how to keep a square hole for a fish tank from caving in

I have a big rectangular hole, 15 feet long and 6 feet wide. It is on a slope, so the deep end is four feet below ground level and the shallow end is two and a half feet. The soil is undisturbed clay, which is very good at holding a wall. Still, I know it will cave in eventually. The hole was supposed to be for a small structure that never got built. Plans changed, and now the structure will have to go 3 feet away from the hole. So if the hole caves in at some point, it will damage the structure.

Right now the hole is full of wood chips with the edged braced by old pallets and metal T posts.

I'd like to have a big fish tank in ground, insulated, with a low tunnel or cold frame over it to keep it warm over the winter. This would also provide lots of thermal mass for seedling plants started on trays over the water.

However, I need a way to keep the hole from caving in and crushing my tank and creating other havoc. What tank materials could stand the strain? I'm on a very tight budget.

The things I've thought of:

EPDM liner over timber shoring. I assume this would rot out over time, EPDM would be hard to fold square, and might get punctured by wood over time.

Ferrocement. Can ferrocement be made square, or would this lack too much strength? Also, it is expensive, lots of work, I have no experience with ferrocement, and I'd probably need temporary shoring to keep the hole together while I cement it. Plus, if it leaked it would be a big problem.

IBC totes. If I put 2 inch polystyrene insulation sheets on the outside of the metal cages, would they take the strain? Of course, they are pricy. And they'd have to have the tops cut off, weakening them.

Concrete block work is out of the question.

What would you do?

I don't have a great answer but I do have some hope.  If the shallow end is 2.5 feet deep, that means the uphill side would have 2.5' underwater with a 1.5' cliff face above the water's surface.

My guess is that the greatest reason for the clay to cave in would be either the fish pond water itself seeping into the clay around it or surface runoff and snow melt sinking into the clay to soften it.

What about if you dug out a bit more on the uphill side to create a terrace at the water level?  So the pond would stay the same size but there would be room on the uphill side for a small retaining wall above waterline to hold back the rest of the hill.  Or a shallower slope to the above water cliff face.

Is the future structure that you are worried about uphill, downhill or sideways from the hole?  I'd be mainly worried if it was uphill.

It is sideways from the hole, paralleling it three feet away. Of course now I wish I hadn't got myself into this position, but plans changed halfway through a bunch of projects.

Your idea is a good one. I'm not sure if I can dig further uphill though. I'll have to see.

I would line it, weigh the edges and bottom down with  stone or concrete.
I would also add a drain pipe,because needing to pump or bail it out could suck.

I went and asked my husband, since he's been keeping fish in aquariums for like 15 years. He's thinking that the soil will likely cave in, despite best efforts. (He doesn't really do dirt or foundations, so perhaps there is a way to support the dirt, but he can't think of any). Also, if any water freezes in the tank over winter, it has a high chance of cracking the tank. We lost a 60 gallon tall tank to a sudden freeze. It took about two days for the ice to kill the tank--he'd figured he'd have time to drain it, but alas did not.

He's wondering if you could just use something like plywood and then spray that with marine epoxy to make a make-shift tank, without the glass.  If you make a box/"aquarium" out of 1 inch thick plywood, and then spray that with epoxy on both sides, wait for it to cure, and then drop it in the hole.

He says the epoxy is long lasting and non-toxic. I, honestly, however, do not know.

If the hole is finding a new use, why not change the shape of the hole to be more accommodating? Instead of reinforcing the walls, dig them out so it becomes a pond.

I agree. A square hole is never going to last as is; you can build a strong wall around it, or change the shape of the walls to be stable. IBC totes will not be strong enough over the long term. I do think the most durable method would be to dig out the top half of the hole to a stable slope (whatever that is for your soil) and let the bottom half fill in at a slope. You can put in some retaining wall or cribbing, depending on what materials are abundant for you, to shore up the side next to the relocated structure.