JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV MINE SUBSIDENCE ENGINEERING Previous Question Paper (July/August - 2022 - R18)

The magnitude and extent of surface subsidence resulting from underground mining are influenced by a combination of geological, mining, and environmental factors:

• Mining Method: The choice of method (e.g., caving, stowing, or room and pillar) significantly dictates the volume of the subsidence trough.
• Extraction Thickness: Subsidence is directly proportional to the thickness of the seam extracted.
• Depth of Cover: Shallower workings typically result in steeper subsidence profiles and more severe surface damage compared to deeper workings.
• Width of Extraction: The ratio of the width of the extraction area to the depth of the seam determines whether the subsidence is sub-critical, critical, or super-critical.
• Geological Conditions: The lithology, strength, and structural features (faults, joints, folds) of the overlying strata influence the caving characteristics and angle of draw.
• Presence of Water: Groundwater can affect the stability of the strata and the rate of compaction in the goaf.
• Time Factor: Subsidence is a time-dependent process; the rate of movement depends on the rheological properties of the rock mass.

The movement of strata above an underground excavation is typically categorized into three distinct zones based on the intensity of rock mass failure:

• Caved Zone: This is the immediate zone above the extracted seam where the roof strata collapse directly into the goaf. The rocks are broken into blocks, and the void space is partially filled by the fragmented rock, which increases in volume due to bulking.
• Fractured Zone: Located above the caved zone, this region experiences intense fracturing and bed separation. While the strata do not collapse completely, the rock mass loses its original integrity, and significant vertical and horizontal cracks develop, creating secondary permeability.
• Continuous Deformation Zone: This is the uppermost zone extending to the surface. The strata here undergo bending and sagging without significant fracturing. The movement is characterized by continuous vertical displacement and horizontal strain, resulting in a smooth subsidence profile on the surface.