UU Triaxial Test Results

[BigEasyGeoSleuth]

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.

 

[moe333]

What were the insitu total stress and laboratory confining pressures used in the tests?

Can you post data/plots?

 

[BigEasyGeoSleuth]

The insitu total stress is about 3000 psf. The confining pressures were 2000, 4000 and 8000 psf. I estimate the preconsolidation pressure at around 9,000 psf.

Attached is the test plot.

 

[moe333]

I didn't crunch the numbers but the saturation is shown less than 70% for all three. You will get a c-Phi for unstaurated samples.

 

[AussieGeoEng]

moe333 is correct. With a partially saturated soil you will experince some consolidation when the cell pressure is applied (even though all the drainage valves are closed).

 

[BigEasyGeoSleuth]

Half of the tests are from samples above the GWT (unsaturated), and I agree that these samples likely underwent volume change. The other half are from below the GWT (saturated). Even the saturated tests show the c-phi behavior.

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.

 

[BigH]

I would have thought you'd run the Atterbergs and Fines content before you would have done the UU test. Our office engineer always doubled checked the field descriptions before assigning lab testing. Things that many field engineers miss (or, heaven help us, drillers who are logging the samples instead of the geotech having his own engineer on site) is laminations. I was involved as an after the fact investigation oh so many years ago in a situation where field drillers missed laminations within a sand deposit which resulted in major stability problems on a deep sewer excavation.

 

[moe333]

The saturation level is important. Soil "below" the groundwater table can be partially saturated as the groundwater may be perched above the soil.

 

[BigEasyGeoSleuth]

Our Client logged the borings and prepared the lab schedule, so we did not have input with regards to index property testing until now when they are now concerned with the UU results.

It is the static ground water table, not perched. The site is near a large body of water.

 

[moe333]

Percent saturation says otherwise.

 

[BigEasyGeoSleuth]

I uploaded the wrong test sheet...that is for one of the unsaturated samples. The others I am referring to indicate saturation.

 

[fattdad]

I understand the relevance of three-point UU tests. I also understand the relevance of replicating the in-situ total stress. Fact remains the in-situ effective stress is the condition of the soil. With increasing total stress, you turn more air voids into more water voids.

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!

 

[Geoengineer67]

Hi
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

 

[molerat2210]

Geoengineer67:

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.