CU Triaxial Test Parameters

[bigtreble]

I recently had a CU triaxial series run on a sample of lean clay with pore pressure monitoring and back pressure saturation. When I look at the results, they varied significantly depending on how the results were presented. Three tests were run for the series, at confining pressures of 1/2, 1 and 2 tsf. When the failure criterion was "maximum stress ratio", the effective friction angle was 32.6 degrees. When maximum pore pressure" was used, it was 31.1 degrees. When "percent axial strain" was used, it was 31.8 degrees, but when "maximum deviator stress" was used it was only 18.9 degrees. Now I'm not sure which values would be appropriate for the effective stress analysis. Also, what could cause such a variation? Thanks

 

[ishvaaag]

It looks to me the close 3 first answers you refer to would represent values close to the average inner friction angle by 3 estimates, whilst the 4th one would represent one -in a from test evaluation- that most differs from that gained from the average of the tests.

If you want, the first 3 ones would be close to average and good for probabilistic prediction of the behaviour, and the last one would be close to the safe engineering value of the inner friction angle, in this case that ascertained for this case to be 100% -or always- warranted, from your tests.

Normally, a safe engineering value of a parameter needs not to be ensured but 95% of the times.

Of course, 3 or 4 tests stay statiscally non-significant and only from more tests you can guess which value of the inner friction angle can be ensured to be got at least 95% of the times.

Respect use, you would use the average value anywhere the true behaviour expected is to be modeled, and the safe engineering value for life safety standard calculations, i.e., you would use the safe engineering value for strength design of structures.

 

[72shortbox]

when determining failure I like to uses 2% strain
and get the axial and confining stress on the soil

you can also get a peak strenght if you plot the stress strain curve and pull your values at the peak