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Layer of Charcoal under basement slab

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?

Re: Layer of Charcoal under basement slab

Highly unlikely that it has anything to do with filtration; more likely, it was simply a convenient way for the previous owner to bury some unneeded charcoal and coal, etc. so he wouldn't have to go through the expense of having it taken away in a rented dumpster.

The only precaution would be that if you have to run any metal pipes adjacent to the material, cover the piping with some insulation material to prevent pipe corrosion.

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