Hanging wall stability of sublevel caving mines in Sweden

Abstract: Mining with sublevel caving implies a drastic destruction of the ground surface in the area conforming the hanging wall of the mine. The extent of this affected zone is of little importance in unpopulated areas, but becomes of utmost economic significance when there are mine installations or a town site located on it. This is the case of the three mines included in this study, Grängesberg, the Zenobia area in Kiruna, and the Kapten orebody in Malmberget mining field. Due to the large volume of rock involved in hanging wall caving, there is poor understanding of the particular failure mechanisms and the strength of the rock mass. This results in large uncertainties in the application of analytical and numerical models for the assessment of hanging wall stability. There is however, a considerable amount of data in the form of surface fracture mapping and surface deformation measurements accumulated in the mines. In the work described in this thesis, some of the experience data from the mines is collected and stored in a computer based databank. The databank is developed based on commercially available software to facilitate the exchange of information among research institutes and mining companies interested in hanging wall stability problems. The information stored in the databank is of two types; graphic information is contained in a Computer Aided Design (CAD) program, and the non-graphic data is stored in a conventional database management program. Records in the database are "coupled" to graphic elements in the CAD program which allows for easy retrieval of the data. The thesis also contains an overview of the analytical methods currently used for the assessment of hanging wall stability and presents a summary of the results obtained in previous works in the form of prognoses of fracture angle as a function of mining depth. A simplified model is proposed for the calculation of geometrical relationships such as shape factor and volume that can be useful as a design tool. These geometrical relationships are related to the stability of the hanging wall. Of the studied geometrical relationships, the hydraulic radius of the vertical projection of the hanging wall seems to have potential for the estimation of fracture angles. Diagrams of fracture angle versus hydraulic radius show less scatter than fracture angle versus mining depth. Hanging walls showing a mining depth /length ratio larger than 0.2 - 0.3 should be analyzed taking into account the friction forces at the perpendicular release planes at the ends of the hanging wall. A rock mechanics classification of the hanging walls is performed with the application of the Rock Mass Strength (RMS) classification scheme (Stille, 1982). The classification gives an assessment of the relative quality of the rock mass in the different mining areas. Based on the geometrical model, the normal and shear stresses at failure have been back-calculated. A Mohr-Coulomb and a power law failure criterion have been fitted using a minimum squares regression method. The constant p of the back-calculated power law failure criterion is lower than the values reported in the literature from shear tests at laboratory scale. The strength parameters cohesion (C) and angle of friction for the Mohr-Coulomb failure criterion are higher than the values from back-analyses of hanging walls obtained by other authors. Finally the back-calculated rock mass strength parameters obtained are used in a limit equilibrium analysis of the geometrical model to produce a prognosis of the expected fracture angle with increasing mining depth. Prognoses were produced for the Kiruna and Malmberget mines. The power law failure criterion gives lower fracture angles than the Mohr-Coulomb, criterion, in particular for larger mining depths. The results of fracture angle calculations using the power law criterion are not bound to the value of the friction angle, but resembles a weakening of the rock mass with increased volume.