[johndallman]

It's important to distinguish between the different types of canals. The classical British industrial canals simply aren't deep enough for the builders to hit bedrock: they're usually only about 5' deep. So explosive aren't much use in building them at TL5.

Ship canals, intended to bring ocean-going ships into cities, are much deeper, and removing bedrock is a normal part of building them. Blasting may well be useful.