The performance of inflatable rock bolts like Swellex is analyzed by taking into account the elastic radial displacements of the rock and the bolt tube during and after bolt installation in this article. The pullout resistance of an inflatable rock bolt is directly associated with the contact stress at the bolt-rock interface, which is composed of a primary and a secondary component. The primary contact stress is dependent on the interaction between the bolt tube and the rock during and after the bolt installation, while the secondary contact stress is determined by the roughness of the borehole wall surface. The analysis reveals that the primary contact stress decreases with the Young's modulus of the rock mass and increases with the borehole diameter and installation pump pressure. The primary contact stress is dominant in the total contact stress in soft and weak rock, while in hard and strong rock the secondary contact stress plays a vital role. The analytical solutions are in agreement in tendency with test results carried out in fields.
Inflatable rock bolts have been used for rock reinforcement, particularly in underground rock excavations, over 30 years since the Swellex bolt, the first type of inflatable rock bolt, was introduced in the beginning of the 1980s (Wijk & Skogberg 1982). A few other types of inflatable rock bolts, for example the Omega (Player et al. 2009) and the EX300 ERB (Bjurholt 2007), have appeared on the market in recent years. An inflatable rock bolt is made of a steel tube that is folded into an omega-shape in the cross section. Bushings are pressed onto the ends of the tube. During installation, the bolt is placed in a borehole and water is injected at high pressure into the folded tube through a small hole in one of the bushings. The tube is inflated and pressed onto the borehole wall under the water pressure. The water pressure disappears after installation and the bolt is stuck in the hole owing to the elastic recovery of the borehole which results in a contact stress and mechanical interlock at the bolt-rock interface, Figure 1.
An inflatable rock bolt is characterized by its instant interaction with the rock after installation, thanks to the frictional anchoring mechanism. The anchorage capacity of the inflatable rock bolt is represented by its pullout resistance, defined as the maximum pull load that a 1-m bolt section carries: