Plaxis 2D Slope Stability Tutorial: Safety Factor

Will this slope stand? Plaxis 2D gives you one number for it: the factor of safety. In this tutorial the answer is 1.65. Below is every step to find it, from an empty project to the final chart.

New to the idea? Read Slope Stability Explained first, then follow this tutorial.

The example

• A slope 10 m high, with a gradient of 1V : 2H
• One soil layer, Mohr-Coulomb, drained
• Goal: the factor of safety (FoS) and the shape of the slip surface

Step 1: Model size

Project properties: plane strain, 15-noded elements. Set x max = 60 m and y max = 25 m, so the slope has room above it and the soil has room below it.

Step 2: Draw the slope

Draw one soil polygon with these points (x, y) in metres:

(0, 0) → (0, 20) → (20, 20) → (40, 10) → (60, 10) → (60, 0)

The drop from y = 20 m to y = 10 m over 20 m gives a 10 m high slope at 1V : 2H.

Step 3: Soil material

Parameter	Value
Model	Mohr-Coulomb, drained
Unit weight γ	18 kN/m³
Young's modulus E'	20,000 kN/m²
Poisson's ratio ν'	0.3
Cohesion c'	10 kN/m²
Friction angle φ'	25°
Dilatancy angle ψ	0°

Drag the material from the Material sets window onto the soil polygon to assign it.

Step 4: Mesh

Generate the mesh with a medium element size. This model gets 166 elements.

Step 5: Initial phase (gravity loading)

In the Initial phase, set the calculation type to Gravity loading. Then right-click the soil and choose Activate, so the soil weight is switched on.

Step 6: Safety calculation

1. Add Phase 1 and set its calculation type to Safety.
2. Tick Reset displacements to zero.
3. Calculate. If Plaxis warns that there are no curve points, ignore the warning and calculate anyway.

In a Safety phase, Plaxis keeps reducing c' and tan φ' until the slope fails. The factor it reaches at failure is the safety factor.

Step 7: Look at the slip surface

In the output, open Deformations > Incremental displacements. For a clearer slip band, switch to Incremental deviatoric strain. The band of high strain shows where the slope wants to slide.

Step 8: Read the safety factor

Create a new curve: x-axis = Step, y-axis = ΣMsf. The curve rises and then flattens. The value where it flattens is the factor of safety: ΣMsf = 1.65.

Why the displacements look so big

In a Safety phase, the displacements in the output are not real movements. Plaxis drives the slope to failure, so they are incremental values at the moment of failure and can look huge. Use the shape of the slip band and the ΣMsf value. For real deformation, use a gravity or plastic phase.

Is 1.65 enough?

Many design codes ask for a factor of safety of about 1.3 to 1.5 for a permanent slope, but the required value depends on your code, the soil data and the consequence of failure. Always check the limit that applies to your project. A quick hand check with a limit-equilibrium method (Bishop) gave a similar answer, about 1.68 in this example.

In short

1. Draw the slope and set the soil.
2. Mesh, then gravity loading.
3. Add a Safety phase and calculate.
4. Check the slip surface.
5. Plot ΣMsf: FoS = 1.65.

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