A Robbins machine has successfully achieved final breakthrough on a complex safety tunnel for Zurich's Milchbuck Road Tunnel in Switzerland.
The 4.15m diameter main beam TBM broke through for contractor Marti Tunnelbau, on behalf of the Swiss federal office ASTRA.
The Milchbuck Safety Tunnel, which runs parallel to the main A1 highway road tunnel, is a heavily-used passage that required urgent updating to comply with new safety standards. Its construction required two methods of excavation due to a split in geology; 1,000m of TBM tunnel were bored through molasse rock of about 80 MPa (11,600 psi) UCS, while a 400m conventionally excavated section was located in unconsolidated rock, mainly moraine.
Breakthrough celebration at Milchbuck
4.15m diameter Robbins main beam TBM
The TBM breakthrough marks completion of the fourth tunnel for the 4.15m diameter Robbins main beam machine, which underwent major refurbishment in 2002. Robbins carried out the machine's latest rebuild as well, which included a diameter change from 4.4 to 4.15m as well as new cutterhead structures that included muck buckets and grill bars. Back-up refurbishment and design was completed by the contractor.
"We are proud the TBM refurbishment was managed in such a short time, and the TBM excavated without any technical interruptions," said Thomas Güggi of Marti Tunnelbau Ltd.
The TBM successfully excavated 400m through compact rock and 600m through fractured zones with overbreak.
"The performance was very good. It was hard work, but we had a special back-up to apply 360° shotcrete in these sections," said Güggi. The design allowed for a 20cm thick wet application using two shotcrete robots for the full length of the tunnel.
With the TBM bore complete, work ramped up in the difficult ground of the conventionally excavated tunnel. In order to excavate this section, crews created a reinforced crown of jet grout. Beneath the jet grout umbrella, crews mined metre by metre with a tunnel excavator, applying a lining of steel arches and shotcrete.
The safety tunnel includes five cross passages, which are still under construction. The main road tunnel, at 1.9km long, is one of the most-used single bore tunnels in Switzerland.
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