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Re: shale backfill

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Well, by sight, it would appear to be a GW/SW mixture, with a small percentage of fines, based on what I've seen around my area, and on the one jobsite I've been at with shale in PA (near Nazareth). I suppose that it might even lean towards GM or GC (the latter a group II) in some cases. I haven't seen a sieve on the stuff, and I suspect it would vary by sight depending on the weathering.

A simple sieve analysis should get you an easy Class I / Class II answer. I've misplaced my classification notes, but I think the percentage of fines needs to be higher than 6%(?) to be considered significant. Anybody fresh from a geotech class have the right number?

My question about shale is that I don't know how stable shale is over time, or what the micro-composition is. Some of the shale around here can be broken down by hand into fines (silt/clay), and ripped shale on building sites often includes a good amount of finer particles (most site here have clay, so it could just be the soil mix). If there ends up being a moisture infiltration problem, will the shale disintegrate into finer particles in the path of the water?


Jim Wilson wrote:

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

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