Behavior Of Rock Bolts Research Articles

Experimental Study on Prestress Loss and Pull-out Behavior of Rock Bolts

Experimental Study on Prestress Loss and Pull-out Behavior of Rock Bolts

Numerical investigation study on tensile-shear failure behavior of rock bolts in inclined strata mining tunnels

In response to the problem of severe deformation and failure of the surrounding rock of the mining tunnels in inclined strata, a tensile-shear failure numerical model of rock bolts was adopted to analyze the rock bolts failure behavior and the surrounding rock mechanical response, and explore the influence of different factors on rock bolts failure. The results indicate that: (1) As angle between the rock bolt and the interface decreases, the shear force of increases, and the rock bolt is easier to fracture. The maximum displacement is 250.2 mm of fractured position. A new evaluation index of shear axis ratio was proposed. The larger the shear axis ratio, the rock bolt is easier to fracture. The shear axis ratio at the interface has increased by 95 % compared to the non interface. (2) The plastic zone and the surrounding rock deformation exhibit the same inclination trend as the strata. In final state, the maximum deformation is 297.7 mm at the bottom and top position perpendicular to the rock bolts at the interface. At the moment of rock bolts fracture, the interface displacement of surrounding rock immediately increased by about 10 mm. And the stress in the surrounding rock decreased about 1 MPa. The weakening effect of surrounding rock stress shifts with the change of rock bolt fracture position. The stress at the arch waist decreases by 44 %, the stress at the arch crown decreases by 25 %. When the fracture of the rock bolts at the arch crown and arch waist ends, the stress at the side wall begin to decrease by 40 %. (3) As the inclination angle increases, the deformation of the surrounding rock and the number of rock bolt failures increase. The range of the plastic zone in the surrounding rock gradually increases, and the degree of rock bolts asymmetric fracture more aggravated. As the diameter of the rock bolt increases, the fractured number of the rock bolts and range of plastic zone decreases.

State-of-the-art on the anchorage performance of rock bolts subjected to shear load

AbstractRock bolts are extensively utilized in underground engineering as a means of offering support and stability to rock masses in tunnels, mines, and other underground structures. In environments of high ground stress, faults or weak zones can frequently arise in rock formations, presenting a significant challenge for engineering and potentially leading to underground engineering collapse. Rock bolts serve as a crucial structural element for the transmission of tensile stress and are capable of withstanding shear loads to prevent sliding of weak zones within rock mass. Therefore, a complete understanding of the behavior of rock bolts subjected to shear loads is essential. This paper presents a state-of-the-art review of the research progress of rock bolts subjected to shear load in three categories: experiment, numerical simulation, and analytical model. The review focuses on the research studies and developments in this area since the 1970s, providing a comprehensive overview of numerous factors that influence the anchorage performance of rock bolts. These factors include the diameter and angle of the rock bolt installation, rock strength, grouting material, bolt material, borehole diameter, rock bolt preload, normal stress, joint surface roughness and joint expansion angle. The paper reviews the improvement of mechanical parameter setting in numerical simulation of rock bolt shear. Furthermore, it delves into the optimization of the analytical model concerning rock bolt shear theory, approached from the perspectives of both Elastic foundation beam theory coupled with Elastoplasticity theory and Structural mechanic methods. The significance of this review lies in its ability to provide insights into the mechanical behavior of rock bolts. The paper also highlights the limitations of current research and guidelines for further research of rock bolts.

Study on the Load Transfer Behaviour and Bond-Slip Model of Fully Grouted Rockbolt

Rockbolts are often subjected to loads during service, and the load transfer behaviour of rockbolts is very important. Therefore, in this study, pullout tests were performed on two kinds of rockbolt systems, with and without defects, and the load transfer behaviour and failure modes were analysed. According to the load transfer process between the rockbolt and cement mortar interface, a bond-slip model considering the yield of the rockbolt was proposed, and the nonlinear behaviour in the softening stage was considered. The test results showed that the evolution of the interface load between the rockbolt and cement mortar in the grouted rockbolt systems without defects underwent four stages of gradual failure, namely, elasticity, yield, softening, and complete slip, and its failure mode involved the rockbolt being pulled out accompanied by splitting cracks and tensile cracks on the concrete surface. The proposed bond-slip models considering the nonyielding and yielding of the rockbolt can accurately reflect the actual load transfer behaviour of a fully grouted rockbolt.

