Bridge building 101: Salmon Creek Interchange Project

Tuesday, October 16, 2012

 by guest blogger Heidi Sause

Take a digital trip to the Salmon Creek Interchange Project in Vancouver for a high-flying journey of rebar, clamshells and cranes – here’s a step-by-step overview of how crews are building the base of the new interchange at Northeast 139th Street. (Note: Don’t try this at home.)

Step one: Dig a hole to China. Alright, not quite, but 130 feet deep is close enough. Crane operators use a giant oscillator to drill large metal casings (nine feet in diameter!) into the ground.  A huge clamshell attaches to the end of a crane and bites mounds of dirt out from the inside of the casings.

Crews assemble one of the 54 rebar cages that
will form the interior of the bride piers on the
new NE 139th Street bridge in Salmon Creek.
The end result is something engineers call a bridge shaft – a deep, cylindrical hole reaching from sun and sky to bedrock – that is, the Troutdale Formation. Trivia fact o’ the day: the Troutdale Formation is a layer of rock and silt several millennia old that was carried more than 500 miles and deposited in this area during the Great Missoula Floods. Cool, huh? But I digress.

Back to bridges… The shafts are important because they form the foundation of the bridge. They are the crucial first step toward getting a bridge off the ground and in the air.  

Step two: Build a rebar cage. Using a variety of large rebar, construct a continuous structural web of metal. Some of the rebar pieces are more than 2 inches in diameter, and each rebar cage weighs up to 80,000 lbs!

Step three: Use two large cranes and a complicated rigging set up, lower the massive metal web of rebar into the drilled shaft.

Step four: Pour 230 cubic yards of concrete into the shaft. Keep in mind, concrete needs to flow at a steady pace in order to set correctly. A well-orchestrated fleet of concrete trucks tags out at the pump truck to keep the pour flowing smoothly.

Step five: Remember the large metal casings mentioned in Step one? Crews will use the same casing pieces to drill 54 separate bridge shafts for the new interchange, and the casing can’t stay in the ground while the concrete sets. An oscillator steadily lifts the casing out of the ground so when the concrete goes in, the casing comes out.

Step six: Detach and remove each casing piece as it’s lifted above ground. Set aside for cleaning.

Repeat steps four through six until the concrete pour is complete and the hole-to-China has been replaced with a concrete bridge shaft, waiting to cure. 

Then brace yourself and get ready to start over – one down, 53 to go!

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