Jawaharlal Nehru Technological University Hyderabad B.Tech IV MINE GROUND CONTROL Previous Question Paper (July - 2021 - R15)

The ground forces acting on a block of in-situ rock are primarily the vertical stress due to the weight of the overlying strata (overburden pressure) and the horizontal stress resulting from tectonic forces or lateral confinement. Vertical stress is calculated as the product of the depth of the rock mass, the average unit weight of the overlying strata, and the gravitational constant. Horizontal stress is often a function of the Poisson's ratio of the rock and the vertical stress, though it is frequently modified by tectonic history.

The influence of these forces is significant in mine design: vertical stress determines the load-bearing requirements for pillars and supports, while horizontal stress often dictates the stability of roof strata, particularly in bedded deposits where it can induce buckling or shear failure. Together, these forces define the in-situ stress field, which dictates the magnitude and orientation of stress concentrations around underground excavations, directly influencing the choice of support systems and the potential for rock bursts or strata instability.

Coal measure roof strata typically consist of a sequence of sedimentary rocks, primarily shales, mudstones, siltstones, and sandstones. Their characteristics are defined by several key factors:

• Lithological Variation: The roof is often heterogeneous, consisting of alternating thin beds of varying strength and stiffness. Sandstones are generally stronger and more competent, while shales and mudstones are weaker and prone to weathering.
• Bedding Planes and Laminations: These act as planes of weakness. The presence of frequent bedding planes reduces the effective thickness of the roof beam, making it susceptible to delamination and bed separation under stress.
• Jointing and Fracturing: Pre-existing geological discontinuities, such as joints, faults, and slips, significantly reduce the structural integrity of the roof, often leading to block falls.
• Moisture Sensitivity: Many coal measure shales and mudstones are moisture-sensitive; they tend to swell or soften when exposed to mine air or water, leading to rapid deterioration of the roof surface.
• Anisotropy: The mechanical properties of the roof strata are often direction-dependent, with different strengths measured parallel and perpendicular to the bedding planes.

Understanding these characteristics is essential for classifying the roof (e.g., immediate roof vs. main roof) and determining the appropriate support density and bolting patterns required to maintain stability.