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Soil-Structure Interaction

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I have a sort of a geotechnical question:

A steel structure stands on a thick concrete slab, which in turn stands on pile footings. When evaluating lateral seismic forces acting on the piles, do I include the weight of the slab, or does it just "move with the ground" as an integral part of it.

My first impression is that one has to include the weight of the slab, when determining the base shear at the top of the piles, because that although the slab will also be ground supported (cast on ground), any soil settlement will cause the slab to rest solely on the piles, and then one can't rely on any friction (sliding resistance) between the slab and the ground.

Our client wants to know if anything can be done to exclude the slab weight, since it has great effect on the pile foundation costs. Geotechnical investigations have not been undertaken yet, apart from some rough estimates, so I don't know if any liquefaction potential exists on the site.

Any thoughts on the subject are appreciated.

Gunnar Hafsteinn Isleifsson