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Home > Forums > Geotechnical Engineers > Activities > Soil mechanics engineering Forum

Unconfined strength and Net all. bearing value

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WWTEng (Structural)

2 Nov 11 11:23

What is the relation between these two numbers, if any?

fattdad (Geotechnical)

10 Nov 11 13:11

If you are running the unconfined compression test, I'll go ahead and assume that you are dealing with a fine-grained (i.e., cohesive) soil.  Therefore, let's discount any contribution from internal friction.

According to the Terzhagi equation, for phi=0 (as appropriate for soils without internal friction), Nc=5.7 so qult=cNc.  For undrained analyses, c=Su (cohesion equals the undrained shear strength.  Undrained shear strength is equal to one-half the unconfined compressive strength.

So, ultimate bearing pressure would be:

qult=5.7*qu/2 or qu*5.7/2 or 2.85*qu

If you consider a safety factor of 3 q-allowable is then equal to:

q-allow=0.95*unconfined compressive strength. Heck that's about like saying the allowable bearing pressure is equal to the unconfined compressive strength!!

Wow!  That was easy, eh?

f-d

¡papá gordo ain't no madre flaca!

FixedEarth (Geotechnical)

11 Nov 11 14:36

Nice explanation fattdad.

I might add, also check settlement & differential settlement, since it can lower your design allowable bearing pressure.

www.FoundEng.com

WWTEng (Structural)

12 Nov 11 20:07

papa gordo, muchas gracias.

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