Jackhammer rocks in Post holes?

[fidowanttobe]

Gentlemen,
My brother and I are installing 240 of cattle windbreak. 16 post holes for total project. Bing-bang done except 2 holes landed on large granite rocks 10 to 18" down. They are in the straight line so no moving them. Also don't want to dig them out because of the crater that will be left. Anyone here rent a jackhammer to hammer rocks into submission in the bottom of a hole? We haven't and would love to hear from experience. Thanks in advance.

 

[Raspy]

It depends on how big they are. If they are several feet in diameter, you'll just be chipping them away a little at a time and not get anywhere fast. Sounds like a lot of time and effort to rent a jackhammer and try it down in a hole. Do you have a backhoe? Then you could just dig them out with that.

 

[prichard]

What are your posts? What kind of rock?

If small enough diameter you could possibly just drill a hole in the rock with a good size hammer drill?

 

[fidowanttobe]

10" dia 12' long treated round poles. Think it is granite. Not sandrock or anything soft. Beating on them with rock bar only produces chips and sparks. A guesstimation of the rock size is a couple/three feet in dia but we are on the edge of it as hogging the hole out on one side gets us down deeper gradually. Afraid of using the neighborhood excavator as we would then have to tamp in a hole the size of a bedroom.

 

[ArlyA]

I've not used one but people here own or rent gas power jackhammers. They say they work well on fine the hard rocks found here.

 

[Cord]

I've used concrete breakers on rock and it can be really slow going. The key thing to make a breaker work, is the ability to displace the material. This displacement is what forms the crack. When the material is too large or too thick, the hammer can't displace it, thus it never cracks. All you get you get are small rocks coming off the edges. The ability of the breaker to crack your rocks is dependent on the size of the breaker and how big the rock is. Even if you managed to crack the rock, you still will not have a hole you can put the post in.

Couple potential solutions:
1. Core drill a hole in the rock. You can now set the post in the hole and use pea gravel to hold it in place. There are mobile outfits that do this on a per job basis. Figure $1,000 per hole.
2. Set the pole with the minimum embedment and then pour concrete around the base to help lower the center of gravity. This post will require a guy pole to help brace it.
3. Drill and blast (or dexpan) and then dig out the remnants.

 

[KennyG]

I would just move down the line on each side of the rock and put in extra posts. I don't know what your windbreak looks like but I'm assuming it has 16 foot sections. You could just bridge across the extra posts. Disclaimer: I'm kind of lazy and I hate rocks.

 

[Ford850]

2 - 3' in diameter? I think I would dig them out and backfill/tamp.

 

[Snobdds]

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.

 

[cmyoung2]

Get the jack hammer, use a combination of drill bits and breaker bits, i.e., drill holes, then chip between them. Posts that size shouldn't take very long, just make sure the hole is big enough the first time. had to put in several posts in a slate bed, about the only thing that would work.

 

[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.