Construction & Contractors/Pool construction


Is there a geological test to determine the depth of rock in the only location on a property to construct a pool?

There are several ways to do this.  If access to digging equipment is a problem, one can run a seismic refraction survey.  This is a technique whereby ground motion sensors (geophones) are arrayed along a line and impulses, generally sledgehammer blows on a steel plate for shallow surveys, are induced.  The various times when the impulses show up at the sensor are recorded by a signal-enhancement seismograph.  The interpretation indicates layers in which seismic energy travels faster or slower, and since rock transmits seismic waves much faster than overburden soil, the technique will indicate the depth to rock.  However, in some cases, if the velocity contrasts in the subsurface are not real clear, the interpretation can be ambiguous. It also may not give you subtle variation in bedrock depth.

Some people advocate electrical resistivity geophysical surveys to indicate the depth to rock but I have found that the data are not specific enough to give you much contractual authority when the time comes for ground breaking.

If you go this geophysical route, I would recommend that you use a seismograph having not less than 12 channels, 24 being better, that you use geophone spacings of not more than 4 ft, and that you run at least two lines, perpendicular to each other and crossing at the centerline of the pool.  A good geotechnical or geophysical company will run this survey for around $2,000.  Be sure to tell them all you are looking for is the depth to bedrock, or they might add data processing that you don't need and it will cost a lot more.

If you have access there is nothing better than test drilling or pot holing.  A small self-propelled percussion hammer drill will give you indisputable information about the depth to bedrock at various places.  You can cover the entire pool area with holes on, say, 5-6 ft centers in a half a day and you will get a good idea of the variation in bedrock surface.  Or, you can pothole with a backhoe.  If you suspect rock isn't too far down you can even scratch around with something small like a Ditch Witch or small Kubota tractor, which you can rent, although with real small machines you can tag on loose boulders and mistake them for bedrock.  

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Robert Cummings, P.E.


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.


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.

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

Mining Engineering, AEG Bulletin.

BS and MS Geological Engineering

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