Extraction of Rocks and Minerals » Subsurface Mining Techniques

What you'll learn this session

Study time: 30 minutes

Different types of subsurface mining techniques
How shaft mining works and when it's used
The process of slope mining and its applications
Solution mining methods and their environmental impacts
In-situ leaching techniques for mineral extraction
Environmental and social impacts of subsurface mining
Case studies of major subsurface mining operations

Introduction to Subsurface Mining Techniques

Subsurface mining involves extracting minerals and resources from beneath the Earth's surface. Unlike surface mining, these techniques allow us to reach deposits that are too deep for open-pit methods. About 60% of the world's mineral production comes from underground mines, making these techniques vital to our modern way of life.

Key Definitions:

Subsurface Mining: The extraction of mineral resources from beneath the Earth's surface through tunnels or shafts.
Ore: Rock containing minerals or metals that can be extracted for profit.
Gangue: The commercially worthless material that surrounds or is mixed with the valuable minerals in an ore deposit.
Stope: An underground excavation made by removing ore from around a deposit, leaving an open space.

⛏ Why Go Underground?

Subsurface mining becomes necessary when:

Mineral deposits are too deep for surface mining
The overburden (surface material) is too thick
Surface restrictions exist (cities, protected areas)
The ore is high-grade but in narrow veins
Environmental concerns make surface mining impractical

📊 Economic Considerations

Subsurface mining is generally more expensive than surface mining because:

More complex infrastructure is required
Safety systems are more extensive
Ventilation and water pumping add costs
Less material can be extracted per day
Higher skilled labour is needed

Major Subsurface Mining Methods

There are several techniques used for extracting minerals from beneath the Earth's surface. Each method is suited to particular types of deposits and geological conditions.

Shaft Mining

Shaft mining involves digging vertical tunnels deep into the ground to reach mineral deposits. This is one of the oldest and most common forms of underground mining, used for coal, gold, diamonds and many other minerals.

🔨 How It Works

Vertical shafts are dug from the surface to the mineral deposit. Horizontal tunnels (called "drifts") extend from the main shaft to access the ore. Miners and equipment travel up and down the shaft to extract the minerals.

✅ Advantages

Can reach very deep deposits (some gold mines extend over 3km deep). Minimal surface disturbance compared to open-pit mining. Can be used for narrow, high-grade veins.

❌ Disadvantages

Expensive to construct and maintain. Requires complex ventilation and water pumping systems. Higher safety risks including cave-ins, gas build-up and flooding.

Slope Mining

Slope mining (also called drift mining) uses inclined tunnels to access mineral deposits. This method is often used when the deposit is located on a hillside or when vertical shafts are impractical.

⛏ Slope Mining Process

In slope mining, an inclined tunnel is created from the surface to the ore body. This sloped entrance allows for easier movement of workers, equipment and extracted materials. It's particularly useful for:

Coal seams that outcrop on hillsides
Deposits that are relatively shallow but not suitable for surface mining
Areas where the water table makes shaft mining difficult

💡 Room and Pillar Method

Once inside the deposit, miners often use the "room and pillar" technique:

"Rooms" (open areas) are created by extracting ore
"Pillars" of untouched material are left to support the ceiling
Typically 50-75% of the ore can be removed
In some cases, pillars can be partially removed as mining retreats
Commonly used in coal, salt and potash mining

Solution Mining

Solution mining extracts minerals by dissolving them in a liquid (usually water with added chemicals) and then pumping the solution to the surface for processing. This method is used for soluble minerals like salt, potash and uranium.

💧 In-Situ Leaching (ISL)

In-situ leaching is a specialized form of solution mining commonly used for uranium extraction:

Wells are drilled into the uranium-bearing formation
A leaching solution (often acidic or alkaline) is pumped down
The solution dissolves the uranium
The uranium-rich solution is pumped back to the surface
The uranium is separated from the solution
The solution is recycled back into the ground

⚖ Environmental Considerations

Solution mining has specific environmental concerns:

Potential groundwater contamination if solutions escape the mining zone
Surface spills during processing
Changes to groundwater chemistry
Requires careful monitoring of injection and recovery wells
Post-mining restoration of groundwater quality is essential

Case Study: Kiruna Iron Ore Mine, Sweden

The Kiruna mine is the world's largest underground iron ore mine, operating since 1898. The mine is so massive that it's causing the nearby town to sink, forcing the entire city to relocate.

