The influence of rockbolt pretension on bolting has not been well addressed,despite its critical importance in drift support systems.In this study,laboratory and numerical simulations of gravel bolting are conducted t...The influence of rockbolt pretension on bolting has not been well addressed,despite its critical importance in drift support systems.In this study,laboratory and numerical simulations of gravel bolting are conducted to investigate the effects of varying rockbolt pretensions.The simulations are developed using the particle flow code(PFC3D),enabling detailed analysis of contact forces between gravel particles under low and high rockbolt pretensions.The results indicate that bolted gravel can maintain stability even without pretension,though bearing capacity is significantly enhanced under high pretension.Two distinct bolting behaviors are identified:a pressure arch structure is formed under low pretension,while high pretension creates a compression zone characterized by intensified particle interlocking and superior load-bearing capacity.Based on these findings,a concept for drift support is proposed,integrating rockbolts and cables to stabilize both shallow and deep rocks.This study advances our understanding of bolting behaviors and provides theoretical guidance for designing effective drift support systems in practical applications.展开更多
To better understand the failure behaviours and strength of bolt-reinforced blocky rocks,large scale extensive laboratory experiments are carried out on blocky rock-like specimens with and without rockbolt reinforceme...To better understand the failure behaviours and strength of bolt-reinforced blocky rocks,large scale extensive laboratory experiments are carried out on blocky rock-like specimens with and without rockbolt reinforcement.The results show that both shear failure and tensile failure along joint surfaces are observed but the shear failure is a main controlling factor for the peak strength of the rock mass with and without rockbolts.The rockbolts are necked and shear deformation simultaneously happens in bolt reinforced rock specimens.As the joint dip angle increases,the joint shear failure becomes more dominant.The number of rockbolts has a significant impact on the peak strain and uniaxial compressive strength(UCS),but little influence on the deformation modulus of the rock mass.Using the Winkler beam model to represent the rockbolt behaviours,an analytical model for the prediction of the strength of boltreinforced blocky rocks is proposed.Good agreement between the UCS values predicted by proposed model and obtained from experiments suggest an encouraging performance of the proposed model.In addition,the performance of the proposed model is further assessed using published results in the literature,indicating the proposed model can be used effectively in the prediction of UCS of bolt-reinforced blocky rocks.展开更多
This article introduces the principles of underground rockbolting design.The items discussed include underground loading conditions,natural pressure zone around an underground opening,design methodologies,selection of...This article introduces the principles of underground rockbolting design.The items discussed include underground loading conditions,natural pressure zone around an underground opening,design methodologies,selection of rockbolt types,determination of bolt length and spacing,factor of safety,and compatibility between support elements.Different types of rockbolting used in engineering practise are also presented.The traditional principle of selecting strong rockbolts is valid only in conditions of low in situ stresses in the rock mass.Energy-absorbing rockbolts are preferred in the case of high in situ stresses.A natural pressure arch is formed in the rock at a certain distance behind the tunnel wall.Rockbolts should be long enough to reach the natural pressure arch when the failure zone is small.The bolt length should be at least 1 m beyond the failure zone.In the case of a vast failure zone,tightly spaced short rockbolts are installed to establish an artificial pressure arch within the failure zone and long cables are anchored on the natural pressure arch.In this case,the rockbolts are usually less than 3 m long in mine drifts,but can be up to 7 m in large-scale rock caverns.Bolt spacing is more important than bolt length in the case of establishing an artificial pressure arch.In addition to the factor of safety,the maximum allowable displacement in the tunnel and the ultimate displacement capacity of rockbolts must be also taken into account in the design.Finally,rockbolts should be compatible with other support elements in the same support system in terms of displacement and energy absorption capacities.展开更多
