Behavior Of Fractured Rock Mass Research Articles

Strength characterization of rock masses, using a coupled DEM-DFN model

A discrete element model (DEM) is used to study the mechanical behaviour of a pre-fractured brittle medium subjected to triaxial loadings where progressive failure can occur. The initial discrete fracture network (DFN) is based on a fractal distribution and it is used to characterize the influence of clustering and size distribution of pre-existing fractures, on the strength of fractured rock masses. The proposed approach captures the progressive failure process occurring in the rock matrix as shearing displacement takes place along pre-existing discontinuities. The results show that the mechanical behaviour of fractured rock masses is mainly dependent on the fracture intensity. However, for a given fracture intensity, the strength can exhibit a 50 per cent variability depending on the size distribution of the pre-existing fractures. This difference can be attributed to the different mechanisms that involve sliding and crushing of blocks in the case of large interconnected fractures or progressive failure of the rock matrix through coalescence of cracks in the case of small unconnected fractures. Clustering of fractures was found to influence the spatial variability of the mechanical properties and therefore to have a scale effect on strength. The results outline the relevance of three parameters, the power-law exponent of the fracture size distribution, the clustering fractal dimension which fixes the fracture-to-fracture correlation number and the fracture intensity, to characterize the mechanical behaviour of rock masses.

Application of Manifold Method (MM) to the stability problems for cut slopes along the national roads

Recently one of the most challenging problems for civil engineers is how to construct new structures/infrastructures adjacent pre-existing ones and how to evaluate the effect of the new construction on the pre-existing structures (so-called neighbouring construction). The Manifold Method (MM) is one of the discontinuum based numerical approaches to simulate the mechanical behaviour of fractured rock masses including large deformation/displacement along fractures as well as stress/strain conditions of the rock blocks/masses. In this study, the MM was used to investigate the stability of the cut slope in the highly fractured rock masses along one of the national roads in Japan, focusing on the effects of new construction on the pre-existing structures/infrastructures. The effect of reinforcement during the construction such as rock bolts, anchors, etc. was also investigated and evaluated quantitatively by numerical simulations using MM.

Correlations Developed for Estimation of Hydraulic Parameters of Rough Fractures Through the Simulation of JRC Flow Channels

The hydro-mechanical response of fractured rock masses is complex, due partly to the presence of fractures at different scales. Surface morphology has a significant influence on fluid flow behaviour of a fracture. Different empirical correlations and statistical models have been proposed to estimate the equivalent hydraulic aperture and determine the pressure drop along a fracture. However, the existing models suffer from not being adequately generalised to be applicable to a wide range of real fracture surfaces. To incorporate the effect of profile roughness in the hydro-mechanical behaviour of fractured rock masses, the joint roughness coefficient (JRC) is the most widely used empirical approach. However, the average JRC of two fracture walls in fluid flow analysis, as is a common practice, appears to be inappropriate. It will be shown how different combinations of pairs of JRCs could lead to a similar JRC value. Also, changing the position of the top and bottom walls of a fracture can significantly change the hydraulic response of the fracture while the average JRC is identical in both cases. In this paper, correlations are developed which are based on the simulation of JRCs using estimated fluid flow parameters of 2D fractures can be estimated. In order to widen the application range of the correlations, JRC flow channels were generated: these are 2D channels with their top and bottom walls being made from two of the JRC profiles. To estimate the JRC of linear profiles, a correlation developed between JRC and a newly developed Riemannian roughness parameter, D R1, is proposed. Considering ten JRC profiles, a total of 100 JRC flow channels were generated. In order to only investigate the effect of surface roughness on fluid flow, the minimum closure between the top and bottom walls of JRC flow channels were considered to be constant. Three cases with minimum closures of 0.01, 0.05 and 0.10 cm were considered in this study. All JRC flow channels were subjected to fluid analysis using FLUENT software. Based on these results, correlations were developed between the geometrical and hydraulic properties of flow channels. Analysis of several real fractures demonstrated the applicability of these correlations.

