Blowing Bridges with Satchel Charges. Wooden Bridges with go down nicely with a SC. However, for the scenario you have in mind, I believe Stone Bridges will only do, as Large Calibre Artillery will take out Wooden Bridges easily. (BTW Large Guns can on occasion take out Stone Bridges as well.)
In order to make what you are doing work I suggest a double bridge or even a rail/road bridge combo. I tried a Bridge at Remeagan type battle once with a road/rail bridge with a couple small islands of impassable terrain to simulate the caissons it worked pretty good and allowed for the bridge to be damaged and still be useable. Now of course this was roughly a 700 to 800 meter river.
Another idea is driving an Ammo Hauler onto the bridge and getting your Engineers to blow it. This requires Engineers with big brass ones of course.
Hope this helps.
