The factor of safety is defined as the ratio of the Ʈf to Ʈ in the limit equilibrium analysis. The factor of safety can be defined in three ways: moment equilibrium, force equilibrium and limit equilibrium. The first definition relies on the shear strength that can be in obtained from two ways: effective stress approach and total stress approach. The strength type depends on the type of the soil and the time elapsed after the excavation and loading conditions. The total stress strength is utilized for the short term conditions in the clayey soils and effective stress strength is utilized in the long term conditions in types of soils and conditions where the pressure pore is known. The third and second definitions are based on the movement equilibrium and force equilibrium conditions to resist and drive moment and force components. The definitions are sometimes confusing but the moment or force components contribute on driving and resisting slides.
The limit equilibrium methods use Mohr-Coulomb expression to evaluate the shear strength along with the sliding surfaces. The shear stress the soil fails is referred to the shear strengths of the soil. The limit equilibrium state exists when the mobilized shear stress is expressed as the fraction of shear strength. The shear strength is mobilized fully at the moment of the failure along with the failure surfaces when the critical conditions state is reached.
The shear strengths rely on the effective normal stress and type of soil. The mobilized shear stress relies on the external forces that act on the soil mass. The factor of safety is the ratio of Ʈ and Ʈf in the limit equilibrium analysis. The factor of safety can be explained in three ways: moment equilibrium, force equilibrium and limit equilibrium. The first definition describes that the shear strength can be obtained in two ways: effective stress approach and total stress approach.