UNEXPECTED GROUND movements around Yorkshire Water's £70M Humbercare sewerage tunnel have been revealed as the cause of its collapse last year.
Engineers now believe that tidal effects on a layer of soft peat caused the ground to move around the tunnel, resulting in fatigue failure of the non-bolted segmental concrete lining.
'Our investigations have 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.'
Extensive ground investigation since the collapse found the layer of heavy peat 40m below the surface, some 18m below the tunnel drive.
Water and silt burst into an access shaft and a 150m long section of the tunnel on the night of 16 November 1999 (NCE 18 Nov 1999). The collapse produced 2m of surface settlement in a 50m wide trough under a car park in Hull.
Repairs are now reaching a critical stage. Work to clear the damaged tunnel has started following recent re-opening of the collapsed shaft.
Ground freezing - using liquid nitrogen rather than conventional brine - will stabilise the tunnel before hand excavation for a sprayed concrete lining.
This will allow access to the back of 'Maureen', 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, nicknamed 'Gloria'.
Miller is optimistic that damage to the trapped TBM - which has been trapped for eight months - will be light. '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.