TBM developments a main focus at NAT
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TBM developments a main focus at NAT Jun 2018

Shani Wallis, TunnelTalk

Major new large diameter TBM excavation projects and developments of the TBM technology and operation were the main focus of proceedings at the 2018 NAT, North American Tunneling conference, in Washington DC. Of the 20 technical sessions held in four simultaneous tracks across two and half days, two were dedicated to TBM technology and in others, including sessions on innovation, transportation case histories, water and wastewater project delivery, and rock tunnelling, TBM application and operation had a high representation. Immersed tube tunnelling was under represented with one paper including details of how the original plan for another immersed tube to increase highway capacity for the Thimble Shoal crossing of Chesapeake Bay being overtaken by the large diameter TBM bore alternative. Sessions on risk management and contract strategies also highlighted these topics from the point of view of TBM tunnel project excavation. As an alternative to TBM operations, a technical session was dedicated to the application of sequential excavation methods.

Stacked station platforms in a single-tube metro configuration
Stacked station platforms in a single-tube metro configuration
Staggered side platforms design for the Toronto Scarborough subway extension
Staggered side platforms design for the Toronto Scarborough subway extension

Other technical sessions included discussions on project controls and challenging design issues as well as dedicated sessions to resiliency of cities and the design of precast concrete segmental linings.

Several papers focused on major new projects for large diameter TBMs including the single bore BART to San Jose project in California on which the platforms for three underground stations will be included in a stacked configuration in the single-tube, double-track tunnel and the large diameter single-tube, double-track tunnel with stacked platforms included at stations, planned for downtown Seattle for its new $5.5 billion West Seattle to Ballard link light rail extension as the first project to benefit from the $54 billion Sound Transit bond measure approved by Seattle area voters in November 2016.

Also designed as a single-bore, twin-track running tunnel will be the new Scarborough subway extension in Toronto. As a significant departure from the traditional arrangement of twin tunnels for subway in Toronto, concerns for operational safety at stations and the need for crossovers lead the Toronto design team to prepare a stacked station platform alternative that would keep the station platforms on each side of the large diameter running tunnel and in a staggered configuration with the TBM being moved sideways by 3.6m and re-launched on the adjusted alignment (Fig 2).

Alignment of bored tunnel for Thimble Shoals crossing
Alignment of bored tunnel for Thimble Shoals crossing

A single-bore double-track TBM tunnel as well as elevated guideways, are being considered for a new cutting edge project to provide maglev train travel between Washington DC, Baltimore and on to New York City. Reaching operating speeds of 500km/hr (311 miles/hr) the maglex trains would shorten travel times between Washington and Baltimore to 15 minutes and Washington to New York City in an hour.

The evolution and challenges of designing the lining of the 17.4m diameter SR99 double deck highway tunnel in Seattle was the topic of a paper in the precast concrete lining design session. Of four papers that discussed the design and construction of the large 43ft 3in diameter TBM bored alternative to the immersed tube options for the Thimble Shoals crossing of Chesapeake Bay, the presenter explained that the bolts of the segmental lining will be removed and only the bolts of the first 21 rings for the tunnel from the two artificial islands, which are specified as stainless steel, will remain in place permanently. In response to a question as to when the standard bolts will be removed and why, the present explained that the timing of taking the bolts out is yet to be confirmed but it would be after the TBM drive is completed. A question as to why take out the bolts was answered after the session because the project owner want to eliminate any potential for the standard bolts to corrode and drop out onto the highway traffic. The discussion prompted discussions over the coffee break as to whether engineers were proponents of taking segmental lining bolts out or leaving them in place. Incidences of segmentally lined tunnel failures came into the discussions including the failure in 2017 of the Rastatt railway tunnel drive in Germany and the Foshan metro tunnel in China.

Another very successful NAT by UCA of SME

As is always the case with the SME-organised tunneling conferences for NAT, and each alternate year, the RETC, all the papers presented were of a very high standard of detail and composition. The detailed engineering and project planning information revealed by each of the presenters in their presentations and by their corresponding printed papers in the conference volume of proceedings is impressive and provoked interesting Q&A periods within each session.

The next tunneling conference to be organised by the SME will be the 2019 RETC conference to be held in Chicago from 16 to 19 June when the program, designed more for the construction side of the industry (NAT focusing more on design and planning) will include sessions on tunnelling in difficult ground conditions, control of ground water and gas infiltration, drill+blast excavation, tunnel rehabilitation, shafts and mining, as well as case studies and the experiences of operating TBMs of all types on projects from across the USA, North America and internationally.

TunnelTalk enjoyed the NAT gathering and program of the conference in Washington DC and looks forward to meeting friends and colleagues again at RETC in Chicago next year.

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