Tracking Bertha, the SR 99 tunneling machine
Bertha, the SR 99 tunneling machine, has resumed digging the SR 99 tunnel beneath downtown Seattle. To make it easier to track her progress, we’ve divided the tunnel route into 10 zones. Each zone has something about it that makes it unique, such as a certain soil type or a noteworthy location beneath the city. The map shows the entire tunnel route. The machine is currently located in zone 1. A full description of all the zones can be found below. Click the menu bar listed below to view each zone.
Zone 1 — Railroad Way South to South Washington Street
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
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
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
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
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 SeattleSMITH.
Zone 6 — Union Street to Stewart Street
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
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
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
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
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 Harrison Street, where she will break through a concrete wall into the north receiving pit.