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Large deformation of surrounding rock makes the internal space of a tunnel cannot meet the requirements of normal use after support, even leads to the instability of surrounding rock and destruction of the tunnel structure. In order to optimize the parameters of the primary support to control the deformation of surrounding rock, field tests and numerical simulations are carried out. Based on the engineering geological conditions and the deformation monitoring data of surrounding rock, four important creep parameters of the improved Burgers model are inversed by numerical simulation, considering the creep of surrounding rock in tunnel construction. The results show that it is feasible to use tunnel crown settlement as a control indicator to determine the thickness of shotcrete. Short rock bolts are preferentially used in the tunnel arches, and a combination of long and short rock bolts should be used on the sidewall to control the large deformation of surrounding rock. When the excavation method of upper-lower bench is used in the single-track railway with a relatively large high-span ratio, the horizontal convergence monitoring points of the upper bench are recommended to be set 0.8-0.9 times the height of the upper bench from the tunnel arch crown, and those of the lower bench are recommended to be set 0.6-0.7 times the total height of the upper-lower bench from the tunnel arch crown. [ABSTRACT FROM AUTHOR]

The mechanical and deformation properties of bolted joints are important research issues in rock mechanics and rock engineering. Quite a few tests have been performed to study the performance of rock bolts in resisting shear loading. The effect of cyclic shear loadings on the bolts in the joints under different joint roughness was studied through a series of shear tests. The results illustrate that the peak shear strength was substantially equal to the residual strength with the increase in the number of cycles, and the slope of the normal displacement curve gradually decreased. When the number of shears is greater than three, the shear stress curve and the slope of the normal displacement curve tend to be stable gradually, which means it has completed the asperity degradation. The cyclic shear loading affects the rock bolt and reduces its bearing capacity. For specimens with larger JRC, the peak shear stress in both the direct shear and the first cycle was significantly elevated compared with small JRC, and so was the normal displacement. An evaluation index of shear energy loss ratio (SELR) was introduced to evaluate the shear performance of the rock bolts and rock joints under a cyclic shear loading condition. When the JRC is large enough, the asperities are difficult to degrade, and the jointed rock still has a more remarkable ability to resist shear deformation. This shows that JRC significantly affects the performance of joint plane and rock bolts under cyclic shear loading. Highlights: The effect of cyclic shear loading on the bolted rock joints under different JRC conditions were investigated. The cyclic shear load affects the rock bolt and reduce its supporting capacity. An evaluation index of shear energy loss ratio (SELR) was introduced to evaluate the shear performance of the rock bolts and rock joints under a cyclic shear loading condition. [ABSTRACT FROM AUTHOR]

Because of its simplicity, cost-effectiveness, and high efficiency, bolt-anchoring technology has been widely used in various fields such as tunnel and slope engineering. In this study, the mechanical behavior and damage pattern of bolted joints are investigated by a direct shear test of bolted joints, considering different bolt numbers, anchoring angles, and arrangements. The results showed that there are five distinct stages in the shear stress–displacement curve of the bolted joint, namely, the compaction deformation stage, the elastic deformation stage, the separation and drop stage, the yield damage stage, and the residual deformation stage. An increase in the bolt number increased the elastic modulus and shear strength of the bolted joint, although the yield deformation of the bolted mass first increased and then decreased. An increase in the anchoring angle caused the shear strength of the bolted joint to first increase and then decrease. The optimal anchoring angle was approximately 45°, and the safety factor was higher under large anchoring angles than under small anchoring angles. In particular, multiple rows of widely spaced bolts arranged perpendicular to the shear direction had significantly higher shear strength than a single row of narrowly spaced bolts placed parallel to the shear direction. These findings offer a guideline for the design of bolted rock mass in engineering projects. [ABSTRACT FROM AUTHOR]

• Simulation of geological laminated strata by 3D concrete printing technology. • Failure mechanism of rock bolt reinforcement 3D printed concrete laminated strata. • Effect of printing and loading directions on bonding failure of 3D printed layers. 3D concrete printing has the potential to simulate physical models for engineering investigations. However, it has not been fully exploited and validated. In this study, 3D concrete printing was used to construct laminated strata, including bolting, to mimic an underground excavation environment. Loading tests were conducted using various experimental techniques, such as digital image correlation (DIC) and acoustic emissions (AE). Monotonic and cyclic loading were applied to investigate the laminate failure in an engineering scenario. Attention was focused on the bonding effect between the 3D printing layers and the orientation between printing and loading directions. This study has three main contributions. Firstly, it exploits 3D printing to accurately mimic laminated strata in a geological scenario. Secondly, it integrates rock bolts with 3D printing models to replicate the interlocking mechanism between rock bolts and strata. Finally, it highlights the benefits of using 3D concrete printing to mimic geological strata, which can be adapted to accurately simulate various engineering failure patterns. The results suggest that 3D concrete printing has the potential to decrease manual labor, enhance precision, and more effectively tackle engineering challenges in comparison to traditional physical models. [ABSTRACT FROM AUTHOR]

Anchoring is a highly effective means to reinforce jointed rock masses. In addition to the axial force from the anchor bolt, the effect of its transverse force in constraining the displacement of the structural plane cannot be ignored. Thus, this paper studies the combined action of the axial and shear forces of an anchor bolt on enhancing the shear strength of the anchored joint. In this work, first, laboratory direct shear tests are conducted to study the evolution of the peak shear strength of the anchored joint with increasing angle between the shear direction and bolt inclination. Second, to further verify this trend, numerical simulation with the finite difference method is used to simulate the same direct shear tests to reveal the lateral and axial stresses and deformations of the bolt. Finally, according to classical beam theory, the mechanical analysis model of a bolt subjected to the combined action of axial tension and lateral shear is established, which takes the realistic stress and deformation patterns of the bolt from the numerical simulation into account. The results show that the lateral restraint of the bolt plays a significant role in the shear strength of the anchored joint, its enhancing effect first increases and then decreases with increasing installation angle, and the optimal installation angle providing the maximum shear strength is approximately 60°. In addition, it is concluded that the shear resistance of the bolt is mainly due to its axial force when the installation angle is relatively small; conversely, the shear resistance of the bolt is mainly due to its shear force when the installation angle is relatively large. [ABSTRACT FROM AUTHOR]

