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I have a piece of land on the rocky coast of Maine, where I plan to build a house. The soil depth there is just 0″ to 24″ to bedrock. In part to save on the cost of blasting, I plan to have just half of the foundation be a full basement; the other half will just be crawl space. What is the best way to anchor or pin the foundation to bedrock? What are the proper kinds of anchors or pins to use? How long should they be? How should they be driven, how deep, and how far apart should they be spaced? What else should I know? I will have the work done by someone else, but I want to know as much as I can before I give the go-ahead on the work.
Scott
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Hi Scott,
Why do you think it important to pin the footings on a residential project?
As you indicated, the house will rest on solid rock.
Gabe
*Scott,If you have a proper set of plans for the house, the foundation sections should answer your questions.Ed. Williams
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Gabe,
I've often wondered about that myself. I mean...where's the footing going to go???? You pour concrete on the rock and it conforms to the irregularities of the rock, locking it in place pretty well. Around here, if the rock slopes, we have to pin the footing to the rock on 6' centers. All they ever do is drill a hole and drive in 5/8" rebar. I guess they figure if the rock slopes, the footing might slide out of position and take the wall with it. Go figure.
Wayne
*Scott-Here in CA engineering would be required and that would give you the specifics (size and length of holes and rebar, depth, spacing, etc). We epoxy each hole and insert rebar pins after drilling with rotary hammer and clearing with compressed air (needle fitting on the end).Last job I did had #5 pins inserted 11" deep and 12" oc in granite with epoxy. Have no idea what would be required in your area (we have earthquakes and high snow loads here), but imagine it would be something of that order or (hopefully) less.In some cases, I've heard of engineering requiring level benches to be cut into the rock in addition to the pins.Best....Don
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Scott, I'm not an engineer but in the past I've worked for builders in Me. doing similar jobs. The footings were stepped to follow the contour of the bedrock and maintain the minimum 8" footing thickness. We used two rows of 1/2" rebar held on 1/2" rebar pins drilled about 8" into the bedrock and held with hydrualic cement (Quickrok is one brand). We washed the bedrock and applied a bounding agent to the bedrock. I'd be careful of cracks and fissures in the bedrock .....when what your house is sitting on moves in two different directions, well.... If it were me, I'd talk to an engineer to get rebar sizes, spacing etc. I think pinning is a good idea ...Maine does have earthquakes (thankfully infrequent and small) and it just might keep your house where it supposed to be.
*Hi Wayne,Because there is so little that we know about this site, I have to assume a few things.There was no indication of any slope.Scott indicated that he was going to have to excavate into the bedrock. This probably includes blasting. If the rock is fractured, either by nature or explosives, the last thing I would do is pin it.Instead I would put a leveling sand or crushed stone buffer down between the rock and the concrete footing. I wouldn't want part of the rock to be stable and the rest to move or settle under my footings.Gabe
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I have a piece of land on the rocky coast of Maine, where I plan to build a house. The soil depth there is just 0" to 24" to bedrock. In part to save on the cost of blasting, I plan to have just half of the foundation be a full basement; the other half will just be crawl space. What is the best way to anchor or pin the foundation to bedrock? What are the proper kinds of anchors or pins to use? How long should they be? How should they be driven, how deep, and how far apart should they be spaced? What else should I know? I will have the work done by someone else, but I want to know as much as I can before I give the go-ahead on the work.
Scott