Construction & Contractors/Rock anchor foundation and frost
I am a contractor doing preliminary research on a home for a client on a solid basalt rock bluff in North Idaho. Max soil coverage is 0" - 6".
If the loose soil is cleared to clean rock can holes be drilled every 4 feet to a 3' depth and then grouted around rod or #6-8 rebar?
If so, how do you account for frost passing through / under the footing into the crawl space or under the slab?
The danger from frost heave results from the permeation of free water into the pore spaces of the substrate, freezing, and displacing the material. A hard intact basalt probably has very poor permeability, so access to the pore spaces by water is very limited. The heave potential is related to the volumetric water content. Furthermore, basalt is very stiff, meaning it can tolerate large stresses with very little strain. What water does enter the shallower discontinuities and freezes will stress the rock, but the rock will strain very little in response. The same goes for the steel used in doweling a foundation to rock.
If the footing and slab are poured in intimate contact with clean rock, the only space for water to occupy is the extremely small concrete-rock interface. (Assuming the basalt is in fact intact and relatively free of vesicles and scoria). Water percolating from the surface into this region will generally pass around a tight interface. If there is concern for significant water intrusion from below, drainage to a free outfall must be provided.