Search

Bearing plates made from plastic composites can be used as an alternative to their steel counterparts in rock bolt or soil nail applications. To achieve this goal, an existing recycled high-density polyethylene bearing plate was investigated and later modified to improve its engineering properties. Laboratory studies were conducted to determine the failure load of the existing and modified plates, and a numerical model was developed for complementary analysis. The results of both efforts clearly showed that the existing bearing plate was not adequate in terms of strength and creep properties, as it quickly yielded with large displacements at relatively low loads. In order to enhance the strength of the plate, both geometric and material modifications are made by our research group to obtain a more efficient plate. Numerical models were used to determine the frame layout, and a series of analyses were performed to evaluate the effects of frame thickness, number and arrangement. Once the design was optimized and finalized, a mold was created to match the new geometry for manufacturing new plates through injection molding. A test setup was also established in the laboratory and numerous compression tests were performed on the manufactured new plates. The measured load-displacement behavior of plates made of polyethylene and polyamide with a variety of additives were discussed separately. It was determined that the new plastic plates reinforced with polyamide through various additives have the potential to reach a strength up to 200–240 kN, which is at least two times higher than the existing one, with distinct economic advantages.

Layered composite roofs are characterized by developed bedding fissures, resulting in severe deformation and damage of rock bolts at the top of the roadway, as well as a poor roadway support effect. Increasing pretension force is an effective way to enhance the stiffness of the rock bolt support system. To clarify the influence and mechanism of the pretension force on the support effect of rock bolts in the layered roof, a roadway model of the layered roof was established using the interface unit of FLAC3D, and the simulation rock bolts were constructed using the pile unit, which can simulate the mechanical behaviors of rock bolts, such as tension, shear, bending, fracture, and anchor failure, and the pretension force was applied. On this basis, the deformation and failure characteristics and influencing factors of rock bolts in the layered roadway roof under different surrounding rock conditions were simulated and analyzed. The research shows the following: ① Field measurements showed minor shear deformation in the rock bolts at the center of the roadway roof, with lateral displacements of 5.7 cm and 5.3 cm. Significant shear deformation occurred in the rock bolts at the roof corners, with lateral displacements of 18.2 cm and 17.6 cm. ② Simulations of rock bolt deformation characteristics matched the field measurements closely, confirming the reliability of the simulation method, parameter selection, and calculation sequence. ③ The primary factors affecting rock bolt deformation are the structural plane strength and surrounding rock strength. Rock bolts are most susceptible to lateral displacement when the structural plane strength is low, the strength difference between rock layers is large, and the weaker layer is below the structural plane. The presented research can provide a reference for the instability mechanism and support treatment of the layered composite roof roadway.

The interaction mechanism between coal and rock masses with supporting materials is significant in roadway control, especially in deep underground mining situations where dynamic hazards frequently happened due to high geo-stress and strong disturbed effects. This paper is to investigate the strain energy evolution in the interaction between coal and rock masses with self-designed energy-absorbing props and rock bolts by numerical modeling with the finite difference method. The interaction between rock and rock bolt/prop is accomplished by the cables element and the interface between the inner and outer props. Roadway excavation and coal extraction conditions in deep mining are numerically employed to investigate deformation, plastic zone ranges, strain energy input, accumulation, dissipation, and release. The effect on strain energy input, accumulation, dissipation, and release with rock deformation, and the plastic zone is addressed. A ratio of strain energy accumulation, dissipation, and release with energy input α, β, γ is to assess the dynamic hazards. The effects on roadway excavation and coal extraction steps of α, β, γ are discussed. The results show that: (1) In deep high geo-stress roadways, the energy-absorbing support system plays a dual role in resisting deformation and reducing the scope of plastic zones in surrounding rock, as well as absorbing energy release in the surrounding rock, especially in the coal extraction state to mitigate disturbed effects. (2) The strain energy input, accumulation is dependent on roadway deformation, the strain energy dissipation is relied on plastic zone area and disturbed effects, and strain energy release density is the difference among the three. The function of energy-absorbing rock bolts and props play a key role to mitigate strain energy release density and amount, especially in coal extraction condition, with a peak density value from 4×104 to 1×104 J/m3, and amount value from 3.57×108 to 1.90×106 J. (3) When mining is advanced in small steps, the strain energy accumulation is dominated. While in a large step, the released energy is dominant, thus a more dynamic hazards proneness. The energy-absorbing rock bolt and prop can reduce three times strain energy release amount, thus reducing the dynamic hazards. The results suggest that energy-absorbing props and rock bolts can effectively reduce the strain energy in the coal and rock masses, and prevent rock bursts and other hazards. The numerical model developed in this study can also be used to optimize the design of energy-absorbing props and rock bolts for specific mining conditions.

