Unconsolidated undrained test

[george69]

Hi
Can some please explain to me in basic terms why the total stress cohesion "c" measure from consolidatd undrained triaxial test is always smaller in value than if measured by the unconsolidated undrained method which gives "cu"

Why are the "c" and "cu" different anyway if they are both measuring undrained conditions?

 

[george69]

A further query to my question and it may be related:

For a Consolidated Undrained triaxial test, the point at which the sloping tangent line intersects the vertical shear stress axis -> is this termed "cu" or "c"? If it is "cu", would this be the same magnitude for "cu" if the testing was done by Unconsolidated Undrained triaxial test?

In other words is the Consolidated Undrained "cu" the same as the Unconsolidated Undrained "cu"

 

[GeoPaveTraffic]

These are good questions that many geotechnical engineers do not fully think through on a regular basis. (Often the differences don't really matter, so that is not necessarily a critisism)

Firstly, cohesion when determined by unconsolidated undrained (Q) tests measure the undrained shear strength at whatever consolidation pressure the sample has been previously consolidated to. Think of it as "a snap shot in time".

If you run group of consolidated undrained tests you get a combination of cohesion and phi. In theory, all of Q tests would plot on this line if you knew the consolidation pressure.

These tests each provide a method of determining the shear stress at given normal pressures. It all depends on how you are planning to use the results as to which test you should run.

Typically, in saturated clays, for foundation design unconfined compression tests are fine. For more involved analysis, other tests are required.

hope this helps.

 

[george69]

Thanks GeoPaveTraffic


The snapshop analogy is a very good way of thinking of the Q test!

Are you saying the consolidated undrained test looks at an envelope of consolidation stresses?

If I am understanding you correctly, after plotting the mohrs circle, and assuming ideal conditions for testing, then the "cu" determined from unconsolidated undrained test, should be the same as the "cu" determined from from a consolidated undrained test? Therefore the "cu" value from CU test can be determined from where the tangent line intersects the vertical shear axis?

Often, in Australia, we get the results for "cu" in a geotechnical report and they do not mention the method of testing. So I have always been curious in knowing if there is actually any difference in value for "cu" when done by either CU or Q test.

 

[moe333]

One problem with UU is you don't know if the sample is completely saturated, and if not it would give very unreliable results. With CU you typically use back pressure saturation, much more reliable.

 

[GeoPaveTraffic]

Your explaniation for comapring CU and unconfined tests is correct.

One miss-statment in my post is the last line where i use unconfined compression, that should have said unconsolidated compression. An unconfined is yet another test to get underained strength.

Moe333 is correct about staturation. With an unconsolidated undrained compression (UU) test the sample is rarley saturated leading to different results.

With all of this said, please note that for most light to medium loaded buildings on shallow foundations, shear strength really doesn't matter. Settlement matters. However, since consolidation tests are expeinsive and slow to run, we rely on strength tests with large factors of safety to reduce the settlement.

 

[moe333]

A couple points; consolidation tests are typically less expensive than CU or UU. If I was concerned about settlement I would opt for a consolidation test, not a strength test.

I have not heard of an unconsolidated compression test..it's not a UU? I am familiar with unconfined compression which is just a UU without the confining pressure.

 

[GeoPaveTraffic]

Should have been an unconfined compression test.

As for costs, our consolidation tests are several times the cost of the an unconfined compression or unconsolidated undrained test.