The longitudinal wave velocity (v L) in a rock bolt is a useful parameter for evaluating the conditions of the rock bolt and the surrounding rock mass. This study investigated the influence of rock properties on the prediction of v L in a rock bolt using numerical and machine learning approaches. Through numerical simulations, we obtained a dataset of the variations in v L according to rock properties, compressional wave velocity (v p), shear wave velocity (v s), density (ρ), Poisson's ratio (μ), porosity (η), uniaxial compressive strength (UCS), and slake durability index (I SD). This dataset was used to design a deep neural network, and the predicted v L was correlated with rock properties. Notably, v L is strongly correlated with v p , v s , ρ , η , UCS , and I SD. Principal component analysis was employed to characterize the relationship between the rock properties, and the retaining rock properties for random forest (RF) were determined. In the RF, the variable importance (VI) of rock properties was assessed. In particular, v s emerged as the most significant predictor of v L. However, relying on v s to predict v L is not sufficient because it accounts for approximately 60–70 % of the VI. For a more reliable prediction of v L , it is essential to incorporate both v s and v p , which collectively account for approximately 80 % of VI. Notably, the VI of physical properties (v p , v s , ρ , and η) accounts for more than 90 %, implying that these properties can be effectively used to predict v L even in the absence of data concerning mechanical properties. [ABSTRACT FROM AUTHOR]

The complex geological structure of Taiwan often makes it difficult to control the deformation behavior of tunnel excavations. To simplify analyses, most traditional empirical formulations and modern numerical analyses do not consider the existence of rock bolts. For this reason, this study aimed to investigate the vertical displacements caused in tunnel crowns under various geological conditions using a three-dimensional finite element analysis with tunnels of different cross sections as an example. The analysis was performed using two rock material models: the Mohr–Coulomb and Hoek–Brown models. The results indicated that there was an insignificant difference between the two models in terms of the magnitudes of displacement and the distribution profiles obtained. When a tunnel was being excavated, the weaker the site was geologically and the larger the cross section, the larger the vertical crown displacement. In addition, the presence of rock bolts had a significant influence on the displacement in areas where the geology was weak and the cross section was large. The numerical analysis results of the Mohr–Coulomb and Hoek–Brown models were compared with the values calculated using the formulas suggested by Unlu and Gercek. At the excavation face, the displacement ratio derived from the prediction formula of Unlu and Gercek was smaller than the numerical analysis result, and the difference between the two became larger when the rock mass quality was worse. [ABSTRACT FROM AUTHOR]

Rock bolting technique is an important reinforcement measure in the geotechnical engineering practice. New rock bolts have been continuously emerging through the development of rock supporting technology. Complex conditions, such as high crustal stress, extremely soft rock, and strong mining disturbance often occur in the deep mining, resulting in large deformation of the surrounding rock masses. Since the deformation of traditional rock bolts is generally below 200 mm, failure often occurs to the rock bolts because of insufficient deformability. To effectively control the large deformation of surrounding rock masses caused by complex conditions, it is necessary to develop large deformation rock bolts with high constant resistance, also called energy-absorbing bolts. This paper systematically reviews the development of large deformation rock bolts and the structure, energy absorption mechanism, anchorage performance, and mechanical properties of several typical large deformation rock bolts. The advantages and disadvantages of existing large deformation rock bolts are compared and the concept of constant resistance large deformation support is introduced. [ABSTRACT FROM AUTHOR]

This study investigated anchorage behavior of geopolymer-grouted rock bolts. Pull-out tests were conducted on rock bolts grouted by geopolymer mortar (GM), geopolymer paste (GP), and cement mortar (CM), respectively. The effect of grout, type of bolts, spacing of bolts, and anchorage depth on anchorage performance was examined. Experimental results have shown that GM achieves better bond than GP and CM under a constant water binder ratio of 0.4. Deformed bolts grouted by GM and GP exhibit a nominal bond strength 2.4 and 1.6 times higher than the plain round ones, respectively. Bolt spacing of more than 150 mm exerts little influence in the anchorage performance. Increasing anchorage depth contributes to both loading capacity and ductility, while reduces nominal bond strength of deformed bolts. The critical anchorage depth ensuring tensile failure of bolts is 12d for both GM-grouted and GP-grouted bolts. GM-grouted deformed bolts with minimum anchorage depth of 12d and bolt spacing of 150 mm are proposed for practical application. [ABSTRACT FROM AUTHOR]

• Sound waves are analyzed to evaluate the unbonded length of the rock bolt. • Cross-correlated sound waves provide a clearer peak in the frequency response. • Resonant effect of reverberant sound waves is identified in the wavelet transform. • Predominant frequency decreases with an increase in the unbonded length. This study aimed to inspect the unbonded length of a rock bolt using the built-in microphone of a smartphone. An experimental study was conducted using rock bolts of different unbonded lengths. Sound waves were produced by impacting the head of the rock bolt, and the reverberation of the sound waves in the unbonded section was recorded. To compensate for the phase difference between the two signals, the stereo audio signals were extracted from the video and cross-correlation was conducted. The frequency spectrum of the cross-correlated signal shows a clear peak compared to that of original signal. Additionally, the predominant frequency decreased with increasing unbonded length. Time-frequency analysis was accomplished using a wavelet transform. Wavelet transform identified that the predominant frequency was the resonant effect of the reverberant sound waves. This study suggests that the predominant frequency of the reverberant sound waves in the unbonded section is an effective indicator for evaluating the unbonded length of the rock bolt. [ABSTRACT FROM AUTHOR]

For road tunnels in most Nordic countries, temporary rock support installed during tunnel excavation usually becomes a part of the permanent rock support. Rock bolts are the most common rock support measure in road tunnels excavated in hard rock, and their conditions over the period of tunnel operation play a significant role in the safety of these tunnels. The rock bolt types and aggressive environmental conditions considered in this research are focused on those used and observed in Norwegian and Swedish road tunnels. Findings elsewhere in similar environments are included to highlight the different kinetics of degradation. Based on the collected data, the corrosion levels in these road tunnels are comparable to those in the most polluted industrial areas. Aggressive groundwater conditions for rock bolts include a groundwater pH below four (which can be caused by sulfuric acid formation due to the oxidation of the mineral pyrite in the rock), marine groundwater, and flowing groundwater with a high concentration of dissolved oxygen. Furthermore, chloride-bearing deicing salts commonly used on roads located in cold climate regions during winter can promote corrosion in rock bolts from the tunnel room. For these environments, this research proposes a lognormal probability function to quantify the expected steel corrosion level for 25, 50, and 100 years of exposure time. The corrosion protection given by cement grouting, hot-dip galvanizing and epoxy coating are also addressed to explore their contributions to the lifespan extension of rock bolts in acidic and chloride-rich environments. [ABSTRACT FROM AUTHOR]

