Friday, October 21, 2011
Building a Freeway: Massive underground caissons give bridges strength
You know when you drive under or over a freeway bridge that it’s a massive structure…
There are the two abutments (the upright supporting structures at each end that carries the load of the bridge span), there are usually center columns or piers, and, of course, the girders and the bridge deck (the part you actually drive across).
The girders and the deck make up the bridge’s superstructure. (You can tell how much goes into a bridge’s superstructure by checking out what goes into taking one down.)
But what you don’t see is that buried beneath the bridge is part of the equally impressive substructure formed by rows of caissons – massive steel-and-concrete pillars that support and lock in place the abutments.
The substructure, which is made up of the caissons and the abutments, ensures that the bridge is secure and strong enough to support not only its own weight, but also the weight of all the vehicles that drive across it every day.
The caissons are formed by tying thousands of feet of rebar into massive steel cages that are then lifted with a crane, lowered into drilled shafts, and filled with concrete. Several feet of rebar is left rising out of the ground, which is what the abutments are ultimately secured to.
There’s no such thing as a “standard size” caisson…their lengths and diameters vary from project to project and bridge to bridge based on several factors, including the soil type and the expected and actual weights the bridge must support.
Generally speaking, caissons for most bridges in Arizona are between 40 and 120 feet deep and 5 to 12 feet in diameter.
There’s also no set number for how many caissons each abutment will require – that, too varies by bridge.
Take the three bridges being constructed for the new phase of Loop 303 in Surprise, for example. The bridge abutments at Waddell and Cactus roads are supported by 23 caissons each (46 total per bridge), while the bridge at Greenway Road, which is only partially elevated over Loop 303, requires just 20 total caissons (10 under each abutment).
The video above shows the fabrication and installation of those Loop 303 caissons. The steel cages for these particular caissons are 95-feet long and weigh about 20,000 pounds. The cages were lowered into 88-feet-deep drilled shafts and filled with 80 cubic yards (8 truckloads) of concrete each.
If you’re keeping tally…that’s 8 truckloads for each caisson, times 106 caissons, for a total of 848 cement truckloads (for just those three bridges)…and that is just part of the substructure!
The concrete for these caissons cures in about 24 hours, clearing the way for the next phase of building a freeway.
Note: This blog has been edited since first posted to correct a technical error.