Slope stability analysis: a decision-grade workflow

Slope stability is rarely “a factor of safety problem.” It is a decision problem under uncertainty: variable stratigraphy, partial groundwater understanding, construction sequencing, and consequences of movement.

This note outlines a workflow that keeps the modelling honest and the outputs useful for design and construction teams.

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1) Start with the decision, not the model

Before opening software, define:

• What decision must be made (e.g., cut slope angle, need for nails/anchors, staged excavation approach).
• What constitutes “acceptable” performance (serviceability movements vs. ultimate failure).
• The critical scenario: long-term drained, short-term undrained, rapid drawdown, seismic, or construction stage.

If you need support framing scope and acceptance criteria, see services.

2) Build a ground model you can defend

The ground model should be a concise statement of geometry + strata + groundwater + material behaviour.

Key checks that typically change outcomes:

• Groundwater: perched water, artesian pressures, seasonal variation, drainage assumptions.
• Stratigraphic continuity: lensing layers and weak seams that control slip surfaces.
• Strength selection: characteristic vs. cautious mean; peak vs. residual where relevant.

In communication, highlight the specific parameters that dominate uncertainty (e.g., (c’), (\phi’), (s_u), (k)).

3) Choose the analysis approach deliberately

Use the simplest method that answers the decision with adequate reliability.

• Limit equilibrium: fast, transparent, ideal for sensitivity and design iteration.
• Finite element (e.g., PLAXIS): staged construction, stress-path effects, deformation, support interaction.

Treat FEM as an extension of the ground model—not a replacement for engineering judgement. A good path is: LEM for bounding + FEM for staging/deformation.

For FEM setup guidance, see PLAXIS modelling guide.

4) Sensitivity is not optional

Run a small set of targeted variations that align with actual uncertainty:

• groundwater level/pressures
• shear strength bounds (including residual where appropriate)
• unit weight / saturation assumptions
• geometry tolerances and temporary stages

Report results as ranges and identify “what needs to be true” for stability.

5) Make outputs buildable

Contractors and designers need actionable constraints:

• allowed temporary cut heights/angles by stage
• required dewatering trigger levels
• monitoring approach and stop-work thresholds
• stabilisation options and their dependencies (access, sequencing, environment)

If you want examples of concise deliverables, see projects.

6) How to write the conclusion

Avoid single-point statements like “FoS = 1.35 therefore stable.” Prefer:

• the governing scenario and why it governs
• the controlling assumptions (especially groundwater)
• sensitivity outcomes and recommended controls
• the residual risks and how they’re managed

If you need a peer review of an existing assessment, reach out via contact.