Expert: Robert Cummings, P.E. - 1/9/2008

Question
QUESTION: Is it remotely practical for me to think I could "excavate" rock ledge in by basement to accomodate a rectangular 100 cu ft hot water storage tank with insulation? This is in an existing home/basement.  I want to find out if it would be possible to do this with an appropriate "jack hamer" - even if the project were to take months to accomplish.  The ledge is granite.  If so, what would you suggest as the appropriate tool?

ANSWER: First let me apologize for not answering you sooner.  I have been preoccupied with personal issues and have not been able to access my Internet for a few days.

I assume the granite is massive without many fractures.   If it is fractured you can often follow the fractures with a spade hammer -- even a large electric hammer will work -- and worry out the rock.

If you need more energy because the rock is tight, fresh, and not well-fractured, you first need to drill holes.   If you are willing to drill, you can do what you need.

The fastest way to simply fracture the rock would be to rent a jackhammer, and drill holes about 1.5 inches in diameter, into which you place expansive grouts specifically manufactured for fracturing rock and concrete.  One is distributed by Daigh Chemical, another had the trade name BriStar, and there are others.  Do a Google search for grouts expansive rock fragmentation and you will find it.  The time required to fracture the rock ranges from a few hours to a day or so.

Another, more dynamic way would be to use the Boulder Buster which is a propellant-actuated rock fragmentation tool.  This can be purchased and used by anyone.  It is a steel implement with a nozzle and a firing breech.  You drill a hole, fill it with water, insert the nozzle with a high-velocity propellant cartridge, cover the apparatus with a 4x4 ft mat (supplied with the tool) and pull a lanyard.  The propellant pops the rock apart.  Search for Boulder Buster on the Internet.  There are US distributors.

You can also use hydraulic splitters.

Any of these methods will involve drilling holes less than 2 inches diameter on spacings of 2 ft or so, to depths of 3-5 ft.  That should meet your requirements.

Your main issues will be drilling.  A suitable drill will be a compressed-air jackhammer so you will need to rent not only the drill (plus bits and steel) but a compressor with not less than 90 cfm capacity, plus the delivery hoses.  This may be inconvenient if you want to spread the job out over months, but you should be able to drill out the job in a weekend.  Then, using the Boulder Buster or the grouts, you just start splitting out the rock.  Be sure to drill deeper than you need!  Rock will not necessarily break out horizontal so I would overdrill 6 inches on a 2 ft pattern.  It sounds like more work but you will regret not doing this if you have to chisel out the bottom.

I favor the Boulder Buster because it separates the rock better than grouts, which may just fracture it and require further wedging for removal.   

If you want to avoid the rental episodes you might be able to buy a large enough electric hammer drill but you will not be able to get as much depth -- probably 2 ft is the maximum for the diameter you need -- and you will not be able to use the Boulder Buster which requires I think a 1-7/8 hole (most electric hammer drills don't go that large) but you would still be able to use the grouts.

You need to have a cogent plan for removing the rock also.  If you drill 2 ft centers to save on drilling you will be handling 2 ft rock.  The jackhammer with a spade bit can break the rock down but handling 2 ft rock in a basement by hand is no small matter.


---------- FOLLOW-UP ----------

QUESTION: Supposing I drill down 2.5 feet on a 2X2 foot grid and that the rock breaks into approximately 2X2X2 ft cubes, would the jackhammer with a spade bit be able to split these into smaller pieces that could be more easily handled?  I am thinking that I may be able to drag lartge blocks out through my basement bulkhead with the Fransgard logging winch on my tractor?

Answer
The trick to breaking rock with a jackjammer is to have the rock on a hard surface and starting close enough to the edges, then work your way into the middle.  If the granite is not too hard you can probably do it readily.  If the chunks are too large and hard to break with a jackhammer you can always use a hydraulic rock splitter (feather and wedge).  As for dragging, you can drill all the way through and run a cable through the hole, or get a heavy eyebolt with a threaded expansion shell (Williams Form Engineering Co. or Dwydag can direct you to a source of expansion shells) and hook your winch cable to the eyebolt.  

If you life in a cold climate you might consider drilling partially through the rock (about 2/3), filling the hole about 1/3 of the way with water, and sealing the hole to the surface with high-strength sanded grout (Portland cement mixed thick to which you have added some silica sand) and leaving them outside.  Cover the rocks with dirt for a day or two so the grout cures, then expose them so the water freezes.  It may break a 2 ft rock.