Feasibility Study and Technical Report - Pretivm

16.5.4 UNDERGROUND ROCK MECHANICSSTOPE DESIGN CRITERIARock mechanics analyses were completed to estimate achievable spans for the proposedmine openings. Stope stability analyses for the observed lower quartile (“conservative”,Q’ = 10) and median (“base case”, Q’ = 40) rock masses were completed. Therecommended maximum unsupported hydraulic radii vary from 1.9 to 3.1 for the backsand from 6.2 to 11.0 for the hanging walls, for the conservative and base case designs,respectively. The recommended maximum supported hydraulic radii vary from 4.1 to 5.6for the backs and from 10.0 to 14.5 for the hanging walls, for the conservative and basecase designs, respectively.A preliminary MAP3D numerical model developed for the VOK Zone shows stressconcentration and yielding proximal to the dense stope clusters in the middle of the VOKZone, indicative of potential instability in the stope hanging walls and footwalls. Thisindicates some potential for increased dilution. Cable bolts could be installed into thehanging walls of dense stoping blocks to "tie" the hanging wall together until backfill isplaced, to help reduce this dilution. Note that currently, the model is not consideredsufficiently calibrated for quantitative design.STAND-OFF DISTANCESMinimum stand-off distances between excavations of 10 m, 25 m, and 50 m arerecommended for the raises, ramps, and underground crusher, respectively. Therecommended stope stand-off distance from all hanging wall drives is 25 m. Theproposed portal decline will be twinned, with a recommended minimum pillar thicknessof approximately 10 m between the two excavations.RIB PILLARSThe rib pillars between cross-cuts were designed to be in waste and will not be recovered,but are considered temporary based on the short-term lifespan required for access to agiven stope. The minimum recommended pillar width to height ratio for cross-cut ribpillars for the “base case” stope design is 1.1:1.0, and 3.3:1.0 for the “conservativecase” stope design. If cross-cuts are developed within the weathered zone, therecommended rib pillar width to height ratio is 1.7:1.The rib pillars between the open stoping blocks are intended to give temporary support tothe mining block until the primary stopes are backfilled and the pillar can be recovered inthe form of a secondary stope. Using the pillar stability graph method developed byHudyma (1988) and tributary loading theory, the minimum recommended secondarystope span (rib pillar thickness) to primary stope span for the “base case” stope design is1:1 for sublevel intervals of 30 m.Analysis of the “conservative case” shows that high stresses may develop in the pillarcore, and that some spalling and dynamic rockmass damage may be expected. This mayresult in spalling in 25% of rib pillars, and difficult drilling in approximately 25% ofsecondary stopes. If stopes are developed within the weathered zone, the minimumrecommended secondary stope span (rib pillar thickness) to primary stope span is 1.5:1.Pretium Resources Inc. 16-29 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC