A microstructural rock model based on the distinct element method employing the Subspring Network contact model with rigid, Breakable, Voronoi-shaped grains (SNBV model) is proposed. The model consists of a mesh (3D Voronoi tessellation) of rigid, breakable, Voronoi blocks. The SNBV model is a microstructural rock model because it is a discrete model that can mimic rock microstructure at the grain scale. SNBV material mimics the microstructure of angular, interlocked, breakable grains with interfaces that may have an initial gap and can sustain partial damage. The model embodies the microstructural features and damage mechanisms that occur at the grain scale: initial microcrack fabric; heterogeneity-induced local tension; and intergranular and transgranular damage. The heterogeneity-induced local tension can be introduced in a controlled fashion that is not tied directly to the shape and packing of the grains and the interface stiffnesses. The synthetic material exhibits behavior during direct-tension and triaxial compression tests that matches the behavior of compact rock. The material can be calibrated to match the standard material properties and characteristic stresses of pink Lac du Bonnet granite. The material properties consist of Young’s modulus and Poisson’s ratio corresponding with uniaxial compression and Young’s modulus corresponding with direct tension, as well as tensile strength, crack-closure stress, crack-initiation stress, secondary crack-initiation stress to mark the onset of grain breakage, crack-damage stress, and compressive strengths up to 4 MPa confinement. The model is suitable for studying the grain-scale micromechanics of brittle rock fracture.

Numerical modelling of tunnels in brittle rock is a challenging endeavour for rock mechanics engineers. Multiple methods have been developed to aid in the design of underground excavations that are prone to brittle failure. For rock mechanics practitioners, the most useful tools are those that adequately and objectively represent the ground reaction, and can be interpreted without excessive qualitative judgement. With these goals in mind, continuum numerical models stand out amongst other methods. Two approaches that make use of continuum numerical modelling are the Cohesion Weakening Friction Strengthening (CWFS) and the Damage Initiation Spalling Limit (DISL). To test their robustness, a sensitivity analysis of the strength parameters was conducted. The approaches were then applied to multiple fictitious stress scenarios to test their capability of reproducing empirical observations of depth of failure. The present investigation shows that the CWFS approach is a robust approach for modelling brittle failure around tunnels that can be easily applied and interpreted by rock mechanics practitioners. The sensitivity analysis of the CWFS parameters provided a full understanding of the practical impact of input parameter selection, some of which were not previously given in the literature. The DISL approach has been shown to also perform suitably, but requires the user to apply a fundamental understanding of numerical modelling for its effective interpretation.

Shearing of rock joints is a critical failure mode in rock masses. The shear strength of rock joints must, therefore, be considered in the design of structures in rock masses. Several criteria for prediction of shear strength have been proposed over the years. However, the possible effect of scale on shear strength is an issue. One possible reason for this is that previous experimental studies on the scale effect contain various sources of uncertainties (mixed test methods, multiple testing of same specimen, application of results to other materials than tested, and omitted handling of statistical dispersion). In this paper, the results from a uniquely comprehensive experimental laboratory program, that handles these uncertainties and also extends the range of previously tested conditions, is presented. 46 direct shear tests on two joint types, natural and tensile induced granite rock joints, have been performed under the constant normal stress and the constant normal stiffness boundary condition at 5 MPa initial normal stress applied over three specimen sizes (35 mm × 60 mm, 70 mm × 100 mm and 300 mm × 500 mm). Analysis of variance shows no effect of the specimen size on the shear strength, whereas the joint type and boundary condition has. Quantitative estimates of the influence of the joint type and boundary condition on the shear strength are presented. A consistent approach for determination of shear strength from the point of time associated with a shear stiffness change of the test system is also presented.

The Barton-Bandis model for the nonlinear shear strength of rock joints is the most commonly used strength criterion in rock engineering practice. There have been advancements in determination of Joint Roughness Coefficient (JRC), such as the use of laser scanning; however, the equally important Joint Wall Compressive Strength (JCS) parameter has not been significantly advanced. The JRC and JCS are effectively linked, to some extent. A sensitive rebound hardness index test, the Leeb Hardness (LH) test, was investigated to provide a quantifiable and repeatable method of JCS determination that offers increased accuracy relative to current methods. The LH test value (LD) correlation to Unconfined Compressive Strength (σc) is proposed for JCS determination. In addition, this study investigates the Influence Zone of the LH test on surfaces with graded hardness profiles (e.g., weathered surfaces). This was done using a series of artificial composite plaster-rock specimens of known hardness to provide insight into the influence effects on the surface LD reading due to underlying material of contrasting hardness. In addition, a collection of natural rock specimens with variable joint wall hardness were collected and LD profiles were obtained by sequential surface grinding and testing. These natural rock specimens included those with wall surface materials softer and harder relative to the underlying intact rock. A Hardness Contrast Type was proposed for classification of hardness contrast conditions. The study findings showed the LH test is a suitable tool for predicting JCS and a proposed methodology was presented.

