Triaxial CIU testing dilemma

[MDirt]

I'm getting conflicting info from two separate labs on running some CIU tests for me. One lab is suggesting that the CIU test is run at one consolidation pressure which they say is typically set for 50psi (I would assume to run it at the estimated preconsolidation is most appropriate). The other lab is suggesting that the test is typically run on 3 or more samples at 3 distinct consol pressures ranging from preconsol stress to about 3x.

Based on Holtz Kovacs it would seem the 2nd lab is confusing the CIU with the CD test which needs to be run at 3 or more consol stresses to define the complete Mohr failure envelope.

It would appear the first lab is correct however the cost for the CIU is more than 3x the cost for the UU, I understand there is a consol phase and the failure rate is much slower and also pore pressure readings and possible back pressure application is needed but does this justify 3x the cost or is the 2nd lab right and the cost increase is associated with the extra samples tested?

 

[BigH]

What exactly are you looking for? Stress-paths - do you want the porewater pressure measured? Undrained shear strength? In the CIU test with porewater pressure measurements, the test does take quite a while (about 8 hours) - as you need the porewater pressure to stabilize across the sample as you are testing (run it too fast and you get garbage results). However, you have a choice - you do one sample, take it to q'1/q'3 max, then reconsolidate the specimen at a higher pressure and do the same - you can get, then, two or three curves off of the same sample and get your effective stress parameters. To do only one test at one confining pressure doesn't make sense if you are looking to effective stress parameters. If you are not taking porewater pressure measurements, you can look at the value giving you the undrained shear strength at that level of consolidation. The second lab, in proposing three tests, wants to give you an idea of the undrained friction angle (like the Su/OB ratio) so you can see how the material behaves with confining pressures. As these tests involve setting up the triaxial cell, rubber membranes on the sample, etc., they will be more costly - without question than the UU test.

 

[TBrandon]

BIGH summed it up pretty well. It matters if you are wanting drained or undrained strengths from ICU tests. If you want undrained strengths (and the soil is overconsolidated), then, as a minimum, you can consolidate one specimen back to the field vertical stress. Bjerrum had a good paper about this in the international conference in Moscow (1973?). If the soil is close to normally consolidated, then determining an undrained strength ratio for specimens consolidated in excess of the preconsolidation stress might be best.

For effective stress or drained parameters, you need to ensure that the effective stress on the failure plane of your lab test specimens brackets what you expect in the field.

I hope this helps!