How could I repair a leaking underground ibc?

Ideas on.. how to seal a leaking ibc. The hard part being- said ibc is underground!

It is a 1000 litre plastic ibc container, underground as part of a rainwater collection system. It has sprung a leak somewhere and the water slowly leaks away. I only have access through the top-opening (15cm diameter circle). Because the ibc has raised beds and a greenhouse built over it I can only access about 50cm square of the top.

What could I try to enable me to still use the container? Or to still use the space for rainwater collection?

I did try contacting some companies who supply industrial ibc-liners. But they all have minimum order quantities of 300 or so, and none want to supply me with a sample (it was worth trying!).

Iam just guessing here. But it may be the Valve at the bottom of the tote that is leaking. You probably have to dig it up to access it. Or you might could drain the tote and pack clay around the valve from the inside to slow down the leak. 🤔

How do you normally drain out the rainwater? Through the bottom valve? Or some other method?

Does the tank empty completely through the leak?

If the valve is not in use, Silas' idea of packing a layer of clay in the bottom is a good one.

Are you drawing water from the top or from the bottom at the valve? Is the leak as low as the valve, in other words does the tank completely empty? Is the leak higher in the IBC?  If you're drawing from the valve level is it the valve or could it be the plumbing down stream of the IBC?

I only fill and empty the ibc from the top access.

The leak must be really low down as it does entirely empty. The bottom valve was both closed, and had an extra screw-cap fitted closed before I buried the ibc!

I can't dig it up- there's a huge greenhouse and massive raised beds built on top of it! I'd have to destroy and the rebuild the greenhouse!

Put putting clay in the bottom... genius! That I could completely do. Not sure I can really pack it down as I can only fit my arm through the access hatch- but I can certainly add clay, poke it a lot with a tamping tool and hope it settles in a waterproof manner. I have plenty of clay from when we dug out the greenhouse heat sink!

Other ideas from outside of the forum have been a 'pillow tank'- ie a 500L top-access only flexible water tank, put inside the ibc. But these are rather costly.

I think the clay will work great. It would be nice if you could put a retainer for the clay inside the IBC around the valve area to prevent stirring it up on fills. Even if the layer covered the entire bottom to cover the leaking valve the loss of available water wouldn't be that great.

It's been a while Charli, did you try the clay? Did it work?

I'm in the UK too, going off grid with IBC's, do you have any photos you can post for inspiration please?

I am suspecting that the IBC as collapsed with ground pressure.
You advise things above it are heavy and those tanks are not designed to be underground.