We discussed in the past the importance of concrete foundations being protected against frost heaving. The frost depth in most of New England is between 4 and 5 feet below finished grade, but sometimes it is not practical to achieve that depth (NH is the Granite State for good reason!). Fortunately, the Building Codes for both one- and two-family homes (the IRC) and commercial buildings (the IBC) permit alternatives.
Most commonly, when achieving frost depth is not desirable, we design Frost Protected Shallow Foundations. By installing properly-designed insulation (yes, we are insulating the ground to protect it from freezing!), shallow foundations can be buried shallower than frost depth. One method is illustrated in IRC Figure R403.3(1):
Frost Protected Shallow Foundations are particularly practical with slab-on-grade foundations, desirable for detached garages and smaller additions where a full basement is not needed.
Both the IRC and IBC reference the publication ASCE 32-01 “Design and Construction of Frost-Protected Shallow Foundations”. We have designed many of these foundations for heated buildings, unheated buildings, strip footings, isolated spread footings, additions, and standalone buildings. With the right combination of insulation and non-frost susceptible fill materials (like gravel or crushed stone), these foundations can be less expensive than and perform comparably to traditional buried foundations like concrete stem walls.