Crown pillar extraction with paste underhand stoping

Technical investigations were conducted for extraction of a high-grade crown pillar in the northern 3500 orebody (N3500) at Glencore’s Mount Isa Mines using cemented paste backfill (CPB) underhand stoping. Several stopes have now been successfully extracted, achieving the planned recoveries with minimal dilution. In addition to standard CPB lab testing, in situ testing was conducted in the area planned for underhand exposure. The objective was to determine the variance between actual and design CPB strengths, to ensure the strengths were suitable for the planned underhand exposure dimensions. The testing results indicated the majority of in situ CPB strengths were higher than the lab cured and design strengths, due to the arching of stresses within the fill mass and curing processes. The strength parameters obtained from the testwork were incorporated in numerical modelling assessments using FLAC3D. Model calibrations were conducted using historical vertical CPB exposures to ensure the adopted methodology and material parameters were suitable. This paper discusses the methodology and results of the technical investigations, and how the data fed into analyses to assist with safe and efficient extraction of the crown pillar.