FEA Seepage Analysis (Experimental)

Experimental feature

FEA seepage is an experimental finite-element workflow. It 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.

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.

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 Experimental (finite element) panel.

1. Mesh generation

Create an external boundary first, then generate the mesh.

Control	Default	Description
Target Element Side Length	Auto	Target triangle edge length (minimum 0.05). Leave blank for an automatic size.
Element Type	6-Node Triangle	6-node (quadratic) or 3-node (linear) triangular elements.
Equilateral Element Length Bias	0.6	0 (uniform sizing) to 1 (stronger bias toward equilateral elements).

Use Generate Mesh to build it and Delete Mesh to clear it.

2. Groundwater boundary conditions

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

Condition	Meaning
Seepage Face	Potential exit face; resolved by an active-set rule. Active segments are fixed to zero pressure head (head = elevation).
No Flow	Zero nodal flow across the segment (impermeable).
Zero Pressure	Pressure head fixed to zero on the segment.
Constant Head	Total head fixed to the entered value.
Nodal Flow	Prescribed nodal flow rate.
Infiltration	Prescribed inflow along the segment.

Unassigned external boundary segments behave as no-flow.

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.

Control	Default	Description
Convergence Tolerance	0.01	Outer-loop tolerance on the maximum head change between iterations.

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:

Advanced option	Default	Effect
Include Negative FEA Pore Pressure	Off	When off, negative (suction) pressures from the field are clamped to zero. When on, suction is retained.
Slice at Phreatic Surface Intersection	—	Adds slice boundaries where the slip surface crosses the FEA phreatic (zero-pressure) surface, improving integration accuracy across the water surface.

Material hydraulic parameters

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

Parameter	Unit	Default	Range	Description
Porosity	—	0.3	0.01–0.99	Effective porosity.
Ksat	m/s	1e-6	> 0	Saturated hydraulic conductivity (primary direction).
K2/K1 Ratio	—	1	> 0	Ratio of the secondary to primary conductivity (1 = isotropic).
Hydraulic Orientation	degrees	0	—	Orientation of the primary conductivity axis from horizontal.