From the Rearview Mirror: Caissons

With hundreds of blog posts in our archives, we understand if you haven’t had a chance to read them all.

However, there’s a lot of interesting content in those early posts and we don’t want you to miss out. That’s why we’re looking back and highlighting some of our favorites in a series called, “From the Rearview Mirror.”

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Originally published on Oct. 21, 2011, this blog post highlights something that most people don’t get to see every day – a bridge’s substructure. In the post and accompanying video, you’ll learn all about caissons and how they give bridges strength. The video also gives a close-up look at how caissons are built and put into place – an interesting process to see.

Bridges are a favorite topic here on the ADOT Blog and in the years since this post was first published, we’ve continued to blog about the structures. If you’d like to learn more, revisit some of our previous bridge blog posts after you’re done learning about caissons.

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 (eight truckloads) of concrete each.

If you’re keeping tally … that’s eight 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.