Experimental study on the pullout behaviour of rockbolts with steel fiber-reinforced grout under constant confining pressures

Deep underground excavation in civil and mining engineering motivates the development of high-strength rockbolting reinforcement. A feasible way to promote the performance of the fully grouted rockbolt is using additives to ameliorate grout properties and thus the bolt-grout interfacial behaviour. Here, we investigated the mechanical behaviour of fully grouted rockbolt specimens in the steel fiber-reinforced grout (FRG) under various constant confining pressures to pullout loading by the self-developed laboratory-scale rockbolt pullout system. We found that the bond strength of the FRG rockbolt specimen can be more than three times high of the plain grout one. The higher the fiber proportion, the greater the bond strength of the FRG rockbolt. When a constant confining pressure was applied, the bond strengths of all the grouted rockbolt specimens were considerably elevated and the bolt rod failure was observed for the FRG rockbolt specimen under 1.0 MPa confining pressure. Compared with the plain grout one, the post-peak failure of the FRG rockbolt specimen of a much higher bond strength was less brittle. The wavy steel fiber performed better in bond strength enhancement and grout cracking mitigation than the straight one mainly due to the crack bridging effect facilitated by its rough geometry. Our experimental studies suggested that FRG rockbolting is promising in reinforcing roadways/tunnels in the deep underground environment.

Influence of joint roughness on the shear properties of energy-absorbing bolt

Many experiments have been performed to study the mechanical behavior of rock bolts in resisting the shear force. However, almost all the tests were conducted on smooth rock joints, which is inconsistent with the field engineering practice. In this paper, the shear behavior of fully encapsulated rock bolts and energy-absorbing rock bolts inserted in the rough joints was investigated with a series of single shear tests under constant normal load (CNL) conditions. For all specimens, when the value of JRC increases gradually, the value of peak shear stress increases gradually. The ultimate shear displacement of an energy-absorbing rock bolt is larger than that of a fully encapsulated rock bolt for the same JRC condition, and they all decrease with the increase of JRC. The sensitivity of the energy-absorbing bolt to JRC change is lower than a fully encapsulated rock bolt. A dimensionless mathematical model was established to predict the ultimate shear displacement of rock bolts inserted in different roughness conditions. The ultimate shear displacement of the rock bolt was evaluated as a linear function of JRC. Two tests with natural rough joints were conducted to verify the applicability of the proposed empirical model for the natural rough joints. The predicted values of the ultimate shear displacement of rock bolts indicated good agreement with the test results. The proposed model is capable of providing an accurate evaluation of the ultimate shear displacement of rock bolts inserted in rough joints. The results of the laboratory test and mathematical model all show that the energy-absorbing bolt can bear a larger shear displacement and adapt to different joint roughness.

Study on the Effect of Loading Method on Mechanical Behavior of Rockbolt Based on DEM Modeling

Fully grouted rockbolt is a form of reinforcement commonly used in underground projects. Its mechanical behavior is of vital importance to enhancing the stability of surrounding rock masses. To investigate the effect of different loading methods on the mechanical behaviors of rockbolt, the distinct element code PFC2D was adopted to simulate the pullout test on rockbolt and a compression test on the bolted rock mass, with Hongtoushan Copper Mine as the engineering background. The stress distribution along rockbolt and the maximum pullout capacity were analyzed. The results indicated that the loading method has significant effects on the mechanical behaviors of rockbolt. In the pullout test, the peak stress in the rockbolt always occurs near the free face. As the rock mass is a passive load-bearing component, the cracks and weak intercalations in the rock mass have little effect on the mechanical behaviors of the rockbolt. Meanwhile, as the external load acts directly on the outer end of rockbolt, the effect of the bearing plate cannot be reflected. However, in the compression test on the bolted rock mass, the peak stress in the rockbolt gradually transfers to greater depth. The existence of cracks and weak intercalations have an impact on the mechanical behaviors of the rockbolt. In addition, with increasing size of the bearing plate, the maximum pullout capacity of the rockbolt gradually increases. Comparison of the simulation results showed that the maximum pullout capacity obtained by the pullout test on a rockbolt is higher than that obtained by the compression test on a bolted rock mass. Therefore, appropriate reduction of the maximum pullout capacity obtained by the pullout test is suggested for rockbolt design, and the reduction factor should be in the range of 0.7–0.8.

