sustainability Review Subsidence Mechanism and Stability Assessment Methods for Partial Extraction Mines for Sustainable Development of Mining Cities—A Review Yang Yu 1 ID , Shen-En Chen 2 ID , Ka-Zhong Deng 1,* ID , Peng Wang 3 and Hong-Dong Fan 1,4 1 School of Environment Science and Spatial Information, China University of Mining and Technology, Xuzhou 221116, China; [email protected] (Y.Y.); [email protected] (H.-D.F.) 2 Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223, USA; [email protected] 3 School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China; [email protected] 4 State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China * Correspondence: [email protected]; Tel.: +86-0516-8388-5986 Received: 14 November 2017; Accepted: 4 January 2018; Published: 6 January 2018 Abstract: Partial extraction methods such as underground strip pillar mining or room-and-pillar mining are widely adopted techniques to control ground subsidence. However, pillar failure in partial extraction mines may introduce violent secondary ground collapses. The stability of partial extraction mines dictates the safety of ground surface structures and the environmental health state of the surrounding mining areas. To reuse mining subsidence lands, it is necessary to evaluate the stability of the land through mine subsidence assessments. This paper summarizes current pillar stability assessment methods and their limitations, and the rock mechanics associated with the stability of abandoned mines. The effects of multiple factors that affect mine stability are discussed in detail; special attention has been extended to discuss the weathering effects associated with infused water and spontaneous combustion, as these are some key reasons for pillar strength degradation in abandoned mines. The mechanism of mine collapse and the corresponding post-mining disasters are also summarized. Finally, suggestions and strategies to improve current mine stability assessment methods are proposed based on the perspective of subsidence control. Keywords: coal mine; stability assessment; pillar strength; pillar load; ground subsidence; room-and-pillar mining; underground strip pillar mining 1. Introduction Mining typically results in substantial abandoned mine lands that are problematic and can cause hazards to the environment [1]. Mining-induced land waste is a major environmental problem in a sustainability context [2]. According to the Bureau of Land Management of the U.S., there are currently about 500 thousand abandoned mines in the U.S. and many of them present serious threats to the environment [1]. A similar problem pervades all mineral rich nations, for example, since 2005, China had over 809.6 thousand hectares of area with abandoned mines and 1439 thousand hectares of surface land area were deemed wasted [3]. The subsidence land resulting from underground mining is an important source of abandoned mine lands. Most of the abandoned underground mines are documented in old mine maps, such as in Figure1, showing massive rooms and pillars. Sustainability 2018, 10, 113; doi:10.3390/su10010113 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 113 2 of 21 Sustainability 2018, 10, 113 2 of 21 Figure 1.1. TypicalTypicalabandoned abandoned mines mines of of partial partial extraction extraction from from China: China: (a) room-and-pillar(a) room-and-pillar mine; mine; (b) strip (b) pillarstrip pillar mine. mine. Massive Massive coal pillarscoal pillars and voids and voids are left are underground. left underground. The partial extraction methods such as undergroundunderground strip pillar mining (some researchers also used the term “strip mining”, caution should be takentaken to note that the “strip mining” usually refers to surfacesurface mining)mining) oror room-and-pillarroom-and-pillar mining (bord-and-pillar(bord-and-pillar mining)mining) were sometimes adopted to control strata movement and reduce miningmining subsidence.subsidence. However, partial extraction methods may result inin more more severe severe post-mining post-mining hazards hazards if mines if mi collapsenes collapse in the future.in the Thefuture. use ofThe mining-influenced use of mining- landinfluenced is restricted land is becauserestricted of because such uncertainty of such uncertainty of ground of surface ground stability. surface stability. Perhaps Perhaps the most the severe most damagesevere damage brought brought about by about thecollapse by the collapse of abandoned of abandoned mines is mines the violent is the andviolent massive and massive ground collapseground incollapse room-and-pillar in room-and-pillar mines. mines. Several Several mining mining areas areas in China in China have have already alread beguny begun to be to subjectedbe subjected to theseto these violent violent disasters. disasters. For For example,example, from 2004 to to 2016, 2016, the the Yuli Yulinn mining mining area area was was subjected subjected to 96 to 96mining-induced mining-induced earthquakes earthquakes with with magnitudes magnitudes exceed exceedinging two two on on the the Richter Richter scale scale [4]. [4]. As As a a result, result, panic has spread amongamong residentsresidents livingliving aroundaround miningmining areas.areas. Besides the engineering safety, there are also long-termlong-term effects on the environment. For example, example, localized pillar failures may introduce atmospheric air into the abandonedabandoned mine shafts and thusthus increase the possibility of spontaneousspontaneous combustion of the residual coal.coal. This will further increase the danger of massive cavity collapse, and will also allow greenhouse and toxic gases to return to the atmosphere. According to incomplete statistics, 2.3 billion tons of coal remained as coal pillars near Ordos, ChinaChina inin 20072007 [ 5[5].]. The The remnant remnant coal coal presents presents a long-terma long-term stability stability issue issue for futurefor future land land use anduse sustainableand sustainable development. development. On the On other the hand,other mininghand, subsidencemining subsidence affects the affects soil physical the soil properties physical (i.e.,properties soil moisture, (i.e., soil hardness,moisture, porosity)hardness, and porosity) chemical and properties chemical (nutrients,properties pH(nutrients, value), whichpH value), will furtherwhich will affect further the soil affect quality the andsoil plantquality community and plan aroundt community the mining around areas the [ 6mi,7].ning The areas degradation [6,7]. The of soildegradation quality and of soil plant quality community and plant structures community induced structures by mine subsidenceinduced by canmine last subsidence at least 10 yearscan last with at naturalleast 10 successionyears with natural [7]. 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A stable mine partial can protect extraction the integrity mine can of protect overburden the integrity and preserve of overburden a water body, and whilepreserve mine a water collapse body, will while lead tomine overburden collapse stratawill le failures,ad to overburden sinkholes orstrata ground failures, fissures, sinkholes and theor waterground from fissures, aquifer and or the from water ground from surfaces aquifer can or from leak intoground the undergroundsurfaces can leak voids into through the underground the fissures, finallyvoids through resulting the in fissures, a decrease finally in water resulting resources in a decrease near the in ground water surfaceresources [8 ,near9]. Moreover, the ground the surface water quality[8,9]. Moreover, in mining the areas water may quality degrade in because mining the area fissuress may can degrade serve asbecause channels the to fissures introduce can hazardous serve as contaminantschannels to introduce from the hazardous ground surface contaminants and underground from the ground voids into surface the aquifer and underground [9]. Therefore, voids stability into evaluationsthe aquifer [9]. of mining-influencedTherefore, stability landevaluations are an importantof mining-influenced part of land land use ar [10e ]an and important the sustainable part of developmentland use [10] and of mining the sustainabl cities. e development of mining cities. When facingfacing minemine subsidence,subsidence, the the key key issues issues of of concern concern are: are: the the stability stability of theof the ground ground surface, surface, the timethe time and extentand extent of collapse, of collapse, and the and associated the associated disasters, disasters, the effect the on effect surface on structures surface structures and effects and on terrestrialeffects on life.terrestrial Figure life.2 shows Figure a scenario 2 shows of a localizedscenario subsidenceof localized during subsidence room-and-pillar during room-and-pillar collapse and ancollapse idealized and subsidencean idealized basin. subsidence Mining basin. subsidence Mining was subsidence studied and