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In the past, the number of CPU cores/threads was usually less than 8/16; now, the maximum number is 128/256. As a CPU-based parallel method, OpenMP has an increasing advantage with the increase in CPU cores and threads. A parallel combined finite-discrete element method (FDEM) for modeling underground excavation and rock reinforcement using OpenMP is implemented. Its computational performance is validated in the two advanced CPUs: AMD Ryzen Threadripper PRO 5995WX (64/128 cores/threads); and 2 × AMD EPYC 7T83 (128/256 cores/threads). Then, its ability in simulating tunnel excavation under rockbolt-shotcrete-grouting support is implemented using the novel solid bolt model, which can explicitly capture the interaction between bolt, grout, and rock. The parallel performance validation of the uniaxial compression test shows: (i) for the speedup ratio, the OpenMP-based parallel FDEM obtains maximum speedup ratios of 30 (33 k elements) and 41 (3304 k elements) on the Threadripper, and 31 and 43 on the 2 × EPYC, respectively; (ii) for the scalability of speedup ratio, when the number of threads used is less than 128, the speedup ratio is always increasing with the increase of the number of threads; (iii) for the stability of speedup ratio, it has a stable speedup ratio, regardless of whether the rock is pre- or post-fractured. Highlights: A parallel FDEM for modeling underground excavation and rock reinforcement using OpenMP is implemented. Its computational efficiency is validated in the two advanced CPUs: AMD Ryzen Threadripper PRO 5995WX; and 2 × AMD EPYC 7T83 (128/256 cores/threads). Its ability in simulating rockbolt-shotcrete-grouting support is implemented using the novel solid bolt model. It obtains maximum speedup ratios of 30 (33 k elements) and 41 (3304 k elements) on the Threadripper, and 31 and 43 on the 2 × EPYC. It has good scalability and stability of speedup ratio, regardless of whether the rock is pre- or post-fractured. [ABSTRACT FROM AUTHOR]

The severe failure of strong supports occurred in the Xinhua tunnel that was a deep-buried tunnel excavated in squeezing ground. In order to address such problem, this study explores the possibility of applying deformable supports in this tunnel. The mechanical response of a circular “rockbolt and yielding lining” supported tunnel is studied from the perspective of the convergence-confinement method. The equations for calculating the elastic modulus, cohesion, and internal friction angle of bolted rock are provided firstly; the mechanical model of the bolted tunnel is established then, where the surrounding rock can be classified into the plastic bolted, elastic bolted, and elastic unbolted regions. The equations for constructing the ground reaction curve are provided considering rockbolt reinforcement and rock shear dilatancy. The reduced case that the surrounding rock does not generate the plastic region is discussed as well, if the support pressure is higher than the critical value. The required minimum support pressure is further determined following the assumptions of maximizing the utilization of rockbolt bearing capacity and generating no loosening rock pressure. The exact equations for determining the lining thickness and length of highly deformable elements are provided with the intention to ensure lining safety and accept rock displacement. Based on the consideration of shotcrete hardening property, the equation to calculate the yielding stress of highly deformable elements is provided. The equations for GRC in this study can be reduced to those without considering rockbolt reinforcement or rock shear dilatancy. The design model of yielding lining is well applied in the Xinhua tunnel. The analysis results show that the rock displaces for 382.6 mm and the lining generates a plastic displacement of 281.8 mm in the Xinhua tunnel using the strong supports, which have a good agreement with the field monitoring data. The required lining thickness is equal to 28 cm and the installation number and length of highly deformable elements are 9 and 41.3 cm, respectively, when the yielding lining is employed in the Xinhua tunnel. Finally, a parametric investigation is carried out, including the cohesion and internal friction angle of rock, rockbolt length, and initial ground stress. Some recommendations for the tunnel design are proposed. [ABSTRACT FROM AUTHOR]

In order to improve bearing capacity of rockbolts in deep-buried coal mine roadways, orthogonal tests were conducted to study influencing factors of rockbolt anchoring effect. Wavelet neural network model was introduced to predict the pull-out force of rockbolt. The activation and output functions of the wavelet neural network were improved, and the scaling and translation parameters were also modified by using the gradient descent method. These improvements enhanced the approximation rate of the wavelet neural network model, and solve the problem that the wavelet transform method is monotonous and difficult to adapt to the complex and variable engineering conditions. Research results illustrated that The value of the ultimate pull-out force is positively correlated with the strength of the specimen and pre-tension value of the specimen. According to the test results, the coal mine roadway support scheme was optimized, and the high prestress full-length anchoring rockbolt support technology was proposed. The effectiveness of research was verified through the engineering applications and in-situ monitoring results. [ABSTRACT FROM AUTHOR]