This study aims to inspect grout defects in rock bolts using electromagnetic (EM) waves. The experiments are conducted using one intact rock bolt and eight defective rock bolts with defect ratios (DR) of 10–50%. A two-conductor transmission line is configured using a pair of rock bolts to transmit EM waves. The EM waves are generated and detected using a time-domain reflectometer. Consequently, the velocity of the EM waves increases with increasing DR. Additionally, almost identical velocities are observed at the same DR, regardless of the defect locations. Moreover, the EM wave reflections are observed at these defects. The defect locations are estimated using the velocity and reflection time of the defects. The estimated location is in good agreement with the actual location. This study demonstrates that EM waves can be an effective tool for evaluating grout defects in rock bolts.

The longwall system is an extraction system commonly used in coal mining in many countries, including Poland. One of the methods for reducing extraction costs is the dual use of the gateroad. In the first instance, the gateroad serves as the tailgate, and during the exploitation of the second coal panel, it functions as the headgate. Such a situation requires maintenance of the roadway behind the longwall face, which is typically challenging, due to significant stress-related loads on the support and its substantial deformation. The support design for this kind of roadway should take into consideration the dual impact of exploitation pressure and the caved zone influence behind the longwall face. This article presents the results of in-situ research conducted on two roadways behind the longwall face. In both roadways, the effectiveness of specially designed steel arch frames and rock bolt patterns were examined to minimize roadway deformations and maintain their functionality. The research project was comprised of several stages. Initially, mining and laboratory studies were conducted to determine the geomechanical parameters of the rocks. Subsequently, excavation stability and functionality forecasts were performed based on the authors’ empirical indicators. Then, numerical analyses were carried out to design support schemes (steel arches and rock bolt) in both roadways. A fully automated monitoring system with programmed data loggers was designed to check the behaviour of a specific rock mass and the support elements. The load on the steel arch support was measured with the help of load cells, while the load on the rock bolt support was carried out with the help of measurement bolts. Behind the longwall face, the loads on the wooden cribs set from the goaf side were also monitored. Additionally, the measurement station was equipped with extensometers to monitor the movement of roof layers and stress meters to determine changes in rock mass stress. Laser scanning or traditional surveying methods were also used to verify the support schemes through roadway convergence measurements. The obtained results allowed us to draw conclusions regarding the optimization of support schemes and to give recommendations for the practical application of specific reinforcements in excavations maintained behind the longwall face.

The arched rock bolt support plate is one of the key components in the rock bolting system, and its structural size and strength directly affect the rock bolting effect. In this paper, using the method of combining indoor experiments and numerical simulation, the diffusion evolution law of anchoring force in the interior of rock mass at different loading stages of arched rock bolt support plate is studied and analyzed, and secondly, the sensitivity of the diffusion evolution of anchoring force diffusion to the arch height and radius of arched rock bolt support plate is analyzed by using orthogonal experiment theory. The study found that: the anchoring force is transmitted to the rock mass through the tray, and the diffusion forms a “stress bubble” type secondary stress field; if the anchoring force is less than the bearing strength of the rock bolt support plate, the secondary stress field range increases with the increase of the anchoring force, and vice versa; factors such as the arch radius and arch height of the arched rock bolt support plate have a significant impact on the secondary stress field range, but the primary and secondary order of sensitivity is the arch radius > the arch height.

