Yazoo Clay

[KUeng2008]

I am designing a 4 story wood structure approx. 60' x 300'. We are sitting on yazoo clay and I was wondering if anyone could help with the typical foundations seen on Yazoo (Jackson, Mississippi). We are trying to decide between piers (auger cast/drilled) with an elevated floor and crawl space or 10'-12' of over excavation then replacing with structural fill. I have read that both can still have issues. I am seeing heaving up to 30" in crawl space situations on one hand and then recommendations of up to 20' of over excavation on the other. Is there a typical system that is "tried and true"?

Respectfully,

Joshua Thorpe

 

[JedClampett]

To state the obvious, you should use whatever foundation system your geotechnical engineer specifies. You're building a structure that's worth about $1,000,000. Pay a Geotechnical Engineer to get you started on the right foot.

 

[KUeng2008]

Yes, that is stating the obvious. We are paying a geotech, but as we all know they make recommendations not decisions. There is a higher amount of risk management than typical with this type of soil. I am just trying to get a feeling for methods that have been used by others in the area while we wait on borings to be completed.

 

[ameyerrenke]

If you want the geotech to make a decision, pay him to make the decision.. (i.e. design a foundation). That's the way I would manage the risk anyway.

 

[KUeng2008]

Your killing me guys. I am just trying to get an idea of what the typical tried and true methods are around Jackson, Mississippi for foundation design as it concerns yazoo clay.

 

[dylansdad]

Kueng2008,
If it is indeed Yazoo clay that comes back in your boring logs (I have designed buildings in the Jackson area, so there is a good chance),the typical foundations there are bell bottom piers supporting grade beams (void formed). The geotechs in that area are very knowledgable in this regard and will give good recommendations.

 

[a2mfk]

I assumpe Yazoo clay is highly plasctic...

Another system to consider is steel auger piles. But these need to be coordinated closely with your SPT results so that you terminate the ends well beyond the active clay zone, but if done right, they have some great uplift resistance. They are fairly inexpensive and fast to install.

 

[Ron]

ameyerrenke...MOST geotechs will not design a foundation, no matter how much you pay them. They will, as the OP noted, provide recommendations. Geotechnical engineers are not typically design engineers. They are analysts. As a structural engineer, you take their analysis and apply it.

Back to the Yazoo issue. Over-excavation is expensive and time consuming. If a deep foundation alternative is chosen, I would suggest that you use a system with low skin friction in the clay region to prevent inordinate uplift stress on the deep foundation system. Casing past the clay region is a consideration, with a combination of augered concrete piles below the casing.

 

[oldestguy]

As with any clay that has significant potential changes in volume with moisture changes, take all precautions possible to avoid either drying or wetting. No trees should be in the area and that means no tree roots can reach the area. No vented crawl spaces. Pave as much of the surrounding area as possible.

This P.E writes a nice summary of what to think about.

http://thecosmiccorner.blogspot.com/2013/01/designing-and-repairing-slab.html

 

[a2mfk]

Ditto with what ron said, and a system of casing and then using steel augers inside (think like an engine piston) isolates the pile from the expansive soil. Upward movement from skin friction when the clay "squeezes" the pile can be very significant, like Ron said.