Technical Aspects of Mining Rate Improvement in Steeply Inclined Coal Seams: a Case Study

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Article Technical Aspects of Mining Rate Improvement in Steeply Inclined Coal Seams: A Case Study

Zbigniew Rak * , Jerzy Stasica, Zbigniew Burtan and Dariusz Chlebowski

Faculty of Mining and Geoengineering, AGH University of Science and Technology, 30-059 Kraków, Poland; [email protected] (J.S.); [email protected] (Z.B.); [email protected] (D.C.) * Correspondence: [email protected]

Received: 27 September 2020; Accepted: 20 November 2020; Published: 24 November 2020

Abstract: This paper presents our experience obtained when mining the thick and steeply-inclined Seam 510 in the Polish Kazimierz-Juliusz coal mine with the use of a unique mechanical face mining system. Seam 510, which is 15–20 m thick and inclined at angles of 40◦–45◦, was initially treated as uneconomical because eﬀective mining systems were not available. However, to extract high-quality coal resources, a completely mechanized variant of the sublevel caving system was designed based on standard machines and equipment applied in coal mining. Extraction was conducted top-down at the levels of the particular mining sub-level drifts with roof caving. The faces in the extracted coal release areas were protected by a single pair of specially designed mechanized mining system sections. One of the basic problems revealed during extraction of subsequent mining panels, was the observed changeability of the resource mining rates. The extraction losses changed in the available resources from less than 10% to about 50%. This paper presents two typical courses of changes in the extractable resource mining rates. Similar rate changes occurred in both cases with continued mining of a single seam section. Our analysis enabled deposit loss estimations and production output planning under the sublevel caving systems applied in the extraction of seam deposits of similar structure.

Keywords: coal mining; thick and steep coal beds; sublevel caving; extraction eﬀectiveness; deposit extraction rate

1. Introduction Mining of thick and steeply-inclined hard coal deposits is one of the most difficult engineering challenges. These coal seams are mainly mined in China [1–3]. For that reason, the topic discussed in this paper has been widely studied in that country [4–13]. Longwall mining with roof caving is the predominant mining system applied to this type of seam; however, backfilling systems are exclusively used for steep coal seams with a narrow or medium thickness [14]. Current underground mining relies on three types of extracting methods for thick and steeply-inclined hard coal deposits. The first two methods are based on longwall top coal caving (LTCC) technology. When the deposit’s dip angle is up to 50◦, LTCC is applied and adjusted to the longwalls that are parallel to stratification, but only in Chinese mines (Figure1)[4,8,15,16].

Resources 2020, 9, 138; doi:10.3390/resources9120138 www.mdpi.com/journal/resources Resources 2020, 9, 138 2 of 16 ResourcesResources 2020 2020, 9,, 9x, xFOR FOR PEER PEER REVIEW REVIEW 2 of2 of 16 16

FigureFigure 1. 1. Diagram Diagram of of an an inclined inclined wall wall mined mined by by the the longwalllongwall longwall top top top coal coal coal caving caving caving (LTCC) ( LTCC)(LTCC) system system system in the in in Dayuanthethe Dayuan Dayuan Coal Coal MineCoal Mine Mine [4]. [4]. [4].

TheThe longwalllongwall longwall heightheight height isis usuallyis usually usually about about about 2.5 2.5 m,2.5 m, andm, and and another another another 2–5 2–5 m2–5 of m m coal of of coal thickness coal thickness thickness is released is is released released from thefromfrom roof. the the Inroof. roof. this In case,In this this the case, case, fully the the mechanized fully fully mechanized mechanized longwall longwall longwall systems systems aresystems additionally are are additionally additionally equipped equipped equipped with devices with with thatdevicesdevices stabilize that that thestabilize stabilize powered the the roofpowered powered support roof roof and support thesupport longwall an andd facethe the longwall conveyors.longwall face face The conveyors. miningconveyors. of steeplyThe The mining mining inclined of of longwallsteeplysteeply inclined inclined faces is, longwall however,longwall faces faces associated is, is, however, however, with manyassociated associated problems with with mainlymany many problems relatedproblems to mainly roofmainly stability, related related asto to wellroof roof asstability,stability, maneuvering as as well well withas as maneuvering maneuvering roof support, with with the conveyor,roof roof suppo suppo andrt,rt, the thethe conveyor, shearer.conveyor, Work and and the safety the shearer. shearer. conditions Work Work require safety safety extensiveconditionsconditions worker require require experience extensive extensive and worker worker the use experience experience of advanced and and process the the use use solutions. of of advanced advanced For those process process reasons, solutions. solutions. mining For For of thickthosethose seams,reasons, reasons, whose mining mining inclination of of thick thick seams, exceeds seams, whose whose 40◦, was inclination inclination abandoned exceeds exceeds in Poland. 40°, 40°, was was abandoned abandoned in in Poland. Poland. TheThe second second typetype type ofof of system system system requires requires requires coal coal coal seam seam seam subdivision subdivision subdivision into into into horizontal horizontal horizontal levels levels levels and and miningand mining mining of the of of particularthethe particular particular level level bylevel subsequent by by subsequent subsequent longwall longwall longwall or shortwall or or sh shortwallortwall top coal top top caving coal coal caving facescaving from faces faces top from from to bottomtop top to to [bottom 17bottom–20]. Levels[17–20].[17–20]. of Levels 2.5–3.5Levels of mof are2.5–3.5 2.5–3.5 mined m m withare are mined amined shearer, with with followed a ashearer, shearer, by dropping followed followed the by bytop dropping dropping coal whose the the thicknesstop top coal coal rangeswhose whose fromthicknessthickness several ranges ranges to a from dozen from several several or so meters to to a adozen dozen (Figure or or so2 ).so meters meters (Figure (Figure 2). 2).

FigureFigure 2. 2. ShortwallShortwall Shortwall top top coal coal caving caving method method with with the the co coal coalal seam seam subdivided subdivided into into horizontal horizontal levels. levels.

InIn this,this, this, typicaltypical typical mechanical mechanical mechanical systems systems systems are are appliedare applied applied in the in in LTCCthe the LTCC technology,LTCC technology, technology, which which is which why theis is why methodwhy the the ismethodmethod fully mechanized. is is fully fully mechanized. mechanized. Such systems Such Such have systems systems been appliedhave have been forbeen severalapplied applied decades for for several several and several decades decades variations and and several several have beenvariationsvariations developed, have have been e.g., been thedeveloped, developed, French sutirage e.g., e.g., the orthe theFrench French Slovenian sutirage sutirage velenje or or the [the21 Sloven– 26Sloven]. Theianian systemvelenje velenje has [21–26]. [21–26]. been usedThe The tosystemsystem extract has has steep been been coal used used seams to to extract extract 20–25 steep m steep thick coal coal in seams the seams Spanish 20–25 20–25 provincem m thick thick in of in the Leon the Spanish Spanish [27] using province province shortwall of of Leon Leon top coal[27] [27] caving.usingusing shortwall shortwall The shortwall top top coal ﬁttingscoal caving. caving. are aThe bitThe dishortwall shortwallﬀerent there fi fittingsttings compared are are a abit withbit different different typical solutions.there there compared compared The system with with typicaltypical solutions. solutions. The The system system uses uses a asingle-drum single-drum shearer shearer and and light light sets sets of of frame frame supports. supports. Due Due to to hardnesshardness of of the the coal, coal, top top coal coal dropping dropping is is preceded preceded by by layer layer destruction destruction us usinging blasting blasting operations. operations.

