Drilling Problem

[Steve Frazier]

My day job is my landscape company and I've been approached by a customer who wants to mount a convex mirror next to his driveway. I'd need to install a post to mount the mirror to, the problem is the property is all fairly hard shale. I'm trying to figure out how to drill a hole in the shale to install a 3" diameter pipe to mount the mirror to.

So far the idea I have is to rent a rotary drill (jackhammer size) and drill a series of holes in a circle and then dig out the fractured rock. I'm open to any other suggestions which might work better. I don't have a lot of experience with augers, are any of these capable of drilling shale? I have a Cat 248, perhaps I could rent an auger.

[Legdoc]

Steve, how deep do you need to drill the hole?

[Pecord Exc]

NM stupid question

[digger242j]

Is there a reason the hole can't be larger?

For instance, we've put in light poles, that have about an 18" base. A couple of passes of the backhoe bucket makes a 24" hole, and we simply put a sonotube in that and fill around it before filling it with concrete. As long as the base is of sufficient mass to keep the pole standing, it shouldn't matter if the surrounding ground has been disturbed. I don't see why you couldn't do the same for a 3" pole. You're still only disturbing a relatively small area...

[Steve Frazier]

I figured on going a bit oversized, but don't want to get crazy, I'll have to mix the concrete by hand. I'm guessing that a depth of 2 to 3 feet will give enough stability to the mirror, I have to find out just how high the customer wants it mounted.

I have a backhoe attachment for the 248, but I doubt it would be very effective in this rock.

[Vantage_TeS]

What if you rented a breaker and pounded out an area about the size you'd need to plant a tree (thats just to give you an idea of about the area you're going to need, it will vary depending on how the rock is formed and what you need to do to get "under" it). Then use your backhoe attachment to scoop out the fairly rough gravel you should now have.

Take a short metal pipe (say around 10"? go to a scrap yard and get a section for free/cheap) and place it in the center of the hole, now backfill around it with your gravel.

When you're ready to erect your mirror simply slide it inside your pipe and pour concrete around it (in the pipe). That should a) solve your drilling problem b) limit the amount of concrete you'll have to hand mix.

[Lashlander]

Could you go smaller? Get a jackhammer and drill a hole. Then weld a smaller pipe or round stock to the piece to fit in the hole?

[earthworkscc]

Try calling a core drilling company (ie. someone who cuts concrete). They should have no problem drilling a 5 or 6 inch hole 2 to 3 feet deep if you need a name of a company I could provide one. I would not recommend renting a machine and trying to do it yourself it can be a little tricky in rock and going 3 feet down. Good luck

[Big Iron]

I figured on going a bit oversized, but don't want to get crazy, I'll have to mix the concrete by hand. I'm guessing that a depth of 2 to 3 feet will give enough stability to the mirror, I have to find out just how high the customer wants it mounted.

I have a backhoe attachment for the 248, but I doubt it would be very effective in this rock.

use 30% of the pole length to put in the ground as a guide line (10' pole = 3.3' in the ground). not familar with the 248 CAT but if it is like a skid steer rent a 6" auger and drill the hole. don't bother to mix the cement outside the hole just pour a bag in, add water and stir with a long piece of rebar/stick/ect. do that untill the annulus of the hole is full and it will be inplace for a long time.

[Squizzy246B]

Steve, I'd just rent a core drill...a 4" or 6" in on a stand (electric) or you use a post hole digger with a core drill bit. We use a petrol powered on limestone with a 5" x 20" long core bit.

[RonG]

How about using a tripod to hold the pole for the mirror and just drill for anchor bolts with whatever you have available.A jackhammer and rock drill would make quick work of the anchor holes.Ron G

[dumptrucker]

I would dig as far as I could with the backhoe then drill the rock with a hilti drill about 18 inches or so probably three holes . Then drop rebar in those holes and put a sono tube in level to grade. Then get four anchor bolts and put them in the concrete in a square pattern and weld a flat plate on the bottom of your pole with the same hole pattern as you set the achor bolts at , then bolt the pole down.

Even if you only get down a foot , as long as it is pinned to the rock it's not going to go anywhere. Have done many light posts and sign's this way.

Good luck.

[Steve Frazier]

dumptrucker, I think you've hit on something. I've done something similar in the past and never considered it for this job. I doubt I will see much more than 6" of soil before I hit the rock. This solution seems the simplest and easiest to do, thus be most cost effective. I'll keep you posted on what works out.

Thanks everyone!!

[stumpjumper83]

dumptrucker has a good idea. You could even use a piece of sidewalk if the rock was too deep.