Can You Dig It?

The specialty construction crew members digging a tunnel underneath Thomas Street at Climate Pledge Arena might not know it or feel like it during dusty or muddy 12-hour shifts, but they are leading the way to glamorous days ahead for Seattleites. That's because the 25-foot-wide tunnel-in-the-making is expansive enough for two semi-trailer trucks to pass each other.
Soon, one fleet of 18-wheelers will be hauling in the staging equipment for a major rock tour (wouldn't we all like to know who) while a formidable line of other semis are exiting with the No. 1 female solo artist on pop music charts in later 2021 (your guess as good as ours). Before it was demolished, save the historic roof and glass-curtain wall, the former arena at Seattle Center had two loading docks at street level.

The loading docks were not nearly enough to accommodate today's concert tours (yes, they will return to our lives) in which a dozen trucks full of equipment and tech gear might be considered on the moderate side of traveling a show. Plus, those trucks clogged up traffic for themselves and motorists alike.

"Why are we doing this?" asked Ken Johnsen, Oak View Group and Seattle Kraken construction executive, standing just outside the south entrance of the tunnel last week. "Because we will have a 360-degree plaza around the entire Climate Pledge Arena building. There will be no visible 'back-of-the-house.' It is an extremely efficient design to separate all of the truck activities from the rest of the arena and pedestrians. The result is to make this not just one of the great arenas in the country in how it looks but how it operates."
Let's pause on Johnsen's mention of a "360-degree plaza." No doubt, the subterranean bowl for Seattle Kraken and WNBA champion Seattle Storm games, plus those world-class concerts and family entertainment shows, will be a sight to see and hear and experience. But the ground-level open space for fans and citizens and visitors to traverse 365 days a year will grow to be a popular civic benefit. The activations and events on that plaza grounds will be plentiful, along pleasing access to all that will unfold on the Seattle Center campus is days and years ahead.
OK, back to the tunnel. The tunnel crew providing all this future glamor and civic ardor are working around the clock to finish the job, splitting into two 12-hour shifts aimed at making the tunnel operable by early January for needed construction trucks and other vehicles to enter and exit the arena event floor more than five stories below ground. Hitting that finish line for tunnel operations means a temporary ramp on the west side of the construction site can be excavated to fully build out the arena's southwest quadrant.

Currently, the tunneling is nearly two-thirds complete. The crew progresses four feet every day in a complex process that demands attention to step-by-step details and, most importantly, safety of the tunnel workers and tenants of the landmark Bressi Garage about the tunnel dig, including non-profit Pottery Northwest organization. The Bressi structure gained protective status in 2017 for its role in the early automobile age and its distinctive exterior and truss system support the roof (the latter more of an interior feature). Digging a tunnel with humans in any building, no less one with a landmark designation, above it is not the norm. The fact is not lost on general contractor Mortenson Company.
"The particular reason why this is a most challenging tunnel project is we are building underneath an active building," said Tyrone Thornton, a Mortenson project engineer who has been collaborating with the tunnel crew. "It is the biggest difference from any other tunnel. That's huge right there. This is first time we are doing something like this."
Thornton explained another northward four feet of dirt and rock is removed every 24 hours, six days a week. The day shift kicks into high gear at 7 a.m. when trucks haul away the soil excavated the previous day. Ready-mix concrete arrives at 7:30 to start "shotcrete" or spraying concrete of the walls and slab where the previous day's excavation occurred. The shotcrete is applied using a Norment shotcrete robot operated with remote control. By 11 a.m., excavator vehicles begin to claw away four more feet toward the northern side of Thomas Street, all followed an engineered sequence.

"First, the upper half of the tunnel is mined out," explained Thornton. "A loader removes the material and stockpiles it. Next, a flash coat of shotcrete is applied to the open face of excavation to stabilize the soil. Next, the lower half of the tunnel is mined out and flash-coated. Once the exposed soil has been covered with 'flashcrete,' the crew begins to set timber footings for the steel set."
"We 'flashcrete' that wall to stabilize it," said Thornton. "Otherwise it would be what we call 'live soil' that could slough off and injure people."
The night crew is responsible for installing "the steel set" that will permanently help support the tunnel structure for those 18-wheelers. Following prep work early in the second 12-hour shift, the steel erection starts about 12 midnight.

"First, each side is set and braced," said Thornton, "then the center is moved into place and bolted up. To help set the steel, a custom-fabricated hydraulic-beam grapple attachment for a rough terrain forklift grips and manipulates the steel pieces."
There is definite "engineered sequence and coordination" that needs to be repeated flawlessly for each of the 46 steel sets, said Thornton. Any complexity he says is "mechanicals" and pipes and utilities need to positioned in the tunnel too.
The goal is break through the north side of the tunnel on the other side of Thomas Street by mid-November, then complete all of the needed installations by early January, allowing trucks to use the route to the arena event level 50-plus feet below.
"There are special skills required to dig tunnels," said Johnsen. "Some of the crew work on tunnels their whole career. It can be dirty and muddy-it is so important to this project."