Experimental study on distributed optical fiber sensing monitoring for ground surface deformation in extra-thick coal seam mining under ultra-thick conglomerate

Optical Fiber Technology 53 (2019) 102006

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Experimental study on distributed optical fiber sensing monitoring for ground surface deformation in extra-thick coal seam mining under ultrathick conglomerate ⁎

T

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Jing Chaia,b, , Wulin Leia,c, , Wengang Dua, Qiang Yuand, Lei Zhue, Dingding Zhanga,b, Hao Lia a

College of Energy Engineering, Xi’an University of Science and Technology, Xi’an 710054, China Xi’an University of Science and Technology, Ministry of Education of the Western Mining and Mine Disaster Prevention and Control of Key Laboratory, Xi’an 710054, China c School of Energy Engineering, Longdong University, Qingyang, Gansu 745000, China d Resource and Environmental Science College, Chongqing University, Chongqing 400044, China e China Coal Xi’an Design Engineering Co. Ltd., Xi’an 710054, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Three-dimensional model Ultra-thick conglomerate Distributed optical fiber sensing technology Surface movement and deformation Numerical calculation

The purpose of this paper is to study the law of ground surface movement and deformation in deep and extrathick coal seam mining under ultra-thick conglomerate. With the engineering geological conditions of Qianqiu Coal Mine as the background, the three-dimensional model with the size of 3600 mm × 2000 mm × 2000 mm (length × width × height) was constructed. The distributed optical fiber sensing monitoring network is set up in the model. The optical fiber sensing signal is collected in real time by NBX-6055 demodulator to characterize the surface movement and deformation. Then based on the Mohr-Coulomb criterion, the numerical calculation model is established by using UDEC 6.0 software to explore the internal relationship between overburden and surface movement and deformation in the process of coal mining. The test results show that the distributed optical fiber monitoring technology can effectively capture the movement and deformation of ground surface. The optical fiber strain distribution curve can accurately locate the formation area and stress state of surface subsidence basin. The surface is controlled by the key strata of ultra-thick conglomerate, and in the whole process of mining, the surface movement and deformation can be roughly divided into three stages: deformation initiation, slow deformation and sudden deformation. The conclusions indicate that the distributed optical fiber sensing monitoring technology has good applicability in the surface monitoring of model test. It provides a method for the monitoring information research of mining surface movement and deformation.

1. Introduction With the mining depth and mining intensity of coal mine continuously growing, the problem of overburden and ground surface movement and deformation become more complex. At the same time, it brings new difficulties in prediction and control of ground surface subsidence [1,2]. The ground surface movement and deformation is a comprehensive reflection of the structural deformation evolution of the overlying rock mass in the progress of coal seam mining. Coal seam mining leads to the gradual movement, bending and destruction of the overburden rock stratum in mine goaf. With the increase of mine goaf range, the deformation of overburden rock stratum begins to spread to the ground surface, thus forming ground surface subsidence basin [3]. Due to the influence of high mining intensity, large mining depth and

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ultra-thick conglomerate, the law of mining surface movement and deformation has obvious particularity in western Henan province mining area [4]. Therefore, grasping the dynamic law of the ground surface movement and deformation in the process of deep and extrathick coal seam mining under ultra-thick conglomerate can provide theoretical basis for prediction of the ground surface subsidence, evaluation of geological hazards and control of ground surface subsidence in mining area. Research on ground surface movement and deformation during mining is the basis for ensuring the green, safe and efficient mining of coal. Many scholars at home and abroad have carried out a lot of research work on it. In the aspect of theoretical prediction method, it can be summarized into three types: influence function method, theoretical model method and empirical method [5–8]. Among them, the influence

Corresponding authors at: School of Energy and Mining Engineering, Xi’an University of Science and Technology, Xi’an 710054, China. E-mail addresses: [email protected] (J. Chai), [email protected] (W. Lei).

https://doi.org/10.1016/j.yofte.2019.102006 Received 12 July 2019; Received in revised form 27 August 2019; Accepted 28 August 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.

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fiber to monitor the ground surface subsidence. The sensing optical fiber has the advantages of small volume, anti-electromagnetic interference, light weight, corrosion resistance, high sensitivity, waterproof and moisture-proof, and it is intrinsically safe. The optical fiber can be implanted into the model or the tested substrate to realize the nondestructive embedding. It senses the deformation of the tested structure like the human nervous system, and realizes remote, distributed fulllength and real-time monitoring of the structure. In recent years, the optical fiber sensing technology has been applied in the field of coal mining, including observation of overburden water flowing fracture zone [28], monitoring of surrounding rock loosening zone [29], monitoring of wellbore deformation and settlement [30], monitoring of roadway stability [31], monitoring of bolt support quality [32], monitoring of mining faults structural [33], monitoring of deformation and failure of stope floor [34] have achieved very good results. With the further development of distributed optical fiber sensing technology, this technology can be applied to long-term real-time monitoring of important structures such as ground surface buildings, dams, highways, electric towers and so on, which can provide a kind of method to solve the problem of “three under” coal mining. So far the thickness of the coal seam in the western Henan mining area is more than ten meters, the depth of the coal seam is nearly one kilometer, and the thickness of the overlying ultra-thick conglomerate is about 400 m. The research on the law of mining ground surface movement and deformation under this special mining geological condition is not enough and deep, and it is urgent to strengthen the research in this field. In this paper, according to the geological and mining conditions of deep buried and extra-thick coal seam under overlying ultra-thick conglomerate, a large-scale three-dimensional model was constructed in the laboratory. The law of ground surface movement and deformation is monitored by distributed optical fiber sensing technology in model test. Then the dynamic process of ultrathick conglomerate and ground surface movement and deformation in the process of coal seam excavation is calculated by using UDEC 6.0. Based on the comparative analysis of the two test results, this paper reveals the internal relationship between the movement and deformation of ultra-thick conglomerate and ground surface migration.

