Say you’re driving west on Interstate 40 through Wilson County headed toward downtown Nashville. Just west of Lebanon you’ll see these big hills rising up out of nowhere right next to the road. The hills are paved on top and capped off with a bridge over the interstate because you’re at the I-40 interchange with SR-840.
Did you ever wonder where all that dirt to build the highway ramps comes from? No?
Well I’m going to tell you anyway, and pay attention because there is a pop quiz at the end.
It’s a pretty elementary question, but I don’t think it occurs to many people the highway business to think much about it. Personally, I find all these little details fascinating in a behind-the-scenes kind of way. It makes you appreciate day to day existence a little more if you know about the work and details that go into it.
Below is an overhead shot of the intersection. Those ramps that go over I-40 are have to get up to at least 20 ft above the level of traffic below, and you can see they’re pretty long. The source of all that dirt fill is also in the picture. See those oddly regular shaped ponds? Those are the borrow pits.
Borrow pits are exactly what the name says they are if you’re willing to be a little liberal about the ‘borrow’ part. When you need fill, it’s sometimes cheapest to just dig a big hole you can ‘borrow’ it from. In middle Tennessee it often doesn’t actually require a pit because there is nearly always a hill somewhere that can stand to be a little shorter. If you do have to dig a pit you have the added benefit of ‘Hey, a pond!’ when the pit fills up with water.
Now that you have been introduced to the concept, I suspect you’ll be noticing them everywhere not unlike most of the trainee engineers who come through our office. I’ll give you two pieces of advice to guide you in your borrow pit spotting…….. (1) If that pond looks too regularly shaped to be natural, AND there is elevated bridge or highway nearby…. Borrow pit. (2) Your wife and other people who travel with you will get tired of you pointing them out a lot sooner than you would have thought.
And now for the pop quiz…… Below is an overhead view of a certain local airport. See if you can find the borrow pit they used to make those runways so flat and long. I’ll provide the answer in the comments if anyone needs it, but it should be pretty self-evident at this point.