Bryan S. A. Tatone

A method to evaluate the three-dimensional roughness of fracture surfaces in brittle geomaterials

Conventionally, the evaluation of fracture surface roughness in brittle geomaterials, such as concrete and rock, has been based on the measurement and analysis of two-dimensional profiles rather than three-dimensional (3D) surfaces. The primary reason for doing so was the lack of tools capable of making 3D measurements. However, in recent years, several optical and mechanical measurement tools have become available, which are capable of quickly and accurately producing high resolution point clouds defining 3D surfaces. This paper provides a methodology for evaluating the surface roughness and roughness anisotropy using these 3D surface measurements. The methodology is presented step-by-step to allow others to easily adopt and implement the process to analyze their own surface measurement data. The methodology is demonstrated by digitizing a series of concrete fracture surfaces and comparing the estimated 3D roughness parameters with qualitative observations and estimates of the well-known roughness coefficient, R(s).

Numerical Modelling of the Anisotropic Mechanical Behaviour of Opalinus Clay at the Laboratory-Scale Using FEM/DEM

The Opalinus Clay (OPA) is an argillaceous rock formation selected to host a deep geologic repository for high-level nuclear waste in Switzerland. It has been shown that the excavation damaged zone (EDZ) in this formation is heavily affected by the anisotropic mechanical response of the material related to the presence of bedding planes. In this context, the purpose of this study is twofold: (i) to illustrate the new developments that have been introduced into the combined finite-discrete element method (FEM/DEM) to model layered materials and (ii) to demonstrate the effectiveness of this new modelling approach in simulating the short-term mechanical response of OPA at the laboratory-scale. A transversely isotropic elastic constitutive law is implemented to account for the anisotropic elastic modulus, while a procedure to incorporate a distribution of preferentially oriented defects is devised to capture the anisotropic strength. Laboratory results of indirect tensile tests and uniaxial compression tests are used to calibrate the numerical model. Emergent strength and deformation properties, together with the simulated damage mechanisms, are shown to be in strong agreement with experimental observations. Subsequently, the calibrated model is validated by investigating the effect of confinement and the influence of the loading angle with respect to the specimen anisotropy. Simulated fracture patterns are discussed in the context of the theory of brittle rock failure and analyzed with reference to the EDZ formation mechanisms observed at the Mont Terri Underground Research Laboratory.

Influence of microscale heterogeneity and microstructure on the tensile behavior of crystalline rocks

This study investigates the influence of microscale heterogeneity and microcracks on the failure behavior and mechanical response of a crystalline rock. The thin section analysis for obtaining the microcrack density is presented. Using micro X-ray computed tomography (μCT) scanning of failed laboratory specimens, the influence of heterogeneity and, in particular, biotite grains on the brittle fracture of the specimens is discussed and various failure patterns are characterized. Three groups of numerical simulations are presented, which demonstrate the role of microcracks and the influence of μCT-based and stochastically generated phase distributions. The mechanical response, stress distribution, and fracturing process obtained by the numerical simulations are also discussed. The simulation results illustrate that heterogeneity and microcracks should be considered to accurately predict the tensile strength and failure behavior of the sample.

Influence of pre-existing discontinuities and bedding planes on hydraulic fracturing initiation

Pressure-driven fracturing, also known as hydraulic fracturing, is a process widely used for developing geothermal resources, extracting hydrocarbons from unconventional reservoirs such as tight sandstone and shale formations, as well as for preconditioning the rock-mass during deep mining operations. While the overall process of pressure-driven fracturing is well understood, a quantitative description of the process is difficult due to both geologic and mechanistic uncertainties. Among them, the simulation of fractures growing in a complex heterogeneous medium is associated with computational difficulties. Experimental evidence based on micro-seismic monitoring clearly demonstrates the important influence of rock mass fabric on hydraulic fracture development, and the interaction between fluid-driven fractures and pre-existing discontinuities. However, these components are not well accounted for by standard numerical approaches. Thus, the design of hydraulic fracturing operations continues to be based on simplified models whereby the rock mass is treated as a homogeneous continuum. The purpose of this paper is to present the preliminary results obtained using the combined finite-discrete element technology to study the interaction between fluid driven fractures and natural rock mass discontinuities.

