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This paper experimentally compares the shear behavior of fiber glass (FG) bolt, rock bolt (steel rebar bolt) and cable bolt for the bolt contribution to bolted concrete surface shear strength, and bolt failure mode. Two double shear apparatuses of different size were used for the study. The tensile strength, the shear strength and the deformation modulus of bolt control the shear behavior of a sheared bolted joint. Since the strength and deformation modulus of FG bolt, rock bolt and cable bolt obtained from uniaxial tensile tests are different, their shear behavior in reinforcing joints is accordingly different. Test results showed that the shear stiffness of FG bolted joints decreased gradually from the beginning to end, while the shear stiffness of joints reinforced by rock bolt and cable bolt decreased bi-linearly, which is clearly consistent with their tensile deformation modulus. The bolted joint shear stiffness was highly influenced by bolt pretension in the high stiffness stage for both rock bolt and cable bolt, but not in the low stiffness stage. The rock bolt contribution to joint shear strength standardised by the bolt tensile strength was the largest, followed by cable bolts, then FG bolts. Both the rock bolts and cable bolts tended to fail in tension, while FG bolts in shear due to their low shear strength and constant deformation modulus.

With the widespread application of small-sized bolts in aerospace and other fields, the demand for measuring their connection structures is increasing. Currently, although ultrasonic longitudinal wave methods are commonly used for bolt pretension stress measurement, their accuracy is limited for small-sized bolts. This paper proposes a piezoelectric acoustic resonance method (PZTAR) for small-sized bolt pretension stress measurement based on acoustic elasticity theory, ultrasonic resonance principles, and a bolt stress–strain model. The method involves analyzing the ultrasonic time-domain signals of small-sized bolts under load in the frequency domain to better evaluate the changes in the ultrasonic frequencies under different pretension stress. The effectiveness of this method is verified through pretension stress measurement experiments. The results indicate that the proposed ultrasonic resonance method achieves an average error of less than 5% for M5 specification bolts. Compared to traditional ultrasonic time delay methods, the proposed method demonstrates higher measurement accuracy. Additionally, the ultrasonic resonance method exhibits better robustness during the measurement process.

This paper proposes a high-strength prestressed bolt that experiences both high strength and high elongation. Its elastic modulus is larger, and spiral ribs appear, which is referred to as the NPR bolt. The influence of different elastic moduli and spiral appearances on the bonding performance of the resin was analyzed. Theoretical analysis and FEM are used to analyze the influence of different elastic moduli on the mechanical distribution of the resin–rock interface. The results of the two methods are consistent. The maximum values of the interface shear stress and the axial force appear at the loading end, the minimum values appear at the end of the bolt, and both decays exponentially. With the increase in the elastic modulus of the bolt, the attenuation rate decreases, and the distribution becomes more uniform. The greater the elastic modulus is, the more positive the bonding performance of the resin–rock interface. The load transfer mechanism of the bolt–resin interface of a spiral rib under axial force was studied. With the same diameter and rib height, the spiral ribs of the NPR bolt can be more fully combined with the bonding agent to bear a greater tension load. Taking the NPR spiral and T-1 bolt used in this paper as examples, the bonding performance of the former is 2.24 times that of the latter. The loads of the spiral bolt and the inclined rib bolt are mainly borne by the ribs, accounting for 98.4% and 96.9%, respectively. The resin bonding performance experiment was carried out on an NPR spiral and commonly used T-1\\T-2 resin bolts. From the experimental results, the spiral rib structure of the NPR bolt has excellent bonding performance with the resin, which can fully guarantee the bonding performance between the bolt–resin interface. With a resin anchor length of 600 mm, the average peak axial force of the NPR spiral bolt reaches 207 kN, and those of the T1 and T2 bolts are 163 kN and 139 kN, respectively. The NPR spiral bolts did not fail at the bolt–resin interface. Failure of the 1/4T-1 bolt and 3/4T-2 bolt occurred at the bolt–resin interface. The research results of this paper can provide a reference basis for the anchorage design of similar high-strength bolts with a spiral appearance. Highlights A new type of prestressed anchor bolt with high strength and high elongation is proposed, which is anchored by resin and mechanical-type anchorage. Its elastic modulus is larger, and its appearance adopts the design of spiral ribs. The influence of the elastic modulus of the bolt on the bonding performance of the resin–rock interface was revealed. Under the same axial load, the larger the elastic modulus of the bolt, the more uniform the interface shear stress distribution. The mechanical transfer model of the spiral rib bolt–resin interface was established and analyzed. In addition, the experiment was carried out. Under the same bolt diameter and rib height, the interaction area between the spiral rib and the resin is larger, which can provide greater load.