In the anchoring system of geotechnical engineering, there are many types of structures and different spatial distributions of rock bolt, which lead to the complicated in BIM design process. In order to realize the rapid deployment of rock bolt and reduce the repetitive work in the BIM design process, an automatic rock bolt layout method was proposed. This method relied on CA TIA Automation, combined with the characteristics of the anchoring system and developed an automatic rock bolt arrangement plug-in by using UDF +VSTA. Taking the bolt layout of excavation face of Huangzangsi Water Control Project on the Heihe River as an example, the effectiveness of the plug-in was verified. The program could automatically read the design parameters in the EXCEL file. According to the type and layout of the bolt selected by the user, it had realized the automatic layout of the rock bolt and improved the quality and speed of the BIM design of the anchor system. [ABSTRACT FROM AUTHOR]

A rock bolt which is grouted into the rock strata may not be installed according to the assumed project. It concerns also cases where due to practical reasons it is impossible to drill a hole with a sufficient deepness. The consequence of that is that it is necessary to cut the rock bolt accordingly. Another case is where the rock bolt is damaged, cut or bent. Than it is desirable to check the quality of supporting elements - installed rock bolts and the length of insertion. In proposed paper an outline of a method for measurement of rock bolt length is presented. The method uses ultrasonic transducers and is based on a pulse emission of an impulse wave. As ultrasonic transducer works both as an emitter and receiver, the reflected wave is measured and the length of the object or the position of a crack or damage may be diagnosed. [ABSTRACT FROM AUTHOR]

• Effect of the rock strength on the fatigue performance of rock bolt systems. • Effect of the cyclic load frequency and amplitude on the damage of rock bolt systems. • Acoustic Emission for the damage evolution in rock bolt systems under cyclic loading. • Cumulative AE energy slope method for macro crack initiation assessment. Rock bolt support is one of the key methods to maintain the stability of underground space. The durability of the rock bolt can be weakened by cyclic loading, resulting in premature failure of the underground structures. A comprehensive understanding of the performance of the bolting system under cyclic loading is essential to assess the safety of the bolting system and underground structures. In this context, this study investigated the mechanical performance of rock bolts installed in simulated soft and medium hard rock and subjected to cyclic loading. The Acoustic Emission (AE) technique was used to monitor internal damage to the bolt system under different loading conditions. A cumulative AE energy slope method has been proposed to determine the onset of the macro crack formation stage. The rock bolt demonstrated an extended fatigue life in soft rock, but its condition worsened when implanted in medium hard rock. The cyclic loading resulted in a slight improvement in the load bearing capacity of the soft rock bolt system and a significant deterioration in the medium hard rock bolt counterpart. The main fatigue failure modes of the rock bolt systems were splitting and shear failure of the surrounding rock, and debonding of the interface. Increasing the amplitude of the cyclic loading reduced the macro crack initiation cycle for both rock bolt systems, whereas increasing the frequency had different effects. The damage variable increased with frequency and amplitude for both rock bolt systems. The toughness of the soft rock bolt degraded during cyclic loading, while the high frequency cyclic loading did not modify the toughness of the medium hard rock bolt. [ABSTRACT FROM AUTHOR]

Due to the effects of discontinuous deformation in fractured rock masses and complex shearing between the rock bolt and the grout, the strain of rock bolt in fractured zone can hardly be measured accurately. A fully-grouted rock bolt was prepared by using 3D printing, and strain gauges were installed inside and outside the bolt respectively. The evolution law of tensile strain in the fully-grouted rock bolt during the fracture propagation process was obtained by uniaxial compression test. It was found that the outer strain gauges were greatly affected by the shear deformation of the grout, and the measurement results are easy to be distorted when the plastic deformation capacity of the grout is high. However, the internal strain gauges could accurately obtain the complete strain of bolt. Finally, the failure process of rock bolt in fractured zone was numerically simulated by FLAC software, the influence of strain gauge location on the measurement results was verified, which further proved the reliability of measurement by strain gauges inside the rock bolt. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A new analytical approach able to predict the mechanical behavior of fully grouted rock bolts subjected to pull-and-shear load is been proposed. The elastic theory of a semi-infinite length beam and approximate differential equation of the deflection curve (elastic and elastoplastic) are employed to analyze the lateral behavior of a bolt under surrounding rock pressures. Bolt axial behavior is based on a realistic tri-linear bond-slip model with residual bond strength at the grout–bolt interface. According to the Tresca yield criterion, important failure parameters such as shear displacement, tangent direction, tensile displacement, and tensile force can be determined via the analytical approach while the shear force and other basic parameters are provided. Results of the proposed analytical approach for 0°, 20°, 40°, 60°, and 90° displacing angles correlate well with experimental test results. Parametric analysis of both the rock and bolt mechanical behavior are also carried out using this new analytical approach. [ABSTRACT FROM AUTHOR]

This article presents the methodology and results of single shear tests of bolt rods under dynamic impact loading generated by means of a drop hammer. Comparative analysis was also performed for bolt rod load capacity, stress and shear work under static and dynamic (impact) loading. The developed method of single shear testing of bolt rods under impact loading makes it possible to obtain repeatable test results concerning maximum bolt rod shearing force, shear stress and shear work values. Comparative shear tests of four types of bolt rods under static and impact loading showed that the APB-type bolt rods made of AP770 steel, which was characterised by having the highest strength, exhibited the greatest shear work. AM22-type bolt rods exhibited a very similar work value. Though the AM22-type bolt rods made of A500sh steel demonstrated lower strength than the APB-type bolts, as well as a smaller diameter and cross section, they dissipated the impact energy better thanks to their higher plasticity. This could indicate the direction of optimisation for bolt rods in order to increase their impact strength. Mathematical relationships were also formulated for selected tests, describing the real single shear courses F d =f(t) of bolts under impact loading. The obtained relationships could be applied in the load assessment process of bolt rods intended for use under roof caving, tremor and rock burst conditions. [ABSTRACT FROM AUTHOR]

