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We have a pipe that is too high and the contractor has proposed a plastic sheet topped by polystyrene pieces.

I have recommended rejecting this solution for a couple of reasons.

Moment slabs are generally directly cast against fine material used here. With the smooth top of fill I expect the plastic to permit premature sliding, if this were a 57stone material, maybe I would consider.

If the plastic doesn’t void the assumptions for sliding then the pieces of form certainly seem to reduce sliding resistance.

Finally, while overturning doesn’t explicitly address soil properties, I have to think that if there is going to be the isolation that the contractor proposed then the material is likely to be significantly softer than what has been tested in the past (even directly on soil embankment).

Are you aware of any research that might contradict my concerns?  I know you are not the testing many (any?) moment slabs, but I thought I would check with you first. Thoughts?

We are not aware an any new research to address this issue. Dr. Mojdeh Pajouh provided here initial thoughts below along with a figure. I also concur that friction may be reduced for resistance to sliding. That is all that we have at this point, which agrees with your concerns.

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As you said no doubt the friction will be gone to a great extent. A schematic view of forces, that friction is a resisting force against sliding (not contributing much to overturning). So there is concern.

Plus, if the soil is so soft and saturated then later large uneven settlement may happen.