The Alaskan Way Viaduct was an elevated concrete structure built in the 1950s to carry SR 99 through downtown Seattle. By the early 2000's, the structure was vulnerable to earthquakes and wearing out. The viaduct was replaced by a 1.7-mile tunnel that opened in February 2019. The tunnel's design and approach to construction allowed SR 99 to remain open for much of construction, thus minimizing closures and disruptions to vehicle and freight traffic. The viaduct was demolished in 2019 to clear the way for new public space along Seattle's downtown waterfront.
The tunneling machine known as Bertha began tunneling beneath Seattle in July 2013 (photo at right: the machine's cutterhead being lowered into the launch pit near Seattle's stadiums). The machine completed its tunneling on April 4, 2017 and was fully dismantled and removed from the tunnel on Aug. 23, 2017.
The SR 99 tunnel was built by one of the world's largest tunneling machines. Nicknamed Bertha, the machine was shipped to Seattle in spring 2013 from her manufacturing facility in Japan. She was then assembled in an 80-foot-deep launch pit to the west of Seattle's stadiums. After a series of tests, Bertha was launched into the soils beneath Seattle on July 30, 2013.
This graphic divided the tunnel route into 10 zones, providing a visual aide to track Bertha's progress during tunneling. Click through the zones to learn about each section of the tunnel drive.
Each of the 10 zones of the tunnel route has something about it that makes it unique, such as a certain soil type or a noteworthy location beneath the city. This simulation (YouTube) shows an early rendering of the full tunnel path.
Zone 1 — Railroad Way South to South Washington Street
Start/end distances: 0 feet | 1,455 feet
Bertha began her tunnel drive at the south end of downtown below sea level. In this first stretch of tunneling, Bertha will dig under fill soil dumped here by Seattle’s early settlers. Because the machine is shallow at this location, crews built underground concrete walls on either side of the tunnel route to hold the ground in place. They also built protected areas underground where Bertha will stop so crews can do planned inspections and maintenance before the machine dives beneath downtown Seattle.
Zone 2 — South Washington Street to Columbia Street
Start/end distances: 1,455 feet | 2,120 feet
Zone 2 is home to Milepost 31, the highway marker on State Route 99 that gives the program information center in Pioneer Square its name. In this zone Bertha passes beneath the Alaskan Way Viaduct, a crossing of the past and future State Route 99. Among other precautionary measures, crews used steel pipes to build angled walls beneath the viaduct’s foundations. These walls will protect the viaduct as Bertha passes 15 feet below its foundation.
Zone 3 — Columbia Street to Madison Street
Start/end distances: 2,120 feet | 2,760 feet
Zone 3 marks Bertha’s entrance into downtown. By this point she’s passed beneath the viaduct and is tunneling through clay, which is dense and uniform, and other glacially compacted materials. Bertha will pass beneath Western Avenue near Marion Street en route to First Avenue.
Zone 4 — Madison Street to Seneca Street
Start/end distances: 2,760 feet | 3,390 feet
The soils in Zone 4 have spent thousands of years sitting under the weight of mile-high glaciers. Bertha’s route takes her past the foundation of the northbound SR 99 off-ramp to Seneca Street, the functionality of which will eventually be replaced by the future Alaskan Way surface street to be built once the tunnel is open and the viaduct is demolished.
Zone 5 — Seneca Street to Union Street
Start/end distances: 3,390 feet | 4,125 feet
Near the end of zone 5, Bertha settles into a steady path beneath First Avenue. After angling down through the first four zones of her drive, Bertha will flatten out and begin to rise here. However, her relative depth to the surface will increase as she tunnels into the hill that is downtown Seattle.
Zone 6 — Union Street to Stewart Street
Start/end distances: 4,125 feet | 5,175 feet
The SR 99 tunnel will get company in Zone 6, as Bertha passes beneath both the railroad tunnel completed in 1904, and a major sewer tunnel (the Elliott Bay Interceptor) that runs from downtown to the West Point Treatment Plant. Bertha will also pass to the east of Pike Place Market.
Zone 7 — Stewart Street to Lenora Street
Start/end distances: 5,175 feet | 6,095 feet
Here, Bertha reaches her deepest point at more than 200 feet below the surface. After following First Avenue for several blocks, she begins to veer east toward Second Avenue.
Zone 8 — Lenora Street to Bell Street
Start/end distances: 6,095 feet | 7,080 feet
In zone 8, Bertha starts to climb toward the surface as she passes underneath Second and Third Avenues. As she does, she encounters a greater variety of soils. The surface above Zone 8 has seen its share of dirt moved over the years, including the massive regrade that leveled Denny Hill in the early 20th century.
Zone 9 — Bell Street to Denny Way
Start/end distances: 7,080 feet | 8,365 feet
An old SR 99 tunnel meets a new SR 99 tunnel in Zone 9, as Bertha digs her way beneath the 1950s-era Battery Street Tunnel. Bertha also passes beneath the foundation of another local transportation icon: the Seattle Monorail.
Zone 10 — Denny Way to the finish line
Start/end distances: 8,365 feet | 9,270 feet
By the time she reaches Denny Way, the top of Bertha’s cutterhead will be less than 100 feet deep. The conveyor belt carrying the tunnel muck out of the machine will be more than a mile and a half long. Her nearly two-mile journey ends near the intersection of Sixth Avenue North and Thomas Street, where she will break through a concrete wall into the north receiving pit.
On April 4, 2017, Bertha completed her journey when she broke through into the disassembly pit at the tunnel's north end, near Seattle Center. This marked the end of the machine's 9,270-foot journey beneath downtown Seattle.
Crews spent four months in summer 2017 dismantling and removing the machine from the ground to make room for the completion of the highway inside the tunnel. The machine was too large to be removed in one piece, so it had to be disassembled and lifted out of the pit in pieces.
The heaviest lift was 70 tons; many pieces were then further cut apart once on the ground before being hauled away by trucks. All told, 580 truckloads hauled away 8,300 tons of equipment and metal. Approximately 6,850 tons of metal were recycled. The final piece was lifted from the disassembly pit on Aug. 23, 2017.
The SR 99 tunneling machine was designed specifically for the ground conditions beneath Seattle. The short video below shows how the machine dug the tunnel. A motorized, 10-foot-long model of Bertha is on display at Milepost 31, the program's information center in Seattle's Pioneer Square neighborhood.
Performing regular maintenance on Bertha was critical to the success of the tunnel drive. Most maintenance tasks could be completed under normal atmospheric pressure, but some tasks at the front end of the machine could only be completed in hyperbaric conditions (conditions in which the air pressure is greater than the atmosphere we live in and breathe every day).
Replacing certain tools on the machine's cutterhead required crews to work in the space between the cutterhead and the ground in front of the machine. Because the machine was underground and below the water table, it was necessary to stabilize the ground in front of the machine and pressurize this space. Crews stabilized the ground by injecting a type of clay, known as bentonite, into the front end of the machine. This created a seal that prevented water and soil from entering - and air from escaping - the work space.
Next, crews over-pressurized the space with compressed air, which pushed against the bentonite to counteract the ground and water pressure at the front end of the machine. This newly created "hyperbaric" work space had pressure levels higher than regular atmospheric pressure, similar to conditions found in an underwater dive. The graphic below illustrates the process.
In December 2013, Seattle Tunnel Partners (STP) stopped tunneling approximately 1,000 feet into the tunnel drive after measuring increased temperatures in the tunneling machine. While investigating the cause of the high temperatures, STP discovered damage to the machine’s seal system and contamination within the main bearing. STP and manufacturer Hitachi Zosen completed repairs to the machine in December 2015.