Assessment on the Decisive Parameters for Open Pit Slopes – A contribution based on a mine in hard rock environment
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Rock slope stability has been one of the major operational and safety challenges in the open pit mines throughout the world. Steeper slope angles provide a higher revenue and less waste material. Gentle slope angles are better for safety. In the hard rock slope environment, instabilities are controlled by discontinuities in the rock mass. Several methodologies haven been developed in the past in order to evaluate slope stability, both numerical and empirical, but none of them have attempted to modify the importance of each variable into rock mass quality assessment based on site specific studies. When evaluating rock mass quality for slope stability probably the most widely known methodology is the RMR (Rock Mass Rating). This method includes information on the strength of the intact rock material, the spacing, quantity and surface properties of the discontinuities and also the influence of groundwater. This system was primarily developed for the estimation of support requirements in tunnels and it has been consequently modified to include the effect of jointing orientation into slope stability. The most accepted modification is the SMR (Slope Mass Rating), which takes in account the relationships between orientation and angle both of the joint and the slope, as well as effects from excavation methods. The overall goal of this research project was to carry out a study on the engineering geological and geo-environmental parameters that influence the stability of open pit slopes and create a methodology for developing stability maps. The methodology should be applicable to any given life time of the mine consisting short, medium and long term stability plans. The research goal is accomplished by developing a methodology for modification of the RMR index that takes in account on which parameters are the most decisive for slope stability conditions in a given area. Such modification has been done following the methodology of pairwise comparison from the AHP (Analytical Hierarchy Process), which allows deriving weights, or degrees of importance, for different parameters involved in a problem. It is expected that a weighted RMR would provide a better basis for calculation of SMR, and therefore, increase the precision into slope stability assessment. In addition, several models were developed through the research in order to study, analyse and organise relevant information associated to instability. The developed models consist; a 3-dimensional structural model (containing jointing direction and the definition of structural domains), a 3-dimensional rock mass model (containing rock mass quality distribution), and an assessment of the major hydrogeological features of the mine (containing water bearing and barrier structures). The combination of information coming from the first two models and the topographical map of the mine was used to create a routine for calculation of SMR surface maps into a GIS environment. Such methodology was used as the base for the calculation of SMR maps from a weighted RMR index. The final maps have been correlated to the major hydrogeological features in the mine. Two methods for weighting RMR were proposed, namely ARMR and WRMR. The results showed that groundwater, roughness and persistence are the most important parameters of the rock mass decisively influencing on the pit slope stability. In general terms, the SMR maps derived from WRMR have provided more accurate depiction of both stable and potentially unstable zones. A correlation of past events to SMR maps derived from RMR, WRMR and ARMR showed that in case of SMR from RMR precision was 92%, from ARMR it was 90%, and from WRMR it was 98%. Furthermore, when taking in account the last two classes of stability into SMR, maps from RMR had a precision of 31%, from ARMR 23%, and from WRMR 84%. In both extremes (stable and unstable zones) SMR maps from WRMR provided a better approximation. The methodology is also easy to replicate in any timespan, as different topographical maps are possible to be used for calculation. Further work is required to investigate the interdependence of each of these variables and how to incorporate these dependencies into slope stability assessment. It is also important to develop structural models in which joint information is stored into each pixel, in order to improve the level of detail of the methodology. In this sense, the main challenge is to highlight areas of the mine influenced by these joints, and not only the surface trace of the unfavourable discontinuity.