In this research, the rockbolt mechanism of a jointed rock mass under uniaxial tension was systematically revealed with a laboratory test and a numerical simulation. It was found that the rockbolt rock mass experienced five stages under uniaxial tension, the densification stage, elastic stage, plastic deformation stage, progressive debonding stage, and complete debonding stage. The stress-strain curve and ultimate tensile strength of a rockbolt rock mass were analyzed by taking the rockbolt spacing and rockbolt angle as variables. It was found that the improvement effect of the reduction of the rockbolt spacing on the ultimate tensile strength was limited. When the rockbolt spacing was reduced to a certain limit, the stress concentration area between adjacent rockbolts was connected and destroyed, resulting in the increase of the rockbolt rock strength becoming smaller, and even having a downward trend. The increase of the rockbolt angle led to the change of the stress mode and failure mode of the whole structure, and the ultimate tensile strength first increased and then decreased. The optimal rockbolt angle was between 60° and 70°. It is worth noting that there was an obvious mechanical occlusion between the thread on the rockbolt surface and the rock mass, resulting in the multi-stage step-down characteristic of the stress-strain curve in the complete debonding stage. The results of this study can provide a reference for the design and construction of similar projects. [ABSTRACT FROM AUTHOR]

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Impact drop tests are routinely used to examine the dynamic performance of rockbolts. Numerous impact tests have been carried out in the past decades on independently designed, constructed and operated testing rigs. Each laboratory has developed testing procedures; thus, the results are often reported in different ways by various laboratories. The inconsistency in testing procedures and reporting formats presents a challenge when comparing results from different laboratories. A series of impact tests of identical rockbolts was carried out using the direct impact method (i.e. the mass free-fall method) on the rigs in four laboratories in different countries. The purpose of these tests was to investigate the level of consistency in the results from the four rigs. Each rig demonstrated a high level of repeatability, but differences existed between the various rigs. The differences would suggest that there is noticeable equipment-dependent bias when test results obtained from different laboratories are compared. It was also observed that the energy dissipated for the plastic displacement of the bolt was smaller than the impact energy in the tests. The average impact load (AIL) and the ultimate plastic displacement (D) of the bolt describe the ultimate dynamic performance of the bolt. In the case where the bolt does not rupture, the specific plastic energy (SPE) is an appropriate parameter in describing the impact performance of the bolt. Two other relevant parameters are the first peak load (FPL) and the initial stiffness (K) of the bolt sample. The information from this test series will guide the formulation of standardised testing procedures for dynamic impact tests of rockbolts.

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.

• An intelligent terminal structure (ITS) of rockbolts was designed for deformable support systems and safety monitoring in rock engineering applications. • The axial stress load exerted on the rockbolt is transferred to the ITS, leading to memory tooth breakage and compression deformation of the yielding ring. • The ITS not only serves as a deformable support system but also plays a significant role in monitoring the stress and deformation of the rock mass. The nonlinear mechanical behavior perception and deformation control of engineering rock masses are crucial technical challenges in the fields of underground engineering support and safety monitoring. An intelligent terminal structure (ITS) of rockbolts was designed for deformable support systems and safety monitoring in rock engineering applications. It is a type of device installed at the end of a traditional bolt. The ITS comprises two key components: a memory tooth and a yielding ring. This study introduces the design principle of an ITS in deformable rockbolt support systems. It proposes design parameters for the ITS in relation to the deformations encountered by engineering rockbolts. Tests were conducted to measure the ultimate bearing capacity and deformation of the ITS under pull-out conditions. The results demonstrate that the ITS of rockbolt undergoes deformation in the form of elongation as opposed to conventional bolts. Furthermore, finite element numerical simulations were conducted to investigate the mechanism of interaction between the memory tooth and yielding ring of the ITS. The numerical simulation results are consistent with the experimental results. The load capacity of the yielding ring exceeded that of the memory tooth. The field test results indicate that the dilatation of the surrounding rock mass causes ITS deformation in the diversion tunnel. The axial stress load exerted on the rockbolt is transferred to the ITS, leading to memory tooth breakage and compression deformation of the yielding ring. This mechanism allows the load and deformation of the surrounding rock mass to be reflected. The ITS not only serves as a deformable support system but also plays a significant role in monitoring the stress and deformation of the rock mass. [ABSTRACT FROM AUTHOR]

