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Workers fought to save collapsing tunnel

TUNNELLERS BATTLED underground to save Yorkshire Water's collapsed tunnel in Hull last week while concrete spalled around them, the client's project manager told NCE this week.

Yorkshire Water's Steve Tindall said workers on the £70M sewer tunnel attempted to reinsert temporary bolts between segments after the precast lining rings began to distort and spew water (NCE 18 November).

Only after the crown had dropped by 0.5m and the invert had risen 0.3m, allowing water and silty sand to enter the tunnel, did they retreat to safety, two hours after the alarm was first raised, said Tindall. Senior site supervisory staff from Miller and Arup were then called out to investigate.

Contractor Miller had been progressing the tunnel at up to 200m per week through silty sand in the weeks leading up to the collapse. Despite the difficult conditions, the project was 12 weeks ahead of programme with nearly 8.5km of the total 10.57km of tunnel completed.

Tindall said no decision had been made about how to proceed. 'One option is to put people in but there are lots of possible options,' he said. 'We are still investigating.'

Up to 15 lining rings are thought to have been damaged.

The £70M tunnel is part of Yorkshire Water's £200M Humbercare wastewater scheme linking west and east Hull with a new wastewater treatment works at Salt End to the east of the city.

Tindall said that there had been no indication of poor soil conditions and emphasised that investigations to discover the cause of the failure were still ongoing. Much of the ground in the area of the collapse is reclaimed from Hull's dock area.

Contractor Miller is carrying out the design and build tunnelling contract under an IChemE Green Book cost reimbursable contract. Ove Arup provides construction management and contract supervision.

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