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According to Chapter 18 of the International Building Code, foundations with slabs over an under-floor space are isolated foundations.  The slabs are supported by deep foundation elements (piers or piles), which are the only elements bearing on the soil.  The piers or piles are designed to deep enough below the active zone, the depth at which soil volume changes from season to season, so as to resist the uplift forces associated with expansive soil swelling into the piers or piles and trying to pull them upward.

If the under-floor space is accessible, the under-floor space is called a crawlspace.  Composite concrete slabs with steel beams and similar structural systems are often installed over crawlspaces, with the plumbing being suspended from the slab-on-crawlspace with conventional hangers after the steel beams are installed but before the metal deck and concrete slab are installed.  Slab-on-voidform foundations are different from slab-on-crawlspace foundations because the voidforms do not allow the under-floor space to be accessible as the voidforms are not removed.  The most common type of voidform is a carton void form that will degrade when exposed to the moisture in the subgrade.  

Isolated slabs protect the slabs from expansive soil damage by isolating the slab so that the only elements in contact with the subgrade are designed to withstand the forces associated with shrinking and swelling expansive soil.  Isolated plumbing similarly protects the plumbing under isolated slabs.  Non-isolated plumbing (buried plumbing or non-isolated void systems) is not effectively protected.


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