Feasibility Study and Technical Report - Pretivm

AMC estimated ore recovery in the weathered zone to be 75% in anticipation of stressinducedmining difficulties. Furthermore, the operational expenditure was escalated forore in the weathered zone in consideration of the increased ground supportrequirements.Analysis of the weathered zone stopes shows low confinement; ground support includingresin-grouted rebar, mesh-reinforced shotcrete, and straps may be required to confinethe pillar rock mass and prevent unravelling. Many of the near-surface stopes willactually extend below the weathered zone into the fresh rock, which will reduce thepotential for rib pillar instability.SILL PILLARSThe current design sill pillar thickness is 30 m. The numerical modelling analysis showssome relaxation in larger stope hanging walls, and stress concentrations in sill pillarswithin areas of the mine with denser stoping. The model shows that the bottom-upsequence concentrates stress in both VOK Zone sill pillars. Yielding is likely to occur priorto recovering the entire sill pillar, and therefore achievable sill pillar recovery may be lessthan 100%. The West Zone sill pillar is interpreted to be stable except for stressconcentration in the sill pillar abutments. Stress concentration in pillar abutments iscommon in mines using centre-out sequencing, and does not necessarily indicatestability problems prior to full extraction of the sill pillar.GROUND SUPPORT REQUIREMENTSThe structural stability of the proposed excavations was analyzed using an empiricaldesign chart after Grimstad and Barton (1993) and Unwedge © (Rocscience 2003) todevelop minimum ground support recommendations. Ground support analyses forprimary (permanent “man-entry”) and secondary (temporary “development”) headingswere conducted in each structural domain. Ground support recommendations areprovided in Table 16.9.Pretium Resources Inc. 16-30 1291990200-REP-R0012-02Feasibility Study and Technical Report on the BrucejackProject, Stewart, BC