Rock bolts are widely applied in undergrounding excavation for strengthening rocks internally. For better understanding the effect of bolt reinforcement, the mechanical behavior of a circular bolt-liner combined supported tunnel is investigated using analytical method in this paper. Viscoelastic mechanical model of bolted rocks is established, and analytical solution for displacement and pressure in the rock/liner interface is provided, accounting for the installation delay of liner. Results show that the variations of displacement and pressure can be generally divided into three stages, rapid increasing stage, slow increasing stage and steady stage. Bolt parameter plays an important role on the evolution time of displacement and pressure. It will take shorter time for displacement and pressure to reach a steady state in the case with greater bolt parameter. In addition, installing rock bolts are able to effectively reduce displacement and pressure and there exists an exponential function relationship between displacement (pressure) and bolt parameter. The displacement and pressure decrease in a high rate with bolt parameter under the relatively low class. But, this decrease trend becomes flatter if bolt parameters reaching a certain value. There should be an appropriate bolt parameter for a certain tunnel project, taking into account mechanical parameters of rocks, load-bearing capacity of liner, or allowed deformation of design. [ABSTRACT FROM AUTHOR]

Rock bolts are widely used in the main structure of the DC transit system and play the role of the main body reinforcement and support. Because of the extensive stray current in the subway system, the corrosion risk of rock bolts is greatly increased. Therefore, the rock bolt was experimentally studied by simulating the input stray current and monitoring the polarization potential in this paper. After the stray current corrosion (SCC) process was finished, a tensile and breaking experiment of the corroded specimens was conducted for the mechanical properties. As an important parameter to indirectly characterize the corrosion strength of the stray current, the polarization potential was analyzed. Two types of rock bolts were compared for their mechanical properties and corrosion parameters. The result shows that the rock bolt with stranded steel wire as the rod has better corrosion resistance than the HRB500 rebar. Hence, the bonded stranded steel wire is more resistant to the stray current corrosion than the HRB500 rebar, which is adaptive for rock bolts in the direct current (DC) transit system. [ABSTRACT FROM AUTHOR]

Shear behavior of the bolt–grout interface basically depends on site-specific boundary conditions, i.e., initial normal stress and normal stiffness of the borehole wall. Few studies have been conducted to investigate the shear behaviors between the bolt and grout material under different boundary conditions. To better understand the effect of boundary conditions on the shear behaviors of the bolt–grout interface, simplified two-dimensional (2D) bolt–grout interface specimens are prepared and tested using direct shear tests at various initial normal stresses and normal stiffnesses in this context. The testing results showed that both initial normal stress and normal stiffness significantly influence the bolt–grout interface shearing behaviors. Increasing the normal stiffness or initial normal stress would increase the peak and residual shear strength. However, the degree of the brittleness after the peak, normal displacement, and peak friction coefficient was reduced. It is noted that the peak and residual shear strength points under the constant normal stiffness conditions located near the peak and residual strength envelopes for the constant normal load tests, respectively. Besides, the shear failure process of the bolt–grout interface was captured by PAC acoustic emission (AE) monitoring and digital camera technology, a good correlation between the evolution of AE parameters and the shear stress curves was obtained. [ABSTRACT FROM AUTHOR]

The jointed rock mass is a multi-crack body, which is cut by discontinuities such as joints and faults. These joints and faults destroy the integrity and continuity of rock mass and reduce the mechanical behavior of rock mass. The rock bolting is a suitable way to reinforce the jointed rock mass. By transferring tension and shear force, the rock bolting can make best use of the strength and stability of rock mass and restrict the deformation of rock mass. In this study, the specimens containing pre-set non-persistent joints were made with granite as prototype. A series of uniaxial compression tests of the reinforced jointed specimens were conducted by rock bolts with various inclinations. The relationships among the stress-strain, anchoring force, bolt strain, bolt inclination were studied, and the change rules of the parameters of the specimens and bolt with crack evolution during compression were analyzed. Results show that the peak strength and the equivalent elastic modulus of the anchored specimens with multiple non-persistent joints first increase and then decrease with the bolt inclination increasing. The optimal anchorage angle is about 65°. The anchoring force is relatively large when the angle between the rock bolt and the minimum principal stress surface is small. There is a strong internal correlation among the stress, anchoring force, bolt strain and crack evolution of the jointed specimens. The obtained conclusions in this study can provide a reference to the similar engineering practice. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• This paper is structurally innovative to proposes a new expanding shell bolt. • The influence of several factors on the characteristics of the bolt is considered. • The group anchor effect is considered and the group anchor factor is formulated. • Comparison of the ultimate pull-out bearing capacity of bolt field and specification. Rock bolt foundation is becoming increasingly popular for transmission line engineering in mountainous areas due to the ability to maximize rock foundation bearing performance, reduce foundation size and burial depth, and save investment. However, failure modes such as pull-out of bolt can lead to insufficient utilization of bearing capacity of rock bolt. To overcome this problem, a new form of expanding shell bolt is proposed and the effect of several variables including grouting materials, bolt diameter, site weathering degree, and anchoring depth are investigated in this study. The results indicate that the new expanding shell bolt may significantly increase the pull-out bearing capacity compared to common rock bolts, due to the built-in effect between the bottom part of bolt and the rock on the hole wall. The effect of grouting material and anchoring length on the shell anchor's uplift bearing capacity is minimal. Considering that the ultimate tensile strength when the anchor bar breaks depends on the diameter of the steel bar, increasing the diameter of the anchor bar will effectively improve the pull-out characteristics of the bolt. The failure mode of anchor rods is also influenced by the rock layers. Rock layers with lower weathering levels have stronger embedding force, and the expansion head can effectively play a role. Due to the group anchor effect, the ultimate uplift bearing capacity of group anchor foundation in rocks is not simply the sum of the bearing capacity of each one, but must be multiplied by the group anchor coefficient. This coefficient is close to 0.9 in moderately weathered and strong-medium weathering sites. Finally, the calculation method for ultimate pullout force of expanding shell bolt is established. The feasibility of this method is verified by comparing on-site test values and calculated values. Author keywords : Rock bolt; Expanding shell bolt; Pull-out bearing capacity; Failure mode; Group anchor effect. [ABSTRACT FROM AUTHOR]

