The gravel in a rubble trench foundation allows any water in the ground under the building to flow to the bottom of the trench and away from the building through the drainage trench, preventing moisture problems or frost heave.
The trench was filled with 1″ to 4″ diamater gravel inside the rigid foam insulation. Outside the insulation, subsoil was simultaneously backfilled to maintain the rigid foam in a vertical position. The gravel was packed to nearly level with the surrounding grade, ready for the concrete cap form work.
