Relationship between cohesion and allowable bearing pressure 2

[valleyboy]

I don't know if anyone out there is able to help.

I'm sitting some professional examinations here in the UK shortly, and have got hold of some typical past exam papers to attempt as part of my preparation.

I have noted in some of these papers that if the ground conditions comprise clays etc that values of effective cohesion are given.

Is anyone aware of an empirical method of converting this to an allowable bearing pressure I can use for preliminary deisgn?

If so, any references I can make use of for the exam would be much appreciated.

Regards VB

 

[jdonville]

VB,

Terzaghi's method works just fine if you assume undrained conditions and phi=0 degrees.

Usually, the coefficients of Nc, Nq and Ng are those developed by Meyerhof and Vesic, or some combination thereof.

When undrained conditions are assumed, Nq=1, Nc=5.14, and Ng=0.

q(all) = (c*Nc + q*Nq) / FS

where the second "q" = gamma'*Df

For shallow footings, FS is usually taken as 3.0.

You may wish to add a soil mechanics text to your prep materials, as this method is about as basic as it gets. (Don't ask me about structural engineering - I'd need quite some time to review and refresh)

Jeff

 

[valleyboy]

Thanks Jeff - that is exactly the type of expression I was hoping for !

I just wanted to clarify - for the second 'q' = gamma * Df

I assume gamma is the density of founding material, and Df = depth of foundation?

Regards

VB

 

[jdonville]

VB,

you should use gamma' - the effective unit weight (not strictly the same thing as density) of the soil above the foundation bearing depth.

Jeff

 

[valleyboy]

Thanks Jeff - star for you.

Regards

VB

 

[BigH]

To be quite honest, for the phi=0 condition, I have always used that the allowable bearing capacity (based on shear - not necessarily on settlement - keep that in mind) is equal to 2xSu. Why? - nearly all the formulas (which you usually have to look up in a book) are in the range of 5.7Su to 6.3Su maybe 6.7 if I remember right) or so assuming reasonably located footings below ground surface - so 6 is a good "typical order of magnitude number) - use a safety factor of 3, you get 2Su - or the unconfined compressive strength! You might have to be careful if you get into very stiff or hard clays when you have to worry about the effect of being heavily overconsolidated nature and/or fissures that might effect the mobilized Su at failure.

 

[fattdad]

I don't account for cohesion in my allowable bearing pressure. Rather for cohesive soils, I account for undrained shear strength (S-sub-U). For the case of undrained shear strength, I do what BigH does.

f-d

¡papá gordo ain’t no madre flaca!