wintertime pool wall flexing

[justjack]

I installed a Doughboy above ground (16x28)with a 7' deep end, last summer. This past spring when I opened it up I noticed there was a small groove on the bottom along one side wall near the edge of the metal plates that progect out and under the bottom of the pool. ( These plates, connected to the side wall vertical posts, keep the walls from flexing out) I assumed that possibly ice pushed on the walls, raising the plates, allowing some sand to fall back under, resulting in these narrow grooves. I had an air pillow in last winter although it was pretty much deflated by spring. I do not want this to happen again as eventually the metal plates might rise up and contact the liner floor. Obviously this would be a bad thing!! Does anyone have any other idea besides using a better quality pillow? Should I lower the winter water level more? any input will be appreciated