# Study on Mechanical Properties and Application of a New Flexible Bolt
## 1 Introduction
### Control Theory and Technology of Roadway Surrounding Rock  
In order to better play the supporting role of bolts, many surrounding rock control theories have been formed at home and abroad. [PERSON] [21] proposed the new Austrian method, which is based on engineering experience and rock mechanics theory, emphasizing that the bearing capacity of surrounding rocks should be fully activated to form the bearing circle together with the support body. [PERSON] et al. [22] put forward the theory of loose circle, which proved that the larger the loose circle, the more violent the roadway bursting and deformation, and the more difficult the support. [PERSON] et al. [23] put forward the theory of surrounding rock strength strengthening, believing that the essence of bolt support is the interaction between bolt and surrounding rock to form an anchorage carrier, so as to jointly resist tunnel deformation. [PERSON] et al. [24] proposed the anchoring force mechanism of the bolt, believing that the bolt changes the strength and stress of the surrounding rock by applying the anchoring force to the surrounding rock. Therefore, many bolt control technologies are proposed, among which bolt and cable combined support technology is one of the most commonly used support forms in the current roadway [25].  
Combined support of bolt and cable generally refers to the combination of short bolt and long cable. The length of bolt is generally 1.8-2.5 m, and the length of the cable is usually 6-9 m. The short bolt compacts and strengthens the shallow surrounding rock, and improves the integrity of the shallow surrounding rock. Compared with the short bolt, the long cable can improve the stability of the surrounding rock by mobilizing the bearing capacity of the surrounding rock deeper in the roof. However, due to the short length of the bolt, the surrounding rock fracture exists extensively in and out of the bolt anchorage zone in practical engineering. Under the action of mining-induced pressure, the fracture is easy to expand and penetrate. The bolt anchorage zone has the characteristics of overall subsidence. At the same time, due to the limitation of construction machinery and tools, the prehension of cable is limited, which cannot effectively reinforce the large range of rock mass. The synergistic effect of cable is relatively limited, which is not conducive to resisting strong impact dynamic load [26]. Therefore, in view of this problem, [PERSON] et al. [27] proposed the control theory of continuous beam in coal roadway, which is to timely construct the thick prestressed anchored rock beam with more than the critical anchorage length in the roadway roof. The core idea is to restrain the expansion and connection of fractures in the anchorage zone, control the flexural deformation of the roof, avoid collapse and instability, and realize the safety of the roadway roof. Long bolts provide the supporting material required to construct thick-layer anchored rock beams, and the length of the bolts should generally reach 3.5-4.5 m. However, the rebar bolts cannot be bent and cannot meet the length requirements. Although the length of the anchor cable can meet the requirements, the installation and tightening time of the cable is too long, generally two to three times the installation time of the bolts, resulting in low tunneling efficiency and affecting the safe and efficient production of coal mines.  
In this paper, by referring to the bolt nut installation method and the flexible rod material of the anchor cable, a flexible long bolt that is not restricted by the height of the roadway and can be installed quickly is developed. The performance test and analysis of the flexible bolt are carried out to determine the best specification. Finally, the flexible bolt is applied to the roadway with a composite roof, and the bearing law of the bolt and the deformation characteristics of the roadway surface are monitored.