After roadway excavation, the deformation and failure of roadway surrounding rocks typically results in roadway damage orcollapse. Conventional monitoring techniques, such as extensometers, stress meters, and convergence stations, are only capableto detect the stress or strain data with the shallow layers of surrounding rocks, and they require arduous manual works.Moreover, in the abovementioned monitoring techniques, the monitoring instruments are installed behind the excavation face;therefore, the strain and deformation occurring in front of excavation face cannot be detected. In order to eliminate theseshortcomings, an innovative monitoring system for surrounding rock deformation control has been developed base on Brillouinoptical time domain reflectometry. Compared with conventional monitoring systems, the proposed system provides a reliable,accurate, and real-time monitoring measure for roadway surrounding rock deformation control over wide extension. Theoptical fiber sensors are installed in boreholes which are situated ahead of the excavation face; therefore, the sensors can beprotected well and the surrounding rock deformation behaviors can be studied. The proposed system has been applied within aTBM-excavated roadway in Zhangji coal mine, China. The surrounding rock deformation behaviors have been detectedaccurately, and the monitoring results provided essential references for surrounding rock deformation control works.
1. Introduction
In recent 20 years, as the depletion of coal resources inshallow strata, coal mining operations have been moving todeeper strata [1]. Approximately 60% of the coal miningworks are conducted with a mining depth of over 800m inChina [2]. Mining works in deep strata are facing the chal-lenges of high ground stresses and complex geological condi-tions [3]. These emerging problems have resulted in largedeformation and failures of surrounding rocks and roadwaycollapse which seriously threaten the safety of miners andlimit output capacity of coal productions. Roadway collapseaccounts for 80% of total coal mine accidents and it contrib-utes 43% fatalities ofminers [4]. The conventionalmonitoringtechniques such as extensometers, stress meters, and con-vergence stations which have been typically applied in shal-low roadways cannot meet the requirement of monitoring
works in deep strata due to the lower precision and exces-sive manual operations.
For overcoming the problems of surrounding rock defor-mation monitoring in deep coal mines, many emerging mea-surement techniques have been conducted at working facesof coal mining and roadway excavation in underground coalmines. Zhao et al. conducted damage process monitoring ofroadway surrounding rock by using microseismic techniques[5]. Zhao et al. proposed a displacement monitoring method-ology of overlying rock layers of coal seam based on fiberBragg grating (FBG) displacement sensors [6]. Kajzar et al.carried out coal pillar deformation and coal roof monitoringby applying 3D laser technology within underground road-ways [7]. Yu et al. investigated surrounding rock deformationand roadway convergence by laser range instruments [8].Martino andChandler studied surrounding rock deformationand damage zone evolution behaviors by using a borehole
HindawiJournal of SensorsVolume 2018, Article ID 8010746, 10 pageshttps://doi.org/10.1155/2018/8010746
camera image [9]. Blümling et al. presented long-term pro-cesses of surrounding rock damage by microfocus X-raytomography [10]. Lubosik et al. proposed measurement tech-niques for axial forces of rockbolts and rock mass displace-ment by applying instrumented rock bolts which embeddedstrain gauges and tensometric sensors [11]. Liu et al. sug-gested transient electromagnetic method (TEM) for detect-ing surrounding rock damage zone range and deformation[12]. Erich discussed collapse feature of coal mine roadwaysby employing seismic reflection investigation method [13].
Despite the fact that the monitoring techniques havegained some advances, the abovementioned monitoringmethods are still flawed in some aspects. Microseismic tech-niques and transient electromagnetic and seismic reflectioninvestigation are able to detect fracture evolution in sur-rounding rocks, while their accuracy on rock displacementmonitoring is unsatisfied (reach to meters). Microfocus X-ray tomography is only capable of measure damages in rocksamples; therefore, it cannot be applied for in situ monitor-ing. Compared with fully distributed fiber optic sensor sys-tems, FBG systems call for an excessive number of sensorsand that leads to high cost. Moreover, most commerciallyavailable interrogators can handle only a fairly small numberof FBGs, setting a limit on the number of sensing points, aswell as on their density along the fiber [14]. Borehole cameraimage can detect the damage and fracture within the sur-rounding rocks while the real-time monitoring cannot beachieved and the image analysis relies on manual operations.Instrumented rock bolt can only be used to measure stressand strain in a shallow section of the surrounding rocksdue to the limitation on length of rock bolts (typically lessthan 2.5m). 3D laser technology provides a high-precisioninstrument for roadway convergence, while the internaldeformation and damage of rocks cannot be measured.
Brillouin optical time domain reflectometry (BOTDR) isa fully distributed sensing technology for distributed strainand temperature measurement along all determined areaswith only one optical fiber which is stimulated by laser pulsesand therefore many discrete sensors can be replaced [15–17].BOTDR provides fast and reliable measurements, and it alsoenables early detection of deformations that may affect thesafety of mining operations, thereby allowing to schedulenecessary works in advance to mitigate the potential risks.In recent years, BOTDR system has been applied in under-ground coal mines by many researchers. Naruse et al. con-ducted BOTDR monitoring in El Teniente mine, Chile. Theoptical fibers are set within the roadway along the roadwayalignment; therefore, the roadway convergence can be mea-sured [18]. Cheng et al. measured the deformation of overly-ing rock layers of coal seam by employing BOTDR-basedmonitoring method [19]. Zhang andWang built a fiber meshstructure on the surface of the roadway and conductedBOTDR strain measurement [20, 21].
In previous BOTDR applications, optical fibers wereinstalled about 5m behind the excavation faces of roadwaysto avoid interference with the installation of supportingstructures (rockbolts, cable bolts, steel meshes, etc.). There-fore, only time-dependent deformation can be measuredand the immediate deformation which occurs soon after
excavation cannot be studied. However, 80% of total roadwaydamage and collapse accidents happened near the excavationfaces [22–24]. Therefore, the monitoring of the whole sectionof the roadway, includes a deeper layer of surrounding rocksand excavation faces, has been a crucial issue of ensuringsafety production in underground coal mines.
This paper focuses on developing a BOTDR-based mon-itoring system for surrounding rock control of roadways inunderground coal mines. The structure of the monitoringsystem is modified so that a real-time monitoring of bothimmediate and time-dependent deformation of surroundingrocks can be detected. In situ monitoring of the system in aroadway is proposed, and the monitoring results are analyzedand compared with measurement results acquired from con-ventional monitoring techniques.
2. Development of the BOTDR MonitoringSystem for Surrounding Rock Control inUnderground Coal Mines
2.1. Basic Principles of the BOTDR Monitoring System. TheBOTDR-based monitoring system implements Brillouinscattering, which is a basic physical process representingthe interaction effect between light and optical medium inpropagation medium [25, 26]. When light is sent throughan optical fiber, most of it propagates along the originaldirection, a small portion of light departure from the originaldirection and results in a scattering. There are three types oflight scattering in optical fiber: Rayleigh scattering caused bythe change of fiber refractive index, Raman scatteringinduced by optical phonon, and Brillouin scattering pro-duced by acoustic phonon [27]. In Brillouin scattering, thescattered light reaches a peak over its spectrum at a frequencyshifted from the pulsed light. This amount of frequency shiftis known as Brillouin frequency shift, vB [26].
vB =2nVaλ
, 1
where n denotes the effective refractive index of the fiber, Vaindicates the acoustic wave velocity of the fiber core, and λ isthe vacuum wavelength of the incident light.
Va =1 − μ E
1 + μ 1 − 2μ ρ, 2
where E, μ, and ρ represent Young’s modulus, Poisson ratio,and density of optical fiber, respectively.
If longitudinal strain ε occurs in the optical fiber, the Bril-louin frequency shift vB changes in proportion to that of thestrain. The relation can be expressed as
vB ε = vB 0 + dvB ε
dεε, 3
where vB ε is the drift quantity of Brillouin scattering lightfrequency when the optical fiber strain occurs, vB 0 indi-cates the drift quantity of Brillouin scattering light frequencywhen the optical fiber does not have strain occurs, dvB ε /dεis the scale factor, and ε is the fiber axial strain.