In this study, laboratory pull tests of rock bolts were conducted to verify and update existing stress transfer models. The rock bolt samples in the study had different bond lengths that were fully encapsulated in cementitious grout with different water-cement ratios. The test results show that the existing models do not correctly describe the shear stress distribution on the bond length in some loading stages. Three new models were proposed for the distribution of shear stress along the bond length. The first model describes cases where the bolt sample has a bond length that is much longer than the critical bond length, the second model describes cases where the bolt sample has a bond length slightly shorter than the critical bond length, and the third model describes cases where the bolt sample has a bond length that is much shorter than the critical bond length. In these models, the bolt is fully debonded along the section between the borehole collar and a depth of approximately one bolt diameter (d b). The bolt is then partially debonded and the residual shear stress on the bolt increases with depth. In the first model, the shear stress reaches the maximum at a depth of approximately 3 d b. The shear stress then attenuates to zero at a depth of 25 d b. In the second and third models, the entire bond length is mobilized to carry load. [ABSTRACT FROM AUTHOR]

The shear performance of bolted rock joints under cyclic loading condition was studied through laboratory shear tests. The tests were performed on a cyclic shear testing apparatus. The fully encapsulated rock bolts were inserted in the rock joints. The upper block was fixed in the horizontal direction, and the cyclic shear load was applied laterally on the lower block. A total of 16 shear tests were conducted for two types of rough joints. The results showed that the failure mode of rock bolts in cyclic shear test was different with the case of direct shear test. When the cyclic displacement was small, the rock bolt played a very small role in the second-to-fifth cycles, the rock bolt remained unbroken during cyclic shearing. The shear resistance of the rock bolt would gradually recover after the shear displacement has exceeded the cyclic displacement in the termination cyclic after five cycles. When the cyclic displacement was large, the rock bolt broke during cyclic shearing, and the shear resistance could not be recovered in the termination cyclic. It is indicated that the rock bolt had completely lost its supporting role after cyclic shear loading. The shear strength reduction of bolted joints under the cyclic loading condition was much significant compared to the un-bolted joints. Therefore, the influence of cyclic loading on the shear resistance of the rock bolt was much larger than its counterpart of the rock joint itself. These results showed that the shear performance of a rock bolt inserted in a rock joint was strongly influenced by cyclic shear loading. [ABSTRACT FROM AUTHOR]

Rock bolts are often used to support reinforcements in transportation, water conservation, coal mining and other industries, to reinforce slopes, tunnels and mines. They also play a key role in ensuring soil stability of foundation pits and slopes. Thus, it is very important to identify the defects of rock bolts. In order to address the issue that it is difficult to obtain the fault information of rock bolts, this paper proposes a method based on a grey cloud model to identify anchor defects. First, the coefficients of wavelet packet energy decomposition of the stress wave signal collected by a hammering anchorage system are selected as eigenvalues. Then, the eigenvalues are expanded into feature intervals, which are characterised by the grey cloud model with the peak value, left and right end-point boundary values, entropy and hyper entropy. Finally, the feature intervals are used for cluster and discriminant analysis of the anchorage quality. Optimisation of the model is achieved and the accuracy of the model in solving the problem of anchor rod defect identification is improved. The results show that the proposed method is feasible and effective in the identification of anchor defects. Additionally, this new method is more convenient and faster than the traditional method. [ABSTRACT FROM AUTHOR]

The reinforcement effect of rock bolts in granular mixtures is studied by the mesoscopic numerical simulation method. The granular mixtures consist of 3D polyhedral granules. The particle size distribution (PSD) is based on stochastic simulation technology. Simulations of rock bolts and granular mixtures are conducted by ABAQUS/Explicit program. The bolted granular material is analysed in macroscopic and mesoscopic scales. Various factors on the mechanical properties of the granular mixture are studied, including spacing, PSD, the prestress, porosity of the granular material, the length of the bolts and the diameter of supporting plates at the bolt ends. The simulated results reflect the deformation and reinforcement effect of different bolted granular structures. The macroscopic results are closely related to the evolution of the mesoscopic fabric. Without considering the bolt length, if the bolt spacing is less than three times the average particle diameter, a stable structure can be formed using the rock bolts. Based on the samples in this paper, the list of ranking of importance of parameters on the degree of reinforcement is porosity > granule size > bolt spacing > supporting plate diameter > bolt prestress > bolt length. [ABSTRACT FROM AUTHOR]

Shear failure is the main failure mode of anchorage bolts in coal mines. To improve ground control, it is crucial to improve the shear strength of roadway anchorage systems. In this study, theoretical analysis, numerical simulation and laboratory experiments were carried out to study the shear characteristics of the cuneiform reaming anchorage. The results show that the cuneiform reaming anchorage can reduce the maximum shear stress on the cross section of the anchorage to improve the shear strength. The stress concentration near the joint planes can be transferred to middle parts of the bolts. Moreover, the cuneiform structure can also prolong the period in which the shear load of the anchorage decreases and shorten the period in which the shear load increases, thus reducing the shear load of the anchorage. This paper provides a new economical and practical method for raising the shear strength of an anchorage system and enhancing roadway stability. [ABSTRACT FROM AUTHOR]

Wide-span openings in horizontally layered jointed rock were analysed using the distinct element method (DEM) to provide additional insight into rock bolt support mechanisms and forces. The rock bolt forces in the DEM calculations exhibited distinct mechanisms, identified here as reinforced rock unit, beam building or suspension mechanisms. Each of these mechanisms was found to be associated with certain characteristics in the jointed rock structure. In jointed rock with thick horizontal beddings, the rock bolt forces at the roof tapered off with distance leading to a reinforced rock unit where each layer exhibited a self-support mechanism akin to a voussoir beam. In jointed rock with thinner horizontal beddings not capable of self-supporting, a beam building mechanism took place. When the rock bolts were extended into a rigid layer above the thinner layers, a suspension mechanism took place. Potential inaccuracies of thrust line predictions and the benefits of rock bolt tensioning were investigated, and their effects were found to be sensitive to the bedding thickness. A case history of the cavern roof of the underground car park for the Sydney Opera House was modelled. The calculated deflections of the cavern roof compared well with field measurements reported in the literature. The findings in this paper highlight the importance to ensure that the reinforcement support for wide-span underground excavations in horizontally layered jointed rock are tailored to the geological conditions being encountered. [ABSTRACT FROM AUTHOR]