Abstract The problem of proper assessment of the technical functionality of rock bolt support systems is still valid. Many research centers have undertaken efforts to diagnose and monitor the technical state of such a support system used in mines and tunneling. With that aim the method of quality assessment of grouted rock bolts was invented and a relevant apparatus was constructed. The method concerns non-destructive identification of discontinuity of a resin layer (grout) surrounding rock bolts. The method is based on an impact excitation of a rock bolt and uses modal analysis procedures. Assuming that the installed rock bolt acts as an oscillator, different lengths and positions of grouting discontinuity alter its modal parameters. The extraction of these modal parameters, of which a resonant frequency is seen as the most valued, enable the relevant identification of grout discontinuity. After constructing a prototype version and validating the results for known cases of resin discontinuity in an experimental coal mine, the apparatus fulfilling ATEX requirements was developed. Subsequently that version was also verified both in laboratory conditions and in an experimental coal mine. As necessary for proper identification of discontinuity length, the reference data base was developed and elaborated consisting of a very large number of finite element models (FE models), namely discontinuity cases. The models encountered different rock bolt lengths and diameters, different rock strata parameters and different positions and lengths of resin layers. Then the method was used in a working coal mine to monitor a technical state of rock bolt support system mounted to reinforce long underground openings. The data base was utilized as reference for investigated rock bolts.

Grouted rock bolts subject to axial loading in the field exhibit various failure modes, among which the most predominant one is the bolt-grout interface failure. Thus, mechanical characterization of the grout is essential for understanding its performance in ground support. To date, few studies have been conducted to characterize the mechanical behaviour of fiber-reinforced grout (FRG) in rock bolt reinforcement. Here we experimentally studied the mechanical behaviour of FRG under uniaxial compression, indirect tension, and direct shear loading conditions. We also conducted a series of pullout tests of rebar bolt encapsulated with different grouts including conventional cementitious grout and FRG. FRG was developed using 15% silica fume (SF) replacement of cement (by weight) and steel fiber to achieve high-strength and crack-resistance to overcome drawbacks of the conventional grout. Two types of steel fibers including straight and wavy steel fibers were further added to enhance the grout quality. The effect of fiber shape and fiber volume proportion on the grout mechanical properties were examined. Our experimental results showed that the addition of SF and steel fiber by 1.5% fiber volume proportion could lead to the highest compressive, tensile, and shear strengths of the grout. The minimum volume of fiber that could improve the mechanical properties of grout was found at 0.5%. The scanning electron microscopy (SEM) analysis demonstrated that steel fibers act as an excellent bridge to prevent the cracks from propagating at the interfacial region and hence to aid in maintaining the integrity of the cementitious grout. Our laboratory pullout tests further confirmed that FRG could prevent the cylindrical grout annulus from radial crack and hence improve the rebar's load carrying capacity. Therefore, FRG has a potential to be utilized in civil and mining applications where high-strength and crack-resistance support is required.

