Numerical investigation of the energy absorption capacity of rockfall protection measures for underground portals

Rockfalls represent a serious hazard in surface mining that affects the safety of working personnel at the base of highwalls and in the vicinity of underground portal entries. The research presented in this thesis focuses on the numerical investigation of the dynamic response of granular and composite rockfall mi-ti-gation structures for the protection of underground portal entries. The study numerically investigates the energy absorption capacity of small scale waste rock piles (or muckpiles) and the application of an innovative damping module system as an alternative mitigation measure. The first part of the research focuses on the dynamic response of a muckpile upon rockfall impact. A simplified discrete element model of a muckpile is developed. The influence on the rebound characteristics of the contact parameters and different mixtures of spheres and elongated rigid aggregates composed of two spheres is investigated. The study highlights that a realistic representation of the granular material using clumps is essential to capture the correct rebound behaviour. These findings are then used for the development of a more detailed model which is used to perform a parametric study on the muckpile geometry and the impact conditions. The study allows presenting critical observations for the design of the portal structure and the exclusion zones around a portal. The second aim of this research is the numerical investigation of the potential application of damping modules as an alternative cost-effective solution to muckpiles for the protection of underground portals against rockfall. A damping module system consists of cylindrical modules made of a wire mesh, steel rings, an optional boundary rope and a geotextile lining filled with granular material. Starting with an accurate numerical representation of each component of the damping module, a novel discrete element formulation for the modelling of deformable structures is presented. In particular, a new deformable triangular element, a so-called PFacet, is developed and implemented into the open-source framework YADE. The model is extremely versatile and its application to the accurate modelling of deformable structures and geotextiles is shown. Then, the model is extended to simulate damping modules by incorporating a stochastically distorted wire model into the PFacet formulation. The dynamic response of the damping module is validated using experimental results and the capabilities of the model are investigated by considering various design and impact configurations. Starting from the initial numerical investigation of the energy absorption capacity of rockfall mitigation measures currently used in surface mining for the protection of underground portal structures, the study articulates a general numerical approach to the modelling of more complex composite structures such as damping modules. By combining experimental observations from the scientific literature, detailed numerical investigations and innovative numerical developments, the study shows the capability of the discrete element method as a useful tool to be used in the design of rockfall protection systems for surface mining applications.