function method is the most widely used, and some optimization methods are proposed based on it [9,10]. However, the practice shows that the predicted results are often different from the field measurements [11,12], so some scholars began to try to study the ground surface subsidence from the dynamic deformation mechanism of overburden. Qian Minggao et al. [13] first put forward the key stratum theory of strata movement and control in 1996, which broke through the limitation of traditional mining surface movement and deformation research, and opened up a new perspective of ground surface subsidence research. Xu Jialin et al. [14] studied the dynamic process of the key strata of overlying strata on the surface subsidence. The bedrock separation grouting bodies, key stratum and isolated section coal pillars co-form load-carrying body, which can effectively slow down surface subsidence, improve the recovery rate of coal seam and promote the development of isolated section-grouting technology for overburden bed separation space [15]. Zhang Zhaojiang et al. [16] compares the relationship between the starting distance of ground surface subsidence and the initial fracture distance of key strata in time and space. A new method for calculating the starting distance of subsidence deformation is proposed. Li Yang et al. [17] found that the filling method effectively slowed down the deformation and failure of the key strata and controlled the ground surface deformation well. Li Chunyi et al. [18] studied on the relationship between the cantilever support and breaking degree of the super-thick key layer and the ground surface discontinuity deformation. In the aspect of model test method, similarity theory, device and technology of the test are relatively mature [19,20], but the test monitoring method is relatively backward. At present, the model test is still based on the traditional methods such as dial gauge, total station, strain sensor and displacement sensor. These detection methods have some defects: poor accuracy, low sensitivity, high labor cost and low degree of intelligence, and they are the single point measurement pattern and non-continuous methods of the ground surface of model. Optical fiber sensing technology solves the technical difficulties of model internal measurement and full-length measurement, and provides a method for overburden and ground surface movement monitoring in the model test. Chai Jing et al. [21–23] monitored the variation of overburden humidity and displacement in model test by the distributed optical fiber technology. Zhang Dan et al. [24] used PPPBOTDA technique to quantify the development height of the abscission rock layer in the model test. Lu Yi et al. [25] effectively captured the law of soil deformation and development under different conditions by using BOTDA technology. Gu Chunsheng et al. [26] used fiber Bragg grating sensor as monitoring means to reveal that the most direct factor for the development of ground fissures was the uneven surface subsidence which was caused by excessive groundwater extraction. Shi Bin et al. [27] proposed a method of drilling full-section distributed optical

2. Working principles of PPP-BOTDA Brillouin optical time domain analysis technique (BOTDA) was proposed by T. Horiguchi and M. Tateda in 1989 [35]. The sensing principle of Brillouin optical time domain analysis is shown in Fig. 1. Based on the Brillouin scattering principle, the PPP-BOTDA sensing technology is that the continuous detection light signal and the pump pulse light signal are injected from both ends of the optical fiber. When

Fig. 1. PPP-BOTDA sensing system. 2

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3.2. Test principle and similar conditions

the frequency difference between the continuous light and the pulse light is equal to the Brillouin frequency shift in a certain region of the optical fiber, this region will produce stimulated Brillouin amplification effect. According to the relationship between Brillouin frequency shift and temperature and strain, the frequency of the two lasers is continuously adjusted to monitor the continuous optical power coupled from one end of the optical fiber. The frequency of the energy transfer to the maximum in each cell of the optical fiber can be determined [36]. The relationship between temperature, strain and Brillouin frequency shift in optical fiber is shown in the following equation [37].

⎧ δVB = CVε + CVT δT δ ⎨ PPB = CPε δε + CPT δT ⎩ B

The similar simulation experiment [38] is a laboratory research method based on similarity theory and dimensional analysis. It can artificially control and change the experimental conditions, simulate the main factors of the entity, and omit the secondary factors. Thus, the law of the influence of single or multiple factors on mine pressure can be determined. The test effect is clear and intuitive, and the test period is short. It is one of the effective ways for people to explore and understand the law of overburden rock and ground surface movement. Its principle is to use similar materials to simulate and construct the physical model of natural or engineering phenomena according to a certain proportion based on the three similarity theorems [39]. Under the action of external factors similar to the actual situation, the change of the model is observed, the law is found out, and this law is applied to practice. According to the geological conditions of the prototype and the purpose of the test, combined with the similar simulation test conditions, the three-dimensional model test-bed with length 3600 mm, width 2000 mm, height 2200 mm was selected for the model test. According to the similarity theory, the experiment must be similar to the prototype system in geometry, kinematics and dynamics. So the design of the experiment adopts the geometry similarity ratio αL 1:400, the bulk density similarity ratio αγ 1:1.6, the stress similarity ratio ασ 1:640, and the time similarity ratio αt 1:20. On the basis of the strength and deformation index of the coal and rock strata in mines, it is determined that sand (particle size 0.15–05 mm) is used as aggregate, gypsum and calcium carbonate is used as cementing material, mica powder is used as layered material and water is used as mixture in this test strata. By changing the weight ratio of sand, gypsum and calcium carbonate (S:G:C), the effects of sand cement ratio, cement ratio and bulk density on uniaxial compressive strength were studied by orthogonal test of similar material proportion. The rock strata with different deformation strength are proportioned, and the proportion number and material consumption of each strata layer are determined as shown in Table 2.

(1)

where δνB is the change of Brillouin frequency shift, δPB is the relative variation of Brillouin power, δε, is the strain variation, δT, is the temperature variation, Cνε is the frequency shift strain coefficient, CνT is the frequency shift temperature coefficient, CPε is the Brillouin power strain coefficient and CPT is the Brillouin power temperature coefficient. The strain and temperature information of the optical fiber is obtained by detecting the frequency shift variation of the Brillouin signal and the normalized signal power variation value to achieve distributed optical fiber sensing monitoring.

3. Three-dimensional physical similarity simulation test 3.1. Engineering background The test engineering background is based on the mining geological conditions of Qianqiu Coal Mine of Yima Coal Industry Group in Henan Province. The mine production capacity is 2.1 million t/a, the main mining coal seam is No. 2 coal, the average thickness of the coal seam is about 24.0 m, the inclination angle of the coal seam is 3°~11°, and the average depth of the coal seam is about 800 m. The mining method is fully-mechanized long-wall top coal caving mining, and the roof is managed by full caving method. The immediate roof of coal seam is shaly-sandstone and mudstone with an average thickness of 25.4 m, and the actual thickness is between 23.1 m and 27.6 m. The Main roof is fine sandstone with an average thickness of 20.4 m. The floor is sandy mudstone with an average thickness of 8.3 m and low strength. The ultra-thick conglomerate with a thickness of about 410 m is developed at about 225 m above the No. 2 coal seam. The integrity of the rock mass is good, the integrity parameter is about 0.88, and there is a weak interlayer with a thickness of about 1 m in the middle to divide it into two groups. It belongs to typical thick and hard rock strata. The ground surface is often incomplete khaki silty clay and clay layer. The middle and lower parts are brick red sandy clay, calcareous sand ginger clay and gravel clay with an average thickness of about 15 m which often appear on the ground surface. The main mechanical parameters of each lithology in the test strata are listed in Table 1.

3.3. Model construction The model building could be divided into the following main processes: (a) weighing the proportion of similar materials, (b) mixing the similar materials evenly in proportion, (c) layered filling and compaction (evenly spreading mica powder between layers), (d) pre-stretching and embedding of optical fibers, (e) template fixing. In order to facilitate coal seam mining, the rectangular galvanized square pipe (60 mm × 40 mm) is selected to replace the coal seam in the test design. One square pipe is pulled out at a time, it means that the model coal seam is excavated once. According to the test object and the research purpose, two working faces was arranged in the model which inclined length and strike length are 800 mm and 2400 mm respectively. The coal pillars with the width of 600 mm and 200 mm are set in the trend and dip direction as boundary in the working face. The 3D model is shown in Fig. 2.

