Negative Pore Pressure Cutoff

Negative pore pressure can be used in shear strength calculations when Unsaturated Shear Strength parameters (Phib and Air Entry Value) are defined for a material. Negative pore pressures may be computed from finite element seepage analysis, water pressure grids, or above water tables or piezo surfaces.

The Negative Pore Pressure Cutoff option allows you to define a maximum allowable value for negative pore pressure. Negative pore pressure cutoff can be specified independently for each material.

To use this option:

1. In the Material Properties dialog, select a Material and select the Water Parameters tab.
2. Select the Maximum negative pore pressure checkbox and enter a value. It does not matter if you enter a positive or negative value, Slide3 will automatically use the negative of the absolute value.

If you are using Unsaturated Shear Strength parameters for a material:

• The Negative Pore Pressure Cutoff value allows you to limit the contribution of negative pore pressure to the material shear strength.
• If you do NOT define a Negative Pore Pressure Cutoff (i.e. the Maximum negative pore pressure checkbox is turned off), then there is no upper limit on the values of negative pore pressure, and you may generate unrealistic high values of shear strength which may lead to unconservative high values of the safety factor.