Academic Editor: Saiied Aminossadati Received: 9 November 2016; Accepted: 22 February 2017; Published: 2 March 2017

Transcription

1 minerals Article Visual Exploration Spatiotemporal Evolution Law Overburden Failure Mining-Induced Fractures: A Case Study Wangjialing Coal Mine in China Hongzhi Wang 1, Dongsheng Zhang 2, *, Xufeng Wang 1 Wei Zhang 3 1 School Mines, China University Mining Technology, Xuzhou , China; cumterwhz@cumt.edu.cn (H.W.); wangxufeng@cumt.edu.cn (X.W.) 2 State Key Laboratory Coal Resources Safe Mining, China University Mining Technology, Xuzhou , China 3 IoT Perception Mine Research Center, National Local Joint Engineering Laboratory Internet Application Technology on Mine, China University Mining Technology, Xuzhou , China; zhangwei@cumt.edu.cn * Correspondence: dshzhang123@cumt.edu.cn; Tel./Fax: Academic Editor: Saiied Aminossadati Received: 9 November 2016; Accepted: 22 February 2017; Published: 2 March 2017 Abstract: The television approach is an effective way detecting mining-induced in overburden strata as it can visualize to facilitate a quantitative analysis size, quantity, length, or features. In this article, television approach is applied on panel Wangjialing Coal Mine in China to investigate overburden movement spatiotemporal evolution law mining-induced from coal seam to ground surface. The results revealed that overburden strata experienced phases ro caving, generation fracture, bed separation, dislocations, fracture propagation, surface subsidence, closing. The process can be divided into initiation stage, active stage, degradation stage along mining direction. For exploited working faces, caved zone height is times mining height, height fractured zone is times mining height. The height range three parts in fractured zone is 24 26, 40 45, m. Significant were observed in bending zone. Step subsidence cracks, which indicate severe damages, were observed on ground surface above goaf. Keywords: overburden failure; mining-induced ; camera; visual exploration 1. Introduction Large-scale longwall mining coal seams may cause movement overburden strata mining-induced. This could lead to a series mine safety environmental issues, including cracks on surface, water loss, vegetation deterioration, l desertification, water inrush [1 4]. Therefore, an accurate effective understing spatiotemporal evolution law mining-induced in overlying strata is critical. It is well known that re are three distinct zones (caved zone, fracture zone continuous deformation zone) movement in overburden rock mass above longwall coal mines. Over past decades, a variety numerical physical simulation methods detection methods have been used to study development characteristics mining-induced in overlying strata [5 9]. Currently, most widely used methods to detect fracture propagation include double end seal with water injection method, transient electromagnetic method, micro-seismic monitoring Minerals 2017, 7, 35; doi: /min

2 Minerals 2017, 7, method Minerals 2017, ultrasonic 7, 35 imaging method. Because mining-induced fracture strata movement 2 16 is buried deep underground, se traditional monitoring methods, which use parameter inversion strata movement is buried deep underground, se traditional monitoring methods, which use to speculate fracture, cannot show visualizing characteristics shape size mining-induced parameter inversion to speculate fracture, cannot show visualizing characteristics shape [10,11]. size mining-induced [10,11]. Borehole television is a device based on optical techniques to provide direct images s in Borehole television is a device based on optical techniques to provide direct images form a film or video. This method has many advantages in determining breakage height s in form a film or video. This method has many advantages in determining time breakage overburden height time strata visualizing overburden strata morphology visualizing size morphology mining-induced size. Hence, mining-induced. television Hence, approach has been television widely applied approach tohas identify been lithology, widely applied determine to coal identify seam thickness, lithology, determine to observe coal seam separation thickness, roadway to observe ros separation deformation roadway surrounding ros rocks [12 16]. deformation Tan et surrounding al. used a rocks [12 16]. camera Tan technology et al. used toa explore camera zonal technology disintegration to developing explore process zonal indisintegration ro strata influenced developing by process multiplein mining ro strata activities influenced [17 19]. by Xiong multiple et al. mining used three- activities [17 19]. method Xiong (multi-point et al. used displacement three- measuring, method (multi-point double endisplacement seal with water measuring, injection method double end seal television with water photographing) injection method for overburden television failure law photographing) an upper protected for coal overburden seam while failure lower law in protective an upper protected seam is mined coal seam [20]. while Zhang etlower al. studied protective distribution seam mined propagation [20]. Zhang et various al. studied mining-induced distribution propagation found that various transverse mining-induced longitudinal intersect found with that transverse each or, while longitudinal a high degree longitudinal intersect with each dominate or, while become a high degree crushing longitudinal as y approach dominate ro [21]. become Gao et crushing al. used results obtained as y approach by ro to [21]. construct Gao et equation al. used results for obtained locationby quantity to construct in overburden equation for strata location demonstrated quantity that most in overburden were concentrated strata behind demonstrated coal wall that [22]. most Currently, were concentrated television behind approach has coal been wall applied [22]. to Currently, investigate distribution television propagation approach has been mining-induced applied to investigate, but not distribution propagation mining-induced, but not dynamic evolution dynamic evolution during mining, which is a key characteristic overburden strata. during mining, which is a key characteristic overburden strata. In this article, television approach is applied on working face In this article, television approach is applied on working face Wangjialing Mine in China to visually detect (1) mining-induced in overburden strata; Wangjialing Mine in China to visually detect (1) mining-induced in overburden strata; (2) (2) movement failure coal seam-overburden stratum-ground surfaces; (3) temporal spatial movement failure coal seam-overburden stratum-ground surfaces; (3) temporal spatial evolution evolution mining-induced mining-induced,, exfoliation, exfoliation, caving caving overburden overburden strata. strata. 2. Mine Conditions Mining Techniques 2. Mine Conditions Mining Techniques Located Located in in western western part part Shanxi Shanxi Province, Province, Hedong Hedong coalfield coalfield is is largest largest coking coking coal coal reservoir reservoir in in China. China. This This coalfield coalfield has has a weared a weared mountain mountain morphology morphology with with limited limited vegetation vegetation a thick a loess thick layer loess ( layer ( m). The m). migrating The migrating direction direction length length dipping dipping direction direction length length panel panel are are m, respectively. m, respectively. The #2 coal The #2 seam coal has seam a thickness has a thickness m (average m 6.18 (average m) 6.18 a dip m) angle a dip 2 4 angle. The ground 2 4. The elevation ground elevation coal seam floor coal seam elevation floor are elevation are m m m, respectively. m, The respectively. longwall The top coal longwall caving top method coal caving (caving method 3.0 m, (caving mining m, m) mining used. 3.2 Figure m) 1 shows used. Figure arrangement 1 shows arrangement panel panel Tailgate Mining Direction Headgate scale 1:2000 Figure 1. Layout Panel Figure 1. Layout Panel