Table 1 Main overburden strata structure and physical parameters. Strata

Thickness/m

Depth/m

Compressive strength/MPa

Tensile strength/MPa

Elastic modulus/GPa

Poisson’s ratio

Remarks

Clay Muddy limestone Conglomerate Fine sandstone Siltstone Mudstone Coal seam Sandy mudstone

15 5 410 40 20 25 24 8

0 15 170 653 722 757 782 806

13 15 52 75 62 37 16 32

0.4 1.8 5.0 6.5 5.5 2.0 0.6 5.0

4.0 5.0 32.0 34.0 28.0 5.0 3.5 32.0

0.40 0.28 0.21 0.22 0.25 0.35 0.30 0.27

Topsoil

3

Primary key stratum Inferior key stratum Main roof Immediate roof Main coal seam Floor

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Table 2 Composition ratio and material consumption of simulated rock strata. Strata

Component ratio (S:G:C)

Sand/kg

Gypsum/kg

Calcium carbonate/kg

Water/kg

Clay Muddy limestone Conglomerate Fine sandstone Siltstone Mudstone Sandy mudstone

982 882 773 764 682 873 782

103.7 102.4 100.8 100.8 98.7 102.4 100.8

9.2 10.2 10.1 8.6 13.2 8.9 11.5

2.3 2.6 4.3 5.8 3.3 3.9 2.9

12.8 12.8 12.8 12.8 12.8 12.8 12.8

3.4. Distributed optical fiber test system (1) Polyurethane jacket cable Considering the requirement of optical fiber force caused by stratum deformation in three-dimensional model test, so the distributed optical sensing cable with 2 mm diameter is selected. This cable is a kind of tight buffer cable with polyurethane jacket, as shown in Fig. 3. The elastic modulus and maximum tensile force of cable is 0.35 GPa and 200 N respectively. In the model experiment, the optical cable is used as a sensing element to monitor the ground surface deformation, and as a transmission line to transmit signals. As the polyurethane is soft and elastic, it can increase the friction with the similarity material. The has good coupling and good strain transfer performance with similar materials, so it is suitable for physical model test. Fig. 3. Polyurethane jacket cable.

(2) Optical fiber sensor demodulator

(4) Optical fiber coupling mechanism

In the experiment, the NBX-6055 optical fiber demodulator produced by NEUBRUX Company of Japan is used to collect the sensing signal. The hardware is set up as follows: the measuring distance range 50 m, the sampling interval 1 cm, the spatial resolution 5 cm, the average count 213, the frequency range 10.65–10.95 GHz, the pulse width 1 ns, and the measurement mode is Normal.

It is assumed that the optical fiber and clay are in direct and comprehensive contact with each other, and there is no bonding material and voids between them. So optical fibers and clay layers are considered to be compatible deformation. The deformation of rock mass can be shown by the deformation of sensing optical fibers. When the clay layer is vertical deformation or horizontal movement, the clay layer will lead to the deformation of the sensing optical fiber. By measuring the strain distribution of optical fiber, the deformation and movement of soil at each point of optical fiber burying position can be obtained. The distributed optical fiber testing system is mainly composed of single mode optical fiber, demodulator and computer, as shown in Fig. 4.

(3) Design of optical fiber senor network In order to comprehensively monitor the law of ground surface movement and deformation in coal seam mining, a mesh sensing optical fiber monitoring system is installed in the clay layer of the test model. The installation mode is “pre-buried pattern”, and then tamped with similar material. The sensing optical fibers are divided into two groups, one group is equipped with two optical fibers along the trend direction of the model, the length is 3600 mm and the buried position height is 1900 mm, and the other group is equipped with three optical fibers with a length of 2000 mm and buried position height of 1900 mm along the dip direction of the model.

3.5. Calibration of optical fiber sensor The strain sensitivity coefficient of optical fiber was calibrated by

Fig. 2. Three-dimensional model design. 4

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Fig. 4. Optical fiber sensing monitoring system.

linear relationship with the distributed fiber test data, and the fitting correlation coefficient of optical fiber data is above 0.99. The first-order coefficient in the fitting function in the graph represents the sensitivity of optical fiber frequency shift and the test strain. The strain sensitivity coefficient of optical fiber used in the test is 0.0497 MHz/με. 3.6. Experimental procedure 3.6.1. Optical fiber fusion and location The optical fibers in the model were laid in sections. All the optical fibers should be connected in series before the beginning of the test to form a loop to facilitate the test optical fiber data acquisition. In this experiment, the KL-280G optical fiber fusion machine was used to weld the optical fiber, and the optical loss at the fusing point should not be greater than 0.02 dB. After the welding was completed, the thermal shrinkage tube was used to protect and fixed the fusion joint, and the optical fiber connectivity tester was used to check the optical path connectivity. The precise positioning of the fiber sensor in the model was the basis of the test accuracy. Only when the positions of the fiber sensors were accurately matched, the test results could accurately reflect the strain distribution of the measured body. Temperature calibration and positioning method was adopted in this experiment. Firstly, the temperature location point was set on the sensing optical fiber outside the boundary of the model. Then the positioning point was heated by the water bath heating method, and when the temperature of the marked point rises to the stable state (2 min), the center frequency of the optical fiber was measured by the fiber demodulator. Finally, the exact position of the marked points was determined by frequency comparison analysis to realize the positional location of the optical fiber sensors in the model. The model test temperature positioning test optical fiber frequency is shown in Fig. 7.

Fig. 5. Calibration experiment system of optical fiber sensor.

the equal strength beam experiment for polyurethane single mode optical fiber of 2 mm, as shown in Fig. 5. The calibration experimental materials are as follows: NBX-6055, equal strength beam, weights, 2 mm optical fiber, FBG, optical fiber fusion machine, optical fiber connectivity tester, adhesive, epoxy glue, scissors, screwdriver, alcohol, etc. In the calibration experiment, the optical fiber sensor with 2 mm diameter and 1100 mm length was bonded to the equal strength beam arm with epoxy resin glue, and a certain pre-tightening force was applied to the optical fiber before bonding. Secondly, three naked packaged FBG sensors was pasted parallel to the side of the optical fiber as a comparative test sensor. Finally, the equal strength beam was loaded step by step. The loading weight of each stage is 0.5 kg, which is loaded from 0.0 kg to 5.0 kg step by step, and the data is collected once at each loading stage. The frequency shift distribution of distributed optical fibers under step-by-step loading of equal-strength beam is shown in Fig. 6(a). When the load is 0.5 kg, the frequency shift is about 2.0 MHz. When the load is 2.0 kg, the frequency shift is about 8.8 MHz. When the load is 5.0 kg, the frequency shift is about 22.9 MHz. The results show that the strain of equal strength beam and the frequency shift of optical fiber increase gradually with the increase of load. The frequency shift of optical fiber will increase with the gradual increase of load, and the frequency shift data values exhibits a gradient arrangement of a certain ratio. The regression analysis of optical fiber and FBG test is shown in Fig. 6(b). The transverse coordinate is the Brillouin frequency shift of optical fiber, and the longitudinal coordinate is the wavelength shift of the FBG sensor. The relationship between the wavelength shift and the strain of the FBG sensor is linear, and the strain sensitivity coefficient of FBG is known. The study found that the test data of FBG sensor has a very high

