Well they enabled digging and dredging, so it would seem logical that dropping dirt would be enabled as well. I do see the technical hurdle, though, in making sure that dropped dirt doesn't exceed the minimum dirt height that was required to plan the bridge in the first place. The particular issue I'm experiencing is that I dug down all the dirt under my bridge so I could do a little surface mining, but once I was done I couldn't cover it up with dirt again.