Terry Ruth wrote:1. FPSF are driven by world wide IRC code and/or ACE 32. ACE 32 points back to the structural requirements of code or a PE analysis, more specifically bearing capacity, plastic, and moisture indexes to resist the loads of the building, or, transfer them to the ground. In most cases, that exceeds what rocks on soils are capable of, yielding deep footing depths past frost lines in cold climates that exceed the minimum 12" FPSF. But it can happen on certain high bearing strength soils or bedrock. Soil test are recommended.
2. If the above structural requirements are meet, a sieve test would determine the needs for insulation. Non-frost susceptible passing #200 sieve or less than 6% granulars by mass = n/a. That is the first step, establishing a requirement. The second step is finding a spec that applies to these foundations or a PE, since ACE 32 or IRC/ACI does not apply.