The impact of fractures on the hydro-mechanical behavior of fractured rock masses is analyzed by means of equivalent Biot \(({\overline{\alpha }})\) and Skempton \(({\overline{B}})\) coefficients. We assume the derivation proposed by De Simone et al. (Rock Mech Rock Eng 56:8907–8925, 2023), in which the equivalent coefficients depend on the combination of fracture size, orientation and mechanical properties, with the mechanical properties of the intact rock. We extend this theory to incorporate more complex and realistic assumptions on fractures, such as the dependence of aperture and normal stiffness on size and confining stress. Under this setting, we explore the range of variability of the two equivalent coefficients with respect to the stochastic distribution of fracture size and orientation in the rock mass, as well as to depth and stress faulting regime. We find that, although \({\overline{\alpha }}\) and \({\overline{B}}\) increase with fracture density, they are larger if the network is populated by a few large fractures than if populated by many small fractures because large fracture are more compliant. Orientation and depth also greatly impact the coefficients. Fractures oriented such that the applied normal stress is maximized, lead to larger equivalent Skempton coefficients and smaller equivalent Biot coefficient. However, the initial confining stress maximizes both coefficients when fractures are shallow and parallel to the maximum principal stress. Therefore, fracture orientation may differently impact the equivalent coefficients depending on the initial and applied stress tensors. Overall, fracture contribution is larger in shallow rocks containing large fractures that are oriented parallel to the largest principal initial stress and normal to the applied stress.
