Identification of instability periods during formation of undermined rock cavings in the tectonically stressed Khibiny massif

The article focuses on studying the state of the rock mass based on assessment of quantitative parameters of seismic process in the undermined rock strata. Using the apatite-nepheline deposits of the Khibiny massif as an example, it is demonstrated that the dynamics of the energy index and cumulative apparent volume parameters reflects the processes of stress changes in the rock mass, including the time after the blast impact. Boundaries of the study area in the rock mass overhang at the Kukisvumchorr and Yukspor fields were determined to exclude seismicity data corresponding to the areas of active mining operations. Periods were identified that characterize the stable state of the rock mass, when the overhang rocks do not cave, as well as the periods when the process of fracturing and caving of the overhang rocks starts actively. It was found that temporal variations in the apparent volume and energy index can be used to assess the state of the rock mass and the potential fragmentation danger of its parts. The dynamics of these parameters for a certain volume of rocks reflects the phases of loading and strength degradation. The survey results were obtained in the most general form and for large volume sections of the rock mass, despite the fact that they correlate well with the data on actual caving of the undermined rock strata.