Good day I was reading an old post started from Acworth NH. After reading I thought due to is age I would add the info below. In short yes piers can be built shallow and has been done to code . We have built numerous slab and pier homes northern NH Maine this way and it IS NOT as difficult as some will lead you to believe, and if you have a code person not up to speed the attached also shows its in the IRC and totally fine, heck you can just use timber too...well not this thread
attached is a pdf outlining the process from American Society of Civil Engineers and the IRC. I am always saddened by those that strike fear to those seeking and economical and GREENER way to build. R factors DO NOT need to be R-14 etc. most of NH and Maine are Freeze days of 2500-3000. The charts show exactly what is needed.
Example: Northern NH home, 2500 freeze factor 28x40 1 story on pier beams, with Unheated crawlspace. R-21 sub-floor, masonry heater etc. cost was 90.00each delivered for pre-cast 3' piers -18, plus 785.00 for ground horizontal and rigid foam(xps) and the vertical 2' enclosing skirt if you will around Piers and crawlspace. The gravel under home and other costs are the same as a conventional DEEP pier. Savings is a DIYer can drop and level piers themselves for 60% savings opposed to pouring anything, or hiring creter.
I know engineers will now go nuts about soil etc. but that info is easily obtained either from Web Soil Survey (free) us govt or hmm in NH DES is AWESOME has reams of data online free with the info needed just takes a little work and reading. Frankly most of NH Maine are pretty much the same and even the worst soil is buildable, with standard 16 " deep footer
Don't let the fear makers scare you it is legal, meets code and done ALL THE TIME. NH PUC energy code speaks to this also as in info the crawlspace MAY be heated we make three vent panals which the customer puts in over the summer then the solid ones put back before winter.
While NOT REQUIRED we do place 3/4 R-3 ridged board over the poly and under the 4" gravel in crawlspace. oldest home is now 13yrs with NO heave no issues and a check of the XPS was / is still intact.
hope this helps a new builder thanks