## 1 Introduction
[19] proposed an effective characterization method for the stress concentration of the surrounding rock in coal mine roadways, replacing the traditional measurement method of the stress of the surrounding rock in the roadway. [PERSON] et al. [20] studied the influence of the loss of the pre-tightening force of high-strength rock bolts on the basic mechanical properties of the structure. The results show that the loss of the pre-tightening force of high-strength rock bolts reduces the connection strength and structural stiffness, but the influence on the ultimate bearing capacity of the structure is very limited. [PERSON] et al. [21] pointed out that the hardness of the surrounding rock of the roadway and the integrity of the rock mass are important factors affecting the anchoring performance of rock bolts in jointed rock. Therefore, a rock bolt pull-out test was designed, and the two influencing factors of rock hardness and integrity were comprehensively considered. The calculation formulas for the ultimate pull-out load and ultimate displacement of mortar rock bolts in jointed rock were established and proposed. [PERSON] [22] introduced the underground rock bolt support mode in American coal mines, indicating that if the roof strata are weak, especially affected by weathering, the rock beams between the rock bolts will be damaged and fall off over time. Engineering rock masses are often affected by frequent geological tectonic movements and complex stress fields, often resulting in many weak structural planes, such as fractures, joints, and faults. These weak structural planes seriously damage the integrity and continuity of the surrounding rock of the roadway [23]. Therefore, most coal mine roadways belong to layered composite roof roadways. The layered composite roof generally refers to the roof rock mass being several layers of heterogeneous soft and hard interlayered strata, which often contain weak interlayers such as thin coal and generally have poor mechanical properties and interlayer cementation. Therefore, the surrounding rock of the roadway roof is prone to large deformations and failures, such as dislocation and separation along the weak structural planes, which is also the decisive factor leading to the failure of rock bolt support [24]. Both [PERSON] and [PERSON] et al. [25; 26] indicated that the composite roof poses a great threat to the safe production of the working face and the stability of the surrounding rock of the roadway. According to relevant data statistics, more than one-third of the coal roadway roofs in China belong to the layered composite roof, and the layered composite roof roadways have typical characteristics such as large surrounding rock deformation, sudden roof collapse, and great support difficulty during the mining process, which greatly weakens the anchoring effect of roadway rock bolts. Due to the significant difference in the mechanical properties of the weak interlayers, the deformation of the rock bolts is very serious, thereby causing difficulties in roadway support and severely restricting the safe production of the mine. According to statistics, the number of accidents caused by composite-roof-type roadways in the coal mine production process accounts for 2/3 of all roadway roof collapse accidents, but at present, scholars at home and abroad have conducted less research on the deformation of rock bolts and its influencing factors in the layered composite-roof-type roadways. In response to the above problems, in this paper, a \"break-tension-suture\" method for applying the pretension force of the pile unit simulation rock bolt was put forward. The weak surface of the layered composite roof structure was simulated through the use of the interface. A reasonable calculation sequence of the stress field of the surrounding rock before and after the rock bolt was devised.