(Not a Teacher)
1) You mean a pontoon bridge. They're essentially "floating bridges" that use something buoyant to keep them afloat such as air filled barrels.
2) A bank is the slope of a hill (in this context).
3) This simply means that the trucks left, with 'ground' (in the sense of 'to grind') suggesting this was done with great effort. Not enough context to tell you what they're driving out of.
4) Since she liked him, she opened the door for him.
5) I'm not sure exactly what the author means by 'heavy' here. It gives me an impression that I can't quite convey in words.
6) They're vehicles powered by motors. It's an unnecessary distinction nowadays.
Interested in Language