Tunnel Technology Has Made New Underground Infrastructure Projects Possible
Going under is in. Advances in tunnel boring machine technology have cleared a path for tunnel projects that in the past involved too much risk, time or cost—or were just outright impossible to execute. That includes recent improvements such as more sophisticated excavation technology that reduces surface disruption, embedded cameras for 3D mapping to increase visibility and track progress, and advanced heat and smoke detection devices for better project safety. These and other improvements are allowing teams to dig bigger tunnels at a faster pace and with more reliable project outcomes.
“A lot of new tunnel projects were originally planned as surface infrastructure because digging a tunnel wasn't considered an affordable option,” says Tom Ireland, major projects director, Aurecon, Auckland, New Zealand. Last year, the 2.4-kilometer (1.5-mile), NZ$1.4 billion Waterview Tunnel project in Auckland was completed and opened to traffic following work from Mr. Ireland's team over a seven-year period. The project was originally planned as a surface highway before new tunnel boring tech made it affordable to dig into the soft, wet earth beneath Auckland. (Before large-diameter tunnel boring technology, digging into soft ground presented the risk of a collapse after excavation.) Mr. Ireland's team is now digging under the city center for a tunnel that will accommodate a NZ$3.4 billion rail network, with the rail due for completion in 2024.
—Tom Ireland, Aurecon, Auckland, New Zealand
It's a global trend. In Asia Pacific, tunnel projects valued at US$563.8 billion are in development, according to the Construction Intelligence Center. In Europe, tunnel projects worth US$574.8 billion are underway. Those include the largest tunnel under construction, the €8.7 billion, 55-kilometer (34.1-mile) Brenner Base Tunnel connecting Austria and Italy, which is scheduled for a 2026 completion. The project is using 3D mapping techniques to identify unexpected underground obstacles.
“It's possible to achieve superior outcomes on these projects,” Mr. Ireland says, in large part because of reduced risk and increased cost certainty provided by developments in tunneling technology. Yet delivery risks remain. In general, better ROI can happen only if owners define risks upfront, set clear expectations for contractors and balance cost against the ability to deliver, Mr. Ireland says. “The more time you spend defining methods for dealing with risks, the more likely you are to deliver the project successfully.”
Here and above, Waterview Tunnel project during construction in Auckland, New Zealand
TOP PHOTO COURTESY OF SICE. BOTTOM PHOTO, COURTESY OF WILSON
In the past, unexpected ground conditions and machine breakdowns could increase costs and schedule overruns on tunnel projects by up to 30 percent, Mr. Ireland estimates. Project teams might also have needed to use multiple machines to address different ground conditions. However, today's boring machines are able to cut through most types of ground, which gives project sponsors, including governments, more confidence to invest.
For a US$2.7 billion project in Washington, D.C., USA, for instance, construction crews dug three massive tunnels under the city to divert sewer overflows that flood local neighborhoods and pollute the Anacostia River. The first phase of the project, which involved construction of the 2.4-mile (3.9-kilometer) Blue Plains and Anacostia River tunnels, was completed in March.
The project is noteworthy for both its size and the fact it was even launched, says George Hawkins. He is the former general manager of the District of Columbia Water and Sewer Authority and a founder of water infrastructure project consultancy Moonshot LLC, Washington, D.C. “We couldn't have built these tunnels 20 years ago,” he says.
With old boring techniques, the tunnel would have begun to collapse in Washington, D.C.'s soft ground before tunnel walls could be put in place. But new pressurized-face boring machines mean the infrastructure is installed, grouted and sealed as the tunnel is bored, preventing the risk of collapse, Mr. Hawkins says. For instance, the new machines spit out sections of precast linings every 6 feet (1.8 meters) while digging. Their highly pressurized faces also turn excavated material into a supportive, precast paste to make the lining more durable, he says.
“The new boring machines made the project possible,” Mr. Hawkins says.
But the new machines alone can't ensure a successful tunnel project, says Robert Goodfellow, senior vice president, Aldea Services, Frederick, Maryland, USA. “You have to take a holistic view of all the risks on a project.”
—Robert Goodfellow, Aldea Services, Frederick, Maryland, USA
He advises project owners to do in-depth assessments of ground conditions upfront and to communicate directly with contractors about potential risks identified on the project and why those risks need to be addressed in the bid. “You can't cover every scenario in a set of written documents. You have to talk to potential bidders about what worries you and why.”
For example, for the ongoing US$110 million Blacklick Creek Sanitary Interceptor Sewer tunnel project in Columbus, Ohio, USA, the city of Columbus insisted that the tunneling process not disturb any local water wells. To meet the sponsor's requirement, the contract restricted the use of dewatering technology in areas that could disrupt local wells. At first, contractors pushed back on the requirements, Mr. Goodfellow says. “But once we explained the issue with the wells, they understood, and they bid the project accordingly.” —Sarah Fister Gale