I am getting permit to build two houses in city of Atlanta on lots that are adjacent to an abandoned granite quarry. Solid rock varies from zero to maybe five feet below grade. I’m wondering how to secure footings to the rock,and if the crawlspace is essentially rock what do I do about the moisture or vapor barrier that we place on the ground under a house. I assume the drainage is poor and there might be small springs uncovered.
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<building on rock>
Sounds Biblical!
Maybe some advice there.
Or maybe Piffin.
Same thing.
Forrest
My gut feel is that there will be a big difference whether it's really solid rock, or fairly significantly fractured. You may need an engineer's or geologist's report for starters, though the info may already be on file with the city.
Many of the lots with the best views out west are rock outcroppings, so we see rock quite a bit and it's great to build on.
Building on rock is great, but building on half rock half dirt is much harder because the rock probably won't give at all, while footings on dirt will settle a slight amount regardless of the amount of compaction.
If it is fairly solid you won't need footers since the footer's sole purpose in life is to distribute weight over a wider area. Holes are drilled in the rock and the verticle rebar that normally comes up from the footer is epoxied in.
If you're half on and half off the rock it's cheap insurance to make extra wide footers, increase rebar at the hinge point and be meticulous about compaction to minimize settling--rock doesn't settle.
Some types of rock transport moisture and some don't or do but very little. If the rock is exposed and fairly smooth a brushed on sealer would be easier to maintain than poly.
Building on good solid rock is quite fun since it eliminates much of the dirty (dirt) work!
Good building
Beer was created so carpenters wouldn't rule the world.
Check this post:
http://forums.taunton.com/tp-breaktime/messages?msg=77934.1
Good luck.
Lots of indoor air quality (and if there's a well, water as well) issues associated with radon gas and building on granitic rock here in New England. I'd check with local or state health departments to see if that's an issue there. Pinning to bedrock is pretty straighforward stuff for most foundation guys, at least around here, but this is the Granite State.
We have built on rock a few times. If you have solid rock under the whole house, there are no structural problems. Clean the rock (air helps), drill rebar pins, scribe the footers (or forms) to the rock, and pour a regular wall.
Your biggest problem in this case is moisture. This will be a problem unless you are perched on a high point. Water will find its way through small cracks and into your basement. The safest thing is to fill in the foundation and pour a slab for the floor. If the people want a basement, lay down poly, cover with sand and pour a rat slab to keep it in place. Make sure that your low point has drainage out of the basement. Both houses where we did this had enough room on the low end for a full basement.
If you have a rock that drops off, you may need to make a joint in your house that will allow for differential settling. As long as this can be incorporated in the design, it need not be a big deal since the settling will probably not be that significant(<1/2"). Good luck.
All this is very helpful, especially the link back to previous discussion. I didn't realize the problem of differential settling if site is not all and uniform rock. What is ICF? I wonder if I might not come out better building on piers, either CMU or poured or even PT 6x6?
That would help the drainage and radon problem but I'd have to place mechanicals--HVAC supply and returns, etc.
ICF is Insulated Concrete Form
Seems to me this would be a good situation to have a steel-framed foundation.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin