JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV PLANNING OF UNDERGROUND COAL MINING PROJECT Previous Question Paper (March - 2021 - R16)

Mine planning is a systematic process of designing an underground coal mine to ensure safety, economic viability, and resource conservation. The key factors considered in this process include:

• Geological Conditions: Detailed assessment of coal seam thickness, depth, dip, faulting, and roof/floor rock characteristics is essential to select the appropriate mining method.
• Reserve Estimation and Quality: The quantity of mineable reserves and the quality of coal (ash content, moisture, calorific value) determine the project's life and marketability.
• Mining Method Selection: Based on seam geometry and geomechanical properties, a choice must be made between bord and pillar, longwall, or other specialized methods to maximize recovery.
• Geotechnical and Hydrogeological Factors: Analysis of in-situ stress, rock mass rating (RMR), and groundwater inflow rates is critical for designing support systems and dewatering strategies.
• Production Capacity and Equipment: Determining the optimal output rate and selecting compatible machinery (e.g., continuous miners, shearers) to meet production targets while maintaining efficiency.
• Ventilation and Safety: Designing a robust ventilation circuit to manage methane, dust, and heat, alongside emergency egress and fire protection protocols.
• Infrastructure and Logistics: Planning for surface facilities, coal handling plants (CHP), power supply, and transport networks to move coal from the face to the consumer.
• Economic and Financial Factors: Capital expenditure (CAPEX), operational expenditure (OPEX), return on investment (ROI), and market price projections are vital for project feasibility.
• Environmental and Statutory Compliance: Adherence to land acquisition laws, environmental clearance regulations, and safety standards mandated by the Directorate General of Mines Safety (DGMS).
• Social and Community Impact: Addressing land rehabilitation, local employment, and the socio-economic development of the surrounding region.

In the Bord and Pillar method, pillars are rectangular or square blocks of coal left in situ to support the overlying strata. The design of these pillars is critical for maintaining stability, preventing pillar crushing, and avoiding surface subsidence.

The design process involves calculating the Pillar Strength and the Pillar Stress (load) to ensure a stable Factor of Safety.

1. Determination of Pillar Stress: The load on a pillar is calculated using the Tributary Area Method. The stress on a pillar (σp) is given by:
   σp = γH * (B + w)^2 / B^2
   where γ is the unit weight of the overburden, H is the depth of cover, B is the pillar width, and w is the width of the gallery.
2. Determination of Pillar Strength: Pillar strength (Sp) is determined using empirical formulas such as the Salamon-Munro formula:
   Sp = k * (w^α / h^β)
   where k is the strength of coal, w is the pillar width, h is the pillar height, and α and β are constants (typically 0.46 and 0.66 respectively).
3. Factor of Safety (FoS): The pillar design is considered safe if the ratio of pillar strength to pillar stress is greater than 1.5 to 2.0.
   FoS = Sp / σp

A neat sketch would illustrate a grid pattern of galleries (bords) intersecting at right angles, creating pillars of uniform size. The sketch should label the 'Pillar' as the coal block, the 'Gallery' as the excavated path, and indicate the 'Pillar Width' (center-to-center distance minus gallery width) and 'Gallery Width'.

Key design considerations include the depth of the seam, the compressive strength of the coal, the thickness of the seam, and the nature of the roof and floor strata. If the pillars are too small, they may undergo 'pillar crushing' or 'creep', leading to instability; if they are too large, coal recovery is significantly reduced.