JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV DEEP SEAM MINING Previous Question Paper (July - 2021 - R15)

Coal deposits are classified based on their geo-mining conditions to determine the feasibility of extraction methods, safety protocols, and economic viability. The primary classifications are based on seam thickness, depth of cover, dip (inclination), and structural complexity.

1. Classification by Seam Thickness:

• Thin Seams: Generally less than 1.2 meters. These require specialized low-profile machinery and are often difficult to mechanize.
• Medium Seams: Ranging from 1.2 to 2.5 meters. This is the most common range for conventional longwall and bord-and-pillar mining.
• Thick Seams: Between 2.5 and 4.5 meters. These often require multi-slice mining or special thick-seam extraction techniques like sublevel caving.
• Very Thick Seams: Exceeding 4.5 meters. These present significant challenges regarding roof control and spontaneous heating, often necessitating hydraulic mining or caving methods.

2. Classification by Depth of Cover:

• Shallow Deposits: Typically up to 150-200 meters. These are often amenable to opencast mining or shallow underground methods where surface subsidence must be managed.
• Moderate Depth: 200 to 500 meters. These are standard for deep underground mining, where strata control and ventilation become critical.
• Deep Deposits: Exceeding 500-600 meters. These require advanced rock mechanics due to high in-situ stresses, potential for rock bursts, and significant methane management.

3. Classification by Dip (Inclination):

• Flat Seams: Dips up to 8 degrees. Ideal for standard trackless or rail-bound mechanized equipment.
• Moderately Inclined Seams: Dips between 8 and 25 degrees. Requires modifications to transport systems and face support stability.
• Steeply Inclined Seams: Dips greater than 25 degrees. These require specialized mining techniques such as gravity-assisted transport or cut-and-fill mining, as conventional face equipment cannot operate effectively.

4. Classification by Structural Complexity:

• Simple/Uniform Deposits: Continuous seams with minimal faulting or folding, allowing for high-productivity longwall panels.
• Disturbed/Complex Deposits: Seams affected by frequent faulting, igneous intrusions (dykes/sills), or folding. These conditions necessitate flexible mining layouts, often limiting the use of longwall equipment and requiring bord-and-pillar or manual methods.

The interaction of these factors determines the Geo-mining Index, which dictates the selection of mining technology, support systems, and safety measures to ensure maximum recovery and worker safety.

Deep-seated deposits are typically developed using underground mining methods, which are categorized primarily based on the support mechanism and the nature of the deposit. The main methods include:

• Room and Pillar Mining: Suitable for flat or gently dipping bedded deposits where pillars of coal or ore are left to support the roof.
• Longwall Mining: A highly mechanized method involving the extraction of a long panel of coal or ore, allowing the roof to cave behind the advancing face.
• Cut and Fill Mining: Used for steeply dipping deposits where the excavated area is backfilled with waste material to provide support.
• Block Caving: A mass mining method where a large block of ore is undercut, causing it to collapse under its own weight.

Longwall mining is the most efficient method for deep-seated, flat-bedded deposits. In this method, a rectangular panel of coal is extracted using a Power-Operated Longwall Face (POLF). The equipment consists of a shearer or plow that cuts the coal, a face conveyor that transports it, and hydraulic powered supports that protect the working area. As the shearer cuts, the supports advance, and the goaf (waste area) behind the supports is allowed to cave in a controlled manner. This method provides high production rates and improved safety by minimizing human exposure to the unsupported roof.

Schematic description: The layout consists of two main roadways (gate roads) connected by the longwall face. The face is equipped with a series of hydraulic props arranged side-by-side. The shearer travels along the face conveyor, cutting coal which is then moved to the main haulage system. Behind the face, the roof strata collapse into the goaf area, which is managed by the yielding capacity of the hydraulic supports.