Historically, the idea of designing and installing rockbolts in rockmasses can be traced back to the construction of the Snowy Mountain hydroelectric scheme in Australia, which was completed in 1974. The use of rockbolts as a ground support system has been widely applied to enhance the stability of structures in mining and to resist the movement of keyblocks in underground excavations. The strength of a rockbolt as a rock support tool lies in its ability to improve the tensile and shear strength needed to maintain the load-bearing capacity of a rockmass. Currently, rockbolts are extensively used as reinforcement methods in mining and civil engineering projects to support deforming rockmasses in coal and hard rock mining, tunnelling and underground openings. The performance of a rockbolt depends on the design parameters such as load-bearing capacity of the bolt, strength of the bolt, bolt diameter, length of the bolt and bolt spacing. This study provides a detailed review of the various applications of rockbolts in mining operations. In addition, the integration of smart sensors in rockbolts to enhance monitoring of displacement measurements, stresses, subsidence of pillars and ground pressure on structures are discussed in this paper. The use of smart rockbolt devices, which has improved the monitoring and evaluation of critical parameters in rockbolts such as the axial force, corrosion occurrence, grout quality and resin delamination, are also discussed. This review shows that with the numerous applications of rockbolts in mining, especially in underground excavations and tunneling engineering, rockbolts can be used as supports in rocks with different geological conditions. Nevertheless, this paper highlights that there are still some areas that need improvement in rockbolting technology. [ABSTRACT FROM AUTHOR]

This paper presents the principles of rock support for rockburst control and three rockburst support systems used in deep metal mines. Before the principles of rock support are presented, rock fracture related to strain burst is first discussed with the help of photos taken on site, and the energy sources and transformations during bursting are illustrated through conceptual models. Surface parallel extension fracture usually occurs in the ejected and surrounding rocks in a strain burst event, while the ejected rock in a fault-slip rockburst is often already pre-fractured before the event. There must be excessive release energy available for rock ejection. The excessive release energy comes from both the ejected rock itself and the surrounding rock. To prevent rock ejection in a rockburst, the support system must be able to dissipate the excessive release energy. All support devices in a support system for rockburst control must be able to dissipate energy, be firmly linked, and be compatible in deformability. A support system for rockburst control comprises surface-retaining devices and yield rockbolts as well as yield cablebolts when needed. Laying mesh on the top of shotcrete liner is a good practice to enhance the surface-retaining capacity of the support system. Energy-absorbing yield rockbolts dissipate energy either by stretching of the bolt shank or by sliding of the inner anchor in the borehole. Mesh, mesh strap and shotcrete are the surface-retaining devices widely used in the current rock support systems. The three types of rock support used for rockburst control at present are soft support system using Split Set bolts, hybrid support system using rebar and two-point anchored yield bolts, and entirely yieldable support system using strong yield bolts.

To clarify the mechanism of the grouting rockbolt with a semi-infinite anchorage length, the interface mechanical property of the rockbolt was analyzed and the corresponding pullout equations were present. Assuming that the shear stress distribution of the elastic part of the anchor rod along the anchoring depth was a quarter elliptical arc, and then the calculation formula of the critical anchorage length of the grouting rockbolt was given. To verify the correctness of the present method, in-situ pull-out tests were carried out on 4 anchor rods with anchoring lengths of 6. 0 m and 8. 5 m, respectively. The results showed that the axial strain distribution curve calculated by the present method was consistent with the experimental results, and the axial strain decreases nonlinearly with the anchoring depth. The critical anchorage length calculated by the present method was 5. 3 m, which was 3. 6% different from the test result. The method presented in the manuscript was universal, and the calculation results obtained by the present method were relatively conservative compared with the results obtained by previous research. [ABSTRACT FROM AUTHOR]

The strength of a rockbolt is one of the key parameters for the design of ground support systems. However, published mechanical performance data by suppliers are usually limited to tensile strength and elongation. To provide additional performance measures, mechanical tests on four commercially available solid reinforcing bar rockbolts have been performed and analysed. In each of the cases, the rockbolts are marketed by the manufacturers as being similar and equivalent products. The mechanical tests have been developed to mimic five modes of failure that include tension, shear, torsion, and nut/thread interface. The results show significant variability in seemingly similarly marketed products. Recommendations are made for suppliers to provide additional testing results (torsion, shear and nut/thread capacity) as standard data within their product specification sheets. A summary of recommended design parameters is provided based on the test results. [ABSTRACT FROM AUTHOR]

