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Question
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?

Answer
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.  

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Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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