sensors Article
Deformation Monitoring of Waste-Rock-Backfilled Mining Gob for Ground Control Tongbin Zhao 1 , Yubao Zhang 1 , Zhenyu Zhang 2, *, Zhanhai Li 1 and Shuqi Ma 3 1
2 3
*
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China;
[email protected] (T.Z.);
[email protected] (Y.Z.);
[email protected] (Z.L.) State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China School of Civil and Environmental Engineering, Nanyang Technological University, 939798 Singapore, Singapore;
[email protected] Correspondence:
[email protected]; Tel.: +86-133-6839-6919
Academic Editors: Maria Marsella and Marco Scaioni Received: 19 March 2017; Accepted: 4 May 2017; Published: 5 May 2017
Abstract: Backfill mining is an effective option to mitigate ground subsidence, especially for mining under surface infrastructure, such as buildings, dams, rivers and railways. To evaluate its performance, continual long-term field monitoring of the deformation of backfilled gob is important to satisfy strict public scrutiny. Based on industrial Ethernet, a real-time monitoring system was established to monitor the deformation of waste-rock-backfilled gob at −700 m depth in the Tangshan coal mine, Hebei Province, China. The designed deformation sensors, based on a resistance transducer mechanism, were placed vertically between the roof and floor. Stress sensors were installed above square steel plates that were anchored to the floor strata. Meanwhile, data cables were protected by steel tubes in case of damage. The developed system continually harvested field data for three months. The results show that industrial Ethernet technology can be reliably used for long-term data transmission in complicated underground mining conditions. The monitoring reveals that the roof subsidence of the backfilled gob area can be categorized into four phases. The bearing load of the backfill developed gradually and simultaneously with the deformation of the roof strata, and started to be almost invariable when the mining face passed 97 m. Keywords: resistance transducer; deformation; backfilled gob; ground control
1. Introduction Solid backfill coal mining is deemed as a scientific and green mining technology, which uses solid waste to backfill the mined-out void (gob) for ground control. This is an effective way to protect underground water resources and surface infrastructure in the vicinity of the mining area [1–3]. So far, dry waste rock, cemented waste rock, hydraulic sandfill and paste backfill have been used for backfill in mining. This study focuses on the backfill of dry coal gangue, simultaneously solving the problems of waste rock disposal, minimizing environmental pollution and saving land resources, as typically a large volume of mined-out waste rock is produced with coal resource extraction [4–6]. In backfill mining, the movement characteristics of overlying rock strata are distinctly different from traditional cave mining methods. Many studies have been conducted to understand the characteristics of overlying strata movement and ground subsidence of backfill mining. Zhang et al. [7] investigated the characteristics of overburden strata movement by monitoring the support resistance of hydraulic support. Huang et al. [8] developed a thin-plate elastic model to describe the mechanical behavior of
Sensors 2017, 17, 1044; doi:10.3390/s17051044
www.mdpi.com/journal/sensors
Sensors 2017, 17, 1044 Sensors 2017, 17, 1044
2 of 13 2 of 13
the immediate roof and main roof decreased with the increase of filling ratios by numerical simulation. overlying strata in backfill mining, and found that the maximum sagging of the immediate roof and In the process of mining over a certain seam thickness, the overlying rock strata movement and main roof decreased with the increase of filling ratios by numerical simulation. surface subsidence start from the immediate roof, and gradually transfer to the ground In the process of mining over a certain seam thickness, the overlying rockupward strata movement and surface. This significantly the stability surface infrastructure [9–11]. numerical surface subsidence start fromthreatens the immediate roof, andofgradually transfer upward to theMany ground surface. studies have been conducted predictofthe stressinfrastructure developed in [9–11]. backfilled gob. Fahey et studies al. [12] found This significantly threatens the to stability surface Many numerical have that an arching effect can result in different horizontal stresses along the vertical direction. been conducted to predict the stress developed in backfilled gob. Fahey et al. [12] found that an Mkadmi et al.can [13] illustrated that drainage was necessary reduce poredirection. pressureMkadmi of backfill, arching effect result in different horizontal stresses alongtothe vertical et in al.order [13] to achieve a high level of frictional stress between the backfill and surrounding rock walls. illustrated that drainage was necessary to reduce pore pressure of backfill, in order to achieve a Doherty [14] found that, with cemented paste backfill, the filling and resting schedule plays high leveletofal. frictional stress between the backfill and surrounding rock walls. Doherty et al. [14] a dominant the development of total stress and pore pressure of backfilled In thisrole study, found that, role withincemented paste backfill, the filling and resting schedule plays a gob. dominant in coal gangue is used, which is mechanically different from paste backfill. Before backfill, blocks of coal the development of total stress and pore pressure of backfilled gob. In this study, coal gangue is gangue were into fine granular particles. In order to better understand the whole process used, which is ground mechanically different from paste backfill. Before backfill, blocks of coal gangue wereof overlying movement andInground in backfillthe mining, evaluation of the ground intorock finestrata granular particles. order tosubsidence better understand whole field process of overlying performance of waste-rock-backfilled gob is irreplaceable for optimum backfill design, even though rock strata movement and ground subsidence in backfill mining, field evaluation of the performance is costly and time-consuming. ofit waste-rock-backfilled gob is irreplaceable for optimum backfill design, even though it is costly In this study, a real-time deformation monitoring system was firstly developed to monitor the and time-consuming. gobIn roof subsidence and the stress of backfilled material. Based industrial Ethernet technology, this study, a real-time deformation monitoring system wason firstly developed to monitor the thisroof system containsand unitsthe of underground data collection, transmission and surface technology, supervisory. gob subsidence stress of backfilled material. data Based on industrial Ethernet Specific deformation sensors were designed monitor the roof strata deformation andsupervisory. the stress of this system contains units of underground datatocollection, data transmission and surface backfilldeformation material under underground conditions. The developed deformation Specific sensorscomplicated were designed to monitor the roof strata deformation and the stress of monitoring system was used to monitor the deformation of waste-rock-backfilled gob in the backfill material under complicated underground conditions. The developed deformation monitoring Tangshan coal mine, Hebei Province, China, and the monitored data was used to analyze the system was used to monitor the deformation of waste-rock-backfilled gob in the Tangshan coal mine, evolution process of theand overlying roof strata of the Hebei Province, China, the monitored data wasgob. used to analyze the evolution process of the overlying roof strata of the gob. 2. Monitoring System Development 2. Monitoring System Development 2.1. Monitoring Scheme 2.1. Monitoring Scheme Deformation monitoring covers the displacement of roof subsidence and the stress gradually Deformation covers displacement roof subsidence and the stress the gradually developed withinmonitoring the backfilled gob.the The layout of theofmonitoring scheme considered specific developed within the backfilled gob. The layout of the monitoring scheme considered the geological conditions, the function of monitoring equipment, the precision of monitoring,specific and the geological conditions, the function of monitoring the precision of monitoring, androck the blind blind spots of monitoring. According to the equipment, rule of thumb in monitoring overlying strata spots of monitoring. According to the rule of thumb in monitoring overlying rock strata pressure, the pressure, the largest subsidence displacement occurs at the roof center of mined-out gob. Therefore, largest subsidencestation displacement thecentral roof center the monitoring the monitoring shouldoccurs cover at the area of of mined-out the gob ingob. the Therefore, fully-mechanized backfill station should cover the central area of the gob in the fully-mechanized backfill mining face. mining face. Three Threerows rowsofofdeformation deformationand andstress stresssensors sensorswere wereinstalled. installed.Figure Figure11shows showsthe thelayout layoutofofthe the monitoring monitoringstations stationsfor forgob gobroof roofdeformation deformationand andstress stressofofthe thebackfill backfillbody. body.InInthe thedirection directionofofthe the advance of the mining face, three monitoring lines were set up with an interval of 10 m between each advance of the mining face, three monitoring lines were set up with an interval of 10 m between each line. line.Each Eachmonitoring monitoringline linewas wasthen thensub-divided sub-dividedinto intothree threezones zoneswith withan aninterval intervalofof2525mmalong alongthe the direction directionparallel paralleltotothe theworking workingface. face.AAset setofofdisplacement displacementsensors sensorsfor forthe thegob gobroof roofand anda aset setofofstress stress sensors sensorsfor forthe thebackfill backfillbody bodywere weremounted mountedatatevery everymonitoring monitoringstation. station.