(7) and (8), so we have 9 HT ht Graph where I k i = I k i SB 0 sb I D i SB b sb and I E i SB b sb is the elastic modulus of rockbolt material. [Extracted from the article]

Rockbolt performance may vary differently under complex geological and stress conditions, especially dynamic disturbance at deep underground mining. Therefore, reinforcement mechanism needs to be further explored to improve the support effect of rockbolt. In this study, static and dynamic Brazilian splitting tests of steel bar reinforced red sandstones were carried out to investigate the rockbolt performance under tension load. The deformation changes on the specimen's surface and the rockbolt were recorded during the test. The strengths and failure modes of unreinforced and reinforced Brazilian disc samples were investigated. The results show that the stress of bolt increases steadily during elastic deformation process, indicating that the bolt shares the tension loading, and increases sharply when the bear capacity of specimen descends caused by crack propagation, indicating that the load is transferred from sandstone specimen. Furthermore, most of the damaged reinforced samples were still bonded by rockbolt, when the test finished. Therefore, the reinforcement mechanism can be divided into two phases. The first phase is the intact rock stage, during which the bolt shares stress reinforcing the strength. The second phase is crack propagation stage, in which the bolt restricts the crack propagation and almost bears the load. Compared with the results of unreinforced samples, it can be seen that the bolt can reduce failure of rock and the strength of the reinforced samples is enhanced. The results may provide a reference for analysis of reinforced mechanism and design of rockbolts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Rockbolts are a critical reinforcement ways which widely used in tunnel engineering. In this paper, an analytical solution of circular tunnel with rockbolts was proposed based on homogenization method, and then the stability of a circular tunnel was investigated by considering the uncertainty based on the proposed analytical solution. Elastoplastic analytical solution for unsupported circular tunnel was presented briefly in hydrostatic stress field with a linear Mohr-Coulomb yield criterion and a non-associated flow rule. An analytical solution of circular tunnel with rockbolts was proposed through considering rock mass and rockbolts as a new homogeneous, isotropic, parameters strengthened equivalent composite material. A numerical example is used to verify the proposed analytical solution. The results show that the proposed solution can effectively characterize the mechanical behavior of rock mass and rockbolts in tunnel. Then, the proposed solution is adopted to calculate reliability index and failure probability of tunnel. The results show that the proposed method can also be effectively used to perform the stability and reliability analysis of tunnel and rockbolts have an important effect on plastic zone size and displacement of tunnel. [ABSTRACT FROM AUTHOR]

Rockbolts have been used to minimize the deformation of underground excavations where the surrounding rock masses contain weak planes such as fractures, joints or faults. Rockbolts installed across rock joints are able to resist the opening and shearing movements of rock joints. One of the rockbolt failure modes encountered in the field is caused by the excessive shear loads. A simple analytical model based on the Beam on Elastic Foundation (BEM) method is proposed in this study to predict the shear responses of a bolt installed perpendicularly to rock joint. The shear load-displacement curve of a double shear test can be divided into three stages: the elastic stage, the elasto-plastic stage and the plastic stage. The foundation stiffness for each respective stage are varied with the curvature influencing zone L c . The pretension effects are taken into account in the proposed analytical model. The model agrees well with the experimental shear tests, suggesting that the analytical model has the capability to predict the shear load-displacement curve of bolts crossing rock joints. [ABSTRACT FROM AUTHOR]

This is a review paper on the performances of both conventional and energy-absorbing rockbolts manifested in laboratory tests. Characteristic parameters such as ultimate load, displacement and energy absorption are reported, in addition to load–displacement graphs for every type of rockbolt. Conventional rockbolts refer to mechanical rockbolts, fully-grouted rebars and frictional rockbolts. According to the test results, under static pull loading a mechanical rockbolt usually fails at the plate; a fully-grouted rebar bolt fails in the bolt shank at an ultimate load equal to the strength of the steel after a small amount of displacement; and a frictional rockbolt is subjected to large displacement at a low yield load. Under shear loading, all types of bolts fail in the shank. Energy-absorbing rockbolts are developed aiming to combat instability problems in burst-prone and squeezing rock conditions. They absorb deformation energy either through ploughing/slippage at predefined load levels or through stretching of the steel bolt. An energy-absorbing rockbolt can carry a high load and also accommodate significant rock displacement, and thus its energy-absorbing capacity is high. The test results show that the energy absorption of the energy-absorbing bolts is much larger than that of all conventional bolts. The dynamic load capacity is smaller than the static load capacity for the energy-absorbing bolts displacing based on ploughing/slippage while they are approximately the same for the D-Bolt that displaces based on steel stretching.

