Hand Penetrometer for Bearing Capacity of Soil

[JohnnyB_]

Hello -

I was wondering if anyone has any experience designing residential foundations using the bearing strength from a hand penetrometer. I understand that the soil may not be uniform but couldn't multiple readings be taken and the average of them be used for design? Does anyone use a factor of safety by reducing the averaged bearing capacity by say 15%?

Thanks in advance!

 

[JoelTXCive]

I'm sure the geotechs can chime in, but I think they use a 50%+ reduction. I thought they had safety factors of 2 to 3 on bearing capacity.

 

[XR250]

Are you talking about one of those pocket penetrometers used for mobile homes?

 

[JohnnyB_]

XR250, yes a pocket penetrometer. Not sure if they are used for mobile homes.

 

[Lomarandil]

Sure, I've done this.

Yes, you want to see that your test location is represetative of the area. And then make sure the result is repeatable (test several places/times, after all, each test takes 10 seconds).

I would not average results. I would take the lowest value obtained, with the allowance for any obvious outliers.

Depending on your structure, foundation type, and local stratigraphy, your zone of influence (depth) will vary. You might need to obtain data from tests at different depths.

And (as you probably know) the result of a pocket penetrometer is not directly an allowable bearing pressure, it is a representation of the EDIT: unconfined compressive strength -- which gets converted to allowable bearing capacity using different correlations, local engineering judgement, and a factor of safety around 3.0.
Often times, that conversion process comes out nearly 1:1 (for strength controlled soils), but not always.
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just call me Lo.

 

[BigH]

Lomarandil - unless things have changed since I started this 45 years ago, the pocket penetrometer (PP) gives the "unconfined compressive strength" - not the undrained shear (cohesive) strength. You end up dividing the PP by 2 to get the undrained shear strength. The hand vane, on the other hand gives the undrained shear strength directly.

 

[Lomarandil]

Nope, BigH, you're right! I was mistaken. Thanks!

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just call me Lo.

 

[JohnnyB_]

Thanks Lo and BigH, very helpful.