Undrained Shear Strength of Clays

[phamENG]

I'm trying to expand my horizons a little (and remember a few things from college that have fallen by the wayside with time), so hopefully you guys can help me.

Say I have a report from a triaxial test run on a normally consolidated clay sample. The report shows results for 3 identical test specimens with varied cell pressures resulting in varied normal stresses. Plots for shear vs. normal stresses (3 semi-Mohr's circles) and deviator stress vs. axial strain are provided.

I want to determine the undrained shear strength of this clay. As I recall, it varies linearly with depth (normal stress). So would I be correct in taking the deviator stress/2 for each and plotting that against depth to approximate the shear strength at any depth within that clay layer? Or am I way off my rocker? Thanks.

And before anyone says it, don't worry - not trying to do the geotech's job. I just want to be more familiar with the source of the data behind the numbers I have to use in structural design.

 

[jdonville]

When determining the undrained shear strength from triaxial testing, the plots should be semi(Mohr)circles with similar radii.
If the radii increase with increasing confining pressure, then you are looking at a drained strength test.

http://www.civil.uwaterloo.ca/maknight/courses/cive353/lectures/shear-strength.pdf

 

[phamENG]

Thanks, jdonville. I looked through that slideshow earlier this morning. The slide on page 9 showing consolidated drained vs. consolidated undrained both show a phi angle. I pulled some historical reports for reference. See the attachment. This is by far the largest angle. Most in this file are between 2 and 6 degrees. As you can see, the report is marked "unconsolidated undrained". Perhaps they mislabeled the report?

 

[phamENG]

So I spent a little time reading in Das today, and I'm not sure if it helped or clouded things.

One takeaway is that in definitely looks like my example report is mislabeled - a UU test should definitely give a phi=0.

There's no particular reason to assume it was a CD test that I can see (somebody please tell me if there's something in the test report that proves me wrong), so let's say it's a CU test. Looking at the Deviator vs. Axial strain plots, the shape of the curves look like they could have come from the text book graphic for a normally consolidated clay. But...if I draw a Mohr-Coulomb failure envelope on the Shear vs. Normal stress plot, I do not pass through the origin. That would suggest an over consolidated clay, wouldn't it?

Can anyone help me out with the concepts and procedures for determining the undrained shear strength of the clay in the file attached to my previous post? Thanks.

 

[GeoEnvGuy]

The initial saturation is labelled as 67.2% this indicates the clay is not in a constant state of saturation and can be fissured which can yield a curved UU result. Additionally heavily over consolidated clays can exhibit a curved undrained failure envelope at low confining pressures. Image below from Craig's soil mechanics.

 

[geomane]

The UU test will only return a straight horizontal line (phi=0) if the soil is saturated (S=100%). If the sample is partially saturated, the strength envelope will increase with increasing confining pressure until the air voids compress and the sample becomes saturated.

 

[phamENG]

Awesome. I figured there was a good explanation staring me in the face. It makes a lot of sense - the report I posted is near the ground surface and has the largest variation (and lowest saturation), while some others I found that are 40+ feet below have relatively flat failure envelopes (with 98.5%+ saturation). Thank you.

So given the partial saturation, how do you go about reading the data to determine the undrained shear strength? I dug some more and found the boring log for the test sample. It shows the shear strength as 2200psf for the sample I posted. This is roughly equal to the average of the three shear strengths: sum(deviator/2)/3. Is that correct, or just an interesting coincidence?

 

[GeoEnvGuy]

So you have identified an unsaturated clay that doesn't have the water content to fail undrained. So you should be looking at modelling the material as a drained failure in the existing condition.

You want to know what happens if you saturate it, load it and then force it to fail undrained, those are your test results.

As for the undrained strength listed on a borehole log it could be just the average or it could be a field vane result listed.

 

[fattdad]

in a triaxial cell, you'd typically consolidate the sample for each cell pressure. When you fail that sample, you'll have both a drained and an undrained stress path. Find the failure of the undrained stress path, plot that circle and the height of the circle is the undrained shear strength for that confinement.

f-d

ípapß gordo ainÆt no madre flaca!