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Resources 2020, 9, x FOR PEER REVIEW 3 of 16 uses a single-drum shearer and light sets of frame supports. Due to hardness of the coal, top coal droppingThe isthird preceded system by is layer called destruction sublevel usingcaving, blasting which operations.was the focus of this study. The system is widelyThe applied third system in underground is called sublevel ore mining caving, [28,29]. which wasHere, the top focus coal of is this not study.dropped The to system the longwall is widely or appliedshortwall, in underground but rather to orethe miningexcavation [28,29 system]. Here, call toped coal the is“sub-level not dropped drift”, to thewhich longwall has a orcross-section shortwall, butranging rather from to the a excavationdozen or so, system to more called than the 20 “sub-level m2. Depending drift”, which on coal has seam a cross-section thickness, ranging one to fromthree 2 asub-level dozen or so,drifts to moreare performed than 20 m .on Depending each sub-leve on coall. seamExtraction thickness, is performed one to three by sub-level dropping drifts the arepreviously performed mined on eachcoal sub-level.using fan-cut Extraction blasting ispatte performedrn operations by dropping (Figure the3) [30–33]. previously Another mined method coal usingis based fan-cut on blastinghydro-cutting pattern instead operations of blasting. (Figure3 )[Wate30–33r is]. Anotherused in methodthe cutting is based process on hydro-cutting for both coal insteadcutting of and blasting. transportation Water is used[34–37]. in the cutting process for both coal cutting and transportation [34–37].

FigureFigure 3. 3.Sub-level Sub-level caving caving in in a a coal coal seam, seam, with with two two drifts drifts [ 33[33].].

TheThe sub-level sub-level height height usually usually ranges ranges from severalfrom several meters tometers about 30to m.about This system30 m. isThis characterized system is bycharacterized low capital investmentsby low capital into investments mine-face equipment into mine-face and low equipment operating costs.and low Sub-level operating drifts costs. are protectedSub-level by drifts steel are supports protected that areby dismantledsteel supports before that top are coal dismantled dropping; thus,before the top supports coal dropping; can be reused thus, manythe supports times. Coal can loadingbe reused is performedmany times. either Coalonly loading by gravity is performed or by scop either feeders, only by with gravity transportation or by scop byfeeders, chain with and belttransportation conveyors. by Two chain to and three belt miners conveyors. are employed Two to three at the miners mine face.are employed In this study, at the wemine constructed face. In athis detailed study, coal-seam we constructed mining methoda detailed and describedcoal-seam amining mechanization method system. and described However, a muchmechanization larger coal system. loss is anotherHowever, typical much feature larger of coal the loss system is another in question. typical The feature coal loss of the usually system does in notquestion. exceed The 15% coal in the loss LTCC usually systems, does not but exceed can be more15% in than the 50%LTCC in systems, the sub-level but can caving be more systems. than 50% in theWe sub-level introduced caving a unique systems. face mechanization method in the Kazimierz-Juliusz coal mine in Poland to extractWe aintroduced bed about a 20 unique m thick face and mechanization inclined at more method than 40 in◦. Thethe Kazimierz-Juliusz coal minemine inin SosnowiecPoland to operatedextract a continuallybed about 20 from m thick 1884 toand 2015. inclined Starting at more in the than 1990s, 40°. coal The was Kazimierz-Juliusz extracted only from coal themine thick in Sosnowiec Seam 510. operated That seam continually had been from mined 1884 by theto 2015. longwall Starting system in the in 1990s, areas wherecoal was inclination extracted didonly not from exceed the 30thick◦, with Seam division 510. That into seam layers had with been roof mined caving by or the backﬁll. longwall The system coal resources in areas where were almostinclination completely did not extracted exceed at30°, the with beginning division of thisinto century. layers with Extraction roof caving of the remainingor backfill. portion The coal of Seamresources 510, withwere an almost inclination completely of 35◦ to extracted 50◦, was theat objectthe beginning of designs of and this tests century. conducted Extraction jointly byof thethe researchersremaining ofportion the AGH of Seam University 510, with of Science an inclination and Technology, of 35° to Faculty50°, was of the Mining object and of designs Geoengineering, and tests inconducted Kraków, andjointly the by engineers the researchers of the Kazimierz-Juliusz of the AGH University coal mine of Science [32,38]. and As Technology, a result, a completely Faculty of mechanizedMining and variant Geoengineering, of the sub-level in Kraków, caving system and the was engineers designed of and the applied Kazimierz-Juliusz with success fromcoal 2003mine until[32,38]. the As coal a mineresult, closed a completely down in mechanized 2015. variant of the sub-level caving system was designed and Theapplied ﬁrst partwith ofsuccess this paper from presents 2003 until the the geological coal mine conditions closed down of Seam in 2015. 510 in the area of extraction conductedThe first by the part mechanized of this paper sub-level presents caving the system, geological as well conditions as the basic of issues Seam related 510 in to the extraction area of technology.extraction conducted We present by the an mechanized example of sub-level a method ca forving making system, coal as well resources as the basic available issues and related the preparationto extraction of technology. coal beds, followedWe present by miningan example works of mechanizationa method for making and a shortcoal resources description available of the extractionand the preparation technology. of The coal second beds, followed part of this by papermining contains works mechanization the extractable and resource a short mining description rates. of the extraction technology. The second part of this paper contains the extractable resource mining rates. In this regard, certain repeatable relationships were identified in that area during about a

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Resources 2020, 9, x FOR PEER REVIEW 4 of 16 In this regard, certain repeatable relationships were identiﬁed in that area during about a dozen of dozenyears of years of mining. of mining. Such Such relationships relationships are illustrated are illustrated on the on basis the ofbasis the of results the results obtained obtained in two in typical two typicaldeposit deposit ﬁelds. fields.

