UNEXPECTED GROUND movements are thought to have caused last November's collapse of Yorkshire Water's £70M Humbercare sewerage tunnel in Hull.
Engineers now believe that tidal effects on a 40m thick layer of soft peat 18m below the tunnel drive caused the ground to move around the tunnel, resulting in fatigue failure of the non-bolted segmental concrete lining.
'Investigations revealed that the soft ground is quite localised, ' said Graham Grundon, managing director of tunnel contractor Miller Civil Engineering. 'We don't expect any further problems, ' he added.
Water and silt burst into an access shaft and a 150m long section of tunnel causing 2m of surface settlement in a 50m wide trough, under a car park.
Repairs are now reaching a critical stage.
The collapsed shaft has been reopened and clearing of the damaged section has started.
Ground freezing - using liquid nitrogen rather than conventional brine - will stabilise the tunnel before hand excavation for a sprayed concrete lining.
Work will allow access to the back of the trapped 3.2m diameter Lovat TBM which became stuck in the collapse. Miller was forced to pressurise and flood 5km of the tunnel, and then plug it with an ice wall to stabilise the ground. The final 2km of the drive has since been completed by a replacement TBM.
Miller is optimistic that damage to the trapped TBM will be minor.'It may need a bit of work, but is a new machine, and well sealed, ' said Grundon. He expected it to finish the drive and be extracted through an existing shaft by the end of December.