Bearing Capacity of Clay

[SuperSandman]

Hi Guys!!!

Simple question...just need some clarity? Terzaghi's general bearing capacity equation makes provision for the term "c'Nc"...Skemptons bearing capacity formula for clays is "Ncu x Cu".

I have carried out numerous triaxial tests and I have determined the cohesion. In drained conditions, I have the results of C'. I would plug in this value of C' into Terzaghis equation to determine bearing capacity of the clay.

Now i have come across a situation where there is no triaxial result but instead the CPT result of the clay layer (Cone penetrations qc in MPa).I have done the necessary correlations and the texts i have used correlate the qc to give me an approximate undrained cohesion Cu.

Am i correct in saying that I need to use Skemptons bearing capacity formula for when i have undrained clay parameters?

 

[egsa]

Hi SuperSandman

if you have calculated the Cu of the clay, why not use the relationship from the TxUU: q=2xCu?

this is what i would do, but i am not sure if it fit in your contest.

 

[GeoPaveTraffic]

I think you need to go back and review all of the assumptions and derivation of the equations you are using.

You are not including all of the terms in Terzaghi's equation. Cohesion (undrained shear strength) * Nc is divided by a factor of safetey (generally 3) is only applicable for undrained analysis. You don't get this strength from drained triaxial tests.

If you want to calcualted the bearing capacity of clay under drained conditions (not sure why you would want to) then you need to use the complete version of Terzaghi's equation.

Mike Lambert

 

[Mccoy]

If you want to calcualted the bearing capacity of clay under drained conditions (not sure why you would want to) ...

Under drained condition though some OC clays can result in lesser bearing capacity. Besides, there is the complication of unsaturated clays for which undrained analysis would not applicable. So there might be a reason to assume drained conditions

 

[GeoPaveTraffic]

Mccoy,

I agree with you on both points. However, given the original questions; I hope SuperSandman is not trying to account for either of these conditions since I think he would be in way over his head.

Since you brought up the topic, I'm curious how often you perform analyses for these conditions? In my exerience, it is rarely done.


Mike Lambert

 

[SuperSandman]

Thanks for the input guys!!!

@Geo: I understand the derivations of the equations. i was only making reference to the "cohesion component" of the equation. Im just making sure im using the correct cohesion (drained or undrained) with the correct formula.

I also need to calculate long term bearing capacity of clay layers. That is why I use C' triaxial results. At junior level i always used terzaghis method and was brainwashed into using C' and phi' until i was given undrained results. Thats were the confusion came up.

 

[fattdad]

It's flawed reasoning to state,

Cohesion (undrained shear strength)

After all, cohesion is the intercept of the failure envelope under zero normal load and undrained shear strength is the shear strength under undrained loading conditions. Terzaghi's equiations are a rational approach to the failure envelope. If you do a "c" and "phi" analysis, they both have some value (I personally discount cohesion). If you do an undrained analysis, then it's "Su" and phi=0 unless it's an unsaturated soil and there is some pseudo-friction angle. You can evaluate whether there is some pseudo-friction angle by doing UU tests at varying confining stresses and see of you actually get the phi=0 condition.

You can get "c," "phi-prime," and Su from a series of undrained test if you measure pore pressures. unfortunatly you can't tell whether there is a pseudo-friction angle under undrained conditions, 'cause you have to saturate the sample prior to shear.

f-d



¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

fattdad, you are correct that cohesion is not theoretically the same as undrained shear strength. However, in many cases it is practially the same for foundation analysis. If the undrained strength testing occurs at the same elevation of as the foundation in question and the undrained friction angle is low; then there is very little contribution to bearing from the friction angle.

There is certainly nothing wrong with performing bearing capacity calculations based on a serries of UU tests and considering both the cohesion intercept and the undrained friction angle in your calculations.

If you are performing a drained bearing capacity analysis; then you would certainly want to include both the cohesion intercept and the drained friction anagle in your calculations.

Unless your experience indicates that one condition or the other will always control, you should check both conditions in your analysis.

The original post included CPT data. CPT's provide undrained shear strength data in clays and plastic silts and correlations may be able to provide some drained data.

Mike Lambert

 

[BigH]

Now, after all that is said and done, do the settlement estimates and adjust your "bearing pressure" down accordingly.

 

[Mccoy]

Geopavetraffic,
presently I'm very rarely calculating bearing capacity since, with the new European regulations, in dynamic conditions it becomes such a messy business that the structurals prefer to use geotechnical add-ons to their structural software.
I provide both drained and undrained values of cohesion though, since the regulations are clear on that. In practice, when we are dealing with moderately small structures, most often only CPTs are carried out so we don't have a phi'-c' pair, we have to provide it regardless to the structurals, from inference or previous experience.
In larger structures we use CPTs and extract samples to be submitted to lab tests.
When c' is not small, as you say cohesion governs.

And BigH, thanks for unflichingly reminding us that most often settlements governs and not bearing capacity, LOL, I agree of course.
Although in very seismic areas with little or no cohesion- silts, bearing capacity may govern. Theoretically, since we often see the results of excessive settlements, most rarely the results of a failure due to lack of bearing capacity .