Underground Mines

Although underground mine operations are not as visible as surface mining, their overall environmental impact can be greater than that of the typical surface mine. A key environmental problem is subsidence. Underground mines are large cavities in the rock, and depending on the strength of the intervening strata, the depth of the mine, and the type of mining and roof support, the rock walls can fail, causing cracks and land collapse at the surface. Typically, coal seams at depths greater than about 200 feet are extracted by underground mining methods rather than by surface mining, with the exact depth principally based on the relative amount of coal and overburden. However, before improved technology made surface mining so affordable, the trade-off occurred at much shallower depths; some abandoned underground mines are only 35 feet below the land surface.

In longwall mines, the subsidence is induced as mining proceeds. Most of the subsidence occurs within a few months after mining. Changes in surface elevation can be significant, affecting highways, waterways, etc., though mining beneath such fea­tures may be restricted. The extensive fractures and settling also disrupt aquifers, though deeper aquifers typically recover once the void spaces fill with water. In many European countries, where mining is centrally planned and directed, subsidence above longwall mines is typically anticipated and planned for (for example, in how buildings are constructed) years ahead of time. Elsewhere, where mining proceeds based on the decisions of mining companies and local and regional regulatory agencies, subsi­dence may be anticipated, but is rarely coordinated with other regional development activities. In con­trast, more traditional room-and-pillar mining may resist subsidence for decades, failing only as pillars, left to support the overburden rock, erode and then collapse. The lateral extent of the subsidence area may not be as great as above longwall mines, but because the collapse is more localized, there is a greater risk of differential subsidence (for example, one corner of a house subsides more than the rest of the house, severely damaging or destroying it (Fig. 4). The ability to predict the extent of subsidence has improved over the past decade, but this is still a very inexact science.

The effect of subsidence on streams and water­ways can be subtle or profound. In the worst cases, subsidence fractures reach the streambed and par­tially or totally drain the stream, diverting the water underground. Such fractures can be detected using terrain conductivity, and inexpensively sealed using an appropriate grout, but until that is done, the stream may go dry, and mining may be impeded or even temporarily halted. Even where waterways are not affected by subsidence fractures, the changes in slope will cause profiles to change, causing erosion in some areas and sediment deposition in others, and locally affecting stream biota.

Depending on the coal seam and location, AMD generation in underground mines can exceed that at surface mines. Because pyrite forms in a swampy environment, coal seams often contain more pyrite than do the overlying strata. At a surface mine, virtually all of the coal is removed, but in an underground mine, significant amounts of coal remain behind to provide roof support; if the coal is pyritic, it is problematic. In addition, alkalinity that may be present in the overburden strata is not as exposed to dissolution as it is when it is disrupted by surface mining. Finally, an underground mine is essentially a void, and behaves like a well; water flows through the surrounding rock into the mine void. When mining ceases and the water is no longer

Underground Mines

FIGURE 4 An abandoned coal refuse pile that leaches acidic drainage; many of these waste piles can be converted into a resource by burning the material in a fluidized bed combustion unit.

being pumped, the mine begins to flood and the water table rises. Thus, the volume of AMD that eventually discharges to the surface can be very great, completely overwhelming the buffering capacity of the receiving waterways. If the coal seam is completely inundated, the large mine pool will gradually improve in quality once all of the acid salts are washed away, because inundated pyrite does not continue to oxidize to a significant extent. However, coal seams typically slope (or ‘‘dip’’); this may mean that only part of the seam is underwater, and the rest is continually exposed to the atmosphere, creating an ideal acid-generating environment that can continue to produce AMD for centuries.

Updated: December 19, 2015 — 6:00 am