2. Material2. Material and Methods and Methods

2.1. Geological2.1. Geological Characteristics Characteristics of the ofResearch the Research Area Area SeamSeam 510 of 510 mine of minesection section M-3 was M-3 deposited was deposited in the in form the formof a trough of a trough.. The trough The trough bottom bottom was was aboutabout 500 m 500 deep, m deep, with withthe deposi the depositt outcrop outcrop located located at a depth at a depth of about of about 50 m 50in mthe in trough the trough wing. wing. MineMine section section M-3 was M-3 cut was through cut through and divided and divided into northern into northern and southern and southern parts partsby a fault by a faultdropping dropping from from25 to 2540 tom. 40 The m. coal The seam coal seam inclination inclination in the in southern the southern part, part,where where the sub-level the sub-level caving caving system system was implemented,was implemented, was 45° was (Figure 45◦ (Figure 4). The4). seam The thickness seam thickness varied variedin that inarea that from area 16 from to more 16 to than more 20 than m. A20 number m. A numberof faults of were faults present were present in the inmine the section, mine section, representing representing various various drop dropand direction and direction parameters.parameters. Fault Faultdrops drops changed changed from fromseveral several to about to about a dozen a dozen meters. meters.

Figure 4.Figure Cross-section 4. Cross-section of Seam of 510 Seam in mine 510 insection mine sectionM-3 of the M-3 Kazimierz-Juliusz of the Kazimierz-Juliusz coal mine. coal mine.

The averageThe average thermal thermal coal coal properties properties of ofthe the mined mined coalbed areare asas follows: follows: 3.5% 3.5% ash, ash, 0.4% 0.4% sulphur, sulphur,and and 32.5% 32.5% volatiles, volatiles, with with a calorific a calorific value value of 5800 of Kcal 5800/kg Kcal/kg and a densityand a density of 1.3 kg of/m 1.33. Thekg/m coal3. The of Seam coal of510 Seam was 510 characterized was characterized by an average by an uniaxialaverage compressiveuniaxial compressive strength (Rstrengthc) of more (Rc) than of more 20 MPa, than with 20 the MPa,maximum with the valuemaximum exceeding value 35 exceeding MPa. The 35 compressive MPa. The strengthcompressive of the strength sandstones of the occurring sandstones in the roof occurringrange in from the 30roof to range more thanfrom 70 30 MPa. to more The roofthan mudstone70 MPa. The displayed roof mudstone strength rangingdisplayed from strength 20 to more rangingthan from 30 MPa, 20 to whilemore thethan strength 30 MPa, of while sandy the schist strength ranged of sandy from 30 schist to 40 ranged MPa. from 30 to 40 MPa. SeamSeam 510 was 510 classified was classified as a Category as a Category I methane I methane hazard hazard(on a four-degree (on a four-degree scale). However, scale). However, the 3 3 methanethe methanecontent of content Seam 510 of Seam did not 510 exceed did not 0.1 exceed m3/Mg 0.1csw m (0.1/Mg mcsw3 of (0.1methane m of in methane one ton in of one dry ton and of dry cleanand coal clean substance) coal substance) in the area in of the the area sub-level of the sub-levelcaving operation. caving operation. The coal from The coal Seam from 510 Seam across 510 the across wholethe coal whole mine coal was mine categorized was categorized as a Group as V a Groupignition V temperature ignition temperature (on the five-degree (on the five-degree scale of fire scale of hazards),fire hazards),representing representing coal with coal a very with high a very tendency high tendency to self-ignite. to self-ignite. The self-ignition The self-ignition indicator indicator of mine ofsection mine M-3 section coal M-3 was coal determined was determined to be 179 to °C be/min, 179 with◦C/min, an activation with an activation energy of energy37 kJ/mol. of 37 The kJ /mol. fire incubationThe fire incubation time amounted time amounted to ca. 30 days. to ca. 30 days. The coalThe deposit coal deposit level levelwas categorized was categorized as a Degree as a Degree II water II water hazard hazard (on a (onthree-degree a three-degree scale). scale). The aquiferThe aquifer located located within within the thesandstone, sandstone, lying lying above above Seam Seam 510, 510, was thethe mainmain source source of of water water hazard. hazard.Coal Coal workings workings and and cutting cutting operations operations caused caused water water dripping dripping and local and leaks.local Increasedleaks. Increased inflow was inflowobserved was observed at the finalat the stage final of stage mining, of mini withng, small with influxsmall frominflux Tertiary from Tertiary sands. sands. SeamSeam 510 was 510 assigned was assigned to Degrees to Degrees I and II Iof and rock-burst II of rock-burst hazard (on hazard a three-degree (on a three-degree scale). Coal scale). miningCoal by miningthe sub-level by the caving sub-level me cavingthod was method performed was performed in accordance in accordance with the withregimes the specific regimes for specific Degree III rock-burst hazard due to the novelty of the mining method and the intention to ensure safety and obtain rock-mass data from the mining field. For that reason, the number of detectors (seismometers and geophones) was increased in the mining areas.

Resources 2020, 9, 138 5 of 16 for Degree III rock-burst hazard due to the novelty of the mining method and the intention to ensure safety and obtain rock-mass data from the mining ﬁeld. For that reason, the number of detectors (seismometers and geophones) was increased in the mining areas. Resources 2020, 9, x FOR PEER REVIEW 5 of 16 2.2. Sub-Level-Caving Mining Method—Assumptions and Practical Aspects 2.2. Sub-Level-Caving Mining Method—Assumptions and Practical Aspects The high strength parameters of coal found at Seam 510 in the Kazimierz-Juliusz coal mine The high strength parameters of coal found at Seam 510 in the Kazimierz-Juliusz coal mine necessitated the use of blasting works for coal mining. Due to the changing coal seam thickness and its necessitated the use of blasting works for coal mining. Due to the changing coal seam thickness and extent, the longwall or shortwall top caving methods were not used. Instead, a typical sub-level caving its extent, the longwall or shortwall top caving methods were not used. Instead, a typical sub-level systemcaving with system a single with sub-level a single sub-level drift was drift applied was inapplied this coal in this seam. coal seam. 2.2.1. General Description of the Sub-Level Caving System Applied in the Kazimierz-Juliusz 2.2.1. General Description of the Sub-Level Caving System Applied in the Kazimierz-Juliusz Coal Coal Mine Mine Mining panels were designed by dividing Seam 510 into horizontal blocks, determined by the Mining panels were designed by dividing Seam 510 into horizontal blocks, determined by the planes perpendicular to the roof and bottom. The blocks, or mining panels, were shaped as cross-section planes perpendicular to the roof and bottom. The blocks, or mining panels, were shaped as squares with a vertically-situated diagonal. The square side length of the panel cross-section was cross-section squares with a vertically-situated diagonal. The square side length of the panel closecross-section to the seam was thickness close to inthe the seam given thickness mining in area, the given i.e., usually mining from area, 16i.e., to usually 24 m [ 32from,38]. 16 The to 24 panel m arrangement[32,38]. The diagram panel arrangement for Seam 510 diagram is shown for inSeam Figure 5105 is. shown in Figure 5.

FigureFigure 5. Subdivision5. Subdivision of of Seam Seam 510 510 into into mining mining panelspanels withwith extraction sequence sequence [32]. [32].

