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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]

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 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]

Physico-mechanical rock properties are typically investigated via laboratory tests using core samples. However, sample coring is time consuming and expensive. The longitudinal wave velocity in rock bolts embedded in a rock mass depends on the surrounding rock properties. Hence, the longitudinal wave velocity in rock bolts can be utilized to predict rock properties. This study presents the relationship between granite rock properties and longitudinal wave velocity (v L) in rock bolts to predict rock properties using the longitudinal wave velocity in a rock bolt. Chemical (saline solution) and mechanical (slake durability test) weathering processes are employed to diversify the properties of the rock specimens. Laboratory tests are conducted on rock specimens to measure the physico-mechanical rock properties, including the velocities (v p and v s) associated with constrained and shear moduli, density (ρ), Young's modulus (E), Poisson's ratio (μ), and porosity (η), compressive strength (f c), and slake durability index (SDI). The measured rock properties are used in the rock mass model, and variations in v L in the rock bolt with different properties are investigated via numerical simulations. Results show that v p , v s , ρ , E , and f c are correlated significantly with v L (R 2 > 0.9), whereas η and SDI are moderately correlated with v L (R 2 > 0.7). However, a meaningful correlation between μ and v L is not obtained. The root mean square and mean absolute percentage errors are estimated to validate the correlation equations, and errors are not considered. Results of the t -test show that the calculated t -value is higher than the critical t -value, and the p -value is smaller than the significance level of 0.05, indicating that the correlation coefficient is significant. This study shows that the velocity of longitudinal waves in a rock bolt can be a useful indicator for predicting in-situ rock properties. [ABSTRACT FROM AUTHOR]

A new analytical model that represents the behavior of reinforced ground near a circular underground opening in a homogeneous, uniform stress field has been developed by considering the interaction behaviors between the grouted rock bolt and ground. Axial force distribution along the rock bolt has been analyzed at first. The coupling and decoupling behaviors of the interface between the ground and rock bolt are taken into account in the model. Thereafter, the mechanical behaviors of the reinforced mass are also evaluated according to the elasto-plasticity concept and the strain softening characteristic of ground. It highlights the influence of the different bolting patterns on the extent yield zone and tunnel deformation. The derivation of the analytical model and the influence of bolts on the stress and displacement field near an opening are illustrated in this paper. The theoretical predictions are verified with the measurement data in field, e.g. a Shinkansen tunnel named as Tawara Zaga Tunnel, which is planned to be open in 2022. Moreover, the applications of the proposed approach to tunnel design are discussed further according to the rock bolting effects of the standard supporting patterns of Japan Highway Public Corporation (JH) in different ground conditions. [ABSTRACT FROM AUTHOR]

In this paper, the impact dynamics of rock bolt with passive friction control is modeled. Numerical investigations are performed for the bolts with frictional energy absorber under various dynamic loads. The results are compared with those from fully grouted rigid bolts. The studies indicate that without the frictional energy absorber, the bolts are highly susceptible to failure under dynamic impact loads, while the bolts with the fictional energy absorber significantly reduce this risk. The studies also reveal that the bolt sliding may occur at the combined effect of vibration frequency and velocity magnitude. Sliding will only initiate above certain threshold value defined by vibration frequency and velocity magnitude. Furthermore, the simulation results show that under certain impact forces when rock mass subject to only elastic deformation, bolt sliding may occur causing the detachment of bolt head and rock face, which may incapacitate the rock bolt in supporting the rock excavation. The investigation signifies that further development and appropriate design of the rock bolt system with a frictional energy absorber are critical in maintaining stability under dynamic loading. [ABSTRACT FROM AUTHOR]

Some mountain tunnels suffer from perennial deformation induced by overburden and confining stresses. Conventionally rock bolts are being used as a countermeasure. Oftentimes the conventional rock bolts fail to deliver their optimum performance, owing to several potential causes. Such as: the ground around the tunnel is too plasticized while transmitting shear forces along with the solidified mortar; and the rock bolts are poorly anchored with grout at the rear end. Considering these facts, we develop a bonding of rock bolt, named CIB-bolt, a combined mechanism deals with ground Improvement, to improve the plasticized ground while preventing further deformation of tunnel and to achieve positive bonding of rock bolts. To confirm the effectiveness of the developed rock bolt which was utilized in the experiment model, a real field application and numerical analysis also were conducted. Observed results indicate that, the developed rock bolt exhibits excellent performance in transmitting axial stresses towards the hard layers of rock. This mechanism can prevent the yielding or plasticization of adjacent ground and control the deformations of tunnel lining. [ABSTRACT FROM AUTHOR]

