Direct Shear - Pore Water Pressure - Undrained and Drained

[ocgeo]

If a sample is undrained, sheared at a rate which pore water pressure cannot dissipate, will the test result in higher shearing stresses? Also, it seems the undrained results will be less impacted by the normal load at the pore water pressure is taking some of the loading, therefore resulting in higher shearing stresses at lower normal stresses (when compared to drained conditions), and lower shearing stresses at higher normal stresses, resulting in a higher cohesion and lower phi.

When drained, the soil particles are driving the results (as opposed to soil/water and soil/soil interaction of undrained conditions). This would result in an increased angle of internal friction and lower cohesion, as the test results would be more significantly impacted by the normal loading (as opposed to undrained conditions).

I know there is much more to it than the above.... Please add, elaborate, and by all means correct me if I am wrong.

 

[oldestguy]

Maybe I am missing something, but I wonder what the set-up would be to run a direct shear test and keep the sample undrained.

 

[ocgeo]

My guess would be that you would have to have the sample enclosed similar to a triaxial test with a confining fluid pressure to get a truly undrained condition. For drained I believe ASTM has a formula for rate based upon consolidation results. Then I would venture to say that anything in-between would be partially-drained. So my above statement should most likely ready "partially-drained", and not "drained".

 

[dgillette]

Just curious - are we talking about direct shear, or direct simple shear? The latter is generally undrained or a pseudo-undrained constant-volume test. I've never heard of a drained DSS test anyway - why would you bother with DS or drained TX being quick and easy, and readily available?

 

[MelCao]

you better get undrained parameters from direct shear test for clayey soils. it takes long time to get completely drained. and for many cases, the retention structure the undrained condition is more critical.

 

[ocgeo]

ASTM specifies 24 hours/point as a default for CH. ASTM also recommend a time rate of consolidation test to determine what speed to run the direct shear test.

Results for points run for 24 hours/point were somewhat similar to CH samples run at 2 hours/point, however 24 hr/pojnt test had lower cohesion and increase phi, as one would expect. Results for the 2 hr/pt. test, however, were much lower than our results for cohesion for undrained Unconfined Compression test.

Conclusion: with a 2 hour test the sample is 'partially drained' (with 'percentage' drained/undrained unknown); with 24 hour/point test sample is closer to 'drained' but still I am not convinced sample is fully 'drained'.

Request: Is there a graph/data set that shows the Phi v. Cohesion for CH soil for, say, a 10 min/point test versus a 48 hour/point test to better understand the results, values, and limitations of the direct shear test? Clearly these parameters are vital to analysis and where do you draw the line for your analysis'.

 

[moe333]

This paper may provide some insight.

Estimating Undrained Strength of Clays from Direct Shear Testing at Fast Displacement Rates

by Andrew D. Bro, M.ASCE, (Fugro Consultants, Inc., 1000 Broadway, Suite 440, Oakland, CA 94607 E-mail: abro@fugro.com), Jonathan P. Stewart, (Dept. of Civil & Environmental Engineering, University of California, Los Angeles, 5731 Boelter Hall, Los Angeles, CA 90095 E-mail: jstewart@seas.ucla.edu), and Daniel Pradel, F.ASCE, (Group Delta Consultants, 370 Amapola Ave., Suite 212, Torrance, CA 90501 E-mail: dpradel@ucla.edu)
Section: Characterization of Soil and Rock, pp. 106-119, (doi:

http://dx.doi.org/10.1061/9780784412787.012)