3.6.2. Model excavation and data acquisition In the model experiment, the rectangular galvanized square pipe (60 mm × 40 mm) was used to instead of the coal seam. Each time a steel rectangular galvanized square pipe will be extracted, it represents one-time coal seam mining cycle. The mining thickness of the coal seam is 60 mm, and the excavation step of the coal seam is 40 mm. The mining progress of the working face is 280 mm/d, and the total advancing length is 2400 mm in 9 days. It is necessary to record the mining time, distance, frequency and optical fiber frequency shift data during the whole mining process of working face. A total of 120 times of the optical fiber frequency shift data were collected in the experiment, and the law of ground surface movement and deformation was 5

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The loading 0.0 kg The loading 0.5 kg The loading 1.0 kg The loading 1.5 kg The loading 2.0 kg

FBG01 original data FBG01 Fitting curve FBG02 original data

400

25

Wavelength drift /pm

Brillouin frequency shift /MHz

30

500

The loading 2.5 kg The loading 3.0 kg The loading 3.5 kg The loading 4.0 kg The loading 4.5 kg The loading 5.0 kg

20 15 10

FBG02 Fitting curve FBG03 original data FBG03 Fitting curve

2

R =0.99014 yFBG02=25.8648x-20.1802

200

2

R =0.99405 yFBG03=27.5141x-28.6787

100

5 0 0

200

400

600

800

1000

0

1200

Fiber location /mm

(a) Fiber frequency shift distribution

yFBG01=24.8999x-28.2766

300

2

R =0.99258

0

5

10 15 20 Brillouin freque ncy shift /MHz

25

30

(b) Regression analysis of optical fiber and FBG testing

Fig. 6. Frequency shift distribution and regression analysis of optical fiber in loading test of equal strength beam. 20

T1(3.491m) 15 Brillouin frequency shift /MHz

temperature effect should be eliminated. In order to ensure the accuracy of the test results, the temperature-compensation fiber is arranged above the boundary coal pillar, and the position is outside the mininginfluence range, so the change of the frequency shift is only influenced by the temperature of the model. In the analysis of the test results, the frequency shift caused by temperature is removed from the measured Brillouin frequency shift [41–43].

T2(7.033m)

10 5 0

4.1. Strain distribution of horizontal sensing optical fiber along trend direction

-5 -10

2

3

4

5

6

7

8

The sensing optical fiber is arranged in the ground surface clay layer along trend direction, and the coal mining face is pushed along trend direction from the open-off cut position. The strain distribution of sensing optical fibers is shown in Fig. 8(a). When the advancing distance is in the range of 0–720 mm, due to the large buried depth of coal seam and the control of ultra-thick conglomerate, the movement of overburden does not spread to the ground surface. The ground surface does not move and deform, and the strain of sensing optical fiber does not change significantly. When the advance distance of working face is 760 mm, the strain of optical fiber begins to change slightly, and the strain curve of the optical fiber is convex in a local range. The optical fiber is subjected to tensile stress as a whole, and the maximum strain of optical fiber is 106.83 με. That is to say, it is considered that the movement of overburden strata begins to spread to the ground, causing ground surface subsidence. The starting distance of the ground surface movement is 760 mm, which is about 0.4 times of the average buried depth. When the advancing distance is in the range of 840–1160 mm, the strain curve of optical fiber changes obviously and basically shows a “single peak” shape, and the whole optical fiber is still subjected to tensile stress. With the increase of mining distance, the convex peak range of optical fiber strain curve increases continuously, and the peak value increases from 183.37 με to 521.84 με. It shows that with the increase of the mining scope, the influence range and the maximum subsidence of the ground surface are increasing, which reflects the dynamic change process of the ground surface subsidence basin. As shown in Fig. 8(b). When the advancing distance is 1200 mm, the strain curve of optical fiber changes from single peak to double peak. The peaks values of the convex peaks on the left and right sides of the optical fiber strain curve are 198.43 με and 244.07 με respectively, and the strain value in the middle of the optical fiber strain curve is lower at about 137.11 με. It shows that the maximum tensile stress appears between the boundary point and the inflection point in the ground surface subsidence basin, the tensile stress gradually changes to the compressive stress in the central part of the basin, and the strain value in the

Fiber location /m

Fig. 7. Frequency shift curve of optical fiber positioning.

observed and recorded in detail in the process of coal mining. 4. Strain distribution of the sensing cables Because the PPP-BOTDA sensing technology is based on the Lorentz fitting of nonlinear least square method to obtain Brillouin frequency shift, this parameter estimation method is very sensitive to the noise generated by optical fiber demodulator [40]. In order to improve the signal-to-noise ratio of the measured signal, the wavelet threshold method is used to perform two-dimensional denoising on the Brillouin scattering spectrum, and the denoising parameters are optimized. In the model test, the amount of distributed optical fiber sensing data is large, and the measurement data is the center frequency. It needs to be manually calculated into Brillouin frequency shift, converted into strain according to the strain sensitivity coefficient, and then processed and analyzed. The data processing process requires a lot of time and energy, and the calculation process is complicated and error-prone, so it is necessary to make a special data processing software for optical fiber monitoring data to solve this problem. Then the distributed optical fiber data processing and analysis software is made by mixed programming with MATLAB and Visual Basic 6.0, and use software to denoise, repair, calculate, analyze and fit the optical fiber data. It effectively simplifies the data analysis and processing process, improves the extraction accuracy of Brillouin frequency shift, and avoids the under-fitting phenomenon which is easy to occur in traditional Lorentz fitting. Finally, the time-space strain curve of optical fiber is drawn to monitor the ground surface movement and deformation. The Brillouin frequency shift of the distributed optical fiber has the dual sensitivity characteristics of strain and temperature. Therefore, when the strain is measured, the frequency shift caused by the 6

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1200

advance to 0mm advance to 400mm advance to 760mm advance to 840mm advance to 920mm advance to 1000mm advance to 1080mm advance to 1160mm

