Underground Coal Mining: Roof Support, Subsidence Control, and Strata Monitoring

Posted on Aug 20, 2025 in Civil Engineering, Transportation, and Urban Services

Power Support in Longwall Mining

This section details different types of power support used in longwall mining and illustrates the structure and working of chock support with an example.

Types of Power Support Used in Longwall Mining:

  1. Chock Supports (Hydraulic Powered Roof Supports – HPRS): Most commonly used, these are heavy-duty, yieldable supports with multiple hydraulic legs.
  2. Shield Supports: Modern variants of chock supports with canopies and base units; they offer better coverage and control.
  3. Chock-Shield Supports: A hybrid between chocks and shields, combining strength and flexibility.
  4. Cantilever Supports: Provide support with an overhanging canopy and are used in special conditions.
  5. Walking Chocks: Mobile supports that can walk forward as mining progresses.
  6. Articulated or Hinged Supports: Used in highly irregular seams or where strata movement is excessive.

Structure and Working of Chock Support:

A chock support consists of a canopy (roof contact), a base (floor contact), and 2-4 hydraulic legs placed between them. These legs are pressurized using hydraulic fluid to exert a strong upward force to support the roof.

Working Principle:

  1. The chock is placed behind the longwall shearer.
  2. As the shearer cuts coal, the chock legs are pressurized to support the roof.
  3. Once the coal is cut and removed, the chocks are depressurized (lowered), moved forward (walking), and re-pressurized.

Example:

In the Longwall Horizon Mining Project, four-leg chock supports with 1000-1500 tons of capacity are used. They ensure roof stability and miner safety in deep, gassy seams.

Controlling Surface Subsidence in Board and Pillar Mining

This section suggests suitable steps to control surface subsidence during board and pillar depillering with the caving method.

Steps to Control Subsidence in Board and Pillar Depillering:

  1. Proper Panel Design: Panels should be designed considering depth, seam thickness, and overburden strength to minimize subsidence impact.
  2. Systematic Extraction: Follow a systematic pillar extraction pattern (e.g., line or split and fender) to control caving progression and reduce sudden collapses.
  3. Controlled Blasting Techniques: Use controlled or cushion blasting to weaken the roof in a gradual and managed way, reducing violent falls.
  4. Stowing Where Necessary: In sensitive areas (near surface structures), use sand or fly-ash stowing in place of full caving to fill voids and reduce surface movement.
  5. Leaving Safety Barriers: Leave unmined barrier pillars around important structures, water bodies, or faults to prevent stress transfer and surface disturbance.
  6. Monitoring and Prediction: Use subsidence monitoring systems and predictive models to track and forecast ground movement, enabling timely preventive action.
  7. Grouting of Cracks: If cracks appear at the surface, grouting with cement or fly ash can help stabilize and prevent further subsidence.

Critical Subsidence and Influencing Factors

This section explains the significance of critical subsidence in relation to surface deformation and analyzes the factors that influence subsidence in underground coal mining.

Significance of Critical Subsidence:

Critical subsidence refers to the maximum or peak vertical displacement that occurs on the surface due to underground coal extraction. It represents the worst-case surface deformation and is essential for:

  • Predicting Surface Damage: Helps in assessing the potential impact on buildings, roads, and infrastructure.
  • Designing Safe Mining Operations: Determines the extent and pattern of caving and surface movement, allowing better planning of pillar sizes and extraction rates.
  • Environmental Management: Guides protective measures in areas with water bodies, forests, or agricultural land.
  • Regulatory Compliance: Ensures mining operations stay within permissible subsidence limits set by authorities.

Factors Influencing Subsidence:

  1. Depth of Mining: Shallower depths lead to more pronounced and rapid subsidence; deeper mines cause gradual and less visible effects.
  2. Extraction Ratio: Higher percentage of coal extraction increases void space, leading to greater subsidence.
  3. Pillar Design and Size: Smaller or closely spaced pillars may delay or reduce subsidence, while full extraction increases it.
  4. Overburden Characteristics: Weak and fractured overburden caves easily, increasing subsidence; strong, massive rock resists movement.
  5. Mining Method: Methods like longwall mining cause uniform subsidence, whereas board and pillar may lead to irregular or delayed effects.
  6. Time Factor: Subsidence may occur immediately or over a long period, depending on overburden movement and caving behavior.
  7. Water Ingress and Moisture: Presence of groundwater can weaken overburden and accelerate subsidence.

Strata Monitoring Instruments and Anchorage Testing

This section differentiates between two commonly used strata monitoring instruments and demonstrates the process of anchorage testing.

Comparing Strata Monitoring Instruments:

1. Extensometer:

  • Function: Measures the displacement or deformation between different layers of rock or strata.
  • Usage: Detects ground movement to assess strata stability, especially in underground mining and tunneling.
  • Type: Can be mechanical, electrical, or vibrating wire type.
  • Output: Provides precise measurements of vertical or horizontal strata movement.

2. Load Cell:

  • Function: Measures the load or force exerted on rock bolts or support structures.
  • Usage: Monitors the load-bearing capacity of support systems and helps assess their effectiveness.
  • Type: Hydraulic or electrical (strain gauge-based).
  • Output: Gives real-time data on support stress levels.

Process of Anchorage Testing (Pull-Out Test):

  1. Drill a Hole: A hole is drilled into the rock to insert the rock bolt.
  2. Insert and Grout Bolt: The bolt is inserted and fixed using resin or grout.
  3. Attach Testing Device: A hydraulic jack or pull tester is attached to the exposed bolt end.
  4. Apply Load: A controlled load is applied incrementally using the jack.
  5. Record Displacement: The movement of the bolt (if any) is measured at each load level.
  6. Assess Results: Compare the bolt’s displacement and load capacity with safety standards to determine anchorage quality.