Figure Figure1.1.Layout Layoutofofthe thegob gobmonitoring monitoringscheme. scheme.
Sensors Sensors 2017, 2017, 17, 17, 1044 1044
33of of13 13
2.2. Online Monitoring System 2.2. Online Monitoring System The online monitoring system contained the ground and underground units, consisting of data The online monitoring system contained the groundmonitoring and underground units, consisting of acquisition, transmission signal cable, communication substation, communication data acquisition, transmission signal communication substation, communication monitoring host, and the computer, ascable, shown in Figure 2. Themonitoring ground unit of the monitoring system monitoring host, andthe theindustrial computer, as shown 2. The host. ground unit of the monitoring system was connected with Ethernet by in theFigure monitoring was connected with the industrial Ethernet by the monitoring host. Transmission media: Cable Transmission distance: 5km
UPS
Printer Monitoring host
Industrial Switch
Transmission media: Single mode fiber Transmission distance: 10km
KDW660/12B DC stabilized power supply
Lightning protection device KJJ12 network switch
Power distance: 3m
Ground unit Underground unit
Transmission media: Single mode fiber Transmission distance: 10km
KDW660/12B DC stabilized power supply
KJJ12 network switch Power distance: 3m Transmission system: RS485 Transmission media: MHYV cable Transmission distance: 3km
KDW660/18B DC stabilized power supply
KJ216-F2 communication substation Transmission system: RS485 Transmission media: MHYV cable Transmission distance: 1km
GUD500 displacement sensors
Power distance: 3m
GPD30 stress sensors
Figure 2. The online deformation monitoring system installed in the backfilled gob. Figure 2. The online deformation monitoring system installed in the backfilled gob.
2.3. Displacement DisplacementSensors Sensors 2.3. A column-type column-type displacement displacement sensor sensor was was used used to to monitor monitor the the roof roof subsidence subsidence of of the the gob, gob, which which A employs resistance transducer technology, as shown in Figure 3a. When the backfill body is gradually employs resistance transducer technology, as shown in Figure 3a. When the backfill body is gradually compacted by by the thesettlement settlementofofthe theroof roofstrata, strata,thethe inner cylinder assembly of the sensor compacted inner cylinder assembly of the sensor can can be be compressed to drive the potentiometer to rotate. By changing the resistance of the precision compressed to drive the potentiometer to rotate. By changing the resistance of the precision potentiometer, aalinear linearoutput outputvoltage voltagesignal signalisisgenerated. generated. The voltage signal is then converted to potentiometer, The voltage signal is then converted to an an RS485 communication signal by the transmitter and communicates with the superior substation. RS485 communication signal by the transmitter and communicates with the superior substation. Some performance performance parameters parameters of of the the displacement displacement sensor sensor are are given given in in Table Table 1. Some 1.
Sensors 2017, 17, 1044
4 of 13
Sensors 2017, 17, 1044
4 of 13
FigureFigure 3. Roof displacement and its itsinstallation: installation: (a) olumn-type displacement sensor; 3. Roof displacementsensor sensor and (a) olumn-type displacement sensor; (b) (b) mounting ofof displacement column-typeddisplacement displacementsensor sensor set-up. mounting displacementsensor; sensor;(c) (c)components components of of column-typed set-up. Table 1. 1. Performance sensor. Table Performanceparameters parametersof ofthe the displacement displacement sensor.
Parameters Parameters
Setting Setting
Working Working voltage voltage DC 18 V DC 18 V Working Working current current mA ≤≤20 20 mA Transmission RS485 Transmission system system RS485 Cross-sectional area of transmission cables 0.43 mm2 2 Cross-sectional area of transmission cables 0.43 mm Transmission maximum distance 1 km Transmission maximum distance 1 km Measuring span 0–500 mm Measuring error ≤0–500 4.0% (F.S.) Measuring span mm Main dimension Φ186 × 1200 mm Measuring error ≤4.0% (F.S.) Main dimension Φ186 × 1200 mm In practice, many difficulties arise in the installation of displacement sensors in a backfilled gob practice, many difficulties arise in the installation of displacement a backfilled gob area dueInto the complicated underground conditions. One challenging sensors issue isin that the backfilled area duetends to the underground conditions. One challenging issue that easily the backfilled waste rock to complicated be soft and loose at the early stage of backfill, and it tends to is move along the waste rock tends to be soft and loose at the early stage of backfill, and it tends to move easily along horizontal direction once loaded vertically. Such movement of backfill can collapse the deformation the horizontal direction once loaded vertically. Such movement of backfill can collapse the sensor. In case of this deformation sensor collapse and data collection failure, the caved-in rock blocks deformation sensor. In case of this deformation sensor collapse and data collection failure, the cavedbehind the hydraulic support were firstly cleaned up, and then nylon bags filled with waste rock in rock blocks behind the hydraulic support were firstly cleaned up, and then nylon bags filled with were packed around the column-type displacement sensor to maintain its upright state, as shown waste rock were packed around the column-type displacement sensor to maintain its upright state, in Figure 3b. In order to minimize its negative effect on roof deformation monitoring, the top of the as shown in Figure 3b. In order to minimize its negative effect on roof deformation monitoring, the packed nylon bags did not contact the gob roof stratum when setting up the column-type displacement top of the packed nylon bags did not contact the gob roof stratum when setting up the column-type sensor and a certain gap was reserved between them so that the packed nylon bags would not provide displacement sensor and a certain gap was reserved between them so that the packed nylon bags anywould support against roof subsidence. not provide any support against roof subsidence. As As shown in in Figure 3c,3c, the displacement positionedin inthe themined-out mined-outgob gob shown Figure the displacementsensor sensorwas was vertically vertically positioned behind the self-pressure support, and its top and bottom were required to be in tight contact with behind the self-pressure support, and its top and bottom were required to be in tight contact with the the immediate immediateroof roofstrata strataand andfloor floorstrata strataofofthe thegob gobsosothat thatthe theroof roofdeformation deformationcould couldbebeeffectively effectively captured. The bottom of the displacement sensor was fixed on the concrete plate using a ground anchor.
