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You are here: Experts > Education > Votech Education > Construction & Contractors > concrete post footing on rock
Expert: Robert Cummings, P.E.
Date: 5/13/2008
Subject: concrete post footing on rock
Question
Hello,
So it seems the perfect little spot for my two story 16x24 cabin is on a 4:1 slope and has solid rock under 4 inches of soil. My plan is to jackhammer eight holes, 16x16 at least one foot deep into the solid rock, then drill four 3/4 inch holes another foot into the rock per hole, then epoxy 3/4 rebar into the holes so they are sticking up through to the top of the concrete 16x16x24" inch tall pier, to which I will connect beams. Is this substantial enough or is it too much, and if not do you have any ideas on the what would be easier, smarter, or using some fastener that I have never heard of.
Thanks, Eric
Answer
You didn't say what the spacing of these pier foundations would be, or the type of construction, but as a method of attachment it sounds realistic, provided the rock is indeed intact bedrock with widely-spaced fractures (more than 3 ft or so), and the cabin is not in an area where high lateral loads can be expected (such as wind or seismic).
The method you are describing is pretty consistent with the state of the art of attaching piers to rock.
One suggestion would be to drill the holes 7/8 so there is room for the epoxy and the rebar. You will inject the epoxy and then beat the rebar into the holes -- there must be room for the epoxy to extrude up around the rebar.
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