Stress-dependent mechanical properties and bounds of Poissons ratio for fractured rock masses investigated by a DFN-DEM technique

Abstract

Stress-dependent mechanical properties and bounds of Poissons ratios for fractured rock masses are investigated using the distinct element method code (UDEC) with the Barton–Bandis (BB) fracture model, with stochastic multiple fracture system models constructed by the discrete fracture network (DFN) approach. Numerical experiments are conducted on both a transversely isotropic model crossed by one parallel fracture set and 10 more realistic random DFN models. The transversely isotropic model is investigated by an analytical solution with constant stiffnesses and by a numerical method using the UDEC-BB approach. Results show that mechanical properties are highly anisotropic and the calculated elastic modulus increases substantially with the increased stresses. The Poisson ratio can be well above 0.5. Numerical experiments on the 10 random DFN models using the UDEC-BB approach suggest that the elastic modulus of the fractured rock masses increases substantially with the increase of stresses (Fig. 1a). A simple empirical equation relating the mean normal stress (s) and rock mass elastic modulus (Em) is proposed in the following form: 1/Em=1/Ei+1/(Sms), where Ei is the intact rock elastic modulus and Sm is a sensitivity parameter. The results from the equation fit well with the numerical results obtained from the 10 random DFN models. The calculated Poissons ratios generally decrease with the stress increase and they are also well above 0.5 (Fig. 1b). The limitation of the two-dimensional (2D) approach is discussed for the case of the transversely isotropic model and the 2D values represent the maximum trace of the 3D results (Fig. 1c). The large Poissons ratios in this particular study are due to a high fracture density and connectivity of the DFN models and the 2D simplification. This paper suggests that engineering practice should consider the stress dependency of the mechanical properties of the fractured rock masses and the common practice of assuming the Poisson ratio as 0.2–0.3 may need careful re-evaluation for specific stress and fracture system conditions.