Full-length grouted bolts play a crucial role in geotechnical engineering thanks to their excellent stability. However, few studies have been concerned with the degrading performance of grouted rock bolts caused by extensive and continuous heat conduction from surrounding rocks in high-geothermal tunnels buried more than 100 m (temperature from 28 °C to 100 °C). To investigate the damage mechanism, we examined the time-varying behaviors of grouted rock bolts in both constant and variable temperature curing environments and their damage due to the coupling effects of high temperature and humidity through mechanical and micro-feature tests, including uniaxial compression test, pull-out test, computed tomography (CT) scans, X-ray diffraction (XRD) test, thermogravimetric analysis (TGA), etc., and further analyzed the relationship between grout properties and anchorage capability. In order to facilitate a rapid assessment and control of the anchorage performance of anchors in different conditions, results of the interface bond degradation tests were correlated to environment parameters based on the damage model of interfacial bond stress proposed. Accordingly, a thermal hazard classification criterion for anchorage design in high-geothermal tunnels was suggested. Based on the reported results, although high temperature accelerated the early-stage hydration reaction of grouting materials, it affected the distribution and quantity of hydration products by inhibiting hydration degree, thus causing mechanical damage to the anchorage system. There was a significant positive correlation between the strength of the grouting material and the anchoring force. Influenced by the changes in grout properties, three failure patterns of rock bolts typically existed. Applying a hot-wet curing regime results in less reduction in anchorage force compared to the hot-dry curing conditions. The findings of this study would contribute to the design and investigations of grouted rock bolts in high-geothermal tunnels.

Aims at the problem that the rock bolt support plate is seriously damaged due to the uneven surface of surrounding rock in the fast tunneling of shallow buried roadway with large section and causes the support failure, the coordinated action mechanism of the bolt support plate composite component was studied by combining laboratory test, numerical simulation and theoretical analysis, and the mechanical model of the bolt support plate composite component was constructed to analyze the force evolution characteristics of the bolt support plate and the self-aligning gasket under partial load and the supporting effect of roadway under different bolt support plate combinations. The industrial test results of large section open-cut roadway of Xiaobaodang working face 112207 show that when the coordination of bolt support plate composite components is good, the stress concentration degree of bolt tail can be effectively reduced, the bolt preload can be improved, the bolt tail is not prone to damage and deformation, and it is helpful to apply the bolt preload, and the support effect is better. The maximum concentrated stress of the supporting plate under the condition of locking each other is greater than that under the condition of good coordination of the supporting plate.

Layered composite roofs are characterized by developed bedding fissures, resulting in severe deformation and damage of rock bolts at the top of the roadway, as well as a poor roadway support effect. Increasing pretension force is an effective way to enhance the stiffness of the rock bolt support system. To clarify the influence and mechanism of the pretension force on the support effect of rock bolts in the layered roof, a roadway model of the layered roof was established using the interface unit of FLAC3D, and the simulation rock bolts were constructed using the pile unit, which can simulate the mechanical behaviors of rock bolts, such as tension, shear, bending, fracture, and anchor failure, and the pretension force was applied. On this basis, the deformation and failure characteristics and influencing factors of rock bolts in the layered roadway roof under different surrounding rock conditions were simulated and analyzed. The research shows the following: ① Field measurements showed minor shear deformation in the rock bolts at the center of the roadway roof, with lateral displacements of 5.7 cm and 5.3 cm. Significant shear deformation occurred in the rock bolts at the roof corners, with lateral displacements of 18.2 cm and 17.6 cm. ② Simulations of rock bolt deformation characteristics matched the field measurements closely, confirming the reliability of the simulation method, parameter selection, and calculation sequence. ③ The primary factors affecting rock bolt deformation are the structural plane strength and surrounding rock strength. Rock bolts are most susceptible to lateral displacement when the structural plane strength is low, the strength difference between rock layers is large, and the weaker layer is below the structural plane. The presented research can provide a reference for the instability mechanism and support treatment of the layered composite roof roadway. [ABSTRACT FROM AUTHOR]

