What is factor of safety in slope stability analysis?

Factor of safety (FOS) in slope stability is the ratio of resisting forces (or moments) to driving forces (or moments) along a potential failure surface. FOS = Shear Strength / Shear Stress. Typical minimum values: temporary slopes 1.25, permanent slopes 1.5, critical structures 2.0. FOS < 1.0 indicates failure. Analysis methods include limit equilibrium (Bishop, Spencer, Janbu) and finite element. FOS varies with water conditions, loading, and assumed failure surface geometry.