OneOne working, working, called called the sub-levelthe sub-level drift, drift, was was cut outcutfrom out from the transportation the transportation ramp ramp in each in mining each panelmining at particular panel at particular mine levels. mine Theselevels. workingsThese work wereings cutwere out cut nearly out nearly horizontally, horizontally, observing observing the directionthe direction of the seam of the extent seam at extent the seam’s at the bottom,seam’s bo startingttom, starting with the with lowest the mining lowest panel.mining The panel. drift The was cutdrift out fromwas cut the out transportation from the transportation ramp to the ramp field to boundary the field inboundary the given in the deposit given wing. deposit wing. OnceOnce the the sub-level sub-level drift drift reached reached the assumedthe assumed mining mining panel panel boundary boundary and and after after the driftthe drift was fitted,was miningfitted, was mining conducted was conducted by caving. by The caving. mining The operations mining operations continued continued by regularly by regularly repeated drillingrepeated of a fan-shapeddrilling of blasting a fan-shaped borehole blasting arrangement, borehole explosive arrangement, charging explosive and blasting, charging gravitational and blasting, release, gravitational release, and coal removal with conveyors and reloading systems in the sub-level drift. and coal removal with conveyors and reloading systems in the sub-level drift. Mining was completed at the run out once the stopping line was reached, usually at the Mining was completed at the run out once the stopping line was reached, usually at the protection protection pillar location designed for the transport ramp. The face equipment was relocated to the pillar location designed for the transport ramp. The face equipment was relocated to the subsequent subsequent panel before the final liquidation of the previous panel by placing an insulation plug. A panel before the final liquidation of the previous panel by placing an insulation plug. A diagram of the diagram of the sub-level caving system applied in the Kazimierz-Juliusz coal mine is presented in sub-levelFigure caving6. system applied in the Kazimierz-Juliusz coal mine is presented in Figure6.

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FigureFigure 6. 6.Sub-level Sub-level caving caving systemsystem diagram.diagram. Figure 6. Sub-level caving system diagram. TheThe details details of the of particularthe particular engineering engineering stages stages of the of sub-level the sub-level caving caving mining mining technology technology applied for SeamappliedThe 510 fordetails of Seam the of Kazimierz-Juliusz510 the of particularthe Kazimierz-Juliusz engin coaleering mine co arestagesal providedmine of are the provided insub-level the sections in caving the sections below. mining below. technology applied for Seam 510 of the Kazimierz-Juliusz coal mine are provided in the sections below. 2.2.2.2.2.2. Development Development 2.2.2. Development TheThe connection connection of of the the mining mining and and ventilation ventilation workingsworkings withinwithin the coal seam seam inclined inclined at at 45° 45 ◦ (with(with aThe di aﬀ differenceconnectionerence in in heightof height the ofmining of ca. ca. 110 and110 and ventilationand 170 170 m)m) workings waswas performedperformed within by the cutting coal seam a a series seriesinclined of of stepped steppedat 45° ramps.(withramps. Sucha differenceSuch ramps ramps were in heightwere composed composedof ca. of110 fairly andof short fairly170 m) sectionsshor wast sectionsperformed (usually (usually ca. by 60 cutting m ca. each) 60 a series situatedm each) of diagonallysteppedsituated ramps. Such ramps were composed of fairly short sections (usually ca. 60 m each) situated withdiagonally respect to with the coalrespect seam to the extent coal andseam with extent an inclinationand with an angle inclination of 25◦ angle(Figure of 725°). This(Figure inclination 7). This ofdiagonally theinclination stepped with of ramps the respect stepped allowed to the ramps forcoal cutting seamallowed extent the for workings andcutting with the withan inclinationworkings roadheader withangle technology roadheader of 25° (Figure using technology 7). typicalThis inclination of the stepped ramps allowed for cutting the workings with roadheader technology miningusing equipment. typical mining Maintaining equipment. the Maintaining 25◦ inclination the allowed 25° inclination for deliveries allowed of for materials, deliveries including of materials, heavy using typical mining equipment. Maintaining the 25° inclination allowed for deliveries of materials, supportincluding systems, heavy chain support conveyor systems, components, chain conveyor and steel components, stands, as welland assteel coal stands, delivery as bywell conveyors. as coal including heavy support systems, chain conveyor components, and steel stands, as well as coal Thedelivery ramps wereby conveyors. placed under The ramps arched were supports placed composed under arched of V25 supports sections. composed of V25 sections. delivery by conveyors. The ramps were placed under arched supports composed of V25 sections. Sub-levelSub-level drifts drifts constituted constituted the the last last link link in thein the preparatory preparatory working working arrangement arrangement designed designed for for the the Sub-levelsub-level cavingdrifts constituted system. The the workings last link werein the cut preparatory from stepped working ramps arrangement with a slight designed elevation for of sub-level caving system. The workings were cut from stepped ramps with a slight elevation of up to 5◦ theup sub-levelto 5° along caving the coal system. seam Thebottom workings in the placeswere cut determined from stepped by the ramps field subdivisionwith a slight into elevation particular of along the coal seam bottom in the places determined by the ﬁeld subdivision into particular mining upmining to 5° alongpanels the (Figure coal seam 5). The bottom distances in the betweenplaces determined the operating by the drifts field depended subdivision on into the particularcoal seam panels (Figure5). The distances between the operating drifts depended on the coal seam thickness; miningthickness; panels they (Figure were usually 5). The similar distances to seam between thickness the operating but no shorter drifts than depended ca. 15 m. on the coal seam theythickness; were usually they were similar usually to seam similar thickness to seam but thickness no shorter but no than shorter ca. 15 than m. ca. 15 m.

Figure 7. Stepped ramp diagram. FigureFigure 7.7. SteppedStepped ramp diagram. diagram. Similar to stepped ramp cutting, the sub-level drifts were cut out by the roadheader technology Similar to stepped ramp cutting, the sub-level drifts were cut out by the roadheader technology usingSimilar typical to steppedtransportation ramp cutting, machines the sub-leveland equipm driftsent. were The cut sub-level out by thdriftse roadheader were executed technology with using typical transportation machines and equipment. The sub-level drifts were executed with using typical transportation machines and equipment. The sub-level drifts were executed with

Resources 2020, 9, 138 7 of 16 Resources 2020, 9, x FOR PEER REVIEW 7 of 16 rectangular supports in compliance with the shape of the powered support setset used in thethe miningmining face at thethe miningmining worksworks stage.stage. A diagram of the rectangular sub-level drift support is presented in Figure8 8..