Experimental investigation on strength and failure behavior of pre-cracked marble under conventional triaxial compression

Fractures in natural rocks have an important effect on the strength and failure behavior of rock mass, which are often evaluated in rock engineering practice. The theoretical evaluation of mechanical behavior of fractured rock mass has no satisfactory answer due to the role of confining pressure and crack geometry. Therefore, in this paper, conventional triaxial compression experiments were carried out to study the strength and failure behavior of marble samples with two pre-existing closed cracks in non-overlapping geometry. Based on the experimental results of a number of triaxial compression tests, the effect of crack coalescence on the axial supporting capacity and deformation property were investigated with different confining pressures. The results show that intact samples and flawed samples (marble with pre-existing cracks) have different deformation properties after peak stress, which change from brittleness to plasticity and ductility with the increase of confining pressure. The peak strength and failure mode are found depending not only on the geometry of flaw, but also on the confining pressure. The strength of flawed samples shows distinct non-linear behavior, which is in a better agreement with non-linear Hoek–Brown criterion than linear Mohr–Coulomb criterion. For a kind of rock that has been evaluated as a Hoek–Brown material, a new evaluation criterion is put forward by adopting optimal approximation polynomial theory, which can be used to confirm more precisely the strength parameters (cohesion and internal friction angle) of flawed samples. For intact samples, the marble leads to typical shear failure mode with a single fracture surface under different confining pressures, while for flawed samples, under uniaxial compression and a lower confining pressure ( σ 3 = 10 MPa), tests for coarse and medium marble (the coarse and medium refer to the grain size) exhibit three basic failure modes, i.e., tensile mode, shear mode, and mixed mode (tensile and shear). Shear mode is associated with lower strength behavior. However, under higher confining pressures ( σ 3 = 30 MPa), for coarse marble, the axial supporting capacity is not related to the geometry of flaw. The friction among crystal grains determines the strength behavior of coarse marble. For medium marble, the failure mode and deformation behavior are dependent on the crack coalescence in the sample. The present research provides increased understanding of the fundamental nature of rock failure under conventional triaxial compression.

Understanding of hydraulic properties from configurations of stochastically distributed fracture networks

AbstractA systematic investigation of the effect of configurations of stochastically distributed fracture networks on hydraulic behaviour for fractured rock masses could provide either quantitative or qualitative correlation between the structural configuration of the fracture network and its corresponding hydraulic behaviour, and enhance our understanding of appropriate application of groundwater flow and contaminant transport modelling in fractured rock masses. In this study, the effect of block sizes, intersection angles of fracture sets, standard deviations of fracture orientation, and fracture densities on directional block hydraulic conductivity and representative elementary volume is systematically investigated in two dimensions by implementing a numerical discrete fracture fluid flow model and incorporating stochastically distributed fracture configurations. It is shown from this investigation that the configuration of a stochastically distributed fracture network has a significant quantitative or qualitative effect on the hydraulic behaviour of fractured rock masses. Compared with the deterministic fracture configurations that have been extensively dealt with in a previous study, this investigation is expected to be more practical and adequate, since fracture geometry parameters are inherently stochastically distributed in the field. Moreover, the methodology and approach presented in this study may be generally applied to any fracture system in investigating the hydraulic behaviours from configurations of the fracture system while establishing a ‘bridge’ from the discrete fracture network flow modelling to equivalent continuum modelling in fractured rock masses. Copyright © 2007 John Wiley & Sons, Ltd.

Influence of persistence on behaviour of fractured rock masses

Abstract Most of the models that simulate fractured rock masses assume fully persistent discontinuities, simplifying the fact that, in nature, fractured rock masses are made of non-continuous sets of joints. A rock bridge gives an effective cohesion to the fracture and a block of rock cannot fall or slide until all the rock bridges fail. This failure involves the failure of the intact rock, which can be orders of magnitude stronger than the shear strength of the rock joint. In this study we focus on how the distribution of rock bridges influences the overall rock mass behaviour, to contribute to the understanding of how the presence of rock bridges influences the ‘scale’ effect that is observed between strength values measured on intact rock in the laboratory and those observed at the rock mass scale. To estimate the influence of spatial fracture, parameters of the rock mass strength and deformation were determined, using the orthogonal arrays method, UDEC and variance analysis. The numerical model was first calibrated on shear tests of samples made of continuous joints, and then used to investigate the shear behaviour of a fractured rock mass with non-persistent joints. The 2D approach was successfully extended to 3D models using 3DEC with the aim of providing a better approach for simulating the stability of an underground cavern in a fractured rockmass.