Sensors 2017, 17, 1044
5 of 13
Sensors 2017, 17,bottom 1044 5 of 13 captured. The of the displacement sensor was fixed on the concrete plate using a ground anchor. captured. The bottom of the displacement sensor was fixed sensor on the are concrete plate using ground Sensors 17, 1044 5 of 13 The2017, specific installation procedures of the displacement as follows: firstly,a the waste anchor. rock was cleaned up and the internal cylinder of the displacement sensor was pushed out using a The specific installation procedures of the displacement sensor are as follows: firstly, the waste spiral push-out device, as shown in Figure 4a. After pushing out the internal cylinder, the three of the displacement sensor are as follows: firstly,out theusing wastea rockThe wasspecific cleanedinstallation up and theprocedures internal cylinder of the displacement sensor was pushed screws of the antiskid sleeve were tightened. The tightening degree was determined by providing rock was cleaned up and the internal cylinder of the displacement sensor was pushed out using a spiral push-out device, as shown in Figure 4a. After pushing out the internal cylinder, the three thespiral internal cylinder with a resistance of approximately 1500 kN while the outer cylinder does push-out device, as shown in Figure 4a. After outdegree the internal cylinder, theby three screwsnot screws of the antiskid sleeve were tightened. The pushing tightening was determined providing appear retract, as shown in Figure 4b. The antiskid sleeve cover was equipped a with wear-resistant of antiskid sleeve were The degree1500 waskN determined providing thedoes internal thethe internal cylinder with tightened. a resistance oftightening approximately while thebyouter cylinder not plastic beltwith to prevent the tightened screws from making direct contact withdoes the not inner surface of the cylinder a resistance of approximately 1500 kN while the outer cylinder appear retract, appear retract, as shown in Figure 4b. The antiskid sleeve cover was equipped a with wear-resistant cylinder, as shown in4b. Figure 4c. At last, thecover displacement sensor was firmly fixed onplastic the base plate as shown in Figure Thetightened antiskid sleeve was equipped a with wear-resistant belt to plastic belt to prevent the screws from making direct contact with the inner surface of the with the bolts of M10 × 30, and a cable wire was mounted into the protective steel tube, as shown prevent the tightened screws from making direct contact with the inner surface of the cylinder, as cylinder, as shown in Figure 4c. At last, the displacement sensor was firmly fixed on the base plate in Figure shown in bolts Figure At×last, the displacement sensor was firmly fixed on the base plate with the bolts with3c. the of4c. M10 30, and a cable wire was mounted into the protective steel tube, as shown in of M10 × Figure 3c.30, and a cable wire was mounted into the protective steel tube, as shown in Figure 3c.
Figure 4. Installation procedures for the displacement sensor: (a) spiral push-out device; (b) set-up of Figuresleeve; 4. Installation Installation procedures for thedisplacement displacement sensor: (a) (a) spiral spiralpush-out push-outdevice; device;(b) (b)set-up set-upof of Figure 4. procedures the antiskid (c). components offor antiskid sleeve. sensor: antiskidsleeve; sleeve;(c). (c). components componentsof ofantiskid antiskidsleeve. sleeve. antiskid
2.4. Stress Sensors 2.4. Stress Stress Sensors Sensors 2.4. The stress sensor was used to measure the vertical stress of the backfilled gob based on the strain The stress stress sensor was used thethe vertical stress of the gob based on theon strain The sensorThe wasmeasurement usedtotomeasure measure vertical stress of backfilled the backfilled gob advance based thethe measuring technique. mechanism is activated when, with the of measuring technique. The measurement mechanism is activated when, withwith the advance of the strain measuring technique. The measurement mechanism is activated when, the advance of mining face and backfill operation, the roof rock strata gradually subsides to compact the backfill. mining faceface andand backfill operation, thethe roof the mining backfill operation, roofrock rockstrata stratagradually graduallysubsides subsidesto to compact compact the the backfill. backfill. TheThe vertical stress from of the backfilled gobisisthen thentransferred transferredtotothe the stress vertical stress fromthe theoverlying overlyingrock rockstrata strataof ofthe thebackfilled backfilledgob gob stress The vertical stress from the overlying rock strata is then transferred to the stress sensor. Such compaction converted to aa voltage voltagesignal signalbybymeans meansofof a sensor. Such compactiontoto tothe thestress stresssensor sensor is is finally finally converted converted to to sensor. Such compaction the stress sensor is finally a voltage signal by means of aa strain gauge, and the voltage signal is converted to an RS485 communication signal by the transmitter straingauge, gauge,and andthe the voltage voltage signal signal is is converted converted to to an an RS485 RS485 communication communication signal signal by by the the transmitter transmitter strain to communicate with the superior substation. Along with the roof roof subsidence, subsidence,the thebackfill backfillundergoes undergoes to communicate with the superior substation. Along with the to communicate with the superior substation. Along with the roof subsidence, the backfill undergoes a a dynamic process of compression and the vertical stress gradually develops. As a consequence, a dynamic process of compression andvertical the vertical stress gradually develops. As a consequence, dynamic process of compression and the stress gradually develops. As a consequence, Young’s Young’s modulus of backfill is a dynamic parameter, which changes with time and space. However, Young’s modulus of backfill is a dynamic parameter, changes with time and space. However, modulus of backfill is a dynamic parameter, which changes with time and space. However, as the as roof thethe roof strata, backfill and stress sensor are continually in contact and there is no separation as roof strata, backfill and stress sensor are continually in contact and there is no separation strata, backfill and stress sensor are continually in contact and there is no separation between any between any two ofof them during subsidence, the verticalthe stress within thebackfill backfill can therefore between any two them duringroof roof subsidence, the within stress within therefore two of them during roof subsidence, the vertical stress backfill canthe therefore becan continually be continually transferred from the top to the bottom sensor, even though the deformation ofof betransferred continually transferred from the top to the bottom stress sensor, even though the deformation from the top to the bottom stress sensor, even though the deformation of the backfill along the backfill along thevertical vertical direction tends to togradients occur in different gradients due in in thethe backfill along the tends occur different gradients due tothe thedifference difference vertical direction tends todirection occur in different due to the difference into Young’s modulus. Young’s modulus. Therefore, this change in Young’s modulus of backfill has little influence on the Young’s modulus. Therefore, this change in Young’s modulus of backfill has little influence on Therefore, this change in Young’s modulus of backfill has little influence on the stress measurement.the stress measurement. Figure5a5ashows shows the components stress and stress measurement. components ofitsthe the stress sensor, sensor, anditsitsperformance performance Figure 5a shows theFigure components of thethe stress sensor, andof performance parameters are listed in parameters are listed in Table 2. Table 2. parameters are listed in Table 2.