Figure 8. Rectangular steel support of the sub-level drift. 1—steel roof bar made of two V25 sections, 2—valent steel friction stand, 3—welded3—welded steel mesh strut, 4—double4—double acting steel brace, 5—timber lining, and 6—steel6—steel anchoranchor with a steel meshmesh strut.strut. 2.2.3. Mining Face Equipment 2.2.3. Mining Face Equipment The face of the sub-level drift was equipped with a system of three basic installations used for The face of the sub-level drift was equipped with a system of three basic installations used for support, coal haulage, and borehole drilling as follows: powered roof support (PRS) powered roof support, coal haulage, and borehole drilling as follows: powered roof support (PRS) powered roof support (BME Novaky, Novaky, Slovakia), Nowomag PZ-1000 chain conveyor (Famur, Katowice, support (BME Novaky, Novaky, Slovakia), Nowomag PZ-1000 chain conveyor (Famur, Katowice, Poland) and VPS-01 portable drilling machine (InterCupro, Ostrava, Czech Republic). Poland) and VPS-01 portable drilling machine (InterCupro, Ostrava, Czech Republic). In addition, the mining faces were equipped with small tools such as pneumatic hammers, In addition, the mining faces were equipped with small tools such as pneumatic hammers, pneumatic or hydraulic screwdrivers, pneumatic blasters, burst charge and clay wad devices, and other pneumatic or hydraulic screwdrivers, pneumatic blasters, burst charge and clay wad devices, and blasting equipment. The protection of working areas at the face was provided by the PRS system. other blasting equipment. The protection of working areas at the face was provided by the PRS The set was composed of two specially-designed powered roof supports whose main task was to system. The set was composed of two specially-designed powered roof supports whose main task protect the area against dropping rocks or uncontrolled relocation of coal or waste rocks (Figure9). was to protect the area against dropping rocks or uncontrolled relocation of coal or waste rocks (Figure 9).

Figure 9. Internal and external sides of a powered roof support (PRS). 1—internal cover of the Figurecanopy, 9. 2—externalInternal and cover external of the sides base, of 3—external a powered roofcover support of the (PRS).canopy, 1—internal 4—internal cover cover of of the the canopy, base, 2—external5—internal coverprotection of the against base, 3—external caving, and cover 6—external of the canopy, protection 4—internal against cover caving. of the base, 5—internal protection against caving, and 6—external protection against caving. The locations of PRS and PZ-1000 are presented in Figures 10 and 11, respectively. The locations of PRS and PZ-1000 are presented in Figures 10 and 11, respectively.

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FigureFigure 10. 10. Locations Locations of of the the PRS PRS and and PZ PZ PZ-1000-1000-1000 in in the the mining mining face. face.

TheThe PZ-1000 PZ-1000 conveyor conveyorconveyor was waswas used usedused for theforfor transport thethe transportranspor of coalt t of releasedof coalcoal released fromreleased the frroof,fromom i.e., thethe by roof,roof, gravitational i.e.,i.e., byby gravitationalloading,gravitational directly loading, loading, after directly directly charge after blasting.after charge charge The blasting. blasting. conveyor The The structureconveyor conveyor allowedstructure structure for allowed allowed retaining for for retaining theretaining conveyor the the conveyorcrossoverconveyor crossover withincrossover the within cavingwithin the areathe caving ca.caving 5 m area fromarea ca. theca. 5 PRS5 mm from protectionfrom thethe PRS system,PRS protectiprotecti facilitatingonon system,system, transportation facilitatingfacilitating of transportationcoaltransportation released inof of thecoal coal caving released released area. in in the the caving caving area. area. TheThe VPS-01 VPS-01VPS-01 hydraulic hydraulichydraulic drilling drillingdrilling machine machinemachine was waswas installed installedinstalled at the atat mining thethe miningmining face. That face.face. device ThatThat was devicedevice designed waswas designedfordesigned drilling for for small-diameterdrilling drilling small-diameter small-diameter blasting boreholesblasting blasting boreho boreho in coallesles and in in rockcoal coal andusing and rock rock the using rotationusing the the method.rotation rotation method. method.

FigureFigure 11. 11. PZ-1000 PZ-1000PZ-1000 conveyor, conveyor,conveyor, with withwith a sliding aa slidingsliding station statiostatio andn PRS.n andand 1—PRS, PRS.PRS. 1—PRS, 2—PZ-10001—PRS, 2—PZ-10002—PZ-1000 conveyor, conveyor, 3—anotherconveyor, 3—anotherchain3—another conveyor, chain chain 4—PZ-1000conveyor, conveyor, 4—PZ-1000 4—PZ-1000 drive, 5—sliding drive, drive, equipment.5—sliding 5—sliding equipment. equipment.

2.2.4.2.2.4. Coal Coal Cutting Cutting Method Method TheThe mining mining technology technology based based on on the the sub-level sub-level caving caving system system design design applied applied in in the the Kazimierz-JuliuszKazimierz-Juliusz coal coal mine mine involved involved three three separate separate stages: stages: installation installation of of face face equipment, equipment, mining, mining, andand sub-level sub-levelsub-level drift driftdrift liquidation liquidationliquidation (backﬁll (backfill(backfill of ofdriftof driftdrift end). end).end). The The followingThe followingfollowing sections sectionssections of this of paperof thisthis only paperpaper describe onlyonly describethedescribe mining the the proper,mining mining whichproper, proper, consisted which which consisted consisted of a typical of of a sub-levela typical typical sub-level cavingsub-level system caving caving application, system system application, application, with regularly with with regularlyrepeatedregularly repeated operations:repeated operations: operations: drilling, explosivedrilling, drilling, explosive explosive charging, charging, charging, and blasting, and and blasting, followedblasting, followed byfollowed gravitational by by gravitational gravitational release of releasecoal,release haulage, of of coal, coal, andhaulage, haulage, relocation and and relocation relocation of the face of of equipment. the the face face equipment. equipment. 2.2.5. Blasting Works 2.2.5.2.2.5. Blasting Blasting Works Works Each time when mining started in the sub-level drift, the start-up blasting works were conducted EachEach timetime whenwhen miningmining startedstarted inin thethe sub-lesub-levelvel drift,drift, thethe start-upstart-up blastingblasting worksworks werewere using a diﬀerent procedure than the standard method applied during regular mining. After completion conductedconducted using using a a different different procedure procedure than than the the st standardandard method method applied applied during during regular regular mining. mining. AfterAfter completion completion of of the the start-up start-up blasting blasting works works (i.e., (i.e., after after full full caving caving in in the the whole whole cross-section cross-section of of the the givengiven mining mining panel), panel), the the coal coal seam seam was was extracted extracted upon upon borehole borehole drilling, drilling, explosive explosive charging, charging, and and

