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A numerical characterization of a fractured rock mass and its mechanical behavior using a discontinuum approach was carried out utilizing lattice-spring-based synthetic rock mass (LS-SRM) models. First, LS-SRM models on a laboratory scale were created to reproduce standard rock mechanical tests on Triassic sandstone samples from a quarry in Germany. Subsequently, the intact rock properties were upscaled to an element volume representative for geotechnical applications, recalibrated and combined with a Discrete Fracture Network (DFN) model. The resulting fractured rock mass properties are compared to predictions from empirical relationships based on rock mass classification schemes and the DFN-Oda-Geomechanics approach. Modeling results reveal a significant reduction in the strength of the fractured rock mass compared to the intact rock, showing a high agreement with empirically calculated values. Results for the deformation modulus reveal a significant reduction induced by the fracture network and a good agreement compared to the results obtained by other approaches. It is shown that the LS-SRM allows analyzing the complex mechanical behavior during failure of rock masses, including crack initiation, propagation and coalescence. The resulting rock mass properties are key parameters for a wide range of geotechnical applications and can be used for large-scale numerical modeling as well.

<p>The Federal Company for Radioactive Waste Disposal (BGE mbH) is responsible for the execution of the German site selection procedure for high-level radioactive waste. The aim is to identify a repository site that ensures the best possible safety for the disposal of high-level radioactive waste for at least one million years. Three potential host rocks are considered for disposal in Germany: rock salt, claystone and crystalline rock. The German site selection procedure consists of three consecutive phases with a continuously increasing level of detail. The first step of phase I served to determine sub-areas based on different geoscientific criteria. In this process 90 suitable areas were identified. The second step of phase I comprises representative preliminary safety assessments to further narrow down the sub-areas (see Figure 1).</p> <p><img src="" alt="" /></p> <p>Figure 1: German Site Selection Procedure</p> <p>For these assessments, a preliminary concept and dimensioning of the deep geological repository is required, among other aspects. The primary input data for the preliminary design of the repository are the respective host rock properties, the inventory data of the high-level radioactive waste as well as a preliminary safety concept for the disposal system. Since there are only limited area-specific data available as no exploration is part of this early stage of the selection procedure, a two-stage method has been developed for the determination of the required area of the potential repository, consisting of a host rock specific and a site specific part. The objective of the first, host rock specific stage is to perform an analysis irrespective of the prevailing geological conditions at the considered locations. The aim is to obtain the areal extend of a potential repository as a function of the depth, the initial temperature in the host rock and mechanical properties of the host rock. Another important topic is the assessment of favorable depth ranges of the repository. Within the second stage, the function is used with available site-specific data to obtain the possible size of the repository at this specific site.</p> <p>This contribution will provide an overview of the first stage within the aforementioned methodology for the development of the preliminary design of the repository as part of the representative preliminary safety assessments. </p>

Understanding the anisotropic hydraulic and mechanical properties of fractured rock masses is of great importance for a safe and optimal utilisation of the subsurface. Two sandstone quarries are utilized to obtain fracture network characteristics by Terrestrial Laser Scanning (TLS) producing 3d point cloud data. Semiautomatic analysis of the point clouds provides the probability density functions for each of the fracture parameters used as stochastic input for a Discrete Fracture Network (DFN) model. Rock mechanical laboratory tests are carried out to determine the mechanical properties of the intact rock and fractures. These parameters are then combined in the DFN model to calculate spatially variable tensors for permeability, Young’s modulus and Poisson’s ratio. Thereby, the spatial resolution of the tensor description is adapted to the grid size which can be used in further hydromechanical models. The approach allows to populate these models with more realistic parameters which incorporate also the effect of fractures on the rock mass behaviour. Obtained results are subsequently compared with conventional engineering rock mass classifications. The applied workflow allows for upscaling of rock properties determined in the laboratory to the anisotropic rock mass properties required for further hydromechanical modelling on larger scales, e.g., the reservoir scale.