Lecture: Mining Induced Surface Subsidence

Subsidence happens when the ground sinks due to underground mining.
Almost all underground mining causes some amount of subsidence.
The ground can shift because of:
Changes in pressure from digging underground.
Activities like removing water from the mine.
Subsidence can be regarded as being of two types –
Continuous
Discontinuous.
Continuous or trough subsidence creates a smooth, gradual sinking of the ground without sudden drops or steps (figure-1).
The amount of surface movement is usually small compared to the size of the mined area or the depth of mining.
This type of subsidence happens when mining thin, flat-lying orebodies covered by weak, flexible rock layers.
It is most common in longwall coal mining but also occurs when extracting minerals like sulfur and evaporites (such as salt and gypsum) found in sedimentary environment.
Pillar collapse in old, shallow mines can cause surface subsidence similar to crown holes. This happens when pillars weaken over time or when extra weight from buildings or construction increases pressure on them.
In active mines, large-scale pillar collapse can lead to widespread discontinuous subsidence, causing severe damage over a bigger area.
A major disaster occurred on January 21, 1960, at Coalbrook North Colliery, South Africa, when a 3 km² room-and-pillar mining area collapsed, killing 437 people.

1. Lecture#01 Conducted by: Mohammad Mamun Ur Rashid Assistant Professor Department of Petroleum and Mining Engineering (PME) [email protected] Thursday, January 23, 2025 www.cuet.ac.bd PME 261 Rock Mechanics L-2,T-I (‘22 Batch) Chittagong University of Engineering and Technology

2. Why We Support Mines? • Keeping miners safe (safety factor) • Ensure continuity of ore production to avoid shortage in profit • Keeping expensive machines in underground mines safe (e.g. machines used in long wall mining)

3. When We Need to Support Mines? • If there are weak or unconsolidated parts of rocks in the roof and walls • If there are geologic structures (e.g., shear zones) weakening rocks • If there are voids and spaces in rocks • If there are clay beds that may shrink and expand • If underground water leads to chemical weathering of rocks and corrosion of machines

4. Factors Controlling Choice of Suitable Mine Support • Availability of support material in the local market • Costs of support materials • Distance to factories (e.g., brick, cement, steel) • Nature of the problem to be solved in the mine • Durability of the mine

5. Types of Mine Support • Wooden logs • Metal rods • Metal pins or roof bolts • Metal fence (net) • Concrete support and using brick walls • Resin support

6. Wooden Logs • Straight and about 4-6 m long • Evergreen trees (e.g., pine trees) • Iron wedges must be used to fix the logs Advantages: • Cheap even when imported • Can be cut to desired length • Gives early warning when it swallows Disadvantages: • Cannot be reused • Needs coating to avoid corrosion

7. Metal Rods Support • Metal rod with two cylinders of different diameters • Length can be controlled Advantages: • Can be reused • Gives high support capacity Disadvantages: • Expensive • Does not give early warning • Corrosion risk due to moisture

8. Metal Fence Support • Used for ceilings and walls in case of rock fall • Metal pins required to fix the fence Advantages: • Gives early warning in case of collapse • Collects falling rocks • Reasonable support capacity Disadvantages: • Expensive • Corrosion risk

9. Concrete Support • Used for friable and brittle bedrocks • Concrete can bear more stress than bricks Advantages: • Perfect support capacity Disadvantages: • Relatively expensive • Difficult to modify or remove

10. Resin Support • Used for friable and porous ceilings and walls • Organic polymers used for injection Advantages: • Ideal for friable and porous bedrocks • Injection is easy and not harmful Disadvantages: • Relatively expensive