Rockbolt is widely employed all over the world as an effective ground reinforcement element in order to secure the underground workplaces.Ordinary encapsulated rebar or rebar rockbolt is most popular and commonly used...Rockbolt is widely employed all over the world as an effective ground reinforcement element in order to secure the underground workplaces.Ordinary encapsulated rebar or rebar rockbolt is most popular and commonly used as reinforcement in a ground support system because of its accessibility,cost effectiveness and easy practicability.Reinforcement elements in a seismic condition such as rock burst have to dissipate the energy release of the dynamic impact via their deformation and ultimate load capacity,knowing that the former is more important.In other words,achieving early stiff behaviour along with large deformation capacity in rockbolts are the goals for new development in rock reinforcement.Yielding rockbolts are expensive while some of them have large deformation capability with low ultimate load capacity.In this paper,modifications were made on encapsulation of rebar rockbolts to utilise it effectively as a yielding reinforcement in seismic conditions.Applying a sufficient decoupled length in the shank of rebar rockbolts which industry has regularly been using to control the bulking of the stress fractured ground,improves the deformation capacity of the bolt.Additionally,leaving a collar bonding underneath of the bearing pad and plate removes the weaknesses of the head anchorage of rockbolt.Therefore the dynamic performance of the bolt is improved by these easily applicable modifications.The behaviour and performance of encapsulated rockbolts have been discussed first,then the effects of modifications are illustrated.The proposed modification of the rebars is not only cost effective but also easy to apply in the field and improves the performance of reinforcements in seismic prone zones.展开更多
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 re...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.展开更多
This paper presented a novel bond-slip model to better reveal the mechanical behavior of the bolt-grout interface for fully-grouted rockbolts under tensile loads by considering the non-linear response in the softening...This paper presented a novel bond-slip model to better reveal the mechanical behavior of the bolt-grout interface for fully-grouted rockbolts under tensile loads by considering the non-linear response in the softening stage.The exponential decay function is adopted for describing the non-linear response in the softening stage.Based on the improved bond-slip model,the corresponding analytical solutions for the interfacial shear stress and the axial force of the bolt under different loading stages are solved.Then,the validity of this proposed model was verified by comparing with the experimental results.The results show that compared with the linear softening model,the proposed model is more suitable for predicting the mechanical performance of fully-grouted rockbolts.Finally,a series of parametric studies are conducted to explore the effect of model parameters on the mechanical properties of fully-grouted rockbolts.The results indicate that compared with the anchor length,the bolt diameter and the bond strength of the bolt-grout interface have a significance influence on the ultimate load of bolt,especially for the elastic and softening stage.Moreover,it can be found that using the linear softening model maybe overestimates the supporting performance of grouted bolt,resulting in an unsafe design for bolt.展开更多
Harmonic wavelets not only possess the traditional advantages of a wavelet function,they also have other merits such as clear expressions,more flexible time-frequency divisions,a simple transformation algorithm,a fine...Harmonic wavelets not only possess the traditional advantages of a wavelet function,they also have other merits such as clear expressions,more flexible time-frequency divisions,a simple transformation algorithm,a finer box-like frequency spectrum and others.Given the frequency distribution characteristics of the nondestructive testing signals from a rockbolt support system and based on the discrete harmonic wavelet transformation theory,we have effectively abstracted signals from frequency ranges concerned by removing useless high and low frequency signals from the testing signals of the rockbolt support system and obtained filtered signals with a reconstruction algorithm of harmonic wavelets.Finally,we applied the harmonic wavelet transformation in filtering analog signals and measured response signals of rockbolts.The results indicate that harmonic wavelets also have excellent filtering characteristics.展开更多
Energy-absorbing rockbolts have been widely adopted in burst-prone excavation support, and their serviceability is closely related to the frequency and magnitude of seismic events. In this research, the splittube drop...Energy-absorbing rockbolts have been widely adopted in burst-prone excavation support, and their serviceability is closely related to the frequency and magnitude of seismic events. In this research, the splittube drop test with varying impact energy was conducted to reproduce the dynamic performance of MP1rockbolts under a wide range of seismic event magnitudes. The test results showed that the impact process could be subdivided into four distinct stages, i.e. mobilization, strain hardening, plastic flow(ductile), and rebound stage, of which strain hardening and plastic flow are the primary energy absorbing stages. As the impact energy per drop increases from 8.1 to 46.7 k J, the strain rate of the shank varies between 1.20 and 2.70 s^(-1), and the average impact load is between 240 and 270kN, which may be considered as constant. The MP1 rockbolt has a cumulative maximum energy absorption(CMEA) of 31.9–40.0 k J/m, with an average of 35.0 k J/m, and the elongation rate is 11.4%–14.7%, with an average of 12.7%, both of which are negatively correlated with the impact energy per drop. Regression analysis shows that energy absorption and shank elongation, as well as momentum input and impact duration,conform to the linear relationship. The complete dynamic capacity envelope of MP1 rockbolts is proposed, which reflects the dynamic bearing capacity, elongation, and distinct stages. This study is helpful to better understand the dynamic characteristics of energy-absorbing rockbolts and assist design engineers in robust reinforcement systems design to mitigate rockburst damage in seismically active underground excavations.展开更多
