UU Triaxial Test Results
UU Triaxial Test Results
(OP)
We've received 4 laboratory test results back from a subconsultant lab. The 3-pt UU tests performed on saturated samples indicate increasing diameter Mohr's circles such that there is a signficiant phi and c (not a horizontal line, phi=0). Stress strain curves demonstrate brittle behavior. Samples are overconsolidated very stiff clay (su between 3 and 4 ksf) from below groundwater table.
I am seeking possible explanations for why the failure envelope is not horizontal. Saturation calculations check out. Water contents before and after do not indicate drainage from specimen? All 4 tests show the same trend.
I am seeking possible explanations for why the failure envelope is not horizontal. Saturation calculations check out. Water contents before and after do not indicate drainage from specimen? All 4 tests show the same trend.





RE: UU Triaxial Test Results
Can you post data/plots?
RE: UU Triaxial Test Results
Attached is the test plot.
RE: UU Triaxial Test Results
RE: UU Triaxial Test Results
RE: UU Triaxial Test Results
Since I originally posted, I spoke with the lab that performed the tests and they said the soils were A LOT siltier and even some fine sand than the field classifications indicated. My guess now is that the observed phi angle for the tests on saturated specimens are a result of a frictional component of the silt and sand in the sample.
We are now running Atterberg limits and 200 washes to confirm our lab's observations.
RE: UU Triaxial Test Results
RE: UU Triaxial Test Results
RE: UU Triaxial Test Results
It is the static ground water table, not perched. The site is near a large body of water.
RE: UU Triaxial Test Results
RE: UU Triaxial Test Results
RE: UU Triaxial Test Results
The test results (i.e., increasing undrained shear strength with increasing total stress) foretell an initial unsaturated condition. Either that or there's a leaky membrane (or some other systematic error) at the lab.
f-d
¡papá gordo ain't no madre flaca!
RE: UU Triaxial Test Results
I mention some points may be of interest for you all
1) in any triaxial test, saturation is highly critical (as some contractors neglect this and pretend they have done saturation stage carefully and B-value of more than 96% has been achieved!)
As you know in unsaturated soils dues to presence of pore air phase, reading of pore water pressure is not reliable and therefore, effective stress parameters are erratic.
2) For heavily over consolidated clays or silts, due to developing negative pore water pressure during shear stage specially at low confining pressures, cavitation might occur and a some friction angle might be deduced due to non-equal radius of resulted Mohr circles. (ref. Mohr Circles, Stress Paths and Geotechnics, R.H.G. Parry, 1995, Page 48, bottom of page)
I hope you these points help you.
Regards
Mehdi
RE: UU Triaxial Test Results
Since when is a UU saturated? It's a test for total stress envelope with no allowance for drainage or saturation. The UU test has limited use except for short term behavour. This has limited value is most geotechnical design since most conditions on long term and drained.
I agree that there could be suction during the shearing phase. If BigEasy wants the residual shear strength go for the CU or direct shear with appropriate very slow shearing rate. The shear rate is the BigEasy tests was 0.05 inch/minute per standard which is very fast for cohesive material.