This paper represent the results of an experimental study handled with developed grouting materials for fully-cement-grouted ribbed rockbolts. These experimental studies cover the effects of some mineral admixtures (silica fume, blast furnace slag, fly ash etc.) on the bond characteristics between grout and rockbolts. The effect of mineral admixtures on the grout strength of fully-grouted rockbolts and block punch index (BPI) and compressive strength (CS) of the grout on the load bearing capacity (LBC) were investigated. Totally 150 rockbolt pull-out tests were performed using ten different grouts in order to investigate and improve the LBC of grouted rockbolts for five different curing times (1, 3, 7, 28 and 90 days). All grout types were prepared same ratios, and silica sand was used as fine aggregate. Grout mixtures that 15% of mineral admixtures replaced with cement by weight were prepared. The results showed that grouts produced with silica fume (SF) and metakaolin (MK) in all mixtures were yielded the highest CS and bond strength at all ages. The all strength tests of fly ash (FA) grout exhibited low values till 28 days of curing time in accordance with reference grout, but a sharp increase was observed after this period. Grouts with perlite (PRL) and blast furnace slag (BFS) showed the similar strength values according to reference grout. In conclusion, mineral admixtures can be used for high-strength and low cost grout in tunneling applications. In addition, the use of waste materials can contribute to the resolution of environmental problems. [ABSTRACT FROM AUTHOR]

Failure of rockbolts as a result of shear or bending loads can often be found in underground excavations. The response of rock anchorage systems has been studied in shear, both by laboratory tests as well as numerical modelling in this study. A double shear test was developed to examine the shear behaviour of a bolt installed across two joints at different angles. To investigate the influence of various parameters in the double shear test, a numerical model of a fully grouted rockbolt installed in concrete was constructed and analysed using FLAC3D code. A number of parameters were considered including concrete strength, inclination between rockbolt and joints and rockbolt diameter. The numerical model considered three material types (steel, grout and concrete) and three interfaces (concrete-concrete, grout-concrete and grout-rockbolt). The main conclusions drawn from the study were that the level of bolt resistance to shear was influenced by rock strength, inclination angle, and diameter of the rockbolt. The numerical simulation of the bolt/grout interaction and deformational behaviour was found to be in close agreement with earlier experimental test results. [ABSTRACT FROM AUTHOR]

A series of rock bolt pull tests were carried out in the laboratory to determine the critical embedment length of a specific type of fully cement-grouted rebar bolt. The rebar bolt is 20 mm in diameter, and it is widely used in underground excavations in Norway. Three water-cement (w/c) ratios were used in the tests. It was discovered that the critical embedment length of the rock bolts was approximately 25 cm for the water-cement ratio 0.40 (the corresponding uniaxial compressive strength (UCS) of the grout is 37 MPa), 32 cm for the ratio 0.46 (UCS 32 MPa), and 36 cm for the ratio 0.50 (UCS 28 MPa), for the specific type of cement, Rescon zinc rock bolt cement. It was found that the bond strength of the rock bolt is not a constant but is related to the embedment length. The bond strength was linearly proportional to the UCS of the grout. [ABSTRACT FROM AUTHOR]

Rockbolt and surface support systems such as shotcrete and thin spray-on liners (TSLs) are widely used as underground support elements to resist the convergence and maintain the stability of excavations. In order to evaluate the bearing capacity of combined reinforced rockbolt and surface support systems in preventing sliding along discontinuities, double shear tests (DST) was carried out using fully grouted rockbolts installed in three separate blocks. These blocks were covered with a 5 mm layer of TSL followed by a 50 mm layer of shotcrete. Two rockbolts were installed at an inclined angle of 45°, and 20 kN lateral constraining force was applied to clamp together the three blocks. Three different support combinations were tested: 50 mm shotcrete only, 5 mm TSL only, and combined shotcrete and TSL, with and without rockbolts. It was confirmed that the shotcrete plays a mechanical role in resisting the shear load, and TSLs increase the bond strength between shotcrete and substrate replicating the side wall of an excavation. The contribution of rockbolt and surface support system in resisting joint movement was also compared. The failure mechanism of rock substrate, rockbolt and surface support system was also analysed. [ABSTRACT FROM AUTHOR]

