I'm a Civil Engineer, so I might know what I am talking about. You are nearly asking the soil to defy physics. Soil has an angle of internal friction, clay soil , maybe 20 degrees measured from the horizontal. Your pics indicate you are trying to get soil to stand at about a 70 degree angle from horizontal. Nearly impossible without some kind of help. That help in your case might need to be a retaining wall , maybe made from flat stone, battered back at about 15 degrees from vertical.
If you don't reinforce somehow, your going to have your water inside the liner pushing outward at 62 pounds per cubic foot and the wet saturated soil pushing inward at 120 pounds per cubic foot. Guess which way your liner is going to go. Inward !
As some else suggested you need to try to exclude water from getting into the backfill soil.
All that being said , I had a similar collapse on a smaller scale, because I knew better , just got lazy and tried to use soil for liner support rather than stone.
If you don't reinforce somehow, your going to have your water inside the liner pushing outward at 62 pounds per cubic foot and the wet saturated soil pushing inward at 120 pounds per cubic foot. Guess which way your liner is going to go. Inward !
As some else suggested you need to try to exclude water from getting into the backfill soil.
All that being said , I had a similar collapse on a smaller scale, because I knew better , just got lazy and tried to use soil for liner support rather than stone.