 σn is the normal stress across the fracture at the instant of failure, σf represents the pore fluid pressure.It is important to point out that pore fluid pressure has a significant impact on shear stress, "especially" where pore fluid pressure approaches "lithostatic pressure", which is the normal pressure induced by the weight of the overlying rock.This relationship serves to provide the "coulomb failure envelope" within the Mohr-Coulomb Theory.