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Flowable fill

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We have a below grade tunnel which the contractor wants to backfill against using flowable fill.  The walls of the tunnel are reinforced concrete and are 14' max. tall.  I have several questions regarding the pressure the flowable fill will generate against the walls.

1)   Is the pressure a fluid pressure or is it dependant on the rate of placement?

2)   If it is a fluid pressure, what would be a typical unit weight for the fill?

3)   Should the fill be placed in multiple lifts?  If so, how much time should there be between the pouring of the lifts and how deep should the lifts be?

Any other input is welcome.

Thank you.

Patrick Ebner, P.E.