USA Latest to Use Large-diameter TBM for Highway Project
By Jim Rush
One of the most talked about tunnel projects in the United States over the past several years has been the Alaskan Way Viaduct replacement tunnel in Seattle. The project has been a hot topic of debate politically, with officials and citizens alike aligning in either the pro-tunnel or no-tunnel camps.
The project has finally been approved and moving forward as a tunnel – a project that will enhance one of the city’s spectacular natural assets: the waterfront. Plans call for a bored tunnel using a 57.5-ft (17.53-m) tunnel boring machine (TBM), which will be the world’s largest. (Temporarily, at least. German TBM manufacturer Herrenknecht has taken an order for a 19.25-m TBM to bore a road tunnel in St. Petersburg, Russia). The Alaskan Way machine has been ordered by Seattle Tunnel Partners (a joint venture between Tutor Perini Corp., Dragados USA, Frank Coluccio Construction Co., Mowat Construction, HNTB Corp., and Intesca) from machine manufacturer Hitachi Zosen in Japan.
There have been several high-profile highway tunnel jobs constructed over the past decade across the globe that have pushed the envelope in terms of large-diameter TBM tunneling. In fact, successful completion of projects in Madrid and Shanghai helped pave the way for the Alaskan Way Viaduct replacement tunnel.
In Madrid, the M30 highway was a massive transportation revitalization project that involved refurbishment of an inner ring artery. The overall project consisted of about 60 miles of new road construction, including 35 miles in tunnels. The project began in 2004 and was completed in 2007. Included in the project was the use of two earth pressure balance (EPB) TBMs, one 15 m and the other 15.2 m, that represented the world’s largest TBMs at the time they were built.
In Shanghai, twin 15.43 m TBMs excavated tunnels connecting Changxing Island and the Pudong District beneath the heavily travelled Yangtze River, further pushing the limits of technology. Tunneling began in 2006 with the approximately 4.5-mile long bores breaking through in 2008.
Back in Seattle, the need for the Alaskan Way Viaduct replacement project became apparent following the Nisqually earthquake that rocked the Puget Sound area in 2001. While the viaduct survived the 6.8 magnitude quake, the structure was damaged and more at risk than ever before, leaving Washington State Department of Transportation (WSDOT) officials looking for a replacement option.
Early plans including constructing side-by-side bores as a tunnel alternative to the viaduct, but as tunneling technology advanced and larger and larger bores like the ones in Madrid and Shanghai were successfully completed, the idea of a large single bore began to take root.
By selecting a tunnel, WSDOT is able to keep the viaduct – one of two north-south routes serving the region – open to traffic while its replacement is being built. “To have that corridor out of commission for several years while the viaduct was being replaced was a non-starter from a community and economy standpoint,” said Matt Preedy, WSDOT’s Alaskan Way Viaduct deputy program administrator. “Putting the tunnel underground outside of where the viaduct is allows us to construct a replacement without disrupting the corridor.”
The large single bore alleviated some concerns planners had regarding the twin-bore plan, namely the construction of cross-passages every 600 ft along the alignment. “The cross passages presented challenges from a risk perspective,” Preedy said. “We have glacial soils in the alignment with some running sand and hardpan – it is a real mixed bag. Constructing a single bore eliminates the risk and cost associated with breaking through the tunnel linings every 600 ft under the downtown core with high-rises and utilities above.”
In designing a tunnel of this magnitude, Preedy says the most important step is to understand the ground. WSDOT developed a checklist of features it wanted to have on the TBM, including redundant systems to monitoring excavation, but it left many of the specifics up to the design-build contractor.
“One of the things we learned as we were researching bored tunnels around the area and around the world is that the entity that is best suited to determine the features of a TBM is the entity that is operating the machine and building the tunnel,” Preedy said. “Ultimately, the risk of performance here is on the design-builders, so it is in everyone’s best interest to let them make the best choices to manage the risk.”
The machine ordered by the contractor is an EPB TBM, measuring 326 ft long and weighing 5,500 tons. The machine will take 14 months to build and is scheduled for delivery in April 2013. The machine includes 28 injection ports pump bentonite behind the shield and prevent ground collapse. Additionally, it has 18 grout ports to fill the annulus behind the lining using a two-component, quick-setting grout.
One novel feature of the machine is the ability to change cutters in atmospheric conditions. Pressurized interventions will be needed for inspection, but eliminating the need for interventions for routine changes will reduce downtime. Completion of the overall project is scheduled for the end of 2015. The tunnel will stretch some 9,190 ft and reach depths up to 200 ft below ground level.
Hitachi Zosen has a long history of building TBMs with more than 1,300 machines delivered around the world, including a 46-ft diameter slurry TBM for the Trans Tokyo Bay Highway project. Currently, a 21-ft Hitachi Zosen EPB TBM is being used on the University Link project for Sound Transit in Seattle. The tunnel, stretching from Capitol Hill to Pine Street, is being built by a joint venture of Jay Dee, Frank Coluccio Construction Co. and Michels.
Jim Rush is editor of TBM.