Use of saturated Mohr-Coulomb failure criterion and a water table

[MiningLad]

Hello all.

I have recently read a report where someone has conducted a slope stability assessment using the rocscience SLIDE software. For this analysis, they defined the material using mohr-coulomb c=40kPa, phi = 25deg which was derived from CU triaxial tests. This test type represents a saturated soil. Therefore, I was surprised to see that they also included a water table.

Now, I am not sure if I am getting confused here, but isn't this double accounting the groundwater within the soil? And if you have MC params from a CU triaxial, you already assume the soil is fully saturated and does not need a water table within the analysis?

Cheers,

MT

 

[geomane]

If you are running an effective stress slope stability analysis, then the water table needs to be included in the analysis because it influences the effective stresses, and thus the shear strength.

Shear strength = c’ + sigma’(tan(phi’))

If you were running a total stress analysis, the water table should not make a difference (a difference in the shear strength) because you are using total stresses.

 

[GeoEnvGuy]

Based on the parameters provided to me this indicates a low plastic clay. Selecting the CU test is an indication of the long term steady state.

As an old engineer told me if you build a dam on soft clay you only need to be concerned for the first two years after construction. If it doesn't fail in the first two years the foundation Clay's have consolidated and gained strength.

The critical case is the end of construction case where all the pore pressure has not dissipated and how that is analyzed. In slope/w I have seen a b-bar used to increase the pore pressure.

 

[MiningLad]

Thank's all for your replies to my question.