After roadway excavation, the deformation and failure of roadway surrounding rocks typically results in roadway damage or collapse. Conventional monitoring techniques, such as extensometers, stress meters, and convergence stations, are only capable to 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 these shortcomings, an innovative monitoring system for surrounding rock deformation control has been developed base on Brillouin optical 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. The optical fiber sensors are installed in boreholes which are situated ahead of the excavation face; therefore, the sensors can be protected well and the surrounding rock deformation behaviors can be studied. The proposed system has been applied within a TBM-excavated roadway in Zhangji coal mine, China. The surrounding rock deformation behaviors have been detected accurately, and the monitoring results provided essential references for surrounding rock deformation control works.
In recent 20 years, as the depletion of coal resources in shallow strata, coal mining operations have been moving to deeper strata [
For overcoming the problems of surrounding rock deformation monitoring in deep coal mines, many emerging measurement techniques have been conducted at working faces of coal mining and roadway excavation in underground coal mines. Zhao et al. conducted damage process monitoring of roadway surrounding rock by using microseismic techniques [
Despite the fact that the monitoring techniques have gained some advances, the abovementioned monitoring methods are still flawed in some aspects. Microseismic techniques and transient electromagnetic and seismic reflection investigation are able to detect fracture evolution in surrounding rocks, while their accuracy on rock displacement monitoring is unsatisfied (reach to meters). Microfocus X-ray tomography is only capable of measure damages in rock samples; therefore, it cannot be applied for in situ monitoring. Compared with fully distributed fiber optic sensor systems, FBG systems call for an excessive number of sensors and that leads to high cost. Moreover, most commercially available interrogators can handle only a fairly small number of FBGs, setting a limit on the number of sensing points, as well as on their density along the fiber [
Brillouin optical time domain reflectometry (BOTDR) is a fully distributed sensing technology for distributed strain and temperature measurement along all determined areas with only one optical fiber which is stimulated by laser pulses and therefore many discrete sensors can be replaced [
In previous BOTDR applications, optical fibers were installed about 5 m behind the excavation faces of roadways to avoid interference with the installation of supporting structures (rockbolts, cable bolts, steel meshes, etc.). Therefore, only time-dependent deformation can be measured and the immediate deformation which occurs soon after excavation cannot be studied. However, 80% of total roadway damage and collapse accidents happened near the excavation faces [
This paper focuses on developing a BOTDR-based monitoring system for surrounding rock control of roadways in underground coal mines. The structure of the monitoring system is modified so that a real-time monitoring of both immediate and time-dependent deformation of surrounding rocks can be detected. In situ monitoring of the system in a roadway is proposed, and the monitoring results are analyzed and compared with measurement results acquired from conventional monitoring techniques.
The BOTDR-based monitoring system implements Brillouin scattering, which is a basic physical process representing the interaction effect between light and optical medium in propagation medium [
If longitudinal strain
Considering the influence of strain and temperature, the Brillouin frequency shift
For typical optical fibers, the scale factor is 493 MHz/% (strain) and the temperature factor is 1 MHz/°C.
The position where the strain occurs can be detected by analyzing the time interval (
A schematic view of time and frequency domain acquisition for strain or temperature detection using BOTDR [
The BOTDR-based monitoring system consists of the ground unit and underground unit, as shown in Figure
The BOTDR-based surrounding rock deformation monitoring system installed in an underground coal mine.
The underground unit of the monitoring system contains the communication system and BOTDR sensing system. The communication system includes communication substations and industrial switch. The communication substations receive monitoring data from different underground working sites, and the industrial switch connects the communication substations and ground unit by using RS485 interface and MHYV cables.
BOTDR sensing system comprises laser light source, pulse modulation unit, optical heterodyne receiver, electrical heterodyne receiver, digital processor, and optical fiber sensors. The digital processor is linked with the nearest communication substation by the MHYV cable. The average distance of the optical fiber sensor to the monitoring host is 6.9 km.
The fiber sensors are capable of sensing the temperature and strain over long distances [
Figure
Schematic view of steel wire reinforced optical fiber sensor.
Before conducting in situ monitoring, the optical fiber sensors need to be calibrated in the laboratory for understanding their mechanical behaviors. The calibration works include strain calibration and temperature calibration.
Stain calibration was conducted by a fiber tensile machine. An optical fiber in length of 1.2 m is stretched on the tensile machine; the strain of fiber and corresponding Brillouin frequency shift are recorded for analysis. Therefore, the linear relationship between strain and Brillouin frequency shift can be obtained. The strain factor of the steel-reinforced optical fibers which are used in this project is 499.8 MHz/%. The laboratory calibration relationship between strain and Brillouin frequency shift is shown in Figure
Brillouin frequency shift versus strain in laboratory calibrations.