Numerical implementation of rock bolts with yield and fracture behaviour under tensile-shear load

To realize the numerical analysis of rock bolt bearing-yield-fracture under tension-shear coupling and to provide an effective means for supporting the design of roadways or tunnels, a new numerical model was developed to analyse the tension-shear coupling fracture behaviour of rock bolts in engineering rock masses. Based on the study of the mechanical behaviour of rock bolts under tensile-shear coupling load, the mechanical criterion and constitutive model of the yield and fracture of rock bolts were proposed, and the secondary development of the FLAC3D platform was realized. At the same time, self-developed equipment was used to conduct the rock bolt tensile shear test. By comparing the numerical simulation results with the experimental results, we determine that the maximum difference in the shear displacement of the rock bolt does not exceed 2 mm during the yielding and breaking of the rock bolt, and the percent difference was approximately 3% and 10%, respectively. In the field of engineering, this result is generally considered acceptable. Using the numerical model we proposed, a numerical simulation experiment was conducted on the rock bolt support design of a coal mining roadway in engineering practice. The results show that under the original support scheme, a large amount of rock bolt tensile-shear failure occurs at the coal-rock interface, which makes the original support scheme unable to meet the engineering requirements with tensile-shear coupling failure tendency. In this regard, we adopted the support method of grouting anchorage at the interface of coal and rock. The results show that the deformation of the roadway surrounding rock has been effectively controlled.

An Improved Analytical Model for the Elastic and Plastic Strain-hardening Shear Behaviour of Fully Grouted Rockbolts

Fully grouted rock bolts are one of the main forms of rock reinforcement used in underground mines, especially in coal mines. Analytical models describing the deformation behaviour of rock bolts under various loading conditions are used in numerical models for designing appropriate rock reinforcement. A reliable analytical model capable of accurately predicting rock bolts’ responses under different loading conditions is, therefore, important for more realistic numerical models and effective reinforcement design. In this paper, a new analytical model for shear behaviour of rock bolts is proposed which can better predict rock bolt behaviour under large shear deformations. A method to consider the effect of post-elastic strain hardening on bending stiffness of rock bolt is presented. The analytical model is validated with results from laboratory shear tests and found to be consistent with the laboratory results. To analyse the effect of post-elastic strain hardening on shear behaviour of a rock bolt, results from the proposed analytical model is compared with analytical model without strain-hardening effect. Without strain hardening, the analytical models overestimate the ultimate shear displacement by up to 35% in the examples shown in the paper.

An analytical model for axial performance of rock bolts under constant confining pressure based on continuously yielding criterion

An analytical model is developed to simulate the load-displacement performance of rock bolts under axial loading based on Modified Continuously Yielding Criterion. Such a model accounts for various surface geometries of rock bolt as well as the various confining pressures at the rock bolt to the grout interface which play significant roles in the behaviour of grouted rock bolts under axial loading. Majority of the defined parameters in the model can be measured within the laboratory environment. The proposed model is validated against the experimental results obtained from FRP and HA25 rock bolts. The good agreement between the experimental results and the analytical predictions highlights the ability of the proposed model to simulate the full load-displacement behaviour of rock bolts under axial loading.

Corrosion Behavior of Rockbolts in Simulated Underground Mining Solutions Using Macro-scale and Micro-scale Electrochemical Measurements

This paper focused on the corrosion of rockbolts with different S contents and yield strengths in simulated underground mining solutions using macro-electrochemical and micro-electrochemical measurements. The effect of Cl- ion on the pitting behavior was also investigated. It was found that the increase in Cl- concentration decreased the corrosion potential and corrosion current of anode polarization. The MnS inclusion was found to be the preferred site for corrosion. Based on in-situ observation and SEM characterization, it was found that the corrosion process started with the formation of corrosion trenches around the MnS, which accelerated the dissolution of the surrounding matrix.

Experimental Study on the Mechanical Behavior of Rock Bolts Subjected to Complex Static and Dynamic Loads

In underground mining practice, the rock bolt support system is the major support pattern to control the deformation and stability of openings. A rock bolt is generally subjected to complex loads including tension, torsion, bending and shear, which result from the deformation of excavations and exposure to dynamic loads that are generated by rockbursts. An understanding of the response of rock bolt under complex conditions is of great importance for rock bolt support design and practice. New sophisticated equipment has been developed for this purpose. This work involved a comprehensive experimental study on the mechanical behavior of rock bolts under complex loads. The results show that rock bolt pre-tensioning by torque application to the nut can result in decreases in tensile strength and elongation because the rock bolt is subjected to a combination of tension and distortion. When a pre-tensioned rock bolt is subjected to a shear load, the maximum shear force can reach up to 80% of the tensile capacity of the rock bolt. Higher impact energy results in a longer period of dynamic loading and a larger irreversible plastic deformation on the rock bolt, in contrast to a rock bolt that is subjected to low impact energy. The capacity and especially the deformation capacity of a rock bolt may decrease significantly after successive containment of the deformation of the surrounding rock mass from rockbursts.

