JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year STRATA CONTROL TECHNOLOGY Previous Question Paper (October/November - 2020 - R16)

Strata control problems in coal mines are significantly influenced by geological conditions. Key factors include:

• Lithology and Rock Strength: The mechanical properties of the roof and floor strata, such as compressive and tensile strength, dictate the stability of excavations. Weak shales or mudstones are prone to rapid deterioration compared to competent sandstones.
• Geological Discontinuities: The presence of faults, joints, bedding planes, and cleats creates planes of weakness. These discontinuities reduce the effective strength of the rock mass, facilitating roof falls and rib spalling.
• In-situ Stress Field: High horizontal stresses, often magnified by tectonic activity or depth of cover, can lead to stress-induced failure, such as floor heave or pillar bursting.
• Groundwater Conditions: The presence of water reduces the effective stress and weakens clay-rich strata, leading to swelling, softening, and loss of cohesion in the roof and floor.
• Depth of Cover: Increasing depth directly increases the vertical stress (overburden pressure), which escalates the risk of strata control issues.
• Presence of Weak Intercalations: Thin layers of soft material (e.g., carbonaceous shale) within a stronger sequence can act as failure initiation zones, leading to roof separation.

To calculate the Rock Mass Quality Index (Q), we use the formula:

First, calculate the average RQD from the given borehole values:

Given parameters:

• RQD = 96.5
• Jn = 9
• Jr = 1.5
• Ja = 2.0
• Jw = 1.0
• SRF = 2.5

Substituting these values into the Q formula:

The calculated Rock mass quality index (Q) is approximately 3.22.