An Improved Support System for Safe and Economical Underground Excavation, A Case Study of Mindolo Deeps Section, Zambia

Abstract

Mopani Copper Mines Plc. (MCM) is sinking a vertical rock and man shaft. The rock hoisting winder will handle 2.0 million tonnes of ore per annum. The project will significantly boost the current productivity of Mindola Mine, as it will enable the company to increase ore production from 800 thousand tonnes to 1.4 million tonnes in the Deeps section. Currently, the Deeps mining method employs up dip Vertical Crater Retreat method with waste rock as back fill material. The method has posed huge challenges in as far as excavation stability problems are concerned. Stability problems range from rock falls between support units to collapse of Vertical Crater Retreat chambers and Ventilation drives. These problems lead to equipment damage, injuries, loss of reserves, loss of tipping space, loss of ore production and disturbance in the ventilation network. The methodology used in this research work involved geotechnical data acquisition, Data analysis and Geotechnical designs. Geotechnical raw data was collected by conducting underground joint survey and core logging. Underground joint survey involved line mapping of joints and window mapping. The output of this exercise was joint spacing, joint orientation and joint condition. Rock samples collected were tested for Point load Index using a Point Load Equipment for the calculation of Intact Rock Strength. Other tools used in data collection were a 30m measuring tape, a Geotechnical compass, a Schmidt hammer, a Clino rule and a Point Load machine. The raw data was evaluated to obtain inputs into geotechnical designs. The methods of analysis included Joint analysis, Rock mass classification and Rock mass stability number. During Joint analysis, the dip and dip direction of joints from mapping were entered into the DIPS Program to determine the number of joint sets and their dip and dip directions. The rock mass quality was determined by using the Rock Mass Rating and the Q system. The Rock Mass Stability number was also determined using the raw data. Geotechnical designs were then carried out for the purpose of v establishing the loads that were induced on excavations and determine the support required to counter the loads so as to ensure stability of the excavations. The design methods involved the use of empirical and numerical methods. The empirical methods included Potvin’s stability graph method for the estimating the stable span of Vertical Crater Retreat chambers and Grinstad/Barton graph method for estimating the support requirements. Mining simulations using Phase 2 and Examine tab numerical modelling programs were carried out. The research recommends measures to improve the stability of Vertical Crater Retreat chambers and Ventilation drives. Among them are, mining the Ventilation drives at not less than 48 meters from the geological footwall contact, mining the Vertical crater retreat chambers at 25.25 meters strike length and supporting them with 6 meters cable bolts and fibre reinforced shotcrete. Furthermore, adopting of a steeper stoping echelon is also recommended. This research gives the details of the problems of excavation instability, research methodology used to combat them and finally the way forward for better stability with economic benefits.