1000

800

800

600

strain /

strain /

600

400

200

0

0

-200

-200

boundary pillar

-400

-600

mining coal seam 0

600

1200

boundary pillar 1800

working face advanced distance

a 1200

2400

boundary pillar

-400 -600

3000

800

mining coal seam

0

600

1200

boundary pillar 1800

2400

3000

working face advanced distance /mm

/mm

Advance 0 mm to 1160 mm

b 1200

advance to 1600mm advance to 1640mm advance to 1720mm advance to 1760mm advance to 1800mm advance to 1840mm advance to 1920mm advance to 1960mm

1000

1000

800

Advance 1200 mm to 1560 mm

advance to 2000mm advance to 2040mm advance to 2080mm advance to 2120mm advance to 2200mm advance to 2280mm advance to 2360mm advance to 2400mm

600

strain /

600

strain /

400

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400

400

200

200

0

0

-200

-200

-400

advance to 1200mm advance to 1240mm advance to 1320mm advance to 1360mm advance to 1400mm advance to 1440mm advance to 1520mm advance to 1560mm

1000

boundary pillar -600

mining coal seam 0

600

1200

boundary pillar 1800

2400

-400 boundary pillar -600 0

3000

working face advanced distance /mm

c

mining coal seam 600

1200

boundary pillar 1800

2400

3000

working face advanced distance /mm

Advance 1600 mm to 1960 mm

d

Advance 2000 mm to 2400 mm

Fig. 8. Strain distribution of sensing cables with the advance of working face.

basically stable in the range of 389 με to 406 με, which indicates that the overburden activity in the goaf tends to be stable, and the ground surface deformation has entered a stable state. The peak of the right side of the optical fiber strain curve increases from 1037 με to 1157 με. With the increase of mining range, the growth rate becomes smaller. It shows that the ultra-thick conglomerate is not unstable after breaking, but forms a certain structure to effectively control the development of ground surface movement and deformation, and the degree of ground surface subsidence deformation gradually weakens with the advance of the working face. Due to the model is limited by mining size, the ground surface subsidence basin under the condition of adequate mining can not be observed in the model experiment. It is predicted that the continuous advance of the working face will lead to the fracture and instability of the ultra-thick conglomerate, then induce the severe unbalanced movement and deformation of the ground surface, and even the ground surface appears the stepped subsidence or cracks.

middle of the optical fiber decreases gradually. The “double peak” of optical fiber strain curve can be considered as the possible location for the development of surface deformation. When the advancing distance is in the range of 1240–1560 mm, the strain curve of optical fiber is still in the shape of double peaks. However, the double peaks range of strain curve increases continuously with the increase of mining range, and the peak value increases from 264.32 με to 555.58 με continuously. The change of optical fiber curve shows that the ground surface subsidence basin is expanding with the increase of mining range, but the strain and its variation are small on the whole. The results show that the ground surface deformation is controlled by ultra-thick conglomerate in the mining range of 0–1560 mm, and the ground surface deformation is always in a slow subsidence state. The fundamental reason is that the ultra-thick conglomerate is in the state of elastic-plastic deformation, and the ground surface subsidence basin is a curved subsidence basin, the ground surface movement and deformation is small, and the subsidence basin is relatively flat. As shown in Fig. 8(c). When the advancing distance is in the range of 1600–1960 mm, the peak value of optical fiber strain increases rapidly to 960.88 με, and the tensile strain of the ground surface optical fiber near the coal wall boundary is larger than that of the goaf boundary. It shows that the ultra-thick conglomerate controlling the ground surface movement occurs large fault deformation, which causes the overburden strata to break and move strongly, resulting in large surface deformation and steep deformation coefficient. At this time, the ground surface subsidence basin is fracture type subsidence basin. As shown in Fig. 8(d). When the advancing distance is in the range of 2000–2400 mm, the left peak value of the optical fiber strain curve is

4.2. Analysis of horizontal sensing optical fiber along dip direction The sensing optical fiber is arranged in the ground surface clay layer along the dip direction, the strain distribution of sensing optical fibers in the mining process of No. 1 working face is shown in Fig. 9. When the advancing distance is in the range of 0–840 mm, due to the movement of overburden does not spread to the ground surface, the strain of sensing optical fiber does not change significantly. When the advancing distance is from 960 mm to 1360 mm, the strain curve of the optical fiber above the goaf changes significantly, and shows a single peak and small influence range. As the mining range increases, the strain value 7

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5. Numerical simulation 5.1. Modeling condition In order to intuitively analyze the internal mechanism of ground surface movement and deformation under the ultra-thick conglomerate situation, the corresponding numerical calculation model is established according to the physical model by using UDEC 6.0 numerical simulation software. The model is 1440 m in length, 800 m in width and 886 m in height. The simulated coal seam height is 16 m and the mining length is 960 m. The mining process is the same as the physical model test. According to the need of the research problem, the lower left corner of the model is set as the origin of the coordinate system (0, 0), the X axis is positively oriented to the right, and the Y axis is positively oriented to the upward. The boundary of model is the displacement boundary, the boundary of both sides limits the displacement in the x direction, the lower boundary limits the displacement in the y direction, and the upper boundary is the free boundary. The Mohr-Coulomb model is used in the overburden constitutive relation, and the Areacontact Coulomb Slip model is used in the joint constitutive relation. Firstly, the model is established to calculate the stress balance of the original rock. Secondly, the stress balance of the model is calculated for coal seam mining, and the standard of the model stress balance is that the ratio of the average unbalance force to the average force applied by the block nodes is less than 10-5. Finally, the data are extracted and post-processed.

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propensity length of working face /mm

Fig. 9. Strain distribution of sensing cables of No. 1 working face.

increases continuously, and the maximum peak value reaches 485.63 με. It indicates that the effect of coal mining begins to spread to the ground surface, and the ground surface movement subsidence basin enlarged with the advance of working face. When the advancing distance is from 1400 mm to 2400 mm, the optical fiber is subjected to tensile stress as a whole, and the strain curve is still in the shape of single peak, but the influence range becomes larger and the peak value increases to 638.56 με. It shows that the ground surface movement and deformation develops slowly under the control of the key layer of ultrathick conglomerate. The strain distribution of sensing optical fibers during coal mining in working face 2 is shown in Fig. 10. Affected by the mining of the working face 2, the position of optical fiber peak value gradually move from the top position of working face 1 to the top position of the middle of working face 1 and 2. When the advancing distance is in the range of 720–2400 mm, the peak strain of optical fiber increases to 972.36 με. However, the strain along the dip direction is 185 με less than the strain along the trend direction. It shows that the movement and deformation of the ground surface in the dip direction is smaller than that in the trend direction. Due to the limitation of the mining size, the ground surface does not form a adequate mining subsidence basin in the dip direction.

5.2. Result analysis The overburden and ground surface movement and deformation of the stope during the mining process of the working face is shown in Fig. 11. As you can see from Fig. 11(a), when the working face is advanced to 288 m, the lower strata of ultra-thick conglomerate appears local caving and fracture, and the maximum vertical displacement of the ultra-thick conglomerate is only about 0.1 m. It shows that the ultrathick conglomerate effectively supports the weight of the overburden in the form of key layers. The deformation of overburden is small due to large thickness and stiffness of the key layers, and the ground surface does not move and deform at this time. As you can see from Fig. 12(a), when the working face is advanced to 288 m, the development height of the plastic area of mining overburden is about 105 m, and the lower part of ultra-thick conglomerate has micro-plastic development. It indicates that the movement and deformation of overburden do not spread to the ground surface due to the control of ultra-thick conglomerate and the limitation of mining size. When the working face advanced to 720 m, the height and width of the overburden fracture gradually expand, and the fracture zone has been developed to the lower part of the ultra-thick conglomerate, showing the trapezoidal distribution, as shown in Fig. 11(b). The maximum displacement of ultra-thick conglomerate increases to 1.2 m. The deformation and failure of mining overburden has been affected to the ground surface, and the maximum subsidence of the ground surface is about 0.9 m. The location of the ultra-thick conglomerate begins to produce the obvious separated layer development area, and the displacement of the rock layer above the super-thick conglomerate is obviously smaller than that of the lower stratum, showing an obvious demarcation line. It indicates that the ultra-thick conglomerate plays a key role in restraining the upward development of overburden failure and effectively limits the upward development of overburden deformation and failure. At this time, the plastic zone of the overburden mainly develops in the lower part of the ultra-thick conglomerate, and there are some slight dislocations or unfolded micro-cracks in the middle of the thick conglomerate strata, as shown in Fig. 12(b). The results show that there is a non-penetrating fracture in the ultra-thick conglomerate, and the step distance of the initial fracture is about 660–720 m. Because the ultra-thick conglomerate is the main key layer

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Fig. 10. Strain distribution of sensing cables of No. 2 working face. 8

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Fig. 11. Vector Distribution of Overburden Vertical Displacement with different mining distance.

of 896 m, the deformation of the upper and lower group conglomerate layers becomes larger, but the magnitude of the increase becomes smaller. It is predicted that the maximum subsidence value of the ground surface is basically stable when the ultra-thick conglomerate structure is not unstable. When the ultra-thick conglomerate structure is unstable, the maximum subsidence value of the ground surface will suddenly increase rapidly, even cracks appear on the ground surface.

of the overburden in the stope, its bending and breaking will lead to the overall cooperative deformation of the overburden. A large number of tensile joint cracks develops in the lower and upper strata of the ultrathick conglomerate, and cracks may appear in the local area of the ground surface. When the working face advanced to 896 m, due to the existence of 1 m weak interlayer in the middle of the ultra-thick conglomerate, the ultra-thick conglomerate is obviously divided into two parts: the upper group conglomerate and the lower group conglomerate, as shown in Fig. 11(c). There is a large separation layer between the upper group conglomerate and the lower group conglomerate. The maximum vertical displacement of the lower group conglomerate is 5.8 m, and the maximum vertical displacement of the upper group conglomerate is 2.5 m. The maximum subsidence of the ground surface is 2.3 m due to the influence of the movement and deformation of the ultra-thick conglomerate. As you can see from Fig. 12(c), the development of joints and cracks in the plastic zone of overburden has penetrated through the ultra-thick conglomerate, the lower group conglomerate has fractured, and the plastic zone of the upper group conglomerate has been locally developed. It indicates that the ultra-thick conglomerate of the main key layer has been locally broken, but the stability is not lost. When the working face advanced to 960 m, the maximum vertical displacement of the lower group conglomerate is 6.9 m, the maximum vertical displacement of the upper group conglomerate is 3.6 m, and the separation layer between the upper group conglomerate and the lower group conglomerate further increases, as shown in Fig. 11(d). The maximum subsidence of the ground surface is 3.3 m. It can be seen from Fig. 12(d) that the plastic zone height in the overburden is further developed upward. Compared with the state of the advancing distance

6. Comparison analysis In order to compare the physical model test results with the numerical simulation results intuitively and reasonably, it is necessary to process the numerical simulation calculation data according to the similar proportion. The comparison curve of the distribution characteristics between the maximum strain of the ground surface detected by optical fiber sensors in physical model test and the maximum vertical displacement calculated by numerical simulation is shown in Fig. 13. In the range of advancing distance from 0 mm to 400 mm, the strain value of optical fiber is 0 με. In the range of advancing distance from 400 mm to 1520 mm, the strain value of optical fiber increases slowly from 0 με to 295 με. In the range of advancing distance from 1520 mm to 2400 mm, the strain value of optical fiber increases sharply, and the maximum peak value is 1157 με. The advancing distance of the working face is in the range of 0–1560 mm, and the vertical displacement of the ground surface is always 0 mm. In the range from 1560 mm to 2400 mm, the vertical displacement of the ground surface increases rapidly from 0 mm to 8.25 mm. The optical fiber strain and the vertical displacement are substantially zero or small before the advancing distance of 1520 mm. After the advancing distance of 1560 mm, the 9

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Fig. 12. Development and distribution of overburden plastic zone with different mining distance.

1000

Model Gauss Equation y=y0 + (A/( 0 Reduced C 0.06568 3028.8053 Adj. R-Squ 0.98975 0.98341 Value Standard E y0 -0.08781 0.07611 xc 2553.1724 64.49049 w 943.6741 70.87947 A A 9818.5081 1279.4952 sigma 471.83705 FWHM 1111.0913 Height 8.30163 y0 31.10313 21.25448 xc 2283.4657 38.2282 w 1110.9246 67.45254 B B 1.5653E6 124797.60 sigma 555.4623 FWHM 1308.0137 Height 1124.2223

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surface vertical displacement huge thick conglomerate displacement GaussFit of B GaussFit of C Model G auss Equation y=y0 + ( A/( Reduc ed C 0.06568 0.0532 Adj . R- Squ 0.98975 0.99219 Val ue Standar d E y0 -0.08781 0.07611 xc 2553.1724 64.49049 w 943.6741 70.87947 B B 9818.5081 1279.4952 sigma 471.83705 FWHM 1111.0913 Height 8.30163 y0 0.05951 0.07411 xc 2682.9711 87.20622 w 1092.8808 84.26062 C C 13068.404 2028.2983 sigma 546.44043 FWHM 1286.7688 Height 9.54091

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Fig. 13. Comparative analysis of optical fiber strain and ground surface displacement.

