How PennDOT District 11 plans to demolish a Parkway East bridge

By ADAM BABETSKI
TribLive

Hundreds of explosive charges detonating with the push of a button.

A blast that echoes like a clap of thunder.

Millions of pounds of concrete and steel crashing into the valley below.

That’s what PennDOT’s John Myler expects to see and hear later this month when workers demolish the old Commercial Street Bridge on the Parkway East to make way for its successor — a bridge that workers will slide into place using a greased rail system.

Myler, PennDOT’s District 11 senior assistant construction manager, has worked for the agency for 22 years and overseen more than a dozen similar bridge demolitions, including the Hulton Bridge demolition in Oakmont in 2016.

But he has never dealt with a project of this magnitude or such a tight timeline. At 9 p.m. on July 10, Parkway East closes between the Squirrel Hill Tunnel and the Edgewood/Swissvale exit. Also know as Interstate 376, the heavily traveled highway is expected to reopen three weeks later, Aug. 3.

PennDOT contracted S&B Fay, a local construction company Myler has worked with extensively in the past and said is “well-versed” in bridge demolition.

S&B Fay’s workers will handle the physical work, while PennDOT officials will join them on site to make sure everything is going according to plan.

“We’re very confident in the procedures of taking this bridge down,” Myler said.

Breaking the bridge

Workers will begin by removing the bridge’s barriers with a specialized vehicle that crushes concrete, Myler said.

From there, four excavators with attached jackhammers known as hoe rams will begin demolishing the bridge deck, or surface, starting in the center and working their way to the edges.

Myler said the excavators need to stay symmetrical to preserve the bridge’s integrity, and beginning on one side and moving toward the other could compromise it.

Four crews will work on the bridge at a time in rotating 12-hour shifts, with eight crews in total keeping the project running around the clock. Each crew consists of operators to drive the equipment and laborers to maintain and refuel it as necessary.

“They’ll have spare parts readily available to keep those machines working as efficiently as possible,” Myler said.

Once the deck is removed, it’s time to bring the bridge down.

PennDOT did not choose to use explosives for show — they are necessary because it would be too tricky and dangerous to demolish the bridge’s three arches using construction vehicles, Myler said.

PennDOT is playing it safe by establishing an 800-foot perimeter around the bridge in every direction ahead of the explosion, protected by police and park rangers. Myler expects the blast radius to be closer to 200 feet.

City police will fly infrared drones over the surrounding woods to make sure no intruders sneak in, Myler said.

Workers will prepare for demolition by inserting hundreds of explosive charges into pre-drilled holes along the bridge. The holes are wrapped in black geotextile to minimize the structure’s degradation.

They will then press a button to trigger all of the charges at once, but they will explode milliseconds apart to shift the bridge away from its nearby successor as it collapses, Myler said.

“It’s one press of a button, [but] the charges are all set off in a very unique pattern to make it fall the way we want it to fall,” he said. “You’ll have the bridge come down in chunks.”

The explosives are strong enough to break through the bridge’s three arches and steel rebar. Chain-link fencing hugging the bridge will limit debris flying from the explosion to pieces no larger than two inches in diameter, Myler said.

While the explosion itself should be fairly confined, it is certainly going to be loud, Myler said.

“I think residents in and around will definitely notice the sound of the blast,” he said.

The foundation piers for the new bridge will quite literally rise from the rubble, since they are already constructed under the current bridge.

Workers have covered the piers with layers of soil, timber and steel plates — “like a cake,” Myler said — to provide as much of a buffer as possible. They will remove the layers after the explosion and inspect the piers underneath before sliding the new bridge into place.

What if something goes wrong?

Because the Commercial Street Bridge carries the vital artery of the Parkway East, any demolition gone wrong could delay its replacement and create disastrous ripple effects across Pittsburgh’s roadways.

PennDOT has not yet determined a date for the explosion, but it will provide 24 hours’ notice when it does, Myler said.

Myler said that emergency services will be available onsite during the blast, but he believes that PennDOT has taken enough safety precautions during the process to ensure the demolition will go smoothly.

To provide another layer of protection, crews will enter the blast site after the explosion to make sure each of the charges were set off as expected, Myler said.

PennDOT will keep a close eye on the weather forecast for the day of the blast. Low-hanging clouds, for example, could carry blast materials outside the expected radius, Myler said.

“If we knew it was going to be heavy storms or something like that, maybe we need to push it a day, but for the most part, we’ll make that call a day in advance,” he said.

Fate of the old bridge

Workers will pile up the remnants of the old Commercial Street Bridge and leave them until the new bridge is open, Myler said. Then, they’ll begin loading the material into trucks and hauling it out, while rubblizing any remaining chunks that are too large for transport.

PennDOT plans to recycle the vast majority of the old material. The rebar is destined for a scrap yard, where it will be cut up and salvaged, while the concrete will most likely become “aggregate,” or fine-ground chunks that could find a new home as part of a driveway.

“It’s the nice thing about all this. … We are able to recycle a lot of the materials that were placed in it,” Myler said.