GROUND FREEZING began last month on the collapsed section of Yorkshire Water's £70M Humbercare tunnel in Hull as the first stage in a recovery operation that could last six months and cost several million pounds.
Tunnelling contractor Miller Civil Engineering said the cause of the collapse last November had not yet been confirmed, although it was thought that unexpected ground conditions was the most likely explanation. The 4m diameter precast lining buckled when high pressure water and alluvium burst through causing ground above to subside by up to 2m (Ground Engineering December 1999).
Ground freezing is being used to seal off a 150m section of the tunnel, allowing it to be pumped dry so repairs can be carried out.
It is hoped the Lovat earth pressure balance TBM trapped in the flooded tunnel can be recovered and refurbished.
A second TBM will complete the remaining 2km of tunnel from an enlarged access shaft 200m from the failed section.