A combined support scheme was proposed to address the issue of large deformation in the deep soft rock roadway at the Tangkou coal mine in Shanxi Province, China. This support scheme includes high constant resistance, elongation, and prestressed negative Poisson's ratio (NPR) bolts as the core element, along with metal mesh, shotcrete, cable, and bottom angle grouting bolt. A numerical simulation test using Flac3D was conducted to verify the feasibility of the NPR bolt-anchored soft rock roadway. In addition, a constitutive model for a rock mass supported by NPR bolts was established. The model is based on a parallel model of a generalised Kelvin body and elastic–viscous sliding body, which was used to reveal the mechanical behaviour of an NPR bolt-anchored rock mass. The results showed that the stress–strain relationship of the NPR bolt-anchored rock mass was obtained as a piecewise function consisting of the initial, slipping, and sticking stages. The stress and strain of the NPR bolt-anchored rock mass exhibit distinct patterns under different stress conditions. Specifically, when the stress remains constant, the strain of the NPR bolt-anchored rock mass increases continuously while the stress decreases in a step-like manner. On the other hand, when the stress increases linearly with time, the strain of the anchored rock mass increases linearly, while the stress of the anchorage rock mass increases slowly in a sawtooth pattern. Furthermore, a field support test fully confirmed the effective control of the NPR bolt support on the large deformations of soft rock roadways. These findings can guide further theoretical research, engineering design, and field applications of NPR bolt-anchored rock masses.HighlightsA new combined support scheme is proposed using the NPR bolt as the core.The mechanical behaviour of an NPR bolt-anchored rock mass is revealed.Both numerical simulations and field tests demonstrated that the combined support scheme effectively controls the large deformations of soft rock roadways.

To study the mechanical responses of large cross-section tunnel reinforced by pretensioned rock bolts and anchor cables, an analytical model is proposed. Considering the interaction between rock mass and bolt-cable support, the strain softening characteristic of rock mass, the elastic-plastic characteristic of bolt-cable support, and the delay effect of installation are considered in the model. To solve the different mechanical cases of tunneling reinforced by bolt-cable support, an analytical approach has been put forward to get the solutions of stress and displacement associated with tunneling. The proposed analytical model is verified by numerical simulation. Moreover, parametric analysis is performed to study the effects of pretension force, cross-section area, length, and supporting density of bolt-cable support on tunnel reinforcement, which can provide references for determining these parameters in tunnel design. Based on the analytical model, a new Ground Response Curve (GRC) considering the reinforcement of bolt-cable support is obtained, which shows the pretension forces and the timely installation are important in bolt-cable support. In addition, the proposed model is applied to the analysis of the Great Wall Station Tunnel, a high-speed railway tunnel with a super large cross-section, which shows that the analytical model of bolt-cable support was a useful tool for preliminary design of large cross-section tunnel.

Selecting and designing the most suitable support systems are crucial for securing underground openings, limiting their deformation and ensuring their long-term stability. Indeed, the rock excavations imposed by the erection of deep tunnels generate various harmful effects such as stress perturbation, damage, fractures, rockbursts, convergence deformation, and so on. To combat such effects by helping the surrounding rocks of these structures to hold up, rock bolts are typically utilized as pioneer support systems. However, the latter must be efficient and sustainable to properly fulfil their vital roles. A thorough understanding of the existing rock bolt types or models and the relevant factors influencing their failure is highly required for appropriate selection, design and applications. It is observed that, despite numerous studies carried out, there is a lack of comprehensive reviews concerning the advances in such rock support systems. This paper provides an insight into the most pertinent rock bolt types or models and describes the potential factors influencing their failure. Additionally, it discusses the durability of rock bolts, which has a huge impact on the long-term stability of deep rock tunnels. Furthermore, the paper highlights some proposals for future trends.

