I think David's lack of additional detail, plus focus on little stuff
that doesn't matter so much, makes it sound dicier
than it probably is. For instance, he says only 1.5' of the foundation
is below ground, but what was done with the gravel and hole is a key
part of the foundation. He doesn't specify that part though.
For these large pad foundations, the overturning moment is mostly
resisted by the soil at the bottom, not the sides, which is opposite
from the normal deep, not so wide, tower foundations. So it doesn't
matter so much whether all the concrete is above or below ground. What
matters is the bearing of the stuff resisting it all at the bottom.
(soil under the gravel)
Packed gravel is a good bearing surface. The concrete bears on the
gravel, then the gravel bears on the soil farther down. It's important
that the gravel can't wash out/move.
It wasn't said how far down a hole was dug, or how much gravel used as
fill before the concrete. I suspect the hole was dug below frost line?
That would seem to be necessary.
Rebar schedule wasn't mentioned.
10x10x5 is bigger than the normal 9x9x5 solid block A.N. wireless
specifies. So assuming good bearing at the bottom, it does sound like it
could be in the right ballbark for a HD-90. (it must be a 70mph or so
wind area, looking at
My guess is that analysis could show it's fine, assuming the rebar was
similar to the A.N. Wireless suggestions here:
http://www.anwireless.com/alt.html (or better..some engineers seem to
like #9 rebar rather than lots of #5 rebar).
In fact it could be better than what a typical ham might have done..i.e.
ignore the water table issues and just install a foundation per drawing
that specifies "normal soil" when you don't have normal soil.
The unknowns are the bearing capacity of the soil at the bottom of the
hole, the gravel size/compaction, and the rebar schedule.
Yeah, you lose some from being essentially above ground so the sides
don't count for resisting the overturning moment. But I think it might
be, that on these type of foundations, the sides aren't included in the
My totally unqualified two cents.
oh p.s. In terms of "too late", there's plenty of things that could
still be adjusted. Like why 90' as opposed to 80' or 70', for the tower.
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