Herrenknecht to face the pressure12 September 2005
The project has had a very high profile in Sweden since it was first started in 1992, having challenged the efforts of two previous contractors (one of which was Skanska) and was the cause of extensive groundwater contamination during the grouting operations of one of those attempts. Previously, both TBM as well as drill and blast techniques have been attempted through the ground that is predominantly gneiss with many fissures and clay bands. Groundwater is a particular problem and the 10.53m diameter Herrenknecht TBM has been designed to operate in water pressures of up to 13 bar during tunnelling.
Simon Taylor, Construction Coordination Manager for the client Banverket, told T&TI that the plan is to run the TBM in open mode for most of the 2 x 5km outstanding balance of tunnelling on the project. Closed mode would be used for softer, unstable ground and especially for high water flows. T&TI was told that conversion from open to closed mode could take up to two days, but the TBM can be "closed-up within an hour".
In open mode, the TBM is run as a hard rock machine without supporting pressure. Cuttings are transported by conveyor after passing through a muck hopper that is moved forward into the excavation chamber as part of the open mode configuration. For closed mode, the hopper and conveyor are moved back, closing the chamber in the process and the TBM is run as a slurry shield. Excavated material is pumped to surface after passing through a stone crusher.
Operated by a crew of 16, the TBM is expected to advance up to 12m per 24hrs in good rock. A 10.12m i.d., 0.54m thick reinforced concrete segmental lining will be installed as the machine advances.
Taylor told T&TI how project environmental limitations imposed by the Swedish courts have been addressed in the specification of the TBM. To ensure drawdown does not affect the overlying flora, fauna or streams, dewatering must not exceed 100l/sec as a rolling monthly average. This means the TBM has to be able to pre-treat the ground to manage leaks into the works and in addition, have the capability of being closed under extreme conditions.
Taylor said if the TBM had to stay in closed mode and any of the 70 cutters be exchanged under pressure, mixed gas diving operations would have to be carried out by specialist divers. The TBM is equipped for hyperbaric works, including a transfer shuttle to take the divers from their living quarters into the TBM.
At the rear of the back-up is a mini water treatment facility including cyclones to remove the bulk of any suspended solids to minimise sedimentation in the dewatering system. During open mode, the surface slurry treatment plant will be used as a water treatment facility for process and ground water that is then discharged to the sea by means of an 8km long pipeline. When operating in closed mode, the plant will revert to a slurry treatment function.