Replies

1. 
   Guest_ | Oct 11, 1999 03:48am | #1

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   Check with your local building official for material and compaction requirements. You will need to cover any rock you have with clay, sand or gravel. This will allow the footing to be independent of the rock in the ground and lessen the chance of cracks and heaves in the footing.

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   Guest_ | Oct 11, 1999 04:51am | #2

   *Get rid of as much of the clay as you can. Clay can be a very detructive force when combined with water. As for the filler, I'd suggest what Mr.Coyner suggested and talk to your local building dept.

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   david_ingle | Oct 11, 1999 07:07am | #3

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   These other guys are 100% correct........Be sure you have drainage away from your excavation, remove as much loose material as you can and I would recommend a layer of gravel with a lot of fines over entire area compacted with a vibrating tamper (you'll get almost 100% compaction with the proper stone and may not have to remove it later). Otherwise you'll have a heck of a mess to deal with in the Spring.

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   MDuval_PE | Oct 14, 1999 07:51am | #4

   *What ever you do, dont use the clay to bear on. no matter how much your loader weighs, you wont be able to compact it. It takes a sheepsfoot roller and alot of time to compact clay. And that only if the water content in the clay is very low. Also, you should either have the footings bear directly on the granite (dowel into granite) everywhere or bring the entire bearing surface up with granular material. If you have part of the foundation on the rock and part on the soil, you will end up with a cracked foundation due to differential settlement. I recommend a minimum of 12 inches of granular material, compacted with something heavier than a plate compactor. If you feel you must use a plate compactor, than come up with very thin lifts (4 inches).I personally feel that this level of detail is needed for foundation work, but rarely happens in practice on residential construction sites. Pounding the soil with the bucket of a backhoe looks impressive but does little to provide a uniformily compacted subbase.Remember that the structure is only as good as the foundation it sits on.... or something like that...

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   david_ingle | Oct 14, 1999 05:06pm | #5

   *Rodger, MDuval is right. I don't know where you're building, but I wouldn't put more than about 4" for gravel in there for now since you're not building until spring. It may loosen from freezing and winter weather, and unless you have access to a big compactor to retamp with through 12" of gravel, I'd wait until spring to add the rest.......really all you would want to do now is keep from having a mush to work with later... Still check with your inspector however.PE in Tennessee

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   Roger_McAllister | Oct 14, 1999 05:06pm | #6

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   I've just finished blasting 100 ton of granite out of the basement excavation for my new home and am left with a very irregular surface. BTW, the area is approx 60' by 90'. It is on top of a hill and has a natural exit (no wall) out the back. Right now, the floor is mostly clay and rock mixed. In some areas the floor is as much as 8" low. I can't dig footing trenches because of the granite near the surface, so will have to set the 12x24" footings on top of the excavation floor. What would be the best material to use to fill the low places that would support the footings? I have plenty of 2" rock and clay handy. My feeling is that if a mixture of the two was tamped in (I have a 7000 lb skid loader with a tamper foot) that it would shed water and be fairly strong, but am open to other, more informed suggestions. One other pertinent fact is that I'm not going to actually lay the footings till next spring.