3 Minerals 2017, 7, Field Monitoring Minerals 2017, 7, Arrangement 3. Field Monitoring Boreholes Minerals 2017, 7, In 3.1. order Arrangement to explore Boreholes propagation overburden cracks, five s were specially drilled, 3. Field Monitoring including three surface s two underground s. Surface s, each 110 mm In order to explore propagation overburden cracks, five s were specially drilled, in diameter, including 3.1. Arrangement were three vertically surface Boreholes s drilled fromtwo underground surface to s. No. 2Surface coal seam s, each 2). 110 They mm were positioned in diameter, in middle working face. Boreholes #1 #1 were 665 m away from main In order were to explore vertically drilled propagation from overburden surface to cracks, No. five 2 s coal seam were specially 2). They drilled, were haulage positioned including roadway. in three Borehole middle surface s #2 working 230 mtwo face. away underground Boreholes from #1 s. main #1 haulage Surface were 665 s, roadway. m away each from The110 stratigraphic mm main columns haulage in diameter, roadway. holes were are Borehole vertically shown #2 drilled in Figure 230 from m 3. away These surface from s to main No. were haulage 2 coal used seam roadway. to monitor The 2). They stratigraphic movement were damages columns positioned from in overburden holes middle are shown strata working Figure to face. 3. surface These Boreholes s during #1 mining. were #1 were used Underground 665 to m monitor away from movement s, main no less than 70 damages mm haulage infrom diameter, roadway. overburden Borehole are drilled #2 strata in 230 to roadway m away surface from during working main mining. haulage face. Underground The roadway. elevation The s, stratigraphic anglesno less than columns 70 mm in diameter, holes are are shown drilled in Figure in #3 #4, within one vertical surface, are roadway These s 30 working were used face. to monitor The elevation movement angles respectively s 2 4). These s damages #3 from #4, within overburden one vertical strata to surface, surface are 22 during mining. 30 respectively Underground s s, 2 no 4). less These were used to monitor movement overburden strata fracture propagation. Boreholes #1, #2, s than 70 were mm in used diameter, to monitor are drilled movement in roadway overburden working strata face. The fracture elevation propagation. angles #3 Boreholes #4 are drilled #1, #3#2, #3 before #4, within #4 are one mining drilled vertical process before surface, are mining 22 TYGD10 process 30 respectively stratum TYGD10 s stratum 2 monitor 4). These 5) is used monitor s record were 5) underground used is used to to monitor record situation movement underground during working overburden situation face during strata mining. working fracture face mining. propagation. Boreholes #1, #2, #3 #4 are drilled before mining process TYGD10 stratum monitor 5) is used to record Surface Borehole underground #1 Surface Borehole situation #1' during working face mining. Surface Borehole #1 Surface Borehole #1' Borehole #3 Borehole #4 Borehole #3 Borehole #4 Tailgate Headgate Column Column Lithology Thickness/m Remarks 1 Loess layer Lithology Thickness/m Remarks 2 1Mudstone Loess layer Fine Mudstone sstone Sy Fine mudstone sstone Fine Sy sstone mudstone Siltstone Fine sstone Sy Siltstone mudstone Fine Sy sstone mudstone Fine sstone Medium sstone Main key stratum 9 Medium sstone Main key stratum 10 Mudstone Mudstone Fine sstone Fine sstone Sy mudstone Sy mudstone Fine sstone Fine sstone Mudstone Mudstone Fine sstone Sub key stratum 15 Fine sstone Sub key stratum 16 16Sy Sy mudstone mudstone Fine Fine sstone sstone Sy Sy mudstone mudstone Medium Medium sstone Sy Sy mudstone Sub key stratum Medium sstone Sy Sy mudstone Siltstone Siltstone Mudstone Fine Fine sstone sstone Sub Sub key key stratum stratum Sy Sy mudstone mudstone Fine sstone Fine sstone Mudstone Mudstone #2 coal seam #2 coal seam 6.46 (a) (a) Tailgate Figure 2. The layout s. Figure 2. The layout s. Figure 2. The layout s. Column Column Headgate Lithology Thickness/m Remarks Lithology Thickness/m Remarks 1 Loess layer Loess layer Mudstone Mudstone Fine sstone Fine sstone Mudstone Mudstone Medium sstone Medium sstone Siltstone Siltstone Sy Sy mudstone mudstone Fine Fine sstone sstone Medium sstone Main Main key key stratum stratum Mudstone Fine sstone Sy mudstone Siltstone Mudstone Fine Fine sstone Sub Sub key key stratum stratum Mudstone Mudstone Fine Fine sstone sstone Sy mudstone Sy mudstone Siltstone Siltstone Mudstone Mudstone 2.00 (b) (b) Figure 3. Cont.

