I have to build a bridge across a creek on my property. The creek banks are 3 feet above the creek and 14 feet wide. I need to be able to get a 2500 pound tractor over it. I was planning on bolting together three 2x10-18' to form a beam and then construct 4 of them. The bridge will be 5 feet wide and decked with 2x10s also. Will this construction hold the tractor? I tend over engineer and don't have an issue with this construction will work and is overkill.
I just ran some structural checks and everything passes if you use Douglas Fir #1 planks for the deck. Anything lower quality will not work structurally.
Beams are fine as is, just make sure they're bolted/screwed together well. Also, make sure that the wheels on either side are centered between the two beams. Don't drive it directly over a single beam. This way each beam is taking 1/4 of the weight. You do not have something that's "over engineered", it's close.
Regarding the soil, the further away you can get from the soil drop off the better. If you can get 20' long beams, do it. I also recommend placing a continuous 2x10 under the end of the beams to act as your foundation, otherwise you may rip up the soil there.