Resources 2020, 9, x FOR PEER REVIEW 9 of 16 blasting, in accordance with one of the blasting procedures designed for regular coal mining. Figure 12Resources presents2020 a, 9sample, 138 diagram of blasting borehole distribution. Blasting boreholes were drilled 9for of16 mining purposes using one VPS-01 portable stand drilling machine. Usually, the boreholes for mining blasting had a diameter of 45 mm and the length of up to 30 m. Barbaryt 4HM explosive (Nitroerg,of the start-up Bieru blastingń, Poland) works was (i.e., mainly after used full caving in blasting in the wholeworks; cross-section it is a nitroglycerine-based of the given mining material panel), appliedthe coal in seam coal wasdeposit extracted blasting upon in the borehole presence drilling, of methane. explosive The charging, boreholes and were blasting, plugged in accordance by clay wadswith packed one of thein paper blasting containers procedures. Detonations designedfor were regular conducted coal mining. with the Figure use 12of presentsthe Nitrocord a sample 8 pentritediagram detonation of blasting cord borehole (Nitroerg, distribution. Bieruń, Blasting Poland) boreholes initiated wereby either drilled Ergodet for mining 0,.5AN purposes instant using or Ergodetone VPS-01 0o45A25M portable millisecond stand drilling electr machine.ic detonators Usually, (Nitroerg, the boreholes Bieruń for, Poland). mining blasting had a diameter of 45At mmthe andbeginning, the length mining of up blas toting 30 m. was Barbaryt performed 4HM using explosive traditional (Nitroerg, coal Bieruń,Poland)rock mass cutting was methods,mainly usedi.e., using in blasting up to works; 20 m long it is aand nitroglycerine-based up to 45 mm diameter material blasting applied boreholes, in coal deposit with manual blasting explosivein the presence charging of methane.and stemming The boreholes. With time, were however, plugged pneumatic by clay wads explosive packed charging in paper containers.was also appliedDetonations in parallel, were conductedalthough the with latter the usemethod of the re Nitrocordquired 75 8 mm pentrite diameter detonation blasting cord boreholes, (Nitroerg, which Bieruń, allowedPoland) for initiated shortening by either the charging Ergodet 0,.5ANprocess instant by half or. A Ergodet vinyl hose 0o45A25M and a pneumatic millisecond blaster electric were detonators used for(Nitroerg, pneumatic Bieruń,Poland). charging.

FigureFigure 12. 12. SampleSample distribution distribution of ofmining mining bl blastingasting works works at atthe the mining mining face. face.

AboutAt the 1 beginning,kg of the explosive mining blasting material was was performed pumped using into traditionala 1 m long coal and rock up massto 45 cuttingmm diameter methods, blastingi.e., using borehole. up to 20For m th longe 75 andmm up diameter to 45 mm boreholes, diameter the blasting quantity boreholes, of the explosive with manual material explosive was aboutcharging 5 kg andper stemming.1 m of borehole. With time,With however, the progression pneumatic of extraction, explosive chargingthe results was of alsocutting applied were in analyzedparallel, and although experience the latter allowed method for requiredselection 75of mma proper diameter blasting blasting procedure. boreholes, In particular, which allowed the distribution,for shortening lengths, the chargingand diameters process of byboreholes, half. A vinylas well hose as charge and a pneumaticsizes, were blasteroptimized. were Both used the for numberpneumatic of boreholes charging. in the fan arrangement and their distribution changed depending on the local conditionsAbout of 1the kg coal of the seam explosive location material (mainly wasits thickness, pumped intotectonics, a 1 m and long coal and R upc). On to 45average, mm diameter 16–19 miningblasting boreholes borehole. were For drilled the 75 mmfor one diameter blasting boreholes, operation. the The quantity boreholes of the were explosive arranged material in three was fans, about with5 kg each per row 1 m of borehole.the fans inclined With the at progressiona different angle, of extraction, and the theboreholes results were of cutting of various were analyzedlengths (Figureand experience 12). Four allowedundercutting for selection boreholes of a were proper dri blastinglled within procedure. the drift In walls particular, (two theon each distribution, side). Accordinglengths, andto Polish diameters mining of law, boreholes, blast holes as well had as to charge be fired sizes, simultaneously. were optimized. The borehole Both the length number did of notboreholes reach the in full the mining fan arrangement panel height, and i.e., their boreho distributionles were changednot drilled depending and blasted on thedirectly local under conditions the coalof theseam coal roof seam or under location the (mainly higher panel its thickness, caving zone. tectonics, A coal and shelf coal about Rc). 2 On m average,thick was 16–19 left, which mining delayedboreholes the wereroof drilledcaving forprocess. one blasting Although operation. the process The boreholescaused loss were of arrangeda certain incoal three quantity, fans, with it reducedeach row the of theloss fans in inclinedquality. atWhen a diﬀ erentcoal angle,was still and hanging the boreholes after wereblasting of various and top lengths coal (Figure release, 12 ). additionalFour undercutting boreholes boreholeswere blasted. were A drilled unit charge within (or the the drift quantity walls (two of explosive on each side). in one According borehole) to Polish mining law, blast holes had to be ﬁred simultaneously. The borehole length did not reach the

full mining panel height, i.e., boreholes were not drilled and blasted directly under the coal seam Resources 2020, 9, 138 10 of 16 roof or under the higher panel caving zone. A coal shelf about 2 m thick was left, which delayed the roof caving process. Although the process caused loss of a certain coal quantity, it reduced the Resources 2020, 9, x FOR PEER REVIEW 10 of 16 loss in quality. When coal was still hanging after blasting and top coal release, additional boreholes Resources 2020, 9, x FOR PEER REVIEW 10 of 16 were blasted. A unit charge (or the quantity of explosive in one borehole) weighed from 5 to 20 kg weighed from 5 to 20 kg depending on the borehole length. During a single face blasting job (with depending on the borehole length. During a single face blasting job (with the total charge blasted), theweighed total charge from blasted), 5 to 20 kg 100 depending to nearly on 300 the kg borehole of explosives length. were During fired. a single The average face blasting consumption job (with of 100 to nearly 300 kg of explosives were ﬁred. The average consumption of the explosives amounted to thethe explosives total charge amounted blasted), to 100 ~0.29 to nearlykg per 300 ton kg of ofcoal. explosives were fired. The average consumption of ~0.29the kgexplosives per ton ofamounted coal. to ~0.29 kg per ton of coal. 2.2.6. Coal Release and Delivery 2.2.6.2.2.6. Coal Coal Release Release and and Delivery Delivery After blasting and drift face ventilation, the miners released the top coal. The process consisted After blasting and drift face ventilation, the miners released the top coal. The process consisted of of the transportationAfter blasting and of thedrift coal face usingventilation, a PZ- the1000 mi nersconveyor released whose the top crosso coal.ver The station process was consisted located theof transportation the transportation of the of coal the usingcoal using a PZ-1000 a PZ- conveyor1000 conveyor whose whose crossover crosso stationver station was locatedwas located within within the release zone (Figures 13 and 14). thewithin release the zone release (Figures zone 13(Figures and 14 13). and 14).

FigureFigure 13. 13. Coal CoalCoal release release diagram [10].[10]. [10].

