The equation that is used to calculate the total shear strength (s) of the material composing a slope, taking account of the pore-water pressure. The shear strength is then equal to the sum of the normal stress (σ‎) and cohesion (c). The Coulomb–Terzaghi equation is:

s = c + σ‎.tan ϕ‎,

where tan ϕ‎ is the coefficient of plane sliding friction, which describes the packing, surface roughness, and hardness of the materials constituting the slope.