Uniaxial Compressive Strength Variation for Multi-point Support Design and Discontinuity Mapping for Tunnel Stability Analyses and Stope Dilution Assessme

Abstract

Introduction : A geotechnical study was undertaken at Lubambe Copper Mine, Zambia, centring on understanding the variations in the uniaxial compressive strength of all rock units coupled with underground discontinuity mapping. The current mining method employed at Lubambe is Longitudinal Room and Pillar (LRP) mining with 30 m strike-wide panels. The Lubambe Copper deposit is located within the Central African Lufilian arc fold and thrust belt. It is located in an area underlain largely by rocks belonging to the Katanga sequence group. Problem: The mine has experienced a number of fall of ground incidents since its inception in 2011, which is against the company policy of zero tolerance. The fall of ground incidents were believed to be as a result of instability controlled by many factors including UCS and the overall jointing system. Significant dilution problems had been encountered in some stopes. Objective: The main objective of the study was to conduct UCS testing and underground discontinuity mapping at Lubambe Copper Mine in order to predict implications of tunnel stability, slope dilution and support design. Methodology: The study involved collection of samples of all rock units at Lubambe. Seventy (70) rock core samples 50 mm in diameter were prepared according to ASTM standard. The UCS was derived by uni-direction loading on the specimen with constant load increment until point of failure. In order to understand the geological structures that control tunnel stability, scan line discontinuity mapping was performed in already developed tunnels, which were plotted and analysed in Dips stereo-net commercial software. Results and interpretation: A summary of variations on rock units was established. Estimation of other rock parameters was performed in RocLab software package. UCS results of all the main mine rock units indicated significant variation, which led to the suggestion of a multi-point support design approach where UCS values were part of numerical indices used for design. Frequently, numerical indices such as RMR and Q, are determined on a contribution from mean UCS, that is, a single point. The situation such as encountered at Lubambe suggests that a multipoint design approach should be adopted, right from the outset. The mapping conducted revealed an interaction of three prominent joint sets which were identified to be controlling the overall tunnel stability as well as stope dilution.