Expansive soil toe and heel pressures

[vato]

My foundation is on expansive soil and I must provide 800psf DL pressure min and 3000 psf maximum.
When I am designing a cantilevered retaining wall foundation, do I need to try and keep the heel pressure above 800psf and toe below 3000psf.

 

[FixedEarth]

If you make the heel about 2/3 of stem height & the toe length about 1/2 of the heel length, you will get nearly rectangular distribution of soil pressure. This assumes you have no axial load and surcharges. Just ensure that this soil pressure magnitude is 800 psf or greater for your case. See attached.

http://www.soilstructure.com/

 

[vato]

Wow, thank you fixed earth. I've been getting there as I refine my spreadsheet thank you.
The discussion with my geotech now concerns whether or not the expansive soil pressure
should be considered in the overturning calculation. Seem to me that it should be included, but
she is saying it is some type of live load. My feeling is if the heel pressure falls below 800, it is
not correct. I think of soil pressure in any direction as a dead load. Any thoughts?

 

[FixedEarth]

Glad to help. An alternative way to look at it is: The upwards swelling pressure = 800 psf. If your average bearing pressure (Rect. Distr.) is 800 psf or more, then "theoretically", no heave will occur. But this is not measured in mircrometer. So if you want add F.S. of 1.25, then you can proportion your heel/toe bearing pressure to be 1000 psf and need not add upward in your overturning clacs. Look at IBC 2009 Sec. 1808.6.3 "Exception" paragraph.

http://www.soilstructure.com/

 

[vato]

fixed earth, I can't find the 1808.6.3 reference, 1808 only goes to 1808.2.
thanks