Fig. 14. Comparative analysis of displacement between ground surface and ultra-thick conglomerate.

optical fiber strain and the vertical displacement begin to increase rapidly, and the trend is monotonously increasing. The variation trend and law of optical fiber strain and vertical displacement are basically the same, which indicates that the optical fiber sensing technology can be used as a reliable method of the ground surface deformation monitoring. With the increase of the advancing distance of the working face, the comparative curves of the distribution characteristics of the vertical displacement of the ground surface and the vertical displacement of the ultra-thick conglomerate by numerical simulation are shown in Fig. 14. When the mining distance is 1520 mm, the ground surface and the

ultra-thick conglomerate began to sink at almost the same time. When the mining distance is 2400 mm, the maximum displacement of the ground surface is 8.25 mm, and the maximum displacement of ultrathick conglomerate is 8.76 mm, the maximum displacement of the both is almost the same. The displacement data of the ground surface and ultra-thick conglomerate are fitted by Gauss method respectively. The results show that the displacement of the ground surface and superthick conglomerate are basically consistent in numerical value and variation law. It is fully proved that the ultra-thick conglomerate as the key layer controls the ground surface movement and deformation. In 10

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Table 3 Comparative analysis of data characteristics. Classification

Optical fiber strain (με) Ground surface displacement (mm) Conglomerate displacement (mm)

Working face advanced distance/mm 0

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2400

0 0 0

0 0 0

106 0 0.25

160 0 0.26

242 0 0.36

295 1.00 1.22

631 1.24 1.36

723 1.93 2.35

960 3.42 3.63

1088 5.23 5.51

1112 5.75 6.25

1157 8.25 8.76

Acknowledgements

the stage of bending deformation of ultra-thick conglomerate, the ground surface subsidence has a good linear law. If the ultra-thick conglomerate is in the stage of fracture deformation, the stepped subsidence deformation will occur on the ground surface. In order to analyze the relationship among ground surface strain, ground surface displacement and conglomerate displacement more intuitively, Table 3 is created as follows.

Thanks to funds supported by the National Natural Science Foundation of China (No. 51174280 and No. 51804244) and the Doctoral Scientific Fund Project of Chinese Ministry of Education (No. 20126121110003). The author would also like to thank Dr. Yuan and Dr. Du for participating in the model test and for valuable comments and suggestions on the improvement of the manuscript. Otherwise, this study would not to be so smooth implementation without them.

7. Conclusions (1) The model test shows that the distributed optical fiber sensing technology can effectively capture the process of the ground surface movement and deformation caused by mining. When the advancing distance is up to 760 mm, the optical fiber monitors the ground surface deformation, and the strain of optical fiber is 106.83 με. It indicates that the movement of mining overburden begins to spread to the ground surface, causing the ground surface to sink, and the starting distance of ground surface subsidence is 760 mm. When the advancing distance is 1200 mm, the strain curve of optical fiber changes from single peak to double peak. The peak values on the left and right sides of the strain curve are respectively 198.43 με and 244.07 με, and the strain value in the middle of the curve is only about 137.71 με. The optical fiber strain curve can accurately locate the formation area of the ground surface subsidence basin. When the advancing distance is from 1200 mm to 1600 mm, the peak strain of optical fiber changes abruptly with the increase of mining distance, and the peak strain of optical fibers increases rapidly to 960.88 με. The results show that the ground surface deformation is increasing sharply due to the breakage of key layer. (2) The comparative analysis of the model test results and the numerical simulation results reveals some basic laws of the relationship between the ground surface and the ultra-thick conglomerate movement and deformation. Under the control of the ultra-thick conglomerate key strata, the whole process of ground surface movement and deformation can be roughly divided into three stages: the stage of deformation start-up, the stage of slow deformation and the stage of deformation mutation. In the deformation start-up stage, the mining area is small, the key layer of ultrathick conglomerate prevents the overburden deformation from transferring to the ground surface, and the mining disturbance does not affect the ground surface deformation. In the slow deformation stage, as the mining area gradually increases, the ultra-thick conglomerate is affected by mining and produces bending deformation, and the ground surface is slowly and continuously deformed by the deformation of the key layer. In the stage of deformation mutation, the mining size is larger than the limit span of ultra-thick conglomerate, the local fracture deformation of ultra-thick conglomerate occurs, and the ground surface deformation increases rapidly. It indicates that the movement of ultra-thick conglomerate is the key factor of the ground surface movement and deformation. (3) This paper explores the construction technology, monitoring effect and applicability of distributed optical fiber sensing technology in the ground surface monitoring of model test, which enriches the detection means of the ground surface deformation in model test and provides a method for the information research of the ground surface movement and deformation.

