Mine backfill is the process of placing engineered fill material into excavated voids, stopes, or mined-out areas within an underground mine or open pit to provide ground support, prevent surface subsidence, manage rock mass stability, enable further ore extraction in adjacent areas, and reduce the volume of waste material requiring surface disposal. It is an integral component of modern underground mining practice and is particularly important in deep, high-stress mining environments where uncontrolled void collapse poses significant safety and production risks. Backfill materials are generally classified into three categories: hydraulic fill (slurried tailings or classified tailings pumped underground), paste fill (a thickened, high-density slurry of tailings, water, and cement binder delivered through pipelines), and rockfill (waste rock or crushed aggregate placed dry or cemented). In underground gold mines employing cut-and-fill or open stope mining methods, cemented paste fill mixed with a percentage of ordinary Portland cement is commonly used to create a stable floor for equipment to operate on and to lock in previously mined voids, allowing adjacent ore pillars to be safely extracted. Underground diamond mines such as those at Cullinan or Finsch in South Africa rely on backfill to maintain hangingwall stability in kimberlite pipe environments. Environmental benefits of backfill include reduced tailings storage facility (TSF) footprints, decreased surface disturbance, and lower acid mine drainage risk from buried sulphidic waste rock.
