Mining method

Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or (coal) seam. The term also includes the removal of soil. Mining of stone and metal has been done since pre-historic times. Modern mining processes involve prospecting for ore bodies, analysis of the profit potential of a proposed mine, extraction of the desired materials and finally reclamation of the land to prepare it for other uses once the mine is closed.

There is a wide variety of methods of extraction which one can choose from as the orebodies have shapes and sizes. The orientation and shape of an orebody, the strength of the ore and the surrounding rock and the form in which the valuable materials are distributed are different for each ore area. These factors among others are what will influence the selection of a mining method and the overall plan for developing the orebody.

Mining techniques can be divided into two common excavation types: surface mining and sub-surface (underground) mining. Surface mining is much more common, and produces, for example, 85% of minerals (excluding petroleum and natural gas) in the United States, including 98% of metallic ores. Underground mining refers to various underground mining techniques used to excavate hard minerals, mainly those containing metals such as ore containing gold, silver, iron, copper and lead, but also involves using the same techniques for excavating ores of gems such as diamonds.

As large open pits reach increasingly greater depths and more frequently involve interaction with underground mines, numerical modelling provides a useful tool to analyse important issues related to both crown pillar and pit slope stability. This section presents examples of a hybrid modelling approach investigating the geotechnical aspects of the interaction between open pit and underground block caving mining. A conceptual model was used in the current study. Further details and material parameters are given in Elmo et al. (2007). The initial scope of the modelling was to characterise the potential effects of block caving mining on the stability of the pit slopes. Simulated horizontal and vertical displacements of the pit walls were analysed as a function of numerical time. Figure 5 shows the potential impact of block caving mining on existing open pit operations.

Underground mining in the world are typically operated using one of two different methods: room and pillar mining or longwall mining. In room and pillar mining, rooms are cut into the coal bed leaving a series of pillars, or columns of coal, to help support the mine roof and control the flow of air. Continuous mining equipment is used to cut the coal from the mining face. Generally, openings are driven 20 feet wide and the pillars are generally rectangular in shape. As mining advances, a grid-like pattern of entries and pillars is formed. Shuttle cars are used to transport coal to the conveyor belt for transport to the surface. When mining advances to the end of a panel, retreat mining may begin. In retreat mining, as much coal as is feasible is mined from the pillars that were created in advancing the panel, allowing the roof to cave. When retreat mining is completed to the mouth of the panel, the mined panel is abandoned. The room and pillar method is often used to mine smaller coal blocks or thin seams, and seam recovery ranges from 35% to 70%, with higher seam recovery rates applicable where retreat mining is combined with room and pillar mining.

Surface mining methods include area, contour, highwall and mountaintop removal. Area mines are surface mines that remove shallow coal over a broad area where the land is fairly flat. After the coal has been removed, the overburden is placed back into the pit. Contour mines are surface mines that mine coal in steep, hilly or mountainous terrain. A wedge of overburden is removed along the coal outcrop on the side of a hill, forming a bench at the level of the coal. After the coal is removed, the overburden is placed back on the bench to return the hill to its natural slope. Highwall mining is a form of mining in which a remotely controlled continuous miner extracts coal and conveys it via augers, belt or chain conveyors to the outside. The cut is typically a rectangular, horizontal cut from a highwall bench, reaching depths of several hundred feet or deeper. A highwall is the unexcavated face of exposed overburden and coal in a surface mine. Mountaintop removal mines are special area mines used where several thick coal seams occur near the top of a mountain. Large quantities of overburden are removed from the top of the mountains, and this material is used to fill in valleys next to the mine.