Purpose. A methodology development for predicting the geomechanical situation when mining an ore deposit with steep-dipping layers, taking into account the uncertainty in determining the rock properties, which is a consequence of the rock mass heterogeneity. Methods. The assessment of the open-pit wall stability is based on a combination of numerical simulation of the rock stress-strain state (SSS) and probabilistic analysis. The finite element method is used to determine the changes in the SSS that occur at various stages of mining operations due to design changes in the overall open-pit slope angle. The elastic-plastic model of the medium and the Mohr-Coulomb failure criterion are implemented in the codes of the 3D finite element analysis program RS3 (Rocscience). Stochastic simulation is used to assess random risks associated with natural object state variations. Findings. The distribution of maximum shear strains, which localizes the real or potential sliding surfaces in the open-pit wall at various stages of ore mining, has been identified. Based on the Shear Strength Reduction procedure, the open-pit wall Strength Reduction Factor (SRF) has been determined. The probabilities of open-pit wall stability loss, as well as the decrease in the strength reduction factor below the standard level at all stages of the ore body mining, have been revealed. Originality. For the first time, for real mining-geological conditions of a deep ore open pit, the dependence of the strength reduction factor on the overall wall slope angle, which changes during mining of each steep layer, has been determined. For each stage of mining operations, for the first time, the probability of a decrease in the open-pit wall stability below the standard level has been determined based on stochastic simulation. Practical implications. The ratio between the open-pit contour characteristic (overall slope angle) and the probabilistic safety factor is the basis for practical solutions to ensure the efficiency and safety of mining at various stages of friable and hard overburden excavation, ore extraction, as well as for the subsequent optimization of the open-pit design contours.
It is advisable to conduct mining operations in the conditions of the ultimate state of the opened massif with an increase in the pit wall slope angles below the limit of effective use of combined motor-conveyor transport in the cleaning-up zone of deep and ultradeep open pit mines to the final depth. Such a design of the pit walls is achieved when mining benches from top to bottom within the boundaries of steep-slope layers with the use of in-bench loaders in the cleaning-up zone. The conditions for the occurrence of irreversible shear deformations in rock layers and the position of the potential sliding surface with an increase in the pit walls slope angle of steeply inclined layers in the conditions of the ultra-deep Kachar iron ore open pit mine to critical values are established. It is advisable to use skips as the load-carrying body of the in-bench loader, the design of the supports of which allows it to be built with a lifting height of more than 30 m with the possibility of moving along the pit wall with variable berm elevations. The main provisions on the selection and justification of the expediency of using a loading device for operation in the deep zone are formulated based on the differentiation of the application zones of cyclic (motor transport) and cyclic and continuous method (combined motor-conveyor transport). In particular, the total costs of transporting rock mass according to the new scheme of combined in-pit transport with the use of a loading device for operation in the deep zone should be less than the costs of transportation according to existing or traditional schemes.
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