Figure 5. Filling body stress sensor and its installation: (a) stress sensor; (b) set-up of stress sensor. Figure 5. 5. Filling body (a) stress stresssensor; sensor;(b) (b)set-up set-upofof stress sensor. Figure Filling bodystress stresssensor sensorand and its its installation: installation: (a) stress sensor.
Sensors 2017, 17, 1044
Sensors 2017, 17, 1044
6 of 13
Table 2. Performance parameters of the stress sensor.
Parameters
6 of 13
Setting
Working DC 18 V Tablevoltage 2. Performance parameters of the stress sensor. Working current ≤20 mA Parameters Setting Transmission system RS485 Working voltage DC V 2 Cross-sectional area of transmission cables 0.4318mm Working current ≤20 mA Transmission maximum distance 1 km Transmission system RS485 2 Cross-sectional area of transmission cables 0.43 mm Measuring span 0–30 MPa Transmission maximum distance 1 km(F.S.) Measuring error ≤4.0% Measuring span 0–30 MPa Main dimension Φ118 × 60.5 Measuring error ≤4.0% (F.S.)mm Main dimension
Φ118 × 60.5 mm
The stress sensor was installed in backfilled gangue as shown in Figure 5b. Before stress sensor installation, thesensor float coal stationas was cleaned and the cables The stress was around installedthe in installation backfilled gangue shown in Figure 5b.output Before signal stress sensor were sequentially labelled. Then, the stress sensor was fixed on the square steel plate with the installation, the float coal around the installation station was cleaned and the output signal cables were clamping screw, and the steel plate was fixedwas to the installation location. thickness the steel sequentially labelled. Then, the stress sensor fixed on the square steelThe plate with theofclamping plate is not less than 5 mm. Finally, the sensor cables were sleeved into a steel conduit or covered by screw, and the steel plate was fixed to the installation location. The thickness of the steel plate is not U-shaped steel and gradually connected into the gate road. less than 5 mm. Finally, the sensor cables were sleeved into a steel conduit or covered by U-shaped steel and gradually connected into the gate road. 3. Field Application 3. Field Application 3.1. Geological Background 3.1. Geological Background The developed online deformation monitoring system was applied to evaluate the ground control of the waste-rock-backfilled miningwas gobapplied in thetoTangshan coalground mine, control Hebei Theperformance developed online deformation monitoring system evaluate the Province, China. Tangshan coal mine hasmining been gob facing extremely strict public government performance of the waste-rock-backfilled in the Tangshan coal mine,and Hebei Province, scrutinyTangshan on ground thefacing coal mine was strict first public founded ago andscrutiny subsequently the China. coal control, mine hasas been extremely andlong government on ground immediate region has become a metropolitan district due to urbanization. Civic buildings have been control, as the coal mine was first founded long ago and subsequently the immediate region has gradually constructed on the surface underground coal seams, as shown in Figureconstructed 6. Also, the become a metropolitan district due toabove urbanization. Civic buildings have been gradually surface storageabove of coal gangue is not the Tangshan The strip mining on the surface underground coalpermitted seams, asat shown in Figure mine. 6. Also, thebackfilled surface storage of coal method is with gangue waste rock) introduced to solvemethod the problems of mininggangue not coal permitted at (mined-out the Tangshan mine. Thewas backfilled strip mining with coal gangue induced ground and mined-out waste rock. Field monitoring of the deformation of (mined-out waste subsidence rock) was introduced to solve the problems of mining-induced ground subsidence backfilled gob is required in order to evaluate its performance in ground control as well as for the and mined-out waste rock. Field monitoring of the deformation of backfilled gob is required in order to optimization of future backfill operations. evaluate its performance in ground control as well as for the optimization of future backfill operations.
Figure coal mine. mine. Figure 6. 6. The The ground ground surface surface buildings buildings of of Tangshan Tangshan coal
Sensors 2017, 17, 1044
7 of 13
Sensors 2017, 17, 1044
7 of 13
Sensors 2017, 17, 1044
7 of 13
At present, thethe backfill mining face is the T3T 292 working face, which is located at the 12th level in At present, backfill mining face is the 3292 working face, which is located at the 12th level the in north of the Iron District of Tangshan mine. The T3 292The working face adopts theadopts fully-mechanized the north ofthe thebackfill Iron District mine. 3292which working face the level fullyAt present, mining of faceTangshan is the T3292 working T face, is located at the 12th strip mining technique with backfill. The coal seam thickness is 4.78 m with the average dipping angle mechanized strip mining technique with backfill. The coal seam thickness is 4.78 m with the average in the north of the Iron District of Tangshan mine. The T3292 working face adopts the fully◦ to the horizon. The mining height is 4.78 m. The mining depth is –700 m. The strike and dip of 12 dipping angle of mining 12° to the horizon.with Thebackfill. mining The height 4.78 m. The mining is the –700average m. The mechanized strip technique coalisseam thickness is 4.78depth m with dimensions of T3of 292 face are 1170 m and 87 m,m respectively. Figure 7 shows strike and dip dimensions Tmining 3292 longwall mining face 1170 and 87 m, depth respectively. Figure 7 dipping angle 12°longwall to the of horizon. The mining height isare 4.78 m. The mining is –700 m. Thethe backfill operation in rear of the hydraulic support. shows thedip backfill operation rearlongwall of the hydraulic support. strike and dimensions of Tin 3292 mining face are 1170 m and 87 m, respectively. Figure 7 shows the backfill operation in rear of the hydraulic support.
Figure (a) schematic representation of mining backfill operation, backfill Figure 7. 7. Backfill Backfilloperation: operation: (a) schematic representation of and mining and backfill(b)operation, photo. (b)Figure backfill photo. 7. Backfill operation: (a) schematic representation of mining and backfill operation, (b) backfill photo.
