Construction & Contractors/Granite Outcropping

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Question
Robert,

We are considering the purchase of a multi-acre property with a Granite outcropping in the Granite Shoals area of Texas beside lake LBJ.The property as enough room to support several home sites avoiding the granite outcrop, but I would like to consider incorporating the outcroping as a natural flooring to the main room(s).

A few questions:

1 - Can the natural outcropping be cut / ground to a level surface for refinishing as a floor?

2 - Are the outcroppings stable enough to consider for a building foundation?  Are there other reasons this should not be considered?

3 - What would be the best way to proceed in terms of finding an appropriate contractor for this effort?

Appreciate your thoughts and guidance,
John

Answer
1.  The natural granite could be leveled but only at great cost.  I know of no technique for sawing rock horizontally, except for wire saws that are used in quarrying operations, and the problem there is always keeping the wire lubricated in a horizontal opening.  You could theoretically start by drilling horizontally through the outcrop from one side to daylight at the other, insert the wire, and start sawing sideways.  The problem there, aside from the fact that the equipment is large and would have to be adapted to your site, and keeping the saw lubricated and cooled, will be controlling orientation on the drill hole and the saw.  If you don't end up with a floor that is level and flat you will be very disappointed, and making a level floor would be extremely difficult.  I suppose you could drill horizontal holes but again controlling the orientation would be next to impossible.  If you don't cut a horizontal slot you would have to start with some technique involving drilling vertically from the granite surface. The problem there is that you need extremely closely-spaced holes whose bottom elevations would be within fractions of an inch of each of each other.  Then, once you break out the columns, you have projections between holes.  Granite is hard and requires even harder polishing media.  The finishing process would be weeks to months of circular grinding with a diamond or tungsten carbide grit to remove the high spots and grind them down.  This is a wet process, so you would need to capture and re-use the grit and water.  When quarried slabs are polished, they are already cut (vertically, so the wire saw stays immersed all the time)so there is not much need to remove asperities.

And when it's all over you'd still have the native weathered cracks and joints, with their gaps and joint-wall discoloration, to deal with.  And you'd have a floor that would stay real cold in the winter.

Altogether I would say you'd be better off, if you want granite flooring, to overexcavate the outcrop and place slabs that you can control and level.

2.  Yes granite will make a great foundation as long as it does not contain inclined fractures that daylight to a free face and can slide., or you are not trying to bear extremely heavy loads on heavily fractured rock or gouge.  But since this is an outcrop, I'd suspect that heavily fractured zones and gouge occur elsewhere.   

3.  Which effort?  Cutting and polishing a granite outcrop in place -- my only advice would be to not waste time with regular excavation contractors and blasters, unless they have connections in the dimension stone business.  Start with dimension stone suppliers (not aggregates look for companies that supply slab products).  As for the conventional construction, find yourself a home builder/architect who works with stone flooring.  You can still do some cool things with the granite exposed in the interior of the house, different levels, and so on.  He will locate a good rock excavation contractor.  

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Robert Cummings, P.E.

Expertise

I can answer questions related to rock blasting, rock and soil excavation (such as tunnels and highway cuts), stability of such excavations, and foundations in rock and soil. I can also answer questions related to geology and mining.

Experience

30+ years as a geotechnical engineer and minerals engineer. Active consulting practice in rock blasting, geotechnical engineering, and rock mechanics for mining and heavy construction.

Organizations
Society of Mining Engineers, Deep Foundations Institute, Association of Engineering Geologists, and International Society of Explosives Engineers.

Publications
Mining Engineering, AEG Bulletin.

Education/Credentials
BS and MS Geological Engineering

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