Thanks for the replies. The pond has a plastic liner and is fed by a well so the level is constant. It does does not catch any run off. Since it has a plastic liner the soil around it is dry. Anchoring or putting legs in the pond is not feasible without damaging the plastic liner.
58,000 lb of concrete is about $1500. Not a deal breaker if I need that much.
I definitely plan to over design it.
30' out puts it over about 12' deep water.
can you drain it, and pull back the liner?
what material is the liner?
does it have any sort of "bottom drains" or "retro bottom drains"? if not when was last time it was drained and cleaned?
is pond setup to be self cleaning in a sense, to help keep the poo and waste from collecting on the bottom?
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make a long 30 foot floating raft, and then make a short ramp between bank and raft. and have long 30 foot dock per say. your asking for concrete and metal, and asking the dock to last for long years from now. three 10 foot floating rafts with correct pressure treated wood. last easy 15 years when taken care of.
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if you could drain and pull back liner. that would be most optimal. and tossing some concrete down. and then put liner back down, then drill through liner into concrete, and set some anchor bolts and attach a flange to the concrete on top of the liner. the flange = clamp with concrete, and liner acting like a rubber o-ring to create a water tight seal.
if you can drain water level "some" if not almost down to were liner is. you can still cut a section of liner out. to drill some holes. and drop some concrete down, or some post anchors down through the liner. then fix liner up with double sided tape. or a mechanical clamp / 0-ring clamp made out of some metal. (( two 4" toliet flanges = a mechanical clamp, with some bolts/nuts)) would say look at "bottom drains" for koi/goldfish ponds. in how they provide a "clamping ring" for hole through liners.
if worst comes to worst. get some thicker rubber mats. and layer them a couple inches thick or more. then build a wide base out of metal, that would fit down on the mats. ((like how a backhoe operator might toss out some plywood under the out riggers, to keep out riggers from sinking)) rubber mats there to act as a "cushion" between liner, metal bottom foot per say of a pipe that extends up.
you could do the "concrete bag" wall method. were ya lay down a row of concrete bags, and then stab them with some rebar. and then make another row of concrete bags, and stab the bags directly over the rebar. (bags and all) concrete never comes out of the bags. over time the bags will rot / tear away. but you can get a nice retaining wall. adjust this idea, in creating a "concrete bag" pad under water. were you have something to place a metal foot / bracing on. to run poles up to a dock.
other option, create a "floating ring" and drop plastic all the way to the bottom with some weights, to help keep plastic in place, and then just start shoveling in concrete right into the water, that is surrounded by plastic. the plastic will help reduce the concrete from spreading out into rest of the pond. ((dredging of rivers, creaks, mud ponds, were you do not want to send all the silt and muddy water to everything else)) the concrete will setup just fine and dandy under water. you would just need to make sure you got your PH testing supplies, and ability to counter act the concrete to make sure your water parameters do not go a miss. once concrete is cured, the concrete can act more of a buffer for pond/lake.
