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RE: Foundations in Swelling Soil

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>From what I have read and heard about expansive soils, I would be very
hesitant to build a structure on grade over it.  I am familiar with a
project that was built on expansive soils where the geotechnical
engineer said that the soils were expansive but that a maximum of 1" of
movement was expected.  The main structure was put on piles but the
slabs were grade supported.  The end result was that due to some poor
construction and water infiltration below the slabs, the slabs moved up
3" in many locations, cracked wall board, broke below grade piping,
effected the use of doors, etc.  The building was structurally o.k. but
the serviceability aspects of the building were not so good.  

We designed a project on expansive soils where we put shafts through the
expansive soils, constructed grade beams on "void form" material (grade
beams supported by the shafts and the "void form" material allowed for
expansion below the beams) and made the grade floor a structural slab
with a "crawl space" below it.  This concept must have worked well
because we have not heard of any problems.

The bottom line is that extreme caution must be exercised in building a
structure over expansive soils.

I'm not sure this helps, but just an opinion.

Jim Hagensen, SE
HNTB Corp 

> -----Original Message-----
> From:	ARW [SMTP:amirr(--nospam--at)paknet1.ptc.pk]
> Sent:	Friday, February 13, 1998 10:04 AM
> To:	'seaoc(--nospam--at)seaoc.org'
> Subject:	Foundations in Swelling Soil
> 
> THE PROBLEM:
> I am currently designing a warehouse for a feedmill building, in
> Pakistan. The soil near the surface (to about a depth of 13 feet) has
> exhibited a slight swelling potential (generating a pressure of the
> order of 1.54 K/sq ft, at an existing moisture content of about 20%)
> in the oedometer test and the geotechnical agency has added a word of
> caution, to the structural designer, in this respect. It has suggested
> the use of grade beams, connecting all columns in both directions. The
> soil is already under an appreciable moisture content and the agency
> does not foresee a further significant increase in moisture content,
> particularly as the water table is also not very deep.
> 
> I have a grid of 50'-0" x 30'-0", with reinforced concrete barrel
> shells, supported on columns that would rest on spread (individual)
> foundations, placed at a depth of 6'-6" from the natural grade. The
> floor is 3'-0" from the natural grade.
> 
> For a typical interior foundation, the unfactored imposed load is
> about 120 K. I need a foundation of 9'-0" x 9'-0" at these locations;
> this foundation size is required, under a permitted 1 inch of
> settlement (shear does not govern for this foundation size) and
> including the floor storage load (about 0.672 K/sq ft) and the weight
> of 3'-0" of soil (about 0.360 K/sq ft) in the column load. For this
> case, the force generated by swelling tends to equal the column load
> in my case, with surcharge acting as the reserve margin.
> 
> On my specific inquiry, the geotechnical engineer has additionally
> suggested (a) adding a sand cushion (to 90% Modified Proctor), of at
> least 12", under the foundation and (b) slightly increasing the
> permitted settlement to achieve a smaller foundation size and hence a
> higher bearing pressure resulting in greater reserve against the
> upward force.
> 
> PRELIMINARY THINKING:
> I intend to go ahead with the use of grade beams. I might consider
> using a T-shaped beam to increase its rigidity. On the 50'-0" side I
> might consider using two intermediate supports for the grade beam,
> again to increase its rigidity. I would also consider some
> differential support settlement (say 0.50 inches), for the design of
> the grade beam. I intend to undertake a spring model for the analysis
> of the grade beam.
> 
> QUESTION:
> What shall be a rational and logical design criterion for the
> structural design of the grade beam and the foundations?
> 
> ONE ADDITIONAL QUESTION:
> I have one additional question: My storage is bagged grain and bagged
> poultry feed, stacked without shelves. The stacks of the bags are
> known to sometimes fall to one side and lean against the infill brick
> masonry walls. How to estimate this loading?
> 
> In other similar projects, for another client, I have been using an
> arbitrarily chosen uniform lateral load, to provide at least some
> degree of partial safety, but I know it is not enough. My previous
> client knows this too and has been willing to accept partial safety in
> this case and in fact does not wish to spend large amounts against an
> unforeseen hazard. On the contrary, he tends to view the infill wall
> as a sort of structural fuse that protects the rest of the structural
> system, rather than transfer the full load to it! 
> 
> I would be extremely grateful for any advice that I may get. 
> 
> Rizwan Mirza
> Consulting Structural Engineer
> Lahore, Pakistan
> 
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