Rockburst;Rockburst damage;Yielding rockbolt;Numerical modeling;UDEC;Underground miningThe assessment of yielding rockbolt performance during rockbursts with actual seismic loading is essential for rock-burst supporti...Rockburst;Rockburst damage;Yielding rockbolt;Numerical modeling;UDEC;Underground miningThe assessment of yielding rockbolt performance during rockbursts with actual seismic loading is essential for rock-burst supporting designs.In this paper,two types of yielding rockbolts(D-bolt and Roofex)and the fully resin-grouted rebar bolt are modeled via the"rockbolt"element in universal distinct element code(UDEC)after an exact calibration procedure.A two-dimensional(2D)model of a deep tunnel is built to fully evaluate the performance(e.g.,capacity of energy-absorption and control of rock damage)of yielding and traditional rockbolts based on the simulated rockbursts.The influence of different rockburst magnitudes is also studied.The results suggest that the D-bolt can effectively control and mitigate rockburst damage during a weak rockburst because of its high strength and deformation capacity.The Roofex is too"soft"or"smooth"to limit the movement of ejected rocks and restrain the large deformation,although it has an excellent deformation capacity.The resin-grouted rebar bolt can maintain a high axial force level during rockbursts but is easy to break during dynamic shocks,which fails to control rapid rock bulking or ejection.Three types of rockbolts cannot control the large deformation and mitigate rockburst damage effectively during violent rockbursts.The rockburst damage severity can be significantly reduced by additional support with cable bolts.This study highlights the effectiveness of numerical modeling methods in assessing the complex performance of yielding rockbolts during rockbursts,which can provide some references to improve and optimize the design of rock supporting in burst-prone grounds.展开更多
This paper presents a numerical study on the pullout behavior of the rockholt grouted system. Among the complicated failure modes of the rockbolt grouted system, the crack of the grout is concerned here. A tri- linear...This paper presents a numerical study on the pullout behavior of the rockholt grouted system. Among the complicated failure modes of the rockbolt grouted system, the crack of the grout is concerned here. A tri- linear cohesive zone model （CZM） is used to simulate the inteffacial behavior of rockbolt-grout interface： and a plastic damaged model is adopted for the grout materials. The feasibility of the numerical method is verified by comparing the calculated results with the test observations. The numerical results indicate that two types of cracks of the grout materials can be identified as the inclined crack and the horizontal crack. The inclined crack forms firstly and then the horizontal crack. Both cracks can reduce the interracial shear stress and thus reduce the load transfer efficiency. Further analysis indicates that the crack of the gout material can induce the obvious drops of load capacity, which is not a safe failure mode. This study leads to a better understanding of the mechanism for rockbolt grouted system.展开更多
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...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.展开更多
This paper proposes an empirical formula to estimate the shear strength of hydraulic expansion rockbolts.The field experimental results were obtained from eleven pullout tests to evaluate the results computed by the p...This paper proposes an empirical formula to estimate the shear strength of hydraulic expansion rockbolts.The field experimental results were obtained from eleven pullout tests to evaluate the results computed by the proposed formula.It was found that shear resistance of hydraulic expansion rockbolts significantly depends on the uniaxial compressive strength and elastic modulus of rock,with high correlation coefficients of 0.7651 and0.8587,respectively.The developed formula enables estimation of the maximum pullout load in an analytical process without pullout tests in the field.Conversely,due to the poor interlocking at the initial pullout load,the measured displacements were higher than the estimated ones.To reduce the interlocking effects between bolt and rock,we recommend preloading of 29.4 kN.Preload allows reducing the distance between the measured and estimated displacement and making two load-displacement curves practically identical with marginal differences of 1.1 to 1.5 mm at the maximum pullout load.展开更多