Temperature calibration is implemented by a water tank. Place the optical fiber into the water tank and then heat the water. The change of temperature and Brillouin frequency shift during the whole heating process are recorded, and the linear relationship between temperature and Brillouin frequency shift is learned. The temperature factor is 1.775 MHz/°C and the laboratory calibration relationship between temperature and Brillouin frequency shift is shown in Figure
Brillouin frequency shift versus temperature in laboratory calibrations.
The monitoring site is located at overlying coalbed methane (CBM) drainage roadway of 1413A longwall panel of Zhangji coal mine (Huainan Mining Industry), China. The roadway has a length of 1598 m, a diameter of 4.5 m, and a buried depth of 505 m. the roadway was excavated by a gripper TBM.
The CBM drainage roadway is situated in coal-bearing strata which consist of fine sandstone, medium sandstone, argillaceous sandstone, and number 1 coal seam. The geometry of CBM drainage roadway cross-section is a circle with a diameter of 4.5 m and the roadway is supported by rockbolts and steel meshes. The number 1 coal seam is 6.5 m in thickness with a dip angle of 2-3° and situated 25–30 m beneath the CBM drainage roadway. The location of Zhangji coal mine and geological setting of the CBM drainage roadway are illustrated in Figure
Location and strata histogram of CBM drainage roadway in Zhangji coal mine.
Rock properties of roadway surrounding rocks.
Rock category | Density (g/cm3) | UCS1 (MPa) | Tensile strength (MPa) | Elastic modulus (GPa) | Limited tensile strain ( |
Limited compress strain ( |
---|---|---|---|---|---|---|
Sandstone | 2.51 | 28 | 7.8 | 42 | 185.7 | 666.7 |
1Uniaxial compressive strength.
The ground stress data had been obtained by borehole stress relief measurements. The measurement results suggested 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, the minimum horizontal stress, and the maximum horizontal stress are 14.5 MPa, 13.4 MPa, and 37.4 MPa, respectively,
It is the first application of TBM in roadway excavation project in an underground coal mine which is operated by vertical shafts. The surrounding rock deformation behaviors of TBM-excavated roadway are estimated to be different with that of roadways constructed by conventional drilling and blasting due to different excavation disturbance effects of two excavation techniques. The monitoring works were conducted for studying deformation and stress filed redistribution behaviors of surrounding rocks of the TBM-excavated roadways, and the monitoring results can provide a reference for surrounding rock control, safety evaluation, and roadway support design.
The excavation works typically result in deformations and damages of roadway surrounding rocks, and continuous deformations and damages occur on both surface and interior section of surrounding rocks under the effect of the disturbed stress field. During the deformation and damage processes, the fractures within the interior sections of the surrounding rocks could expand themselves to the surface of surrounding rocks and result in damages or collapses of roadways. Therefore, the deformation and damage behaviors of roadway surrounding rocks are demanded to evaluate the stability of roadways during the excavation and utilization of roadways. In previous researches, optical fiber sensors were typically installed behind excavation faces within excavated roadways and the strain data of roadway surface can be obtained after excavation works. Nevertheless, the conventional monitoring systems cannot record strain variation during excavation works and the strain distribution in the interior sections of surrounding rocks.
In order to understand disturbance behaviors of roadway surrounding rocks under TBM excavation, two monitoring boreholes were drilled from an adjacent roadway (main gate of 1413A longwall panel) to the CBM drainage roadway of 1413A longwall panel. Optical fiber sensors were installed within the boreholes; therefore, as the TBM pass through the monitoring area, the strain values at various depths of roadway surrounding rocks along the radial direction of the roadway can be detected. As shown in Figure
The monitoring borehole layout.
Optical fiber sensors were installed with monitoring boreholes. The diameter of boreholes should fulfill the requirements of sensor installation. In this study, two boreholes with a diameter of 127 mm were drilled from the main gate of 1413A longwall panel for optical fiber sensor installation. The distance from the main gate entrance to the number 1 borehole and number 2 borehole is 867 m and 912 m, respectively.
The installation procedure of optical fiber sensors is as follows:
Installing the optical fiber sensors: Tie the steel wire-reinforced optical fiber sensor on a polyvinyl chloride (PVC) tube which has a diameter of 40 mm then place the PVC tube into the borehole. The PVC tube is used as an orienting device for optical fiber sensors. Place the exhaust pipe and grouting pipe into the borehole and seal the borehole by installing a sealing plate. Start grouting. Injecting the cement grout into the borehole, after the cement hardens, it is capable of providing protection for optical fiber sensors as well as insurance of essential coupling effect between sensors and surrounding rocks. Connect transmission lines and power supplies. The preparation of monitoring works is finished. The structure of monitoring boreholes is shown in Figure
A schematic view of monitoring borehole and optical fiber sensor installation.