ROCKTOPPLE: A spreadsheet-based program for probabilistic block-toppling analysis

Uncertainty and variability are inherent in the input parameters required for rock slope stability analyses. Since in the 1970s, probabilistic methods have been applied to slope stability analyses as a means of incorporating and evaluating the impact of uncertainty. Since then, methods of probabilistic analysis for planar and wedge sliding failures have become well established in the literature and are now widely used in practice. Analysis of toppling failure, however, has received relatively little attention. This paper introduces a Monte Carlo simulation procedure for the probabilistic analysis of block-toppling and describes its implementation into a spreadsheet-based program (ROCKTOPPLE). The analysis procedure considers both kinematic and kinetic probabilities of failure. These probabilities are evaluated separately and multiplied to give the total probability of block toppling. To demonstrate the use of ROCKTOPPLE, it is first verified against a published deterministic result, and then applied to a practical example with uncertain input parameters. Results obtained with the probabilistic approach are compared to those of an equivalent deterministic analysis in which mean values of input parameters are considered.

Hybrid Finite-Discrete Element Simulation of the EDZ Formation and Mechanical Sealing Process Around a Microtunnel in Opalinus Clay

The analysis and prediction of the rock mass disturbance around underground excavations are critical components of the performance and safety assessment of deep geological repositories for nuclear waste. In the short term, an excavation damaged zone (EDZ) tends to develop due to the redistribution of stresses around the underground openings. The EDZ is associated with an increase in hydraulic conductivity of several orders of magnitude. In argillaceous rocks, sealing mechanisms ultimately lead to a partial reduction in the effective hydraulic conductivity of the EDZ with time. The goal of this study is to strengthen the understanding of the phenomena involved in the EDZ formation and sealing in Opalinus Clay, an indurated claystone currently being assessed as a host rock for a geological repository in Switzerland. To achieve this goal, hybrid finite-discrete element method (FDEM) simulations are performed. With its explicit consideration of fracturing processes, FDEM modeling is applied to the HG-A experiment, an in situ test carried out at the Mont Terri underground rock laboratory to investigate the hydro-mechanical response of a backfilled and sealed microtunnel. A quantitative simulation of the EDZ formation process around the microtunnel is first carried out, and the numerical results are compared with field observations. Then, the re-compression of the EDZ under the effect of a purely mechanical loading, capturing the increase of swelling pressure from the backfill onto the rock, is considered. The simulation results highlight distinctive rock failure kinematics due to the bedded structure of the rock mass. Also, fracture termination is simulated at the intersection with a pre-existing discontinuity, representing a fault plane oblique to the bedding orientation. Simulation of the EDZ re-compression indicates an overall reduction of the total fracture area as a function of the applied pressure, with locations of ineffective sealing associated with self-propping of fractures. These results are consistent with hydraulic testing data revealing a negative correlation between pressure values and an increase in the EDZ transmissivity.

Quantitative Measurements of Fracture Aperture and Directional Roughness from Rock Cores

The hydro-mechanical behavior of a blocky rock mass near the surface and at shallow depths is more dependent on the characteristics of the system of discontinuities within the rock mass than the characteristics of the intact rock. Discontinuities represent planes of weakness and conduits of enhanced hydraulic conductivity relative to the intact rock. The spatial aperture distribution and roughness of these fractures can have a significant influence on their hydromechanical behavior. In terms of mechanical behavior, the aperture distribution and roughness directly affect the spatial distribution and inclination of contact areas, which in turn influence the stress distribution, deformation, and asperity damage, under normal and shear loading (e.g., Re and Scavia 1999; Gentier et al. 2000; Grasselli and Egger 2003). In terms of hydraulic behavior, the spatial aperture distribution and roughness directly affect the tortuosity and connectivity of flow paths, which in turn influence the hydraulic transmissivity of the fracture (Zimmerman and Bodvarsson 1996; Berkowitz 2002). The evaluation of fracture roughness requires measurements or observations of surface topography coupled with some empirical (e.g., Barton and Choubey 1977), statistical (e.g., Reeves 1985; Maerz et al. 1990) or fractal (e.g., Seidel and Haberfield 1995; Kulatilake et al. 2006) analysis methodologies, which yield one or more roughness parameters. Several contact and non-contact tools and techniques have been used to measure surface topography of discontinuity surfaces in rock. Contact techniques include: