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Underground Mining

Underground Mining and Geomorphic Effect


Underground mining is undoubtedly associated with earth movement on a major scale. It is therefore understandable to assume that underground mining will have great metamorphic effects. The truth of the matter is that if underground mining is done correctly it can succeed with
minimum geomorphic consequences. The reality is that geomorphic scars are left by underground mining, in part due to accidental improper application of mining methods and process and also sometimes due to intentional processes. This paper will show some of the most geomorphic features left behind by underground mining processes, both during and after the mining process.

Stope and Retreat Mining

Underground mining tends to have a better reputation when it comes to geomorphic impacts, compared to other mining methods, i.e. open pit mining. The truth of the matter is that more often than not, open pit mining and underground mining go hand-in-hand. When open pit mining, the most geomorphic intrusive mining method, is no longer cost effective, underground mining takes over (where possible). However, the damage has been done to the surface of the land. The underground mining method tends to continue this intrusive action by using a stope method. The stope method continues the geomorphic scar on the landscape by digging out cavities collapsing stopes.  

The stope and retreat mining method is used to extract the resources/rocks from the stopes leaving a void behind; this way the rock walls cave in to the extracted stope, after the ore removal.  The images below indicates the stope and retreat mining method, as-well as what the surface (geomorphic) consequences can be.

These methods are commonplace in diamond mining, in Kimberlite pipes. The cost effectiveness of a Kimberlite pipe is first determined, and if needed a switch from an expensive open-pit mine is made to a more cost effective underground mine. The Kimberlite pipe and host rock is further exploited by weathering and decomposing methods through an underground tunnel system. This all occurs underneath the older open pit mine adding to possible collapses, some intentional some accidental. (ref -1) (ref – 6)

 

 

Stopes forming during underground mining


http://www.emt-india.net/process/mining/img/19.gif


Geomorphic effect, due to stope and retreat mining


http://en.wikipedia.org/wiki/File:Elura.png


GOAF STOWING DUE TO LONGWALL MINING

Longwall mining is a method of underground mining, usually used in coal mining. The method is fairly basic. A tunnel is created by basically cutting back the coal with a rotating drum and shearers. The coal fragments are then conveyed up to the surface. A series of hydraulic jacks (aka., chocks, are used to support the cavity wall, temporarily). As the shearer excavates the coal a cavity is created. The chocks advances with the shearer supporting the open space it created. The cavity behind the shearer and chocks increases and eventually collapses under the weight of the rocks above it. These gaofs can then be excavated and mined. (Ref – 2) (ref – 6)

Goafs forming due to longwall mining


http://www.georges-river-macarthur.com/longwall_process.html


Roof support in longwall mining


http://www.fhwa.dot.gov/engineering/geotech/hazards/mine/workshops/iawkshp/mcbfigs.cfm


Trough due to room-and-pillar mining

Room-and-pillar mining is a mining process that is exactly what the name suggests it to be. The mining area is dug out, divided the underground area into an assortment of rooms. The spaces between the rooms are called pillars; they are left behind to support the mined roof, (to keep the underground mine from collapsing). When the rooms eventually collapse, they create troughs. This is done deliberately in the mining process in order to loosen the ground and mine the loosened troughs; however it should be perfectly controlled.  This process can be clearly seen in the figures below. As seen in the figures, the weight of the overlaying ground can cause spontaneous (uncontrolled) collapses of the pillars themselves. These collapses cause a depression on the surface above, leaving a geomorphic scar. (ref – 3)(ref – 6)


Troughs forming due to room-and-pillar mining

Common forms of subsidence in room and pillar coal mining [Whittaker and Reddish, 1989]

http://www.fhwa.dot.gov/engineering/geotech/hazards/mine/workshops/iawkshp/mcbfigs.cfm 


http://www.dep.state.pa.us/msi/whatisms.html#9pics 

Surface mining vs underground mining (ref – 4)

Even though underground mining causes major amounts of earth movement, the geomorphic effect is minimum, compared to surface mining. When one takes coal mining in Kentucky as a practical example, this effect is evident. In 2000, 131.8 million tons of coal was mined in Kentucky; 62% (81 million tons) through underground mining and 38% (50 million tons) through surface mining (according the Kentucky Department of Mines and Minerals). Even though the underground mining moved almost double the amount of coal, the above image is evident that surface mining has a much bigger impact on the geomorphology of the area/environment.

The only noticeable geomorphic impact made by the underground mining, (besides the construction of man-made buildings), are the opening holes, due to drift-, slope- and shaft mining.  

The two major coal underground mining methods are pillar mining and long-wall mining.

Both methods have different possible geomorphic impacts. In both cases empty spaces are created in the mining process and supported by either pillars (pillar mining) or bolted (long-wall mining). Once the mining in a certain area is completed, the empty spaces (rooms) are allowed to collapse. If done correctly, the impact on the surface should be minimal, however, that isn’t always the case, as seen below:

 

Underground mining vs surface mining

 (http://www.uky.edu/KGS/coal/coalform_download.htm)

 

Environmental impact and recovery

Underground mining has numerous environmental risks to the geomorphology of the surrounding areas. These hazards include both physical and chemical factors, however it’s the physical hazards that have the greatest impact on the geomorphology of the environment, i.e. open mine shafts, collapsed mine shafts, and subsidence areas. The main cause of most of these physical hazards and collapses are known as “robbing the pillars”. This involves recovering the pillars responsible for supporting the underground mine roofs. Without these supports the mine will collapse, causing subsidence.
Centuries of mining can leave an environment in serious dismay. The abandoned mine tunnels tend to fill up with contaminated water from the mining remnants. These contaminated water, abandoned tunnels, collapsed tunnels, etc. leave great scars on a landscape.
In many cases efforts are put on recovering these environments with a natural or sometimes even an anthropogenic effort to recover some sort of normality to the environment and its surroundings. Efforts towards a natural recovery can be seen in the abandoned Galena mine in Kansas. (ref – 5)
An anthropogenic (zoogeomorphic) success story of reusing an old abandoned gold mine can be seen in Gold Reef City, Johannesburg, South Africa. This abandoned gold mine is now an urban geomorphic feature, (an entertainment area and theme park).

 

                                                                                                    

 


 

Galena area before reclamation.

http://www.kgs.ku.edu/Extension/ozark/mining.html

Reclamation of an old gold mine (Gold Reef City< South Africa)



Galena area after reclamation





http://showme.co.za/tourism/gold-reef-city-johannesburg/



http://pathwaysthruafrica.com/2011/gold-reef-city-2/


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