OK to Drill

Even before work began on the Hot Metal Bridge project, I noticed a number of painted survey markers along the Eliza Furnace Trail. It only makes sense when you are going to build something like a bridge, you need to know where everything is. Recently, I've noticed that a few of those survey marks have a mysterious message "OK TO DRILL" painted next to them. Drill what? Why? These things are quite a way from the bridge itself, what possible reason could they have to drill there?

Well, today on the way back after work, I saw a work crew that appeared to be engaged in drilling at an OK place. Foolish me, I neglected to stop and ask what it was they were drilling for. Maybe I'll get lucky later this week and run across them again. This time I'll ask.