An often-utilized solution in terms of providing support to underground excavation, the fully grouted rebar rock bolt system presents optimization potential due to existing technological limitations in capturing and understanding its composite system response. In order to address these limitations, the development and application of Distributed Optic Fiber Sensing (DOS) allows for continuous strain monitoring (at a spatial resolution of 0.65 mm utilizing the technique defined herein) across a full spectrum of loading. A robust laboratory investigation was conducted featuring 24 rock bolt specimens. This examined the effects of two selected independent variables: rib spacing (from 13 mm to 68 mm) and grout annulus (from 7.7 mm to 22.8 mm). This body of research provides valuable insight into the performance of grouted rebar rock bolts and the effects of the selected parameters (rib spacing and size of the grout annulus), while also highlighting an advanced monitoring technique. Results indicated that rib spacing was a negative predictor of bond performance. No definite conclusions were drawn in terms of the effect of the size of grout annulus; however, findings provide limited support for an optimal sizing in relation to rib height. The results were also compared to analytic and numerical models. These insights can aid in calibrating and validating numerical models, and improve monitoring and rock bolt design within the overall goal of improving and optimizing ground support design arrangements.

A parametric study was conducted to determine the influence of different bolt parameters (bolt length and diameter) on the maximum induced boundary displacements in jointed rockmass, using a numerical method based on finite element code. Three different types of jointed systems (Type A, Type B, and Type C) were considered and analyzed. Type A displays normal joint closure, Type B demonstrates a combination of normal and shear joint closure, and in Type C, the majority of the joint is loaded in shear condition. The ground surface settlement profiles for these three joint systems, under unsupported conditions, are presented in this work. For the same rockmass properties, the cases in Type A and Type B exhibit the maximum and minimum settlement, respectively. For each joint type, the effect of variable bolt parameters on the tunnel boundary is studied in terms of the total displacement. The simulation results confirm that an increase in the bolt length does not significantly reduce the boundary displacement, whereas an increase in the bolt diameter substantially reduces the tunnel boundary displacement.

Rock bolts have been widely used to enhance the structural stability of underground infrastructures. Careful tracking of rock bolt positions is highly significant since it assists with operational success of ground support and has applications to predictive maintenance practices. This paper presents a practical algorithm, CFBolt, to detect rock bolts from a 3D laser scanned point cloud. Considering that rock bolts are relatively tiny objects, CFBolt follows a two-step coarse-to-fine strategy. It first computes a single-scale proportion of variance (POV) for each point as the local point descriptor and filters out near 95% not-bolt points with a simple but effective classifier, Linear Discriminant Analysis (LDA), which allows for the pruned point cloud to be then used as a compatible input to a deep neural network, designed and trained to precisely detect rock bolts from the pruned point cloud. CFBolt was tested for detecting rock bolts from LiDAR scan data collected from Sydney’s civil tunnelling project site. The entire dataset contains more than 160 million points. The obtained scores of Intersection over Union (IoU) and precision for individual bolt points were 89.33% and 92.04%, respectively. For rock bolt objects, the precision and recall were 98.34% and 98.73%, respectively. The detection quality of CFBolt is superior to the state-of-the-art 3D object detection algorithms and the newest rock bolt detection algorithm, demonstrating the robustness and effectiveness of CFBolt.

Previous studies rarely involved the mechanical properties of anchorage system under the condition of internal absolute displacement of surrounding rock and the solution of important parameters of anchorage system. In this paper, the absolute displacement law of rock mass under the anchorage system is analyzed by the fiber grating (FBG) multi-point displacement meter and multidirectional displacement conversion formula. Based on the coordinated deformation principle for rock bolt and rock mass, the mechanical analysis model for rock bolt-rock mass system is established, and the influence rules of rock bolt length, rock bolt diameter and rock bolt preload on the internal force distribution of rock bolt is studied based on the stress analysis of full-length rock bolt in a tunnel engineering. The results show that: (1) with the increase of rock bolt length, the anchoring effect gradually enhances, but the improvement degree decreases gradually; (2) with the increase of rock bolt diameter, the anchoring effect is enhanced; at the same time, however, the shear stress of rock bolt is larger, and the possibility of rock bolt sliding also increases; and (3) with the increase of preload, the increase of “anchor length” makes the anti-sliding ability of rock bolt increase correspondingly, and the supporting effect of rock bolt increases gradually. Then the full-process analysis for the mechanical mechanism of rock bolt sliding failure and the iterative calculation method for rock bolt displacement are studied. At last, the numerical simulation for pull-out test is carried out through the finite element analysis to verify the calculation results of mechanical model, and according to the iterative operation, the two important parameters such as the interface shear stiffness K and the interface peak shear strength τP are solved. The analysis show that the numerical simulation results are in good agreement with the mechanical model calculation. This study can provide a theoretical basis for rock bolt support technology and support structure design.