4 Minerals 2017, 7, Minerals 2017, 7, Minerals 2017, 7, 35 Column Lithology Thickness/m Remarks Column 1 Loess Lithology layer Thickness/m Remarks 4 16 Minerals 2017, 7, Sy Loess layer mudstone Sy Siltstone mudstone Column 34 Siltstone Sy Lithology mudstone Thickness/m Remarks 45 1 Sy Fine Loess sstone mudstone layer Sy mudstone Fine Medium sstone sstone Siltstone Medium Mudstonesstone Sy mudstone Mudstone 5 Fine Fine sstone Sy Fine Medium sstone mudstone sstone Sy Fine Mudstone sstone mudstone Fine Mudstone Fine sstone Sy mudstone Mudstone Fine sstone Main key stratum 10 Fine sstone Fine Mudstone sstone Main key stratum 11 Mudstone Siltstone Mudstone 10.6 Fine sstone Main key stratum Siltstone Fine Mudstone sstone Fine Mudstone Siltstone sstone Siltstone Mudstone Fine sstone Sub key stratum Mudstone Siltstone Fine sstone Sub key stratum 17 Siltstone 17.7 Sub key stratum Fine 18 Mudstone sstone 8.65 Fine sstone Mudstone Fine Mudstone sstone Fine Mudstone Fine sstone Siltstone Mudstone Sub key stratum Sub key stratum 23 Siltstone Medium sstone Sub key stratum Medium Medium Mudstonesstone Mudstone Mudstone 25Fine Fine sstone 6.15 sstone Sy Fine sstone mudstone Sub Sub key stratum key stratum Sy Siltstone mudstone Sub key stratum Siltstone Fine Fine sstone Sy mudstone Sy Fine sstone mudstone Mudstone Sy Mudstone mudstone #2 coal seam Mudstone #2 coal seam #2 coal seam (c) 5.97 (c) (c) Figure 3. Stratigraphic columns surface holes: (a) #1; (b) #1 ; (c) #2. Figure 3. Stratigraphic columns surface holes: (a) #1; (b) #1 ; (c) #2. Figure 3. Stratigraphic columns surface holes: (a) #1; (b) #1 ; (c) #2. Figure 3. Stratigraphic columns surface holes: (a) #1; (b) #1 ; (c) #2. Borehole #4 25m 80m m 80m m Borehole #4 Borehole Borehole #4 #3 40m 28m 28m 28m 40m 40m advancing direction Tailgate advancing direction Figure 4. Layout schemes underground holes. 25m advancing direction 25m Tailgate Tailgate Figure Figure Layout Layout schemes schemes underground underground holes. holes. Figure 4. Layout schemes underground holes. 64.6m 64.6m Borehole #3 Borehole #3 Figure 5. TYGD10 stratum monitor. Figure 5. TYGD10 stratum monitor. Figure 5. TYGD10 stratum monitor. Figure 5. TYGD10 stratum monitor.

5 Minerals 2017, 7, Minerals 2017, 7, x FOR PEER REVIEW Monitoring via Surface Borehole 3.2. Monitoring via Surface Borehole Strata Movement Fracture Propagation in Borehole #1 # Strata Movement Fracture Propagation in Borehole #1 #1 Borehole #1 drilled from 6 May 2013 to 2 June The drill core relatively intact (with a few exceptions), Borehole #1 drilled core recovery from 6 May rate2013 exceeds to 2 70%. June The observation The drill core started when relatively working intact face (with a 143 few mexceptions), ahead core recovery endedrate when exceeds working 70%. The face observation 160 mstarted past when ; working face moved face forward 143 m at aahead rate 5 m/day. ended when working face 160 m past When ; working face moved face forward 96.7 mat away a rate from 5 m/day. #1, cracks 6a) first appeared in area When around working face #1 its 96.7 size m increased away from as working #1, cracks face moved 6a) forward. first appeared As a result, in area around #1 its size increased as working face moved forward. As a result, opening divided in half 6b). Then, deformation cracks were observed in opening divided in half 6b). Then, deformation cracks were observed epipedon 6d), vertical in mudstone beneath epipedon appeared. in epipedon 6d), vertical in mudstone beneath epipedon appeared. When working face 29.2 away from, opening deformed When working face 29.2 m away from, opening deformed epipedon deformation exacerbated with exfoliation wall 6e f). epipedon deformation exacerbated with exfoliation wall 6e f). Meanwhile, vertical appeared on deep bedrocks. Then, opening Meanwhile, vertical appeared on m deep bedrocks. Then, opening deformation deformation 6c) 6c) exacerbated. exacerbated. The The quantity quantity size size vertical vertical on on bedrocks bedrocks continuously continuously increased, increased, while while horizontal horizontal were were observed. observed. However, However, no noobvious obvious appeared appeared on on bedrocks bedrocks under under m deep. deep. (a) (b) (c) (d) (e) (f) Figure 6. Observations in #1: (a) crack at opening; (b) crack propagation; (c) Figure 6. Observations in #1: (a) crack at opening; (b) crack propagation; opening deformation; (d) in loess layer; (e) fracture propagation; (f) (c) opening deformation; (d) in loess layer; (e) fracture propagation; (f) exfoliation. exfoliation. When When working working face face m away from, opening opening divided divided into into two two parts, parts, with with a height a height difference cm. cm. Fractures 7a) 7a) were observed in mudstone at depths at depths m. Shortly m. Shortly after after working working face passed face passed,, loess layer loess subsided layer severely subsided above severely goaf, above goaf, height difference height difference increased increased to 30 cm. to 30 The cm. influence The influence mining mining to fracture to propagation fracture propagation in in significantly significantly increased increased propagation propagation rate rate obviously faster obviously than faster before. than before. When working face 13.2 m past, caving immediatero ro strata strata at at depth a depth m m 7d) 7d) observed. When working face m mpast past, failure failure main main ro ro ( ( first first sub sub key key stratum) observed caving height reached 21.6 m. Dislocations m. Dislocations 7e), accompanied 7e), accompanied by propagation by propagation vertical vertical irregular irregular 7b), were observed 7b), were on observed strata at on depths strata at depths m, a bed m, separation a bed separation between between second