Abstract Flexural toppling failure is a widespread issue in anti-dip bedding rock slopes. Rock bolts are usually employed to reinforce bedding rock slopes as they are low-cost and efficient. In this technical note, a mechanical model based on limit equilibrium theory is proposed to predict the stability of anti-dip bedding rock slopes locally reinforced by rock bolts. By considering a practical case, the proposed model is subsequently verified by comparing its results with numerical results using a discrete element method. Finally, we investigate the influence the positions of rock bolts have on the reinforcement effect. The results show that, for a given bolt angle, the reinforcement effect is controlled by the positions of the rock bolts. In particular, the installation position must start from the superimposed toppling zone (the zone within which the rock layers resemble superimposed cantilever beams). On the other hand, there is no reinforcement effect if the installation position starts from the cantilevered toppling zone (the zone within which the rock layers resemble individual cantilever beams). The method proposed provides theoretical guidance for the design of the reinforcements applied to anti-dip bedding rock slopes. The results produced should also help engineers to gain a better understanding of the reinforcement mechanisms underlying the action of the rock bolts used in anti-dip bedding rock slopes. Highlights • A theoretical model reflecting the effect of local rock bolts is proposed. • Reinforcement effect of rock bolts relies on the reinforcement position. • Influence of the height of rock layer on the reinforcement effect is discussed. [ABSTRACT FROM AUTHOR]

A rock bolt is a reinforcing bar commonly used in geotechnical engineering, and it plays an important role for the safe operation of the reinforced structures, such as tunnels, underground stations, and stabilized slopes. The length of an installed rock bolt is an important quality index that ensures the performance of a rock bolt for the stability of the reinforcing structure. In this paper, based on the general principle of stress wave propagation, the method of Automatic Extraction Algorithm (AEA) is developed using self-correlation analysis to measure the length of an in-service rock bolt, whose length is mostly embedded. The propose method has advantages of simple computation and high accuracy. Through laboratory experiments and field tests, we compared the performances of AEA with the traditional Phase Analysis Method (PAM) and the results demonstrated that the proposed AEA showed improved performance in terms of both detection speed and accuracy in rock bolt length estimation. In addition, the AEA method was successfully implemented in several engineering scenarios with high accuracy. [ABSTRACT FROM AUTHOR]

The purpose of a rock bolt is to improve the mechanical performance of a jointed-rock mass. The performance of a rock bolt is generally evaluated by conducting a field pullout test, as the analytical or numerical evaluation of the rock bolt behavior still remains difficult. In this study, wide range of field test was performed to investigate the pullout resistance of rock bolts considering influencing factors such as the rock type, water bearing conditions, rock bolt type and length. The test results showed that the fully grouted rock bolt (FGR) in water-bearing rocks can be inadequate to provide the required pullout resistance, meanwhile the inflated steel tube rock bolt (ISR) satisfied required pullout resistance, even immediately after installation in water-bearing conditions. The ISR was particularly effective when the water inflow into a drill hole is greater than 1.0 l/min. The effect of the rock bolt failure on the tunnel stability was investigated through numerical analysis. The results show that the contribution of the rock bolt to the overall stability of the tunnel was not significant. However, it is found that the rock bolt can effectively reinforce the jointed-rock mass and reduce the possibility of local collapses of rocks, thus the importance of the rock bolt should not be overlooked, regardless of the overall stability. [ABSTRACT FROM AUTHOR]

Full-length bonded rock bolts are commonly used in mining, tunneling and slope engineering because of their simple design and resistance to corrosion. However, the length of a rock bolt and grouting quality do not often meet the required design standards in practice because of the concealment and complexity of bolt construction. Non-destructive testing is preferred when testing a rock bolt's quality because of the convenience, low cost and wide detection range. In this paper, a signal analysis method for the non-destructive sound wave testing of full-length bonded rock bolts is presented, which is based on the Hilbert-Huang transform (HHT). First, we introduce the HHT analysis method to calculate the bolt length and identify defect locations based on sound wave reflection test signals, which includes decomposing the test signal via empirical mode decomposition (EMD), selecting the intrinsic mode functions (IMF) using the Pearson Correlation Index (PCI) and calculating the instantaneous phase and frequency via the Hilbert transform (HT). Second, six model tests are conducted using different grouting defects and bolt protruding lengths to verify the effectiveness of the HHT analysis method. Lastly, the influence of the bolt protruding length on the test signal, identification of multiple reflections from defects, bolt end and protruding end, and mode mixing from EMD are discussed. The HHT analysis method can identify the bolt length and grouting defect locations from signals that contain noise at multiple reflected interfaces. The reflection from the long protruding end creates an irregular test signal with many frequency peaks on the spectrum. The reflections from defects barely change the original signal because they are low energy, which cannot be adequately resolved using existing methods. The HHT analysis method can identify reflections from the long protruding end of the bolt and multiple reflections from grouting defects based on mutations in the instantaneous frequency, which makes weak reflections more noticeable. The mode mixing phenomenon is observed in several tests, but this does not markedly affect the identification results due to the simple medium in bolt tests. The mode mixing can be reduced by ensemble EMD (EEMD) or complete ensemble EMD with adaptive noise (CEEMDAN), which are powerful tools to used analyze the test signal in a complex medium and may play an important role in future studies. The HHT bolt signal analysis method is a self-adaptive and automatic process, which can be programed as analysis software and will make bolt tests more convenient in practice. [ABSTRACT FROM AUTHOR]

