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shale backfill

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It is common for local builders to use shale backfill under garage slabs.  The largest pieces are perhaps 6" typically.  The material of course does not meet any gradation requirements.
Is there a way to roughly classify the type of material under the Unified Soil Classification System (per IRC Table R405.1) for purposes of determining its performance characteristics?
This is not for an engineered design.  The builder that asked the question has said that all of the inspectors they deal with (except one) accept it as a Group I material.  That lone hold-out was looking for some type of data to substantiate the classification.
Jim Wilson, PE
Stroudsburg, PA