# Shear Mechanical Properties of Bolt-Grout Interface under Different Bolt Surface Profiles
## 1 Introduction  
Rock bolting is an important means of improving the mechanical properties of a rock mass, and fully grouted bolts, which have the advantages of good support effects and low support costs, are widely used in the field of civil, tunnels and mines, as well as other elements of rock engineering [1, 2, 3, 4]. The fully grouted bolting system is made of surrounding rock, grout, bolt, Bolt-Grout and grout-rock interfaces. The bolting effect of fully grouted bolts are mainly influenced by the type of bolt [5, 6], the grout material [7], the type of surrounding rock [8, 9], and the interaction at the Bolt-Grout interface [10, 11]. Extensive pullout experiments show that the Bolt-Grout interface is the weakest component, and its failure is one of the main forms of failure of the bolting system [10, 11, 12], as seen in Figure 1. The Bolt-Grout interface is an important part of the load transfer of the bolting system, and its shear strength is primarily determined by the bonding force, frictional force, and mechanical interlock force between the interfaces [13]. In addition to the surrounding pressure, the frictional force and mechanical interlock force are influenced by the bolt profile [2, 11, 13, 14]. Therefore, studying the shear characteristics of the Bolt-Grout interface is essential to improving the support impact of rock bolting systems [15]. Many scholars have conducted a great deal of research on the failure process of the Bolt-Grout interface. Numerous techniques are used in this research, such as laboratory testing, theoretical analysis and numerical simulations [16; 17; 18; 19; 20; 21]. The pull-out test is a common approach to investigating bolt systems that is frequently employed to investigate shear mechanical characteristics. It was found that the mechanical properties of the Bolt-Grout interface were influenced by many factors, including the anchorage agent, confining pressure, and the bolt profile [22; 23; 24; 25]. According to [PERSON] et al. [1; 26], the anchorage agent material significantly affects the bolting effect of the rock bolting system. By appropriately adjusting the anchorage agent material and optimizing the material ratio, it is possible to improve the Bolt-Grout interaction and raise the bolting system's bearing capacity. Through tests, [PERSON] et al. [27] discovered that the higher the confining pressure, the higher the bonding capacity and the lower the dilation. According to [PERSON] et al. [28], increasing the bolt's surface roughness coefficient can improve the Bolt-Grout interface's mechanical interlock force and frictional force, enhancing the interface's shear strength and serving the intended objective of enhancing the bolting effect. [PERSON] et al. [13] discovered a strong correlation between the bolt profile and the mechanical behaviors of the interface. [PERSON] et al. [22] conducted experiments on rebar bolts with five different rib spacings, finding that the bolting force increased with rib spacing and reached a maximum of 48 mm rib spacing. According to [PERSON] et al. [24], the peak stress and displacements of the bolts are directly related to the rib spacing and rib height. [PERSON], [PERSON] et al. [29] concluded that the interaction between the anchor and the rock can be influenced by changing the main geometric parameters of the bolt. The profile parameter that matters most for rebar bolts that are subjected to axial loads during load transmission is the rib spacing [22]. While the pull-out test has been used extensively in bolting system studies, it does have several disadvantages. Firstly, the rock bolting system is a classical composite system; the bolt is wrapped in the rock, and the pull-out test reflects the comprehensive effect of the support system, which makes it difficult to assess the mechanical properties and failure characteristics of the Bolt-Grout interface [16; 30; 31]. Secondly, the shear stress along the axial direction of the bolt in the pull-out test is exponentially distributed rather than uniformly distributed.