JNTU B.Tech IV MINE GROUND CONTROL Previous Question Paper (September 2020 - R15)

Strata mechanics in Longwall Mining involves the systematic control and redistribution of stresses around the longwall face as the coal seam is extracted. The process is characterized by the formation of distinct zones of strata movement and the interaction between the powered roof supports and the surrounding rock mass.

The mechanics of strata behavior can be categorized into the following key zones:

1. Immediate Roof: The strata directly above the coal seam that collapses into the goaf (gob) immediately after the powered supports are advanced. The thickness and strength of these beds determine the caving characteristics and the load on the supports.
2. Main Roof: The more competent, thicker strata layers above the immediate roof. These layers undergo periodic weighting as they cantilever over the goaf. When the span exceeds the critical length, the main roof fails, causing sudden increases in pressure on the face supports, known as periodic weighting.
3. Abutment Pressures: As coal is extracted, the vertical stress is redistributed to the solid coal ahead of the face (front abutment) and to the sides (side abutments). The front abutment pressure is critical as it influences the stability of the face and the coal seam ahead of the shearer.
4. Goaf/Gob Area: The area behind the face where the roof has caved. The compaction of this broken rock eventually provides passive support to the main roof, reducing the convergence of the overlying strata.

The interaction between these zones is managed by Powered Roof Supports (PRS), which are designed to maintain a safe working environment by providing active resistance to the immediate roof. The support capacity must be sufficient to prevent excessive convergence and to control the caving process, ensuring that the main roof breaks behind the support line rather than over the face area. Effective strata control relies on matching the support resistance to the geological conditions, specifically the caving behavior of the immediate roof and the periodic weighting interval of the main roof.

Ground control practices in mines are systematic methods employed to maintain the stability of underground excavations and surface slopes by managing the stresses induced by mining activities. These practices are essential to prevent rockfalls, strata control failures, and surface subsidence.

The primary ground control practices include:

• Rock Reinforcement: The use of mechanical or chemical means to strengthen the rock mass. This includes rock bolting (tensioned or untensioned), cable bolting for deep-seated reinforcement, and the use of dowels or friction stabilizers like split sets to bind rock layers together.
• Support Systems: Active and passive support structures installed to carry the load of the immediate roof or sidewalls. This involves the use of steel arches, rigid or yielding props, hydraulic props, and powered roof supports in longwall mining.
• Surface Support: Techniques to prevent the unraveling of fractured rock between support elements. This includes the application of shotcrete (sprayed concrete), wire mesh, steel straps, and fiber-reinforced linings.
• Pillar Design and Extraction: Strategic planning of pillar sizes and layouts to ensure global stability. Practices include leaving barrier pillars, yield pillars, or adopting systematic caving methods where the roof is allowed to collapse in a controlled manner behind the working face.
• Backfilling: The process of filling mined-out voids with waste rock, tailings, or hydraulic sand fill. This provides regional support, reduces surface subsidence, and helps in managing the stress distribution across the mine workings.
• Monitoring and Instrumentation: The use of sensors to track rock mass behavior. This includes convergence meters to measure wall movement, load cells to monitor support pressure, and micro-seismic monitoring to detect impending rock bursts or major structural failures.
• Stress Management: Planning the sequence of extraction to minimize stress concentrations. This involves techniques such as destressing, where high-stress zones are artificially relieved, or adopting specific retreat mining sequences to avoid leaving isolated, high-stress pillars.

Effective ground control requires an integrated approach combining geological assessment, numerical modeling, and real-time monitoring to adapt support strategies to the specific geotechnical conditions of the mine.