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RE: augered piles

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Michael (and Bruce):

Having practiced structural engineering in Texas (the land of expansive clay soils) for more than 30 years, I feel compelled to offer a bit of advice.  Insist on a site-specific soils investigation by a local geotechnical engineer, and follow his/her recommendations without deviation.  To do otherwise will place you at odds with the Texas PE Board, and that is never a good thing.

Auger-cast piles are typically frowned on down here because of the inability to monitor the quality of the installation.  The most common and most economical solution is straight shaft drilled piers penetrating several feet into rock, with appropriate rebar to resist tension.  Unless you encounter groundwater (unusual in areas of expansive clay), there is no need for temporary or permanent casing.  Keep the shafts relatively small (3 ft. is typical) in order to minimize the uplift forces.

Do not get overly enthralled with first floor subgrade stabilization techniques such as water injection, lime injection, or pre-loading with several feet of select fill.  Each of these methods has limitations, and (at best) will only lessen the heave potential, not eliminate it.  At worst, they will increase the potential for damage.

Finally, there is the relatively expensive approach of isolation, with structural slabs suspended over a void space.  Even this approach is not fool-proof, as plenty of fools around here can attest.  I recall one project where the first floor of an office building was a structural slab over a 12 in. void space.  The subgrade eventually heaved more than 12 in., pushing the floor slab and beams upward and, consequently, the first floor columns outward.  More commonly, failures occur when slabs are placed over cardboard or styrofoam void boxes.   These void boxes often fail locally before or while the concrete is being placed, causing unintended and highly detrimental spread footings (i.e., drop slabs).  Consequently, our practice is to only use void boxes beneath grade beams and walls.  Beneath structural slabs, we insist on a true crawl space.

In closing, you should be aware that structural engineers in Texas annually experience more and bigger claims related to expansive clays than all other causes combined.  Don't take on liability that you can avoid.  Work with a knowledgeable local geotechnical engineer, and follow his/her advice.


Stan Caldwell, in Dallas