I think that depends heavily on the type of soil involved, and how the
soil is compacted. When I backfilled under the slab of my house (behind
the six foot high retaining wall) I removed all rocks, laid the soil
down in 3 inch layers, wet each layer down, let each layer sit until it
was just damp, and then compacted each layer with a heavy duty plate
compactor. It took literally months to finish, but the resulting adobe
was so hard that when a 50 ton crane drove across it to install a large
rock it didn't leave any tread impression at all ... only a dust print
on the surface. Even as hard as the natural undisturbed soil is here on
this rocky hillside, that adobe I created under the slab is several
times more resistant to flow.
Most contractors, however, just dump in the backfill dirt and tamp it
every couple of feet with the backhoe bucket, and that is almost for
sure not the equivalent of undisturbed soil.
The biggest issue, though, even if your friend goes to extraordinary
means to compact the backfilled soil, is whether the building inspector
will bless it. Signing off on something that doesn't have engineering
approval (and that cannot be independently verified later) puts the
inspector in a pretty bad position. It also gives the P.E. an easy out
if any liability problems should arise later. In a nutshell, your
friend's contractor didn't follow the engineering drawing and, in my
opinion, has hung your friend out to dry.
73,
Dave AB7E
Dick Flanagan wrote:
> A friend of mine is installing his first tower. It is a US Tower
> HDX454. (I thinks that's the right number. I have an HDX572 and his
> is the next one shorter than mine.)
>
> He had the tower specs checked by a local PE for the requisite wet
> stamp needed for his building permit. The PE calculated and
> specified a tower footing of 5'x5'x7.5' in undisturbed soil.
>
> His contractor dug him a 10'x10'x8' hole and is going to put a
> 5'x5'x8' plywood frame in the hole enclosing the rebar cage. He is
> then going to fill the frame with concrete and when it cures is going
> to remove the frame and back-fill the hole with compacted soil.
>
> My friend is being told that properly compacted back-fill is the
> equivalent of undisturbed soil. Is that true??
>
> Dick
> --
> Dick Flanagan K7VC
> dick@k7vc.com
>
>
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>
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