Analysis of the Combined Load Behaviour of Rock Bolt Installed Across Discontinuity and Its Modelling Using FLAC3D

Determining the rock bolt response under the typical in situ loading conditions of tension, shear and bending is an important part of the support design process. Calibrated numerical modelling can be an important tool for simulating the rock bolt response under in situ loads. However, currently available structural support models do not accurately model the rock bolt behaviour under shear loads. These models need to be modified to simulate the actual rock bolt response as determined under laboratory testing. Results from double shear laboratory tests by Kostecki (2019) have been used to calibrate the numerical models in ANSYS (ANSYS® Academic Research Mechanical 2019). The calibrated numerical models were used to study the state of stress along the rock bolt under different concrete strength. These numerical models are compared with the analytical model proposed by Pellet and Egger (1996). The results show that the rock reaction and consequently the shear load on rock bolt in the analytical model is underestimated (by up to 50% in case of high strength concrete). In high strength concrete, the yield and failure behaviour is determined by the shear stress rather than the bending and tensile stress as predicted by the analytical models. The analytical model is modified to better predict the shear forces in high strength concrete. The pile structural element available in FLAC3D (FLAC3D — Fast Lagrangian Analysis of Continua in Three-Dimensions 2017) finite difference method neglects the transverse shear strain in the element. In this paper, the pile structural element is modified to simulate the correct response of rock bolt under shear. The FLAC3D code is also modified to simulate the bolt yielding and failure under combined axial, bending and shear load. The modified model is validated with laboratory results from Mchugh and Signer (McHugh and Signer 1999) and is shown to be better at simulating the shear response and failure of rock bolt under shear compared to the original pile model.

3D bolted cohesive element for the modelling of bolt-reinforced rough rock-shotcrete interfaces

In this paper, we develop a novel 3D computational method that describes the behaviour of rock bolts and rough rock-shotcrete interfaces as support systems. Specifically, a Finite Element model has been formulated and implemented using the user-defined element (UEL) subroutine with ABAQUS. The input parameters of the bolted cohesive model are determined experimentally. Once the location of the bolts is defined, the overall force-displacement performance of the system can be simulated. In addition, the rough interface is modelled as a combination of ‘bond’ and ‘Coulomb-friction’ laws where the cohesive stiffness reduces with degradation. To validate the model, we compared its outputs to the results of direct shear and bolt pull-out tests; a good agreement between the two approaches has been obtained. In addition, a mesh independent result is observed for the overall force-displacement response, which makes our model particularly fast and efficient. Furthermore, we conducted a parametric study by investigating the effects of number of bolts, bolt installation angle and pretension on the overall behaviour.

A Laboratory Study of Shear Behaviour of Rockbolts Under Dynamic Loading Based on the Drop Test Using a Double Shear System

Rockbolts are widely used as an underground support element to resist the convergence and maintain the stability of excavations. However, shear failure of rockbolts is increasingly observed in jointed rock mass of underground tunnels, especially after being subjected to seismic events. At present, understanding of the mechanical response of rockbolts subjected to seismic or dynamic loading in shear is still unclear. To investigate the shear performance of rockbolts under dynamic loading condition, a series of tests were conducted using a drop mass of up to185 kg from a maximum height of 3 m based on a double shear test (DST) system. Three variables were examined in the laboratory test including rockbolt diameter (8 mm and 16 mm), installation angle (90° and 45°) and input energy (drop height). The duration of the impact was 10–12 ms from release of the drop mass to first contact. By evaluating the DST system’s displacement/velocity/acceleration–time characteristic and the amount of energy absorption, the shear performance of rockbolt was assessed. When sufficient energy is applied into the DST system, the deformation of the rockbolt is dominated by localized shear force. The transient force can rupture the rockbolt with little bending and without any obvious tensile elongation. It was found that the averaged dynamic shear load is less than the peak static shear load whether horizontally installed or installed at an angle. In conclusion, the effectiveness of rockbolts in resisting shear stress can differ significantly under static loading and dynamic loading condition; the difference is reflected in the level of shear deformation and amount of energy absorption. The shear capacity of a rockbolt under 1 s−1 strain rate can be determined by the energy absorbed and average dynamic load. This approach can be applied to the support system design in rockburst-prone condition.

