JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III SURFACE MINING TECHNOLOGY Previous Question Paper (February/March - 2022 - R16)

In opencast coal mining, benches are the horizontal layers or steps created to facilitate safe and systematic extraction of coal and overburden. The formation process involves drilling and blasting the rock mass to create a working face, followed by the removal of fragmented material using excavators and dumpers.

Key parameters in bench formation include the bench height, which is determined by the reach of the loading equipment, and the bench width, which must be sufficient to accommodate drilling machinery, loading equipment, and haulage vehicles. The bench slope angle is maintained to ensure the stability of the highwall and prevent slope failure.

A typical bench layout consists of a crest (top edge), a toe (bottom edge), and a working floor. The sketch would illustrate a series of stacked benches forming a stepped profile, where each bench serves as a platform for drilling rigs and loading shovels, ensuring that the overall slope angle of the mine remains within safe geotechnical limits.

The suitability of a mining block for opencast extraction is evaluated based on several geological and surface characteristics:

• Stripping Ratio: The most critical factor is a low stripping ratio, where the volume of overburden to be removed per unit of coal is economically viable.
• Coal Seam Characteristics: Shallow depth of cover, thick and continuous coal seams, and low dip angles are ideal for large-scale mechanized opencast operations.
• Geological Structure: Minimal faulting, folding, or igneous intrusions simplify the mining process and reduce dilution. A competent roof and floor strata are preferred for stability.
• Overburden Properties: Soft or moderately hard overburden allows for easier excavation, whereas very hard rock requires extensive drilling and blasting, increasing costs.
• Topography and Hydrology: Relatively flat or gently undulating terrain is preferred to minimize drainage issues and facilitate easier haul road construction. Low groundwater inflow is essential to reduce pumping costs.
• Surface Constraints: The absence of major surface infrastructure, water bodies, or dense forest cover within the block significantly reduces environmental and rehabilitation costs, making the site more suitable for open-pit methods.