Drilling in bedrock

Question

I need to drill five 2.5" holes 1 foot deep in bedrock to accept 2" pipe that will be used to anchor the footings for my deck on a steep hill. I have a Bosch SDS Max rotary hammer drill.

I'm not sure if I can shut off the hammer part. The rock is in solid layers many feet deep.

Do I use a 2.5" rotary hammer bit, or a 2.5" diamond core bit?

I will be drilling on a 50-degree slope, so anchoring a wet core drill is not an option.

If I use a long core bit how do I get the core out?

Any tips, tricks, or advice would be appreciated.

Answer 1

Don't know about your local market, but in our regions a matching SDS-max tool for roughly 63mm+ bedrock is more costly than the estimanted rent for an diamond wet core drill.

It isn't impossible with an SDS max tool, they exist. But the core drill option might cost less and go faster.

I don't think that you need to anchor a core drill in this size. I had two guys make an 100mm hole with an handheld drill. On dude held the drill, the other pumped the water. Perhaps you could chisel a step into the bedrock to provide an even starting point for the core drill.

Answer 2

I'd use a quad carbide bit. I've done 2" diameter hole in concrete with my bosch sds-max. The bit I used wasn't great in rebar so when it hit rebar I spent a lot of time (15m instead of 1m for a similar hole). You want the hammer and the drill mode. Depending on the rock type you might want some kind of vacuum or powered air to clear out the debris as you drill. Use a plunge up and down motion can help clear the dust as well.

I've used these previously on a rock to do a feather and wedge split. Granite I made holes in no time. Some other rock I kept breaking the carbide cutting edges off the bit. It was my first time doing that kind of work though so I didn't realize why the bit wasn't working until I inspected the head and saw the cutting edges were gone. So using these kind of bits generally works well but I guess there are certain rock types that eat carbide like crazy - I broke 3 bits ( the cutting edges ) before I gave up and just used a 60# jack hammer for 8 hours to break the rock into pieces. Bring more than 1 bit !!

The core bit will take longer and you'd have to keep clearing the cores assuming you have enough extensions to keep drilling. If you get out of plumb you are also kind of stuck.

1' is pretty deep your challenge is going to be drilling more or less plumb unless you don't care so much and only need so much projection into your footing or can bend the projection to be the proper edge setback from your concrete surface. You don't need to buy the 18" carbide bits as you can get sds max extensions - though they are pricey !

Be interested to hear how it goes. Good luck !