Finding an optimum reinforcement layout for underground excavation can result in a safer and more economical design, and is therefore highly desirable. Some works in the literature have applied topology optimization in tunnel reinforcement design in which reinforced rock is modeled as homogenized isotropic material. Optimization results, therefore, do not clearly show reinforcement distributions, leading to difficulties in explaining the final outcomes. To overcome this deficiency, a more sophisticated modeling technique in which reinforcements are explicitly modeled as truss elements embedded in rock mass media is used. An optimization algorithm extending the solid isotropic material with penalization method is introduced to seek for an optimal bolt layout. To obtain the stiffest structure with a given amount of reinforced material, external work along the opening is selected as the objective function with a constraint on the volume of reinforcement. The presented technique does not depend on material models used for rock and reinforcements and can be applied to any material model. Nonlinear material behavior of rock and reinforcement is considered in this work. Through solving some typical examples, the proposed approach is proved to enhance the conventional reinforcement design and provide clear and practical reinforcement layouts. [ABSTRACT FROM AUTHOR]

The present study applies the numerical manifold method (NMM) as a tool to investigate the rockfall hazard in underground engineering. The crack evolution technique with crack initiation and propagation criterion, which has been successfully applied to handle cracking problems in rocks, is used in this study. A rockbolt element is introduced, which is first validated by a simple case. The mechanism of the rockbolt in reinforcing a layered rock mass is then investigated through a four-layered rock beam example. The developed NMM is then used to investigate the rockfall instability caused by either natural joints or mining induced fractures in an underground power station house or a tunnel. The results illustrate that the developed NMM can not only capture the entire dynamic process of the rockfall but also locate the keyblock successfully. As such, corresponding reinforcement methods can be chosen reasonably. [ABSTRACT FROM AUTHOR]

Highlights: [•] A rockbolt model is developed within the frame work of the two-dimensional discontinuous deformation analysis (DDA). [•] The mechanism of a rockbolt relies on the anchored system and the bolt material adopted. [•] The proposed rockbolt models can predict the bonding forces and axial loads along the length of different kinds of rockbolt. [•] The D-bolt model shows its advantages in energy dissipation under dynamic loading conditions. [Copyright &y& Elsevier]

Highlights: [•] The ambiguities in factors of safety resolved in reliability analysis. [•] A transparent computational approach for the first-order reliability method. [•] Probabilistic analysis of tunnels reinforced with rockbolts. [•] Reliability-based design of rockbolts for a target reliability index. [•] Design-point of reliability analysis compared with partial factors of Eurocode. [Copyright &y& Elsevier]

• The study is the first to explore the method of correcting the stress wave velocity in the case of incomplete anchoring to deal with the problem of excessive detection errors. • To design rockbolts specimens with various anchoring ratios that will be able to simulate possible defects in construction. • Generally, for rock bolts anchored by grouting, the wave velocity of the anchoring layer material must be obtained first, and then the reasonable and the correct rockbolt length can be obtained by the method. This study examines the spectral responses resulting from the use of the impact-echo method for rockbolt length detection. A rockbolt is a thin long member that is often used in tunnels or slope protection works to fasten structures and soil/rock mass tightly together. It involves embedding a long steel bolt fully anchored to the original structure to resist strong pulling forces and stabilize the original structure or serve as a connecting member between two materials. Inadequate embedment length or poor grouting can prevent rockbolts from serving the function they were designed for and result in rockbolt failure. This study employed finite element analysis to conduct numerical simulation analyses of the stress wave signals in rockbolts and then used the impact-echo method to measure rockbolt length and grout integrity. Our results indicate that the impact-echo method can be used to determine rockbolt length but requires prior knowledge of whether the rockbolt is grouted. If not, then its detection signals will be similar to those of a bare steel bar. The rockbolt length can then be calculated using the theoretical wave velocity in a steel bar and the fundamental frequency derived from measurements. If the rockbolt is grouted, then the wave velocity in the grout material must be known so that the accurate length of the rockbolt can be derived. [ABSTRACT FROM AUTHOR]