Methods for simulating the mechanics of debonding and predicting global load-slip ( P -Δ) response from local bond stress versus slip (τ-δ) relationships can vary significantly in their complexity. It is generally accepted that an adequate representation of intermediate crack debonding can be achieved by considering only mode-II (shear) fracture along the interfacial bond, a problem that can be solved by accounting for one-dimensional fields of elastic stress and strain in the substrate and plate, and nonlinear interfacial slip and shear stress along the bond. In this paper, full-slip-range analytical solutions are presented for the bilinear τ-δ rule with allowance for residual friction. The procedure is capable of modelling the entire debonding process over both long and short bonded lengths. This is an extension of previous works which are either inapplicable to all bonded lengths or do not allow for residual strength. Applicability of the formulation can range from externally-bonded or near-surface-mounted FRP plates, to embedded bars or bolts in brittle substrates such as concrete, rock or masonry. The versatility and low computational effort required to apply the developed formulation makes it ideal for both directly predicting the P -Δ relationship from known τ-δ parameters, or conversely for extracting a τ-δ relationship from a reference P -Δ curve using inverse calibration. While it is not the purpose of this paper to propose a bond model for any specific type of system, a framework is proposed for doing so. Significantly this framework addresses the difficulty in identifying a unique solution of local properties from experimental data, and highlights that the bonded length has an important influence on the reliability of extracted results. [ABSTRACT FROM AUTHOR]

In this paper, a unified rock bolt model is proposed and incorporated into the two-dimensional discontinuous deformation analysis. In the model, the bolt shank is discretized into a finite number of (modified) Euler-Bernoulli beam elements with the degrees of freedom represented at the end nodes, while the face plate is treated as solid blocks. The rock mass and the bolt shank deform independently, but interact with each other through a few anchored points. The interactions between the rock mass and the face plate are handled via general contact algorithm. Different types of rock bolts (e.g., Expansion Shell, fully grouted rebar, Split Set, cone bolt, Roofex, Garford and D-bolt) can be realized by specifying the corresponding constitutive model for the tangential behavior of the anchored points. Four failure modes, namely tensile failure and shear failure of the bolt shank, debonding along the bolt/rock interface and loss of the face plate, are available in the analysis procedure. The performance of a typical conventional rock bolt (fully grouted rebar) and a typical energy-absorbing rock bolt (D-bolt) under the scenarios of suspending loosened blocks and rock dilation is investigated using the proposed model. The reliability of the proposed model is verified by comparing the simulation results with theoretical predictions and experimental observations. The proposed model could be used to reveal the mechanism of each type of rock bolt in realistic scenarios and to provide a numerical way for presenting the detailed profile about the behavior of bolts, in particular at intermediate loading stages. [ABSTRACT FROM AUTHOR]

The present study focuses on the load transmission law along the rock bolt and the efficacy for different anchored conditions. Shear displacement along the joint face is considered on the basis of traditional rock bolt stress calculation method with regard to joint open displacement. Combining the elastic foundation beam model and the rock bolt pull-out model, an analytical solution for the stress distribution along the rock bolt through a single joint face is derived. The performance of the proposed analytical solution is compared with an experimental result and a previous study. In addition, joint displacement, joint location, and dip angle of the joint face are set as variables to further explore the corresponding anchored effect. The results indicate that larger joint displacement along the joint face tend to cause a distinct increase of the shear and compressive stress along the rock bolt. When the joint face moves inward, the interaction force between the rock bolt and the surrounding rock mass decreases, suggesting that the strength of the rock bolt is not taken full use. Moreover, for a certain joint displacement and joint location, larger anchored angle between the rock bolt and the joint face contributes to better reinforcement effect. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Rock bolts ensure structural stability for tunnels and many other underground structures. The pre-load on a rock bolt plays an important role in the structural reinforcement and it is vital to monitor the pre-load status of rock bolts. In this paper, a rock bolt pre-load monitoring method based on the piezoceramic enabled time reversal method is proposed. A lead zirconate titanate (PZT) patch transducer, which works as an actuator to generate stress waves, is bonded onto the anchor plate of the rock bolt. A smart washer, which is fabricated by sandwiching a PZT patch between two metal rings, is installed between the hex nut and the anchor plate along the rock bolt. The smart washer functions as a sensor to detect the stress wave. With the increase of the pre-load values on the rock bolt, the effective contact surface area between the smart washer and the anchor plate, benefiting the stress wave propagation crossing the contact surface. With the help of time reversal technique, experimental results reveal that the magnitude of focused signal clearly increases with the increase of the pre-load on a rock bolt before the saturation which happens beyond a relatively high value of the pre-load. The proposed method provides an innovative and real time means to monitor the pre-load level of a rock bolt. By employing this method, the pre-load degradation process on a rock bolt can be clearly monitored. Please note that, currently, the proposed method applies to only new rock bolts, on which it is possible to install the PZT smart washer. [ABSTRACT FROM AUTHOR]

Special considerations are required for the support of tunnels in fault zones. On the one hand, standard support elements are not compatible with the imposed displacements. On the other hand, local shearing can cause damage to the lining already at rather small displacements magnitudes. Cracks in the lining frequently are observed, when the geometry of the tunnel changes, or at the intersection with cross passages. As strengthening of the lining in many cases cannot prevent damage, temporarily leaving an open gap in the primary lining. Implementing yielding elements into the lining is as well feasible, as economical increasingly replaces the traditional method of leaving open gaps. This results in a better utilization of the lining's capacity and a considerable reduction of displacements. As the lining with integrated yielding elements develops considerable thrust, a transfer of the loads via the construction joints is required. Thus, careful construction of those joints is essential. Another problem can be the bond between bolt, grout, and rock mass, as the bolts are installed, when the displacement rate is highest, and the strength of the grout still low. This paper deals only with phenomena caused by discontinuities and large displacements, and does not address other hazards, like flowing ground. [ABSTRACT FROM AUTHOR]

Rock bolts are one of the most prominent and economical techniques of rock stabilization used widely in various applications such as mining, tunneling, surface slope stabilization. Various researchers have found that efficacy of rock bolt system depends on the many factors such as annulus space, type of bolt, grouting media, installation mechanism and positioning. Since its inception, many researchers have studied the behaviour of bolts experimentally and analytically. At present in Indian mining industry, the use of fully grouted rock bolts are increasing rapidly owing to the number of accidents in underground mainly due to roof fall. This paper reviews the performance of fully grouted rock bolts under axial loading conditions and presents a case study of an Indian underground metal mine. The review provides an overview of the need for rock bolts to support the roof rock and the bolting system used in Indian mines along with characteristic parameters such as ultimate load, displacement and failure mode under axial load. A study was undertaken to understand the behaviour of fully grouted un-tensioned rebar bolts in an Indian metal mine having competent host rock. The case study describes the performance of fully grouted un-tensioned rock bolt supporting the hanging wall at a depth of 150 m from the surface. The anchorage strength of the bolts were determined in field conditions by conducting pull out test at different durations from the time of installation of the bolt. The generated load-displacement curve shows the debonding nature of the installed bolts under axial loading conditions. The safety factor was calculated for the designing of support system of the mine and the rock load was found using empirical equations using rock mass rating. The safety factor calculation resemble successful application of un-tensioned roof bolt system in supporting an underground metal mine. [ABSTRACT FROM AUTHOR]

