I am at a summer cottage about 100 miles north of Toronto. We are trying to repair foundations under a 12′ x 18′ cabin. The cabin is now on 10″ diameter poured concrete posts that gradually over 7 or 8 years have frost heaved and then tipped over. No huge deal as the posts only extended about one foot out of the ground.

This area was heavily glaciated and mostly granite with little soil, but the cabin appears to be in a very wide, very deep “pool” in the rock that is filled with clay. The clay is almost always wet and there is standing water if you dig down 2 feet. The plan is to dig in new foundation posts beside the cabin and then move it over (easier than working under it as it sits now). I believe that the problem with the old posts was there was no footing under them, so we intend to put a 20″ diameter footing under a 10″ diameter poured concrete post and make sure it is below frost line (about 4 feet here).

My question is what about backfill? Should it be the clay that we dig out? Should it be gravel? I have one opininion that says no gravel as it will just collect the water around the posts. There is no way to trench and drain this area short of blasting a lot of rock.

I have heard another idea that the post should be wrapped tightly in 5 to 6 turns of 6 mil poly to make it harder for the frost to get a hold of them. Has anyone out there any experince that might help me? Thanks for any help.