Have a customer wanting this work done but I can't say I have ever done such a thing to date and wanted to get ideas. this pond is very remote and he wants a slot cut to both park a pontoon boat, and have a ramp at the end so I can retrieve the boat as well. He was thinking rubber horse mats in the bottom but I can't see that working at all! The notch will be roughly 10ft wide x 20ft long probably have 4 ft of water so I am literally expected to build this thing blind. What I am hoping to do is work what I can dry, then break my little dam open and flood it. I just don't know how I will make anything resembling a floor and ramp in this thing. I thought about aggregates but would need a pretty good layer.
Why does it need a floor? The ramp could be 3" rock, or concrete, depending on what they have to pull out with. If the water level is steady, then it makes it easy as it doesn't have to slope, the vehicle can be on flat while the trailer is sloped. Sounds like a job for 2x2x6' concrete blocks, angled back against the wall a bit, and maybe coarse rock packed between for a little lateral support. The horse mats could work if he just wants it for a non mucky surface to walk on for some reason? I wouldn't use them for support for the trailer.
I would check to make sure that no one (gov. agency) has issue with you working in the water. I am dealing with an issue now, even after getting approval from the Army Corps of Engineers, Dept. of Lands now 5 months later has issues. So word to the wise, make sure of that your clear to work below high water mark. On a Forest Service job we are on now, we are daming the water and pumping out the water for a new ramp into the water. Its a lot of work but maybe something similar would work for you.
Dig what you need into the bank but leave a strip of dirt to hold the water back.do the prep work when done dig the strip of dirt out and let the the water in
Ah where would we be without the environment. One of our sub contractors was replacing a 4 foot pipe across a road. It was a small brook, but there were small fish in it. Had to dam it on both sides, block the up stream side with hay so the tiny fish didn't get sucked into the pump. Had to do an electroshock fish rescue in the work area. People from the fishery dept had to come and scoop fish that were unable to make the commute up or down stream. Maybe they work, not sure if fish have jobs. When the pipe was replaced, someone removed the sand bags too soon and muddied the water, just as the highway engineer was driving by. Had to re dam, another fish rescue, follow the proper procedure to let the water go, and the sub got a $10,000 fine.. Next day heavy rain, brook muddy, but you can't fine mother nature.