The likelihood that a slope will fail. This depends on the relative strengths of the forces tending to make slope material move and the forces resisting such movement. The safety factor for a slope is expressed as the ratio between the shear strength and shear stress. F = s/t, where F is the safety factor, s is total shear strength along a specified shear plane, and t is the total shear stress along that plane. For shallow slopes, F = c + [(gz cos2 b – u) tan F]/gz sin b cos b, where c is cohesion, g is the unit weight of the regolith, z is the vertical depth of the shear plane, b is the angle of the shear plane, u is the pore-water pressure at the shear plane, and F is the angle of internal friction. A slope is stable if F > 1.3 and unstable if F < 1.