Seattle's Little Dig

The proposal to replace the dangerous, 53-year-old Alaskan Way Viaduct with a tunnel along Seattle's central waterfront already has a disparaging nickname: the Big Dig. That nickname, of course, originated in Boston, where it described the replacement of an elevated portion of Interstate 93, which separated downtown Beantown from Boston Harbor, with a complicated series of tunnels. Boston's Big Dig also earned infamy as the nation's most expensive public-works project as its cost rose from an estimated $2.6 billion in 1983 to $14.6 billion today. While there are enough similarities between the two projects that the nickname for Seattle's undertaking will undoubtedly stick, the differences are important for Seattle voters to grasp as the city prepares for November's vote on how, or whether, to replace the viaduct: with another elevated structure, or with something on or below the ground.

From an engineering standpoint, it is absurd to compare Seattle's proposed cut-and-cover tunnel to Boston's Big Dig. The Boston project, writes Dan McNichol, its former deputy director of public affairs, in his book The Big Dig, "is the largest and most complex urban infrastructure project ever undertaken in the modern world. It is bigger in scale than the Panama Canal or Hoover Dam and more complex in its planning, engineering, and construction than the two combined." Boston's project consisted of three tunnels—a three-mile tunnel under saltwater Boston Harbor, a 1.5-mile tunnel under Boston's commercial district, and a 1,100-foot, 11-lane tunnel connecting downtown and South Boston. Oh, they also built the world's widest cable-stayed bridge. Seattle's tunnel would be just over a mile long, would be under a street with no structures above—much less water—and would have six lanes. "It's kind of a Small Dig," says Clark Williams-Derry, a researcher with Northwest Environment Watch and an adamant tunnel opponent.

Boston's Big Dig is eight miles long with 121 lane miles, 80 of which are in a tunnel. Seattle's full project "corridor" would begin in the north on Aurora Avenue North at the intersection of Ward Street—four blocks north of Mercer Street on the eastern slope of Queen Anne Hill—and would run south to a point on Highway 99 three blocks south of Qwest Field. That corridor is four miles, with 20 lane miles, 6.9 of which will be in a tunnel (5.2 lane miles in the new tunnel, 1.7 lane miles in the existing Battery Street tunnel).

Lately, just to make things more confusing, Mayor Greg Nickels, the most prominent backer of an underground solution, and the Washington State Department of Transportation have proposed a "core tunnel" project that would essentially be the first stage of the work proposed for the entire corridor, leaving out the expensive sinking of Aurora Avenue north of the Battery Street tunnel. Comparisons in this article are to the entire project corridor, not just the core tunnel.

Boston's Big Dig involved digging up 16 million cubic yards of soil; Seattle's entire project, including the tunnel, would displace around 2 million cubic yards.

University of Washington civil engineering professor Steven Kramer specializes in geotechnical earthquake engineering and first exposed the dangers of soil liquefaction during an earthquake under the existing viaduct. Kramer says the soil in Boston presented much greater challenges than the soil along Seattle's waterfront. "The level of risk is a lot lower than in Boston, where they are dealing with that Boston clay," says Kramer, pronouncing the final two words as if they were the vilest insult imaginable. Kramer explains that Seattle's tunnel area has between 20 feet and 60 feet of fill on top of compacted glacial till. WSDOT plans to scoop out the fill—all 2 million cubic yards of it—and build directly on the glacial till. "The glacial till material would be very competent," Kramer says. "Twenty-five thousand feet of ice was sitting on it about 12,000 to 13,000 years ago. It is very dense and strong." Kramer says this construction technique involves no actual boring, so he hesitates to even call it a tunnel, preferring to refer to it as an excavation. "We build excavations all the time," he says. Kramer compares the process to building a Seattle skyscraper, where a huge trench 60 feet deep is dug for a building's foundation, to be tied into the glacial till underlying the city. "You are using conventional construction techniques," says Kramer.

Kramer did a study of the seawall that keeps Elliott Bay from flooding the filled-in central waterfront and found it decrepit and, like the viaduct, in danger of collapsing in an earthquake. He expresses confidence in WSDOT's plan to use the tunnel's western wall to replace the part of the seawall that runs from King Street in Pioneer Square to the proposed tunnel's northern terminus near Pike Street and the Seattle Aquarium. "It gives you a real strong, beefy wall," says Kramer.

WSDOT documents explain, "The wall would be constructed of 4-foot-diameter drilled (concrete) shafts that would extend up to 90 feet below the ground. The shafts would overlap to form a continuous wall. . . . " This is a common structure called a secant pile wall. Essentially, the wall is built before excavation. Kramer says water leakage through a secant pile wall is not much of a concern. In contrast, Boston's Big Dig has had major problems with leaks. In Boston, contractors used an experimental technique to save money and space in an I-93 tunnel under Boston's commercial district. Most tunnels like this are built by constructing reinforced concrete walls called "slurry walls" as you dig a trench. Then a full concrete box is poured between the slurry walls to form the final walls of the tunnel. Seattle's tunnel would be constructed in this conventional fashion. Boston's I-93 tunnel skipped the final step of pouring a concrete box and relied on slurry walls alone. Boston's slurry walls, which are 12 stories deep in some places, have not performed well.