Transportation Troubleshooting: Lessons About Sharing Risks in Major Tunnel Projects
As Washington State Secretary of Transportation, I oversaw the development phase leading up to construction of the State Route 99 Tunnel, also known as the Alaskan Way Viaduct replacement tunnel. The 1.7-mile underground tunnel features a state-of-the-art earthquake-resilient double-deck thoroughfare, and it replaced one of Seattle’s most earthquake-vulnerable highways.
The project’s headlines included its many successes and challenges. But beyond the headlines, some key best practices in design-build delivery—especially the approach to apportionment of risks—made the project a winner for the state’s taxpayers.
The Importance of Risk Management
Along with the project’s ambition, complexity and public benefits, its risk-management provisions in contracting figured in the American Public Works Association’s decision to honor the State Route 99 Tunnel as a Project of the Year after it opened in 2019.
Replacing the aged and seismically vulnerable Alaskan Way Viaduct, the State Route 99 Tunnel was built by one of the largest-diameter tunnel boring machines (TBM) in the world: 57.5-foot-wide “Bertha,” named after Bertha Landes, mayor of Seattle from 1926-28 and the first female mayor of a major U.S. city. The TBM method was chosen over a cut-and-cover approach to allow the viaduct to continue handling traffic during construction and avoid massive disruptions to businesses along Seattle’s waterfront. May 31, 2023, marks the 10th anniversary of Bertha’s cutter head being lowered into place to begin her journey under the city.
While Bertha stopped advancing and required two years of highly complex engineering and construction work to repair, the Washington State Department of Transportation (WSDOT) and the design/builder disputed the causes of that stoppage. Because WSDOT took great care to identify risks and apportion them appropriately between the state and the design-build team, Washington taxpayers didn’t have to pay for the cost overruns triggered when Bertha broke down. As America’s transportation infrastructure managers plan new tunnel projects, many of which will be funded by the Bipartisan Infrastructure Law, these principles should guide procurement and their resulting contracts.
“The State Route 99 Tunnel really clarified the importance of delineating risks,” says my colleague Ian Chaney, senior vice president and WSP’s geotechnical and tunneling national market leader. “For large tunnel projects, owners cannot reasonably expect to shed all risk, but rather to share risks as efficiently as possible and assign to the contractor those risks they are most well-suited to cover.”
Improved Prospects for Tunnels
While Bertha still ranks as one of the widest TBMs ever built, TBM capabilities have increased dramatically in other ways since she was deployed. “Today’s TBMs are significantly more capable of accommodating adverse subsurface conditions than older technology,” according to Chaney.
Chaney also is enthusiastic about new fiber-reinforced concrete linings that are replacing traditional steel reinforcement in many tunnel projects. “Because they can be constructed without conventional bar reinforcing, tunnels built with this technology have the potential for a greater design life that can exceed 100 years,” he says. And as vehicles shift from internal combustion to electric drive, expect our existing infrastructure to be evaluated and potentially retrofitted to better accommodate the different design fire characteristics of battery-electric vehicles.
Of course, tunnels usually have cost disadvantages relative to bridges, and modern technology hasn’t fundamentally changed that. But as right-of-way acquisition and permitting costs become more expensive, tunnels—especially machine-bored tunnels that avoid surface disruptions—will become relatively more economical in many situations.
In many cities and urban neighborhoods, choosing a tunnel instead of a bridge provides a chance to redress inequities in urban planning. “Think about the phrase ‘on the wrong side of the tracks’ and what that connotes,” says Chaney. “As trains, viaducts, highways and other surface infrastructure provided a physical divide in many urban areas, economic consequences followed. Putting that infrastructure underground allows governments to reconnect what has been divided by legacy infrastructure and avoid that type of isolation going forward.”
In addition, tunnels can be more resilient to flooding and other severe weather that will worsen with climate change, because they’re less exposed to environmental conditions. “Tunnels generally have longer lifespans,” adds Chaney. “Whereas many bridges constructed 50 to 60 years ago are currently being completely replaced, many tunnels of the same era are still in useful service today and are instead requiring some rehabilitation and modernization. Many of the subway tunnels around the world are more than 150 years old, and modern tunnels are being designed and constructed with service lives in excess of 100 years.”
So, there’s a lot to like about tunnels—especially with properly risk-apportioned procurement strategies. “It’s vitally important for agency managers and their private-sector partners to carefully explore the appropriate distribution of risk and which parties have the most control over risks and negative impacts,” says Chaney.
And there’s a lot to like about the Alaskan Way Viaduct replacement program—also named the American Council of Engineering Companies’ winner of the prestigious “Grand Conceptor” Award, signifying the most outstanding engineering achievement in 2019.
About Paula Hammond
Paula Hammond is senior vice president and national multimodal market leader, WSP USA; email: [email protected].
