Second Stage Shear Strengths - Rapid Drawdown

[A10warthog]

I would like to confirm my assumptions on input values for shear strength values and methods for the second stage condition using UTexas.

Based on the manual for UTexas, it is my understanding that a composite shear stength envelope should be developed based on the effective stress and the R envelopes. The effective stress values are simply the values obtained from the CU test. However to develop the R envelope, the shear at failure for each modified mohr circle is determined based on the normal failure stress difference (deviator stress) divided by 2 and multiplied by the cos of the effective stress angle of friction. After computing and drawing the envelope the R envelope cohesion can be determined along with the angle of friction. Here is were I hit a gray area. To determine the intercept, one must determine the angle between where the envelope intercepts a circle and the corresponding max shear for a given circle. Once I do this and solve for the resultant intercept fiction angle and cohesion, these values are very close to the total stess envelope provided in the CU test results. Therefore, is the total stress envelope and the R envelope the same? Thanks!

 

[dgillette]

"Total stress envelope" is, in my opinion, a bit of a misnomer. It would be better to call it an "undrained strength envelope," since it gives undrained strength as a fn. of effective consolidation stress, not of total stress.

If I understand your question properly, the R envelope is the same animal, giving Tau-ff as a fn. of Sigma'c for the CIUC test.