Open Cut » Geology
Rockfall is a major safety hazard in open pit mines which can also have financial consequences should the production be temporarily stopped for safety issues. The safety of the workers in all the areas potentially affected by rockfall has to be rigorously ensured and the risk of not being able to recover coal from hazardous areas with reduced/constrained access should also be considered. Access to some slopes cannot be restricted (e.g. portal entries) and appropriate mitigation measures become necessary to reduce the hazard to an acceptable level. A rigorous rockfall hazard assessment is therefore of main importance to identify the severity of the risk associated to rockfall to different areas. In mining, this rockfall hazard assessment entails a zoning of constantly evolving worked areas. The zoning establishes zones with different hazard levels that are generally associated with different restrictions.
Several zoning methodologies have been developed in the past decades, however, these approaches have been studied for hazard assessment along transportation corridors and mountain slopes. In surface mining, these approaches present several limitations as detailed records of previous events are not always available (especially at an early stage of mining activities) and there is still some uncertainty around the choice of the representative event size.
This research focuses on the development of a new Evolving Rockfall Hazard Assessment (ERHA) method specifically designed for coal mining. The new methodology involves a first qualitative identification of the most hazardous areas for which a second more robust quantitative analysis is requested. The qualitative step intends to be a simple tool for a quick estimation of the rockfall hazard, without requiring detailed measurements and rockfall simulations.
The proposed method was validated by applying ERHA to several Australian mine sites and data gathered from the scientific literature. Thanks to the contribution of the monitors and colleagues from various mine sites, the method was also applied to four trial mine sites in NSW and QLD. The sites were visited for data acquisition and the method was applied to different hazard condition areas.
The qualitative method allows practitioners to quickly assess the hazard level at the base of a given highwall and to conduct a first qualitative and rapid estimation of the rockfall energy range expected at the bottom of the highwall and the range of the distance of the first impact at the base of the wall.
A free user‐friendly web app was developed for a quick application of ERHA methodology. The tool can be launched from any web browser and is platform independent and it allows for a detailed report of the input data chosen by the user and for the exact location of the rock surface object of investigation.
A quantitative rockfall hazard assessment method was also developed to rigorously assess the most hazardous zone identified in the first qualitative phase. According to the surface coal mining industry requirements, the developed quantitative method allows accounting for different times of exposure, failure frequency and mitigation measures within the hazard assessment procedure.
The method provides hazard maps at the base of a given highwall based on the likelihood of occurrence and maps with an allowed time of exposure. The method can be applied to single highwall profiles or whole highwall sections considering various rockfall scenarios.