I am digging a hole in my basement to install a sump pump. Under the 4" slab of concrete is a 4" layer of material composed of ~30% charcoal briquettes, ~20% coal, and ~50% cynder or ash.
The house was built in 1952, in Union County NJ. My first thought (and fear) is that this layer of material is some sort of filtration between the house and the ground.
Any idea what the actual function of this layer of material is?