Layout of Monitoring Stations 3.2.3.2. Layout of Monitoring Stations 3.2. Layout Monitoring Stations sensors for the overlying rock strata and the stress sensors for the In theoffield, displacement In the field, thethe displacement sensors for the overlying rock strata and the stress sensors for the backfilled gob the were used to monitor for thethedeformation of strata the backfilled mining gob. The In the displacement overlying rock and the stress for the backfilled gobfield, were used to monitor sensors the deformation of the backfilled mining gob. Thesensors communication communication route and the layout of the monitoring stations are schematically shown in Figure backfilled were used to monitor the are deformation of shown the backfilled gob. The8. route and the gob layout of the monitoring stations schematically in Figuremining 8. The deformation The deformation sensors were firstly connected to the communication substation located the aircommunication route and the layout of the monitoring stations are schematically shown in at Figure 8. sensors were firstly connected to the communication substation located at the air-return gate road. return gate road.sensors The communication cable wastosequentially laid intosubstation the air-return gateatroad, the The deformation were firstly connected the communication located the airThe communication cable was sequentially laid into the air-return gate road, the walking level, and walking level, and thecommunication main haulage roadway. total length the the cable extendedgate to 3road, km. In return gate road. The cable wasThe sequentially laidofinto air-return thea the high-voltage main haulage roadway. The total length theshaft cableunderground extended toopening, 3 km. Inas a high-voltage power distribution room of theof10# shown Figure walking level,power and the main haulage roadway. The total length of the cable extended to in 3 km. In a8, distribution room of the 10#was shaft underground opening, as shown in Figure 8, the communication the communication cable connected to the switch to theopening, master communication station. high-voltage power distribution room of the 10#central shaft underground as shown in Figure 8, cable was connected to the central switch to the master communication station. Finally, the cable Finally, the cable was incorporated to the industrial Ethernet and the collected data was transmitted the communication cable was connected to the central switch to the master communication station. was to theincorporated industrial andstations theEthernet collected data wasgob transmitted the ground toincorporated the ground monitoring host. TheEthernet monitoring of theand backfilled were in Storage Finally, the cable was to the industrial the collected datalocated wasto transmitted monitoring host. The monitoring stations of the backfilled gob were located in Storage 88 + 3, Storage 88the + 3,ground Storagemonitoring 89 + 3 andhost. Storage + 2, respectively. data was recorded every 5 s and the preto The90 monitoring stationsThe of the backfilled gob were located in Storage 89 88 +warning 3+ and Storage 2, respectively. The was for recorded every s andevery the pre-warning value value89 of90 subsidence was setrespectively. todata 500 mm thedata stability of5surface infrastructure. 3, Storage +roof 3+and Storage 90 + 2, The was recorded 5 s and theFigure preof warning roof subsidence was set to 500 mm for the stability of surface infrastructure. Figure 9 shows 9 shows the physical set-up of this field monitoring system. value of roof subsidence was set to 500 mm for the stability of surface infrastructure. Figurethe physical set-up of this field monitoring 9 shows the physical set-up of this fieldsystem. monitoring system.
Figure 8. Communication route and the layout of monitoring stations. Figure 8. Communication route and the layout of monitoring stations. Figure 8. Communication route and the layout of monitoring stations.
Sensors 2017, 2017, 17, Sensors 17, 1044 1044
of 13 13 88 of
Sensors 2017, 17, 1044
8 of 13
Figure 9. Physical diagram of the monitoring system. Figure 9. Physical diagram offield the field monitoring system.
Figure 9. Physical diagram of the field monitoring system.
3.3. Results and Discussion 3.3. Results Results and and Discussion Discussion 3.3. The complexity of the underground backfilled gob environment is the main difficulty for such The complexity complexity of of the the underground underground backfilled backfilled gob environment environment is is the the main main difficulty difficulty for for such The deformation monitoring. Generally, with the advance ofgob mining, the immediate roof initially cracks such deformation monitoring. Generally, with the advance of mining, the immediate roof initially cracks deformation monitoring. bending Generally, with cracking the advance mining, immediate roof and caves in. Subsequently or even of theofmain roof the would occur until theinitially backfillcracks and caves in. Subsequently bending or even cracking of the main roof would occur until the backfill caves in. Subsequently bending or shown even cracking theSuch main roof would occur until the backfill isand concretely compacted, as schematically in Figureof10. movement of the overlying roof is concretely compacted, as schematically shown in Figure 10. Such movement of the overlying roof is concretely compacted, schematically shown in Figuremonitoring 10. Such movement overlying strata is detrimental to the as effectiveness of the deformation system, asofit the changes the roof strata is detrimental to the effectiveness of the deformation monitoring system, as it changes the strata is detrimental to theand effectiveness of thecable deformation system, as it changes deformation sensor position direction, breaks lines, etc.monitoring In this study, even though nine the deformation sensor position and direction, breaks cable lines, etc. In this study, even though nine sensors were installed for both and deformation and stresscable monitoring, onlyInone one nine deformation sensor position direction, breaks lines, etc. thisdeformation study, evenand though sensors were installed for both deformation and stress monitoring, only one deformation and one one stress sensor successfully monitored the deformation data due to the reasons presented above. sensors were installed for both deformation and stress monitoring, only one deformation and stress sensor successfully monitored the deformation data due to the reasons presented above. stress sensor successfully monitored the deformation data due to the reasons presented above.
Figure 10. Schematic demonstration of overlying rock strata movement after backfilled mining.
Figure 10. Schematic demonstration of overlying rock strata movement after backfilled mining. Figure 10. Schematic demonstration of overlying rock strata movement after backfilled mining.
Sensors 2017, 17, 1044 Sensors 2017, 17, 1044
3.3.1.Sensors Displacement 2017, 17, 1044 Monitoring 3.3.1. Displacement Monitoring
9 of 13 9 of 13 9 of 13
The normal mining process involvesthe theprocess process mining normal mining processwith withthe the backfill backfill technique technique involves of of thethe mining 3.3.1.The Displacement Monitoring operation, backfill, maintenance, etc. Therefore, the advance of the mining face does not proceed operation, backfill, maintenance, etc. Therefore, the advance of the mining face does not proceed in a in Thetime normal mining with the backfillroof technique involves the process the a continual Figure 11shows shows the monitored roof subsidence with regard toIttime. can be continual timeline. line. Figureprocess 11 the monitored subsidence with regard to time.of can mining beItseen operation, backfill, maintenance, etc. Therefore, the advance of the mining face does not proceed in is a gob backfill, thethe monitored maximum roofroof subsidence of theofbackfilled gob seenthat, that,over over5858days daysafter after backfill, monitored maximum subsidence the backfilled continual time line. Figure 11 shows the monitored roof subsidence with regard to time. It can be seen 354.2 mm.However, However,around around 74.4% 74.4% of occurred in in thethe first four days andand is 354.2 mm. of the the total totalroof roofsubsidence subsidence occurred first four days that, over 58 days after from backfill, the monitoreddays maximum roof subsidence of the backfilled gob is around 22.0% occurred four to thirty-two after backfill. around 22.0% occurred from four to thirty-two days after backfill. 354.2 mm. However, around 74.4% of the total roof subsidence occurred in the first four days and around 22.0% occurred from four to thirty-two days after backfill.
