Tunnel work continues despite Bertha stoppage
by Bill Lucia
Although the boring-machine digging the Highway 99 Tunnel remains at a standstill, work is moving ahead on the Alaskan Way Viaduct replacement project.
Looking south toward Bertha's "launch pit" at the Highway 99 tunnel construction site. Photo: Bill Lucia
The South Atlantic Street Overpass will open on Monday. The bridge will allow truck traffic to bypass a set of railroad tracks that separates South Atlantic Street from a busy freight terminal at the Port of Seattle. The BNSF Railway Company uses the tracks when linking together train cars, which can cause delays up to 20 minutes, according to Washington State Department of Transportation deputy program administrator Matt Preedy.
Map of the South Atlantic Street Overpass, which will allow truck traffic to bypass a busy set of BNSF railroad tracks when delivering freight to the Port of Seattle. Source: WSDOT
South of the tunnel’s entrance, two tiers of concrete roadbed, which will eventually help connect the deep-bore section of the project with an existing portion of Highway 99, are about 40 to 50 percent complete, Preedy said. The approximately 1,000 foot section of roadway will sit beneath a "cut and cover" tunnel and will contain about 18,000 cubic yards of concrete.
Unfinished and finished concrete walls mark the divide between Bertha's "launch pit" and a segment of the "cut-and-cover" approach tunnel that is nearing completion. Photo: Bill Lucia
A total of about 275 workers are constructing the road and other non-deep-bore portions of the $3.1 billion dollar project.
Workers exit the under-construction "cut-and-cover" tunnel via a stairway in the "launch pit," with a crane overhead. Photo: Bill Lucia
Also underway is the tunnel’s south operations building. What is currently an 80-foot-deep pit will be capped by a two-and-a-half story building. The building will function as a “nerve center” for the tunnel’s lighting, traffic and fire safety systems and will also contain a garage for maintenance vehicles. A similar operations center will be built at the north end of the tunnel in South Lake Union.
“There's a huge amount of work still going on on the site,” Preedy said.
Curved concrete panels that will be bolted together to build some of the 1,450 "rings" that will form the tunnel's interior. Each ring requires 10 panels. Each panel is roughly 6-and-a-half feet wide, 2-feet thick and 18-tons. The panels are prefabricated in Frederickson, WA and hauled by truck to the job site. Photo: Bill Lucia
Meanwhile, about 60 feet beneath the ground near South Main Street, two 12-person crews have been working around the clock in 12-hour shifts since late last week, investigating why the tunnel-boring machine, known as Bertha, ground to a halt in early December. The work involves five-person teams entering into a pressurized workspace around the machine’s “cutter head.” The area is sealed with a material called "bentonite clay" and then injected with air to keep out intruding groundwater and water from Elliott Bay. Those teams can only stay in the high-pressure space — which is similar to being underwater — for four hours at a time. On Friday, Preedy said the teams had entered the pressurized space 24 times.
Containers housing an above ground pressurized habitat, which is on standby in case workers get sick during "hyperbaric interventions." The crews performing the interventions are working in a pressurized space at the front of the tunnel boring machine, which is similar to being underwater. Ailments that can occur in high pressure environments include the bends. A pressurized shuttle can be attached to Bertha's built-in airlock to transport workers to the containers in the event of an emergency. Photo: Bill Lucia
So far the investigation has found a small boulder, and pieces of PVC and steel pipe, but no conclusive evidence as to why exactly Bertha encountered unusual resistance while digging.
On Friday morning, from an area near the tunnel’s headwall, a group of four workers could be seen standing in the dimness near the rear of the five-story tall machine. Bertha’s backside sits about 600 feet into the tunnel. The machine itself is 326 feet long, and its cutter head sits at a mucky terminus roughly 1,000 feet down the planned 1.7-mile tunnel path.
Looking into the Highway 99 Tunnel from the "launch pit." Lights on Bertha's back end can barely be seen about 600 feet down the tunnel. Photo: Bill Lucia
As he made his way through the job site, Preedy wouldn’t speculate on Bertha’s problems. “At the end of the day Bertha belongs to Seattle Tunnel Partners,” he said, adding that it’s “critically important” that the contractor thoroughly inspects the stalled machine before moving forward. “If there’s an issue out there,” he said, “they need to resolve it.”