Material Water Properties (Hu)
Per-material pore-pressure parameters including the Hu coefficient.
When the model uses a defined phreatic surface (water table or piezometric line), each material specifies which water surface applies to it and how the head above the slice base is converted to pore pressure via the Hu coefficient. These controls are on the Hydraulic Parameters tab of the Define Material Properties modal.
Where these fields appear
The water-surface and Hu controls only appear when the analysis Pore Pressure Source is a defined phreatic surface. With FEA Groundwater selected, the Hydraulic Parameters tab instead shows hydraulic conductivity and porosity — see FEA Seepage Analysis.
Water surface assignment
| Setting | Value | Description |
|---|---|---|
| Water Surface | None | The material is treated as dry; pore pressure is zero. |
| Water Surface | Water Table | The material uses the model's global water table. |
| Water Surface | (named piezo line) | The material uses a specific piezometric line, e.g. Piezo Line 1. |
Water tables and piezometric lines are drawn in the geometry editor — see Water Table and Piezometric Lines. Assigning surfaces per material lets different layers reference different pressure surfaces in the same model.
The Hu coefficient
Hu scales the pressure head used to compute pore pressure. It accounts for the
inclination of the water surface and seepage conditions: a value of 1 gives full
hydrostatic head, and lower values reduce it.
| Mode | Value | Description |
|---|---|---|
| Auto | computed | Hu = cos²(beta), where beta is the inclination of the water surface from horizontal at the slice. Ranges 0–1; equals 1 for a horizontal surface. |
| Custom | 0–1 | A fixed user-entered value (default 1). |
The Hu controls are disabled when the water surface is None, and the custom value field is disabled when the mode is Auto.
Pore-pressure calculation
For a slice whose base lies below the assigned water surface, the pore pressure is:
u = Hu * gamma_w * (y_water - y_base)where gamma_w is the pore-fluid unit weight, and (y_water - y_base) is the
vertical distance from the slice base up to the water surface (negative distances,
i.e. base above the surface, give zero pressure).
The pore pressure is evaluated at the slice base using the water surface sampled at the slice mid-point (with the left, mid, and right base points checked to confirm the slice base lies under the surface). If only part of the slice base is under the surface, that slice's pore pressure is treated as undefined for that configuration.
Note
Hu is applied per material, so the same water table can produce different pore pressures in different materials if they use different Hu modes or values.