In Fairfax, Virginia, the installation of a new water main came with more than its share of challenges. Diabase rock, exceeding 50,000 psi (345MPa) UCS, halted initial excavation attempts by the project’s first contractor (using a modified Lovat-built double shield). The Corbalis to Fox Mill Water Main Project ultimately required a new contractor, Southland, and a new machine. When Southland was awarded the new construction contract, the Lovat TBM was stranded 2,400ft (730m) into its drive, with the majority of tunneling still left to go. However, on May 8, 2014, a 7.3ft (2.2m) Robbins double shield TBM broke through, completing 1.7-miles (2.7km) of hard rock tunneling and setting an unofficial North American record for excavation of high rock strengths in the 6.5ft to 9.8ft (2m to 3m) TBM size class.
For John Marcantoni, Project Manager at Southland Contracting, the accomplishment cannot be overstated: “The rock that we encountered was greater than 50,000 psi in many places. From start to finish, it took about 1.5 years to go 1,890m (6,200ft).”
The 1.66-mile (2.68km) tunnel is part of a larger 2.14-mile (3.44km) scheme, approximately 2,500ft (762m) of which was open cut. The remainder of the tunnel required a TBM due to its depth and close proximity to highways, buildings, and other structures. Drill and Blast was prohibited due to the presence of two active petroleum lines running parallel to the tunnel and providing service to nearby Dulles Airport.
Multiple challenges presented themselves during tunneling: “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 a rigorous maintenance schedule,” said Marcantoni.
The strength of the rock not only tested the machine, 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 continued to push forward. Overall, the TBM averaged 12ft-13ft (3.7m- 4m) per day, with 10-man crews working in 10-hour double shifts.
“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. After extensive research on the record, the only similar project found in North America was excavated by a 7ft (2.1m) Robbins double shield in Montreal, Canada. Bored in 1998, the St. Jerome Tunnel was excavated through rock at maximum strengths of 42,600psi (294MPa) UCS.
Now that tunneling is complete in Fairfax, it will take around four months to install the final carrier pipe, a 54in (1.4m) water main, and complete all tie-ins. Project owner Fairfax Water Authority expects to be able to expand capacity of their existing water transmission system by the end of 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 tough TBM will go on to be used by Southland on another rock tunnel in Atlanta, Georgia.