Assessment of geotechnical considerations for ground control and stability at Nkana Synclinorium copper mine Mopani Plc, Zambia.
Date
2024
Authors
Mpaka, Barnabas
Journal Title
Journal ISSN
Volume Title
Publisher
The University of Zambia
Abstract
Nkana Synclinorium Mine is in the Southern part of the Nkana mining license area in Kitwe. The mine has been an underground operation since 1930 when the first shaft was sunk at Central Shaft. Vertical Crater Retreat (CVR) and Sub-Level Curving (SLC) are the main copper ore mining methods used at the mine. Due to increase in mine depth by 137m, from 3510ft to 3960ft (1.070 to 1.207km), mining operations have been encountering geotechnical challenges related to ground control and ground stability. This study applied Empirical methods (Barton’s Q system, 1974, Bieniawski (1973) classification and Hoek, 1994) for rock mass classification. Scanline mapping of geological structures was done to identify joints, bedding planes, schistosity and folds. The mapping was carried out along the crosscuts or tunnel walls at a 1.50m grade line elevation with geological features being picked along a stretched 100m tape. Borehole cores were examined for geological formations, grain size, colour, joints, and RQD. The assessment of rock reinforcement and surface support elements like cable bolts and shotcreting were done by using pull tests and uniaxial compressive strength (UCS) tests. Pull Testing was done in selected mine excavations to provide a quantitative measure of the relative performance of different anchor systems and compressive strength tests on concrete cubes were used to assess the quality and strength of the material used in shotcreting underground. Collection of survey data for over-break and under-break in tunnels mined underground was done by the author with the assistance of mine surveyors. The Theodolite was the major instrument used. Numerical modelling using MAP3D and Finite Element Analysis (FEA) softwares was used to assess ground stability around underground excavations. Evaluation of design systems for ground support and pillars was carried out with the reviewing of current design flow charts in comparison with design charts from literature for other mines around the world. Results of study indicate that the rock mass ratings (RMRs) for the samples of Basal Quartzite (BQ), Foot Wall Conglomerate (FCON), South Orebody Shale (SOBS) and Hanging Wall Argillite (HWA) rock formations compared well with Barton’s Q system RMR ranges except for Foot Wall Sandstone (FSAN) which was out of range. All pull tests conducted at 3 sites had an average failure rate of 22.3% while compressive tests gave a high failure rate of 83.3%. Numerical analysis using Finite Element Analysis (FEA) software indicate more stresses and displacement around the excavation before rock bolt support, and less stresses and displacement after rock bolts are installed around the same excavation. Over-break and under-break data did not show any correlation with mined linear meters of the tunnels. Geological factors like jointing, weathering and shearing as well as drill-and-blast challenges were attributed to over-break and under-break. In conclusion, folds and joints are major factors affecting ground and stability at Synclinorium mine, and the geotechnical database has insufficient data. The confidence of the current geotechnical database at Synclinorium mine needs to be improved through more data collection. Design excavation and ground support designs need to be improved and matched with the real situation underground. The key issues affecting drill-and-blast like varying powder factors in the shots, lack of pragmatic consideration of geological and geotechnical information in the design, and diligent adherence to the blast designs need to be addressed.
Description
Thesis of Master of Engineering in Rock Mechanics