Figure 14. View of the face when coal was released [10]. Figure 14. ViewView of of the face when coal was released [10]. [10]. The conveyor supplied the coal to subsequent chain and belt conveyors that were arranged in seriesThe inconveyor conveyor the sub-level supplied supplied drift the theand coal coal other to to subsequent subsequentworkings toward ch chainain and the and beltshaft. belt conveyors conveyorsReleased thatcoal that werestreaming were arranged arranged was in seriesincontrolled series in inthe the bysub-level sub-levelopening drift and drift closingand and other other of the workings workings internal towardprotection toward the the against shaft. shaft. theReleased Released caving coalof coal the streaming streaming PRS section was controlled(Figure 9).by opening and closing of the internal protection against the caving of the PRS section (Figure 9The9).). periods of coal release were diverse and usually continued for several dozens of minutes. DuringThe periods that process, of coal the release face minerswere diverse properly and op us usuallyenedually the continued continued PRS internal for for severalprotection several dozens dozens against of of minutes.caving minutes. (Figure 9, Item 5) to control the feed to the conveyor and prevent jamming of the face zone under the During that process, the face miners properly op openedened the PRS internal protection against caving PRS. The miners were also responsible for observation of the coal stream and stopping delivery (Figure 99,, ItemItem 5)5) toto controlcontrol thethe feedfeed toto thethe conveyorconveyor andand preventprevent jammingjamming ofof thethe faceface zonezone underunder thethe when oversize coal boulders fell. Such boulders would be cut by pneumatic hammers or crushers PRS. The miners were also responsible for observation of the coal stream and stopping delivery mounted on the PZ-1000 conveyor. Occurrence of oversize coal boulders depended on the when oversize coal boulders fell. Such boulders would be cut by pneumatic hammers or crushers effectiveness of the blasting procedure applied. When blasting works were poor, the conveyor had mountedto be stopped on the often, PZ-1000 which conveyor.caused reduced Occurrence face yield of and oversize output. coal boulders depended on the effectiveness of the blasting procedure applied. When blasting works were poor, the conveyor had to be stopped often, which caused reduced face yield and output.

Resources 2020, 9, 138 11 of 16

PRS. The miners were also responsible for observation of the coal stream and stopping delivery when oversize coal boulders fell. Such boulders would be cut by pneumatic hammers or crushers mounted on the PZ-1000 conveyor. Occurrence of oversize coal boulders depended on the effectiveness of the blasting procedure applied. When blasting works were poor, the conveyor had to be stopped often, which caused reduced face yield and output. The essential experience gain from coal release in the Kazimierz-Juliusz coal mine was the direct roof caving behind PRS, which was also associated with the extent of blasting works. The effect was visible in the void created after the release of cut coal. Roof caving delay also contributed to the very low losses in coal quality. The roof caving occurred periodically and the goaf often filled the whole excavation void behind the PRS. When that was the case, the continuation of coal extraction and release works were usually not justified because of quality losses. The losses were mainly caused by a considerable difference between the coal density (1.3 Mg/m3) and the roof rock density (2.5 Mg/m3), which was transported quickly in the released coal stream. In such cases, once a large roof caving was observed and rocks were found on the PZ-1000 conveyor, regular operation was stopped at the mining face. Instead of drilling boreholes, the face equipment was relocated by 3–5 m, followed by the replacement of the collected steel stands with wooden ones. Consequently, a small coal pillar was left, separating the face in the new position away from the roof caving site. That operation was followed by regular mining until another full roof caving. The necessity of using protection pillars increased with the progress of mining in the given mining field. Consequently, the deposit use indicator gradually reduced with the progress of extraction in the mining field. Coal left in the gob zone and pillars constituted the mining losses of extractable resources. Together with the progress of mining operations in the coal mine, such losses increased until the loss rate of 50% was achieved. The course of the extractable resource mining rate changes is described in the next section.

3. Results and Discussion—Extractable Resource Mining Rate During about a dozen years of extraction of Seam 510, a repeatable changeability of the extractable resource mining rate changes was identified. Our priority was to maintain a low level of quality losses during coal extraction. The level of extracted coal contamination with gangue did not exceed about 3%, and maintaining the required regime was associated with the generation of quality losses. As mentioned in Section 2.2.6, the appearance of a continuous gangue stream on a conveyor ended each time with immediate interruption of coal release. When large-volume gangue appearing in the gob zone created a stream, it prevented further coal release. Our observations were confirmed by model testing [39]. The continuous mining procedure would cause losses of coal remaining in the gob zone (unreleased coal) and losses of coal remaining in the pillar (3–5 m thick). On each lower sub-level drift, extraction losses increased, affecting the extractable resource mining rate. Simultaneously, typical models of rate changes were observed. That phenomenon presented in two mining fields: Fields A and B. These two fields were selected due to the regularity of seam locations within their areas and, primarily, the lack of significant faults at the end of sub-level drifts that would have obviously distorted the results of observations of the extractable resource mining rate changes. The seam inclination was constant in both fields (ca. 45◦), with the seam thickness ranging from 16 to 24 m. The mining panel length (or sub-level drift length) generally did not exceed 200 m in the two fields. The mining depth ranges from 280 to 550 m. Tables1 and2 describe the particular sub-level drifts in both fields together with their extractable resources, extracted resources, and percentage rates of extractable resource mining. The sub-level drifts are arranged in the tables in accordance with the mining sequence or from the shallowest to the deepest drifts. Resources 2020, 9, 138 12 of 16

Table 1. Extractable resource mining rates in Field A.

Sub-level Sub-Level Drift Extractable Output Extractable Resource Drift No. Length (m) Resources (Mg) (Mg) Mining Rates (%) A/1 82 27,211 23,891 87.8 A/2 84 27,848 27,208 97.7 A/3 120 45,738 44,063 96.3 A/4 134 41,459 27,309 65.9 A/5 136 54,264 32,261 59.5 A/6 170 66,640 40,921 61.4 A/7 172 75,852 45,921 60.5 A/8 186 84,369 37,974 45 A/9 142 60,832 33,908 55.7 A/10 161 71,520 39,550 55.3 A/11 174 69,950 38,850 55.5 Σ Field A 1561 625,683 391,856 av. 67.3

Table 2. Extractable resource mining rates in Field B.

Sub-Level Sub-Level Drift Extractable Output Extractable Resource Drift No. Length (m) Resources (Mg) (Mg) Mining Rate (%) B/1 107 31,158 26,794 86 B/2 142 41,350 29,977 72.5 B/3 155 58,590 31,389 53.6 B/4 169 67,431 41,594 61.7 B/5 191 74,872 50,712 67.7 B/6 200 84,000 53,337 63.5 B/7 178 65,789 29,031 44.1 B/8 179 67,662 38,030 56.2 B/9 185 66,245 35,520 53.6 B/10 187 69,881 38,554 55.2 Σ Field B 1693 626,978 374,938 av. 61.4

Figure 15 presents the extractable resource mining rate changes in both ﬁelds. Our analysis considered 11 subsequent mining panels (or sub-level drifts) in Field A and 10 in Field B. The exploitable resources were recognized in particular ﬁelds, at both the sub-level drifts cutting and mining operations stages. The seam thickness was determined by the drilling method. Resources 2020, 9, x FOR PEER REVIEW 13 of 16

Figure 15. TheThe course course of extractable resource mining rate changes in Fields A and B.