This paper presents a damage mechanics method for the evaluation of the elastic damage of jointed rock mass and its application in predicting damage distribution in the jointed rock mass in the excavation for an underground structure in the framework of continuum damage mechanics. The damage variable is a second-order damage tensor which is assumed to describe the damage effect of jointed rock mass. The elasticity tensor defined in the model is verified by employing laboratory cement mortar specimens with various cases of joint distribution. The same specimen is analyzed using the developed damage mechanics method as well as the normal FE method, which employs joint elements to simulate jointed cracks. In order to apply the developed damage elasticity theory to three-dimensional damage analysis in underground structure excavation, a three-dimensional grouted rockbolt element is introduced, which considers the resistance of the grout to the relative shear displacement between the bolt and the rock mass around it in the axial direction. Plastic behaviors of the steel bar and the grout of the grouted bolt are considered in the model by using various constitutive laws. Finally, an engineering example for damage analysis is presented, namely an underground theater assumed to be excavated in a geological environment of jointed rock. [Copyright &y& Elsevier]

For stabilizing of rock cavern roof arches, tensioned rockbolts should be used with caution since they may have a negative influence on the stability of the arch. A proper design of rockbolts should therefore be based on a clear understanding of both the features of rockbolt reinforcement and the mechanism of the rock roof arch structure. For this purpose the mechanism of a rock cavern roof arch is studied and effects of different types of rockbolts are evaluated. Numerical modeling of rockbolt support of the Xiaolangdi powerhouse cavern as well as application of arching theory are carried out with the objective to study the effects of fully grouted rockbolts and tensioned cable anchors in reinforcing the cavern roof and walls, and on the forming of the roof arch. [Copyright &y& Elsevier]

• Deep-buried TBM-excavated coal mine roadways were supported by rockbolts. • Support schemes of installing inclined and perpendicular rockbolts were compared. • Stress concentration and damage of surrounding rocks were quantitatively analyzed. • Loads applied on rockbolts were monitored by self-developed instrumented rockbolts. There is an increasing number of TBMs applied in coal mine roadway excavation projects due to the higher safety, faster tunnelling speed and fewer labor works. Nevertheless, the experiences gained from TBM tunnelling projects of transportation or water conveyance tunnels are not well applicable in underground coal mines because of their special engineering and geological conditions. This paper proposed a case study of the excavation disturbance behaviors and support design of TBM-excavated roadway in Zhangji coal mine in Anhui Province, China. Mechanical properties of roadway surrounding rocks were obtained based on parameters of intact rocks and Rock Quality Designation (RQD) values of rock masses. The Universal Discrete Element Code (UDEC) software was used to establish numerical model of the roadway and micro-parameters are also calibrated. The excavation disturbance behaviors of unsupported roadway were firstly simulated. Moreover, the excavation disturbance behaviors and rockbolts loading characters under two different support schemes were studied and compared based on simulation and monitoring results. In this case, simulation results are in good agreement with monitoring results. Research results and experiences obtained from the working site suggest that installing rockbolts perpendicularly to rock surface can better satisfy the requirements of TBM tunnelling in weak grounds of coal mine roadways. [ABSTRACT FROM AUTHOR]

• Two new dynamic rockbolts were developed. • The rockbolts have both reinforcing and yielding capacities. • Laboratory tests were conducted to validate the design. • The rockbolts can be used for safe and rapid tunneling in bursting grounds. Strainbursts are the most frequently encountered rockburst in underground mines. Conventional approach to support strainburst-prone ground is to install rock reinforcement system using rebar and wire mesh first and then install yielding support system using dynamic rockbolts at a later stage. This two-stage rock support installation process is not safe and effective because it can adversely impact worker's safety and mine production schedule. Two new patented dynamic rockbolts for reinforcing and holding rock masses, which are called Superbolts, were developed for safe and rapid tunneling in burst-prone grounds. Laboratory testing was conducted to confirm the design. The new rockbolts are characterized by high reinforcement load capacity, high shear resistance capacity, high dynamic energy absorption capacity, and the ability to withstand repeated dynamic loadings. The new rockbolts can be used in cost-effective support systems in burst-prone grounds. Most importantly, they are quick to install and can be used in a one-pass rock support system to facilitate safe and rapid tunnel development in underground mines and increase mine safety and productivity. [ABSTRACT FROM AUTHOR]