An overview of slope failure mechanisms, groundwater, material strength and stability assessment methods.
Slope Stability
Slope stability assessment considers whether the available shear resistance is sufficient for the potential failure mechanisms in a natural slope, excavation, embankment or fill. The controlling mechanism may be shallow or deep, circular or non-circular, translational, structurally controlled or influenced by erosion and groundwater.
The simplified formula and worked example for a shallow translational mechanism are on the separate Infinite Slope Stability Calculation page.
Define the slope model
A stability model should represent:
- Surface geometry and relevant construction stages
- Soil, fill and rock stratigraphy
- Drained or undrained strength parameters
- Groundwater, perched water, seepage and possible rapid changes
- Surcharge, structures, traffic and temporary plant
- Excavation, erosion, toe support and nearby services
- Existing cracks, scarps, deformation and previous instability
The model should cover credible conditions, not only the surveyed condition on one date.
Groundwater commonly controls stability
Pore-water pressure reduces effective stress and can materially reduce available shear resistance. Rainfall infiltration, leaking services, irrigation, perched water, blocked drainage and rapid drawdown may create conditions that differ from a routine investigation measurement.
Groundwater assumptions should be based on observations, site history and sensitivity analysis. A single water-table line may not represent transient or perched conditions.
Strength selection
Drained effective-stress parameters are generally used for long-term conditions where drainage occurs. Undrained total-stress parameters may be appropriate for relevant short-term loading in saturated fine-grained materials. Residual or softened strength may control along existing shear surfaces or in susceptible materials.
Selecting an analysis method
The infinite-slope model is suited to shallow failure approximately parallel to a long, uniform slope. More complex geometries and deep-seated mechanisms commonly require limit-equilibrium methods of slices, numerical analysis or other specialist methods. The search strategy and interslice-force assumptions should be documented.
Factors beyond a calculated ratio
The result should be considered with deformation, erosion, rockfall, debris movement, consequences, monitoring and constructability. Temporary works and staged excavation can govern even when the completed profile is stable.
Calculation resources
Engineering note
There is no universal factor-of-safety requirement for every slope and condition. Acceptance criteria, actions, parameters and analysis methods must follow the project design basis and applicable requirements.