The Cerro Lindo Mine is the largest underground mine in Peru, currently mining more than 20,000 tonnes/ day. The sublevel stoping method was used with paste backfill, which had been experiencing overexcavation issues at the hangingwall and the front wall of the stopes.
The geology can be described as a volcanogenic massive sulphide (VMS) deposit that strikes NW and dips 65° SW on average. Its dimensions are 1,350m long, 83m wide and 245m deep. It features a jointing system that predominantly strikes NE-SW and a secondary orthogonal system that strikes NW-SE at the mineralised zone. The rock mass quality is mainly rated as fair (RMR = 40 to 50) and in some specific sectors as poor (RMR = x to y).
Our scope was to design the openings and dilution control for stopes without personnel entry.
For this purpose, the Potvin Modified Stability Graph method was used to correlate with Clark and Pakalnis’s Equivalent Lineal Over- Excavation (ELOS).
Stope dimensions were recommended for both fair and poor quality rock, taking into account the results of the rock mass geomechanical model and the current status of the mining activity. Based on the back analysis results of mined stopes and on the kinematic wedge analysis, we prepared a standard support design for the roof of the stopes.
After making a comparative analysis of the over-excavation height at the dome of the stope versus the stope width without support, we observed that the height of the unsupported dome is almost half the width of the stope. This indicates that the fault depth is mostly controlled by the rock mass quality.
Regarding the stopes with cable bolting (fan-type) support at the roof, we observed that the over-excavation in the stopes had been eliminated. Also, reducing the stope lengths helped control ELOS efficiently at the front walls.
Conclusions:
- The stability graphic method and ELOS were complemented with a back-analysis of the mined stopes. These methods were useful in determining efficiently the dimensions of the stopes
- Understanding the over-excavation type and its magnitude, measured at the existing stopes, helped to determine efficiently the dimensions of the cable bolting support needed at the stope domes.
- The over-excavation control at the front walls and hanging wall of the stopes favors the continuity of the mining cycle and the mining of secondary or adjacent stopes.
