## 2 Establishment of a Similarity Simulation Test Bed of a Composite Rock Stratum and Test Scheme  
As the physical model is a real physical entity, the similarity simulation test can accurately reflect the field situation of underground engineering and give more intuitive test results under the condition of basically meeting the similarity principle. With the further development of similar materials and the progress of monitoring methods, similarity simulation testing has become an indispensable research method for geotechnical engineering problems, especially for underground engineering problems. The self-developed stress field test bed of anchor bolt supports is used for the purpose of the test. The geometric parameters of the test bed are length \(\times\) width \(\times\) height = 3000 mm \(\times\) 2000 mm \(\times\) 2000 mm. Four anchor bolts can be arranged in the middle of the test bed, with a row spacing of 1000 mm between the anchor bolts, and the distance between each bolt and the boundary is 500 mm. The materials used in this simulation experiment are all the longest support material models used underground in major mining areas in China. The diameter of the anchor rod is 22 mm, and the material is MG-500 (Yongqing Branch, CCTEG Coal Mining Research Institute, Langfang, China), with a yield strength of no less than 500 MPa. The tray specification is 150 mm \(\times\) 150 mm \(\times\) 10 mm. The test bed for composite rock stratum is divided into two parts with the same volume but different mechanical properties. Amongthem, the compressive strength \(\sigma_{\text{hard}}\) = 2.65 \(\sigma_{\text{soft}}\) (\(\sigma_{\text{soft}}\) = 16.56 MPa, \(\sigma_{\text{hard}}\) = 43.56 MPa); the elastic modulus is E\({}_{\text{hard}}\) = 3.51 E\({}_{\text{soft}}\) (E\({}_{\text{soft}}\) = 3.27 GPa, E\({}_{\text{hard}}\) = 11.47 GPa); and the hard and soft interface is placed in the middle of the model. The layout of the composite rock stratum test bed and sensors is shown in Figures 1 and 2. The data acquisition system consists of two parts: one is the stress acquisition system which applies pretightening force to the bolt, and the other is the stress acquisition system inside the test bed. The pretightening force of anchor bolt is collected by ZHC-35 load sensor (Dandong Electronic Instrument Factory, Dandong, China), and the internal stress of the test bed is collected by the XYJ-2 vibrating wire sensor (Dandong Electronic Instrument Factory, Dandong, China). The technical indicators of embedded strain sensors are shown in Table 1.  
In order to better simulate the actual working state of the mine, the most commonly used anchor bolt in the construction site is the left-screwing threaded steel anchor bolt without longitudinal rib, which has a diameter of 22 mm and a length of 2400 mm. The W steel guard plate and metal mesh commonly used in underground works are used as surface protection components. Two anchor bolts are installed on the stress field test bed of anchor bolt support. Under the condition of applying 400 N-m pretigttening torque, the stress field distribution of anchor bolt support is tested for end anchorage and lengthening anchorage, and the influence of the length of the anchoring section on the prestress field of the anchor bolt support is explored. One MSZ2335 (Shanxi Qinxin Coal Industry Co., Ltd., Changzhi, China) resin anchoring cartridge is used at the end, and the anchoring length is 445.1 mm. A two-speed resin anchoring cartridge is used in lengthening anchorage. The anchoring cartridge is composed of one MSCK2340 and one MSZ2360 (Shanxi Qinxin Coal Industry Co., Ltd., Changzhi, China). The anchorage length is 1271.7 mm. At the same time, the distribution of prestress field of anchor bolt support in soft rock and hard rock in the anchoring section is tested under the condition of end anchorage, and the influence of anchoring surrounding rock lithology on the prestress field of bolt support is explored. The specific test scheme is shown in Table 2.