EDMUND NUTTALL last month had to halt tunnelling work for Southern Water after cracks appeared in the concrete lining of its 4km long tunnel in Portsmouth.
The 2.85m diameter Lovat earth pressure balance tunnel boring machine ran into trouble as ground conditions deteriorated only 200m from completion of its run at Eastney pumping station.
No one was hurt in the collapse which took place below allotments and there is no risk of property damage above. Compressed air is now being used as a temporary support for the tunnel.
Ground movement in the surrounding silty sand and clay is thought to have caused the cracking in the concrete segment lining.
But project manager Damon Elliott said it was too early to tell the precise cause.
Ground freezing is the favoured rescue option, although other approaches are being considered.
Work will be carried out from the allotments after agreement has been negotiated with the holders, some of whom have tended plots for more than 20 years.
The tunnel is part of Southern Water's £100M water improvement scheme for Portsmouth and will transfer treated wastewater from Budds Farm tratment works to the Eastney pumping station.