Underground Mining

Underground mining is used for deep seams, and the mining methods vary according to the site conditions. Underground mines can be classified by the types of access used to reach the coal (i.e., shaft mines, slope mines, and drift mines) but are primarily classified by the coal removal system: room and pillar mining, pitch mining, or longwall mining [3]. A shaft mine uses a vertical hole dug straight down from the surface to the coal seam. A slope mine provides access at a slant and is used to follow a seam along its pitch or to cut through a mountain to reach the coal. A drift mine accesses coal outcrops on a mountainside. Room and pillar mining is used when the coal seam lies relatively level, and it is carried out by using one of two processes: conventional mining or continuous mining [3]. In 2002, over four million short tons of the coal produced in underground mines were obtained by conventional mining, but it is being replaced by other mining methods [2]. In conventional mining, explosives are used to shatter the face of the seam, and the broken pieces are manually loaded onto tram cars or conveyors and hauled out of the mine. In continuous mining, used to produce more than 175 million short tons of coal in 2002 [2], a machine that moves along on caterpillar tracks cuts the coal from the face of the seam and automatically loads the coal onto tram cars or a conveyor. Pitch mining is a technique used when the coal seams are inclined and is frequently used in anthracite mining [4]. In pitch mining, the bottom of the seam is accessed, and the coal is dropped into chutes that are gravity-fed into tram cars. Longwall mining is the removal of coal from one, long continuous face rather than removal from a number of short faces as occurs in room and pillar or pitch mining. Annual longwall mining production is similar to that of continuous mining and accounted for nearly 190 million short tons of coal produced in the United States in 2002 [2]. Regardless of the mining system used, the health and environmental impacts are common to every underground mine: land subsidence, generation of methane and other gases, liquid effluents, dust, solid waste, and miner safety.

Subsidence

Subsidence can have a major effect on the topography of the land surface. Following the removal of the coal from an underground mine, the roof materials may cave, causing collapse of the overlying rock strata and resulting in subsidence of the surface. The degree of collapse of the overlying rock strata can vary from practically no collapse with no resulting surface impacts to total collapse with more pronounced changes at the surface [5]. In general, mine subsidence problems develop where post-mining pillar support systems and coal barriers ultimately fail. Many interrelated factors control when, where, and how failure will occur, including [1]:

• Thickness of coal removed;

• Size, shape, and distribution of pillars and rooms;

• Percent extraction of coal;

• Thickness and physical characteristics (e.g., strength) of the overburden;

• Method of mining, such as longwall, shortwall (which is a slight modification of longwall mining), room and pillar, room and pillar with full or partial retreat;

• Actual or potential level and degree of fracturing in the overburden;

• Mineralogy of the overburden (e.g., clay minerals that swell when water is added, sulfide minerals that chemically and physically change in the presence of oxygen and moisture, minerals that react with water to form new minerals).

Over areas that have been longwall mined, the subsidence is often a shallow trough. In flat terrain, this trough is usually quite visible and can cause local changes in drainage. Subsidence from active coal mining has the largest effect on the land surface in terms of area undermined, although the effects are often small in terms of overall topography [5]. Subsidence impacts on surface structures and subsurface hydrologic resources are generally of more importance than the impacts on topography or surface features.

Subsidence from shallow, abandoned coal mines often results in abrupt but localized changes in topography that reflect the collapse of individual rooms or voids. This type of subsidence can be an isolated, single collapse or can involve a larger area with many individual subsidence pits.

Generation of Gases

Methane (CH4) is produced during coalification, and only a fraction of this gas remains trapped under pressure in the coal seam and surrounding rock strata. This trapped methane is released during mining when the coal seam is fractured. The amount of methane released during coal mining depends on a number of factors, including coal rank, coal seam depth, and method of mining [6]. As coal rank increases, the amount of methane produced also increases. The adsorption capacity of coal increases with pressure, and pressure increases with depth of the coal seam; consequently, deeper coal generally seems to contain more methane than shallow seams of the same rank. Underground coal mining releases more methane than surface mining because of the higher gas content of deeper seams. The methane that is contained in the coal seams is referred to as coalbed methane (CBM). The CBM that is released from the coal during coal mining is referred to as coal mine methane (CMM) and is a subset of CBM.

Methane is highly explosive in air in concentrations between 5 and 15%, and operators have developed two types of systems for removing methane from underground mines: ventilation systems and degasification systems [6]. At present, almost all ventilation air is emitted into the atmosphere. Although the methane concentrations are low, the amount of methane released into the atmosphere each year is significant. Emissions factors for underground mining range from 10 to 25 cubic meters emitted per ton of coal mined, compared to 0.3 to 2 cubic meters per ton of coal that is surface mined. As a greenhouse gas, methane is more than 21 times as potent as carbon dioxide (CO2). Table 3-1 lists global estimates of methane emissions from coal mining in 1990 along with coal production that year [7].

Some of the gassiest mines in the United States (as well as Russia, Australia, and other countries) have installed desgasification systems to extract the methane from the coal seams in advance of mining, during coal recovery, and during post-mining operations [6]. Depending on the quality of this gas, mine operators can use recovered methane for on-site electricity generation or can sell it to a local pipeline.

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