Rock spalling is a brittle failure process that occurs around tunnels excavated in hard rock under high in-situ stress states. In nuclear waste disposal, spalling in fractured crystalline rock could create connected fractures, potentially providing pathways for radionuclides. Robust numerical models are therefore needed to evaluate the extent of rock spalling so that the design and layout of a prospective deep geological repository can be optimized and made fit for purpose. With this motivation in mind, this study proposes a methodology for the numerical analysis of rock spalling based on zero-thickness interface elements with a visco-plastic-fracture constitutive law, combined with a workflow for finite element removal/excavation. To simulate spalling, zero-thickness interface elements are pre-inserted along a sufficient number of mesh lines with random orientation within the rock mass. A uniform initial stress state is generated and the excavation of the circular tunnel is performed by removing the corresponding elements, which leads to stresses in excess of the elastic limit in some of the interfaces, and subsequent visco-plastic fracture openings. A criterion for excavation of the finite elements around the tunnel is established when a block which is totally surrounded by failed interfaces is totally detached or can slide off the mesh following a kinematically admissible path. The excavation of blocks causes a stress redistribution around the tunnel and this leads the need for new excavation steps, until a new equilibrium configuration is reached. The proposed methodology is applied to assess rock spalling in the Mine-by Experiment at the Atomic Energy of Canada Limited's (AECL's) Underground Research Laboratory in the massive Lac du Bonnet granite. The focus of the analysis is to understand the mechanisms and the influencing factors that lead to brittle failure, and to calibrate material properties to reproduce both the final stress state of the tunnel and its spalling depth.

Simulation of coupled thermo-hydro-mechanical-chemical (THMC) processes in fractured rocks is relevant to many areas of applied geoscience. Groundwater flow and reactive transport (hydro-chemical processes) in hard rocks are primarily controlled by networks of rock fractures, and therefore best modelled in a discrete fracture network, or DFN. By contrast, heat and stress (thermo-mechanical processes) are mediated via the rock mass, and thus best represented as a continuum. Here, we present an approach using dual, coincident meshes, each simulating the relevant processes in their ‘natural’ media. A sequential coupling between thermo-mechanical and hydro(-chemical) processes is achieved by computing the effective stress on individual fracture planes, and updating their mechanical apertures and hydraulic properties at selected times during model simulations. The simulated processes are modelled in PFLOTRAN, using a linear elastic constitutive model for rock deformation. Several updates to the PFLOTRAN code were required to enable this, as verified against analytical test cases and benchmarked against alternative finite element codes. We present an example application using data from a spent nuclear fuel repository in Finland, which simulates the evolution of thermal stress due to radiogenic heating from spent fuel canisters. The results demonstrate how continuum-based thermo-mechanical processes exert an important influence on near-field flows around the repository that can only be accurately captured in a DFN, thus demonstrating the advantages of a dual-media approach.

It is necessary to obtain reliable estimates of the in situ stress state for the design of any underground engineering project in rock, but it is of paramount importance for safety-critical projects such as deep geological repositories for nuclear waste. It is widely considered that in situ stress is a function of depth below ground surface. This often leads to rock masses being partitioned into depth-based domains, but there are no universally agreed and statistically robust methods for doing so. In this paper we present a novel method that uses Bayesian linear segmented regression of Cartesian stress components to probabilistically characterize the variability and uncertainty in the depth of non-crisp stress domain boundaries, and the in situ stress state within each domain. We demonstrate the efficacy of the method using synthetically generated stress data, and then apply the method to overcoring stress measurements obtained at the Forsmark site in Sweden.

Understanding the shear strength characteristics of rock fractures is crucial for a wide range of rock engineering applications. The strength of rock fractures is significantly dependent on fracture geometry that can be altered during historical shearing process. This study presents a brief analysis of repeated direct shear of a mated fracture. We conducted five repeated shear test simulations under constant normal load conditions using a predictive shear model presented in our previous work. The fracture surface used in the first round of shear simulation is scanned from a natural granite fracture surface. After shearing, the fracture surfaces are repeatedly used for the next rounds of shear simulations. The results generally show that the repeated shear induces irreversible surface degradation, which reduces the shear strength and normal displacement. The findings of this study are helpful for understanding the shear behavior of rock fractures.

The presence of fractures in rock masses plays a major role in its stress state and its variability. Each fracture potentially induces a stress perturbation, which is correlated to its geometrical and mechanical properties. This work aims to understand and quantitatively predict the relationship between fractured systems and the associated stress fluctuations distribution, considering any regional stress conditions. The approach considers the rock mass as an elastic rock matrix into which a population of discrete fractures is embedded—known as a Discrete Fracture Network (DFN) modeling approach. We develop relevant indicators and analytical solutions to quantify stress perturbations at the fracture network scale, supported by 3D numerical simulations, using various fracture size distributions. We show that stress fluctuations increase with fracture density and decrease as a function of the so-called stiffness length, a characteristic length that can be defined as the ratio between Young’s modulus of the matrix and fracture stiffness. Based on these considerations we discuss, depending on DFN parameters, which range of fractures should be modeled explicitly to account for major stress perturbations in fractured rock masses.