37. Underground mining

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Contents

1. Scope

2. Environmental impacts and protective measures

2.1 Environmental impacts on the deposit and the surrounding rock

2.1.1 Exploitation of resources
2.1.2 Disruption of rock structure
2.1.3 Disruption of groundwater flow
2.1.4 Alteration of groundwater quality

2.2 Underground environmental impacts

2.2.1 Air / climate
2.2.2 Noise
2.2.3 Dust
2.2.4 Mine waters

2.3 Aboveground environmental impacts

2.3.1 Air / climate
2.3.2 Water
2.3.3 Subsidence
2.3.4 Dumps, land consumption, landscape

2.4 Other consequences of underground mining

3. Notes on the analysis and evaluation of environmental impacts

3.1 Air / climate
3.2 Noise
3.3 Dust
3.4 Water
3.5 Soil

4. Interaction with other sectors

5. Summary assessment of environmental relevance

6. References

1. Scope

Mining is defined as the extraction of mineral resources from the earth. Underground mining is the extraction of raw materials below the earth's surface (deep mining) and their conveyance to the surface. Access to the vein or lode is by shafts and tunnels with links to the surface. (The subsequent stages of raw material processing are dealt with in a separate brief: Minerals - Handling and Processing.) The present brief examines only the underground extraction of solid mineral resources.

There are some 70 individual types of useful minerals that occur in minable concentrations either alone or in combination with other minerals, frequently as natural mixtures (aggregates).

Underground mining includes all work involved in the winning of raw materials by people using technical contrivances. Apart from the actual extraction and conveyance processes, the term underground mining also covers development of the deposit and provision of the requisite infrastructure (transportation/handling, storage facilities, surface plant, e.g., administration building, workshops, etc.) and all measures devoted to ensuring the safety of the miners. This includes:

Small-scale mining activities in many countries frequently include a transitional form of extraction referred to as trench mining, or burrowing.

In special cases, the mineral can be made transportable and hauled off from its natural surroundings with no need of exploratory work (brine mining, in-situ leaching and in-situ gasification of coal).

Deep mining creates underground spaces in which people work. Their working conditions with regard to air temperature and humidity, presence of harmful or explosive gases or radiation, as well as moisture, dust and noise, can be specific to the mined mineral and/or the surrounding rock, the depth of the mine, and the type of machinery in use.

The locations of deep mines are dictated by the presence of potentially profitable raw materials. Underground extraction is practiced in all climate zones, in remote areas as well as under large cities, on the ocean floor and in alpine regions. The size, or output, of such mines ranges from less than 1 to more than 15 000 tons a day, and the depth at which extraction takes place ranges from a few meters to more than 4 kilometers.

2. Environmental impacts and protective measures

Deep mining impacts the environment in three different areas: in the deposit itself and the surrounding rock, in the underground spaces created by and for the mine, and aboveground. Optimal exploitation of the resource with attendant limitation of environmental effects is dependent on detailed planning of the sequence of operations and on the mining methods and technology to be employed.

2.1 Environmental impacts on the deposit and the surrounding rock

2.1.1 Exploitation of resources

The most important environmental consequence of underground mining is that it involves the exploitation of a nonrenewable resource. The process of extracting the raw material necessarily also involves mining losses and impairment of other parts of the deposit. The best way to counter the latter effects is to carefully plan the extraction operations, stowing measures, etc.

Some raw materials (coal and several sulfidic ores) can under certain circumstances ignite spontaneously and cause mine fires.

2.1.2 Disruption of rock structure

The opening up of underground workings creates cavities and leads to stress and motion in the surrounding rocks. The effects of mining on the rock structure can include:

– subsidence due to cave-ins in the cavities. The resultant settling can propagate to the surface, possibly causing damage to structures and facilities (subsidence damage; cf. section 2.3.3 for protective measures);
– destruction of hanging parts of the deposit (most likely as a result of inadequate extraction planning).

