Tuesday, December 6, 2011

Building a Freeway: Moving Dirt

Before you can build a freeway, you’ve got to move a little dirt...

Make that a LOT of dirt, especially in the case of the Loop 303 project under construction right now in the West Valley.

Crews are working to construct additional lanes and plan to move about 3 million cubic yards of dirt by the time the project is finished. (By the way, a lot of work is planned for the Loop 303. The portion of the project we’re talking about runs from Mountain View Boulevard, south to Peoria Avenue.)

That much dirt translates to roughly 200,000 truck loads – and these are some big trucks (take a look at them in action in the video above).

Other projects may, or may not require this much earthwork. It depends on the road design and the original shape of the terrain.

Take the Loop 303 project for example...

Part of the freeway (starting slightly south of Greenway Road headed north) is going to be depressed ... in other words, freeway traffic will pass underneath the nearby local streets. The rest of the freeway (heading south of Greenway down to Cactus Road) is going to be elevated and will pass over local traffic.

So, parts of this project need to be dug out and other portions need to be lifted up. The dirt that crews dig out in some areas helps to build the overpasses in other spots.

If you’re lucky, it all balances out. Otherwise, purchasing additional dirt is an option. Or, if there is a surplus, the extra dirt often can be utilized in landscaping and to build berms.

Moving the dirt...
Before any digging can start, the dirt is pre-wet to make it easier to move and help with compaction and dust control.

After that, there’s some heavy-duty equipment that comes into play.

Typically, an excavator is used to dig up the dirt. The excavators being used on the 303 project have buckets that hold about seven cubic yards worth of material at a time. Those load onto something called a belly dump truck (see photo below).

Scrapers also are used to dig up the dirt. Depending on their size, scrapers can hold and transport about 14-18 cubic yards at a time.

Contractors have geotechnical reports that tell them the type of soil they’re going to be dealing with. Crews sometimes will run into rocky terrain on projects, which requires blasting

This belly dump truck is transporting dirt. 
Whether they need to blast or just dig, crews have to put the dirt somewhere...

If it’s being used on the project (as is the case with the Loop 303), the dirt is hauled to where it is needed.

It is dumped in 8-inch lifts (or layers) and is knocked down with a compactor and roller until it reaches the necessary compaction rate – again, this is where pre-wetting the dirt comes in handy.

Crews will continue to place the dirt and compact it until they get it to the grade (or height) they need it to be.

After that, all that’s left to do is put down an aggregate base course and some concrete ... but we’ll save that for another blog post!
Posted by Angela DeWelles   |  Labels:  Belly-Dump-Truck, Building-a-Freeway, Excavator, Loop-303, Moving-Dirt, Video


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