PILING RESTRICTIONS resulting from fears of unexploded bombs have forced a major redesign on the Hungerford footbridge across the Thames in London, pushing construction costs up by two-thirds to about £50M.
London Underground Ltd places onerous restrictions on piling activities in the Thames near its tunnels because of the potential damage from undetonated Second World War bombs buried in the river bed (Ground Engineering September).
LUL claims the project consortium Cross River Partnership ran into problems because it had not realised the full cost of these restrictions. LUL insists on closing its tunnels if piling work is to take place within 15m of them, for which it charges dearly.
Designers have opted to sidestep the problem by redesigning the bridge so that foundation work is outside the restricted zone.
Most of the extra cost of the redesign will be met by a £16. 5M grant from Transport For London, ratified last month by the Greater London Authority.
Another £3M of extra funding is coming from Westminster and Lambeth councils and Railtrack. LUL has agreed to absorb £1M of the costs.
The new design, by the bridge's engineer WSP and contractor Costain, will move the upstream pylon nearest the north bank of the Thames on to the river bank which avoids having to pile in the river near the Bakerloo Line. This backstayed pylon will be replaced by an Aframe structure, fundamentally altering the symmetry of the twin cable stayed footbridges.
Foundation designs for the next nearest pier to the north bank will also be changed from bored pile to a concrete-filled, hand-dug shaft within a sheet pile cofferdam.
Piling work by Norwest Holst was halted in August but is expected to restart next month.