2.1.3 Disruption of groundwater flow

The opening up of underground workings modifies the formerly stable water balance of the rock structure by creating new water conduits. Water drainage, for example, can cause significant recession of the groundwater level with substantial attendant detriment to vegetation within the affected area (cf. section 2.3.2).

2.1.4 Alteration of groundwater quality

Mining activities can pollute groundwater in several ways: mine waters (cf. item 2.2.4), for example, can enter the groundwater system, and various alkaline and other solutions used in in-situ dressing processes, as well as leakage losses of refrigerants used in the sinking of shafts, all can contaminate the groundwater, just as the leaching of dumps produces percolating water that can alter the character of groundwater. Effective preventive measures include the sealing off of soils, shafts and worked-out parts of the deposit, drainage and/or canalization.

2.2 Underground environmental impacts

Man, machine, rock and climate all interact underground, whereas man is impacted most significantly. Matters concerning the health and safety of miners are therefore given priority consideration.

2.2.1 Air / climate

The underground climate is influenced by the elevated temperature of deep rock and by the gases and liquids it contains.

Table 1 - Factors influencing the atmosphere in underground mines

2.2.2 Noise

In underground workings, noise is generated by drilling and blasting, by internal-combustion engines and pneumatic and hydraulic motors, and by various means of conveyance (conveyor belts, trains, vehicles) and fans.

Machine-generated noise can be reduced by various design measures, and ear protectors are mandatory beginning at certain sound intensity levels.

2.2.3 Dust

Exposure to dust (stone dust in coal mines, for example) must be limited to minimize the incidence of related diseases, the most dangerous of which is silicosis resulting from the inhalation of silica particles. Dust forms when rock is destroyed by mechanical means (drilling, blasting, crushing, handling, etc.).

Dust consisting of the following mineral substances poses a hazard to human health: asbestos, beryllium, fluorspar, nickel ores, quartz, mercury, cinnabar, titanium dioxide, manganese oxide, uranium compounds and tin ores. Pulverized asbestos and respirable dust containing nickel ore and/or beryllium, as well as soot from diesel engines, are carcinogenic. Coal dust can cause dust explosions.

Countermeasures against dust pollution include its consolidation during drilling and conveying, either by spraying it with water or by saturating the face through appropriately arranged boreholes prior to extraction. Gas masks prevent the inhalation of dust, and filters on engines bond soot particles.

2.2.4 Mine waters

Mining activities alter the characteristics of mine waters.

Appropriate safety clothing protects miners against aggressive mine waters, and appropriately resistant materials prevent corrosion of material goods.

Table 2 - Pollution of mine and surface waters

2.3 Aboveground environmental impacts

The aboveground environmental consequences derive from communication between the mine and the surface in the form of ventilation, mine pumping and conveyance of the product, in combination with establishment of the requisite aboveground mining infrastructure. Vibrations caused by blasting and ground movement are also perceptible aboveground.

2.3.1 Air / climate

The harmful effects of air pollution, particularly on nearby vegetation can be alleviated by filtering the outgoing air from the shafts and tunnel faces. Dumping and wind-induced erosion of dumps can cause substantial air pollution, most notably in the form of dust.

Dust evolution can be controlled by appropriate sprinkling in connection with dumping and by immediate greenbelting, oversowing and protective dams. In arid regions where land planting is hardly possible, preventive measures must be taken in the form of restricted use in the prevailing wind direction.

Coal mining releases large quantities of methane (CH₄), one of the most notorious "greenhouse gases". The best way to control methane is to "drill and extract" (with subsequent utilization). Particulate solids in the vitiated air from underground mines can be extensively eliminated by filtration.

2.3.2 Water

The pH of mine waters, particularly in the presence of sulfidic ores, can range below 5.5 (acidic). Adherence to the limits prescribed for sulfates, chlorides and metals is essential.

