldershot and Farnborough. The road was designed and constructed in three sections, with the centre section being the last to be completed, in 1996. The horizontal and vertical alignments of this section were dictated by numerous constraints so that the very difficult ground and groundwater conditions present had to be overcome rather than avoided. This paper describes the ground and groundwater conditions, the problems that these presented and the solutions adopted.