It’s finished, but what the heck is it?
For more than a year, drivers on state Route 167 near the Emerald Queen Casino have been eyeing a slowly growing mountain of earth directly north of the casino parking lot and along Interstate 5.
The oddly sculpted pile now is as tall as a six-story building and nearly as long as two football fields. With its terraced sides and flat top, it looks like a cross between an ammunition bunker and an Aztec temple.
According to the state Department of Transportation, it will start making sense very soon.
The structure is a bridge abutment, the first stage of a $528 million construction project that over the next six years will move the I-5 freeway onto two new bridges over the Puyallup River. As the bridges are being built, the existing northbound and southbound bridges will be torn down and hauled away.
The earthen structure standing near the casino eventually will provide a toehold on the Tacoma side of the river for the northbound bridge, the first of the pair to be built.
It might look like a dirt pile, but it represents $1.3 million worth of complex stabilization engineering, according to the state’s project manager, Neal Uhlmeyer. The extraordinary effort was necessary, state engineers say, because of the instability of soils in the Puyallup’s flood plain.
“The entire valley there is alluvial deposits,” said Mark Frye, an engineer in the Transportation Department’s geotechnical division. “They don’t hold together well, and they contain lots of organics, which continue to decompose and decay after long periods. When you put something very heavy on them, they’re inclined to settle.”
Soil along rivers is nearly always unstable, Frye said, but in the case of the Puyallup, which flows off an active volcano, the stability problems are worse.
Lahars and avalanche debris flows that peeled off Mount Rainier during past centuries deposited unstable mixes of mud, silt and organic material throughout the Puyallup River delta – material that’s hundreds of feet thick in places.
The abutments for the new freeway bridges will be built on top of the Osceola Mudflow, the largest Rainier has produced in 10,000 years, and which carried shattered forests all the way to Commencement Bay.
To stabilize the soil beneath the new bridge abutment – and the other three abutments in the project, when their times come – construction crews buried 857 “stone columns,” each 3 feet in diameter and extending to solid ground 40 feet to 60 feet below the surface.
The columns are vertical stacks of compacted gravel, created by vibrating probes driven underground by vibration and jets of water. When they reach solid ground, gravel is fed to the tip of the vibrator and compacted as the probe withdraws and repenetrates.
The technique, also known as “vibro-displacement,” generates such violent lateral ground movement that engineers were afraid to use it at the new bridge abutment’s western end, which lies just a few yards away from a major City of Tacoma utility corridor.
“There’s a lot of movement associated with stone columns,” Uhlmeyer said. “We had to use a different technique when we got within a certain distance of the utilities.”
In that area, crews created 175 “soil-cement” columns – each 6 feet in diameter and 60 feet to 80 feet deep – by stirring together cement and existing soil with giant augers equipped with mixing paddles.
Once the stabilizing columns were in place, crews began building the above-ground portion of the abutment by laying down layer after layer of compacted gravel, 12 inches at a time, onto reinforced landscape fabric and then folding the edges over, like soft tortillas, to hold the gravel filling in place. That accounts for the structure’s terraced sides.
Even with all that effort, bridge engineers expect the structure to settle at least 2 feet. To hasten compaction and squeeze water out, they built the structure 10 to 15 feet higher than eventually will be needed, to weigh it down. When settling is complete, they’ll remove the top layer.
Engineers are monitoring how much the pile settles by surveying markers on its exterior and by monitoring pressure on a fluid reservoir buried beneath it. They expect the settling process to take about three months.
“We could have done the same thing with big concrete blocks,” Frye said. “Dirt is easier to move.”
Rob Carson: 253-597-8693
rob.carson@thenewstribune.com
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