2 Journal of Sensors
Considering the influence of strain and temperature, theBrillouin frequency shift vB can be expressed as
vB ε, T = vB 0 + dvB ε
dεε + dvB T
dTT , 4
where vB ε, T is the drift quantity of Brillouin scatteringlight frequency when the optical fiber strain and temperaturechanging occur, vB 0 indicates the drift quantity of Brillouinscattering light frequency when the changing values of strainand temperature are zero, dvB ε /dε is the strain factor,dvB T /dT is the temperature factor, ε is fiber axial strain,and T is the temperature changing.
For typical optical fibers, the scale factor is 493MHz/%(strain) and the temperature factor is 1MHz/°C.
The position where the strain occurs can be detected byanalyzing the time interval (t) between launching the pulsedlight and receiving the scattered light at the end of the opticalfiber. The distance between the position where the strainhappens and the end of the optical fiber can be expressed as
Z = ct2n , 5
where Z is the distance between the position where the strainhappens to the end of the optical fiber, t is the time intervalbetween launching the pulsed light and receiving the scat-tered light at the end of the optical fiber, c is the light velocityin vacuum, and n is the refractive index of the optical fiber.The basic principle of BOTDR is shown in Figure 1.
2.2. System Structure. The BOTDR-based monitoring systemconsists of the ground unit and underground unit, as shownin Figure 2. The ground unit is connected with the industrialEthernet by the monitoring host. The monitoring host pro-cesses data obtained from underground monitoring devicesand the monitoring results can be shown on various termi-nals (computer, pad, smartphone, etc.). Devices of theground unit are connected from each other by single-modecable, mobile network, or Wi-Fi. The ground unit is alsoconnected with the Internet for remote monitoring.
The underground unit of the monitoring system containsthe communication system and BOTDR sensing system. Thecommunication system includes communication substationsand industrial switch. The communication substationsreceive monitoring data from different underground workingsites, and the industrial switch connects the communicationsubstations and ground unit by using RS485 interface andMHYV cables.
BOTDR sensing system comprises laser light source,pulse modulation unit, optical heterodyne receiver, electricalheterodyne receiver, digital processor, and optical fibersensors. The digital processor is linked with the nearest com-munication substation by the MHYV cable. The average dis-tance of the optical fiber sensor to the monitoring host is6.9 km.
2.3. Optical Fiber Sensor. The fiber sensors are capable ofsensing the temperature and strain over long distances[29, 30]. While the bare optical fibers are usually used inlaboratory tests because they are easily fractured under the
deformation of measured structures. Therefore, fiber encap-sulation is required before monitoring for optical fiber sen-sor protection as well as insurance of essential couplingeffect between sensors and geotechnical structures [31–34].Many fiber encapsulation methods have been proposed byresearchers, such as PVC or glass fiber-reinforced polymer(GFRP) encapsulation, and steel and wire encapsulation.Usually, PVC or GFRP encapsulation are applied in labora-tory tests and soil body monitoring due to their lowstrength and stiffness. In this paper, steel wire-reinforcedoptical fiber sensors are applied for two reasons: (1) theoptical fiber sensors could be damaged in undergroundstructures under huge tectonic stress and ground stresswhich can reach up to 30MPa; therefore, only steel wireencapsulation is capable to provide a suitable protectionfor optical fiber sensors. (2) The optical fiber sensors needto be installed in deep boreholes for surrounding rockdeformation measurement, and the steel wire-reinforcedfibers are easier to insert into boreholes due to higher stiff-ness of the steel wire.
Figure 3 shows the structure of the steel wire-reinforcedoptical fiber sensor. The optical fiber is enclosed by a set ofsteel wires which provides protection for the fiber, and thesteel wires and fiber are sealed by a polymer sheath in orderto isolate them from groundwater and dust. Therefore, reli-able performance of optical fiber sensors can be obtained inharsh work conditions of the underground working site.
