passive pressure

huangyhg

passive pressure
i am looking for opinions others may have for "clean" stone acting to withstand building lateral loads.  the site condition i am analyzing is the passive pressure of a 12" layer of clean stone as a result of lateral forces from grade beams.  to describe the constraint conditions, there are multiple grade beams with finished floor 6" above clean stone.  the 6" is a gap, no soil.  
can this clean stone be compacted to an affective density to provide passive pressure?  does the gap create an unrestrained condition, i.e. the stone will tend to push upwards during a lateral event?  
thanks in advance.  
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i have asked the soils engineers that i work with the same basic question, and they showed me that in order to develope full passive pressure you need significant displacement of the structure into the resisting soils, even if you thoroughly compact the soil.  however, you don't need much displacement in order to be able to reliably count on half of the passive pressure.  the army corps. of engineers in a guideline for the design of retaining walls recommends using the at rest soil pressure for sliding resistance to be conservative, but says to never use more than 1/2 of the full passive pressure.  this assumes that the resisting soils will not be removed.
not like, since it has no confinement.
thanks guys.
i thought a little about how i worded the question and the stone pushing up now does not make sense to me.  the grade beams are rigidly connected so if one grade beams moves, they all do.  either way, clean stone with a gap under the plank, to me, seems to not provide support.  i have been authorized to use 400 lb/ft^2/ft for this passive pressure?  seems high to me.