The axial loading in rockbolts changes due to stress redistribution and rheology in the country rock mass.Such a change may lead to debonding at rockbolt to grout interface or rupture of the rockbolt.In this study,bas...The axial loading in rockbolts changes due to stress redistribution and rheology in the country rock mass.Such a change may lead to debonding at rockbolt to grout interface or rupture of the rockbolt.In this study,based on laboratory experiments,ultrasonic guided wave propagation in fully grouted rockbolt under different pull-out loads was investigated in order to examine the resultant debonding of rockbolt.The signals obtained from the ultrasonic monitoring during the pull-out test were processed using wavelet multi-scale analysis and frequency spectrum analysis,the signal amplitude and the amplitude ratio(Q)of low frequency to high frequency were defined to quantify the debonding of rockbolt.In addition to the laboratory test,numerical simulation on the effect of the embedment lengths on ultrasonic guided wave propagation in rockbolt was conducted by using a damage-based model,and the debonding between rockbolt and cement mortar was numerically examined.It was confirmed that the ultrasonic guided wave propagation in rockbolt was very sensitive to the debonding because of pull-out load,therefore,the critical bond length could be calculated based on the propagation of guided wave in the grouted rockbolt.In time domain,the signal amplitude in rockbolt increased with pull-out load from 0 to 100 kN until the completely debonding,thus quantifying the debonding under the different pull-out loads.In the frequency domain,as the Q value increased,the debonding length of rockbolt decreased exponentially.The numerical results confirmed that the guided wave propagation in the fully grouted rockbolt was effective in detecting and quantifying the debonding of rockbolt under pull-out load.展开更多
A rockbolt acting in the rock mass is subjected to the combined action of the pull-out load and confining pressure, and the bond quality of the rockbolt directly affects the stability of the roadway and cavern. Theref...A rockbolt acting in the rock mass is subjected to the combined action of the pull-out load and confining pressure, and the bond quality of the rockbolt directly affects the stability of the roadway and cavern. Therefore, in this study, confining pressure and pull-out load are applied to grouted rockbolt systems with bond defects by a numerical simulation method, and the rockbolt is detected by ultrasonic guided waves to study the propagation law of ultrasonic guided waves in defective rockbolt systems and the bond quality of rockbolts under the combined action of pull-out load and confining pressure. The numerical simulation results show that the length and location of bond defects can be detected by ultrasonic guided waves under the combined action of pull-out load and confining pressure. Under no pull-out load, with increasing confining pressure, the low-frequency part of the guided wave frequency in the rockbolt increases, the high-frequency part decreases, the weakening effect of the confining pressure on the guided wave propagation law increases, and the bond quality of the rockbolt increases. The existence of defects cannot change the strengthening effect of the confining pressure on the guided wave propagation law under the same pull-out load or the weakening effect of the pull-out load on the guided wave propagation law under the same confining pressure.展开更多
Underground mine pillars provide natural stability to the mine area,allowing safe operations for workers and machinery.Extensive prior research has been conducted to understand pillar failure mechanics and design safe...Underground mine pillars provide natural stability to the mine area,allowing safe operations for workers and machinery.Extensive prior research has been conducted to understand pillar failure mechanics and design safe pillar layouts.However,limited studies(mostly based on empirical field observation and small-scale laboratory tests)have considered pillar-support interactions under monotonic loading conditions for the design of pillar-support systems.This study used a series of large-scale laboratory compression tests on porous limestone blocks to analyze rock and support behavior at a sufficiently large scale(specimens with edge length of 0.5 m)for incorporation of actual support elements,with consideration of different w/h ratios.Both unsupported and supported(grouted rebar rockbolt and wire mesh)tests were conducted,and the surface deformations of the specimens were monitored using three-dimensional(3D)digital image correlation(DIC).Rockbolts instrumented with distributed fiber optic strain sensors were used to study rockbolt strain distribution,load mobilization,and localized deformation at different w/h ratios.Both axial and bending strains were observed in the rockbolts,which became more prominent in the post-peak region of the stress-strain curve.展开更多
基金supported by the National Natural Science Foundation of China(Grant Nos.52274124,52274123)funding of the Coal Mining and Designing Department,Tiandi Science&Technology Co.,Ltd.(Grant No.2022-2-TD-QN008).
文摘The influence of rockbolt pretension on bolting has not been well addressed,despite its critical importance in drift support systems.In this study,laboratory and numerical simulations of gravel bolting are conducted to investigate the effects of varying rockbolt pretensions.The simulations are developed using the particle flow code(PFC3D),enabling detailed analysis of contact forces between gravel particles under low and high rockbolt pretensions.The results indicate that bolted gravel can maintain stability even without pretension,though bearing capacity is significantly enhanced under high pretension.Two distinct bolting behaviors are identified:a pressure arch structure is formed under low pretension,while high pretension creates a compression zone characterized by intensified particle interlocking and superior load-bearing capacity.Based on these findings,a concept for drift support is proposed,integrating rockbolts and cables to stabilize both shallow and deep rocks.This study advances our understanding of bolting behaviors and provides theoretical guidance for designing effective drift support systems in practical applications.
