PILING RESTRICTIONS have forced a major redesign on the Hungerford footbridge across the Thames in London, contributing to a £20M cost escalation, it emerged this week.
The extra cost of the redesign - along with knock on costs - will be met by a £16.5M grant from Transport for London, ratified last week by the Greater London Authority.
Another £3M of extra funding is coming from Westminster and Lambeth councils and Railtrack.
London Underground has agreed to absorb £1M of its costs, which include loss of revenue resulting from weekend possession work on the Bakerloo Line tunnels running close to the bridge site.
The footbridge is the second new Thames crossing to suffer an embarrassing setback this year. It is just under 2km from the Ove Arup/Foster designed Millennium footbridge at Bankside which remains closed after its wobbling deck raised fears over its safety soon after it opened in June (see page 14).
On the Hungerford bridge LU has forced a major redesign on Cross River Partnership after stopping piling work in the Thames last month (NCE 10/17 August).
The current design involves construction of two footbridges, one on each side of the Hungerford railway bridge. Each cable stayed span is suspended from seven pylons founded on six piers which are piled into the river.
The new design, by the bridge's engineer WSP and contractor Costain, will move the upstream pylon nearest the north bank of the Thames onto the river bank to avoid having to pile in the river near the Bakerloo Line.
This backstayed pylon will be replaced by an A-frame structure, fundamentally altering the symmetry of the 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.
'Moving the pylon onto the bank takes us out of LU's restrictions on piling into the river bed within 15m of the Bakerloo Line, ' Westminster City Council project director Gareth Hardwick told NCE.
LU has imposed onerous restrictions on piling in the Thames near its tunnels. Anyone piling within 15m of LU's tunnels must pay LU for possession time and close the tunnels. Closures are needed because of the risk that piling may set off unexploded Second World War bombs.
These are known to have penetrated deep into the river bed and could flood the tunnel if detonated.
The designers will sidestep this restriction by moving the pylon to the river bank, where LU allows piles to come within 3m of its tunnels.
LU claims that Cross River Partnership ran into problems with the piling restriction because it had not realised the full cost of the 15m restriction on piling in the river.
Piling work by Norwest Holst halted a month ago and most plant has been idle since then.
CRP is now seeking planning permission for the A-frame pylon.