References [1] S. Abdikan, M. Arıkan, F.B. Sanli, et al., Monitoring of coal mining subsidence in peri-urban area of Zonguldak city (NW Turkey) with persistent scatterer interferometry using ALOS-PALSAR, Environ. Earth Sci. 71 (9) (2014) 4081–4089. [2] Q. Wu, J. Pang, S. Qi, et al., Impacts of coal mining subsidence on the surface landscape in Longkou city, Shandong Province of China, Environ. Earth Sci. 59 (4) (2009) 783–791. [3] L.J. Donnelly, D.J. Reddish, The development of surface steps during mining subsidence: “Not due to fault reactivation”, Eng. Geol. 36 (3–4) (1994) 243–255, https://doi.org/10.1016/0013-7952(94)90006-X. [4] G. Wei-Jia, K. Ling-Hai, C. Shao-Jie, Correlation between rock burst and movement of strata and ground surface, Rock Soil Mech. 30 (2) (2009) 447–451. [5] Ximin Cui, Kazhong Deng, Research review of predicting theory and method for coal mining subsidence, Coal Sci. Technol. 45 (1) (2017) 160–169. [6] Baochen Liu, Jiasheng Zhang, Stochastic method for ground subsidence due to near surface excavation, Chin. J. Rock Mech. Eng. 14 (4) (1995) 289–296. [7] J. Litwiniszyn, Przemieszczenia gorotworu. ws wietle teorii prawdopodobienstwa, Arch. Gor. Hut. 2 (1954) 45–68. [8] M. Hood, R.T. Ewy, L.R. Riddle, Empirical methods of subsidence prediction—a case study from Illinois, Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 20 (4) (1983) 153–170. [9] Hua-yang Dai, Jin-zhuang Wang, Mei-feng Cai, Extraction-vectorized prediction method for rock and surface movement, J. China Coal Soc. 27 (5) (2002) 473–478. [10] Yin-fei Cai, V. Thierry, O. Deck, et al., Improving the influence function method to take topography into the calculation of mining subsidence, J. China Coal Soc. 41 (1) (2016) 271–276. [11] Jixiong Zhang, Qiang Zhang, Qiang Sun, Surface subsidence control theory and application to backfill coal mining technology, Environ. Earth Sci. 74 (2) (2015) 1439–1448. [12] X. Cui, J. Wang, Y. Liu, Prediction of progressive surface subsidence above longwall coal mining using a time function, Int. J. Rock Mech. Min. Sci. 38 (7) (2001) 1057–1063. [13] Ming-gao Qian, Xie-xing Miao, Jia-lin Xu, Theoretical study of key stratumin ground control, J. China Coal Soc. 21 (3) (1996) 225–230. [14] J.L. Xu, M.G. Qian, W.B. Zhu, Study on influences of primary key stratum on surface dynamic subsidence, Chin. J. Rock Mech. Eng. 24 (5) (2005) 787–791. [15] Weibing Zhu, Jialin Xu, Wenqi Lai, et al., Research of isolated section-grouting technology for overburden bed separation space to reduce subsidence, J. China Coal Soc. 32 (5) (2007) 458–462. [16] Z.J. Zhang, W.U. Kan, A.B. Zhang, Determination on starting distance of subsidence deformation base on key strata theory, Coal Eng. 41 (2) (2009) 70–72. [17] L. Yang, B. Qiu, Investigation into key strata movement impact to overburden movement in cemented backfill mining method, Procedia Eng. 31 (2012) 727–733. [18] L.I. Chunyi, C. Ximin, H.U. Qingfeng, et al., An analysis of extra-thick coal mining influence on ground surface deformation under the condition of massive conglomerate stratum in Changcun colliery, J. Min. Safety Eng. 32 (4) (2015) 628–633. [19] T.N. Singh, I.W. Farmer, A physical model of an underground coal mine prototype, Int. J. Min. Eng. 3 (4) (1985) 319–326. [20] M. Tao, X. Chen, Q.Q. Ding, A method for subsidence monitoring of similar material simulation test in coal mining, Adv. Mater. Res. 765–767 (6) (2013) 2172–2175. [21] J. Chai, Q. Liu, J. Liu, et al., Optical fiber sensors based on novel polyimide for humidity monitoring of building materials, Opt. Fiber Technol. 41 (2018) 40–47. [22] J. Chai, W. Du, Q. Yuan, et al., Analysis of test method for physical model test of mining based on optical fiber sensing technology detection, Opt. Fiber Technol. 48 (2019) 84–94. [23] J. Chai, W. Du, Experimental study on the application of BOTDA in the overlying strata deformation monitoring induced by coal mining, J. Sens. (2019) 1–9. [24] D. Zhang, J.C. Wang, P.S. Zhang, et al., Internal strain monitoring for coal mining

11

Optical Fiber Technology 53 (2019) 102006

J. Chai, et al.

[25] [26]

[27]

[28]

[29]

[30]

[31]

[32] [33]

similarity model based on distributed fiber optical sensing, Measurement 97 (2016) 234–241. Yi Lu, Bin Shi, Jun Yu, et al., Model test on distributed optical fiber monitoring of land subsidence and ground fissures, J. Eng. Geol. 23 (5) (2015) 896–901. Chunsheng Gu, Xulong Gong, Qiang Sun, et al., Model tests on the ground fissures based on the optical fiber sensing technology, Hydrogeol. Eng. Geol. 45 (3) (2018) 118–123. S. Bin, G.U. Kai, W. Guangqing, et al., Full section monitoring of land subsidence borehole using distributed fiber optic sensing techniques, J. Eng. Geol. 26 (2) (2018) 356–364. Dan Zahng, Pingsong Zhang, Bin Shi, et al., Monitoring and analysis of overburden deformation and failure using distributed fiber optic sensing, Chin. J. Geotech. Eng. 37 (5) (2015) 952–957. S.L. Liu, D. Zhang, P.S. Zhang, et al., Deformation monitoring of overburden based on distributed optical fiber sensing technology, J. Eng. Geol. 24 (6) (2014) 1118–1125. J. Chai, L. Zhu, D.D. Zhang, et al., Study on settlement deformation of unconsolidated strata during low pressure water injection process of multi borehole, J. China Coal Soc. 38 (10) (2013) 1720–1727 1728. Pingsong Zhang, Dan Zhang, Binyang Sun, et al., Optical fiber monitoring technology for evolution characteristic of rock stratum deformation and failure in space of mining field, J. Eng. Geol. 27 (2) (2019) 45–51. Minfu Liang, Xinqiu Fang, Guangzhe Xue, et al., Development of anchor dynamometer of FBG and its field test, J. Min. Saf. Eng. 34 (3) (2017) 549–555. Pingsong Zhang, Haifeng Lu, Biwu Han, et al., Monitoring and analysis of deformation characteristics of fault structure undermining condition, J. Min. Saf. Eng. 36 (2) (2019) 351–356.

[34] K. Wan-Jun, Z. Gen-Yuan, G. Wei, et al., Research and application of dynamic testing technology for floor rock damage depth under mining condition, Coal Eng. 50 (10) (2018) 96–100. [35] T. Horiguchi, M. Tateda, BOTDA-nondestructive measurement of single-mode optical fiber attenuation characteristics using Brillouin interaction: theory, J. Lightwave Technol. 7 (8) (1989) 1170–1176. [36] T. Horiguchi, K. Shimizu, T. Kurashima, et al., Development of a distributed sensing technique using Brillouin scattering, J. Lightwave Technol. 13 (7) (1995) 1296–1302. [37] T. Kurashima, T. Horiguchi, H. Ohno, et al., Strain and temperature characteristics of Brillouin spectra in optical fibers for distributed sensing techniques, European Conference on Optical Communication, IEEE, 1998. [38] H.R. Zhao, T.N. Wang, W.B. Tang, The developments of geophysical modeling, Chin. J. Geophys. 37 (a01) (1994) 267–276. [39] Dongqi Zuo, Theory and method of model test, Hydraulic and Electric Power Press, 1984. [40] Q. Shang, Y. Hu, W. Liu, Research on Brillouin frequency shift extraction method of BOTDA sensing system, Semicond. Optoelectron. 38 (5) (2017) 633–638 669. [41] R.K. Palmer, T.E. Blue, Modulation transfer function for distributed temperature measurements using an optical fiber sensor system, IEEE Sens. J. 18 (5) (2018) 1911–1918. [42] H. Wang, L. Jiang, P. Xiang, et al., Improving the durability of the optical fiber sensor based on strain transfer analysis, Opt. Fiber Technol. 42 (2018) 97–104. [43] W. Huaping, X. Ping, Lizhong Jiang, Optical fiber sensor based in-field structural performance monitoring of multi-layered asphalt pavement, J. Lightwave Technol. 36 (17) (2018) 3624–3632.

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