Figure Monitored roofsubsidence subsidenceof of backfilled backfilled gob after backfill. Figure 11. 11. Monitored roof gobover over5858days days after backfill. 11.the Monitored roofroof subsidence of backfilled gob over 58 days backfill. Figure 12Figure shows monitored subsidence of the backfilled gob at after the early stage of backfill.
Figure the 12 shows the monitored roof subsidence of the backfilled gobAfter at the stage of backfill. During first hydraulic support advance, the roof subsided by 130 mm. theearly support contacted Figure 12 shows the monitored roof subsidence of the backfilled gob at the early stage of backfill. During hydraulic advance, thebyroof subsided bythe 130second mm. After the support contacted the the rooffirst strata, the roof support continued to subside 70 mm. During and third backfill mining During the first subsided by 130 mm. Afterand the support contacted the roof strata, the hydraulic roof continued toadvance, subsidethe 70 mm. the second third mining operations, amount ofsupport roof subsidence isby 25roof mm andDuring 12 mm, respectively. Figure 12backfill also shows the the toduring subside 70 mm. During the respectively. second and third backfill mining thatroof the strata, amount of roof roof subsidence theby is relatively small. Therefore, the operations, the amount ofcontinued roof subsidence is 25maintenance mm and 12periods mm, Figure 12 also shows operations, the amount of roof subsidence is 25ofmm and 12 mm, respectively. Figure 12 alsoinshows roof subsidence mainly occurred in the period support advance. Caution should be taken these that the amount of roof subsidence during the maintenance periods is relatively small. Therefore, the that the amount ofroof rooffailure subsidence during the maintenance periods is relatively small. Therefore, the in case of roof periods subsidence mainly occurreddisaster. in the period of support advance. Caution should be taken in these roof subsidence mainly occurred in the period of support advance. Caution should be taken in these periods in case of roof failure disaster. periods in case of roof failure disaster.
Figure 12. Monitoring the roof subsidence of the backfilled gob at the early stage after backfill. Figure 12. Monitoring the roof subsidence of the backfilled gob at the early stage after backfill.
Figure 12. Monitoring the roof subsidence of the backfilled gob at the early stage after backfill.
Sensors 2017, 17, 1044
10 of 13
Sensors 2017, 17, 1044
10 of 13
Figure 13 shows the cumulative displacement of the roof strata and the rate of roof subsidence with Figure the advance of the mining face. The total displacement of the goband roof subsidence is 353.30 mm. 13 shows the cumulative displacement of the roof strata the rate of roof subsidence Accordingly, the dynamic evolution thetotal roofdisplacement strata deformation subdivided into stages: with the advance of the mining face.ofThe of thecan gobbe roof subsidence is four 353.30 mm.
Accordingly, the dynamic evolution of the roof strata deformation can be subdivided into four stages: (1) Rapid subsidence stage. In this stage, the total roof subsidence of the mined-out gob was (1) Rapid stage. In this stage, the totaloccurred roof subsidence of thepart mined-out gobsubsidence, was 303.30 303.30 subsidence mm, and the cumulative deformation in the main of the roof mm, and the cumulative occurred in rapid the main part ofof the subsidence, approximately 85.8% of thedeformation total deformation. This subsidence the roof was mainly approximately of the contact total deformation. This rapid of the roof caused by the 85.8% insufficient between the roof and subsidence backfilled waste rock,was andmainly rapid caused by the insufficient contactoccurred betweendue thetoroof and backfilled compaction of the backfill material its initial soft nature.waste rock, and rapid of the backfill material due toroof its initial soft nature. (2) compaction Slow subsidence stage. In this stage, occurred the cumulative subsidence was 28.70 mm, approximately (2) Slow subsidence stage. In this stage, the cumulative roof subsidence was 28.70 mm, 8.1% of the total deformation. During this stage, the roof subsidence rate significantly slowed approximately 8.1% of the ittotal deformation. During this stage, the roof subsidence down. According to statics, is found that the roof subsidence is dramatically affected by rate the significantly slowed down. According to statics, it is found that the roof subsidence is rate of backfilled mining, and a faster mining speed can produce greater roof subsidence during dramatically this stage. affected by the rate of backfilled mining, and a faster mining speed can produce greater roofstable subsidence during this In stage. (3) Relatively subsidence stage. this stage, the cumulative roof subsidence was 17.40 mm. (3) Relatively stable subsidence stage. In thisand stage, cumulative roof rate subsidence wassubsidence 17.40 mm. This indicates that roof is relatively stable thethe influence of mining on the roof This indicatesreduced. that roof is relatively stable and the influence of mining rate on the roof subsidence is obviously is obviously reduced. (4) Long-term stable stage. In this stage, the cumulative roof subsidence value was only 3.90 mm, (4) Long-term stable stage. In total this stage, the cumulative roof subsidence was mainly only 3.90 mm, approximately 1.1% of the roof subsidence. The slight subsidence value value was caused approximately 1.1% of the total roof subsidence. subsidence value was mainly caused by the movement of the overlying strata in the gobThe andslight was mainly determined by the mechanical by the movement of the overlying strata in the gob and was mainly determined by the properties and developed structure of the overlying rock strata after deformation. mechanical properties and developed structure of the overlying rock strata after deformation.
Figure 13. Roof subsidence and roof subsidence rate of the backfilled gob with the advance of the Figure 13. Roof subsidence and roof subsidence rate of the backfilled gob with the advance of the mining face. mining face.
