Pier and Beam, piers only 18"?

I am having a manufactured home put up in North Texas, in the middle of a drought.
My site is on the side of a hill, and the mobile home retailer contracted some guys to dig out the the hill and use that dirt to level a spot for the home. On one end of the pad, the non-native soil is about 18"-24" deep. On the other end, almost solid rock.
All they did to pack the soil was to drive over it with a small dozer.
I have no experience in this and asked several people if this will work, most said it is a bad idea.
So on the suggestion on many, I decided to go pier and beam. All the pier and beam guys I called will only go down to 18" piers max.
My questions is, if you put 18" piers in soil that isn't packed that good, am I getting any benefit from the piers, or am I still gonna have alot of problems?
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On 16 Jan 2006 16:36:55 -0800, snipped-for-privacy@yahoo.com wrote:

If the piers go down 18 inches to rock substrate, you're home n dry.
Brian Whatcott Altus OK
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Iff u r on solid rock outcropping no foundation piers at all is required. Up here in Central canada where frost heave reaches down 4-1/2' frost projections must go down 4' in soil. or until solid rock.
wrote:

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Piers and footings where I live are considered by depth into undisturbed soil.
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Dems the buzz words I heard from a town inspector/engineer.
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I suppose what I'm having trouble believing is that if the piers are not on undisturbed soil or on rock then how do they help any, won't they shift and settle? Isn't the point to put them on rock or at least undisturbed soil so that they don't shift or settle as much ?
On one side of the pad, they will hit rock quick, but down the hill, at the other end of the pad, if they only go down 18", then there is still 6"+ of disturbed soil between the pier and the undisturbed soil surface.
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snipped-for-privacy@yahoo.com wrote in

Wose yet. Codes are often stated that footings and the like have to put xx" into undisturbed soil.
He said "the non-native soil is about 18"-24" deep." So if the soil is disturbed 24" and say the code said 24" into undisturbed soil then they would have to do down 48".
Anyone feel free to correct me on this. I'm not a builder, engineer, inspector or the like. Just a professional wanna-be. And a damn good one at that!
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Like others stated, "undisturbed soil" are keywords for piers.
18" is minimum soil to subfloor clearance.
Admittedly, on the contractors point of view, leveling out the building site soil prior to pouring the piers is a bit easier initially. Most don't do it this way. Rather survey, designate pier locations, dig the holes, pour the piers, then backfill.
A easy way to dig holes for piers is to use a machine used for utility pole placement. A few places rent these. They need the proper auger bit to drill through rocky soil. The diameter of the piers depends on the weight of the structure and anticipated weight within after move-in, number and placement of the piers. Auger bits come in 12, 18, and 24 inches diameter.
The depth of the pier in undisturbed soil should be equal or greater than the amount of pier in length above it. The bottom should be slightly flared for a footing. A chair of some sort should be fabricated out of rebar at the bottom pier location in the undisturbed soil. The tops of the piers should be at the proper height to support the subfloor. All the piers should be in proper alignment horizontally to support the subfloor without veering in/out demonstrably, as this will affect support and any skirt applied afterwards. The subfloor should be physically tied to the piers, not just resting on the piers. The tying methods and hardware vary, but should hold up to exterior weather environments for the life of the structure it supports.
............ Jonny

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On 16 Jan 2006 16:36:55 -0800, snipped-for-privacy@yahoo.com wrote:

I think you should drive pilings all the way to bedrock, on both ends. How far is it to rock, on the downhill side?
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wrote:

crawling under this thing on a regular basis. I'd put a slab in, if the concrete truck is gonna be there anyway. If you have pilings or piers taking the weight, the slab doesn't need to be thick.
aem sends....
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