Tunnel boring machine named Bertha begins digging new State Route 99 tunnel underneath downtown Seattle on July 30, 2013.

  • By Jennifer Ott
  • Posted 1/02/2015
  • HistoryLink.org Essay 10685

On July 30, 2013, Seattle Tunnel Partners begins digging a new State Route 99 tunnel underneath downtown Seattle using a tunnel-boring machine named Bertha. The machine is to dig a 57.5-foot-diameter tunnel -- the largest in the world at the time work starts, and designed to carry a double-decker highway with two lanes in each direction -- from south of S King Street to Thomas Street. The pieces of the Hitachi Zosen Corporation-built machine arrived from Osaka, Japan, in April 2013 and it was assembled in the launch pit at the tunnel's south portal over the summer. Students from around Washington participated in a naming contest and the machine was dubbed Bertha in honor of Seattle's first (and thus far only) female mayor, Bertha Landes (1868-1943). Plans initially call for the tunnel boring to be completed in late 2014, but tunneling will be halted in December 2013. After Seattle Tunnel Partners and Hitachi Zosen Corporation make repairs and modifications, excavation will resume in December 2015 and Bertha will complete the tunnel boring on April 4, 2017.

Getting Ready

In 2009, after eight years of public discussion and planning, and several votes, the City of Seattle, the State of Washington, and the Port of Seattle agreed to build a deep-bore tunnel to replace the Alaskan Way Viaduct, which carried SR 99 through Seattle along the waterfront but had long been considered by many a blight on the waterfront and was damaged in the 2001 Nisqually Earthquake. Several alternatives, including a cut-and-cover tunnel along the Alaskan Way seawall, which also needed to be replaced; a new viaduct; a cross-bay bridge; and a street-only option, were considered. Some legal wrangling and environmental review followed the January 2009 decision to build a tunnel and work did not begin on the project until 2011.

In preparation for construction of the tunnel, the state removed the southern mile of the Alaskan Way Viaduct in the fall of 2011. In its place, Seattle Tunnel Partners, a joint venture company made up of contractors Dragados USA and Tutor Perini Corporation, and subcontractors Frank Coluccio Construction, HNTB Corporation, and Intecsa-Inarsa, dug the launch pit for the tunnel-boring machine, a piping system for debris removal, and facilities at Terminal 46 for barge loading. The Washington State Department of Transportation built an overpass over the railroad tracks at S Atlantic Street for truck traffic traveling between the freeways and Terminal 46.

The launch pit at the south portal measured 80 feet wide by 400 feet long. More than five stories deep, it provided space for assembling the tunnel-boring machine when it arrived from Osaka in April 2013 in 41 pieces. Seattle Tunnel Partners had chosen Hitachi Zosen Corporation to manufacture the machine because of its extensive experience building more than a thousand tunnel-boring machines, though none this big. Hitachi Zosen also provided the tunnel-boring machine for the Capitol Hill Station to Pine Street segment of Sound Transit's light rail tunnel through Seattle.

Big Bertha

Bertha was enormous. The boring machine was 57.5 feet tall, or about the height of a five-story building, and 326 feet long, and weighed 7,000 tons. In addition to the tunneling parts, there were restrooms and a kitchen for the crew of 25. The cutter head had three different types of teeth: one to grind up softer soils, another to pull soil back into the slurry pipe so that the screw conveyor could carry it up to the surface, and the third to grind up boulders. Some bigger boulders would have to be broken up by the operators.

As the machine was readied for shipment to Seattle, the Washington State Department of Transportation launched a naming contest open to kindergarten-through-twelfth-grade students in Washington. The name had to be female, in keeping with traditional machinery-naming practices, and it had to "have Washington state heritage, life, nature, transportation or engineering" significance (Gunn). The judges chose the entries submitted by Darryl Elves' fifth grade class at Poulsbo Elementary and by Elijah Beerbower, a second grader at Hoquiam's Lincoln Elementary: Bertha, in honor of Bertha Landes, elected in 1926 as Seattle's first female mayor.

Once named, the machine got its own Twitter account: @BerthaDigsSR99. Staff regularly tweeted about progress with clever plays on words and tongue-in-cheek comments. The first tweet, on December 10, 2012, said "So nice to finally have an identity. Maybe now the passport agency will take my application. Which photo should I use?" (@BerthaDigsSR99). A year later, Bertha had more than 8,000 followers.

