slab subgrade

[LMG66]

A note in the plans says " 6" porous material" for use under slab.
What is the reasoning behind this?
What constitutes a porous material? 3/4" crushed stone? other?

 

[JAE]

Many times there is a "porous" or free draining type granular material specified to form a moisture barrier of sorts based on the idea that a uniformly graded rock (think lots of marbles all the same size) will have small voids between them that will inhibit capillary action of ground moisture from wicking up to the bottom of the slab and then create upward vapor pressure through the slab.



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[Ron]

This is to provide a capillary barrier. Essentially anything with a fineness modulus of 3.0 or above would be good.