6 Minerals 2017, 7, Minerals 2017, 7, x FOR PEER REVIEW 6 16 sub key stratum sstone beneath it observed. And propagated severe second sub key stratum sstone beneath it observed. And propagated damages severe damages wall 7c,f) wall appeared. 7c,f) When appeared. working When face working 42.4 m face past 42.4, m past second sub, key stratum second failed. sub Bedkey separation, stratum dislocation, failed. Bed separation, vertical fracture dislocation, propagation vertical stratum fracture above propagation second substratum key stratum above were second observed. sub key Thestratum height were observed. caving zone The increased height to 78 m. caving zone increased to 78 m. (a) (b) (c) (d) (e) (f) Figure 7. Observations in #1: (a) vertical fracture; (b) irregular fracture; (c) damages Figure 7. Observations in #1: (a) vertical fracture; (b) irregular fracture; (c) damages wall; (d) strata caving ; (e) dislocations; (f) severe damages wall. wall; (d) strata caving; (e) dislocations; (f) severe damages wall. When working face 72.8 m past, third sub key stratum failed. The mining-induced When working face propagated 72.8upwards m pastwhile, density third subdecreased. key stratum The failed. bed Theseparation mining-induced dislocation were propagated not very upwards obvious. while Before long density that decreased m The deep bed separation completely dislocation blocked were not strata verydislocations obvious. Before at depth long m that increased After m deep working completely face blocked 120 m past strata, dislocations movement at depth overburden strata mgradually increased. weakened, After working slowly face sinked 120 munder past gravity., Part movement bed separation overburden strata gradually were closed weakened, to compaction. slowly sinked The under fractured gravity. zone Part reached bed section separation beneath main key were strata closed with to a maximum compaction. Theheight fractured zone m. reached section beneath main key strata with a maximum height m. Owing Owing to to delay delay in movement in movement overburden overburden strata, strata, #1 #1 drilled drilled three months three after months working after face working reached face reached #1. A 50 m#1. seamless A 50 m steel seamless pipe steel inserted pipe toinserted mitigateto influences mitigate severe influences damages severe in damages loess layer. in Discontinuous loess layer. Discontinuous vertical vertical partial damages partial damages appeared on m deep bedrocks, indicating that tensile strength appeared on m deep bedrocks, indicating that tensile strength bedrocks not strong bedrocks not strong enough to prevent breakage. Most parts drill core on m deep enough to prevent breakage. Most parts drill core on m deep undisturbed. Circular undisturbed. Circular were observed on wall at a depth 170 m were observed on wall at a depth 170 m 8c); fracture density 8c); fracture density increased with depth. The results indicated that fractured zone Panel increased with depth. The results indicated that fractured zone Panel #20105 propagated to #20105 propagated to fine stone layer at a depth m a maximum height 120 m. fine stone layer at a depth m a maximum height 120 m. Drill sticking, accompanied Drill sticking, accompanied by fractured rock core wall failure 8d), observed in by fine fractured sstone rock layer core (depth wall varied failure from d), m to observed m). Severe in dislocations fine sstone layer 8e) were (depth varied observed from in strata m to at depths m). between Severe210 dislocations 220 m. Severe 8e) were drill observed sticking in strata considerable at depths between consumption drilling m. Severe fluid forced drill sticking drilling considerable process to end consumption at a depth 231 drilling m. The fluid observation forced drilling results process obtained trom end at a depth#1 are 231 shown m. The in Figure observation 8. results obtained from #1 are shown in Figure 8.

7 Minerals 2017, 7, Minerals 2017, 7, x FOR PEER REVIEW 7 16 Minerals 2017, 7, x FOR PEER REVIEW 7 16 (a) (b) (c) (a) (b) (c) (d) (e) (f) Figure 8. Observations in #1 : (a) burial depth m; (b) burial depth m; Figure 8. Observations (d) in #1 : (a) burial (e) depth m; (b) burial depth(f) m; (c) burial depth m; (d) burial depth m; (e) burial depth m; (f) s (c) Figure burial burial depth 8. depth Observations in m; m. (d) burial #1 : depth (a) burial depth m; (e) burial m; (b) depth burial depth m; (f) burial m; depth (c) burial depth m m; (d) burial depth m; (e) burial depth m; (f) s burial Strata depth Movement m. Fracture Propagation in Borehole # Strata Movement Fracture Propagation in Borehole # Borehole Strata Movement #2 drilled Fracture from 26 Propagation March 2013 in Borehole to 30 April # The drill core relatively intact Borehole (with#2 a few drilled exceptions), from 26 March core 2013recovery to 30 April rate exceeds The drill 80%. core The characteristics relatively intact (with overburden aborehole few exceptions), movement #2 drilled crack from core development 26 recovery March 2013 rate in exceeds to 30 April #280%. were The similar The characteristics drill to that core overburden relatively #1. movement intact When (with a crack working few development exceptions), face 87.5 in m away core #2 from recovery were similar rate #2, exceeds tocracks that 80%. The 9a) characteristics #1. first appeared in overburden When area around movement working face #2 crack 87.5 its development size m away quantity from in increased #2 #2, were as cracks similar working to that face 9a) moved first appeared forward. #1. in Then area When around epipedon working deformation face #2 its 87.5 size observed m away quantity from increased 9b). Whereafter #2, as cracks slight working damage 9a) face first appeared moved appeared forward. near in Then area interface around epipedon between deformation epipedon #2 its size observed rock. quantity Vertical increased as 9b). Whereafter (width working slight = 1 2 damage mm) face moved appeared horizontal forward. near Then epipedon interface appeared deformation between epipedon at depths rock observed Vertical m. With 9b). working Whereafter (width = 1 2 face slight mm) advancing, damage appeared horizontal epipedon near appeared deformation interface depths between exacerbated epipedon m. with Wixfoliation rock. Vertical working face (width advancing, wall = 1 2 mm) epipedon 9c), deformation horizontal size exacerbated on appeared with exfoliation bedrocks at depths continuously increased. m. With working face advancing, epipedon wall 9c), size on bedrocks deformation exacerbated with exfoliation wall 9c), size continuously increased. on bedrocks continuously increased. (a) (b) (c) Figure 9. (a) Observations in #2: (a) crack (b) near opening; (b) (c) epipedon deformation; (c) exfoliation wall. Figure 9. Observations in #2: (a) crack near opening; (b) epipedon Figure 9. Observations in #2: (a) crack near opening; (b) epipedon deformation; deformation; When working (c) exfoliation face 19.0 m away wall. (c) exfoliation wall. from, loess layer subsided above goaf. However, no obvious mining-induced cracks were observed ahead working face, indicating When that working strength face 19.0 main m ro away from only strong, enough to loess prevent layer cracking subsided until above goaf. When longwall However, working face no right obvious face under mining-induced 19.0 m away from it. After breakage cracks were, immediate observed loess ahead layer ro subsided working above main ro, face, goaf. However, indicating nothat obvious strength mining-induced main cracks ro were observed only strong ahead enough to working prevent cracking face, indicating until that longwall face right under it. After breakage immediate ro main ro,

8 Minerals 2017, 7, Minerals strength 2017, 7, x FOR main PEER ro REVIEW only strong enough to prevent cracking until longwall face 8 16 right under it. After breakage immediate ro main ro, caving height reached caving height reached 25.5 m 10a). Propagation vertical 10b) 25.5 m 10a). Propagation vertical 10b) irregular 10c) irregular 10c) accompanied by dislocations 10d) were observed, a bed accompanied by dislocations 10d) were observed, a bed separation 10e) between separation 10e) between second sub key stratum medium sstone beneath it second sub key stratum medium sstone beneath it observed. As working observed. As working face moved forward, propagated severe damages face moved forward, propagated severe damages wall 10f) were wall 10f) were observed. When working face 56.9 m past, observed. When working face 56.9 m past, second sub key stratum failed. second sub key stratum failed. Bed separation, dislocation, vertical fracture propagation Bed stratum separation, above dislocation, second sub key vertical stratum fracture were propagation observed. The height stratum above caving zone second increased sub key stratum to 82 m. were observed. The height caving zone increased to 82 m. (a) (b) (c) (d) (e) (f) Figure 10. Observations in #1: (a) strata caving; (b) vertical fracture; (c) irregular fracture; Figure 10. Observations in #1: (a) strata caving; (b) vertical fracture; (c) irregular fracture; (d) dislocations ; (e) bed separation; (f) damages wall. (d) dislocations ; (e) bed separation; (f) damages wall. When working face 80.6 m past, third sub key stratum failed. The mining-induced When working face propagated 80.6upwards m pastwhile, density third subdecreased. key stratum The bed failed. The separation mining-induced dislocation were propagated not very upwards obvious. while As density working face moved decreased. forward, The bed separation that dislocation m were deep not verycompletely obvious. As blocked working face strata moved that were forward, m that deep had increasing m deep dislocations. completely After blocked working face strata that 115 were m past , m deep had increasing movement dislocations. overburden After strata gradually workingweakened, face 115slowly m past sinked, under gravity. movement Part overburden bed separation strata gradually weakened, were closed to slowly compaction. sinked The under fractured gravity. zone Part reached bed separation section beneath were main closed key strata to compaction. with a maximum The fractured height zone reached m. section beneath main key strata with a maximum height m Underground Observation Borehole 3.3. Underground The observation Observation started Borehole when working face 58.5 m away from opening The 33.5 observation m from started hole bottom. when No significant working face 58.5 were mobserved away from in. opening 33.5 m When from hole working bottom. face No significant 43.5 m away from were observed opening,. axial (length = 3 4 When cm, width working = 1 mm) first faceappeared 43.5in m away #3. from As working face opening, moved forward, axial both (length density = 3 4 cm, 11a) width = 1 average mm) first size appeared axial in increased #3. As local working breakage face moved coal forward, wall were observed 11c). When working face 19 m away from opening, both density 11a) average size axial increased local breakage increased up to 20 cm height increased to m. After that, fracture coal wall were observed 11c). When working face 19 m away from propagation significantly accelerated maximum fracture exceeded 50 cm. Irregular opening, increased up to 20 cm height increased to m. After that, fracture were observed 11b), significant wall breakage were observed, quantity propagation significantly accelerated maximum fracture exceeded 50 cm. Irregular size crushed coal in increased 11d). Half cross section were observed 11b), significant wall breakage were observed, quantity

9 Minerals 2017, 7, size Minerals crushed 2017, 7, x coal FOR in PEER REVIEW increased 11d). Half cross section 9 16 occupied by crushed coal, indicating that working face had significant effects from mining activities. occupied Additionally, by crushed fractured coal, indicating zone spread that to working m. face had significant effects from mining activities. When Additionally, working face fractured 11.1 m zone away spread from to m. opening, severe coal wall damages When working face 11.1 m away from opening, severe coal wall damages considerable crushed coal were observed. The observation significantly damaged, considerable crushed coal were observed. The observation significantly damaged, resulting in severe deformations 11e). The camera stuck at location 1.9 m behind resulting in severe deformations 11e). The camera stuck at location 1.9 m behind working face 14.5 m above channel ro. Dislocations caused by ro caving into working face 14.5 m above channel ro. Dislocations caused by ro caving into goaf were observed on. The height caved zone m. During six-day goaf were observed on. The height caved zone m. During six-day idling idling period, period, #3 #3 blocked blocked by by crushed crushed coal coal 11f) 11f) that that located located m from m from opening. opening. (a) (b) (c) (d) (e) (f) Figure 11. Observations in #3: (a) axial ; (b) irregular ; (c) wall breakage; Figure 11. Observations in #3: (a) axial ; (b) irregular ; (c) wall breakage; (d) crushed coal; (e) deformations; (f) blocked by crushed coal. (d) crushed coal; (e) deformations; (f) blocked by crushed coal. When working face 19 m away from opening, axial were observed When in working face#4 19 m away 12a). During from four-day opening, idling period, axial density were observed axial in increased, #4 while 12a). damages During coal four-day wall were idling not affected. period, The propagation density axial increased, during this while period damages can be attributed coal wall to were fact not that affected. propagations The propagation mining-induced during were this period slightly canbehind attributed ro tocaving. fact The that height propagations water-conducting mining-induced fracture were m. slightly behind rowith caving. The working height face advancing, water-conducting density fracture axial 17 19increased m. maximum length Withreached working 35 cm. face Irregular advancing, were density observed axial 12b) increased damages coal maximum wall length reached 10d) were 35 cm. exacerbated. Irregular The fractured werezone observed propagated upwards 12b) damages height coal wall m. Then 10d) one were axial exacerbated. reached The fractured 60 cm in zone length. propagated Deep upwards propagated height along internal m. Then wall one axial 12c). Observations reached 60 in cm insix-day length. idling Deepperiod revealed propagated severe along damages internal coal wall wall 12c). considerable Observations crushed in coal six-day in idling. period Deformations revealed severe damages were observed coal wall at a depth 58.5 m. This location 7.2 m behind working face 29.3 m above roadway considerable crushed coal in. Deformations were observed at a depth ro. It can be concluded that significantly affected by mining activities. The 58.5 m. This location 7.2 m behind working face 29.3 m above roadway ro. It can be height fractured zone reached m. When working face 5.6 m away from concluded that significantly affected by mining activities. The height fractured opening, breakage coal wall exacerbated blocked by zone reached m. When working face 5.6 m away from opening, breakage crushed coal at a depth 13.5 m 12f). coal wall exacerbated blocked by crushed coal at a depth 13.5 m 12f).

10 Minerals Minerals 2017, 2017, 7, 7, 35 x FOR PEER REVIEW Minerals 2017, 7, x FOR PEER REVIEW (a) (a) (b) (b) (c) (c) (d) (d) (e) (f) (f) Figure 12. in (b) irregular ; (c)fracture Figure 12. Observations in #4: (a) axial ; (b) (b) irregular ; (c) (c)fracture propagation; (d) wall breakage; (e) crushed coal; (f) blocked by crushed coal. propagation; (d) wall breakage; (e) crushed coal; (f) blocked by crushed coal Heights Caved Zone Fractured Zone Heights Caved Zone Fractured Zone Zone The heights change caved zone fractured zone in s during mining prosess The The heights heights is shown change change in Figure caved caved 13. The zone zone cutting fractured fractured caving zone zone heights in in s s panel during during were 3.0 mining 3.2 prosess mining m, is prosess shownis respectively; inshown Figurein height 13. Figure The cutting 13. The fractured cutting caving zone heights caving panel heights m panel (see Table were ). 3.0 The were height 3.2 m, 3.0 respectively; 3.2 m, caved respectively; zone height height fractured m, while zone fractured height zone m caved (seezone Tablem in 1). (see The Table height 1). The #1 height not caved obtained zone caved as it zone didn t reach m, while coal m, seam. height while The heights height caved zone caved caved in zone zone in #1 fractured not zone obtained #1 s not as it obtained didn t #3 reach #4 as it didn t were coal reach 18.5 seam. The 30 coal m, heights seam. respectively. The heights caved Notably, zone caved measured fractured zone heights zone fractured incaved s zone zone in #3 s fractured #4 were #3 zone 18.5 #4 were appeared m, respectively. to 30 be m, smaller respectively. Notably, than Notably, actual measured values measured heights due to heights caved early deformations zone caved fractured zone blocking zone fractured appeared in zone to appeared underground be smaller to be than s smaller actual than caused values by actual dueabutment to values early due pressure. deformations to The early measured deformations blocking heights in blocking underground caved zone in s underground caused fractured s byzone abutment were caused by pressure. abutment The measured pressure. times heights The mining measured height. caved heights zone caved fractured zone zone were fractured zone were times mining height. times mining height. Height,m Height,m caved zone fractured zone caved zone fractured zone Distance from working face to surface,m Figure 13. Change Distance from caved working zone face fractured to surface,m zone in s. Figure 13. Change caved zone fractured zone in s.

11 Minerals Minerals 2017, 2017, 7, 7, 35 x FOR PEER REVIEW Table 1. The height caved zone fractured zone. Table 1. The height caved zone fractured zone. Borehole #1 Borehole #1 Borehole #2 Borehole #3 Borehole #4 ratio to ratio to ratio to ratio to ratio to Zone Borehole #1 Borehole #1 Borehole #2 Borehole #3 Borehole #4 height/m mining height/m mining height/m mining height/m mining height/m mining Ratio to Ratio to Ratio to Ratio to Ratio to Zone height height height height height Height/m Mining Height/m Mining Height/m Mining Height/m Mining Height/m Mining Caved Height Height Height Height Height zone Caved zone Fractured zone zone 3.5. Ground Surface Subsidence When working face 96.7 m away from surface, that are parallel to working face inclination were first observed in in vicinity. The advance affecting range mining mining approximately approximately 100 m, 100 m, movable movable basin area basin significantly area significantly exceeded exceeded mining area. mining Based area. onbased advance on affecting advance affecting range range mining depth, mining it depth, is deduced it is deduced that advance that advance affecting affecting angle angle 71. As 71. working As working face moved face forward, moved forward, quantity quantity size cracks size increased cracks increased furr. furr. When working face face mm ahead ahead surface surface,, step step subsidence subsidence appeared appeared in in vicinity vicinity.. The ground The ground surface surface subsided subsided gradually gradually continuously. continuously. After After mining process, mining process, severe damages severe damages to ground to ground surface surface step subsidence step subsidence (maximum (maximum subsidence subsidence = 4.2 m) = were 4.2 m) observed, were observed, surface cracks surface were cracks distributed were distributed all over all goaf. over Thegoaf. inter-fracture The inter-fracture inter-step distances inter-step varied distances from varied 1.0 to 6.2from m. The 1.0 process to 6.2 m. The surface process subsidence around surface s subsidence around shown in s Figure 14. shown in Figure 14. (a) (b) (c) (d) (e) (f) Figure 14. Surface subsidence around s: (a) cracks; (b) quantity cracks increased; (c) Figure 14. Surface subsidence around s: (a) cracks; (b) quantity cracks increased; (c) surface surface subsidence; (d) step subsidence; (e) subsidence increased; (f) final subsidence. subsidence; (d) step subsidence; (e) subsidence increased; (f) final subsidence. 4. Discussion 4. Discussion 4.1. Movement Overburden Strata Fracture Propagation Ro caving, as well as separation dislocation, were observed as a result underground mining. Mining-induced were were propagated propagated overburden overburden strata movement strata movement transferred upwards transferred to upwards ground to surface ground in this surface process, in this resulting process, in surface resulting deformation. in surface deformation. Eventually, Eventually, movable basin area exceeded mined-out area. Based on differences

12 Minerals 2017, 7, movable basin area exceeded mined-out area. Based on differences surface subsidence velocity, extent strata movement propagation, whole process can be divided into three stages 15): Minerals 2017, 7, x FOR PEER REVIEW (a) Initiation stage: This stage started when working face 100 m away from surface ended when subsidence working velocity, face extent 20strata m away movement from. propagation This stage, characterized whole by process cracks distributed can be divided oninto three ground stages surface, 15): appeared on loess layer strata (a) beneath Initiation it. The stage: density This stage average started when size se working cracks face increased 100 m as away from working face moved forward. ended Surface when deformation working face local 20 m wall away exfoliation from were. observed. This stage Additionally, axialcharacterized circularby cracks distributed were observed on ground on near-surface surface, strata, while appeared deep on strata loess were not significantly layer affected. strata beneath it. The density average size se cracks increased as working face moved forward. Surface deformation local wall exfoliation were (b) Active stage: This stage started when working face 20 m away from observed. Additionally, axial circular were observed on near-surface strata, while ended when working face 120 m past. Propagation deep strata were not significantly affected. (b) in Active overburden stage: This strata stage started movement when working ground face surface 20 m away werefrom significantly accelerated. Mining-induced ended when working appeared face in120 m coal past rock. aheadpropagation working face withinin a certain range. overburden When strata working face movement reached, ground surface step subsidence were significantly observed, accelerated. resulting in severe Mining-induced surface ground deformation. appeared in Large-sized coal rock ahead working local damages face within were a certain observed in range. bedrocks When beneath working face epipedon. reached Separation, step subsidence dislocation observed, overburden resulting strata werein observed severe surface after ground failuresdeformation. immediate Large-sized ro main ro, local damages mining-induced were observed in bedrocks beneath epipedon. Separation dislocation overburden strata propagated. The overburden movement, which characterized by movement key were observed after failures immediate ro main ro, mining-induced strata, transferred upwards to ground surface gradually. propagated. The overburden movement, which characterized by movement (c) Degradation key strata, stage: This transferred stage started upwards when to ground working surface face gradually. 120 m past. (c) During Degradation this stage, stage: This movement stage started when overburdened working face strata degraded 120 m past (re. a slow subsidence During this duestage, to gravitation). movement The bed separation overburdened strata degraded that (re had developed a slow in overburden subsidence strata due were to gravitation). partially closed The bed to compaction separation rate that surface had developed deformation in slowed down. overburden Surface cracks strata were step partially subsidence closed to were compaction partially mitigated. rate surface deformation slowed down. Surface cracks step subsidence were partially mitigated. It should be pointed out that duration each stage varies as a result geological conditions It should be pointed out that duration each stage varies as a result geological (e.g., conditions coal seam(e.g., thickness, coal seam depth, thickness, overburden depth, overburden strata properties) strata properties) mining area mining area mining conditions mining (mining conditions depth (mining moving depth rate moving working rate face). working face). Initiation stage Active stage Degradation stage -100m -80m -60m -40m -20m 0m 20m 40m 60m 80m 100m 120m 140m 160m Figure 15. Three stages overburden movement. Figure 15. Three stages overburden movement Distribution Fractures in Overburden Strata 4.2. Distribution Fractures in Overburden Strata Borehole television approach can be used not only to reveal conditions in for Borehole qualitative television identification, approach but also can be to used obtain not pertinent only to information reveal conditions for quantitative in analysis, for qualitative especially identification, for fracture but number, also to fracture obtainangle pertinent fracture information width. for In this quantitative field measurement, analysis, especially re