In addressing limitations of conventional ground support measurement, a novel optical strain sensing technique is presented that is capable of measuring strain at increments as low as 0.65 mm along the entirety of an optical sensor affixed to a support element. The technique considers monitoring three sensing lengths along the profile of a fully grouted rock bolt element using a single optical fiber, which, in turn, allows the derivation of both the principle strain and principle strain direction along the bolt. A series of three experiments, which include: symmetric bending, combined axial load and bending, and double shear loading are presented and highlight the potential of the technique to capture bolt behaviour under such generalized rock bolt loading conditions. Analogized as a distributed strain rosette, the optical technique can distinguish both the coaxial and bending induced constituents of the total strain in the bolt regardless of load orientation. [ABSTRACT FROM AUTHOR]

Apparent discontinuities can be easily found in natural rock medium owing to the constant motion and change of the Earth's crust, which contains large amount of discontinuous surfaces such as faults, joints, cracks and so forth. Rock bolt is one of the most effective and economical reinforcing tools in practical geotechnical engineering for a long time. This paper investigates the mechanical properties, anchorage effect, cracking, and coalescence process of intact rock-like specimens, rock-like specimens containing flaws, and bolted rock-like specimens containing flaws. A series of uniaxial compression tests, splitting tests, and biaxial compression tests are performed on these specimens. Some findings can be observed from this study. (1) The number of rock bolt(s) and the anchoring angle have a great influence on the anchorage effect of rock bolt(s). With the increasing number of rock bolt(s), the uniaxial compressive strength (UCS), the splitting peak strength (which can be converted to the tensile strength), and biaxial compressive strength (BCS) can all be improved, whose variation tendencies do not follow a linear relationship. (2) The contributions to the tensile strength for rock bolt is greater than that to the UCS for the same-type specimen. (3) In the biaxial compression tests, with the increasing of the lateral pressures, the anchorage effect of rock bolt gradually declines and the lateral pressure plays a dominant role in improving the strength of the specimen. The failure characteristics of three types of laboratory tests have also been systematically analyzed in this paper. [ABSTRACT FROM AUTHOR]

In underground coal mining, high abutment loads caused by the extraction of coal can be a major contributor to many rock mechanic issues. In this paper, a large-scale physical modeling of a 2.6 × 2.0 × 1.0 m entry roof has been conducted to investigate the fundamentals of the fracture mechanics of entry roof strata subjected to high abutment loads. Two different types of roof, massive roof and laminated roof, are considered. Rock bolt system has been taken into consideration. A distinct element analyses based on the physical modeling conditions have been performed, and the results are compared with the physical results. The physical and numerical models suggest that under the condition of high abutment loads, the massive roof and the laminated roof fail in a similar pattern which is characterized as vertical tensile fracturing in the middle of the roof and inclined shear fracturing initiated at the roof and rib intersections and propagated deeper into the roof. Both the massive roof and the laminated roof collapse in a shear sliding mode shortly after shear fractures are observed from the roof surface. It is found that shear sliding is a combination of tensile cracking of intact rock and sliding on bedding planes and cross joints. Shear sliding occurs when the abutment load is much less than the compressive strength of roof. [ABSTRACT FROM AUTHOR]

Rock mass classification has played a crucial role in underground construction and mining projects in the past fifty years, especially the Rock Mass Rating (RMR) and Rock Tunneling Quality Index (Q) systems, which have been applied in many cases. The parameters of discontinuity conditions in the RMR system, along with the related ratings, were used to develop a new equation in order to introduce an intrinsic quality of a given rock mass, namely, rock bolt supporting factor (RSF). The RSF was used to develop a mathematical theory of the rock bolt supporting mechanism as a new principle in explaining rock bolting effects. Finally, using the parameters of the discontinuities in the Q-system, the QRSF is defined to present the capability of a given rock mass to be reinforced by rock bolting. The RSF leads to more reliable judgment of rock bolting capability of a given rock mass than the QRSF since it uses five parameters of discontinuity conditions while the QRSF uses only two parameters. [ABSTRACT FROM AUTHOR]

• The necessity of a bolt–cable support for large-span tunnelling was analyzed. • Rock mass deformation characteristics were investigated by numerical modelling. • The mechanical behaviors of pretensioned rock bolts and anchor cables are analyzed. • The load evolution and lining responses of large-span tunnel were monitored. This study focused on a new support method, a pretensioned bolt–cable combined support system, for large-span tunnelling. The Badaling tunnel is a high-speed railway station tunnel that has a span of 32.7 m, a cross-sectional area of 494 m2, and a maximum depth of 102 m. The super large cross-section and unfavorable geological conditions around the tunnel resulted in considerable difficulties for tunnel construction. Because this tunnel was successfully excavated using the pretensioned bolt–cable support, the mechanical characteristics of this new support system are analyzed in this study. First, the necessity of a bolt–cable support for large-span tunnelling was analyzed using a numerical model. The deformation characteristics of rock mass in the large-span tunnel are discussed based on the modelling results of the tunnelling process. Second, based on field monitoring results, the mechanical behaviors of pretensioned rock bolts and anchor cables were analyzed. The anchoring forces of the rock bolts and anchor cables were determined by the pretension forces, loss of pretension forces, and passive support effects. The development of the anchoring force can be divided into three phases: rapid loss of the pretension force, a fluctuation phase, and a stable phase. Finally, The loading effect and mechanical responses of the primary lining were analyzed according to the monitoring results of the primary lining, which indicated that the rock mass was effectively reinforced by the pretensioned bolt–cable system and that the primary lining was safe. [ABSTRACT FROM AUTHOR]