The idea is very narrative and creates a scenario that players would want to explore. It's like the climatic scene in a movie and playing to find out what happens is a great idea. Not following the template is a bit of an issue and might cost you when OPR reviews the submissions. There could be a map showing how this is to be set up. There are a lot of terrain pieces that need to be set up just right, so showing how you want it done would be helpful.
Some of the rules are a bit confusing. Do you roll every turn for each charge planted?
Also, it seems that the ones blowing up the bridge are going to lose each time. They have to spend an entire turn setting the charge? You'll need 3 charges to have a chance at blowing it up - that's a hefty amount of time not using units while the opponent can just keep fighting as usual. I think this is a good idea, but might need some play testing to iron out kinks and discover potential potholes.