On Dec. 4, 2012, a steadfast Robbins TBM made its final breakthrough in Zurich, Switzerland. The Milchbuck Safety Tunnel required two methods of excavation due to a split in geology—1,000 m (3,280 ft) of TBM tunnel were bored through molasse rock of about 80 MPa (11,600 psi) UCS, while a 400-m (1,300-ft) conventionally dug section was located in unconsolidated rock, mainly moraine. The machine, for contractor Marti Tunnelbau AG, completed tunneling in March and was stopped in order to allow the completion of the conventionally excavated tunnel.
The TBM breakthrough marks the completion of the fourth tunnel for the 4.15-m (13.6-ft) 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 4.15 m (14.4 to 13.6 ft) as well as new cutterhead structures, including muck buckets and grill bars. Back-up refurbishment and design was done 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 through 400 m (1,310 ft) of compact rock and 600 m (1,970 ft) of fractured zones with over-break. “The performance went very well. It was hard work, but we had a special back-up to apply 360 degree shotcrete in these sections,” continued Güggi. The design allowed for a 20 cm (8 in) 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 meter by meter with a tunnel excavator, applying a lining of steel arches and shotcrete.
The safety gallery runs parallel to Zurich’s Milchbuck Road Tunnel, a heavily used passage that needed to be updated to the country’s modern safety standards. The safety tunnel, for Swiss federal office ASTRA, includes five cross passages currently under construction.