GROUND FREEZING began this week on the collapsed section of Yorkshire Water's £70M Humbercare tunnel in Hull, in the first stage of a recovery operation that could last six months and cost several million pounds.
Main design & build contractor Miller Civil Engineering said the cause of the collapse last November had not been confirmed. The 4m diameter precast lining buckled when high pressure water and silt burst through, causing ground above to subside by up to 2m (NCE 25 November 1999).
Miller managing director Graham Grundon said: 'Unexpected bad ground conditions seem more likely than any problem with the lining - we will have a better idea once we are able to get back into the tunnel.'
The ground freezing will seal the 150m long damaged section with ice 'plugs' and freeze ground surrounding the tunnel bore. The tunnel will then be pumped dry through an existing access shaft to enable repairs to begin.
Grundon said it was hoped the Lovat earth pressure balance tunnel boring machine currently trapped in the flooded tunnel could be recovered and refurbished. 'We will be starting a fresh drive with our other TBM from an enlarged existing access shaft 200m from the failed section,' he added.
This second TBM will complete the outstanding 2km of tunnel, leaving the first TBM to eventually drive 200m to complete the tunnel.
Progress on the 10.6km sewage tunnel had been ahead of programme, despite working through difficult ground conditions. Miller was tunnelling at a depth of 21m in silty sands when the section of precast concrete tunnel lining 150m behind the TBM began to deform.
But Grundon refused to speculate on the final cost of the rescue operation. 'We are operating under a partnering, cost re-imbursable contract,' he said. 'Until we know exactly what went wrong, we have to assume the extra costs will be re-imbursable.'