About this title - Rock Physics and Geomechanics in the Study of Reservoirs and Repositories

The study of reservoir and repository performance requires the integration of many different fields in Earth sciences, among them rock physics and geomechanics. The aim of this book is to emphasize how rock physics and geomechanics help to get a better insight into important issues linked to reservoir management for exploitation of natural resources, and to repository safety assessment for hazardous waste storage in geological environment. The studies presented here deal with the hydromechanical coupling in fractured rocks, the key experiments in safety assessment of repositories, the development of damaged zones during excavation in a shaley formation, the influence of temperature on the properties of shales, the poroelastic response of sandstones, the development and propagation of compaction bands in reservoir rocks, imaging techniques of geomaterials, the characterization and modelling of reservoirs using 4D seismic data, the mechanical behaviour of fractured rock masses, the petrophysical properties of fault zones, models for rock deformation by pressure solution and the elastic anisotropy in cracked rocks.

Stress dependent mechanical properties and bounds of poisson's ratio for fractured rock masses investigated by a DFN-DEM technique

The equivalent elastic modulus is a parameter for controlling the deformation behavior of fractured rock masses in the equivalent continuum approach. The confining stress, whose effect on the equivalent elastic modulus is of great importance, is the fundamental stress environment of natural rock masses. This paper employs an analytical approach to obtain the equivalent elastic modulus of fractured rock masses containing random discrete fractures (RDFs) or regular fracture sets (RFSs) while considering the confining stress. The proposed analytical solution considers not only the elastic properties of the intact rocks and fractures, but also the geometrical structure of the fractures and the confining stress. The performance of the analytical solution is verified by comparing it with the results of numerical tests obtained using the three-dimensional distinct element code (3DEC), leading to a reasonably good agreement. The analytical solution quantitatively demonstrates that the equivalent elastic modulus increases substantially with an increase in confining stress, i.e. it is characterized by stress-dependency. Further, a sensitivity analysis of the variables in the analytical solution is conducted using a global sensitivity analysis approach, i.e. the extended Fourier amplitude sensitivity test (EFAST). The variations in the sensitivity indices for different ranges and distribution types of the variables are investigated. The results provide an in-depth understanding of the influence of the variables on the equivalent elastic modulus from different perspectives.

Numerical determination of the equivalent elastic compliance tensor for fractured rock masses using the distinct element method

Abstract The purpose of this paper is to establish a methodology to determine the equivalent elastic properties of fractured rock masses by explicit representations of stochastic fracture systems, and to investigate the conditions for the application of the equivalent continuum approach for representing mechanical behavior of the fractured rock masses. A series of numerical simulations of mechanical deformation of fractured rock masses at different scales were conducted with a large number of realizations of discrete fracture networks (DFN), based on realistic fracture system information and using the two-dimensional distinct element program, UDEC. General theory of anisotropic elasticity was used for describing the macroscopic mechanical behavior of fractured rock masses as equivalent elastic continua. Verification of the methodology for determining the elastic compliance tensor was conducted against closed-form solutions for regularly fractured rock mass, leading to very good agreements. The main advantage of the developed methodology using the distinct element method is that it can consider complex fracture system geometry and various constitutive relations of fractures and rock matrix, and their interactions. Two criteria for the applicability of equivalent continuum approach were adopted from the investigations: (i) the existence of a properly defined REV (representative elementary volume) and (ii) the existence of an elastic compliance tensor. For the problems with in situ conditions studied in this paper, the results show that a REV can be defined and the elastic properties of the fractured rock mass can be represented approximately by the elastic compliance tensor through numerical simulations.

New directions in rock mechanics — report on a forum sponsored by the American Rock Mechanics Association

The American Rock Mechanics Association (ARMA) and the ARMA Foundation sponsored a forum on New Directions for US Rock Mechanics which was held at the Asilomar Conference Center in Pacific Grove, California, USA, during 18–20 October 1998. The goal of the Forum was to focus on: (1) a strategic vision for the future of rock mechanics in the United States; (2) the identification and delineation of critical issues facing the rock mechanics community; (3) the role of research in addressing these issues; and (4) critical areas of research in each of the topic areas addressed at the forum including examples of specific research initiatives. To a large degree, rock mechanics has been used successfully to model and predict the behavior of fractured rock masses for building large structures. However, improvements on present success, and future advances in modeling and construction, are predicated on better characterization of fractured rock masses. This will require increasing emphasis on non-linear and discontinuity-based models to reflect the mechanisms at work in fractured rock masses. For example, relevant characterization schemes, as well as field, laboratory, and logging techniques have yet to be developed for weak rock conditions and for all but the simplest fluid flow conditions. This report summarizes two days of discussion held during the Forum. It was the sense of the Forum participants that fundamental improvements in in situ rock mass characterization is of the utmost importance. Every session emphasized that obtaining valid information from large volumes of rock is the prerequisite for the effective and improved practice of rock mechanics. Research and application of remote imaging and non-destructive evaluation of the subsurface should provide a source of economically realizable data from large volumes of rock. This area should be a primary goal of future research. Associated with the question of site characterization is the degree of uncertainty associated with the data, and with the chosen interpretive model. Forum participants also recommended conducting research to find implementations of stochastic techniques, which would allow uncertainty to be dealt with in a rational manner.

