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.
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.
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.
