I had the same problem, specialy that walk-way bridge was above a pond with a very deep and soft ground - allmost imposible to build additional suport. Solution - SUSPENDED bridge. 2 posts on each side of the wather- in a havy concrete base. Galvanized 2 cable each 5/8" will support 14 tons and its not expensive. Atach hanging cables every 6 feet with "U" shape 4x4 ( left to right) for additional support you may need 4x6 on a butom and than conect each "U" frame with walking surface - board walk. with my estimate suspended bridge is less expensive than traditional wood only and its easy and faster to be finished.All conectors for cables are avaliable from your local steel - cable deeler.