Tucked away on a small jobsite in a quiet neighborhood bordering a busy interstate highway, a Robbins Earth Pressure Balance (EPB) machine underwent Onsite First Time Assembly (OFTA). The OFTA method allows for TBMs to be initially assembled onsite, and results in time and cost savings to the contractor. On Monday, Nov. 17, 2014, unbeknownst to commuters driving by, the 6.65-m (21.8-ft) TBM rumbled to life and began its journey south into the city of Seattle.
The machine, for contractor JCM North Link LLC (a joint venture of Jay Dee, Coluccio, and Michels) is excavating for the North Link Project, which is an extension of Seattle’s light rail system. The project required twin 3.6-mile tunnels through glacial till and
sand. JCM utilized a refurbished Hitachi Zosen machine in its fleet for the first tunnel, which launched in summer 2014, and the Robbins EPB for the second. This isn’t the Robbins machine’s first project; prior to North Link, it bored mixed ground tunnels for Singapore’s Downtown Line.
Urban tunneling brings unique challenges to the project. The tunnel will travel under a university campus, where there is concern about noise and vibration from the machine’s movements. Research and preparation have been done to mitigate these foreseeable issues. The compact jobsite also caused complications during assembly, with a long, narrow setup that required creative storage methods for parts and systems.
JCM is confident that refurbished TBMs are the best solution given the tight project schedule. Designed for use on multiple tunnels in mixed ground, the Robbins EPB features a steel frame 30% heavier than other EPBs on the market, with components intended for 10,000 hours of workable life.
“Ten years ago, EPB tunneling in mixed ground below the water table was not that common…it would have been considered a big risk to use a refurbished machine. Now, many EPBs have been specified to deal with more challenging conditions, so there are a quite a lot more out there to be refurbished. This [solution] increases our ability to get a TBM to launch sooner, and is also more cost-effective,” said Glen Frank, Project Manager for JCM North Link, on using a refurbished machine.
Robbins continuous conveyors are running behind both TBMs. “We’ve had great success with Robbins conveyors [on past projects],” said Frank. “Without the conveyor, we couldn’t do this job. They cut down on costs and
vibration, and allow us to use rubber-tired vehicles, rather than trains, for transport in the tunnels. We feel that Robbins conveyors are the best quality out there.”
Both machines will travel from the North Link site in northern Seattle south to the Roosevelt station site near the University of Washington campus, adding length to the recently completed University Link tunnels. The tunnels are all part of a larger transit system scheme for project owner Sound Transit, intended to provide a quick and alternative transportation option to Seattle’s outdated surface bus system, and to help alleviate the city’s traffic congestion. The tunnels are expected to go online in 2021.