How to build a culvert

I pass by this work site walking from IU House to the hospital or town. This is where they are building one of those wide pedestrian paths.

The first day I passed by, I noticed what was going on. They were building a culvert where the walkway will cross a ditch right after the walkway crosses a road. They placed this long, inflatable tube in the ditch. I'm not sure if it was full of air, water, or what. Then they built forms around the tube at each side of the walkway.

The next day, they were mixing concrete on site and pouring it on and around the tube.

And the next day, the tube had been deflated and removed. The last picture shows a couple of deflated tubes laying near the worksite.