Direct Shear
Direct Shear
(OP)
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.
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RE: Direct Shear
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.
RE: Direct Shear
RE: Direct Shear
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.
RE: Direct Shear
RE: Direct Shear
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.
RE: Direct Shear
RE: Direct Shear
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).
RE: Direct Shear
Sorry, should have rolled my eyes after my first post. ;) I figured mine was even more of a wisecrack statement.
RE: Direct Shear
RE: Direct Shear
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.