112 movement of surrounding rock. For example, in a coal mine longwall panel, a lowdensity foam injection was used to completely fill an abandoned roadway above the panel that had been causing periodic roof falls; once filled, the overlaying strata stabilized, ensuring the longwall face could pass beneath safely. In hard-rock mines, void filling is often combined with stabilization: a foam might fill the majority of a cavity, and then a stronger grout or resin is injected around the edges to bond the fill to the rock. This strategy was applied in a copper mine where a large stope void from a previous mining phase needed to be rehabilitated – foam was used in the central open volume while a cementitious grout solidified the caving perimeter. The composite fill prevented further caving and served as a stable wall for adjacent new stopes. Figure 3. Filling a natural Geode Cavity 2.4 Typical scenarios by deposit type: different deposit types tend to manifest the above challenges in varying combinations. Table 1 summarizes common geotechnical issues for several underground deposit contexts and the corresponding injection solutions typically employed. Deposit Type Key Geotechnical Challenges Injection/Consolidation Solutions Narrow vein high-grade (e.g. gold) Fractured host rock; Faults with water; Small excavation spans sensitive to vibration. Pre-injection with microcement to seal fractures and dewater faults; Polyurethane resin to quickly stop water inflows; Resin injection to bind broken vein walls and reduce dilution. Massive sulfide or skarn (base metals) Large open stopes leading to dilution from weak hanging walls; Alteration (clays) causing weak back; Possible aquifers in host rock. Grout injection of hanging walls and sill contacts to improve competence; Resin injection in altered zones to reinforce ground before stopping; Curtain grouting around orebodies to cut off aquifers. Block/panel caving (porphyry Induced widespread fractures and subsidence; Potential water and mud Pre-grouting of perimeter faults and waterbearing zones before cave initiation; Fastreacting resin injections to stabilize draw bells or
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