Effective elastic properties of fractured rocks
The objective of the project is to predict the scaling evolution of rock mass effective elastic properties for conditions relevant to the POSE (Posiva’s Olkiluoto Spalling Experiment) niche surroundings at ONKALO, the Finnish site for underground storage of nuclear waste.
The project is done in collaboration with the Géosciences lab (UMR 6118) from the University of Rennes (France). We develop an approach based on the statistical representation of Discrete Fracture Networks (DFN). A Coulomb-Slip model is used for the fractures and the intact rock is elastic. Theoretical relations are first derived and validated through numerical simulations, to define the strain, stress and displacement fields induced by the DFN. They are then used to predict effective elastic properties (Young’s Modulus E and Poisson ratio) for the POSE niche surroundings at a 100 m scale.
The decrease of E with scale is controlled by a DFN factor related to the sum of fracture sizes either to the cube (𝑙3) or to the square (𝑙2). Which regime is prevalent depends on the ratio between fracture size, intact rock modulus and fracture shear stiffness. The consequences are critical for predicting the scaling laws of effective properties.
For the selected DFN model and mechanical properties, the main results are:
- Given the large variability within each fracture set, the effective properties are isotropic.
- The modulus decrease with scale is significant (see Figure). Its extent and intensity are dependent on the fracture shear stiffness (ks): the smaller ks, the larger the scale effect. The extent of the scale effect for ks equal to 12 Mpa/mm is a decrease of about 20% from the intact rock Young’s Modulus and up to a typical domain size of 30 m.
- Le Goc, R., Davy Ph. and Darcel C., (2015). Scaling effects on elastic properties of jointed rock mass. Eurock, Salzburg, Austria.
- Darcel, C., Davy Ph., Le Goc R. (2015). Onkalo POSE experiment - Effective elastic properties of fractured rocks. Posiva 2015-17. Eurajoki, Finland, Posiva Oy. WR 2015-17