INVESTIGATIONS INTO the tunnel collapse that brought construction of the Humbercare sewerage tunnel in Hull to a halt this week were due to begin today.
A spokesman for client Yorkshire Water said the tunnel filled with slurrylike material following collapse of a completed section of tunnel wall at 1.30am on Tuesday morning. No one was hurt in the accident.
Tunnelling contractor Miller Civil Engineering immediately shut down operations and is awaiting delivery of compressed air equipment before venturing into the damaged tunnel.
Miller's £50M contract is to construct a 10km long tunnel as part of Yorkshire Water's £200M Humbercare sewage collection and processing scheme. Two Lovat earth pressure balance tunnel boring machines were being used to construct the 4m diameter tunnel at a depth of 21m through sand and silty ground at 2 bar pressure with grouted trapezoidal precast concrete segments.
Tunnelling had been through poor ground, but was advancing well and ten weeks ahead of schedule when the col-lapse occurred. A rupture was noticed over a 15m long completed section 150m behind the TBM. Residents in properties above the tunnel were evacuated after 2m deep subsidence occurred at the surface.
A spokesman for Yorkshire Water said the contractor was focusing on recovering the situation and was working to seal the tunnel either side of the damaged section and cap the two access shafts adjoining it. Compressed air, he said, would then be applied to clear water and allow access for inspection and eventual repair.
Miller tunnelling director Chris Hughes said he was unwilling to speculate on the cause of the collapse. However, he confirmed that the second TBM on site had recently completed its 5km drive and may be re-deployed to work back towards the damaged tunnel section to make up time.
Miller managing director Graham Grundon also confirmed that once the ground around the damaged tunnel was stabilised, the TBM could then be re-driven to repair the damaged tunnel.
Tunnelling experts expressed sur-prise that a completed section of tunnel should collapse and claimed the problem was more likely to lie with construction rather than design. Poorly aligned or inadequately sealed joints between segments giving way under external pressure are likely to be investigated, they said.
Yorkshire Water's Humbercare project should meet a December 2000 deadline imposed by the European Union Urban Wastewater Directive, demanding all sewage for coastal discharge receives secondary treatment.
A Department for the Environment Transport and the Regions spokesman said failure to meet the deadline could make Yorkshire Water liable for prosecution by both the EU and Environment Agency.