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Mine Size Effects on Coal Pillar Stress and Their Application for Partial Extraction

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Mine Size Effects on Coal Pillar Stress and Their Application for Partial Extraction sustainability Article Mine Size Effects on Coal Pillar Stress and Their Application for Partial Extraction Yang Yu 1 ID , Ka-Zhong Deng 1,* ID and Shen-En Chen 2 ID 1 School of Environment Science and Spatial Information, China University of Mining and Technology, Xuzhou 221116, China; [email protected] 2 Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223, USA; [email protected] * Correspondence: [email protected]; Tel.: +86-0516-8388-5986 Received: 30 January 2018; Accepted: 9 March 2018; Published: 13 March 2018 Abstract: Coal is a nonrenewable resource. Hence, it is important to improve the coal recovery ratio and ensure the stability of coal mines for sustainable development of mining cities. Partial extraction techniques, such as strip pillar mining or room-and-pillar mining, are efficient methods to extract coal. Pillar stress is a critical property for pillar design and for the assessment of mine stability after partial extraction. Current pillar stress calculation methods can sometimes overestimate the pillar stress and unnecessarily large coal pillars may be left underground, which leads to a waste of coal resources. In this paper, the size effects of mining activity on the maximum vertical pillar stress were investigated using numerical simulations. Both strip pillar mining and room-and-pillar mining were considered as possible mining scenarios at different mining depths. The results show that the maximum pillar stress of a mine is primarily controlled by four factors: the mine size to mining depth ratio, the mining width to pillar width ratio, the overburden elastic modulus, and the mining depth. The maximum pillar stress of a mine gradually increases to an ultimate value as the mine size increases. Simplified formulas and methodology have been derived for stress calculations under consideration of mine size effects and, therefore, can reduce the waste of coal resources from the overestimation of pillar stress. Keywords: strip pillar mining; room-and-pillar mining; pillar stress; pillar design 1. Introduction Underground mining activities can result in severe ground subsidence and damage man-made structures (buildings, railways, or other infrastructure) on the surface [1–5]. Mining subsidence is an important factor that restricts the sustainable development of mining cities [5]. To reduce mining subsidence and protect surface and ground structures, partial extraction methods such as strip pillar mining or room-and-pillar mining are widely adopted [5–7]. Figure1 shows typical layouts of room-and-pillar mining and strip pillar mining techniques. In a partial extraction operation, massive coal pillars will be left underground to support the overburden, where the effectiveness of subsidence control for the partial extraction will depend on the stability of these pillars. Additionally, the failure of a coal pillar may result in violent ground movements [8,9]. Therefore, the stability of the coal pillars in a partial extraction is very important during and after mining activities. The pillar stress is a critical factor affecting pillar stability, and the pillar stress calculation is an important aspect of mine design. The stress calculation for room-and-pillar mining is a three-dimensional problem because both the pillar length and the pillar width are relatively small (Figure1a), whereas the stress calculation for strip pillar mining can be simplified to a two-dimensional problem because the pillar length is usually far longer than the pillar width (Figure1b). In coal mine Sustainability 2018, 10, 792; doi:10.3390/su10030792 www.mdpi.com/journal/sustainability SustainabilitySustainability2018 2018,,10 10,, 792x FOR PEER REVIEW 22 of 1211 design and pillar stability evaluation, the pillar stress is usually determined by using TAT (tributary designarea theory), and pillar PAT stability (pressure evaluation, arch theory), the pillaror numerical stress is simulations usually determined [5,10–16]. by TAT using assumes TAT (tributary that the areaoverburden theory), load PAT will (pressure be evenly arch distributed theory), or numericalamong coal simulations pillars, and [ 5the,10 –overburden16]. TAT assumes load above that thethe overburdenmined room load is equally will be shared evenly by distributed neighboring among pillars coal [5]. pillars, On the and other the overburdenhand, PAT loadassumes above that the a minedpressure room arch is will equally form shared between by large neighboring barrier pillarspillars [and5]. Oncarry the most other of hand, overburden PAT assumes weight, that the aabutment pressure angle arch willis utilized form