Design and engineering practice of rebar bolt for large deformation roadway

Rebar rock bolt has been widely used in the mining industry since 1990′s, however, its thread geometric parameters are still the same as those of steel tendon used in civil engineering, which is designed for the steel-concrete structure without ground stress and the deformation of the structure is normally within elastic stage. With the development of deep resource mining, the surrounding rock of the underground roadway has large deformation. The geometric parameters of the rebar bolt designed for the ground building should be optimized for the geological conditions such as high ground stress and large deformation of soft surrounding rock. Based on the load transfer mechanism of anchorage mechanics, this paper clarifies that the rib of the rebar is the bearing body of bolt support, and the anchorage performance of the bolt is closely related to the geometric parameters of the rib. By analyzing the failure mode of anchorage, the failure mode of the anchorage section is classified as parallel shear failure and dilational slip failure. Furthermore, by analyzing two failure processes, the influence of the geometric parameters of rebar rib (including rod diameter, rib slope angle, rib width, rib rise angle, rib height and rib spacing) on anchorage performance are clarified. Aiming at deep resource exploitation, a new type of rebar bolt designed for the large deformation of surrounding rock is developed by comprehensively considering the deformation control and separation control of surrounding rock. The design improves the residual strength of anchorage section through thread geometry optimization, it effectively improves the strata control ability of large deformation surrounding rock by increasing the energy absorption. Pullout tests were conducted in the laboratory and in the field. Results show that the average peak pull-out forces of the new bolt are 13% and 16% higher than that of the left-spiral bolt, respectively. From the perspective of energy absorption, the average energy absorption of the new bolt is 15% and 55% higher than that of the left-spiral bolt, indicating that the anchoring performance of the new bolt designed for large deformation surrounding rock is better than that of the left-spiral bolt. Furthermore, the application of the new designed bolt is carried out. Three sections of rock bolting support roadway are designed and installed in a deep mining roadway. Through the comparative analysis of roadway convergence, roof separation, bolt bearing force and the failure mode of the bolt, it is further verified that the new designed bolt support is more conducive to the strata control of deep underground roadway.