Slope Stability Analysis Explained: Factor of Safety

Slope Stability Analysis Explained: Will This Slope Stand?

Engineers answer that question with one number, the factor of safety (FS). It compares the forces that resist sliding along a slip surface with the forces that drive it. Above 1 the slope stands. Below 1 it slides.

The resisting side comes from the soil's shear strength (Mohr-Coulomb), and water pressure in the soil lowers it. That is why many landslides happen after heavy rain.

The analysis follows five steps: investigate the site, assume a slip surface, compute FS, search for the critical surface, and check every condition (long-term, short-term, earthquake, rapid drawdown).

The main methods are:

• Infinite slope, for shallow slides parallel to the surface
• Ordinary method of slices (Fellenius)
• Bishop's and Janbu's simplified methods
• Spencer and Morgenstern-Price, the rigorous limit equilibrium methods
• Finite element strength reduction, which needs no assumed surface

In practice we never design for FS = 1.0. Typical targets are about 1.3 to 1.5, because soil strength is uncertain. A good rule: the input data matters more than the method you pick.

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