pt ground slab cracked

huangyhg

pt ground slab cracked
i recently looked at a pt foundation slab supporting a 3 story building.  the footiprint is rectangular but necks down at the middle.  at the middle there is a 1/2" wide crack through the 4" slab (from front to back).  buidling is 5 years old, on clay with excessive water around the foundation.  have not accessed the top of the slab yet. no sign of broken cables.  i suspect settlement. anything else i should look for ?

what about the possibility of expansive soils?
i think it would be worthwhile to model this structure with it's foundation and soil interaction. a 4" pt slab just doesn't have the stiffness to transfer loads excepting to a rock foundation without excessive deformation.