In order to study the influence of the number of cables on the simulation results when the same length of the bolt is prepared, the bolt is created by using the Cable structure element in FLAC3D, and the simulation experiment is carried out under the same surrounding rock properties, mesh density and boundary conditions. The effect of the number of cables and the effect of grout stiffness was further explored. The results show that the recommendation to set the number of cables according to the development length in FLAC3D help manual is not reliable. When the same length of the bolt is divided into different numbers of cables, the simulation results are significantly different. When the length of a single cable is consistent with the length of the mesh or shorter than the mesh, the bolt can obtain a large pulling axial force. The ultimate pull-out force of the bolt can be determined according to the on-site pull-out test, and then a single cable length is set to be between 0.5 and 1 times the length of the grid in the numerical simulation to find a simulation scheme consistent with the field test. The change of grout stiffness has no significant effect on the peak force of the bolt axial force, but it will change the post-peak variation characteristics of the axial force.

A D bolt, an energy-absorbing rock bolt, is a smooth steel bar with a number of anchors along its length. The anchors, which can be spaced evenly or unevenly along its length, 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. A series of numerical simulations have been conducted using the finite difference method to investigate the effects of D bolt on the displacement increase of rock mass around a roadway in comparison with normal fully encapsulated rebar. As a result, the displacement of 49 mm at the top of roadway roof in the D bolts supported model is much larger than 30.08 mm in the fully encapsulated rebar bolts supported model so that the former is capable of absorbing potential deformation energy of rock mass around a roadway to tolerate the large deformation of rock. Plans of spacing arrangement of D bolt’s anchor have a significant effect on stress redistribution of the bolt. The numerical simulation result shows that for the D bolt with its whole length of 2.4 m, the length of its exposed section of 0.1 m, and the 4 anchors with the length of 0.1 m, the maximum tensile stress of 3.25 GPa generated in the D bolt with the ratio of the spacing between anchors (RSA) of 30 : 40 : 50 : 70 is lower about 1.13–1.31 times than the other D bolts with different ratio of spacing, and the changing range of stress is also the smallest, where the ratio of 30 : 40 : 50 : 70 indicates a ratio of lengths of deformable sections which is determined by turns from the innermost section of rock mass around roadway to the outermost section of roadway space. This study demonstrates that it is reasonable to employ the RSA of D bolt which makes it bring out its energy-absorbing capability to the full.

Abstract Hollow grouted rock bolts are widely used in deep soft rock roadways. The density of grout has a significant influence on the anchorage quality. However, there is still a lack of effective methods to detect the grouting quality fast, accurately, and extensively. Based on the wave velocity and the law of stress wave transmission and reflection in the impedance mismatch, non-destructive experiment focused on the defect in the anchorage was conducted. The wavelet transform was used to analyze the test signals. The test results show that the free length of hollow grouted rock bolt could be accurately detected through the analysis of the stress wave signal by using the low-pass filter in the wavelet toolbox. The grouted length, defect length, and specific position of the anchorage section can be accurately detected through the analysis of the stress wave signal by using the high-pass filter in the wavelet toolbox. The research can provide experiment foundation for further study of grouting quality based on the non-destructive methods.

At present, the suspended monorail systems constitute a very common means of transportation in the Polish hard coal mines. The main advantages of the suspended monorail include the independence of the route from the working floor surface irregularities and the possibility to transport cargo of significant mass and size.

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.