Tracking Bertha, the SR 99 tunneling machine
On July 30, Bertha, the world's largest tunneling machine, began 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 interesting, 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
The tunnel drive starts in a controlled environment at the south end of downtown. Fill soils dumped here by the city’s early settlers have been removed or strengthened ahead of tunneling. Additionally, crews have built three protected areas — we call them safe havens — where they can crawl through the front of the machine to inspect it and make sure everything is working properly. Think of it this way: if Bertha were learning to ride a bike, Zone 1 would be her training wheels.
Zone 2 — South Washington Street to Columbia Street
Zone 2 is home to Milepost 31, the highway marker on State Route 99 that is so significant we named our project information center in Pioneer Square after it. Milepost 31 matters because it’s where the tunneling machine passes beneath the viaduct—literally the intersection of the past and future SR 99 through downtown Seattle. Bertha's route takes her about 15 feet below the viaduct’s foundation. In addition to other precautionary measures, crews built angled walls beneath the structure’s foundation to protect it during tunneling.
Zone 3 — Columbia Street to Madison Street
Zone 3 marks Bertha's entrance into downtown. By this point she's passed the viaduct and is tunneling through clay, which is dense, uniform and easy on Bertha’s cutterhead — all good things for crews digging their way beneath the city. With less groundwater here than in previous zones, crews must rely heavily on soil conditioners, which minimize the amount of wear on the machine’s cutting tools.
Zone 4 — Madison Street to Seneca Street
Zone 4 finds Bertha directly between Western and First avenues. The soils here have spent thousands of years sitting under the weight of mile-high glaciers, making them ideal for tunneling. Bertha’s route takes her past the foundation of the northbound SR 99 off-ramp to Seneca Street — her last look at the viaduct she’s replacing.
Zone 5 — Seneca Street to Union Street
Near the end of Zone 5, Bertha settles into a steady path beneath First Avenue. The soils should continue to be solid, but let's say Bertha hits a boulder. No problem. She can swallow anything under three feet in diameter. Larger rocks will be broken down to a more manageable size.
Zone 6 — Union Street to Stewart Street
The SR 99 tunnel isn’t the only tunnel in town. Far from it, actually. Proof is in the heart of Zone 6, where Bertha passes beneath a railroad tunnel dug by hand in 1904, as well as a major sewer tunnel serving neighborhoods to the east. Zone 6 also passes to the east of Pike Place Market, one of Seattle’s biggest tourist attractions. Crews are taking great care to protect structures above and near the tunnel route. Nearly 200 buildings are being monitored around the clock by tunneling crews. While we don’t expect much settlement, we’re taking every precaution, including the monitors.
Zone 7 — Stewart Street to Lenora Street
Bertha reaches her deepest point — more than 200 feet below the surface — in Zone 7. Here, she starts to veer east toward Second Avenue. The soils in her path continue to be a good mixture of glacially compacted sand, clay and gravel.
Zone 8 — Lenora Street to Bell Street
In Zone 8, Bertha starts to climb toward the surface. 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. Bertha’s dirt-moving is considerably less visible — but no less transformational — than that project.
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. The old tunnel will close for good as soon as the SR 99 tunnel opens to traffic in late 2015. Bertha also passes beneath the foundation of another Seattle transportation icon — the monorail.
Zone 10 — Denny Way to Harrison Street
Bertha’s whole journey is about this moment: the big finish. By the time she reaches Denny Way, Bertha is less than 100 feet deep. She continues climbing until she emerges from the soil near the intersection of Sixth Avenue North and Harrison Street, a few blocks north of the Battery Street Tunnel.