Investigation into depressurisation of highwalls at Mogalakwena Mine, Limpopo Province, South Africa
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Date
2022
Authors
Mogatusi, Itumeleng
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Publisher
University of the Free State
Abstract
As the magnitude of open pit mines like Mogalakwena Mine, gets bigger and deeper, the assessment and management water in the pits and particularly in the high walls is becomingly increasingly important for water and rock engineers. For over the years, Mogalakwena mine in its older pits has been employing a general mine dewatering method. The popular method involves pumping from vertical wells to reduce groundwater levels and groundwater inflows to the pits. The newer pits use sump pumping programs that target the removal of groundwater collected at the bottom of the pit using mobile pumps in sumps. Groundwater on the slopes also results in low calculations for Factor of Safety (FoS), increasing the number of geotechnical high-risk zones. This means fewer mining areas to excess ore. Loss of mining due to loss of excess is not good for any profitable mine operation because valuable ore will be left behind. The need for depressurisation is then considered to improve slope stability performance and increase Factor of Safety. The most applied method of dewatering in open pit mines is usually by drilling vertical boreholes outside the pit and pumping out the groundwater. This technique for Mogalakwena have been investigated to be ineffective for the mine because of the low permeability of the hanging walls rock mass made up of Norites. This hinders efficient and successful pumping using the classic dewatering techniques. Dewatering programs aim to achieve lower levels of the groundwater table in the mining area, ideally to be below the level of the working pit floor. The current water control taking place in the mine is operational dewatering, which pumps out groundwater seepage and rainfall water that collects at the bottom of the pits in sumps. This water is then pumped out from the sump. In a fractured aquifer setting such as that in Mogalakwena another method such as depressurisation to manage saturated slopes is needed. The dissertation through a hydrogeological and geotechnical analysis of the hydrogeological setting shows a mixed low to moderate hydraulic conductivity (K) system of low storage and moderate precipitation (mean annual precipitation of approximately 700 mm). Very low K occurs in the strong rock mass Hanging Wall (HW) and Foot Wall (FW) slopes and moderate to high K occurs in the Platreef and shear zone generating seepages and discrete inflows into the open-pits. No relevant recharge locations have been identified from nearby aquifers and groundwater inflows during pit development will occur along strikes within the Platreef and along potentially permeable regional structures. The drainable porosity of all rock units are low and the groundwater system has assumed the low overall storativity. Thus, showing that significant reduction in water levels and inflows could be achieved through slope depressurisation and draining of the slopes and not classic dewatering from vertical wells. The need for a slope depressurisation program for Mogalakwena mine highwalls was established through the investigation outlined in this dissertation. The need and timing was determined by applying the Slope Depressurisation Action Trigger Response Questionnaire (SDATRQ). While the timing of a slope depressurisation program requires further investigation and therefore the timing was not fully determined by SDATRQ. The relationship between dewatering and slope depressurisation was established to be incompatible for Mogalakwena Mine, dewatering the slopes did not depressurise the pore pressures on the highwalls. This was established also through the Read and Stacey (2009) dewatering and depressurisation broad categorization. Although the dissertation largely focuses on the depressurisation of highwalls of Mogalakwena Mine, it is important to note that the topic of highwall depressurisation is an important discussion point at several open pit mines in the world. The development and further testing use of the Slope Depressurisation Action Trigger Response Questionnaire can prove to be of value to other open pit mines, experiencing the build-up of pore pressure behind the highwalls and mining advancing deeper. The dissertation demonstrated through an investigation of Mogalakwena Mine highwalls that the need for a slope depressurisation program can be determined by using SDATRQ. Hence proving the dissertation statement to be true and further investigation on the timing needs to be completed.
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Keywords
Dissertation (M.Sc. (Geohydrology))--University of the Free State, [2022], Depressurization of highwalls, Mogalakwena Mine, Limpopo Province, South Africa, Hydrogeology -- South Africa -- Limpopo Province, Slope Depressurisation Action Trigger Response Questionnaire (SDATRQ), Mogalakwena Mine -- South Africa -- Limpopo Province