'Digging in the dirt...'

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.
a
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Jackhammer
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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?
Cheers, Wayne
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You might use an old tire instead of the sonotube. A clothesline post doesn't have to be below the frostline.
Nick
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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.
Good luck

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This isn't a valid comparison. What a telephone pole weighs and how it's installed are considerably different than a residential deck post.
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a wrote:

Using the standard rule of thumb that three foot would be good for 6 foot above ground.
--
Joseph Meehan

Dia duit
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I may be wrong but i believe you need to get down below the "frostline" which i believe is 4ft. This is so the pole won't heave up and down when it freezes.

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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.
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a wrote:

you need a sledgehammer if you gonna do that !
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Even if you get below frost, it will still come out a couple inches every spring if you dont have a bigfoot on the bottom or enlarge the hole sufficiently at the bottom before pouring.
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Salutations:

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.
--
Radio Free Dexterdyne Top Tune o\'be-do-da-day
Ayub Ogada - En Mana Kuoyo
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