1 INTRODUCTION
The engineering practice shows that the disturbance of tunnel excavation changes the stress state of the surrounding rock and causes the deformation of the surrounding rock. If the surrounding rock deformation is not controlled by constructing a support structure, it will often be difficult to stabilize the surrounding rock of the tunnel.1 In fact, after the continuous accumulation of engineering experience, theoretical study, and experimental research, the modern tunnel support theory has reached a consensus, that is, the tunnel is a structure that integrates load, material, and structure.2 The surrounding rock of the tunnel is the primary source of the load borne by the support structure, and it also shows a certain bearing capacity. In the process of diagenesis, there are great differences in the physical fields such as stress, groundwater, and temperature, which lead to different properties of the rock and the soil, and the mechanical response will significantly vary after being disturbed by tunnel excavation. Developing the corresponding excavation methods and support plans for different types of the surrounding rock is considered an effective and controllable measure to control the deformation of the surrounding rock and ensure the stability of the surrounding rock of the tunnel.
The supporting mechanism of the support structure, the interaction relationship between the surrounding rock and the support structure, the selection of the timing of support, and the supporting effect of the support structure under different surrounding rocks have been the focus of researchers around the world, 3-8 among them the action mechanism of the support structure is considered to be particularly complex. The bolt is considered a very effective support system in tunnel engineering. Although bolts are widely used, the supporting mechanism of bolts has not been clearly identified. There are many kinds of bolts for tunnels, among them, the most commonly used are systematic rock bolts and steel pipes. Because of their grouting function, steel pipes are often used to support or treat weak–broken surrounding rocks. Sometimes when the tunnel excavation has a great impact on the shallow surface, steel pipes are often used for grouting the surface or the slope. The systematic rock bolts are mostly used for the initial support of tunnels. However, the existing support theories mostly investigate the support effect of systematic rock bolts9-11 and rarely use the support mechanism of steel pipes. Investigating the action mechanism of systematic rock bolts and steel pipes is considered the basis of understanding their supporting effect on surrounding rocks. Yanyi Yang 12,13 studied the anchoring effect and the mechanism of systematic rock bolts on the layered rock mass and proposed the constitutive relationship of the anchored layered rock mass. He also pointed out that the bolt induces a toughening and crack arrest effect on rock fractures, which thus reduces the damage of the rock mass and improves the strength of surrounding rocks. Yang et al 14,15 used the anchored layered rock mass as the equivalent continuous medium and derived the constitutive equation of the anchored layered rock mass. According to the analysis of the formula, it is believed that the reinforcement effect of bolts on the jointed rock mass primarily improves its shear strength and the deformation capacity of joints. Zhang et al 16believed that the anchored rock bolts work together with the surrounding rocks, and the bolt and the fractured rock mass reinforced by the bolt can be considered as a damaged cylindrical bolt element, which along with the surrounding damaged rock mass forms an anchoring damaged rock mass element. During calculation, the stiffness of the cylindrical bolt element is added to the stiffness matrix of the corresponding damaged rock mass element in order to reflect the confinement effect of the bolt on the deformation of the surrounding rock. Yang et al17 studied the deformation process of the anchoring body of a layered rock by using laboratory experiments and concluded that the anchoring effect of bolts on the layered rock mass primarily improves the post-peak bearing capacity of the anchoring body and the toughness of the anchoring layer and induces little influence on the pre-peak mechanical properties. Wang et al 18 believed that the existence and changes in the seepage pressure and fractures change the damage characteristics of the rock mass, and its stiffness decreases during the deformation process, whereas the bolts can improve the mechanical and damage characteristics of the rock mass; this improves its stiffness. They proposed the solution by using the concept of added stiffness to reflect the anchoring effect of the bolt on the fractured rock mass. Wei Zhang and Quansheng Liu 19-21have reported a comprehensive and systematic summary of the existing theories of rock bolt reinforcement in the jointed rock mass and pointed out that the study of the anchoring mechanism of the deep fractured rock mass should collectively consider all factors, such as the effect of the actual engineering application, the behaviors of the anchored rock mass, the effect of anchoring components, etc. Zhang et al22,23 studied the mechanical characteristics and failure features of the bolt-supported jointed rock mass by using the bolt reinforcement tests on the rock mass with pre-existing flaws in the laboratory. In order to solve the problem of the anchoring mechanism of the rock and the soil, international scholars have also carried out a lot of research work, primarily by applying the following two aspects:22,24-32 the load transfer mechanism of bolts and the anchoring effect of bolts, by using methods such as laboratory simulation, theoretical analysis, in-situ field test, etc., and the research results have enriched and deepened the understanding of the reinforcement mechanism of bolts on the rock and soil structure.
Therefore, similar simulation materials were used to form anchored specimens with pre-existing holes. Then, laboratory uniaxial compression tests and CT scan were carried out to study the strength, deformation characteristics, and crack propagation laws. The numerical simulation method is used for the coupled stress-damage simulation of fracturing in the rock. By the analysis of different supporting characteristics, in this paper, we investigate the supporting mechanism of support structures on the layered and weak rock tunnel, which provides a theoretical basis for field application.