I had a very similar need a couple of years ago, on a military base and in some oil field applications -- both of these were environmental cleanup projects, so the concerns were similar--solutions were not real cheap, but certainly have potential to be the best solution for your situation.
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This sounds like a very difficult situation to me.
The groundwater will be a big problem--you may want to check with your geotechnical expert to model what type of piping you can expect. The "step" will mean there are more locations that water can seep in, and must be controlled.
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