Done with drilling, Bertha will be removed, and then road decks, walls, lights and fireproofing will quickly take shape inside the new Highway 99 tunnel under Seattle.
Now that tunnel machine Bertha has finished digging through downtown, work will accelerate behind the giant drill to complete the Highway 99 tunnel’s double-decked traffic lanes.
Already, more than half the top deck is cast in concrete.
The finished tunnel, to open by early 2019, will expose all 1,426 of its concrete rings to motorists heading south toward Sodo. They will see hundreds of lights hung from a sweeping arch, two stories overhead.
A cathedral for cars.
Of course, that view is no substitute for glimpsing Elliott Bay and the Olympic Mountains from today’s Alaskan Way Viaduct. And this bypass tunnel offers no downtown exits.
Joe Hedges, state Highway 99 administrator, recently said the completion of the tunnel itself represents halftime for the project. Nonetheless, workers were busy installing roadways even during Bertha’s two-year breakdown, and some 600 people remain on the job.
That by itself prevented delays from getting worse, said Brian Russell, vice president for HNTB, which wrote the technical designs for the tunnel structures.
The tunnel’s relatively new “design-build” contract, which hands final engineering to the bidders, gave Seattle Tunnel Partners (STP) flexibility to juggle its work schedules so the upper deck and walls kept advancing north, behind the tunnel machine. Under conventional bidding, where the state writes the design, the decking might be done by other companies, and might not begin until after the underground tube is finished.
“The uniqueness is, having this builder innovate as they go,” Russell said.
Bertha finished digging April 4. Another four to five months will be required to cut up and hoist away its front-end steel, while its rear conveyors, walkways and hoses are pulled 1.7 miles back through the Sodo portal.
The lower deck can’t be added until those parts are cleared out.
Then the job should move quickly, because the lower lanes will be built of precast concrete slabs, trucked from Tacoma and fastened onto ledges already in place. And supplies can arrive from the north end, too.
Roadways could be done by early next year, followed by several months of work to fasten lights and signals, test sprinklers and fans, and build connecting ramps.
Here are the tunnel’s key features.
The tunnel is designed for travel at 50 mph, though an official speed limit isn’t determined yet.
Two 11-foot lanes will be on each deck, plus an eight-foot shoulder on the bay side and a two-foot margin on the inland side.
Eleven feet is less than the 12-foot federal standard but considered acceptable, and it’s common on Seattle-area highways.
Decks will be banked 2 degrees around curves, and cars will ascend at 4 degrees as they leave the tunnel at either portal.
Extra-bright transitional lights at the two portals will help drivers make the adjustment between sunlight and the darker tunnel, Russell said.
Hundreds of rows of redundant tunnel lights are known to lull some motorists to sleep — so much that in Norway, a 15-mile tunnel contains four chambers that simulate driving into daylight.
That’s not considered a problem in the 1.7-mile Seattle passage.
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That kind of tragedy shouldn’t occur here.
There are no such false ceilings in the Highway 99 tunnel. Lights and signs also won’t be held in place with adhesives, as in Boston, where epoxy gradually cracked and allowed anchor bolts to slip loose.
In Seattle, fixtures for the northbound level will be bolted into steel frames that are embedded inside the southbound deck above, with preset anchoring holes.
Above the southbound level, lights and signs will be bolted into curved frames, which in turn will be mounted flush onto the concrete tunnel rings.
The steel frames will be secured into the top of the tunnel using special undercut anchors. A custom drill creates a notch about six inches deep where the tip of the anchor spreads and bites into the concrete when a bolt is threaded into an anchor.
More importantly, the Highway 99 tunnel lighting fixtures will exert no more than 25 pounds per foot of downward tension on their curved frames, according to HNTB. That’s one-hundredth the weight per bolt that federal investigators found in some of the failed Boston panels.
The tunnel is expected to provide safe airflow in normal traffic, without continuously running fans. Fresh air enters through the portals, while toxic air can be sucked away through a large ventilation duct by nine jet fans at the Sodo end and eight at the South Lake Union end.
Fans will trip on when carbon-monoxide sensors show elevated levels.
Air modeling suggests fans are needed most during traffic jams moving less than 20 mph, to keep carbon monoxide below a limit of 35 parts per million, according to a chart by Highway 99 engineers in Tunnel Business Magazine.
Polluted air will be removed through dampers on the east side of the tunnel, where square openings to the ventilation passage are built into the concrete walls. Fans will pull the toxic air north or south.
There are 200 dampers overall, and electrostatic filters near the portals will catch particles before the air is released through yellow exhaust stacks.
For routine air exchange, the four dampers closest to each portal will open and suck out dirty air through the ventilation passage, while fresh air fills the tunnel.
Fire is the greatest safety threat in tunnels worldwide.
Petroleum tankers will be banned, because of their potential to generate concrete-blistering temperatures in a fire. (Tankers will be able to use the new waterfront surface boulevard.)
Operators on-site or at the state regional control center in Shoreline can lower entry gates to halt a truck, or halt all traffic, if they spot trouble on security cameras.
A serious fire would take just under two minutes to trigger the tunnel’s heat detectors, which are set at 190 degrees, Highway 99 engineers say. Once the heat detectors are tripped, it takes another 80 seconds to activate the sprinklers, unless human operators override them, designers say.
Sprinklers would rain like a typhoon.
“Within five seconds, you would be completely drenched,” Russell said.
Seattle’s Battery Street Tunnel was the first U.S. tunnel to use a deluge system, rather than mist.
Also, the upper three feet of the tunnel walls, and all ceilings, will be coated with spray-on lining one inch thick. It’s a mixture of Portland cement, mica and quartz that is supposed to shield the tunnel’s concrete walls from exceeding 716 degrees, to meet national codes.
And firefighters will find standpipes every 275 feet or less, to attach hoses and extinguish flames directly.
Under extreme heat, concrete can burst when its embedded moisture explodes like a popcorn kernel.
That phenomenon was discovered when a truck carrying margarine, flour and diesel burned for two days in a French Alps tunnel in 1999.
The Highway 99 tunnel is designed to avoid a fire-induced collapse, or forestall it until the Seattle Fire Department can fight back.
Arrows and green running-man icons will direct people to 15 escape doors on each level, spread along the waterfront side’s eight-foot shoulder.
Engineers assume people will need four minutes to recognize a fire and flee through a door.
From there, people will climb or descend stairs to the exit concourse, and walk to Sodo or South Lake Union. People who can’t use stairs would wait in a refuge zone to be rescued, under the eye of security cameras.
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Fresh air would be pumped through the exit concourse, creating positive pressure to keep smoke from following people through the emergency exits.
Meanwhile, smoke that accumulates at the tunnel ceiling will start to be sucked downward by the dampers immediately ahead, obscuring vision along the road deck.
“Traffic not blocked downstream of the fire will have driven out of this area before untenable conditions develop,” state tunnel design manager Susan Everett and HNTB engineers wrote in TBM magazine.
So if you’re in front of a crash or fire, continue driving to daylight.
Four times a year, the white-painted tunnel walls will be scrubbed by a truck named Mr. Bubbles.
The same truck cleans the Battery Street and other local tunnels. The lights and ceilings require pressure-hose soaping and rinsing by somebody standing on a platform over the truck cab. Tests at the north portal confirmed Mr. Bubbles wouldn’t chip away the fireproof lining, Russell said.
“It’s like a carwash. But instead of you driving into the carwash, the carwash travels inside the tunnel.”