A narrow-vein gold overhand blasthole stoping operation that is geologically and geotechnically complicated, and seismically active, is exploring alternative mining approaches to manage these adverse conditions. Transitioning from overhand to underhand mining is being considered because the top-sill (high occupancy mining activities) is developed in the destressed paste backfill of the overlying stope and the stress concentrations will be in the floor, as opposed to the immediate back in overhand mining.
An integrated SRK team evaluated the backfill strength requirements, mining and support guidelines for re-mining the top-sill, and prepared preliminary Standard Operating Procedures (SOPs) for the initial trials. Others evaluated the seismic risk (possible floor heave) and increased stress levels that would impact stope drilling and lower-sill development. These assessments include pre-conditioning opportunities.
Although underhand blasthole stoping methods are being used elsewhere in Canada, this mine’s geotechnical context presents a different case as the narrow-vein mining, in contrast to thicker orebody mining, does not afford the destressing of subsequent stopes, using a cut-off concept, on the same level.
SRK reviewed case studies for analogue mines and found that backfill strengths determined using empirical guidelines resulted in lower strengths than those typically used in practice for main entry excavations. However, designing backfill that is too strong can result in backfill slabbing as experienced at the Lucky Friday mine, a good analogue operation, when the backfill became stressed and spalled.
Re-establishing the top-sill in the bottom-sill of the overlying backfilled stope requires the mining and support of backfill, but also that existing rock surfaces are supported using dynamic loading-capable support. The backfill is weak enough to be free dug, but other operations have used careful drilling and blasting to maintain the drift profile and maintain production rate requirements. Mechanical mining is possible but will be challenging if substantial fall-off has occurred before backfilling. The support system in the backfill consists of Swellex/Split-sets, mesh and shotcrete. A flash-coat of shotcrete is planned but requires evaluation and testing as issues with shotcrete adhesion on paste have been cited. Installing Swellex and Split-sets in paste requires testing and continuous improvement to verify that support capacity is being achieved. In complicated development situations, spiling and shotcrete arches/steel arch sets will be applied.
Mining through paste mixed with rock from stope fall-off will add complexity and impact advance rates. It will be important to understand if a material has sloughed-off into the stope after mucking and prior to backfilling commencing. SRK recommended LiDAR drone surveys be used to accurately scan the stope just prior to backfilling. Then a decision can be made to remove the barricade and re-muck the stope or implement a change to the re-sill mining strategy/schedule..
