This giant checker board is actually the “guts” of an unbonded post tension slab.
Post tension slabs are used to create a monolithic slab that is stronger than the typical slab and foundation that is poured in stages. Post tension slabs are used in many parts of the country where the soil has high shrink / swell potential, also commonly know as expansive soil because they resist soil movement. Many parts of the Phoenix area have soil that is expansive and exhibits a high shrink / swell potential. The USDA has published a shrink / swell potential map for Phoenix which identifies the cities or towns most affected by this condition.