How deep must compacted earth be for a concrete slab on ground floor located at the first floor above grade?

The correct specification for the depth of compacted earth beneath a concrete slab on ground that is situated at the first floor above grade is typically around 8 inches. This depth is significant as it helps ensure that the concrete slab has a stable and well-supported foundation, which is crucial for preventing cracking, settling, and other structural issues. Adequate compaction of the earth not only provides this stability but also helps manage moisture levels that could adversely affect the slab over time.

Moreover, this depth is aligned with common construction practices and codes that dictate the minimum requirements for subgrade preparation when placing a slab above grade. This depth can vary based on the specific conditions of the site and local building codes, but 8 inches is a generally accepted baseline in many construction scenarios.

While the other options present various depths, they tend to either exceed typical requirements or fall below the necessary specifications that offer a balance of stability and support for the concrete slab. Therefore, 8 inches represents a well-established standard in the industry for this type of construction.