Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behavior of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the containment-providing rock zone in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. BARIK is the German acronym for the research project on which this paper is based. This contribution provides an overview of the development and verification of the BARIK constitutive model, an extended Hoek–Brown model for fractured crystalline hard rock that takes into account up to three fracture systems. The model enables the consideration of the matrix and joint behavior of the rock separately, with each component having unique strength characteristics and failure criteria. These criteria are formulated such that suitable consideration of the strength-reducing properties of the respective fracture systems during barrier integrity verification is possible. The BARIK model has been implemented into two computer codes, FLAC3D and MFront for OpenGeoSys, allowing for the identification and evaluation of any inaccuracies that may arise from the use of different codes. The model enables isotropic–elastic, orthotropic–elastic, isotropic–elasto-plastic, and orthotropic–elasto-plastic calculations of the matrix, making it a valuable tool for the site selection process and for the construction and long-term safety of underground repositories. Furthermore, this poster presentation will show how the constitutive model was evaluated in relation to the dilatancy criterion and how the BARIK constitutive model's suitability for conducting an integrity assessment was validated. In conclusion, the development of BARIK is a significant step forward in the understanding and modeling of the complex material behavior of fractured crystalline hard rock. This contribution will provide insights into the development and verification of this model for the safe disposal of radioactive waste.
Discontinuum and continuum modelling approaches were used to simulate the single- and multi-stage hydraulic fracturing. Two-dimensional discontinuum modelling of single-stage fracturing revealed a marked modification of the stress field in the vicinity of the fracture (stress shadow effect) during fracture propagation which was identified as the reason for the significant asymmetry of fracture propagation about the wellbore observed for multi-stage fracturing. Allowing the fracturing fluid to flow back after each fracturing stage appeared to minimize this stress shadow effect on fracture propagation geometry. Three-dimensional discontinuum modelling results showed similar behaviors as two-dimensional modelling. Inclined orientation of wellbores with respect to principal stresses and enabling fluid backflow after each fracturing stage were found to be effective to minimize the stress shadow effect on fracture propagation geometry after three-dimensional discontinuum modelling. Our three-dimensional continuum modelling of hydraulic fracture propagation so far revealed some promising results demonstrating its capability to simulate hydraulic fracturing applications. Finally, the specific and contrasting features of both approaches are discussed here giving some useful insights into both approaches to assist users with choosing the appropriate method for a particular simulation project.
Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behaviour of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the “containment effective rock area” in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. This contribution provides an overview and first results of the laboratory programme, which was performed as a part of the research project. The aim was to generate a fundamental and extensive dataset for an anisotropic material used for verification and validation purposes of the newly developed constitutive model. The rock material for the test programme was “Freiberger gneiss” because of its pronounced anisotropic properties regarding deformation and strength as well as the good access to obtain a larger amount of sample material. A wide range of basic tests have been performed, e.g. determination of density and porosity, measurement of ultrasonic wave velocities to get dynamic elastic properties, Brazilian tests, and uniaxial compression tests to obtain strength data. Additionally, a large number of more complex multi-stage triaxial compression tests with examination of the post-failure region and hydromechanical coupled triaxial compression tests have been conducted or are in progress. The hydromechanical part, in particular, plays an important role in examining and quantifying the evolution of the micromechanical damage process, which changes the permeability and Biot coefficient and therefore the effective stresses inside a saturated rock material. Tests were conducted with different orientations of the structural planes in relation to the loading directions.
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