Mining Method: Sub-level caving, a form of shaft mining
Depth: Currently operating at depths of 1,365 metres
Production: Approximately 26 million tonnes of iron ore per year
Environmental Challenge: The mining has caused ground deformation, forcing the relocation of the entire town of Kiruna at a cost of over $1 billion
Adaptation: The town is being moved 3 kilometres east, with historic buildings physically relocated

This case demonstrates how large-scale subsurface mining can have significant surface impacts despite occurring underground and how social and environmental considerations must be balanced with economic benefits.

Environmental and Social Impacts

While subsurface mining generally has less visible surface impact than open-pit mining, it still presents significant environmental and social challenges that must be managed.

🌏 Environmental Impacts

Acid mine drainage from exposed minerals
Groundwater contamination and depletion
Subsidence (ground sinking)
Waste rock and tailings disposal
Energy use and carbon emissions
Habitat disruption at mine entrances

👥 Social Impacts

Worker safety concerns
Community displacement
Noise from ventilation systems
Truck traffic for ore transport
Changes to local employment patterns
Boom-bust economic cycles

📈 Mitigation Strategies

Water treatment systems
Backfilling mined areas
Groundwater monitoring
Mine reclamation planning
Community benefit agreements
Strict safety regulations

Modern Innovations in Subsurface Mining

Technology is transforming subsurface mining, making it safer, more efficient and less environmentally damaging.

🤖 Automation and Robotics

Modern mines increasingly use automated equipment:

Remote-controlled drilling machines
Autonomous loading and hauling vehicles
Robotic systems for dangerous areas
Reduces worker exposure to hazards
Improves efficiency and precision

🔍 Advanced Monitoring

New technologies help miners "see" through rock:

Seismic monitoring to detect potential cave-ins
Real-time air quality sensors
Ground-penetrating radar to map ore bodies
Digital twins (computer models) of entire mines
Reduces environmental impacts through precision

Case Study: Olympic Dam Mine, Australia

The Olympic Dam mine in South Australia demonstrates modern subsurface mining techniques for multiple minerals.

Resources: One of the world's largest deposits of copper, gold, silver and uranium
Mining Method: Sublevel open stoping, a sophisticated shaft mining technique
Depth: Operations extend to approximately 500 metres below the surface
Scale: Over 450 kilometres of underground tunnels
Water Management: Uses a groundwater monitoring system with over 400 monitoring wells to prevent contamination
Innovation: Implementing autonomous vehicles and remote operation to improve safety

Olympic Dam demonstrates how modern subsurface mining can extract multiple resources from a single deposit while implementing environmental safeguards and technological innovations.

Summary: Comparing Subsurface Mining Methods

Each subsurface mining technique has specific applications, advantages and challenges:

Method	Best For	Advantages	Challenges
Shaft Mining	Deep deposits, narrow veins	Can reach great depths, minimal surface disturbance	Expensive, complex ventilation, safety risks
Slope Mining	Hillside deposits, shallow resources	Easier access, natural drainage, lower initial cost	Limited depth, potential landslides, surface scarring
Solution Mining	Soluble minerals (salt, potash)	Low surface disturbance, fewer workers needed	Only works for soluble minerals, potential groundwater impacts
In-Situ Leaching	Uranium, copper in permeable rock	No waste rock, no open pits or tunnels	Groundwater contamination risk, limited to certain deposits

As we continue to need minerals for modern technology, subsurface mining will remain essential. The challenge is to extract these resources while minimizing environmental impacts and ensuring worker safety through improved techniques and technologies.

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