A Stepwise Approach to Verification of the Combined Finite-Discrete Element Method for Modelling Instability Around Tunnels in Brittle Rock

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Authors
Markus, Simone
Keyword
Stepwise Verification , Finite-Discrete Element Method , Finite Element Method , Tunnel , Numerical Modelling , Tunnelling , Hard Rock , Hybrid Numerical Model , Geomechanics , Rock Mechanics , Pseudo-Discontinuum , Voronoi , Brittle Rock
Abstract
Numerical modelling of excavations in rock has advanced considerably in recent decades. While continuum numerical models form their basis in methods which can be verified by analytical solutions, discontinuum and hybrid numerical modelling software are challenging to verify. This necessitates the development of processes that can verify individual aspects of complex models. The combined finitediscrete element method (FDEM) allows for the numerical representation of progressive fracture in a simulated elastic material. The FDEM is a powerful tool for modelling instability around tunnels in brittle rock; however, significant verification of the method is required for its use in predictive modelling in critical engineering projects. To verify the FDEM for the purpose of modelling instability around tunnels in brittle rock, a multi-method and multi-scale stepwise verification approach is proposed. Multi-scale verification is required due to the practical limitations of current computational power. Simulation of a laboratory scale tests generally requires a mesh size not significantly larger than the median grain size of the material, limiting the size of elements to a few millimeters for most rock types. In tunnel scale models, a larger element sizes must be used. To relate the input parameters obtained from the laboratory scale calibration to parameters which can be used in tunnel-scale modelling, a gradual upscaling process for Unconfined Compressive Strength (UCS) test simulations is developed. The results of the upscaling process provide guidance for input parameter selection for tunnel-scale models, and insight into scale effects in FDEM models. In multi-method verification, equivalent modelling scenarios are represented using analytical and numerical methods of increasing complexity, to allow individual model behaviours to be progressively verified. A pseudo-discontinuum finite element method (FEM) approach is compared with the FDEM for modelling fracture propagation. Agreement of results is found for simulated non-frictional materials; however, for frictional materials, results agreement is not achieved. Further assessment of tunnel model response to pseudo-discontinuum FEM and FDEM input parameters will lead to improvement of input parameter and result equivalency between methods.
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