Robbins TBM Completes Swiss Safety Tunnel
The crew celebrates the breakthrough of the Robbins TBM at the Milchbuck Safety Tunnel, December 2012.
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.
Sounds like a great geotechnical project and congratulations to all of the associated crews for bringing this project to a successful completion. It is quite difficult to conduct TBM operations in geologically complex areas such as Swiss Safety Tunnel. Encountered rocks with molassic, glacial and competent bedrock characteristics certainly provided a complex lithological anomaly and warranted adjustments in TBM operations due to both geological and structural constraints. My experience involving geotechnical consulting assignment with the New York City Water Tunnel # 3 enabled me to gain deep understanding of the hardrock geology in reference to geotechnical attributes. Often TBM drilling rate was less than 100 feet a day due to structural and lithological anisotrophism associated with the subsurface rocks (600 to 800 feet below the surface). I am just curious to find out if TBM drilling rate was remarkably different with respect to molassic, glacial, and competent bedrock. How about other features involving degree of weathering, jointing, fracture trend, or sharp or gradual lithological contrast that might have forced to redesign or recalculate TBM setting. Also using over 11,000 psi in thick molassic section, were there any large-scale roof collapse or side-wall caving? How about water flow associate with molassic unit which I suppose had higher permeability. TBM operations in both soft and hardgrounds always pose a great geoengineering challenge in terms of drilling rate and apparently were resolved by having meticulous geotechnical design in place.