September 12, 2007

7 Engineering Wonders: Transportation/Infrastructure — Tunnels keep traffic flowing out of sight

  • The Mount Baker Ridge Tunnel and lid conceals more than a half-mile of I-90 coming into Seattle.

    Photos courtesy of HNTB
    A cut-and-cover tunnel extends another 1,900 feet out of the Mount Baker Ridge Tunnel.

    In June 1989, the Mount Baker Ridge Tunnel opened, qualifying it as the largest diameter soft-soil tunnel in the world at that time. The 63-foot-diameter tunnel and the adjoining lidded structures provide a 3,400-foot stretch of concealed traffic for the Seattle portion of Interstate 90. The $139 million project was a critical component of the $1.46 billion program to complete a seven-mile stretch of I-90 from Seattle to Bellevue.

    The major components for the project were the 1,500-foot-long tunnel through Mt. Baker Ridge and the design and construction of 1,900 feet of cut-and-cover tunnel in Rainier Valley that reconnects the community that had been divided by the freeway. The final design was pivotal to forging a compromise between pro- and anti-freeway forces; the resulting complex became a neighborhood asset rather than an eyesore, while at the same time met improved transportation goals.

    The Mount Baker Ridge Tunnel carries westbound I-90 traffic into Seattle. Reversible express lanes below the main roadway share the tunnel.

    Providing one tunnel with five lanes on two levels considerably reduced corridor impacts. A third level reserved for pedestrians and bicyclists was built at no additional cost. The single tunnel required less right-of-way acquisition and was less costly than two or three smaller tunnels placed side-by-side. The tunnel and lid interiors match so that drivers do not notice they are traveling through two distinct structures. Wall and ceiling treatments, roadway widths and vertical clearances are all the same.

    Because of the difficulty of constructing such a large tunnel by conventional methods, a “stack-drift” method was developed. An articulated or semi-flexible tunnel lining consisting of 24 concrete-filled drifts was first constructed, forming a compression ring, followed by removal of the soil core. Since less surface is exposed at one time and each drift is immediately backfilled with concrete, the method resulted in minimal distortion of the liner and little disturbance to the ground above.

    The tunnel required:

    • 136,000 cubic yards of concrete
    • 15,000 tons of steel
    • 600,000 cubic yards of excavation
    • 400,000 cubic yards of backfill
    • 29,000 feet of precast concrete girders
    • 370,000 square feet of precast concrete panels
    • 28,000 square yards of concrete paving
    • 21 ventilation fans
    • 4,000 lights

    The lid required:

    • 154,000 cubic yards of concrete
    • 15,500 tons of steel
    • 736 precast concrete girders
    • 453,000 cubic yards of backfill
    • 21 ventilation fans
    • 2,800 lights

    One of the most extensive instrumentation systems ever installed around a tunnel under construction was used to monitor soil movements.

    Thanks to the bored tunnel and lidded roadways, freeway noise and air pollution no longer dominate surrounding communities. Neighborhoods once physically divided by the freeway are now joined by landscaped open spaces atop the lidded areas. Motorists enjoy improved access and travel amid state-of-the-art life safety systems, including one of the largest fire suppression sprinkler systems in the United States.

    The tunnel is equipped with ventilation, fire suppression, emergency telephones, fire detection and alarm, closed-circuit TV monitoring, and an AM/FM rebroadcast system. Equipment for these systems, along with the electrical distribution, is housed in underground buildings alongside the cut-and-cover tunnel.

    To build the tunnel, 24 concrete-filled drifts were built to create a compression ring and then soil was removed from the center.

    HNTB was the lead designer for both the tunnel and lid, assisted in the construction and wrote a manual for the structural systems of the tunnel complex to serve as a guideline for WSDOT inspection and maintenance personnel.

    The project in 1987 won ACEC’s highest achievement, the Grand Conceptor Award. It went on to win four other engineering awards.

    Robert Josephson, PE, is associate vice president and director of the Construction Services Group at HNTB’s Bellevue office. Josephson was WSDOT’s project engineer for design and assistant construction engineer for the Mt. Baker Ridge tunnel and lid complex project. He also was responsible for the I-90 Mercer Island tunnel and the Lacey V. Murrow Floating Bridge replacement.

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