Behaviour of rock joint reinforced by energy-absorbing rock bolt under cyclic shear loading condition
Introduction
Rock bolts are one of the most commonly used support equipment for fractured rock mass.1 However, the conventional fully encapsulated rock bolts are easily damaged when subjected to shearing movement of rock mass.2, 3, 4 Li proposed a concept of “energy-absorbing rock bolt”, which is a promising way to improve the shear displacement of rock bolt inserted in the rock joints.5
The energy-absorbing rock bolts should not only have high strength to reinforce the rock mass, but also be deformable to adjust to the movement of rock mass. There are many types of energy-absorbing rock bolts. According to the yielding mechanism, they can be summarized as structural components sliding type and steel deformation type as shown in Fig. 1. The structural components sliding type mainly include Cone bolt,6 Roofex, He-bolt7 and Cold drawing bolt.8 They were designed according to steel-rock or steel-steel interaction. However, the “energy-absorber” unit makes the bolt inherently cost expensive due to its complex structure. The steel deformation type includes a smooth segment which is free of grout, and D bolt is a typical representative of them. It is a smooth steel bar with a number of anchors along its length. The anchors are fixed in the borehole with either cement grout or resin, while the smooth sections of the bolt between the anchors can deform freely in response to rock movement.9 Because of the simple structure and low cost, a lot of research has been done on the steel deformation type energy-absorbing rock bolts.
Many experiments have been conducted to study the behaviour of rock bolts in resisting the shear loading. However, most of the experiments was conducted on the fully encapsulated rock bolt.10, 11, 12, 13, 14, 15, 16, 17 Chen and Li conducted some experiments and confirmed that the behaviour of energy-absorbing rock bolt was much better than the conventional fully encapsulated rock bolt in resisting shear loading.18, 19 However, all these experiments were conducted under a direct shear loading condition. Direct shear loading condition means that the shear load was applied in a single direction until the failure of rock bolt.
When a joint is subjected to loading and/or unloading during a seismic event, it will undergo a sequence of cyclic shearing loadings.20 In engineering practice, the cyclic shear loading during earthquake activity could cause a great threat to the stability of the surrounding rock mass.21, 22 It is an important issue in terms of assessing the stability of reinforced slopes during and after earthquakes. It is inappropriate to ignore the effect of cyclic shear loading on the behaviour of rock bolts in support design where earthquakes may occur.
According to the experiment results by the authors for the fully encapsulated rock bolts, cyclic shear loading can cause a significant impact on their mechanical properties.23 However, the effect of cyclic shear loading on the energy-absorbing rock bolts is still unknown. The effect of cyclic shear loading on the energy-absorbing rock bolts was investigated in this paper.
Section snippets
Experiment arrangement
Laboratory shear experiments were conducted to study the shear behaviour of rock joints reinforced by the energy-absorbing rock bolts (steel deformation type) under cyclic loading condition.
The failure mode of energy-absorbing rock bolts in cyclic shear tests
The energy-absorbing rock bolt specimens after the cyclic shear experiment are shown in Fig. 5b. The broken section of the rock bolts were marked by dashed lines. Generally, the rock bolts would be broken into two sections. A very interesting phenomenon has been found in the last energy-absorbing rock bolt specimen. When the cycle distance is relatively large (8 mm), the rock bolt was broken into three sections. Three-dimensional damaged area of the rock specimen at the joint is shown in Fig. 6
A new index to describe the shear behaviour of the energy-absorbing rock bolts
Due to the discreteness of the peak shear strength and maximum shear displacement, neither the peak shear strength nor the maximum shear displacement can accurately describe the shear behaviour of an energy-absorbing rock bolt inserted in a rock joint under cyclic loading condition. In order to evaluate the shear resistance of bolted rock joints more objectively, the shear energy can be used as an evaluation index. It is has been verified that energy-absorbing ability is a more effective
Conclusions
The effect of cyclic shear loading on the energy-absorbing rock bolts inserted in rock joints were investigated by a series of shear experiments. The results showed that the asperity damage at the joint surface was relatively minor under the direct shear condition, and the asperity damage was increased significantly under the cyclic loading condition. In the case of relatively large JRC values, the damaged areas around the energy-absorbing rock bolts were relatively large.
When the cyclic
Acknowledgments
The author would like to thank Prof. R. Sterling from Louisiana Tech University for his critical suggestions during his visit to our laboratory. This study was funded by the Natural Science Foundation of China (No. 51379117) and the open found research project of State Key Laboratory of Mining Disaster Prevention and Control (No. MDPC201812).
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