2.4. Laboratory Calibrations. Before conducting in situ mon-itoring, the optical fiber sensors need to be calibrated in thelaboratory for understanding their mechanical behaviors.The calibration works include strain calibration and temper-ature calibration.
Stain calibration was conducted by a fiber tensilemachine. An optical fiber in length of 1.2m is stretched onthe tensile machine; the strain of fiber and correspondingBrillouin frequency shift are recorded for analysis. Therefore,the linear relationship between strain and Brillouin fre-quency shift can be obtained. The strain factor of the steel-reinforced optical fibers which are used in this project is499.8MHz/%. The laboratory calibration relationshipbetween strain and Brillouin frequency shift is shown inFigure 4.
Temperature calibration is implemented by a watertank. Place the optical fiber into the water tank and thenheat the water. The change of temperature and Brillouinfrequency shift during the whole heating process arerecorded, and the linear relationship between temperatureand Brillouin frequency shift is learned. The temperaturefactor is 1.775MHz/°C and the laboratory calibration rela-tionship between temperature and Brillouin frequency shiftis shown in Figure 5.
3. Field Application and Results
3.1. Description of the Study Site. The monitoring site islocated at overlying coalbed methane (CBM) drainage road-way of 1413A longwall panel of Zhangji coal mine (HuainanMining Industry), China. The roadway has a length of
3Journal of Sensors
1598m, a diameter of 4.5m, and a buried depth of 505m. theroadway was excavated by a gripper TBM.
The CBM drainage roadway is situated in coal-bearingstrata which consist of fine sandstone, medium sandstone,argillaceous sandstone, and number 1 coal seam. The geom-etry of CBM drainage roadway cross-section is a circle with adiameter of 4.5m and the roadway is supported by rockboltsand steel meshes. The number 1 coal seam is 6.5m in thick-ness with a dip angle of 2-3° and situated 25–30m beneaththe CBM drainage roadway. The location of Zhangji coalmine and geological setting of the CBM drainage roadway
are illustrated in Figure 6. The roadway surrounding rockmainly consists of sandstone, and the rock properties havebeen obtained through laboratory tests. The rock propertiesare shown in Table 1.
The ground stress data had been obtained by boreholestress relief measurements. The measurement results sug-gested that the ground stress is controlled by tectonic stress.The orientation of the maximum horizontal stress is 116.3°
(NWW-SEE), and the magnitude of vertical stress, theminimum horizontal stress, and the maximum horizontalstress are 14.5MPa, 13.4MPa, and 37.4MPa, respectively,
Pad
Industrialswitch
Wi-Fi
Printer
Monitoringhost
Mobilenetwork
Groundunit
Undergroundunit
Industrialswitch
Digital processor
Electricalheterodyne receiver
Opticalheterodyne receiver
Pulsemodulation unit
Referencelight
Laserlightsource Probe light Pulsed light
Electrical signal conversation
Optical fiber sensor
BOTDR
v0
v0−vB
v0−vB
Brillouinscattering lightv0
v0
vB
Communicationsubstation
PC
PC Smartphone
Figure 2: The BOTDR-based surrounding rock deformation monitoring system installed in an underground coal mine.
Distance
Light frequency
Ligh
t pow
er
Pulse light
Strain or temperature rise
Inten
sity
Inten
sity
Time
TimeProbe light Frequency rise
Figure 1: A schematic view of time and frequency domain acquisition for strain or temperature detection using BOTDR [28].
4 Journal of Sensors
σv :σh :σH ratio approximately at 1.08 : 1 : 2.8. This directionof maximum horizontal stress is in agreement with local geo-logical setting.
It is the first application of TBM in roadway excavationproject in an underground coal mine which is operated byvertical shafts. The surrounding rock deformation behaviorsof TBM-excavated roadway are estimated to be different withthat of roadways constructed by conventional drilling andblasting due to different excavation disturbance effects oftwo excavation techniques. The monitoring works were con-ducted for studying deformation and stress filed redistribu-tion behaviors of surrounding rocks of the TBM-excavatedroadways, and the monitoring results can provide a referencefor surrounding rock control, safety evaluation, and roadwaysupport design.
3.2. Sensor Layout. The excavation works typically result indeformations and damages of roadway surrounding rocks,
and continuous deformations and damages occur on bothsurface and interior section of surrounding rocks under theeffect of the disturbed stress field. During the deformationand damage processes, the fractures within the interior sec-tions of the surrounding rocks could expand themselves tothe surface of surrounding rocks and result in damages orcollapses of roadways. Therefore, the deformation and dam-age behaviors of roadway surrounding rocks are demandedto evaluate the stability of roadways during the excavationand utilization of roadways. In previous researches, opticalfiber sensors were typically installed behind excavation faceswithin excavated roadways and the strain data of roadwaysurface can be obtained after excavation works. Nevertheless,the conventional monitoring systems cannot record strainvariation during excavation works and the strain distributionin the interior sections of surrounding rocks.
In order to understand disturbance behaviors of roadwaysurrounding rocks under TBM excavation, two monitoringboreholes were drilled from an adjacent roadway (main gateof 1413A longwall panel) to the CBM drainage roadway of1413A longwall panel. Optical fiber sensors were installedwithin the boreholes; therefore, as the TBM pass throughthe monitoring area, the strain values at various depths ofroadway surrounding rocks along the radial direction of theroadway can be detected. As shown in Figure 7, two monitor-ing boreholes were set between the TBM-excavated roadwayand its adjacent roadway with a space of 45m. The horizontaland vertical distance of the two sides of monitoring boreholesare 40m and 26.5m, respectively. Both two boreholes have alength of 48m and dip angle of boreholes is 34°. In futuremonitoring operations, the positions, number, and space ofsensors can be determined based on the demands of moni-toring and site conditions.
3.3. Sensor Installation. Optical fiber sensors were installedwith monitoring boreholes. The diameter of boreholesshould fulfill the requirements of sensor installation. In this
Steel wires
Optical fiber
Polymer sheath
Figure 3: Schematic view of steel wire reinforced optical fibersensor.
Experimental data
0.0 0.1 0.2 0.3 0.4 0.5 0.6Strain (%)
Linear regression fitting
250
200
150
100
50
Brill
ouin
freq
uenc
y sh
ift (M
Hz)
0
300
y = 499.8xR2 = 0.987
Figure 4: Brillouin frequency shift versus strain in laboratorycalibrations.
Experimental data
0 10 20 30 40 50 60 70 80Temperature ( oC)
Linear regression fitting
100
80
60
40
20Brill
ouin
freq
uenc
y sh
ift (M
Hz)
0
120y = 1.775x − 0.327R2 = 0.998
Figure 5: Brillouin frequency shift versus temperature in laboratorycalibrations.
5Journal of Sensors
study, two boreholes with a diameter of 127mm were drilledfrom the main gate of 1413A longwall panel for optical fibersensor installation. The distance from the main gate entranceto the number 1 borehole and number 2 borehole is 867mand 912m, respectively.
The installation procedure of optical fiber sensors is asfollows:
(1) Installing the optical fiber sensors: Tie the steel wire-reinforced optical fiber sensor on a polyvinyl chloride(PVC) tube which has a diameter of 40mm thenplace the PVC tube into the borehole. The PVC tubeis used as an orienting device for optical fiber sensors.
(2) Place the exhaust pipe and grouting pipe into theborehole and seal the borehole by installing a sealingplate.
(3) Start grouting. Injecting the cement grout into theborehole, after the cement hardens, it is capable ofproviding protection for optical fiber sensors as wellas insurance of essential coupling effect betweensensors and surrounding rocks.
(4) Connect transmission lines and power supplies. Thepreparation of monitoring works is finished. Thestructure of monitoring boreholes is shown inFigure 8.