The implications of joint deformation in analyzing the properties and behavior of fractured rock masses, underground excavations, and faults

Abstract For a given stress state, joint deformation depends on the joint’s geometry, including surface roughness, spatial geometry of the contact area, and large-scale topographical features such as dips. Under normal stress, contacting asperities compress, and the half spaces bounding the joint are deformed. A very significant consequence of half-space deformation is that it allows mechanical interaction among all contact points between the joint surfaces. As a result, the contact area’s overall spatial geometry plays an important role in determining the distribution of stress across joint surfaces and the change in geometry of the void space between surfaces that occurs with changes in stress. Mechanical interaction among contact points is important in determining normal joint stiffness: two joints with the same total contact area can have substantially different stiffnesses depending on the spatial geometry of their contact areas. Modeling results indicate that joints with small contact areas uniformly distributed across the surfaces can be nearly as stiff as a perfectly mated interface. These results have significant implications for almost any endeavor in fractured rock, including designing underground excavations, predicting the hydraulic response of a rock mass to changes in stress, understanding the deformation and failure of joints under shear stress, and analyzing the stability of faults. In underground excavations, for example, deformation of the roof and floor means that the load acting on any supporting pillar and the distribution of stress throughout the pillar depend on: the pillar’s size and shape; the size, shape, and proximity of neighboring pillars; and the spatial geometry of the pillar array. Purely elastic deformation can lead to either catastrophic or progressive failure. Similarly, to accurately predict fluid flow through jointed rock, changes in void space geometry that result from changes in stress must be considered; these changes in geometry are not predicted by methods that assume that aperture changes uniformly across the joint. For joints and faults, the nonuniform distribution of normal and shear stresses resulting from surface roughness and mechanical interaction between contact points suggests a progressive form of shear failure. Failure is initiated at points of low normal stress and propagates as stress from failed asperities is redistributed to neighboring asperities. Consistent with observations on many faults, modeling and analytical results predict that earthquakes on a fault would be clustered in time and space because of mechanical interaction between persistent asperities.

Numerical modeling of the nonlinear behavior of fractured rock masses under compression

Numerical modeling of the nonlinear behavior of fractured rock masses under compression

Numerical modelling of the hydro-mechanical behaviour of fractured rock masses

Abstract Results are presented from an investigation into the hydro-mechanical response of a rock mass to changes in effective stress associated with fluid extraction. Numerical models of regularly and randomly fractured reservoirs have been created using UDEC, the distinct element code. Relationships between permeability and stress state have been established in regions immediately adjacent to simulated wellbores and more generally throughout the fractured reservoirs. The effects of in-situ stress state, hydraulic boundary conditions and scale have been investigated. The results demonstrate that numerical modelling combined with analytical solutions can be employed to investigate coupled flow-deformation analysis realistically for simple geological situations. The relationships presented here relating fracture pemeability to stress field changes will be helpful in understanding the behaviour of naturally fractured reservoirs during pressure depletion.

Numerical modelling of the hydro-mechanical behaviour of fractured rock masses

The Pont-Ventoux underground Power Plant is a 150 MW hydroelectric project in the North-West part of the italian Alps. During the geological survey, necessary for the definition of the mechanical parameters, it was found that the original project location was unsuitable due to a fault zone. The power station complex was therefore relocated to a new area with better mechanical conditions. As a result the project of the cavern was modified and a new shape created. The project engineers designed three-dimensional egg-shaped cavern with two abses on both ends of the cavern and an elliptical plan well. To study and research such an innovative shape several numerical analysis were carried out. An hybrid approach F.E.M.-B.E.M. (finite elements method-boundary elements method) was used. The results obtained from these numerical analysis confirmed the validity of the whole idea. The new cavern is therefore supported mainly by means of untensioned grouted dowels and shotcrete.