In my view, the concept of foundations and frost depth where it is "taken" that foundations MUST go below is a codal issue that has woven its way into the books without justification. If I have a well graded clean sand and gravel and the frost level is 3 ft, why is it necessary to put a footing below frost depth? The sand and gravel is not frost-susceptible so it is a non-issue. Yet I am sure 95% of the geotechs and civil designers will do it anyway.
For the rock, some rock, say granite and volcanic without joints would not be frost susceptible and in my view would not need, from a technical reason, to extend below frost depth. Shale, on the other hand, in my experience does frost heave when there is a source of water that can be drawn up into the shale, so again, I would be prudent and make the frost correction.
For frost to be critical, one needs (1) a frost susceptible soil (clean sands and gravels are not; (2) in a frost susceptible soil, a source of water at or near the ground surface or which may be drawn up by capillary and heat transfer action so that the effects of ice lensing, etc. can occur; and (3)a freezing plane (i.e., cold weather to freeze the soil/rock. (see:
Massive clay soils of low permeability are not necessarily "frost susceptible" in their natural state because water would not be able to move quick enough during the frost months to reach the footing to cause lensing.
In your case, if you have massive rock (little to no jointing or fracturing) a low water table (say 2B below the footing level) I do not see a need to provide frost protection from a technical point of view - BUT you may run into bureaucrats who insist on it because of the "code" - so this is why I suggested a blanket of soil over the footing rather than blasting rock to lower the foundation - clearly, in my mind, a layer of soil placement is less costly than blasting.
But, these are only my views . . .
![[cheers] [cheers] [cheers]](/data/assets/smilies/cheers.gif)