Expert: Robert Cummings, P.E. - 1/22/2008

Question
QUESTION: Job Info: Norther NJ job site very  tight site and well populated. The rock is Basalt very dense about 5,000 lbs + per CY we are about -12 feet from street level all the weathered rock that could be hammered (Cat 345/ 12,000 lb hammer) was removed there is a 19'x35'x9' deep cut must be made for elevator shafts it is souroned by rock with no free face. Blasting is not allowed in this town. we have been drilling and using hydraulic splitters with good success for some high spot in other areas also tried expansive mortar it was hit or miss. My question to you is i need to create a free face for the rock to go to . do you any tips on creating a slot? I was going to bring a track drill and drill 2" pilot holes 6" OC and then use a hole opener  and create a slot. Do you have any other suggestions or ideas.

ANSWER: I think you have the right idea, but my first reaction is, if you can bring in a drill for 2 inch holes, can you bring in a little bit better drill and drill 4 or 5 inch holes?

If I understand correctly, you need to dig out a slot in hard, dense basalt 19 ft wide, 35 ft long, and 9 ft deep, right?  So your 2 inch holes on 6 inch centers would amount to at least 70 holes (you would need to overexcavate the ends to allow clearance for the drill head) to 10 ft deep each.  Once you have the slot (2 inches wide) drilled, which would take the better part of a day, then what?  Drill more holes for expansive grout or splitters?

I think if you drilled 4 or 5 inch diameter holes 10 ft deep (the drilling rate should not be that much less with the right bit and a decent hydraulic tracked drill) on 24 inch centers, you could swiss cheese the center and attack the rock with that hoe-ram full depth.  If successful, and in brittle rock I thionk you would be, you would have the equiupment to extend the drilling pattern all the way to the sides.  You have 665 sq ft which at 4 sq ft per hole (and you may find out you can increase the spacing with 4 inch holes to 2.5 r even 3 ft) is 166 holes.  You could have it all drilled out in 2 days, at 800 ft/day.

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

QUESTION: The swiss cheese method was tried 5" holes were drilled on a 16" OC pattern and 12,000 lb was used with little success. That why I was contacted to do the job i specialize in hydraulic splitting and expansive grout. I need to drill a slot to create a free face. I was thinking of drilling 2" pilot holes 6" OC an after all the pilot holes where done redrill them with a holeopener/reamer bit to 6" size this way a had clear slot the full dept and width to split the rock too.

Answer
That's the first time i have heard of the swiss cheese method not working.  Sometimes the feasibility of hoe-ram work depends on the skill of the operator.  Oh, well...

What you are proposing should provide enough relief, IF you can ream the holes out accurately enough to leave a small web between the 6 inch holes that would crush out when splitting.  That depends on replicating the setup between the 2 inch and 6 inch drilling.

You will also have to address how to remove the first few rocks that work their way into the slot.  Eyebolts, probably.

Happy splitting.