FemSoilCalc

Set up and stage the excavation

1. Define the wall x coordinate, domain widths and mesh. Retained soil is on the left; excavation is on the right. y is elevation. Inputs use kN and m.
2. Enter material properties and layer tops in Materials. The first layer starts at ground level and the mesh fits every layer boundary; near-coincident levels require a finer mesh or corrected input.
3. Enter hydrostatic water levels, wall stiffness, normal/shear penalties, friction and explicit wall adhesion (default zero). Horizontal support stiffness and prestress are per metre of wall.
4. Stage 1 describes unexcavated ground. Define subsequent descending excavation levels. Supports are installed at the previous wall position at the start of their activation stage, before that stage's excavation or water changes. An off stage can remove a support or surcharge.
5. A passive-side surcharge stays at its specified surface elevation. Blank means original ground level, so excavation below that level removes the surcharge. Enter the excavation elevation to load a later platform.
6. All stages are calculated in their original order; Analyse selects stages for reporting and the envelope. Warnings check every stage. Review force and moment balance, wall axial/shear/moment actions, support reactions, principal strain and effective-stress mobilisation. Save the input model and export a separate result report after running.

What the results mean

The model uses plane-strain constant-strain triangles with elastic effective-stress stiffness, stored initial K₀ stresses and separate contact on each wall face. Compression is negative; mobilisation includes all three principal effective stresses. The envelope covers selected reported stages, while warnings check all construction stages.

No soil yield correction, dilation, strength reduction, global factor of safety, seepage or excess undrained pore pressure is calculated. Mobilisation at or above 1, tensile effective stress or principal strain above 1% flags results beyond or near the limits of the elastic small-strain model.

Each layer has one unit weight. Inclined anchors, remote support movement, wall self-weight and toe bearing are not modelled. Independently check plastic behaviour, hydraulic and basal stability, mesh sensitivity, boundary sensitivity and design against a validated geotechnical model.

In-app help

Calculation method

Plane-strain constant-strain triangles with layer boundaries fitted to the mesh; compression is negative.

Initial vertical total stress is integrated from soil unit weights. Initial effective stresses subtract hydrostatic pore pressure; both horizontal initial effective stresses use K₀.

Total equilibrium uses initial effective stress plus elastic strain increments and the current pore pressure. Excavation removes soil elements; remaining initial stresses release through the exposed boundaries.

Independent soil nodes on both wall faces, joined beneath the toe. Normal contact is compression-only; tangential contact has a friction/adhesion cap (explicit wall adhesion, default zero) and carries plastic slip history. Water traction acts separately from effective soil contact.

Horizontal supports use displacement since installation. Unilateral supports resist positive wall movement towards the excavation; bilateral springs can also reverse their force. Supports and surcharges can be removed at a specified stage. Stiffness and prestress are per metre of wall.

All construction stages are equilibrated, including stages unticked for reporting. Linear and contact force residuals and global force and moment balance must pass before results are accepted.

Wall moments and shears come from beam element end forces. Ux/Uy envelopes include only active soil and wall nodes.

Engineering limits

Soil is linearly elastic; Mohr–Coulomb mobilisation is a stress diagnostic using all three principal effective stresses. There is no soil yield correction, dilation, strength reduction or global factor of safety. Mobilisation ≥ 1 or tensile effective stress marks an elastic extrapolation.

Groundwater is a prescribed hydrostatic field on each side. Seepage, piping, consolidation, excess undrained pore pressure and basal stability are not calculated. Each soil layer has one unit weight; separate dry and saturated weights are unavailable. Clay examples are elastic diagnostics rather than undrained analyses.

Side boundaries have fixed horizontal displacement and the base is fixed in both directions. Supports act horizontally at the wall; inclined anchors, remote support movement, wall self-weight and toe bearing are not modelled.

Use mesh and domain sensitivity checks and independent engineering benchmarks. CST elements can be too stiff near incompressibility; ν ≥ 0.49 is rejected. Direct dense solves are limited to small models. Treat these results as preliminary until independently checked against a validated geotechnical model.

Tips and field guidance

Widths are measured from the wall x coordinate. Retained soil is to the left; excavation is to the right. y is elevation.

The top layer starts at ground level. Every layer top is included in the mesh. Soil is linearly elastic in effective stress. Strength is checked diagnostically; soil plasticity, dilation and undrained pore-pressure generation are unavailable.

Supports act horizontally. Enter an off stage to remove one before that stage; leave blank to retain it.

A passive-side surcharge stays at its entered surface elevation. Blank means original ground level; it is removed once excavation passes below it. To load an excavated platform, enter that stage’s excavation elevation. Off stage removes the load before that stage.

Every construction stage is calculated in its original order. “Analyse” selects stages for the results envelope; unticked stages still carry their construction history. Supports are installed at the start of their activation stage, at the previous wall position, before excavation or water changes in that stage.

Areas in the app

Use these sections to locate the inputs, checks and results shown in the app:

• Project
• Geometry
• Materials
• Wall / Interfaces
• Loads
• Stages
• Summary
• Wall Results
• FEM Results
• Theory

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