betweento describe large the barriershape of pillars the pressure and carry arch most [5,15]. of overburdenFor narrow mine weight, panels, the abutmentTAT overestimates angle is utilized the pillar to describestresses since the shape pressure of the arches pressure may archcarry [5 the,15]. weight For narrow of the mineoverburden. panels, TATIn such overestimates cases, PAT is the usually pillar adop stressested sinceto calculate pressure pillar arches stress may [5,14–16]. carry the However, weight the of the abutment overburden. angle Inused such by cases, PAT is PAT mostly is usually summarized adopted from to calculate longwall pillar mini stressng [5,10–20], [5,14–16 which]. However, is not thealways abutment accurate angle for usedpartial by extraction. PAT is mostly On the summarized other hand, from the longwall shape and mining height [5 ,of10 –the20 ],pressure which is arch not alwaysmay change accurate during for partialexcavation extraction. [21], indicating On the other that hand,the portions the shape of andthe overburden height of the load pressure that transfer arch may to changethe pillars during are excavationaffected by [the21], mine indicating size. Therefore, that the portions the existing of the methods overburden fail to loadprovide that a transfer reliable tostress the pillarsestimation are affectedfor partial by theextraction. mine size. Inaccurate Therefore, pillar the existingstress calculations methods fail usually to provide result a reliablein unnecessarily stress estimation large coal for partialpillars extraction.that are permanently Inaccurate left pillar underg stressround; calculations the coal usually resource result is wasted. in unnecessarily large coal pillars that areTo permanentlyaddress this leftissue, underground; empirically theestablishe coal resourced techniques is wasted. and methodologies which estimate pillarTo stress address have this been issue, suggested empirically [22]. established He et al. techniques[23] observed and that methodologies the mine subsidence which estimate and pillarstrata stressmovement have beenare affected suggested by [the22]. mine He et size al. [23. ]Additionally, observed that they the minefound subsidence that the mine and size strata movement to mining aredepth affected ratio by the(Figure mine 1c, size D⁄ .) Additionally, is an important they parameter found that to the det mineermine size whetherD to mining the subsidence depth H ratio can (Figurereach its1c, peakD/H value.) is an This important observation parameter suggests to determine that the deformation whether the subsidenceand stress of can strata reach may its peakbe a value.function This of observationthe mine size suggests . Roberts that et the al. deformation [11] reported and similar stress observations of strata may that be the a function pillar stress of the is mineaffected size byD . Roberts⁄ and et proposed al. [11] reported a stress similarcalculation observations formula for that room-and-pillar the pillar stress mines is affected that included by D/H andmine proposed size effects. a stress calculation formula for room-and-pillar mines that included mine size effects. InIn this paper, paper, the the effects effects of of mine mine size size variation variation on on coal coal pillar pillar stress stress are investigated are investigated for both for bothstrip strippillar pillar mining mining and androom-and-pillar room-and-pillar mining mining using using numerical numerical software software UDEC UDEC (Universal DistinctDistinct ElementElement Code)Code) and and 3DEC 3DEC (3 Dimension(3 Dimension Distinct Distinct Element Element Code). Code). Simulation Simulation of mines of includes mines differentincludes pillardifferent sizes pillar and differentsizes and minedifferent depths. mine The depths. mine sizeThe refersmine size to the refers mined to areathe mined of the area coal seamof the with coal coalseam pillars with coal and pillars mined and rooms mined (Figure rooms1c). (Figure Simple 1c). formulas Simple to formulas calculate to the calculate maximum the maximum pillar stress pillar for differentstress for mine different sizes mine are derived sizes are and derived proposed and for pr mineoposed design. for mine Based design. on the proposedBased on formulas,the proposed the recoveryformulas, ratio the recovery may be improved ratio may ifbe the improved mine size if isthe not mi toone size large, is not and too the large, waste and of coal the resourcewaste of coal can beresource reduced. can be reduced. FigureFigure 1. TypicalTypical layout layout of of partial partial extraction extraction sites sites [5]: [ 5(a]:) Planar (a) Planar view view of the of working the working face of face a room- of a room-and-pillarand-pillar mine; mine; (b) Planar (b) Planar view view of the of theworking working face face of ofa astrip
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