Giovanni Grasselli

Constitutive law for the shear strength of rock joints based on three-dimensional surface parameters

A new constitutive criterion, relating stress and displacements, is proposed to model the shear resistance of joints under constant normal load conditions. It is based on an empirical description of the surface, and on the results from more than 50 constant-normal-load direct-shear tests performed on replicas of tensile joints and on induced tensile fractures for seven rock types. This constitutive model is able to describe experimental shear tests conducted in the laboratory. Moreover, the parameters required in the model can be easily measured through standard laboratory tests. The proposed criterion was also used to estimate the joint roughness coefficient (JRC) value. The predicting values were successfully correlated with JRC values obtained by back analysis of shear tests.

Quantitative three-dimensional description of a rough surface and parameter evolution with shearing

Note: Roches Reference LMR-ARTICLE-2002-005View record in Web of Science Record created on 2006-11-09, modified on 2016-08-08

Y-Geo: New Combined Finite-Discrete Element Numerical Code for Geomechanical Applications

AbstractThe purpose of this paper is to present Y-Geo, a new numerical code for geomechanical applications based on the combined finite-discrete element method (FDEM). FDEM is an innovative numerical technique that combines the advantages of continuum-based modeling approaches and discrete element methods to overcome the inability of these methods to capture progressive damage and failure processes in rock. In particular, FDEM offers the ability to explicitly model the transition from continuum to discontinuous behavior by fracture and fragmentation processes. Several algorithmic developments have been implemented in Y-Geo to specifically address a broad range of rock mechanics problems. These features include (1) a quasi-static friction law, (2) the Mohr-Coulomb failure criterion, (3) a rock joint shear strength criterion, (4) a dissipative impact model, (5) an in situ stress initialization routine, (6) a material mapping function (for an exact representation of heterogeneous models), and (7) a tool to i...

Y-GUI : A graphical user interface and pre-processor for the combined finite-discrete element code, Y2D, incorporating material heterogeneity

Numerical modelling of a discontinuous medium has gained much popularity in recent decades. The combined finite-discrete element method (FEM/DEM) is a state-of-the-art numerical modelling technique pioneered in the mid-1990s. Y2D is a robust two-dimensional FEM/DEM research code developed by Munjiza in 2004. The major limitations of this code are (1) the lack of a graphical user interface (GUI) meaning that all pre-processing has to be made directly on an ASCII input file and (2) the inability of dealing with heterogeneous media. This contribution presents the first GUI and pre-processor, known as Y-GUI, developed for Y2D and the implementation of a new algorithm that allows for the use of heterogeneous materials. In the text all major FEM/DEM concepts are described, together with the main features available in the Y-GUI. The use of Y-GUI is presented in detail and some of its functionalities, including the heterogeneity module to be used to randomly assign materials to a mesh, are introduced. At the end of the manuscript, four case studies, including Brazilian tests of a homogeneous and a layered rock sample and a rock avalanche, are presented.

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.

Rock Slide Simulation with the Combined Finite-Discrete Element Method

AbstractThis paper describes the application of the combined finite-discrete element method (FDEM) for the simulation of two different rock slides observed at the Alpetto Mine in Cesana Brianza (Italy). Previous numerical modeling conducted to study the stability conditions of the high rock cut are used as comparison for the results obtained. The paper presents how the FDEM simulation was performed and discusses the ability and limits of the currently available combined method to simulate rock slide problems for practical purposes.

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.