Overcoming an American hard rock challenge 11 June 2014
Desiree Willis, Technical Writer, The Robbins Company
- Eighteen months after a Lovat TBM became trapped trying to bore through some of the most challenging hard rock conditions ever faced on the North American continent, a replacement contractor with a refurbished Robbins machine completes excavation of a water main project in Fairfax, Virginia.
Southland crew celebrates after 18 months of hard rock tunneling
- Diabase rock exceeding 345 MPa UCS stopped the initial attempt of the original contractor using a modified Lovat-built double shield machine. A new contractor – Southland - and a new machine, was appointed by project owner, Fairfax Water Authority, to rescue the situation, and last month (May 2014) the 2.2m double shield TBM broke through to complete 2.7 km of hard fought hard rock tunneling. In doing so, the small diameter machine set an unofficial North American record for excavation of high rock strengths in the 2-3m TBM class.
- Drill+blast excavation was prohibited due to the presence of two active petroleum lines running parallel to the tunnel that provide service to nearby Dulles Airport.
- “The rock that we encountered was greater than 50,000 psi in many places. From start to finish, it took about one and a half years for us to excavate the remaining 1,890m,” said John Marcantoni, Project Manager at Southland Contracting. “Our issue was not water inflows, faults, or seams, it was actually the consistently hard rock. The machine was working at its highest capacity just to cut the rock. We kept pushing - a lot of our success was in maintaining the cutters and the ability to keep pushing as hard as we could. We had to operate a rigorous maintenance schedule.”
- When Southland was awarded the Corbalis to Fox Mill Water Main Project, they first had to remove the trapped TBM of the previous contractor at the 730m mark. The 2.7 km tunnel is part of a larger 3.44 km scheme, approximately 762m of which is open cut. The remainder of the tunnel required a TBM due to its depth and close proximity to highways, buildings and other structures.
- The strength of the rock not only tested the equipment, but also generated a lot of heat, making working conditions and cooling the equipment difficult. Despite the equipment challenges and much higher than normal cutter wear, the machine averaged an advance rate of 3.7m-4.m/day, with crews of 10 working in double shifts of 10 hours each.
Refurbished Robbins TBM replaced a Lovat machine
- “I don’t know for sure if this project is a record, but given the rock strengths, the number of cutters we used, and the raw heat being generated, I wouldn’t be surprised,” said Marcantoni.
- The record is all the more remarkable considering the history of the TBM. Originally built in 1999 for a sewer tunnel in Cleveland, Ohio, the Corbalis to Fox Mill Water Main is the machine’s seventh project.
- Following extensive research into the records, the only similar project found in North America was excavated by a 2.1m Robbins double shield in Montreal, Quebec, Canada. Bored in 1998, the St Jerome Tunnel was excavated through rock at maximum strengths of 294 MPa (42,600 psi) UCS.
- Now that tunneling is complete in Fairfax, it will take around four months to install the final carrier pipe, a 1.4m water main, and complete all tie-ins. Fairfax Water Authority expects to be able to expand capacity of its existing water transmission system by the end of this year (2014). The new pipeline will extend the existing water main to Fox Mill pumping station, providing additional transmission capacity to local customers.
- Marcantoni says he’s most proud of the hard work on behalf of Southland’s employees. “Their perseverance made the difference, in maintaining the machine and dealing with the heat that was generated. In particular our General Superintendent, Norm Gray and Superintendent Lamar Haynie deserve some credit. We were able to push through and make the project a success with this machine.”
- The TBM used in Fairfax will now go on to be used by Southland for excavation of another rock tunnel in Atlanta, Georgia.
Caterpillar sells all Lovat assets to China – TunnelTalk, February 2014
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