The purpose of this study is to demonstrate the effect of virgin rock stresses on the deformability and permeability of fractured rocks. Geological data were taken from the site investigation at Forsmark, Sweden, Conducted by SKB. A set of numerical experiments was conducted to determine the equivalent mechanical properties (essentially, elastic moduli and Poisson's ratio) and permeability using the DFN-DEM (Discrete fracture Network - Distinct Element Method) approach. The results show that both mechanical properties and permeability are highly dependent on stress. Stress-induced anisotropy of mechanical properties was caused by hyperbolic nature of stiffness of the individual fractures. Stress induced anisotropy of permeability was observed due to different closure behavior of fractures, and in situ stresses influence the fluid pathways in the fractured rock mass. This study shows that proper characterization and consideration of in situ stress is important for the understanding of both mechanical and hydraulic behavior of fractured rocks.
Fractures provide the main fluid pathways and major sources of deformation in fractured rock masses, especially in crystalline rocks. When it comes to the equivalent properties of fractured rock masses, permeability, is stress-dependent due to the stress-induced deformation of fractures (Bai & Elsworth 1994). The mechanical properties is also stress-dependent because use deformation of fracture is highly non-linear much larger deformability at lower stress (Min & Jing 2004). Given the difficulties of in situ experiment on fractured rock masses at a representative scale, numerical experiments can be an alternative (Hart et. al.1985, Bhasin & Hoeg 1998, Zhang & Sanderson 2002).
The purpose of this study is to demonstrate the effect of in situ rock stresses on the deformability and permeability of fractured rock masses. Geological data were taken from the ongoing site investigation conducted by Swedish Nuclear Fuel and Waste Management Company (SKB) for the disposal of the spent nuclear Fuel at a candidate area Forsmark, Sweden (SKB 2004).
A set of numerical experiments was conducted to determine the equivalent mechanical properties (essentially, elastic moduli and Poisson's ratio) and permeability of fractured rock masses using a DFN-DEM (Discrete Fracture Network - Distinct Element Method) approach. The approach uses DFN for the geometry of fractured rock masses and DEM as the numerical technique for the numerical experiments (Min & Jing 2003, Min et al. 2004). The Distinct Element Method program, UDEC (Universal Distinct Element Code, Itasca, 2000) is used for the numerical experiment due to its capability to consider coupled mechanical and hydraulic behavior in fractured rock masses.
Discrete Fracture Network (DFN) Statistics, in situ stress and characterized properties of fractures (JRC, JCS, and friction angle) were the main data used for this study.
Table 1 shows the parameters of intact rock and fractures that were used for the present study. Some stress-dependent parameters of fractures were approximated by Barton-Bandis fracture model (Barton 1982) and transmissivity measurement at the site (Table 2).