Assessment of rock mass quality and its effects on charge ability using drill monitoring technique

Abstract: For an efficient mining operation, it is essential to have as much information as possible aboutthe ore to be excavated and the rock masses surrounding the ore. Geological information andthe content and distribution of extractable minerals, are central concerns for long term mineplanning. However, for mine stability and production scheduling, the mechanical conditionsof ore and side rock are also very important. The underground mining process normallyconsists of a number of unit operations, such as drilling, charging, blasting, loading,transportation, hoisting etc., linked in a production chain. The quality of the initial operations(drilling, charging and blasting) normally defines the pre-conditions for the following loadingand transportation processes in the mine. The ability to fully charge holes as planned has beenidentified as one of the major obstacles for smooth fragmentation. Course or unevenfragmentation will, for example, significantly affect the loading and transportation efficiencyin the downstream production chain.Earlier studies in LKAB’s Malmberget mine have shown that the chargeability is on averagearound 90%. However, individual levels can have an average chargeability of only 70% andindividual rings, at those levels, can suffer from chargeability as low as 50%. A significantpart of these problems has its origin in geo-mechanical problems in the rock mass. Therefore,detailed knowledge of the rock mass condition surrounding the boreholes is essential toimprove the planning and execution of the charging works in a mine and to improve overallfragmentation and production efficiency.The focus of this thesis is therefore to define and evaluate geo-mechanical features in thedrilled rock mass effecting chargeability, and to evaluate drill monitoring technique for theassessment of rock mass quality and its effects on borehole’s chargeability using hydraulic In-The-Hole (ITH) percussive drilling.The research is based on literature review, drill-monitoring data, borehole filming, on-lineproduction database and monitoring of charging operation. Statistical methods are used toanalyse drill data. The data have been collected from LKAB’s underground mine inMalmberget, Sweden.Several rock mass conditions including caving, shearing, deformation, fracturing, cavities,solid rock, etc., have been identified during filming of 361 production boreholes.Measurement While Drilling (MWD) technique has been used to assess the quality of thepenetrated rock mass. In order to do so, a detailed analysis of the drilling system and thedrilling control including how monitored parameters relate to each other and to the penetratedrock mass conditions, has been performed. The results show that the MWD data containpronounced hole length dependent trends, both linear and step-wise linear, for mostparameters. By combining the borehole filming and the analyses of monitored drillparameters, the drilling responds to each geo-mechanical features in the rock mass is furtherdemonstrated. High correlation has been found between the geo-mechanical rock properties(fractures, cavities, solid rocks, etc.,), and the registered drilling system’s response. Theanalyses show that the responses from the drill monitoring system can distinguish between solid rock, fracture zone, cavity and cave-in. Based on the correlation between the registereddrilling system’s responses and the geo-mechanical features, a geo-mechanical model isdeveloped to assess the borehole chargeability. Principal Component Analysis (PCA) isperformed to model this relationship. The developed model can distinguish fans, or parts offans, with solid, non-fractured rocks where no chargeability problems can be expected, fromfans, or part of fans, with fractures, cavities or cave-in risks, where chargeability problemscan be expected. The model shows high potential for identifying charging problems in theborehole, and has been verified and validated by following an actual charging operation in thereal production environment.