## 1 Introduction  
Rockbolts have been used to provide ground support in mines for more than one century [1, 2, 3, 4, 5]. Rockbolts are characterized by highly effective reinforcement and simple operation, which can guarantee the rapid development of roadways, the efficient mining of working faces, and the safe operation of mines [6]. The bolting system consists of four components, namely, surrounding rocks, bolts, and internal and external fixtures [7; 8]. External fixtures are trays and nuts, which connect the reinforcing elements at the roadway surface. Internal fixtures can be divided into three types on the basis of the bolt-rock coupling structure, i.e., continuous frictionally coupled (CFC), continuous mechanically coupled (CMC), and discretely mechanically or frictionally coupled (DMFC) [9], corresponding to mechanical, fully resin-grouted, and partially grouted bolts, respectively. The reinforcing effect is based on load transfer from unstable surrounding rock to stable rock mass through the bolt. The load-transfer capacity of the bolt is one of the most important indicators of the bolting effect. Therefore, the accurate estimation of the bolting force is fundamental for reinforcement-system design and an important parameter in the numerical modeling of the deformation of underground roadways. The pull-out test is commonly used to evaluate anchorage performance. In practice, the rockbolting failure of fully resin-grouted rockbolts is often at the rock-resin interface, the resin, or resin-bolt interface failure. The tensile failure of the bolt shank is rarely observed. Thus, the short encapsulation pull-out test (SEPT) is normally employed to evaluate anchorage performance in the laboratory and in the field. As the SEPT measures the anchorage performance of rockbolting exclusive to the deformation of the bolt shank, the design criterion in SEPT is accordingly that the peak anchorage force should be lower than the yielding load of the bolt shank [10]. The most straightforward method to measure anchorage performance is the in situ pull-out test. According to the GB/T35056-2018 standard [11], in situ pull-out testing holes should be drilled at different depths into the surrounding rock. The bolting materials, equipment, and procedure should be identical to those on-site. The anchorage length should be less than 300 mm, and the test should be conducted within 1-24 h. However, the in situ test has the following disadvantages: (1) it is extremely time-consuming and laborious; (2) the accuracy of the measurement is low due to the limitation of the instrument; (3) the variation of the testing results is largely due to the complexity of the site conditions; and (4) many variables can hardly be controlled, such as the mechanical properties of the surrounding rock and ground stress. As a result, the repeatability and university of the testing result are limited. Field testing is useful in the case of rockbolting design for a specific site, but it is inefficient for rockbolting research that focuses on the generality of the testing result. The laboratory pull-out test is the most popular measure in rockbolting study due to its simplicity and high consistency of the data. Currently, there is no universal standard for the laboratory SEPT. Researchers often design their testing sample on the basis of the studied object and the availability of the material [12; 13; 14; 15; 16; 17]. Concrete blocks or steel tubes are normally used as the confining material to simulate the surrounding rock in the field. For instance, [PERSON] et al. used cement specimens in investigating the influence of borehole diameter, rock joint, and rock strength on the anchoring effect, the shear- and axial-stress distribution along the bolt, the performance of the rebar and D bolts, and the anchorage-failure model [18; 19; 20]. By using steel-tube confinement, [PERSON] et al. studied the effect of the bolt profile, resin temperature, resin mixture, installation procedure, and cyclic loading on anchoring force [21; 22; 23; 24; 25; 26; 27].