We were very pleasantly surprised to find that the rear wall that holds back the earth bermed north side of the house has a caisson foundation.
I had never heard of this type of foundation, and it is not generally listed among the foundation choices. It is the best foundation for expansive soils, which is why there was no evidence of cracking in the foundation of the house. Mark Benjamin suggested replacing the void as he has seen the dirt lift the walls right off the caissons. The dirt was all excavated from underneath, mostly with the claw of a hammer, and the space will be isolated with insulation. We considered replacing the cardboard forms but they are used to establish the space in the first place and since they rotted, we decided that the expense was not justified in addition to the insulation.
There was a board under the void form that rotted. The insulation does not seem to come all the way down into the void, so the rocks at the bottom of the french drain are spilling into the void here.
Most of the cardboard void forms were completely rotted away and dirt had heaved into the voids.
The void form impression at the bottom of the concrete wall has a honeycomb pattern.
Tar paper, plastic and foam on the outside. I measured the foam and it was 4″ thick.
After the dirt was excavated from the void, it created a clean open space to protect with the plastic and foam preparation for the new floor.