FEA Seepage Analysis

FEA seepage solves a steady-state Darcy flow problem on a triangular mesh and produces a pressure-head field. When the analysis Pore Pressure Source is set to FEA Groundwater, the limit-equilibrium slices sample pore pressure from that solved field instead of from a phreatic surface.

FEA seepage is more involved to set up than a defined phreatic surface and is intended for two-dimensional seepage problems that a simple water table cannot represent. Standard limit-equilibrium analyses do not require it.

The workflow is: build the mesh, assign groundwater boundary conditions, solve, and then run the LEM analysis against the resulting field. The relevant controls are on the Finite Element panel.

1. Mesh generation

Create an external boundary first, then generate the mesh.

Groundwater and stress each solve on their own mesh, with their own settings. The Groundwater and Stress tabs at the top of the Meshing window pick the mesh you edit, generate or delete, and the one the Geometry view draws. Groundwater results always draw on the groundwater mesh and stress results on the stress mesh. Stress reads groundwater pore pressures from the groundwater mesh by location, so the two meshes do not have to match. Changing or regenerating the stress mesh keeps the groundwater result.

Use Generate Mesh to build the selected mesh and Delete Mesh to clear it. A groundwater solve generates a missing groundwater mesh first. While a mesh generates, Stop and Keep stops it and keeps the previous mesh, and Stop and Clear stops it and clears the mesh.

2. Groundwater boundary conditions

Select external boundary segments and assign a condition type. A boundary value is required for head, flow, and infiltration conditions.

Unassigned external boundary segments behave as no-flow.

Clearing conditions from a segment also clears its end points. A condition on a neighbouring segment then covers its own segment except that shared corner, on any mesh, and the condition's text lists the corners it leaves out. Infiltration keeps its corners because it acts along the segment rather than at nodes. Assigning the segment again covers its whole length. Slide2 conditions set on every node of a segment except a corner import the same way.

3. Solving

The solver runs a steady-state Darcy analysis with anisotropic conductivity and optional unsaturated behavior:

• A saturated precursor solve is run first, then an adaptive unsaturated continuation marches toward the unsaturated conductivity law.
• Each step assembles the finite-element system and solves the reduced linear system with a Conjugate Gradient solver (Jacobi-preconditioned).
• Seepage faces are handled as a damped active set.

The internal CG linear solve uses a fixed relative tolerance (1e-5) and is not user configurable. A solver dialog reports progress, per-step iterations, and the convergence metric, with pause / stop-and-keep / stop-and-clear controls.

4. Outputs

The solved field can be displayed in the viewport:

• Pressure-head field and total-head field, shown as filled contours.
• Velocity / flow vectors from the solved gradients.

These overlays are selected from the result display controls (e.g. Total Head / Pressure Head).

5. LEM integration

With FEA Groundwater as the pore pressure source, each slice base samples the solved pressure-head field (using the left, mid, and right base points) and multiplies by the pore-fluid unit weight to get pore pressure. Two advanced options affect this:

Material hydraulic parameters

When FEA Groundwater is the source, each material's Hydraulic Parameters tab shows the seepage inputs: