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RE: Sec 1815 UBC (slabs subject to expansive soils)

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Title: RE: Sec 1815 UBC (slabs subject to expansive soils)

Eric Green wrote:

How do you actually operate your practice in design of slabs on expansive clay? What design methods do you use?


According the Construction Research Center at UT-Arlington (a.k.a., UT-Almost), expansive clay soils in the USA annually cause more economic loss to the built environment than all natural disasters combined.  Whether or not that is true, it is undisputed that expansive clay soils annually result in more lawsuits against Texas' structural and geotechnical engineers than all other causes.  In suburban Dallas, there are areas where seasonal moisture changes can result in more than 12 inches of differential heaving/settling.  Obviously, this can be a serious problem.

Most residential structures, including apartments and single family houses (even large custom ones), are constructed on stiffened slab foundations.  This means a monolithic slab with perimeter and interior drop beams, sometimes conventionally reinforced, but most often post-tensioned to provide greater flexural stiffness.  The idea of this design approach is not to resist heaving and settling, but rather to allow the structure to more-or-less "ride" through the movement as a reasonably rigid body.  As an analogy, think of a ship riding through the waves at sea.  Many years ago, the common design methodology was HUD's prescriptive BRAB Slab.  For at least the past decade, most designers have relied of the excellent literature published by PTI.  Speaking of designers, very few residential structures in Texas involve mainstream structural engineering practitioners, and the foundation slabs are typically "designed" more-or-less for free by the post-tensioning tendon suppliers.

All other types of structures rarely involve stiffened slabs.  Instead, care is taken to isolate all columns and bearing walls from near-surface soils, and to support them on deep foundations (typically drilled shafts or belled piers).  Most warehouse and retail floor slabs are isolated from the building shell, and placed directly over several feet of "stabilized" subgrade (several feet of chemically injected native soil and/or several feet of select (PI < 15) fill).  Most office and institutional floors are designed as structural slabs suspended several inches to several feet above the subgrade. 


Stan R. Caldwell, P.E., F.ASCE
Vice President
Halff Associates, Inc.
8616 Northwest Plaza Drive
Dallas, Texas  75225
Phone:  (214) 346-6280
Fax:  (214) 739-0095
Email:  scaldwell(--nospam--at)