13 Minerals 2017, 7, for fracture number, fracture angle fracture width. In this field measurement, re are 106 totally in surface s, removing that were induced during drilling. The number mining-induced can reflect degree influence by mining to rock mass. As shown in Figure 16a, with increase drilling depth, degree mining influence Minerals 2017, 7, x FOR PEER REVIEW deepening, fracture number has increased dramatically. The width reflects propagation degree. As seen in Figure 16b, fracture number decreased with increase width. Fractures with are 106 totally in surface s, removing that were induced during width between mm account for 76%, which proves that are mainly drilling. low width. As shown in Figure 16c, number at all orientations relatively average, The number interval mining-induced can reflect degree influence by mining to accounted for 62%. It can be concluded that most mining-induced rock mass. As shown in Figure 16a, with increase drilling depth, degree mining are vertically oriented. influence deepening, fracture number has increased dramatically. The width reflects The fractured zone can be divided into three parts: rock blocks, through-going vertical propagation degree. As seen in Figure 16b, fracture number decreased with increase horizontal caused by bedding layer separation. According to different attributes, width. Fractures with width between mm account for 76%, which proves that with field observations, we confirm range three parts (see Figure 17). In most cases, are mainly low width. As shown in Figure 16c, number at all in bending subsidence zone exhibited small apertures. However, with large orientations relatively average, interval accounted for 62%. It can be concluded apertures were also observed due to limited tensile strength certain strata, especially those near that most mining-induced are vertically oriented. ground surface (large step subsidence) y = e x Percentage(%) Fracture width, mm (a) Percentage(%) Fracture width, mm (b) Figure 16. Cont.

14 Minerals 2017, 7, Minerals 2017, 7, x FOR 25 PEER REVIEW Percentage(%) Percentage(%) Fracture angle( ) (c) Figure 16. Fracture attributes: (a) number versus depth; (b) proportion 5 versus fracture width; (c) proportion versus fracture angle. 0 The fractured zone 0-20 can be divided into three 40-60parts: rock blocks, through-going vertical horizontal caused by Fracture bedding angle( ) layer separation. According to different attributes, with field observations, we confirm (c) range three parts (see Figure 17). In most cases, in bending subsidence zone exhibited small apertures. However, Figure 16. Fracture attributes: (a) number versus depth; (b) proportion with Figure large 16. apertures Fracture attributes: were also (a) number observed due to versus limited tensile depth; strength (b) proportion certain strata, versus fracture width; (c) proportion versus fracture angle. especially versus fracture those near width; (c) ground proportion surface (large versus fracture step angle. subsidence). The fractured zone can be divided into three parts: rock blocks, through-going vertical horizontal caused by bedding layer separation. According to different Bending zone(150~165m) attributes, with field observations, we confirm range three parts (see Figure 17). In most cases, in bending subsidence zone exhibited small apertures. However, with large apertures were also observed due to limited tensile strength upper section certain strata, (30~36m) especially those near ground surface (large step subsidence). middle section Fractured zone (40~45m) (95~105m) lower section (24~26m) Bending zone(150~165m) Caved zone(18~25m) Figure 17. Distribution in overburden strata. Figure 17. Distribution in overburden strata. upper section (30~36m) middle section Fractured zone (40~45m) (95~105m) 5. Conclusions 5. Conclusions (1) Field observations showed that overburden strata generally experienced lower phases section ro (1) Field observations showed that overburden strata generally experienced (24~26m) phases caving, generation fracture, bed separation, dislocations, fracture propagation, surface ro Caved zone(18~25m) subsidence, caving, generation closing. fracture, The bed entire separation, mining process dislocations, can be fracture divided propagation, into initiation surface subsidence, stage, active closing stage,. The degradation entire mining stage process according can be to divided activities into initiation overburden stage, strata active stage, propagation Figure degradation mining-induced stage 17. Distribution. according to activities overburden strata in overburden strata. (2) propagation The advance affecting mining-induced range. angle working face were 96.7 m 71, respectively. (2) 5. Conclusions The advance caved zone affecting height is range times angle mining working height, face were 96.7 length m fractured 71, respectively. zone is The caved zone times height is mining height. times The height miningrange height, three parts length in fractured zone zone is is (1) Field observations showed that overburden strata generally experienced phases ro , 40 45, times m. Significant mining height. Thewere height observed range in three bending parts zone. fractured Step zone subsidence is 24 26, caving, generation fracture, bed separation, dislocations, fracture propagation, surface 40 45, cracks, 30 35which m. Significant indicate severe damages, were observed were observed in on bending ground zone. surface Step above subsidence goaf. (3) subsidence, closing. The entire mining process can be divided into initiation cracks, The which indicate television severe approach damages, is an were effective observed onrapid ground way to surface observe above goaf. dynamic morphology stage, active s stage, facilitate degradation investigation stage according temporal to activities spatial evolution overburden (3) The television approach is an effective rapid way to observe dynamic mining-induced strata propagation mining-induced overburden strata. morphology s facilitate investigation during mining temporal process. spatial Neverless, evolution (2) The advance affecting range angle working face were 96.7 m 71, respectively. mining-induced in overburden strata during mining process. Neverless, The caved zone height is times mining height, length fractured zone is application this approach has been limited by several issues. For instance, resolution times mining height. The height range three parts in fractured zone is obtained images may be significantly affected by air humidity in underground , 40 45, m. Significant were observed in bending zone. Step subsidence When air is very damp in s, observed images are ten blurred. Also, cracks, which indicate severe damages, were observed on ground surface above goaf. observation cannot be achieved in presence wall damage or horizontal dislocation (3) The television approach is an effective rapid way to observe dynamic, which lead to collapse blockage. morphology s facilitate investigation temporal spatial evolution mining-induced in overburden strata during mining process. Neverless,

