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Question
Mr. Cummings--

I own 8 acres of land just ENE of Lake Elsinore, CA.  There exists a pad on top of the hill that was cut in rock (I think it is shale rock) that I was told ruined 2 "rippers" on a D9 Cat.

My question:  Is it possible to "drill" casson footings in the cut rock and build the house on an elevated (2 1/2') "metal trussed foundation"?

If so, how much "easement" is needed around the pad?

Thank you in advanced for your time,

Michael Reister

Answer
If the pad was cut in rock, especially rock that was so resistive to ripping with a D-9, you have plenty of bearing capacity, why not just use spread foundations instead of a suspended system of caissons and trusses?

I wonder what kind of rock it actually is -- most shales are not so tough to rip that they would defeat a D-9 (provided of course that the cat skinner knew what he was doing).  Possibly a schist?  There are some tough black shales, but usually a dozer with enough maneuvering room can find some way to rip them.

Anyway, your question was whether you can drill caissons.  Yes, you certainly can.  What you would do is hire a driller, preferably one with a down-the-hole hammer, to drill 6 inch or larger holes 5-6 ft deep, then run rebar into those and use sonotubes above grade as forms, and fill then with concrete, screed the tops of the forms off at the same elevations, and off you go.  A structural engineer can design the connections between these rock-socketed caissons and the trusses, and can give you minimum sizing on the rock sockets in order to maintain sufficient cover over the reinforcement plus adequate friction against uplift loads.

You would need about 10 ft minimum outside the footprint of the caissons for maneuvering room, and on one side more, to permit the drill to track in and out, and turn.  You also need space for a concrete truck to get close enough to each of the caissons that you can deliver concrete without having a remixer and a pump.  Later you will need space to handle the trusses and that will involve a cherry-picker which probably will not be able to operate from within the foundation perimeter with all the caissons sticking up.  If he has a 10-ft perimeter he should be able to traverse the outside of the foundation area and reach into the inside, setting outriggers between caissons.

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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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