If the groundwater is being used as drinking water and ore is being discharged into a body of surface water, the relevant values must be monitored. It is important to know which anions and cations can occur in mine water and which of them constitute potential hazards on the basis of their concentration or toxicity.

It is also important to mention that heaps of material extracted from an underground mine are liable to contain high concentrations of chlorides and sulfates and that, in a humid climate, such salts can be leached out by precipitation.

Whenever minewater is discharged into a body of surface water, care must be taken to avoid damaging any sensitive ecosystems and to ensure that no long-term accumulation of pollutants occurs in the sediment and that overall use of the water in question, e.g., for fishing purposes, is not impaired.

Marine pollution and alteration of the ocean floor or fishing/spawning grounds can result from the conveyance of polluted water through rivers leading to the coast.

Finally, underground mining consumes water for such activities as drilling, gobbing/stowing, hydro-mining, etc.

The measures described in section 2.2.4 (table 2) should be adopted to prevent pollution of surface and groundwater by mine waters.

2.3.3 Subsidence

For the day surface, the most frequent danger resulting from underground mining activities is subsidence, or settling. Subsidence-induced tilt, curvature, thrust, stretch and compression of the day surface can cause damage to buildings and infrastructural facilities as well as to the natural environment. Watercourses such as canals and rivers - and rice paddies, for example - react very sensitively to the slightest change in ground inclination.

Protective measures begin with early regional planning with due consideration of the potential mining-induced consequences of ground subsidence.

Settling can also be avoided or at least reduced by properly lining the mine with support material and backfilling the face workings with rejects and/or the use of certain suitable extraction techniques. Well-planned and controlled extraction allows slow areal settling that is unlikely to damage buildings or public utility lines and facilities.

2.3.4 Dumps, land consumption, landscape

Underground mining activities are usually accompanied by the appearance of large rubbish heaps within the immediate vicinity of the mine, where rejects and other useless material are dumped. The residual metal contents of such material should be ascertained, even though the metal burdens emanating from dressing heaps can be expected to be higher. Frequently, rubbish dumps are difficult to recultivate, and appropriate measures therefore should be included in the working plans.

Underground mines require a certain extent of surface area for the requisite infrastructure (hoists, buildings, workshops, storage areas, power generating equipment, access road, etc.). The aboveground facilities can impair the appearance of the landscape, and relevant architectural measures have limited effects. The establishment of any such industrial complex is bound to alter the landscape in the vicinity of the mining facilities. To the extent that resettlement is necessary, the affected parties must receive appropriate compensation.

Lowering the groundwater level can have detrimental effects on the local vegetation, including the drying out of ponds, streams, etc. Moreover, the local fauna and human population can be adversely affected by a diminishing supply of drinking water as a result of the altered water regimen.

Adequate protection of wetlands against such negative impacts may require the artificial recharge of groundwater, particularly since receding groundwater tends to cause settling, with damage to structures as one likely result.

Finally, vibrations caused by blasting and ground movement are also perceptible aboveground.

2.4 Other consequences of underground mining

Establishing mining operations in remote areas can have the inadvertent effect of opening the area up to uncontrolled settlement and land use. Appropriate planning-stage backup measures are therefore called for.

The intensive use of wood for timbering mines can trigger the large-scale felling of trees and, hence, erosion of the exposed soil. Orderly silvicultural activities in the area around the mine can help prevent such problems, especially if fast-growing species of trees are planted. Nonetheless, long-term effects on the ecosystem remain unavoidable. The use of anchoring techniques and steel supports in underground mines can extensively reduce wood consumption.

The world over, underground mining provides employment almost exclusively for men, because cultural and traditional conceptions forbid women to work underground. If at all, jobs for women are to be found in the areas of mineral processing, marketing and attendant services. Children should never be allowed to work in underground mines. Other social problems can arise in connection with mining if the housing for the miners and their families is either inadequate or not accompanied by the appropriate infrastructure (water, markets, schools, etc.) and if the miners are not covered by social insurance.

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