1699 include old stopes or drifts from historical workings, as well as broken rock zones created by caving methods. Leaving such voids unfilled can pose a collapse hazard or a source of air and water flow. Injection techniques provide solutions here through cavity filling foams and grouts. High-expansion and lightweight urea-silicate foams can be injected to fill large volumes, forming a solid but relatively light support that occupies the space and prevents further movement of surrounding rock. For example, in a coal mine longwall panel, a low-density 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 104. Filling a natural Geode Cavity 2.5 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 copper, blocky ore) Induced widespread fractures and subsidence; Potential water and mud inflows from overlying strata; High stress and seismicity loosening joints. Pre-grouting of perimeter faults and water-bearing zones before cave initiation; Fast-reacting resin injections to stabilize draw bells or fragmentation zones if they become too weak; Backfill of large caves or sinkholes with foaming grout to control subsidence.
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