JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III UNDERGROUND COAL MINING TECHNOLOGY Previous Question Paper (February - 2022 - R13)

A shaft pillar is a solid block of coal left unmined around the shaft bottom to provide structural support and protect the shaft, winding equipment, and surface infrastructure from the effects of ground movement and subsidence caused by underground mining operations. It ensures the stability of the shaft lining and prevents damage to the hoisting system.

The size of the shaft pillar is determined using several empirical, analytical, and numerical methods:

1. Empirical Methods: These rely on historical data and rules of thumb. A common approach is the Angle of Draw method, where the pillar boundary is defined by projecting a line from the shaft at a specific angle (usually 35 to 45 degrees) from the vertical, extending to the surface or the seam level to encompass the zone of influence.
2. Rule of Thumb Formulas: Various formulas based on shaft depth (D) are used. For example, a common rule is that the radius of the pillar should be at least 0.35 times the depth of the shaft, or a minimum of 30 to 50 meters, depending on the strata conditions and seam thickness.
3. Analytical Methods: These involve stress analysis calculations. The pillar must be large enough to ensure that the vertical stress induced by the weight of the overburden and the redistributed stresses from mining do not exceed the compressive strength of the coal seam, considering a suitable Factor of Safety (typically 2.0 to 3.0 for critical infrastructure).
4. Numerical Modeling: Modern methods use software (such as Finite Element Method or Boundary Element Method) to simulate the stress distribution and displacement around the shaft. This allows for a site-specific assessment by modeling the geological properties, seam thickness, and mining sequence to optimize pillar size while ensuring safety.
5. Statutory Requirements: In many jurisdictions, mining regulations specify minimum pillar dimensions based on the depth of the shaft and the nature of the overlying strata to ensure legal compliance and safety standards.