I don't see a picture.

There are endless possible fixes, and I do mean possible. There is a kind of bottom line, if a crack developed once it would be reasonable to guess more cracks will develop in the future. And the existing crack is likely to grow, shrink and/or move. Normally I hear about this type of issue with a poorly made concrete pond. When the underlying structure (concrete and rebar) isn't stable these ponds crack, people patch, more cracks, more repairs. Never really ends.

Clay could work if the crack can be plugged. If the crack allows water to flow out into the earth or out a side I assume clay would wash way with the water. Normally clay is used to plug almost microscopic pores in a less dense soil.

So the idea would be to make the cracks smaller and smaller. Wide cracks can be filled with expanding foam or anchor cement. Anchor cement can be made watery to get into small cracks and it expands slight to seal well. Very hard and durable too. You could try to get sand into the cracks, then dirt and finally bentonite. The sand makes smaller holes which the soil fills making smaller holes which the clay fills.

Would like to see a picture, sounds pretty cool.

I'm trying again with the picture, hope it works. My computer is dying, will be offline for 2-3 weeks while it goes back to manufacturer.

meanwhile, thanks for the tips!

Bentonite is used for sealing wells and artesian wells which have a significant amount of force to them and it can be used for a project like yours. However it must be applied dry so that it can expand to seal those tiny cracks as it absorbs water.

If you can dry and crush the clay you have access to it is doable but will be a long and frustrating task.

Enough clay can seal most anything. Like in a well there could be 100' of clay.