Apologies to Mr. Gabriel.
I'm installing a 4X4X16 pole for a clothesline. I'm down to about 3 feet and I
hit solid rock. Not 'a' rock - just rock. I'll be using 10" sonotube (I have 4'
of it) and concrete.
Any rules of thumb for how deep I *should* be for good lateral stability? I can
obviously cut the pole shorter, but I'd like to have 10'->12' above ground to be
somewhat level with the mounting point on the house.
I don't know anything about this, but I was wondering, if you've hit a
solid ledge, does it make any sense to drill holes into the ledge,
grout or epoxy in some rebar, and then pour your footing?
If you are going concrete you can forget the sono tube. if you are 3' down
by 10+" across you are fine. Just pour away.
Your biggest problem will be in the long run, keeping the base from rotting
off in the concrete. This is why some have said add tar, treat, leave top of
concrete above grade and sloping away. All of those are fine suggestions.
Especially do not put the cut end into the concrete.
Some may suggest putting 6" of gravel in hole first then another 6" around
bottom of post. Then filling with concrete. That will allow drainage around
the bottom of the post, where rot will begin. Could be a good idea.
Also, by not using the sono tube you will have a bit more mass to you base.
A good thing if there will be a lot of pull on it. If you want to dress up
the pour a bit, just use a few inches of sono tube where the concrete is
above grade, no tube below grade.
Someone may also suggest not using concrete at all! If the ground is tamped
properly, in layers, as you fill you shouldn't need concrete at all. Have
you ever seen a telephone pole set into concrete? Nope. Look at those metal
digging bars, heavy steel, with a 2 or 3" blade. The opposite end is about a
3" round area. That is great for tamping around a post in a hole and digging
end may break the rock you found.
I hit solid rock. Not 'a' rock - just rock. I'll be using 10" sonotube (I have
4' of it) and concrete.
can obviously cut the pole shorter, but I'd like to have 10'->12' above ground
to be somewhat level with the mounting point on the house.
Three feet is probably deep enough to get you below the frost.
Four feet is the code around here. That has a margin of safety. I
find the frost won't penetrate more that around two feet.
You might be all right with the 10" Sono tube. Personally, I'd
want the concrete around the post to be a little thicker, to
prevent cracking due to flexing in the post, but your tube might
be enough. After all, you're not building the Brooklyn Bridge
here, you're just supporting one line of wet clothes. I'd go with
either a 12" tube, or build a square box from 12 to 16 inches
across and fill it with concrete, just to be sure. That would give
more mass at the base of the post, and would eliminate the danger
of the concrete cracking due to flexing in the post. The concrete
should project slightly above grade, and be sloped for water runoff.
I should mention that I erected a two-story clothes post a few
years ago designed to carry four clotheslines, for an apartment
building. I set it around four feet down, and reinforced the lower
part of the post that went into the concrete with steel L angle
brackets up a couple of feet along each leading edge of the post.
It projected some 20 feet into the air, and had two cross masts. I
had to special-order the post from Hefler's -- it was six inches
thick. They milled a tree for me.
You should be sure to pentox the entire post, if it is not already
pressure treated. I would go further, and paint the lower section
that goes into the concrete with tar -- use liquid roofing sealer.
Actually, unless you widen the hole and find it drops off, you have hit
the holy grail of Household Engineering there brother - Bedrock. It is
relatively unaffected by frost.
Clear a larger opening around the bottom if the soil allows it, sort of so
the hole forms up as a up ended bolt - or simply dig a wider hole - then
make two casts. A footing around 2/3 larger than the final proposed
concrete column poured directly on the rock face (clean it off as best you
can) with a short stand of rebar to secure the column should be perfect.
Personally, I would cast so that I had a 26(inch wide) x 12(inch high)
footer with three 36 inch rebar stands set through to the rock face - then
I would cast a column on the 24 inch rebar studs remaining about 30 inches
above the footer with a mounting plate cast in wet.
This would leave you about 6 or so inches of column above the ground -
directly laid on the Canadian Shield - which is always very handy should
you need to tie the house down next hurricane. If you are looking for that
o'natural lawn look - just reduce the column and rebar according so you
are about a inch below the lawn.
In both cases I really recommend the mounting plate (galvanized, and I
think Piercy's in Halifax has a good selection) because it will make
changing out the pole, if ever needs be, much more easy. Use a straight
deck post type plate that allows you to bolt from the sides - it is more
than strong enough.
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