Tunnel-Shield Construction Techniques

The type of tunnel construction that uses a tunnel-shield machine like Bertha was developed in the nineteenth century. French-born architect and civil engineer Marc Isambard Brunel (1769-1849) designed the technique in 1818 to enable the construction of a tunnel through the water-saturated soils under the Thames River between the Wapping and Rotherhithe districts of London, England. The technique involves using a shield to support an excavated section of a tunnel while the walls and ceiling are built. The shield is then moved forward and the process repeated.

In 1874 engineers Peter M. Barlow and James Henry Greathead modified Brunel's design with a circular shield. Instead of building a box-like structure, the circular shield creates a tube-shaped tunnel. This is the type of machine built by Hitachi Zosen, though the design has been significantly refined since 1874. As the shield moves forward behind the cutter head, two erector arms place 2-foot-thick curved concrete panels in a ring around the perimeter of the bored hole. The panels have rubber gaskets to ensure a watertight seal between them. Once a complete 10-panel ring is in place, hydraulic jacks braced on the ring push the entire tunnel-boring machine forward.

During excavation, debris is removed from a pressurized area between the cutter head and the shield. The pressure helps support the face of the cutting area until the ring is in place and the cutter head moves forward again. The dirt and rocks move through the cutter head and into the pressurized void where they are mixed with chemicals and water, creating a slurry that can be carried out of the tunnel on a screw conveyor in a pipe. When Bertha was at the southern end of the tunnel route, the conveyor was folded accordion-style into the launch pit. It was designed to stretch out as the tunnel-boring machine moved north, until it reached the length of the entire tunnel.

Through the Dirt

Seattle Tunnel Partners moved the first excavated material by truck to several locations around Puget Sound. It included fill that contained various types of materials and soils that had been injected with grout to increase their stability. Once the tunnel-boring machine reached soil that did not contain contaminants, that material was loaded onto barges and taken to the Mats Mats Quarry in Port Ludlow, Jefferson County, where it was to refill mined pits. A labor dispute between the International Longshore and Warehouse Union (ILWU) and Seattle Tunnel Partners over who would load the barges led to a work stoppage that idled Bertha for a month before being resolved in September 2013.

Bertha's intended route under the city traversed many different types of soil. Clay soils would be the easiest to pass through, soft and stable. Sandy soils are the most unpredictable and would be hardest to keep in place as the machine moved through. Glacially compacted soils are the hardest and rockiest, but would be easiest to keep in place while the tunnel rings were built.

The tunnel's planned route traveled about 200 feet below the surface at its deepest parts, reducing concerns about running into basements and underground parking garages. Planners did have to take into account the built environment in other ways, however. First, the route had to avoid existing transportation infrastructure. Plans called for the Alaskan Way Viaduct to be removed once the tunnel was completed, but its foundation pilings had to be protected during the tunneling process. Moving laterally as the tunnel headed north, the route had to travel below the Great Northern railroad tunnel, which runs from Alaskan Way at Pike Street to King Street Station, and the Battery Street Tunnel.

Planners also implemented several measures to address concerns about settlement that could lead to even minute shifts in the buildings above the tunnel route. Six hundred soil samples were taken along the route to identify where potential problem areas might be. Sensors were attached to buildings to detect any shifting that might occur and grout was injected into the soil around vulnerable buildings to harden and stabilize the soils. Some buildings had rows of grout-filled pipes sunk into the ground alongside them to create an "underground micropile wall" intended to hold up the soil directly under the buildings in the case of a large shift in the soil farther below them (Newcomb).

On December 5, 2013, Bertha marked completion of the first 1,000 feet of tunnel excavated, nearly one-ninth of the total tunnel length. Seattle Tunnel Partners stopped tunneling the next day due to increasing resistance at the front of the machine. Inspections found problems with the seals protecting the boring machine's main bearing. A repair pit was dug to access the damaged machine and in March 2015 the cutterhead was raised to the surface. After nine months of assessment, repairs, and testing, tunnel-boring resumed in December 2015.

By October 3, 2016, the machine had reached the half-way point of its 9,270-foot tunnel route, deep beneath the Pike Place Market. Six months later, and nearly two and a half years after the initial target date of October 2014, Bertha broke through to the previously constructed disassembly vault at the north end of the tunnel on April 4, 2017. With much work remaining, including months to disassemble and remove the machine now that its work was done, as well as building the double-decked highway inside the tunnel, the new SR 99 was not scheduled to open to traffic until early 2019.


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Note: This essay was updated on July 28, 2015, July 29, 2016, and July 26, 2017.


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