As coal mining progresses to greater depths, the rate and severity of coal burst hazards have greatly increased, resulting in significant operational and safety challenges. Appropriate rock support design for coal burst-prone roadways requires the understanding of the mechanical characteristics of the support system under impact loads and the underlying damage mechanisms. In this study, the response of rock bolts under coal burst conditions is assessed, aiming to advance the knowledge concerning the application of rock bolt systems in coal burst-prone mines. This is achieved by a review of typical failure patterns of rock bolts caused by coal bursts in a typical coal burst coalfield in China, a field study on the response of rock bolts subjected to a simulated coal burst via blasting and a laboratory study on the mechanical properties of rock bolts that have suffered coal bursts. It is believed that the outcome of this study has added to the improved understanding of the performance and design of rock bolt support systems in coal burst-prone mines.

Rock burst hazards are a common occurrence during deep coal mining, resulting in the collapse of roadway wall rock and the failure of support structures. In response, in situ testing was conducted in underground mine areas affected by rock burst disasters to systematically assess the impact of rock burst on wall rock of roadways and the performance of support systems. The findings of the study suggest that rock burst events lead to a substantial decrease in the strength of the coal mass of the roadway roof and ribs, as well as an increase in fractured zones. Additionally, there is a reduction in wave velocity, attenuation of anchoring and pretension forces of rock bolts, and varying degrees of damage to anchoring interfaces between rock bolts and their anchoring agents and between anchoring agents and wall rock. The parameters defining the mechanical and impact resistance properties of rock bolts and anchor cables, including extension rate and tensile strength, exhibit a significant decrease. In light of the aforementioned research results, it is recommended that the existing support system for roadways vulnerable to rock burst be enhanced and reconfigured in three key areas: improving support design, choosing materials with superior impact resistance capabilities, and bolstering monitoring and warning mechanisms.

Roof failure related to laminated and jointed rock masses has been recognized as a major cause of fatalities in underground coal mines. This paper presents numerical simulations of the failure of a laminated and jointed roof using the discrete element method, while focusing on the initiation and propagation of (and the interactions between) microcracks and macroscopic fractures and the development of the associated damage, stress and deformation in the laminated and jointed roof. Numerical modeling shows that the behaviors of bedding planes and vertical joints governed the progressive failure of the laminated and jointed roof. Microseismic activity corresponding to the development of microcracks in the roof intensified before major roof displacements and subsided in the later stages; such activity could serve as a precursor of roof fall events. The effectiveness of various support elements (rock bolts, metal meshes, and anchor cables) on the roof stability was numerically evaluated. Rock bolts and/or anchor cables produced a banded compressive stress in the laminated roof, restricting the separation of bedding planes. Rock bridges in the bolted roof suppressed the formation of macroscopic fractures and significantly reduced the interactions between fractures. The roof skin support with a metal mesh provided confinement to the fractured roof, thereby preventing fragmented rocks from falling and restricting progressive spalling between bolts. The numerical simulations presented in this paper can be helpful for understanding the likely failure mechanisms of laminated and jointed roofs and for finding potential solutions.

In response to the problem of debonding failure of anchor rods crossing coal-rock interfaces in coal-rock roadways in Guizhou Province, China, based on bolt pull-out tests, the failure characteristics and bearing characteristics of coal-rock composite anchor bodies under different height ratios of rock to coal were analyzed. The results show that the bearing capacity of coal-rock composite anchor bodies gradually weakened with the height (proportion) of the coal, and that the load-bearing strength of the composite anchor body became closer to that of pure coal. Numerical simulation experiments were conducted using ABAQUS to reveal the debonding mechanism of coal-rock composite anchor bodies under different height ratios of rock to coal from the perspective of differences in coal-rock cooperative failure. The main stages included stress concentration zone expansion; coal failure and debonding while the rock still had bearing performance; the rock reaching critical gradual debonding; and residual strength. In addition, a load transfer model of bolts on coal-rock composite anchor bodies under different height ratios of rock to coal was constructed. This model could derive the pull-out load and displacement curves of coal-rock composite anchor bodies with different height ratios of rock to coal, which showed the same trend as the pull-out test and numerical simulation results and had high credibility. These findings can guide further theoretical research, engineering design, and on-site application of bolt support in coal-rock roadways. Highlights Load bearing capacity of the composite anchor body differs from full-coal (or full-rock). There is a time differential development of the destruction between coal and rock. A load transfer model for bolt pull out on coal rock composite anchor was constructed.