# Application of Fiber Bragg Grating Sensing Technology for Bol Force Status Monitoring in Roadways
## 4 Application of FBG Sensors for Bolts Force Monitoring in Coal Mine Roadway
### Analysis of Monitoring Results  
One hundred days of continuous monitoring was conducted at the selected observation station. The working face advanced 200 m during this period, i.e., an average of about 2 m a day. The temperature in the installation place of the sensor was basically constant, so the effect of temperature on the sensor could be omitted and it was approximately supposed that the FBG wavelength shift was caused by only the force change of the bolt. Figure 9 shows the curve of the BFSM results versus the working face distance (first advance is positive and lagging advance is negative).  
Figure 8: Arrangement of roadway bolt support and monitoring system in the coal mine.  
The FBG bolt force sensor can clearly monitor the dynamic force status of the bolt in the roadway. The feasibility of FBG monitoring can be verified by the observation data on the pressure gauge.  
It can be seen from the monitoring results of Figure 9 that the bolt force at the monitoring observation station shows a change rule of being stable first, gradually increasing to a peak, and then decreasing. The monitoring results obtained by two methods, namely, the FBG sensor method and the pressure gauge method, are very different but have the same change trend. In the range of 105-150 m from the front of the working face, the bolt force is basically stable, the monitoring value is the initial pretightening force of the bolt of about 37 kN, and the FBG sensor is much more sensitive than the pressure gauge. With the advancement of the working face, the change curve between the bolt force and the relative working face distance is more obvious. The bolt force is gradually increased in the range of 60-105 m from the working face. The bolt force is changed greatly in the range of \(-\)50-60 m from the working face, because the roadway in this area is drastically affected by mining  
Figure 9: Monitoring results of roof bolt force in the No. 14301 track roadway: (**a**) monitoring results of Sensor-1; (**b**) monitoring results of Sensor-2.  
in the working face and the roof pressure is changed abruptly. The axial force of the bolt reaches its maximum of 128 kN at the point about 25 m from the front of the working face. Therefore, reinforcing measures shall be taken in the range of 60 m in front of the working face to reduce the roadway roof subsidence and prevent roof accidents.  
The reasons for the difference in monitoring results obtained by the FBG sensor and the pressure gauge are mainly as follows: The FBG sensor has a stronger immunity to interference, while the pressure gauge is more easily affected by the downhole humidity, temperature, and electromagnetism. The measuring results of the pressure gauge are very volatile, while the FBG sensor are more smooth and reliable. Therefore, monitoring by the FBG sensor has the advantages of continuity, and the measuring results can be used to accurately evaluate roadway roof stability and reduce the risk of roof accidents. In addition, the FBG sensing technology can provide automatic data collection and efficient transmission, effectively avoiding arduous and frequent downhole manual observation and improving the timeliness of monitoring results and the miners' safety. To sum up, monitoring any force change of bolts in roadways by the FBG technology can reflect the affected mining range and provide a reference basis for determining the working face forepooling range, evaluating the real-time support quality of bolts and taking stability measures.