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Re: Foundation on Expansive Clay

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Prairie engineer here.

Expansive clays containing montmerillinite (sp?) or even worse bentonite are a loose loose situation for slab on grade.  No matter how deep you back fil, you can always get these varying moisture contents.  In our experience the volatility of the clays settles down after seven years or so.  It's about at this time that the soil has found a new equilibrium.  But if you pull out a major tree, pave over some extra virgin land for parking, have a water main break or a flood, it can take another 7 years for the soil mass to find a new equilibrium.  If you add into the mix frost penetration due to winter you have a great technical problem to solve.

I'd recommend getting a report from a geotechnical engineer to assess the risks and make recommendations based on those risks.

If the client wants to pay for guaranteed no movement, then a structural slab on c.i.p. piles designed for skin friction interaction with the clay only will solve this.  One of the nice things about clay is that yes they have terrible expansive properties but good skin friction values.  Typically around here I would design to 300-350 psf skin friction.

If the client doesn't want pay for that, and doesn't mind rehanging doors and cosmetic repairs then a slab on grade on poly and 6" crushed gravel or as recommended by geotechnical will do it.  If you decide to go this route, and most people do, make sure your drawings indicate that there will be significant movements which may incur costs to the owner.  We do this often.  But it is also a common reality in these parts.

I am not aware of an easy way to deal with this.


>>> "Fountain Conner" <fconner(--nospam--at)> 04/04/01 12:01PM >>>
Unfortunately, it appears that these expansive clays extend to China.

1.  What would a couple feet of undercut and backfill gain me?

2.  How likely am I to get differential movement in excess of 1/2" in a
yard foundation 7ft x 8ft?


> From: Bill Polhemus <bill(--nospam--at)>
> To: seaint(--nospam--at) 
> Subject: RE: Foundation on Expansive Clay
> Date: Wednesday, April 04, 2001 11:02 AM
> Oops, I didn't catch that (I'm used to an active zone of 10 ft or so here
> Southeast Texas).
> You're right, John. If it's only a 6 inch active zone, excavating it out
> the best alternative. In fact, I'd recommend doing that up to about 5 or
> feet or so. After that, it's worth looking at a cost comparison.
> William L. Polhemus, Jr., P.E.
> Polhemus Engineering Company
> Katy, Texas
> Phone 281-492-2251
> Fax 281-492-8203
> -----Original Message-----
> From: John P. Riley [mailto:jpriley485(--nospam--at)] 
> Sent: Wednesday, April 04, 2001 9:55 AM
> To: seaint(--nospam--at) 
> Subject: Re: Foundation on Expansive Clay
> "My thinking is to turn down the outside edge of the footing an
additional 6
> inches.  If this clay is expansive, it shouldn't be given to capillary
> action, and will provide a seal, preventing the soil beneath the
> from seeing the varying moisture content."
> Is the expansive clay layer only 6" thick?  If this "turn-down" method is
> effective in protecting the soil within the 6" zone, what about the soil
> beneath IT?
> My first inclination is to overexcavate and backfill.
> __________________
> John P. Riley, PE, SE
> Riley Engineering
> 20 Oakwood Drive, Blue Grass, Iowa 52726
> Tel & Fax:  319-381-3949
> jpriley485(--nospam--at)