Expert: Robert Cummings, P.E. - 2/10/2008

Question
QUESTION: Hi, Mr. Cummings,
Our pool contractor hit a rock that is approximately the size of a rhinoceros.  Should I encorporate the tactics you described on January 8th for the granite ledge removal? Thank you very much.

ANSWER: This is the ideal application for the Boulder Buster, RocKracker, or NONEX.  The Boulder Buster would be my first choice, if you can find one locally, great.  http://www.boulderbuster.com/

Paul Kunze, an acquaintance of mine from the International Society of Explosives Engineers, has McCarthy Industries, which is a distributor.

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

QUESTION: Hi, Mr, Cummings.  Our pool contractor ended up using a 500 hoe ram to break up the rock.  But now we've now hit bed rock (likely granite (I live in Louisville, KY)) and the 500 hoe ram was ineffective.  I've tried using a concrete cutter on the bed rock and hitting the cut lines with a sledge hammer -- this works but it is really slow.  Really slow.  The pool contractor has suggested using a 750 hoe-ram, but he doesn't believe that will work.  He believes that a 1500 hoe ram will be sufficient.  
We need to bust off between 4 and 8 inches of the  bedrock to get to a safe pool depth.  The 750 hoe-ram will cost me $800.  The 1500 hoe-ram will cost me $2500.  The 1500 is really out of our price range.  Do you think there is any hope in a 750 hoe ram working in our situation?  Thank you so much for your time.

Answer
I am not sure what the difference is between your hoe-ram designations.  If 500, 750, and 1500 are the energy in ft-lb, then I am surprised your got anywhere with the 500.....  Because of this and not seeing the rock, I really can't say if the 750 will work or not. If it were me, I would avoid the uncertainty and use the bigger machine.

But I also wonder about the huge price difference going from the 750 to the 1500.  Even the very large hoe-rams (10,000 ft-lb +) that I see regularly on construction projects are less than $400 per hour, operated, on a large excavator, and could probably chew through your pool in less than an hour.  Little hoe-rams (in the 3,000 ft-lb range) are like $275/hr.  So maybe the cost is in the mobilization (e.g., does the 750 fit on his current tractor but the 1500 would not and therefore require a full, additional mobilization)?  Or the larger machines are a premium because they work when the others don't and he knows you have a problem only he can solve?

At any rate I would suggest going back to the contractor and seeing if he has figured the increased productivity rate of the larger machine into his cost estimate.  If so, then ask why the cost is so much different. Perhaps he has to obtain the larger machine on a full day rental basis.

It's too bad the additional cost of the larger machine is out of your price range because in my experience, if there is any doubt, the cost of a larger machine (assuming it is operated correctly) is justified.  If the 750 turns out to be marginal, you could gobble up the savings in operation hours -- three days and $2,000 later, he will tell you "we almost got 'er done".  With the larger machine, the increased productivity means more certainty in meeting the cost estimate and also in downstream savings resulting from the avoidance of delays.

Of course you do have options.  What you are doing with the concrete saw is introducing weakness planes into the rock that you can expolit, but a saw does this in a very inefficient way.  It would be much faster if you rented a jackhammer drill and compressor and drilled 1.5 inch holes 1 or 1.5 ft deep on 1 ft centers.  Then I bet the 750 would work.  

On the other hand, since you have only 4-8 inches to go, have you considered the feasibility of just raising the pool deck?  Depending on the application, the difference might be negligible.