Design of initial shotcrete lining for a mine shaft using two-dimensional finite and hybrid finite-discrete element methods
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Shotcrete is widely used as a temporary support element in the construction of underground mines and tunnels. It is sprayed on the excavation walls close to the face to provide a safe working condition for mine personnel and equipment until the permanent support elements are installed. Therefore, the ability of the shotcrete lining to resists falls of ground is of paramount importance. The mechanical properties of shotcrete, including strength and stiffness, change with time as the excavation advances. Shotcrete may also be subject to plastic deformation, if it is loaded beyond its maximum capacity, especially in fast advancing excavations. Conventional methods for the design of shotcrete lining, based on analytical and empirical approaches, do not consider the influence of advance rate on the time-dependent properties and deformation behavior of shotcrete. In this research, two-dimensional finite and hybrid finite-discrete element methods are utilized to develop new methodologies for the design of initial shotcrete lining for a mine shaft by considering the excavation advance rate. These methods are also used to gain further insight into the damage evolution leading to the failure of shotcrete liner during the excavation advance. Through the analyses of the load and strain factors of safety calculated for the shotcrete liner based on the results of finite element models, the minimum shotcrete thickness is suggested for the mine shaft. From the results of the hybrid finite-discrete element model, it is concluded that the shotcrete liner cannot be used as a sole support element for the mine shaft as it fails during the excavation advance due to the shaft convergence. However, it can be used as a temporary support, although it may be damaged until the final, permanent concrete liner is installed.