Why are pore pressures above the groundwater table negative, and how is this affecting the analysis?

The Interpreter plots total stress minus pore pressure for effective stress, regardless of whether the point is above or below the phreatic surface. 

If you use the Negative Pore Pressure Cutoff in the Project Settings, this cutoff will be applied to all materials. When calculating the effective stress the cutoff will limit the maximum suction for all materials to that value.

Example

If the Maximum negative pore pressure is set to 20 kPa in the Project Settings, then the maximum suction for all materials is 20 kPa.

neg_pore_pressure_cutoff_rs2.png

Global Negative Pore Pressure Cutoff in Project Settings

 

For Mohr-Coulomb materials you are able to define Phi_b and Air Entry Values. When the suction is less than the Air Entry Value, the calculation of effective stress proceeds as usual – the suction will affect the effective stress as if it’s a saturated media. When the suction is more than the Air Entry Value, the corrections are applied such that the suction beyond the Air Entry Values is cutoff from the effective stress calculation for that particular material. However, for values of suction between the Air Entry Value and the global Maximum Negative Pore Pressure, the effect of Phi_b will be added to the strength.

Example

Phi_b = 10
Air Entry = 5 kPa
Maximum Negative Pore Pressure = 20 kPa

Suction < 5 kPa: suction affects the effective stress
5 kPa < Suction < 20kPa: effective stress is modified by Phi_b
Suction > 20kPa: suction effect is capped at the effect of Phi_b for 20 kPa

unsat_shear_strength_rs2.png

Material-specific suction effect