Expert: Robert Cummings, P.E. - 10/14/2008

Question
My house was built about 30 years ago on rock.  There is a room inside my house that has cement walls covering rock that was not originally excavated so there are a few levels.  I would like to have the cement and rock removed to flatten the room so I can use it as a family room.  Is this possible without damaging my house and foundation?

Answer
By cement I presume you mean concrete, although it really does not matter.

The answer is yes, you can remove the rock provided the rock is stable enough to sustain the imposed loads.  Presumably only the uppermost level is actually imposing any structural loads on the rock so you should be able to remove the rock from in front of the uppermost level and have only one stairstep in the room.  If the rock is reasonably strong with respect to the structural loads, you should be able to remove it to a vertical surface without inducing settlements.  The presumption is that the removal of lateral confinement from the rock on which the main structure is bearing will not cause crushing or sliding displacements in the rock.  If, however, the rock is very weak and friable, heavily fractured, or heavily weathered, it is possible that you will have fallouts of ground below the bearing surface.  Although a few small ones probably will not present a threat to the structure, very many fallouts and you will start seeing foundation cracks.  If you have doubts about the rock condition, you may want to seek the assistance of a geotechnical engineer to both assess the imposed loads and assess the condition of the rock.

The removal technique that will most likely prevent disturbance is labor-intensive.  The technique is to line-drill the rock using a percussion drill (jackhammer) and then hammer the rock out or drill more holes and use expansive grouts like BriStar to break the rock just back to the line of holes.  

When line-drilling you drill a row of parallel, vertical, closely-spaced (less than 2 diameters apart) holes along the line of intended breakage.  This leaves webs between holes that are approximately one diameter wide and which should readily fragment when you chisel the rock back to it, and prevent migration of disturbance behind the line of drilled holes.  You have to leave room for the drill so you cannot drill vertically exactly below an existing concrete wall - you will have a slight offset whose dimensions depend on how much clearance the drill requires.  The trick is getting the holes vertical and parallel. With a little creativity, you can rig up a guide or slide so the drill travels against the vertical existing wall, thereby assuring that the holes will be in a vertical plane parallel to the existing wall.  Then you only have to worry sbout plumbness within the vertical plane.  You can usually eyeball that.  

I am a little suspicious about the rock because the builder went to the effort of covering the rock with concrete.  Two possible issues come to mind.  One is that they may have been concerned about weakness due to intense fracturing or spalling (very weak or weathered rock) or air slacking (disintegration of exposed rock surfaces, common in claystones, mudstones, and many shales).  The other issue may have been the presence of water.  If they were trying to seal the rock surface by covering it with concrete, whatever you put back there will need to accomplish the same seal; it would be advisable to have some kind of drainage at the base of the new concrete (on the outside)and also prepare whatever existing surfaces the new concrete will adjoin so that a tight, waterproof bond is created, or you will have a dank and musty room.

Hopefully the concrete is not reinforced because its removal will be a lot easier if it is not. If it is, you really may want to consider expansive grout, like those offered by Daigh Chemical.