3.3.2. Stress Monitoring 3.3.2. Stress Monitoring The vertical stress development of the backfill body in the gob as a function of time is shown in The vertical stress development of the backfill body in the gob as a function of time is shown in Figure 14. Over 60 days of stress sensor installation, the vertical bearing load of backfill increased to Figure 14. Over 60 days of stress sensor installation, the vertical bearing load of backfill increased to 0.4 MPa. In the first 13 days after installation, the movement of the overlying rock strata, including 0.4 MPa. In the first 13 days after installation, the movement of the overlying rock strata, including roof strata bending, cracking, rotation and cave-in, was violent due to the advance of the mining face roof strata bending, cracking, rotation and cave-in, was violent due to the advance of the mining face and hydraulic support. However, the growth rate of vertical stress in the backfill is not the largest and hydraulic support. However, the growth rate of vertical stress in the backfill is not the largest during this period, where it increased with time following a linear trend. Therefore, the vertical stress during this period, where it increased with time following a linear trend. Therefore, the vertical stress development of backfill lags behind the movement of the overlying roof strata during this period. development of backfill lags behind the movement of the overlying roof strata during this period. This may be because the initial contact between the hydraulic support and backfill is not tight in practical operation. Furthermore, the backfilled waste rock is of a soft and loose nature at the early stage of vertical loading. These characteristics and factors determine such a low growth rate of
Sensors 2017, 17, 1044
11 of 13
This may be because the initial contact between the hydraulic support and backfill is not tight in Sensors 2017, 17, 1044 of 13 practical operation. Furthermore, the backfilled waste rock is of a soft and loose nature at the 11 early stage of vertical loading. These characteristics and factors determine such a low growth rate of vertical vertical stress in theFrom backfill. 13–20sensor days installation, after sensorthe installation, the in vertical stressrapidly in the stress in the backfill. 13–20From days after vertical stress the backfill backfill rapidly increased to 0.2 MPa at a greater growth rate. This is because, with the increase of the increased to 0.2 MPa at a greater growth rate. This is because, with the increase of the gob area due to gob area due to mining advance, a larger volume of the overlying rock strata has broken and mining advance, a larger volume of the overlying rock strata has broken and compacted the backfill, compacted thedenser backfill, which becomes denser and stiffer. load Therefore, theroof overburden from the which becomes and stiffer. Therefore, the overburden from the strata can load be effectively roof strata can bestress effectively transferred to the stress A slightfrom vertical stress drop transferred to the sensor. A slight vertical stresssensor. drop occurred 20–24 days afteroccurred sensor from 20–24 days after sensor installation, but this time period was very short. Such a stress may installation, but this time period was very short. Such a stress drop may be caused by thedrop regional be caused by the regional rotation of collapsed large roof blocks, as shown in Figure 10. After that, rotation of collapsed large roof blocks, as shown in Figure 10. After that, the vertical stress of the the vertical stressagain of thebetween backfill increased between and day 44,by due to the load transfer backfill increased day 24 andagain day 44, due today the 24 load transfer continual subsidence by continual subsidence of the overlying roof strata. During the period of day 44 to day 58, the of the overlying roof strata. During the period of day 44 to day 58, the growth of vertical stressgrowth of the of vertical stressdown, of the and backfill down,stress and the final to vertical stress at around backfill slowed the slowed final vertical tended stabilize at tended aroundto 0.4stabilize MPa, indicating 0.4 MPa, indicating that the overlying rock strata tends be stable with little movement. that the overlying rock strata tends to be stable with littletomovement.
Figure14. 14.Stress Stressdevelopment developmentof ofthe thebackfill backfillwith withthe thetime. time. Figure
Figure 15 shows the stress changes of the backfill with the advance of the mining face. In Figure 15 shows the stress changes of the backfill with the advance of the mining face. accordance with the roof deformation, the developed stress of the backfilled material can also be In accordance with the roof deformation, the developed stress of the backfilled material can also divided into four stages: be divided into four stages: (1) Rapid stress development stage I: In this stage, the stress of backfilled rock grew linearly to 0.10 (1) Rapid stress stage I: In this the stress of backfilled rock grew linearly to MPa with thedevelopment advance of the mining face.stage, Generally speaking, the stress development of the 0.10 MPa with therock advance of the mining face. Generallydue speaking, the stress development of the backfilled waste lags behind the roof subsidence to the initial loose and soft nature of backfilled waste rock lags behind the roof subsidence due to the initial loose and soft nature of the backfill material. backfill material. (2) the Stress regulation stage II: The stress of the backfilled material increased rapidly from 0.10 MPa (2) Stress regulation stage II: Thedecreased stress of the increased from MPa to 0.20 MPa, then gradually to backfilled 0.18 MPa. material This fluctuation is rapidly caused by the0.10 regional to 0.20 MPa, then gradually decreased to 0.18 MPa. This fluctuation is caused by the regional cave-in of the roof strata. After the regional cave-in of the roof strata, the pressure of the cave-in of the roof strata.could Afternot the regional cave-in of thetoroof thebody. pressure of the overlying overlying rock strata effectively transfer thestrata, backfill Consequently, the rock strata could not effectively transfer to the backfill body. Consequently, the overlying strata overlying strata were co-supported by the backfill and the collapsed roof rock. the III: backfill and the collapsed roof (3) were Slow co-supported stress growthby stage In this stage, the stress of therock. backfill slowly increased to 0.38 MPa. With the continual advance of the mining face, the roof strata continued to subside and contact the backfill and collapsed rock again. Therefore, the stress on the sensors started to increase again.
Sensors 2017, 17, 1044
12 of 13
(3)
Slow stress growth stage III: In this stage, the stress of the backfill slowly increased to 0.38 MPa. With the continual advance of the mining face, the roof strata continued to subside and contact backfill again. Sensorsthe 2017, 17, 1044and collapsed rock again. Therefore, the stress on the sensors started to increase12 of 13 (4) Stress stable stage IV: The stress of the backfill reached the maximum of 0.40 MPa and tended (4) Stress stable stage stressface of the backfill the maximum of 97 0.40 and stage, tendedthe to to be stable whenIV: theThe mining passed thereached monitoring stations by m.MPa In this be stable when mining of face stations bythe 97 mining m. In this the cumulative stressthe increment thepassed backfillthe wasmonitoring only 0.020 MPa, when face stage, advanced cumulative stress increment oftothe was only 0.020 MPa, MPa/m, when thethus mining face advanced by 30 m away, corresponding thebackfill stress growth rate of 0.0067 indicating that the by 30 m away, to the stress of 0.0067 MPa/m, thus indicating that the overlying rockcorresponding strata of the backfilled gob growth tends torate be stable. overlying rock strata of the backfilled gob tends to be stable.
