Tunneling Begins on HNTB-Designed SR 99 Tunnel With Launch of World’s Largest Tunnel Boring Machine

Aug 23, 2013

Five-story-tall TBM will complete 1.75-mile journey under Seattle in 14 months. Tunneling recently began on the SR 99 Tunnel, which at 57.5 feet excavated diameter, will be the world’s largest TBM-bored tunnel. HNTB Corporation is lead designer for the Washington State Department of Transportation project as part of Seattle Tunnel Partners, a joint venture of Dragados USA and Tutor Perini Corporation.

"This unique design-build project will be delivered nearly a year ahead of the initial estimated schedule,” said Phil Petrocelli, HNTB president West Division. “The alternative technical concepts developed by HNTB, and vetted within the design-build team, represent cutting-edge thinking. These industry-first efforts drew from staff and projects from every division within HNTB.”

The $2 billion SR 99 Tunnel Project — a major component of the Alaskan Way Viaduct Replacement Program — will build a tunnel featuring a two-level roadway to replace the aging Alaskan Way Viaduct along Seattle’s downtown waterfront. Work began in January 2011 and the tunnel will be open to traffic in December 2015.

HNTB is responsible for the design of the tunnel structure — including cut-and-cover sections, open cut sections and bored tunnel structure — mechanical and electrical components of the tunnel structures, ventilation buildings, fire life safety, civil work, roadway design and building settlement mitigation measures for structures along the tunnel alignment.

The nearly 7,000-ton TBM, nicknamed “Bertha,” was assembled and launched in an 80-foot-deep launch pit west of Seattle’s sports stadiums. It will take approximately 14 months to complete its 1.75-mile route under Seattle, emerging about four blocks east of the Seattle Space Needle. “Bertha” will cross under 154 buildings, numerous utilities and other infrastructure assets in downtown Seattle through various soft ground conditions along the route.

HNTB has more than 45 years of experience in the design, construction and restoration of tunnels and underground structures, including soft ground, rock and underwater crossing tunnels. The firm has proven its capabilities on numerous award-winning tunnel projects around the world, including the Devil’s Slide Tunnels project in San Mateo County, Calif., which earned HNTB the Tunneling Advisor/Program Manager of the Year Award at the 2012 International Tunneling Awards.

HNTB Corporation is an employee-owned infrastructure solutions firm serving public and private owners and construction contractors. With nearly a century of service, HNTB understands the life cycle of infrastructure and addresses clients’ most complex technical, financial and operational challenges. Professionals nationwide deliver a full range of infrastructure-related services, including award-winning planning, design, program delivery and construction management

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