# Application of Fiber Bragg Grating Sensing Technology for Bol Force Status Monitoring in Roadways
## 1 Introduction  
In recent years, with the improvement of production capacity and the depletion of shallow coal resources, underground coal mining has been extending to deeper and deeper levels, facing high in situ stress conditions [1]. High in situ stresses can cause roof subsidence and floor heaving in coal mine roadways, thus increasing the difficulty and costs of roadway support and seriously affecting mine production and the miners' safety [2]. Bolt support is characterized by efficient reinforcement and easy construction and has become the main active support method in coal mine roadways to control roadway roof failure and improve surrounding rock structure stability. Therefore, it is very important to perform real-time online monitoring of the force status of bolts in coal mine roadways to ensure long-term safety and stability of roadway surrounding rock structure and personnel safety. However, the concealment and force status complexity of bolt anchorage systems in surrounding rocks make it more difficult to measure mechanical status in the bolt support process. The current method that uses mechanical or electronic sensors of bolt force status monitoring (BFSM) has poor anti-interference performance, is easily affected by harsh downhole environments, and cannot implement distributed measurement and remote transmission, failing to meet the requirements for long-term real-time monitoring of anchoring quality and force status of bolts in roadways [3]. The fiber Bragg grating (FBG) sensing technology as a new measuring technology has become one of the most promising optical fiber passive device applications [4]. The FBG as a sensing elementand transmission medium can be used to measure physical quantities such as temperature, pressure, displacement, flow rate, and acceleration [5]. It has the following unparalleled advantages over the traditional sensing method: intrinsic safety, corrosion resistance, immunity to electromagnetic interference, high reliability, high sensitivity, good stability, strong multiplexing ability, and easy implementation of quasi-distributed long-term online measurement [6]. It is being widely applied in a growing number of sensing application fields, including geotechnical engineering [7; 8; 9], structural engineering [10; 11; 12; 13], civil engineering [14; 15], and tunnel construction engineering [16; 17]. The FBG has all-optical characteristic during transmission and signal detection, so it is very suitable for operating in coal mine environments. [PERSON] et al. [18] used the simulation experiment with analogous material for mining and studied the application of fiber optical sensors for monitoring rock failure. [PERSON] et al. [19] presented FBG sensors for online monitoring of stress variation and detection of the micro-seismic signal in coal mines. [PERSON] et al. [20] concluded that optical fiber sensing technology is an effective method for strata strain monitoring in similarity model test of coal mining. [PERSON] et al. [21] designed an overall temperature measurement system based on optical fiber online monitoring mechanism and carried out field tests in a coal mine goat area. The FBG has drawn an increasing amount of attention in the coal mine safety monitoring field. Nevertheless, there are few studies on the applications of FBG sensors for BFSM in coal mine roadways. This paper reports the design of an FBG bolt force sensor using a cantilever and a circular diaphragm as elastic elements. Two FBGs of the sensor as sensing elements are arranged on the top and bottom surfaces of the cantilever, respectively, and the center wavelength difference between the two FBGs is utilized as a measuring signal, thus improving measuring precision. This sensor has a sensitivity of 38.79 pm/kN and a linear fitting coefficient of 99.98% in the measuring range. It provides the functions of real-time online monitoring and on-site reading, and it was applied in coal mine roadways to monitor bolt force status.