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
wrote in message
oups.com...
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?