Check this thread:

http://www.poolforum.com/pf2/showthread.php?t=8857

I think I remember measuring 17" of ice in my IG. To add a little more, the underground skimmer line pipe would freeze if filled with water. I have a rectangular plug of foam for the skimmer opening to stop an inrush of water to give me enough time to use a shop-vac and suck the skimmer line dry. Then use teflon tape and silicone seal on the gizmo threads (it peels off easily in the spring) to plug the pipe at the bottom of the skimmer. Then from the pump basket in the poolhouse I fill the line with RV antifreeze with the level in the pump basket higher than the pool water level. This keeps a positive pressure at the skimmer in the remote chance the gizmo should leak at the threads. If it does then the AF will prevent pool water from getting into the skimmer line. I know....overkill. The skimmer is then packed with foam as I said in the referenced thread. The skimmer does fill with water and ice and no damage so far.
Al