A FIREBALL caused by polystyrene fill has caused serious damage to a key bridge structure on the Channel Tunnel Rail Link (CTRL).
An investigation has started to find ways of salvaging parts of the blackened structure at Brewers Road near Cobham in Kent.
The bridge is part of Alfred McAlpine/Amec joint venture's contract 330.
The fire also raises wider safety concerns about the risk of accidental fire breaking out in polystyrene fill used for road and rail embankments.
The fire occurred on Wednesday 15 February. It broke out in a void between the top of a 75m long railway tunnel and a section of road which crosses the CTRL route. The void was filled with polystyrene blocks and enclosed in a protective plastic membrane.
Vandalism was initially suspected but Kent police told NCE its investigations have not found any evidence of the cause of the blaze.
Security staff at the site noticed the fire but abandoned attempts to put it out once it became an inferno.
Kent Fire Brigade said it was a serious fire and six units attended. 'It was going well when we got there, although we controlled it quickly enough, ' said a fireman.
The intensity of the heat is understood to have damaged the 1.2m thick road slab beyond repair, burning off the blacktop surfacing. Its intensity spalled concrete and twisted reinforcement. The surface of the railway roof slab below is also believed to have been damaged, along with the edges of the bored piles which form the void sides.
Reconstruction work is likely to prove complex because the original construction was 'top down', making installation of temporary works for reconstruction work difficult.
The structure has been cordoned off while police and engineers examine the scene.
This has caused further disruption to construction work because the rail tunnel slab, which was being used as a road to ferry materials between different sections of the project, is closed at the bridge.
Investigators will also examine the broader safety risks posed by using polystyrene as fill for road and rail embankments. The fill extended beyond the road slab to form an embankment and was to have been covered with topsoil.
Investigators will be expected to examine the risks of a vehicle coming off the road catching fire, puncturing the fill and setting it alight.
A CTRL spokesman played down the fire risk. 'At Brewers Road, the area in question is protected from vehicles by a road safety fence. The polystyrene will be covered with an impermeable membrane which is in turn protected by at least 700mm of soil cover and vegetation.'