A BRIDGE FOR ALL SEASONS

The article describes the design and construction of the aqueduct carrying the Basingstoke Canal over the new Blackwater Valley Route (BVR) on the Surrey-Hampshire border in England. The BVR is a 10km link between the M3 motorway and A30 trunk road in the north and the A31 trunk road in the south. An asymmetric cable-stayed design for the aqueduct was rejected by the Royal Fine Arts Commission, so that an alternative design was developed, essentially a post-tensioned concrete structure with a flat-bottomed stretched U-shape cross section. It spans 17m from each abutment to a central 40m span, supported on two pairs of V-shaped piers in fluted concrete. There are also two 15m transition spans at each end, to connect to the existing canal earthworks. There was a very tight construction schedule, from 30 September 1994 to 30 June 1995, but it was just met, despite a very wet winter. The first task was to install clay dams alongside the route, then drain the 200m long cut-off section. Then the canal embankment was removed and excavated down to the level required for the piling and new roadway. After piling, extensive falsework was erected to carry the timber formwork over the aqueduct's external surface. Finally, the aqueduct's concrete was cast with rebar and post-tensioning ducts in place.