Foundation on Big Rocky Soil

[larsengineer]

I am designing a 22' x 66' residential foundation on 25% slope of soil having 40% silt (yuk) mixed with sand & gravel. (Haven't done soil test drilling yet) Plan on pouring slab-on-grade beam (12"x30") sitting on 12" dia x 18' deep piers with belled ends spaced 10' apart.
At one end, below grade there is a very big limestone slab, part of which I had jackhammered away to make room for the grade beam to pass over. Elsewhere I have removed 10 inch thick limestone pieces the size of pianos. There are more, I fear.
In drilling the piers - I expect to hit that same big slab in several places 4 to 8 feet deep. At which time, I suppose we will just "socket in" and quit. This makes a strong unyielding support. Elsewhere I will hopefully have conventional piers which I guess could have 1/2" or more of settlement.
Wisdom says either get all "on the rock" or get all "off the rock" but I don't know how. If "on the rock" - I could make the piers deeper to reduce (eliminate?) settling. If "off the rock" I could make wide spread footings over the rock without touching it (instead of piers) with piers elsewhere.
Either way I want to make the grade beam a "raft" so maybe increase the size?
Sorry for the lengthy post - any ideas? thanks guys and gals.

 

[rowingengineer]

Sounds like your doing a nice home project, the project sounds complicated, with belled piers and pads all over the place, suggest you talk with the local engineer so he can get a handle on the situation.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[Bluefoot]

Geotechnical engineer is the one you should ask about at this case. The house may be little loading to worry, the different bearing characteristics is. It is hard to say that the limestone is 'good' for supporting, it depends on where you are and what kind of limestone you encounter. Ground water and local temperature variation are other interests for the geotechnical engineer to know.
Regarding the foundation design, you may sit your building on different bearing materials, i.e. soil and rock, just to make sure either treatment be taken in the foundation (transition zone applied), or structurally integrate your footing, or both.
Again, if thing goes complicated, you should retain a geotechnical engineer. That will worth the money.

 

[msucog]

hire a local, qualitified geotech