Effect of grout strength on the stress distribution (tensile) of fully-grouted rockbolts

The paper deals with and axial stress distribution (ASD) obtained from pull-out tests of strain gauged rockbolts in high strength rock media. For this purpose, an experimental research was conducted considering the only axial forces acting on fully grouted rockbolts (FGR). Strain gauges were used to observe the behavior of rockbolts and to determine the load transfer mechanism of the bolts to rock mass. In the test, five grout mixture having different properties was prepared and stress–strain mechanism of the bolt-grout interface was investigated for these conditions. ASD was demonstrated elastic load transfer behavior from the rockbolt to the outlying rock under the applied load (for each grout type) and stress concentration was rather a high level at 70 mm distance from rock surface. Axial stress curves drawing with the aid of strain readings were showed the regular distribution with increasing of grout stiffness. The test results of strain gauged rockbolts show that the shear stress distribution (SSD) is uniform and, the style of the distributions along the rockbolt was similar under various loading levels for all grout types. Shear stress was rather a small amount at the pulling end of rockbolt but it reached the peak after a short distance away from the pulling end of rockbolt and subsequent to peak value decayed rapidly with the load increase. The results were showed that strength properties of grout played an important role in SSD and ASD along the rockbolt. The increase in grout strength and rigidity made the stress distributions regular along the rockbolt.

Mechanical behavior of rock bolts under a high temperature environment

Mechanical behavior of rock bolts under a high temperature environment

Influence of temperature and roughness of surrounding rocks on mechanical behavior of rock bolts

In order to solve the degradation problem of rock bolts and the safety of a support structure for a high geothermal tunnel in a dry heat environment, various tensile tests of rock bolts were conducted at different temperatures and different levels of roughness of the surrounding rocks in this paper. The roughness of the surrounding rocks was simulated through different types of steel tubes, and dry heat environment was simulated through different temperatures of a drying oven. Based on the first interface (bolt to grout interface) and the second interface (grout to pipe interface) pullout tests, the load-displacement curves of rock bolts were obtained at different temperatures and roughness. The results showed that temperature had little effects on the failure mode, but roughness of the surrounding rock had a great influence on the pullout strength. With an increase in roughness, the tensile strength increased, and the bolting effect was enhanced. For the non-threaded specimens, the interface bond strength of the first interface was larger than that of the second interface, and the second interface showed a kind of shear failure. Finally, it was revealed that the dependence of ultimate tensile force on temperature and roughness of surrounding rocks obeyed a cubic and quadratic polynomial function, respectively.

Verification of the Analytical Model for Fully Grouted Rock Bolts Based on Pull-out Test (Case Study: Tabas Coal Mine)

A new analytical approach able to predict the mechanical behavior of rock bolts subjected to pull-out tests was proposed in this paper. Input parameters of such approach are: rib profile shape, height, width, angle of wrap and spacing, axial bolt pull-out force, and material properties. As for the analytical part, the basis of the new analytical solution for the prediction of the load displacement curve was developed. Derived mathematical equations enable calculations of grout displacement in elastic and elasto-plastic behaviour. In the experimental part, four examined bolts in situ pull-out tests were compared with the predictions of the new analytical approach, and this comparison led to the validation of this approach.

Numerical Modeling of Fully Grouted Rockbolts Reaching Free-End Slip

AbstractIn this study, numerical simulation was conducted to model the behavior of rockbolts with free-end slip in tension. A nonlinear bond–slip relationship was input to the numerical model to represent the interaction mechanism of the interface between rockbolt and grout. The results of the model were validated against laboratory tests. The numerical model matched the experimental results in terms of load–displacement relationship, rockbolt strain distribution, and interfacial shear-stress distribution. This paper indicates that (1) the rockbolt element in the numerical model is capable of describing the strain-softening behavior of the rockbolt–grout interface; and (2) the pullout tests of rockbolts with free-end slip can be used to derive the bond–slip relationship, and thus provide insights into the mechanical behavior of rockbolts with free-end slip.