Direct Shear

[adidasmayo]

Does anyone know why I would get negative values for cohesion for a Direct Shear Test? I am pretty sure that this should not happen.

 

[theCorkster]

mnoorzay;

To answer this, provide soil classification, density at placement, density/settlement under confining pressures prior to testing, load/strain data for each specimen during testing, your selected failures for each specimen, and mohr diagram.

 

[adidasmayo]

Please see the attachment on what we do for a Direct Shear

 

[moe333]

If the strength at the higher confining stresses is higher than what it "should" be, a best fit line of the data can result in a negative cohesion value. It is the result of an error and has no physical meaning.

Two conditions that could cause this are:

The points at higher stresses were run too fast resulting in an artifically higher strength.

The points at higher stresses were run on a higher strength material than the the lower point.

 

[dgillette]

I think negative cohesion might violate the laws of thermodynamics. B-)

 

[TDAA]

dgillete, it is not a violation. It can as the material changes phases. It is all due to the pressure increases from the normal and shear stresses, and temperature fluctuations in the lab. This is a little known secret since the material changes back to the original state after the pressure is removed and the samples are observed.

On another note:

Items to think about:
1. You have run a test that will have only a certain amount of precision (as in the actual numbers could be +- the number you obtained).
2. You only have two data points, and are basing your decisions off of them. Often a third point is done and a best fit line is drawn.
3. You have extrapolated the result back to zero, and this negative result is not based on an actual data point.
4. There is potential that there is a nonlinear strength envelope, even though we normally make it so.
5. The result is only below zero by 18psf, and it is not like it is 500psf.
6. Perhaps some engineering judgment should be applied. Test results are meaningless without it.

 

[moe333]

I didn't look at your plots. I had assumed you ran 3 points, which is what I would always normally do. As TDAA states, -18 psf is well within the range of error/accuracy. This is a non-issue IMHO.

 

[dgillette]

TDAA - You miss the point. If you project the strength envelope back to very low normal pressure, the predicted shearing resistance goes negative - hence violation of thermodynamics, the first law thereof I believe. Strength can't really be negative, or exactly zero for that matter, so it all has to be an artifact of testing and variability in test specimens, and the limitations of assuming a linear MC strength envelope. I guess my wisecrack was too obscure.

BTW - What "phase change" are you talking about in a direct shear test (which is necessarily drained)? The only phase change I've ever heard of in soil shear resistance is in undrained testing of initially contractive material that under goes "phase transformation" and begins to dilate (seen in a p'-q path as initially curving left, then turning back to the right, roughly following the critical state line).

 

[TDAA]

dgillette, per the first law, as you apply work to the system, you must be increasing the internal energy of the system. This change results in the solids changing phase directly to gas. You do have to watch that it does not become ionized, resulting in plasma. That would really throw the results off.

Sorry, should have rolled my eyes after my first post. I figured mine was even more of a wisecrack statement.

 

[dgillette]

Lucky we have engineering to fall back on for income. Neither one of us is Dave Barry or Mark Twain.

 

[oldestguy]

I'm curious. Does your device make allowance for the reduced area at any particular point of the displacement? (Thus, increasing the unit pressure as the displacement takes place.) If so, is your value for shear strength and the plot taking that into account also?

Does the device keep the loading centered on the resulting area?

These effects may explain some things.

Also, only two plots seems inadequate for any reliability on the results.