Jackhammer rocks in Post holes?

[Captain Dirty]

In my neck of the woods what passes for soil is till that was bulldozed by the glaciers. Two to three foot boulders are common; digging a post hole often results in a grave-sized pit. The normal procedure is to dig, by hand or machine, to expose enough of the rock to assess the situation. Usually the most expedient course of action is to enlarge the pit. Moving the post location often results in discovery of a new boulder. If the bolder can be lifted, lift it; if it can be moved within the pit, move it; the rock will later be used to brace the pole. While tamping or puddling may be desirable, the spoil can be mounded around the post and will eventually subside.
As others noted most alternatives are impractical or expensive. Cord's observation about displacement is spot on. Because of the buttressing and supporting soil around it, a cobble or even a pebble may feel like ledge when struck with a pick or digging bar.

 

[Stimw]

For those that have never used an air powered jack hammer, Hammering is not really the problem using them.
GETTING THAT HEAVY POS HAMMER BACK OUT OF THE HOLE IS!!
I learned that going in/down was easy but then you have to pick the hammer back up to do it again.
Sounds like you are in the classic rock and the (dig them out) hard place.

 

[Leejohn]

Why remove the rocks, let them act as an anchor. Drill a hole in the rock for one large piece of rebar. Epoxy it in. Then put the post in to get an imprint where the rebar is. Then drill a hole the size of the rebar in the wood post. Concrete everything in.

That's the way to go , works good.

 

[PILOON]

Or drill a bit larger and pound in a 'T' bar.
A 'T bar' sized hole can easily be drilled with a Kango or Hilti type hammer drill ,even in granite, besides being pounded into a drilled hole the T bar need not go very deep, like 6" pounded won't go anywhere.

 

[sixdogs]

I put some posts in Maine granite by drilling a hole in the bottom of the post and then the ledge and put a metal rod in there. Then I side braced the posts either in an "H" high tensile pattern or just a couple of side posts on an angle from the ground. It was strong enough that I had high tensile fencing tied to them and it held fine. A lot of my posts were like this since granite was everywhere.

It's been a number of years but I think I got the info from a Kencove Fence flyer from Pennsylvania.

 

[lennyzx11]

I put some posts in Maine granite by drilling a hole in the bottom of the post and then the ledge and put a metal rod in there. Then I side braced the posts either in an "H" high tensile pattern or just a couple of side posts on an angle from the ground. It was strong enough that I had high tensile fencing tied to them and it held fine. A lot of my posts were like this since granite was everywhere.

It's been a number of years but I think I got the info from a Kencove Fence flyer from Pennsylvania.

Google "rock jack fence post" and see if it will work for your needs.
Lenny

 

[QRTRHRS]

I have a HF electric hammer that I use on rocks but on granite, no way. You can rent rotary drills that are about the same size as an air hammer but will bore a round hole. It's been many years since I was elected to drill "boo coo" holes for a blaster to come in but as I recall, there is a hand valve that allows you to blow air through the bit periodically to cleanse the hole. Can we say dirt in the face?

I worked on some windmill sites where we had to bring in a rotary drill to honeycomb some granite then it would break apart with a standard jackhammer. (Small company, most cost efficient way though not much fun) Maybe this would work for your needs with the hand held drill and hammer?

You are going to need to rent some long bits. They make an already heavy tool even more so. One issue you will have is the small diameter hole and getting debris out of it. Also, if you do not break completely through the granite and water does not drain, your post will eventually rot if it's wooden.

 

[Sprinklerman]

Explosives would be fun

 

[QRTRHRS]

Explosives would be fun

Hold my beer and watch this....

 

[crazyal]

I wouldn't mess with trying to drill a hole that big in granite. Either do like others have said, drill a hole and put rebar or some other rod into the hole and drill a hole into the post and call it good. if you (or a friend) has a welder and a little skill you could easily make something you could bolt onto the rock that you could put the post into. If you really want to go the jackhammer route then I would rent a large pneumatic jackhammer that can also drill holes. I would then drill as many small holes as I could by starting off by going around the outside edge of the outline of the hole I wanted. Then drill as many in the middle as possible. Then use the jackhammer to break apart what's left. I would also go an inch or two larger than I wanted and fill the hole with cement to allow room to level the post.