Slope stability is a fundamental concern in geotechnical engineering. Failures can range from small, localized slips to catastrophic landslides that endanger lives and infrastructure.
Slopes become unstable when driving forces exceed resisting forces. Common triggers include:
The classical approach to slope stability analysis uses limit equilibrium methods. These methods assume a failure surface and calculate a factor of safety (FoS) as the ratio of resisting to driving forces (or moments).
Common methods include:
| Method | Assumptions | Typical Use |
|---|---|---|
| Ordinary Method of Slices | Neglects interslice forces | Simple hand calculations |
| Bishop Simplified | Horizontal interslice forces | Circular failures |
| Janbu | Applicable to non-circular surfaces | General slip surfaces |
| Spencer | Satisfies both force and moment equilibrium | High-accuracy analyses |
| Morgenstern-Price | Variable interslice force function | Complex geometries |
A factor of safety greater than 1.0 indicates theoretical stability. Design values typically range from 1.3 to 1.5 for permanent slopes under static conditions, with higher values required for critical infrastructure.
Modern software allows automated search for the critical failure surface.
Finite element and finite difference methods have become increasingly common. These approaches:
The strength reduction method (SRM) is widely used to compute a factor of safety with continuum numerical models.
Slope stability analysis combines soil strength theory, geometry, and groundwater conditions to evaluate safety. While limit equilibrium methods remain the workhorse of practice, numerical methods provide deeper insight into deformation mechanisms and progressive failure.
In the following post, we will examine earth retaining structures and the calculation of lateral earth pressures.