The latter factor is considered the trigger of the landslide, even though its impact may be less significant with respect to others. At some point, one of those factors will result in reducing the FoS to less than one and subsequently, the landslide occurs. However, with time, there are certain processes or events (weathering, rainfalls, earthquakes, etc.) that tend to destabilize a slope, reducing temporarily or permanently the FoS of the slope. A stable slope has, by default, an FoS greater than 1. Once the concept of FoS has been defined, it is important to assess a slope failure in terms of FoS reduction. Finally, when the FoS is high enough (typically more than 2), an instability is less likely to occur. When FoS is slightly higher than 1 (e.g., 1.30), the slope is stable but a small disturbance (e.g., an earthquake) may temporarily increase the driving forces (or reduce the resistance forces) and lead to landsliding. If the FoS is less than 1, a landslide occurs since the driving forces exceed the resistance forces. The Factor of Safety ( FoS or FS) is defined as the ratio between the aforementioned two components, as: They depend on the shear strength of the soil/rock materials, the discontinuity planes between two ground layers, as well as any additional forces that can act in favor of the stabilization of the slope (e.g., a buttress at the lower section of the slope). The impact of gravitational forces is significant, and thus, driving forces highly depend on the inclination of the slope, the unit weight of the ground and the slope height.Ĭontrary, the forces that resist slope instability are referred to as resisting forces. The driving forces tend to destabilize a slope, leading to landslide phenomena, and are typically associated with the triggering causes of the landslides (the weight of the ground and the additional loads) (see Introduction to Slope Stability). Slope stability is controlled by 2 main factors: the driving and the resisting forces.