Residual Shear Strengt by Multi-reversal Direct Shear 2

[GTNOP]

Hi all
My project is Residual shear strength along Shear layer.
I plan to run Multi-reversal Direct Shear in multistage(vary normal stress on same sample).Have anybody any good comments or ideas??? , thank you
!! I don't have a Ring Shear Test

 

[Focht3]

You probably won't find the true lower bound strength - reversing direction tends to cause surface to "re-roughen", resulting in increased resistance. If you want to get closer to a true lower bound, you'll have to disassemble after each cycle, separate the two halves of the shear box, put it back to the starting position, reload it, etc. It's a real hassle, and you have no guarantee that you will achieve the absolute lowest residual values. But the values achieved this way should be lower than you will get by the multi-reversal technique.

Multi-stage may be okay if you don't have any granular material in the sample.

 

[KULTUR]

In reversing the shearbox most of the disadvantage is the orientation of the shear zone. If it is horizontal there is no problem but if not then a stress accumulation occurs especially on the corners.
I may propose dividing the two halves by using a wire saw (to get a smoot flat shear surface) after you run the test once and got the peak values and then wait for the soil to consolidate at least 12hrs and then shear it again.

 

[Focht3]

KULTUR makes a good point; also if the soil being tested has pre-existing planes of weakness (like slickensides) then you need to carefully trim the sample so that the shear box failure surface coincides with the "plane" of a large slickenside (or whatever weakness is present.) Very tedious work, but crucial to the success of your endeavor.

 

[geekengineer]

I have limited experience using torsional ring shear and therefore when estimating residual shear, that is the test that I perfer. Realize that while running this test, you will have to "remold" the sample and then run each point out to large strains after consolidation and at rates that are related to the 95% consolidation time rate. There is an excellent paper detailing expected residual strength results of "clayey" soils based on effect normal stress, clay size fraction, and liquid limit. The reference is Stark and Eid (1994). By running this test to large strains, you are dictating the location of the shear plane and also ensuring that "slickensides" develope in the sample. Note also that you will get a curved or non-linear envelope if you run the test over a large range of normal stresses. It is important to start with low stresses so that you do not plot the results and discover an aparent cohesion intercept which is not expected. Good lock