Groundwater Overview
Model pore-water pressure with water tables, piezometric lines, and pressure grids.
Pore-water pressure on the slip surface reduces effective normal stress and therefore shear strength. JW Slope offers two Pore Pressure Source settings, selected in the analysis settings, which between them cover three ways of generating slice pore pressures.
Pore Pressure Source
| Source | Description |
|---|---|
| Defined Phreatic Surface | Pore pressure comes from a water table and/or per-material piezometric lines, scaled by each material's Hu coefficient. |
| FEA Groundwater | Pore pressure is sampled from a solved finite-element seepage field. Experimental. |
The three pore-pressure mechanisms
1. Water table + Hu
A single global water table is drawn in the geometry. Materials that reference the water table get a pore pressure from the head between the slice base and the table, multiplied by their Hu coefficient:
u = Hu * gamma_w * (y_water - y_base)2. Piezometric lines + Hu
Instead of (or in addition to) the water table, you can draw one or more named piezometric lines and assign each material to a specific line. The pore-pressure calculation is the same as the water table, using the assigned line's elevation and the material's Hu. This lets different materials see different pressure surfaces in one model. Both mechanisms 1 and 2 are active under the Defined Phreatic Surface source; the choice between them is made per material on the Hydraulic Parameters tab.
3. FEA seepage field
With FEA Groundwater selected, a steady-state finite-element seepage analysis solves for a pressure-head field across a mesh. Each slice base then samples the solved field directly, with no Hu factor. See FEA Seepage Analysis. This source is experimental.
How each feeds slice pore pressures
- Defined phreatic surface: the assigned water surface is sampled at the slice mid-point; the head above the base times Hu times the pore-fluid unit weight gives the slice pore pressure. Materials set to None are dry.
- FEA seepage: the pressure head is interpolated from the solved field at the slice base (left, mid, and right base points combined), then multiplied by the pore-fluid unit weight. Negative (suction) pressures are clamped to zero unless the Include Negative FEA Pore Pressure advanced option is enabled.
Choosing a source
- Use a defined phreatic surface for routine analyses where a known water table or piezometric line adequately represents the pressure field. It is fast, transparent, and the standard approach for limit-equilibrium design.
- Use FEA Groundwater when seepage is genuinely two-dimensional — for example with anisotropic conductivity, infiltration boundaries, or a seepage face — and a simple phreatic surface cannot capture the pressure distribution. It is experimental and more involved to set up.