3.4. Monitoring Results and Analysis.Monitoring works werestarted after cement grout curing. The initial strain along theoptical fiber sensors was recorded as reference values. There-fore, quantitative evaluation of damage degree of surround-ing rocks under the disturbed effects of TBM excavationcan be studied by measuring the changing of strain values.The limited compressive and tensile strain of rocks had beenobtained from laboratory tests of rock specimens. Once thestrain values exceed the limited strains, it can be deemed thatdamages occur in surrounding rocks and the positions ofdamages can be detected. Consequently, an early warningof surrounding rock damages can be provided.
The optical fiber sensors were set on January 28, 2015,and the monitoring works finished on March 1, 2015. Thestrain values of surrounding rocks which were induced byTBM excavation has been clearly measured by optical fiber
RockcategoryChina
Mediumsandstone
Argillaceoussandstone
Finesandstone
Mediumsandstone
Number 1coal seam
Averagethickness (m) Columnar
7.6
4.0
7.01000 km
200 km
Huainan Anhui
Zhangjicoal mine
21.9
6.5
CBM drainageroadway
Figure 6: Location and strata histogram of CBM drainage roadway in Zhangji coal mine.
Table 1: Rock properties of roadway surrounding rocks.
sensors when the TBM passed through the monitoring area.The monitoring results of numbers 1 and 2 boreholes areshown in Figures 9 and 10, respectively.
The TBM approached the number 1 borehole firstly. Inthe 8th of February, the TBM is 12m to the number 1 bore-hole, the tensile strain increased to 629.85με at a depth of35.8m in the number 1 borehole, and the compress strainascended to −649.2με in 43.3m. From February 9 to Febru-ary 11, the TBM passed through the monitoring area of thenumber 1 borehole. The maximum tensile strain happenedat a depth of 35.9m with the tensile strain value of 879.4με,and the maximum compressive strain occurred at a depthof 43.2m with the compressive strain magnitude of−865.1με. After February 12, as the TBM passed throughthe borehole and advanced forward, the strain fell back tonormal values which range from 100με to −100με.
Figure 10 indicates that strain data obtained from thenumber 2 borehole are in agreement with monitoring resultsof the number 1 borehole. The TBM passed through thenumber 2 borehole in February 12 and that resulted in theincreasing of surrounding rock strains from February 12 toFebruary 15. The maximum tensile strain was detected at adepth of 35.4m in the number 1 borehole with a value of660.7με and the maximum compressive strain was mea-sured at a depth of 42.2m with a value of −623.6με. AfterFebruary 16, the roadway surrounding rock strain fell backto a low level.
According to the monitoring results of two optical fibersensors, the maximum tensile and compressive strain weredetected at depth of 35.9m and 43.2m in the number 1 bore-hole and 35.4m and 42.2m in the number 2 borehole, respec-tively. In this study, sites at a depth of 35.5m and 43m withinboth monitoring boreholes were chosen as critical monitor-ing points of tensile and compress concentration. Figure 11illustrates the relationship between TBM advancing andstrain changing at tensile and compressive concentrationpoints within two boreholes. In the figure, Δx=0 representsthe arrival of TBM face at the monitoring plane, while thedistance of the TBM behind and ahead the monitoring planeis indicated as −Δx and +Δx, respectively. As shown inFigure 11, when Δx= 5m, both tensile and compressivestrain increased significantly. As TBM passed through theborehole, the strain values remained on a high level for 3 to4 days, and then the strain values dropped back to normallevel (−100με to 100με) when Δx=−40m.
It can be learned from the monitoring results that the dis-turbance range ahead the TBM excavation face is around 5m.There are tensile concentration zone and compressive con-centration zone located in 35.5m and 43m depth withinthe boreholes. Comparing the monitoring results with thelimited tensile and compressive strain which obtained fromlaboratory tests, the TBM-induced damage zone can beacquired in real-time by taking advantage of the BOTDR-based online monitoring system. Damage zones had beendetected about 5m and 12.5m away from roadway surface,and these damage zones cannot be found by conventionaltechniques such as extensometers or instrumental bolts dueto their limited monitoring ranges. The strain values within5m to roadway surface are relatively low due to the effectof roadway support facilities such as rockbolt and cable bolt.The monitoring results also indicate that the roadway sur-rounding rock disturbance mainly happened after excava-tion. The surrounding rock typically gets rebalance within3-4 days after excavation and that indicates the excavationdisturbance of TBM is significantly less than that of conven-tional drilling and blasting.
