Undrained Shear Strength from Unconsolidated Undrained Triaxial Test

[slhc]

Hello everyone,

I recently reviewed the test results from the Unconsolidated Undrained Triaxial Test. In theory, the undrained shear strength (Su) should be consistent across different cell confining pressures. However, my test results indicate that, as the confining pressure increases from 200 kPa to 400 kPa, the Su varies significantly—by a factor of three. Could there be any possible explanations for this discrepancy?

Thanks in advance.

 

[geotechguy1]

Well, you'd expect it to double at least. Undrained shear strength increases with confining strength / or depth eg. commonly Su is given as 0.1-0.4 * sigmav' or Su = Su(0) + k*z where k can range from 1-10.

 

[EireChch]

GG1- its 3 mohr circles from the same sample at one elevation, i think .

An undrained shear strength should not be dependent on confining stress. You should have 3 circles with the same y intercept and different x intercepts.

Are you sure you are not looking at a CU test with pore pressure measurements or something.

You can get a little increase in cu but this is likely due to a non saturated sample which consolidates and gains a little strength with each stage. Please plot you data

 

[slhc]

GG1- its 3 mohr circles from the same sample at one elevation, i think .

An undrained shear strength should not be dependent on confining stress. You should have 3 circles with the same y intercept and different x intercepts.

Are you sure you are not looking at a CU test with pore pressure measurements or something.

You can get a little increase in cu but this is likely due to a non saturated sample which consolidates and gains a little strength with each stage. Please plot you data

Hi EireChch,

Thank you for your message. We currently do not have any results from CU tests.

Two of the UU test results, which include plots of deviatoric stress against strain and shear stress against strain, are attached in the following Excel file.

 

[EireChch]

yeah somethings not right, two tests on the same sample should not perform like that. There has been some consolidation (and gain of strength) between the two test pressures.