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Spread Footing Settlements

Spread Footing Settlements

Spread Footing Settlements

(OP)
I am trying to design a  square spread footing on a very stiff sandy silt with clay N >50. I dont have any consolidation test results. I assume that the soil is over consolidated but not sure about OCR??. My boring goes 18 ft. below B.O.F. I am trying to calculate the consolidation settlement and not sure what assumptions to make and what H value should I use in the OC settlement equations. Do I take it as one layer just to the bottom of boring (H=18) and calculate effective pressure (P0') and use Boussinesq  equation to calculate Pv at the middle of the H layer. Is there any design example around..?. Thanks

RE: Spread Footing Settlements

I'd base the calculation on soil modulus. For N=50 you may have a modulus value of 500 tsf, let's say.  Also, let's say it's that way for the full depth of the stress influence.

Using Boussinesq, calculate the change in vertical effective stress with depth.  Integrate this stress curve to depth (this will essentially be solving for the area of the delta-sigmaV v. depth curve.  Your units will be psf*ft, or pounds/ft.  If you then divide this by the soil modulus (using consistent units), you'll get units of feet, which will be the anticipated compression under the foundation load.  Some folks multiply this by 1.2 to account for long term soil creep.

Hope this helps.  My professor (Mike Duncan) agreed with this approach when I was in graduate school and it's simpler than using the Schmertman method (especially if you don't have cone data).

f-d

¡papá gordo ain't no madre flaca!

RE: Spread Footing Settlements

I think that what fattdad is also implying is that with N values of that magnitude, there is very little chance that you won't be "way over" consolidated and wouldn't worry about virgin consolidation - only the recompression.

RE: Spread Footing Settlements

Quote:

I think that what fattdad is also implying is that with N values of that magnitude, there is very little chance that you won't be "way over" consolidated and wouldn't worry about virgin consolidation - only the recompression.

Yes.  

Additionally, in many instances a sandy silt displays elastic behavior, expecially if the silt is non plastic.  Normally, I'd want to know LL and PI, but for N>50 it's kind of a moot point.

f-d

¡papá gordo ain't no madre flaca!

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