A rate drop from 80–90% to the final stable level exceeding 50% was also observed in other mining fields. Due to the seam distortions in the form of faults and local seam thickness reductions, other local distortions appeared within the trends presented above, generally in the form of local drops in the extractable resource mining rates. The observed course of the extractable resource mining rate changeability may be associated with the effects of rock pressure. Upon extraction of each subsequent mining panel, pressure on the undercut roof layers increased, followed by roof cracking and the appearance of caving. The roof cracking intervals depended on the roof-rock strength parameters, the span of the undercut roof, and the depth of the specific mining panel. Sub-level drift roof cracks migrated toward the coal release area at the drift face, producing gangue streams on the conveyors in cycles depending on the respective roof-rock strength parameters. Based on the experiences collected in the Kazimierz-Juliusz coal mine, building a precise and general model of extractable resource losses under the sub-level caving system would be difficult. However, we can assume that, in terms of quality, the process will occur similarly to that identified in deposits of similar structure. The relevant observations presented here should be useful in the production planning process and or prediction of the extractable resource mining rates when using the sub-level caving system in the extraction of thick and steeply inclined deposits.

Consequently, a high accuracy of resource recognition was attained. In both cases, at the initial stage of field mining (in the first sub-level drifts), the extractable resource mining rate exceeded 80% (and even 90% in Field A). In the fourth sub-level drift of Field A and the third in Field B, we observed a considerable increase in resource losses. The rate dropped to ca. 66% and 54%, respectively. Later, rate stability was observed at the level of more than 60% in both sub-level drifts. This was followed by a sudden drop in the rate (below 45%) in both mining fields and, again, rate stability at a level exceeding 50%. The average extractable resource mining rates of those fields were 67.3% and 61.4%, respectively. A rate drop from 80–90% to the final stable level exceeding 50% was also observed in other mining fields. Due to the seam distortions in the form of faults and local seam thickness reductions, other local distortions appeared within the trends presented above, generally in the form of local drops in the extractable resource mining rates. The observed course of the extractable resource mining rate changeability may be associated with the effects of rock pressure. Upon extraction of each subsequent mining panel, pressure on the undercut roof layers increased, followed by roof cracking and the appearance of caving. The roof cracking intervals depended on the roof-rock strength parameters, the span of the undercut roof, and the depth of the specific mining panel. Sub-level drift roof cracks migrated toward the coal release area at the drift face, producing gangue streams on the conveyors in cycles depending on the respective roof-rock strength parameters. Based on the experiences collected in the Kazimierz-Juliusz coal mine, building a precise and general model of extractable resource losses under the sub-level caving system would be difficult. However, we can assume that, in terms of quality, the process will occur similarly to that identified in deposits of similar structure. The relevant observations presented here should be useful in the production planning process and or prediction of the extractable resource mining rates when using the sub-level caving system in the extraction of thick and steeply inclined deposits.

4. Conclusions The sub-level caving extraction system presented here constitutes a unique face mining mechanization solution for extraction from thick and steeply inclined coal seams. In our opinion, the system presented here should realistically allow for maintenance of the extractable resource mining rate of 50–95%, depending on geological conditions. When designing mining operations, the increase in mining losses to the level of even 50% should be considered, remembering that the final rate will mainly depend on the roof-rock strength parameters and the mining depth. The application of the proposed mining system conception allowed for extraction of more than 12 million tons of coal from Seam 510 in 2003–2015. The effective solutions regarding the face protection systems, with maximum possible work mechanization, resulted in no serious accidents being recorded in the mining areas. Although the coal from Seam 510 was classified as belonging to self-ignition Group V, which is the highest in Poland, with a fire incubation time of about 30 days, no fire outbreaks occurred. The coal resources remaining in gobs did not present any hazard due to the relatively fast progress of face advance. In addition, the sub-level drift was ventilated by forced ventilation ducting. This system blocked the air flow through the gobs and coal within because, otherwise, breeding fire would be initiated. The ventilation performance was limited to the value of ca. 400 m3/min. Plugging and sealing the entrances to the abandoned sub-levels was the other equally essential factor that reduced the combustion hazard. A stopping system composed of mineral and cement binders was applied. Consequently, no drafts occurred from active sub-levels to goafs. Despite the low output, the coal mine obtained positive financial results each year due to the extraction of high-quality coal from Seam 510 with relatively low operating costs and expenditures on drift-face protection systems. In the initial mining period, the mine-face costs (i.e., the operating costs borne in a single sub-level drift), including labor, materials, and electricity, were USD 11.8 per ton and the costs regularly decreased. In the third year of mining, the costs were about USD 7.2 per ton and that level was maintained until the end of the coal mine’s operation. The capital investment associated with the infrastructure in a Resources 2020, 9, 138 14 of 16 single mine face, considering all the machines and equipment described in Section 2.2.3, was about USD 520,000. The mining method discussed here was subjected to regular modifications regarding machine, equipment, and blasting operation designs during the first several years of mining. Gradual improvements allowed the engineers to obtain a high level of system efficiency, except for one component. Blast hole drilling was the weakest process, and it was never improved. The mine face was equipped with only one VPS-01 portable drilling machine, which is why the drilling works required a whole shift. Fitting the mine face with a mobile two-arm drilling machine, suspended on the steel support of the sub-level drift, would considerably improve work efficiency. Even better results could be obtained by the implementation of two independent compact crawler drilling machines. Such machines could operate in parallel on two sides of the Nowomag PZ-1000 chain conveyor. In this case, it would be necessary to increase the sub-level drift width from 4.5 to 5.5 m, which would slightly increase the costs of preparatory works. The system has developmental potential. We therefore hope to both improve system efficiency and reduce mining costs with respect to the values that were attained in the Kazimierz-Juliusz coal mine.

Author Contributions: Conceptualization, Z.R., J.S., Z.B. and D.C.; methodology, Z.R., J.S., Z.B. and D.C.; validation, J.S. and D.C.; formal analysis, Z.R. and Z.B.; investigation, Z.R., J.S., Z.B. and D.C.; resources, Z.R., J.S., Z.B. and D.C.; writing—original draft preparation, Z.R., J.S., Z.B. and D.C.; writing—review and editing, Z.B. and D.C.; visualization, Z.R. and J.S.; supervision, Z.R. and Z.B. All authors have read and agreed to the published version of the manuscript. Funding: The article was funded by AGH University of Science and Technology, Faculty of Mining and Geoengineering; subsidy number: 16.16.100.215. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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