Michael Goins wrote:
> I based it upon my original decision to locate the tower where it is.
> Once we found rock and did the initial hole (such as it is), I decided
> it was not practical to continue to chip away rock to replace it with
> concrete. The factory dimensions are obviously engineered, and we all
> know they are likely slightly over-engineered from a litigious society
> legal aspect. They are also engineered for "average soil" - whatever
> that is supposed to be -
I'm not a soils guy, but I'm pretty sure there is an actual definition
for what "average soil" is from a foundation design standpoint.
Probably something to do with density and resistance to movement (I
vaguely recall something like 2500 or 3000 pounds/square foot bearing
Obviously, sand isn't "average soil", but neither is rock. In an
engineered application, the engineer would take the actual soil
properties (either by testing or by local knowledge..) and scale
appropriately. If your soil were, say 1250 psf bearing capacity, and
"average" was defined as 2500 psf, your foundation would need twice the
area to resist the same forces. If your soil were 5000 psf, then half
would do (assuming all the other design requirements are met.. pushing
soil out of the way is only part of it)
There's also the density part of it. If the foundation design depends on
the weight of the soil on top of something, then if the density is low
(you live in Pumice Flats) you'd need more depth.
It might seem that is is pretty common sense if you've ever pounded in a
fence post or tent stake or built sand castles at the beach, but where
the engineering comes in is reducing the common sense to actual numbers,
so you know how to adjust, rather than just taking the "add a few yards
of concrete and make it wicked strong" approach. At some point the few
hundred bucks for the Engineer is cheaper than the few hundred bucks for
the concrete and rebar.
In your situation, you've got lots of land, presumably nothing really
bad happens if you estimate wrong, so either way works.
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