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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.