Monitoring works were started after cement grout curing. The initial strain along the optical fiber sensors was recorded as reference values. Therefore, quantitative evaluation of damage degree of surrounding rocks under the disturbed effects of TBM excavation can be studied by measuring the changing of strain values. The limited compressive and tensile strain of rocks had been obtained from laboratory tests of rock specimens. Once the strain values exceed the limited strains, it can be deemed that damages occur in surrounding rocks and the positions of damages can be detected. Consequently, an early warning of 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. The strain values of surrounding rocks which were induced by TBM excavation has been clearly measured by optical fiber sensors when the TBM passed through the monitoring area. The monitoring results of numbers 1 and 2 boreholes are shown in Figures
Monitoring results of surrounding rock strain along the optical fiber in the number 1 borehole during the approaching of the TBM.
Monitoring results of surrounding rock strain along the optical fiber in the number 2 borehole during the approaching of the TBM.
The TBM approached the number 1 borehole firstly. In the 8th of February, the TBM is 12 m to the number 1 borehole, the tensile strain increased to 629.85
Figure
According to the monitoring results of two optical fiber sensors, the maximum tensile and compressive strain were detected at depth of 35.9 m and 43.2 m in the number 1 borehole and 35.4 m and 42.2 m in the number 2 borehole, respectively. In this study, sites at a depth of 35.5 m and 43 m within both monitoring boreholes were chosen as critical monitoring points of tensile and compress concentration. Figure
Strain variation of surrounding rocks along with TBM advancing.
It can be learned from the monitoring results that the disturbance range ahead the TBM excavation face is around 5 m. There are tensile concentration zone and compressive concentration zone located in 35.5 m and 43 m depth within the boreholes. Comparing the monitoring results with the limited tensile and compressive strain which obtained from laboratory tests, the TBM-induced damage zone can be acquired in real-time by taking advantage of the BOTDR-based online monitoring system. Damage zones had been detected about 5 m and 12.5 m away from roadway surface, and these damage zones cannot be found by conventional techniques such as extensometers or instrumental bolts due to their limited monitoring ranges. The strain values within 5 m to roadway surface are relatively low due to the effect of roadway support facilities such as rockbolt and cable bolt. The monitoring results also indicate that the roadway surrounding rock disturbance mainly happened after excavation. The surrounding rock typically gets rebalance within 3-4 days after excavation and that indicates the excavation disturbance of TBM is significantly less than that of conventional drilling and blasting.
Compared with conventional monitoring techniques, optical fibers which are used in BOTDR-based monitoring system are both sensors and propagation medium of signals. One optical fiber can detect strain in a large area of surrounding rocks and the monitoring area can be controlled by simply adding or reducing the number of optical fibers. The disturbance and damage behaviors of rocks under TBM excavation had been learned by using BOTDR-based monitoring system. The BOTDR monitoring system is capable of measuring ± 15000
For discrete strain monitoring techniques (FBG, electric resistance strain gauges, vibrating wire strain gauge, etc.), it is difficult to obtain strain data over long distances because this requires installing a sufficient number of sensors and a corresponding increase in costs. The total cost of BOTDR monitoring operation in this study is 300,000 RMB, whereas if FBG sensors had been used as alternatives, in this case, the cost would increase to over 600,000 RMB due to the high price of FBG sensors (700 RMB). Moreover, the cost of BOTDR monitoring could be even lower in future coal mine applications because abandoned boreholes (such as sampling boreholes, surveying boreholes, methane drainage boreholes, and water drainage boreholes) near roadways can be used to install optical fiber sensors.
In previous BOTDR monitoring applications in coal mines, optical fiber sensors were typically installed on the rock surface. Therefore, the deformation and damage behaviors of surrounding rocks are hardly acquired by the conventional layout of sensors. Installing sensors within boreholes which are capable of providing well protection of sensors and the installation works can be conducted in adjacent roadways so that the interference between sensor installation and roadway supporting works can be avoided.
A BOTDR-based monitoring system has been proposed and successfully applied in Zhangji coal mine, China. An accurate, reliable, large-scale, and real-time monitoring of surrounding rocks had been obtained by means of the special layout of sensors and installing optical fiber sensors within boreholes. The deformation and damage behaviors of surrounding rocks under the disturbance of TBM excavation had been learned. An early warning of surrounding rock damages can be provided when the strain values of rocks exceed the limits and additional roadway supporting works can be conducted. Consequently, the prevention-oriented strategy of roadway safety and stability control can be realized.
The authors declare no conflict of interest.
The work presented in this paper is financially supported by the National Natural Science Foundation of China (Grant nos. 51674006 and 51474004) and Youth Fund of Anhui University of Science and Technology (Grant nos. QN2017211 and QN2017222). The authors would like to express their appreciation to the staff of Zhangji coal mine for their substantial support.