Swell Pressure and Minimum Deadload for Foundation Design 1

[drewdavis2424]

Hello all,

I recently came across a situation where the bearing capacity of a site was 2,000 psf. However, based on the swell-consolidation test results the swell pressure (applied load on the swell-consolidation curve that prevents swelling) was 3,100 psf. Does this mean that my minimum site deadload is 3100 psf? Or is there a different way to calculate minimum deadload?

Thx

 

[MTNClimber]

The minimum dead load is selected by determining a value that would reduce the anticipated heave to an acceptable level. If 0”, which the 3100 psf would provide in your example, is an acceptable level, then shallow foundations on expansive soil are the wrong foundation choice.

The geotechnical engineer would have to run the numbers to see if a minimum dead load would reduce the heave to an acceptable limit. 0”-2” is satisfactory for most residential developers I’ve worked for in CO. Minimum deadloads I’ve recommended were in the ballpark of 500-800 psf.

 

[drewdavis2424]

Thanks. That helps alot. In this above example the above Volume Change for the Consolidation test was 0.50%. Does that correspond to a 1/2" anticipated movement then? If not...how does one correlate the volume change of a swell test to anticipated movement?

 

[MTNClimber]

0.5% is not very expansive if it is the entire soil profile for the assumed depth of wetting. Do you have a boring log to post with the lab test results?

Most firms in Colorado calculate the anticipated heave differently. Some do calculations, some use experience, and some use both. I've even seen the same firm calculate the anticipated heave differently from office to office. It would be wise to speak to your local geotechnical engineer on the project for their thoughts.

 

[Jimpat]

The PVR is the critical issue here, this can not accurately be determined by a single swell/consolidation.
The entire soil profile would have to be reviewed test should include current MC, gradations, PI, current dry/wet density and fully saturated/swollen dry/wet densities.

However, if you have an active zone of 15' and you assume a 0.5% swell (at which overburden pressure?) across the board that would equate to ~0.9" PVR : 0.005*12"*15'.