15 Minerals 2017, 7, Acknowledgments: The research is financially supported by National Basic Research Program China (No. 2015CB251600) National Natural Science Foundation China (Nos ). We wish to thank Wangjialing Coal Mine for supporting to conduct this important study, as well as Yang Zhang, Shenglong Zhang, Jiang Wen for assistance data collection. Author Contributions: All authors contributed to publishing this paper. Hongzhi Wang performed technological development prepared edited manuscript. Dongsheng Zhang revised reviewed manuscript. Xufeng Wang, Wei Zhang partially participated in literature research data processing during research process. Conflicts Interest: The authors declare no conflict interest. References 1. Qian, M.G.; Shi, P.W.; Xu, J.L. Mining Pressure Strata Control; China University Mining Technology Press: Xuzhou, China, 2010; pp (In Chinese) 2. Zhang, D.S.; Fan, G.W.; Ma, L.Q.; Wang, X.F. Aquifer protection during longwall mining shallow coal seams: A case study in Shendong Coalfield China. Int. J. Coal Geol. 2011, 86, [CrossRef] 3. Ma, L.Q.; Du, X.; Wang, F.; Liang, J.M. Water-preserved mining technology for shallow buried coal seam in ecologically-vulnerable coal field: A case study in Shendong coal field China. Disaster Adv. 2013, 6, Fan, G.W.; Zhang, D.S.; Zhai, D.Y.; Wang, X.F.; Lu, X. Laws mechanisms slope movement due to shallow buried coal seam mining underground gully. J. Coal Sci. Eng. 2009, 15, [CrossRef] 5. Lai, X.P.; Cai, M.F.; Xie, M.W. In situ monitoring analysis rock mass behavior prior to collapse main transport roadway in Linglong Gold Mine, China. Int. J. Rock Mech. Min. Sci. 2006, 43, [CrossRef] 6. Palchik, V. Localization mining-induced horizontal along rock layer interfaces inoverburden: Field measurements prediction. Environ. Geol. 2005, 48, [CrossRef] 7. Palchik, V. Formation fractured zones in overburden due to longwall mining. Environ. Geol. 2003, 44, Palchik, V. Experimental investigation apertures mining-induced horizontal. Int. J. Rock Mech. Min. 2010, 47, [CrossRef] 9. Zhang, S.T.; Liu, Y. A simple efficient way to detect mining induced water-conducting fractured zone in overlying strata. Energy Procedia 2012, 16, [CrossRef] 10. Yu, J.C.; Liu, Z.X.; Yue, J.H.; Liu, S.C. Development prospect geophysical technology in deep mining. Prog. Geophys. 2007, 22, (In Chinese) 11. Kang, H.P.; Lin, J. Geomechanical tests ir applications in rock anchorage design. In Proceedings 11th Congress International Society for Rock Mechanics, Lisbon, Portugal, 9 13 July 2007; pp Kang, H.P.; Si, L.P.; Su, B. Borehole observation methods in coal rock mass ir applications. J. China Coal Soc. 2010, 35, (In Chinese) 13. Wang, C.Y.; Law, K.T. Review camera technology. Chin. J. Rock Mech. Eng. 2005, 24, (In Chinese) 14. Han, Z.Q.; Wang, C.Y.; Hu, S. Digital camera technology its applications in mining geology exploration. In Proceedings 24th International Mining Congress Exhibition Turkey, Antalya, Turkey, April 2015; pp Han, Z.Q.; Wang, C.Y.; Liu, S.B.; Zhu, H.Y. Research on connectivity deep ore-lodes based on digital camera. Disaster Adv. 2013, 6, Han, Z.Q.; Wang, C.Y.; Zhu, H.Y. Research on deep joints lode extension based on digital camera technology. Pol. Marit. Res. 2015, 22, [CrossRef] 17. Tan, Y.L.; Zhao, T.B.; Xiao, Y.X. In situ investigations failure zone floor strata in mining close distance coal seams. Int. J. Rock Mech. Min. 2010, 47, [CrossRef] 18. Tan, Y.L.; Ning, J.G.; Li, H.T. In situ explorations on zonal disintegration ro strata in deep coalmines. Int. J. Rock Mech. Min. 2012, 49, [CrossRef] 19. Tan, Y.L.; Yu, F.H.; Chen, L. A new approach for predicting bedding separation ro strata in underground coalmines. Int. J. Rock Mech. Min. 2013, 61, [CrossRef]

16 Minerals 2017, 7, Xiong, Z.Q.; Wang, C.; Zhang, N.C.; Tao, G.M. A field investigation for overlying strata behavior study during protective seam longwall overmining. Arab. J. Geosci. 2015, 8, [CrossRef] 21. Zhang, Y.J.; Liu, Z.G.; Guo, D. Study on simulation monitoring overburden failure height multifull-mechanized caving mining. In Proceedings International Conference on Mine Hazards Prevention Control, Qingdao, China, October 2010; pp Gao, B.B.; Wang, X.L.; Zhu, M.L. Dynamic development characteristics overburden strata two-zones under conditions compound ro, highly gassy thick coal seam in full-mechanized top coal caving faces. Chin. J. Rock Mech. Eng. 2012, 31, (In Chinese) 2017 by authors. Licensee MDPI, Basel, Switzerl. This article is an open access article distributed under terms conditions Creative Commons Attribution (CC BY) license (