As stated on the home page, this structure utilizes a shallow frost-protected, rubble-trench foundation. Essentially what that means is the foundation's depth does not need to extend to or below the frost line for this area. Rather, a predetermined thickness of rigid insulation is added to the outside perimeter of the trench extending from the bottom of the trench to the top of the grade beam (and horizontally outside and against the grade beam, if required for the area). This serves as a thermal break, preventing any moisture that finds its way beneath the foundation from freezing, and potentially heaving. Also, the foundation utilizes rubble (in this case one inch broken stone) in place of a footing/stem-wall system, thus much less concrete is used - only an 8 inch deep by 19 inch wide reinforced grade beam - roughly the size of a footing for such a structure. This approach saves time and money, as digging is reduced due to the shallow trench, as well as the time required to form and pour a stem wall. The concrete volume is also reduced, as the broken stone rubble replaces much of it.
Diagram of the foundation including the trench, insulating rigid foam, perforated drain pipe, broken stone, or "rubble", reinforced grade beam with j-bolts and bale curb.
Trench depth and insulation requirements were determined using the Design Guide for Frost-Protected Shallow Foundations, prepared by the NAHB Research Center for the U.S. Department of Housing and Urban Development. Adobe Acrobat is required to view the document.