Steel tube is used to simulate rock mass in laboratory impact tests of rock bolts. The steel tube must be equivalent to the borehole in the rock mass in radial stiffness in order to obtain valid test results. The stiffness of the steel tube is directly associated with the thickness of the steel tube. The importance of the tube thickness has not been paid enough attention in the laboratory tests of rock bolts at present. This technical note derives a formula for calculating the required thickness of the steel tube used for laboratory tests of rock bolts. It also provides a formula for calculating the stiffness of the steel tube under plane stress which is the realistic loading condition of rock bolts in the field. The analysis reveals that the grout in the tube hole does not play a role in the calculation of the thickness of the steel tube. The required thickness of the steel tube is solely dependent on the Young's moduli and Poisson's ratios of the tube steel and the rock mass to be simulated. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a novel numerical approach to simulate the longwall mining in detail, aiming to investigate the stability of gates and the loading process to rock bolts. The study site is the Svea Nord coalmine in Svalbard. In the proposed approach, progressive cave-in and fracturing of the roof strata, consolidation of the cave-in materials and stress changes are simulated in detail. The results of the simulations are used to analyse the loading process of the rock bolts in the gates. The numerical model is calibrated with the measurement data. The simulations reveal that the stability of the gates and the loading to the rock bolts are closely related to the width of the chain pillars. With slender pillars, shear displacements along weak interlayers and bedding planes results in heavy loading to the rock bolts. Therefore, the locations of weakness zones should be taken into account for rock bolt design. The proposed numerical approach can be used to find an optimal combination of the dimension of the chain pillars and the rock reinforcement in the gates. [Copyright &y& Elsevier]

Abstract: An energy-absorbing rock support device, called a D bolt, has been recently developed to counteract both burst-prone and squeezing rock conditions that occur during underground excavation. The bolt is a smooth steel bar with a number of anchors along its length. The anchors are firmly fixed within a borehole using either cement grout or resin, while the smooth sections of the bolt between the anchors may freely deform in response to rock dilation. Failure of one section does not affect the reinforcement performance of the other sections. The bolt is designed to fully use both the strength and the deformation capacity of the bolt material along the entire length. The bolt has large load-bearing and deformation capacities. Static pull tests and dynamic drop tests show that the bolt length elongates by 14–20% at a load level equal to the strength of the bolt material, thereby absorbing a large amount of energy. The impact average load of a 20mm D bolt is 200–230kN, with only a small portion of the load transferred to the bolt plate. The cumulative dynamic energy absorption of the bolt is measured to be 47kJ/m. D bolts were tested in three deep mines. Filed measurements show that D bolts are loaded less than rebar bolts. This paper presents the layout and principle of the D bolt, and corresponding results from static, dynamic, and field tests. [Copyright &y& Elsevier]

Abstract: To ensure safety in underground excavations, it is important that the support systems used are capable of resisting the dynamic loads produced, for example, by rock bursts. In this paper, a dynamic simulation model for a cone bolt is proposed based on an experimental study. Drop weight tests were performed on resin-based cone bolts. These experiments revealed that the bolt has two energy absorption mechanisms: sliding in the resin and plastic deformation. To simulate this behaviour, a two degrees-of-freedom lumped-mass model is proposed. Experimentally, the proportions of sliding and plastic deformation were found to vary significantly from one test to another. To account for this variability, two methods are proposed to determine the value of the parameters governing the sliding of the bolt in the resin, whereas a dynamic force–elongation model is used to simulate the plastic deformation. Comparing the results of a simulation to experimental data proved that the constitutive elements of the model are appropriate to simulate the dynamic response of the cone bolt. [Copyright &y& Elsevier]

