Underground mining is the process of extracting minerals and ores from deposits located deep beneath the Earth’s surface. It involves creating networks of tunnels, shafts, and chambers to reach these resources. This method is selected when the ore is too far down for surface excavation to be practical or economical, requiring significant engineering to access materials buried deep underground.
Underground Mining vs Surface Mining
The choice between underground and surface mining depends on the depth and geology of the mineral deposit. Surface mining, including methods like open-pit mining, removes layers of soil and rock to access resources near the surface. This approach is used for shallower deposits, typically those less than 60 meters deep, and is more cost-effective due to simpler logistics. While surface operations can extract large volumes of material quickly, they cause significant land disruption.
Underground mining is reserved for deposits too deep for surface methods. While this approach has a smaller surface footprint and produces less waste rock, it is more expensive. The higher costs result from the extensive infrastructure required to access the ore body safely and efficiently.
Types of Underground Mining Methods
Room and Pillar Mining
Room and pillar mining is used for horizontally layered deposits like coal. The technique involves excavating a grid of tunnels, or “rooms,” into the ore seam while leaving behind “pillars” of the mineral to support the mine roof. A continuous miner cuts the rooms, which can be 20 to 30 feet wide. After the initial extraction is complete, retreat mining can begin, where pillars are systematically removed to maximize ore extraction as the roof collapses in a controlled manner.
Longwall Mining
Longwall mining is a highly productive technique used for flat, extensive seams, most commonly coal. This method carves out a large rectangular block of the mineral, known as a longwall panel. A shearer moves back and forth across the coal face, cutting slices of the mineral that fall onto a conveyor belt. As the shearer advances, hydraulic-powered roof supports temporarily hold up the ceiling. Once the machine passes, these supports advance, and the roof behind them collapses into the void, or “gob.” The high mechanization allows for a very high rate of resource recovery.
Cut and Fill Mining
Cut and fill mining is a selective method used for steeply dipping or irregularly shaped orebodies. The process is cyclical, involving mining the ore in horizontal slices from the bottom of the orebody upwards. After a slice of ore is excavated, the open space, or stope, is backfilled with waste rock, sand, or cement. This backfill provides a stable working platform for miners to extract the next slice and supports the surrounding rock walls to prevent collapse. This method offers good ground stability and is advantageous for minimizing waste rock dilution.
Sublevel Stoping
Sublevel stoping is designed for large, steeply dipping orebodies. This bulk mining method begins with developing multiple levels, or sublevels, within the orebody at regular vertical intervals. From these sublevels, miners drill long holes into the ore block between the levels. Explosives in the holes blast the ore, causing it to break apart and collapse into the open space below. The fragmented ore is then collected from the bottom of the stope using loaders.
The Underground Mining Process
The life of an underground mine follows a distinct sequence of stages, beginning long before any minerals are extracted.
- Exploration: Geologists use techniques like drilling and seismic surveys to locate and define a valuable mineral deposit. This phase assesses the size, grade, and economic viability of the orebody to determine if it warrants the investment.
- Development: Once a deposit is proven profitable, this construction phase creates the infrastructure to access the ore. Miners excavate vertical shafts for moving workers and materials, and horizontal tunnels, known as adits or declines, to reach the deposit.
- Production: This is the active process of extracting the ore using a specialized mining method. The extracted ore is transported to the surface for processing, where valuable minerals are separated from waste rock. This phase can last for many years or even decades.
- Closure and Reclamation: When the deposit is exhausted or no longer economical, operations cease. The mine is decommissioned by sealing tunnels and shafts, and the surrounding land is rehabilitated to mitigate environmental impact and return the site to a stable condition.
Safety and Environmental Considerations
Underground mining presents significant safety challenges. Ground instability is a concern, with the potential for rockfalls and roof collapses. To manage this, mine operators install extensive ground support systems, such as rock bolts, mesh, and shotcrete, to reinforce tunnels and working areas.
The underground atmosphere also poses dangers. Hazardous gases, such as methane in coal mines, can accumulate and create explosion risks. Air quality is another issue, as drilling and blasting release fine dust particles. Ventilation systems are engineered to circulate fresh air, dilute dangerous gases, and filter out harmful dust.
Underground mining has notable environmental impacts, such as land subsidence, where the ground level sinks due to empty voids left by extraction. This can damage surface structures and alter drainage patterns. Methods like backfilling can reduce this risk, but it remains a concern with methods that involve planned roof collapse, like longwall mining.
Water contamination is another environmental consideration. Water flowing through the mine can become acidic and leach heavy metals, creating acid mine drainage. This can contaminate both groundwater and surface water sources if not properly collected and treated. The disposal of mine tailings, the waste material from processing, also requires careful management to prevent releasing pollutants.
Working in an Underground Mine
A career in an underground mine involves adapting to a demanding work environment. Miners operate in confined spaces far below the surface, with light sourced from their helmet lamps and machinery. The work is physically intensive and takes place in rotating shifts to keep the mine operational 24 hours a day, requiring a significant adjustment for workers and their families.
The workforce in a modern mine is diverse and includes specialized roles. Production miners operate the heavy machinery that drills, blasts, and transports ore. Geologists map the orebody and identify new reserves to guide the mining process. Mining engineers are responsible for the overall design and planning of the mine, from ventilation to extraction logistics.
Despite the challenging conditions, many are drawn to the industry. The work involves operating advanced technology and machinery, and the close-knit nature of the crews fosters a strong sense of teamwork. The specialized skills required lead to opportunities for career advancement and compensation that reflects the demanding nature of the job.