Abstract: The anisotropic characteristics of rock mass are related not only to the properties of the joints in the rock, but also closely related to the stress state of rock mass. According to the parametric statistics of rock mass, combined with strain energy, a series of constitutive equations of rock mass containing single structural planes is derived to determine the deformation parameters of rock mass, such as elastic modulus. The reliability of the theoretical formula of the deformation parameters is verified by comparison with engineering examples. On this basis, the concept of anisotropic index is developed to quantitatively describe the anisotropic characteristics of rock mass deformation with a single set of joint. An expressway example is introduced to discuss the influence of rock mass structural characteristics and stress state on the anisotropic deformation of rock mass. The research results show that the anisotropy index of deformation increases as well as a significant weakening of elastic modulus with the increase of normal density of the joints. As the change of joint inclinations, the elastic modulus of the rock mass varies with a V-shaped trend, while the anisotropy index of the rock mass deformation will not be affected. In addition, with the increase of confining pressure, the anisotropic feature will weaken until it completely disappears. In regard to the anisotropic deformation and failure phenomenon in carbonaceous slate tunnels of Lan-Yu Railway, the paper finally presents the interpretation in the light of the above conclusions. The intense anisotropic weakening of elastic modulus of rock mass in tunnels was triggered mainly on great discontinuity density and excavation caused unloading of confining pressure. The orientation of minimum modulus is controlled by the occurrence of structural plane, which also determines the position where the local excessive deformation arises in tunnel heading.

Key words: layered rock mass, statistical mechanics of rock mass, elastic modulus, anisotropy index