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Numerical Simulations of Scale Effects Under Varying Loading Conditions For Naturally Fractured Rock Masses And Implications For Rock For Rock Mass Strength Characterization And the Design of Overhanging Rock Slopes
Elmo, D. (Golder Associates) | Schlotfeldt, P. (Golder Associates) | Beddoes, R. (Golder Associates) | Roberts, D. (Golder Associates)
ABSTRACT It is largely recognized that the uncertainty in predicting the mechanical behavior of a naturally fractured mass is associated with scale effects. In this paper, a coupled Discrete Fracture Network (DFN) - Fracture Mechanics approach is applied to the characterization of rock mass strength at different scale under different loading conditions, including compression, shear and tensile loading. The strength of the approach is that the anisotropic, inhomogeneous spatial distribution and influence of the jointing can be fully considered and the resulting deformation and failure mechanisms simulated in a more realistic way. The analysis provides an example of the definition of both a representative elementary volume (REV) for a naturally fractured rock mass and reduction of its unit mass strength with increasing volume up to the REV. This paper demonstrates that he numerical simulations of rock mass behavior strongly depend on the geological assumptions used to build the DFN models and the mechanical properties of the natural discontinuities. Ultimately, it is believed that to be effective, the use of synthetic rock mass properties requires a correct balance of engineering judgment, the integration of characterized field data, and numerical modeling. INTRODUCTION The influence of specimen size on the strength of intact rock has been long recognized [1]. For an intact rock sample it is expected that the reduction in strength will be associated with the number of microdefects included in the sample. Similarly, it is suggested that the uncertainty in predicting the behavior of a fractured mass is also clearly associated with scale effects [2, 3]. Except for a either a very closely fractured or an almost massive rock mass, the mechanical response is non-uniform due to the orientation, spacing and persistence of the discontinuities [4]. To accurately simulate this behavior by numerical modeling, the current analysis incorporates the use of an integrated Finite/Discrete Element (FEM/DEM) - Discrete Fracture Network (DFN) approach. By using an integrated FEM/DEM - DFN approach it is possible to study the failure of rock masses in tension and compression, as a combination of failure along pre-existing fractures and/or through intact rock bridges. The approach also fully accounts for potentially complex kinematic mechanisms and it has been shown [5] to capture the anisotropic and inhomogeneous effects of natural jointing, and accordingly is considered to be more realistic than methods relying solely on a continuum or discontinuum representation of rock masses. The hybrid FEM/DEM approach combines aspects of both finite elements and discrete elements with fracture mechanics principles. The finite-element-based analysis of continua is combined with discrete-element-based transient dynamics, contact detection and contact interaction solutions. The use of fracture mechanics principles allows the realistic simulation of brittle-fracture driven processes and a full consideration of kinematics processes. The proprietary code FracMan [6,7] is used in the current paper for DFN data analysis, whilst the fracture mechanics component of the approach is simulated using the code ELFEN [8], which is a 2D/3D FEM/DEM package that has recently found increasing use in rock mechanics [5,9,10].
ABSTRACT In recent years numerical modeling has been shown to provide the opportunity to better investigate caving mechanisms and to increase our understanding of the factors governing caving induced subsidence. Cave development and surface subsidence are the products of complex rock mass response, including brittle fracture driven failure of the rock mass and complex kinematic mechanisms. The authors present a review of numerical modeling of caving problems carried out to date using a hybrid Finite/Discrete technique (FEM/DEM) incorporating fracture mechanics principles and discuss the approaches adopted to-date to simulate the impact that a number of factors play on both cave development and surface subsidence, including the presence of major geological structures and draw control. Preliminary 3-dimensional models are also presented in which caving is indirectly simulated by using a continuum based strain softening approach integrated with mesh adaptivity to reproduce the large strain deformations typically associated with surface subsidence. INTRODUCTION Both continuum and discontinuum modeling techniques provide a convenient framework for the analysis of many complex engineering problems. Whereas finite element and finite difference methods model the rock mass as a continuum medium, distinct/discrete element methods model the rock mass as a discontinuum, consisting of an assembly or finite number of interacting singularities. The physical processes and the modeling techniques chosen will eventually influence the extent to which features such discrete fractures can be incorporated in the model. Parameter representability associated with sample size, representative elemental volume and upscaling represent additional fundamental aspects associated with numerical modeling of rock masses. In this context, cave mining involves complex kinematic mechanisms and comprises widespread failure of the rock mass in tension, and shear, along both existing discontinuities and through intact rock bridges. The need to numerically model such a complex problem certainly requires a better consideration of both continuous and discrete computational processes to provide an adequate solution. THE HYBRID FEM/DEM APPROACH Amongst the different numerical codes currently available, the hybrid finite/discrete element techniques (FEM/DEM) incorporates a coupled elasto-plastic fracture mechanics constitutive criterion that allows realistic modeling of cave mining processes through simulation of the transition from a continuum to a discontinuum, with the development of new fractures and discrete blocks, and a full consideration of the failure kinematics. In the FEM/DEM method the finite element-based analysis of continua is merged with discrete element-based transient dynamics, contact detection and contact interaction solutions [1]. The ELFEN code [2] used in the analyses is a multipurpose FEM/DEM software package that utilizes a variety of constitutive criteria and is capable of undertaking both implicit and explicit analyses in 2-dimensional and 3-dimensional. Use of fracture mechanics principles integrated within the ELFEN finite-discrete element method allows rock mass failure processes to be simulated in a physically realistic manner. The ELFEN methodology has been extensively tested and validated fully against controlled laboratory tests [3,4]. Among others, research by various authors [5,6,7,8,9], has demonstrated the capabilities of the code to analyze various rock engineering problems including but not limited to Brazilian, UCS and direct shear laboratory tests, analysis of slope failures, and analysis of underground pillar stability.
- Europe > United Kingdom (0.47)
- North America > Canada > British Columbia (0.29)
- Energy > Oil & Gas > Upstream (0.50)
- Materials > Metals & Mining (0.34)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Integration of geomechanics in models (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)