4. Discussions
Compared with conventional monitoring techniques, opticalfibers which are used in BOTDR-based monitoring systemare both sensors and propagation medium of signals. Oneoptical fiber can detect strain in a large area of surroundingrocks and the monitoring area can be controlled by simplyadding or reducing the number of optical fibers. The distur-bance and damage behaviors of rocks under TBM excava-tion had been learned by using BOTDR-based monitoringsystem. The BOTDR monitoring system is capable of
Monitoring borehole
Exhaust pipe
Groutingpipe
Optical fibersensor
Sealing plate
Figure 8: A schematic view of monitoring borehole and optical fibersensor installation.
1000
800
600
400
200
0 5 10 15 20 25 30 35 40 45 50Borehole depth (m)
0
Feb 2Feb 9Feb 11Feb 26Limited tensile strain
Limited compressive strainFeb 8Feb 10Feb 12
Stra
in (𝜇
𝜀)
−200
−400
−600
−800
−1000
Figure 9: Monitoring results of surrounding rock strain along theoptical fiber in the number 1 borehole during the approaching ofthe TBM.
7Journal of Sensors
measuring± 15000με with an accuracy of ±15με. In con-trast, the resolution of the mechanical monitoring devicessuch as extensometers typically is 1mm in a measuringrange of 5m, which is equivalent to 200με (strain). Theaccuracy of the BOTDR-based monitoring system is signifi-cantly higher than that of conventional monitoring devices.
For discrete strain monitoring techniques (FBG, electricresistance strain gauges, vibrating wire strain gauge, etc.), itis difficult to obtain strain data over long distances becausethis requires installing a sufficient number of sensors and acorresponding increase in costs. The total cost of BOTDRmonitoring operation in this study is 300,000 RMB, whereasif FBG sensors had been used as alternatives, in this case, thecost would increase to over 600,000 RMB due to the highprice of FBG sensors (700 RMB). Moreover, the cost ofBOTDR monitoring could be even lower in future coal mineapplications because abandoned boreholes (such as samplingboreholes, surveying boreholes, methane drainage boreholes,and water drainage boreholes) near roadways can be used toinstall optical fiber sensors.
In previous BOTDR monitoring applications in coalmines, optical fiber sensors were typically installed on therock surface. Therefore, the deformation and damage behav-iors of surrounding rocks are hardly acquired by the conven-tional layout of sensors. Installing sensors within boreholeswhich are capable of providing well protection of sensorsand the installation works can be conducted in adjacent road-ways so that the interference between sensor installation androadway supporting works can be avoided.
5. Conclusions
A BOTDR-based monitoring system has been proposedand successfully applied in Zhangji coal mine, China. Anaccurate, reliable, large-scale, and real-time monitoring ofsurrounding rocks had been obtained by means of the spe-cial layout of sensors and installing optical fiber sensorswithin boreholes. The deformation and damage behaviorsof surrounding rocks under the disturbance of TBM exca-vation had been learned. An early warning of surroundingrock damages can be provided when the strain values ofrocks exceed the limits and additional roadway supportingworks can be conducted. Consequently, the prevention-oriented strategy of roadway safety and stability controlcan be realized.
Conflicts of Interest
The authors declare no conflict of interest.
Acknowledgments
The work presented in this paper is financially supported bythe National Natural Science Foundation of China (Grantnos. 51674006 and 51474004) and Youth Fund of AnhuiUniversity of Science and Technology (Grant nos.QN2017211 and QN2017222). The authors would like toexpress their appreciation to the staff of Zhangji coal minefor their substantial support.
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0 5 10 15 20 25 30 35 40 45 50Borehole depth (m)
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Feb 12Feb 14Feb 16
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Figure 11: Strain variation of surrounding rocks along with TBMadvancing.
8 Journal of Sensors
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