Miller Civil Engineering is in the final stages of recovery work on the Humbercare sewer tunnel in northern England following large ground movements around shaft T3 (see News, Oct ’00).
The underground scene is bizarre even for experienced tunnellers as the wail of escaping nitrogen from the ground-freeze pipes greet you in one leg of the recovery excavation, while oily slime from the, now drained, flood water coats the opposite leg of the tunnel to the abandoned Lovat TBM ‘Maureen’.
Crews were making preparations for a horizontal freezing of the final 20m stretch to the east of T3 when T&TI visited the site. This final excavation will link the affected section with the sound tunnel and shaft T3A. Miller is using a Schaeff excavator with interchangeable tools on the articulated boom to excavate the face. A hydraulic hammer is necessary in hard frozen ground but a rotary drum cutter can excavate the softer central material.
To the west of T3 a trapped loco was something of a blessing in disguise as it acted at a partial dam to the solid material entering the tunnel from the collapse, leaving water to fill the tunnel. Miller’s second Lovat TBM, ‘Gloria’ had been used to complete the drive, in the opposite direction, from shaft T2 which was originally planned for ‘Maureen’. This was recovered from a new shaft T2A, leaving 50m to be excavated.
Once the hydraulic pipes and components of ‘Maureen’ have been flushed and checked, the TBM will be reactivated using a portable hydraulic power pack, making it unnecessary to renovate the electrical systems which are bound to have been damaged beyond repair by standing in water for 15 months.
Steve Tindall, project manager for Yorkshire Water forecast that it should only take about five days (ten shifts) to complete the 50m of tunnel once the TBM was operational. ‘Maureen’ is in remarkably good condition considering its recent neglect. Even leakage through the tail seal is negligible, although tubs of the Condat tail-seal grease, abandoned in the rush to leave the tunnel, have joined with hydraulic fluid to contribute to the slippery conditions.
All parties to the project – Miller (with geotechnical partner Beton und Monierbau), Arup and Yorkshire Water – are keen to emphasise the spirit of partnering in which the recovery has been carried out to the benefit of all. The Miller contract is of a ‘cost reimbursable’ type. On the question of the extra cost Peter Chamley, project manager for consulting engineers Arup, commented, "The insurers are responding."
There is still no final report on the cause of the collapse, but Chamley remarked that if there was a single cause the results of a separate investigation conducted by Arup’s Cambridge office would probably be known by now. A combination of circumstances seemed likely.
