Base material under a below grade concrete floor, if required, shall consist of?

The specified choice correctly highlights that the base material under a below-grade concrete floor should be a 4-inch base composed of sand, gravel, crushed stone, or crushed slag that is smaller than 2 inches in diameter. This option is appropriate because it aligns with industry standards for creating a stable foundation for concrete floors, which is crucial for preventing cracking and settling.

Using a mixture of these materials ensures proper drainage and support beneath the concrete slab. The specified dimensions of 4 inches are typically sufficient to provide the necessary support while also accommodating drainage needs, which is essential in below-grade applications where moisture can accumulate.

Other options suggest alternatives that don't meet standard construction practices for below-grade settings. For instance, a 6-inch mixture of sand and gravel may not provide the same level of stability and regulatory compliance as the specified 4-inch option. Similarly, using clay soil, although it may seem like a cost-effective solution, is not appropriate due to its tendency to retain moisture and not provide adequate drainage. Lastly, a 9-inch base of gravel exceeds what's generally necessary, which may unnecessarily increase costs and be impractical for the design of the structure.