Abstract: The shotcrete–rock interaction is very complex and is influenced by a number of factors. The influence of the following factors was investigated by a series of numerical analyses: the surface roughness of the opening, the rock strength and Young''s modulus, the discontinuities, the extent and properties of the excavated disturbed zone, the mechanical properties of the interface between shotcrete and rock, and the thickness of the shotcrete lining and the rock bolts. The study was carried out as a sensitivity analysis. The results showed that the rock strength and the surface roughness had significant impact on the number of failures at the rock–shotcrete interface and in the shotcrete lining. Furthermore, the behaviour of the lining is sensitive to small amplitudes of the surface roughness. In all the cases investigated, a high interface strength was favourable. The results indicate that if a thick shotcrete lining is dependent on the bond strength. The benefit of using a thicker lining can be doubtful. The analyses showed that for an uneven surface the extent of the EDZ had a minor effect on the behaviour of the shotcrete lining. Furthermore, if rock bolts were installed at the apex of the protrusion instead of at the depression, the number of failures decreased both at the interface and in the lining. [Copyright &y& Elsevier]

Abstract: Rock bolting is the most effective and also the most economical means of supporting excavations in rock. Various types of bolts are used today, and an understanding of the way in which these bolts work is essential for an optimal, safe, and economical use. Fully grouted, untensioned bolts have been commonly used in rock mechanics (i.e., mines, rock fall stabilisation, underground works) for many years. In the 1980s a new type of bolt, called Swellex, was developed, becoming more and more widespread because of their easy and fast installation. However, regardless of the type, the mechanical behaviour of the bolted rock joint is not fully understood, and only the experience accumulated on rock bolting gives the know-how for the reinforcement calculation and execution. In this paper the different mechanical responses of full steel bars as opposed to the frictional Swellex are discussed. The study was done through experimental tests coupled with numerical simulations. The analysis of the results obtained both from finite element (FEM) modelling, and from large-scale (1:1) shear tests on rock joints, reinforced with fully grouted rods and Swellex bolts, clearly shows that the two bolt types deform in dissimilar ways, responding very differently to shear load. [Copyright &y& Elsevier]

The research outlined in this paper is aimed at the development of a portable non-destructive testing instrument for evaluating the condition of rock bolts. In applications such as coal mine roof reinforcement, the opportunities for rock bolt inspection are currently limited to destructive techniques such as the pull-out test. It is proposed that guided ultrasonic waves can be used to solve this inspection problem, using a pulse-echo test carried out from the free end of the bolt. Suitable test frequencies have been identified through the use of modelling software, and successful laboratory and site trials have been undertaken. The research has shown that the proposed approach is capable of determining the bolt length, and of identifying major defects such as necking, deformation, and loss of resin encapsulation. [Copyright &y& Elsevier]

To analyze rock-bolt dynamic mechanics, one theoretical model has been put forward based on several assumptions. At the interface of the bolt and surrounding rock-soil body, the coupling of a linear spring and a damper is used for the bolt system. The coefficient k of the spring and the damper coefficient η are key parameters between the rock-soil body and bolt. The elastic and viscoelastic kinematic equations are established, and the displacement control equations are binary second-order partial-differential equations that can be simulated by numerical methods. One example shows the displacement curves with input waves; the displacement decays with different distributions. The curves of different times were simulated, and it was found that as the ratio value of the coefficient k of the spring and elastic modulus E increases, the displacement-decay-ratio speed increases per unit length. The same is true for the ratio values of the damper coefficient η and elastic modulus E. The theoretical model can also be used to solve the impact-force problems. [ABSTRACT FROM AUTHOR]

Rock bolts have been widely used to reinforce the jointed rock mass. Modelling the mechanical shear behaviors of the bolted rock joints are difficult due to the complex interactions between bolts and rock joints. The applied pretension forces combined with the axial loads developed in the bolt act as the normal forces which are applied to the rock joints. However, these combined normal forces are not considered in the existing analytical models. An analytical model is proposed in this study to predict the shear behavior of the bolted rock joints, by taking into account the pretension forces, the axial forces developed in the bolt, the interfacial bond stress between the bolt and grout, and dowel shear loads acting transversely to the bolt axis. The proposed analytical model is able to provide complete curves of the dowel shear loads, axial loads, and the global shear loads as a function of the joint shear displacement. The analytically predicted axial load vs shear displacement curves and the global shear load vs shear displacement curves are verified by available experimental tests. The validation shows that the proposed model has the capacity to predict the global shear load evolution as well as the axial load evolution. The factors such as the pretension forces, the bolt inclination angles, the concrete strength and the rock joint friction are successfully accounted for in the analytical model. [ABSTRACT FROM AUTHOR]