Figure 15. Stress of the the backfill backfill with with the the advance advance of of the the mining mining face. face. Figure 15. Stress development development of
4. Ground Surface Subsidence 4. Ground Surface Subsidence The ground surface subsidence above the backfilled gob was also continually monitored to The ground surface subsidence above the backfilled gob was also continually monitored to determine the ultimate ground subsidence and evaluate the effect of this backfill mining on surface determine the ultimate ground subsidence and evaluate the effect of this backfill mining on surface infrastructure. It was found that the maximum ground surface subsidence is 28 mm. Also, the ground infrastructure. It was found that the maximum ground surface subsidence is 28 mm. Also, the ground surface subsidence mainly occurred in the first three months after backfill mining. After that, the surface subsidence mainly occurred in the first three months after backfill mining. After that, the ground surface tended to be stable, indicating that it is not influenced by the backfill mining any ground surface tended to be stable, indicating that it is not influenced by the backfill mining any more. more. Furthermore, no obvious cracks or damage were detected in the surface infrastructure, and the Furthermore, no obvious cracks or damage were detected in the surface infrastructure, and the surface surface infrastructure was still able to be normally used. Therefore, backfilled strip mining in infrastructure was still able to be normally used. Therefore, backfilled strip mining in Tangshan Mine Tangshan Mine can successfully meet the target goal of ground control. can successfully meet the target goal of ground control. 5. Conclusions 5. Conclusions In this thisstudy, study, an online deformation monitoring system was developed the In an online deformation monitoring system was developed to monitor to the monitor deformation deformation of waste-rock-backfilled mining gob. The online monitoring system is composed of of waste-rock-backfilled mining gob. The online monitoring system is composed of a data acquisitiona data acquisition transmission signal monitoring cable, communication monitoring monitoring substation, sensor, transmissionsensor, signal cable, communication substation, communication communication monitoringThe hostdeformation and the computer. The deformation sensors, including host and the computer. sensors, including roof displacement sensors roof and displacement sensors and stress sensors of the backfill body, were developed employing resistance stress sensors of the backfill body, were developed employing resistance transducer technology. transducer technology. sensors and their installation procedure specifically The deformation sensorsThe anddeformation their installation procedure were specifically designedwere so that they can designed so that they can be used in the complicated underground environment of a mined-out gob. be used in the complicated underground environment of a mined-out gob. The developed monitoring system was practically used to monitor the deformation and stress The developed monitoring system was practically used to monitor the deformation and stress of waste-rock-backfilled mm depth in the Tangshan coalcoal mine, Hebei Province, China. The of waste-rock-backfilled gob gobatat−700 −700 depth in the Tangshan mine, Hebei Province, China. results show that industrial Ethernet technology can be reliably used for long-term data transmission The results show that industrial Ethernet technology can be reliably used for long-term data under complicated underground mining conditions. The monitoring reveals that the roof subsidence of the backfilled gob area can be categorized into four phases, that is, rapid subsidence stage, slow subsidence stage, relatively stable subsidence stage and long-term stable stage. When the mining face passed by the monitoring stations by 90 m, the subsidence of the roof strata reached the maximum value, 353.3 mm. Similarly, the developed stress of the backfill can also be divided into four stages,
Sensors 2017, 17, 1044
13 of 13
transmission under complicated underground mining conditions. The monitoring reveals that the roof subsidence of the backfilled gob area can be categorized into four phases, that is, rapid subsidence stage, slow subsidence stage, relatively stable subsidence stage and long-term stable stage. When the mining face passed by the monitoring stations by 90 m, the subsidence of the roof strata reached the maximum value, 353.3 mm. Similarly, the developed stress of the backfill can also be divided into four stages, including the rapid growth stage, the stress regulation stage, the slow growth stage and the stress stable stage. When the mining face passed by the monitoring stations by 97 m, the stress of the backfill increased to its maximum value of 0.40 MPa, and started to be almost invariable. Acknowledgments: The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 51474136, 51674160 and 51674047) and Taishan Scholar Talent Team Support Plan for Advantaged & Unique Discipline Areas. The support of Tangshan coal mine during the field investigation is acknowledged. Author Contributions: Tongbin Zhao and Zhenyu Zhang proposed the idea; Tongbin Zhao, Yubao Zhang and Zhenyu Zhang participated in processing the results, and writing the manuscript; Yubao Zhang and Zhanhai Li contributed to field monitoring and data acquisition; Shuqi Ma analyzed the data and editing the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3. 4. 5. 6.
7.
8. 9. 10. 11. 12. 13. 14.
Miao, X.X.; Zhang, J.X.; Guo, G.L. Study on waste-filling method and technology in fully-mechanized coal mining. J. China Univ. Min. Technol. 2010, 35, 1–6. Zhang, J.X.; Miao, X.X.; Guo, G.L. Development status of backfilling technology using raw waste in coal mining. J. Min. Saf. Eng. 2009, 26, 395–401. Qian, M.G.; Xu, J.L.; Miao, X.X. Green technique in coal mining. J. China Univ. Min. Technol. 2003, 32, 343–348. Yao, Y.; Sun, H. A novel silica alumina-based backfill material composed of coal refuse and fly ash. J. Hazard. Mater. 2012, 213, 71–82. [CrossRef] [PubMed] Al Heib, M.M.; Didier, C.; Masrouri, F. Improving short- and long-term stability of underground gypsum mine using partial and total backfill. Rock Mech. Rock Eng. 2010, 43, 447–461. [CrossRef] Zhang, J.; Zhang, Q.; Huang, Y.; Liu, J.; Zhou, N.; Zan, D. Strata movement controlling effect of waste and fly ash backfillings in fully mechanized coal mining with backfilling face. Min. Sci. Technol. 2011, 21, 721–726. [CrossRef] Zhang, Q.; Zhang, J.X.; Kang, T.; Sun, Q.; Li, W.K. Mining pressure monitoring and analysis in fully mechanized backfilling coal mining face-A case study in Zhai Zhen Coal Mine. J. Cent. South. Univ. 2015, 22, 1965–1972. [CrossRef] Huang, Y.; Li, J.; Song, T.; Kong, G.; Li, M. Analysis on filling ratio and shield supporting pressure for overburden movement control in coal mining with compacted backfilling. Energies 2017, 10, 31. [CrossRef] Yin, W.; Chen, Z.; Quan, K.; Mei, X. Strata behavior at fully-mechanized coal mining and solid backfilling face. SpringerPlus 2016, 5, 1611. [CrossRef] [PubMed] Li, H.Z.; Zha, J.F.; Guo, G.L.; Zhao, B.C.; Wang, B. Compression ratio design and research on lower coal seams in solid backfilling mining under urban areas. Soil Mech. Found. Eng. 2016, 53, 125–131. Zhao, H.; Ma, F.; Zhang, Y.; Guo, J. Monitoring and assessment of mining subsidence in a metal mine in China. Environ. Eng. Manag. J. 2014, 13, 3015–3024. Fahey, M.; Helinski, M.; Fourie, A. Some aspects of the mechanics of arching in backfilled stopes. Can. Geotech. J. 2009, 46, 1322–1336. [CrossRef] Mkadmi, N.E.; Aubertin, M.; Li, L. Effect of drainage and sequential filling on the behavior of backfill in mine stopes. Can. Geotech. J. 2013, 51, 1–15. Doherty, J.P.; Hasan, A.; Suazo, G.H.; Fourie, A. Investigation of some controllable factors that impact the stress state in cemented paste backfill. Can. Geotech. J. 2015, 52, 1901–1912. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
