Slab heaving from expansive soils

Posted on May 10, 2019 by Jeremy Livermore, S.E., P.E.
The scope of work indicates a 5” slab with 8 helical piles (I presume equally spaced in a manner as shown below in the concept)….there are 2 ways to interpret this proposed design.

1. the slab is meant to span to the 8 helical piles like a “2-way” spanning slab (such as in a building) in the event that the soil subsides below it (which is extremely improbable given the compaction that will occur) or soil expansion/heaving lifting the slab. If this interpretation is correct, the 5” is not thick enough and will fail in flexure at mid span and punching shear at the supports.
2. the slab is not meant to span to the 8 helical piles and is meant to sit directly on the imported base without the consideration for heaving or subsidence of the soil. If this interpretation is correct, there is no purpose of the 8 helical piles as the slab is being supported uniformly by the soil without the piles.