基金supported by the National Key Research and Development Projects of China(No.2021YFB2600402)National Natural Science Foundation of China(Nos.52209148 and 52374119)+1 种基金the opening fund of State Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences(No.SKLGME023023)the opening fund of Key Laboratory of Water Management and Water Security for Yellow River Basin,Ministry of Water Resources(No.2023-SYSJJ-02)。
文摘To better understand the failure behaviours and strength of bolt-reinforced blocky rocks,large scale extensive laboratory experiments are carried out on blocky rock-like specimens with and without rockbolt reinforcement.The results show that both shear failure and tensile failure along joint surfaces are observed but the shear failure is a main controlling factor for the peak strength of the rock mass with and without rockbolts.The rockbolts are necked and shear deformation simultaneously happens in bolt reinforced rock specimens.As the joint dip angle increases,the joint shear failure becomes more dominant.The number of rockbolts has a significant impact on the peak strain and uniaxial compressive strength(UCS),but little influence on the deformation modulus of the rock mass.Using the Winkler beam model to represent the rockbolt behaviours,an analytical model for the prediction of the strength of boltreinforced blocky rocks is proposed.Good agreement between the UCS values predicted by proposed model and obtained from experiments suggest an encouraging performance of the proposed model.In addition,the performance of the proposed model is further assessed using published results in the literature,indicating the proposed model can be used effectively in the prediction of UCS of bolt-reinforced blocky rocks.
文摘This article introduces the principles of underground rockbolting design.The items discussed include underground loading conditions,natural pressure zone around an underground opening,design methodologies,selection of rockbolt types,determination of bolt length and spacing,factor of safety,and compatibility between support elements.Different types of rockbolting used in engineering practise are also presented.The traditional principle of selecting strong rockbolts is valid only in conditions of low in situ stresses in the rock mass.Energy-absorbing rockbolts are preferred in the case of high in situ stresses.A natural pressure arch is formed in the rock at a certain distance behind the tunnel wall.Rockbolts should be long enough to reach the natural pressure arch when the failure zone is small.The bolt length should be at least 1 m beyond the failure zone.In the case of a vast failure zone,tightly spaced short rockbolts are installed to establish an artificial pressure arch within the failure zone and long cables are anchored on the natural pressure arch.In this case,the rockbolts are usually less than 3 m long in mine drifts,but can be up to 7 m in large-scale rock caverns.Bolt spacing is more important than bolt length in the case of establishing an artificial pressure arch.In addition to the factor of safety,the maximum allowable displacement in the tunnel and the ultimate displacement capacity of rockbolts must be also taken into account in the design.Finally,rockbolts should be compatible with other support elements in the same support system in terms of displacement and energy absorption capacities.
基金funding organisations, Curtin International Postgraduate Scholarship (CIPRS)/Department of Mining and Metallurgy Scholarship, and the Mining Education Australia (MEA), Collaborative Research Grant Scheme [grant number RS-59041]
文摘Rockbolt is widely employed all over the world as an effective ground reinforcement element in order to secure the underground workplaces.Ordinary encapsulated rebar or rebar rockbolt is most popular and commonly used as reinforcement in a ground support system because of its accessibility,cost effectiveness and easy practicability.Reinforcement elements in a seismic condition such as rock burst have to dissipate the energy release of the dynamic impact via their deformation and ultimate load capacity,knowing that the former is more important.In other words,achieving early stiff behaviour along with large deformation capacity in rockbolts are the goals for new development in rock reinforcement.Yielding rockbolts are expensive while some of them have large deformation capability with low ultimate load capacity.In this paper,modifications were made on encapsulation of rebar rockbolts to utilise it effectively as a yielding reinforcement in seismic conditions.Applying a sufficient decoupled length in the shank of rebar rockbolts which industry has regularly been using to control the bulking of the stress fractured ground,improves the deformation capacity of the bolt.Additionally,leaving a collar bonding underneath of the bearing pad and plate removes the weaknesses of the head anchorage of rockbolt.Therefore the dynamic performance of the bolt is improved by these easily applicable modifications.The behaviour and performance of encapsulated rockbolts have been discussed first,then the effects of modifications are illustrated.The proposed modification of the rebars is not only cost effective but also easy to apply in the field and improves the performance of reinforcements in seismic prone zones.
文摘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.
基金This work was financially supported by the National Key Research and Development Program of China(No.2021YFC2902103)Meanwhile,the authors would also like to acknowledge the support provided by Beijing Gold-Bridge Funds and Beijing Excellent Young Engineer Innovation Studio.
文摘This paper presented a novel bond-slip model to better reveal the mechanical behavior of the bolt-grout interface for fully-grouted rockbolts under tensile loads by considering the non-linear response in the softening stage.The exponential decay function is adopted for describing the non-linear response in the softening stage.Based on the improved bond-slip model,the corresponding analytical solutions for the interfacial shear stress and the axial force of the bolt under different loading stages are solved.Then,the validity of this proposed model was verified by comparing with the experimental results.The results show that compared with the linear softening model,the proposed model is more suitable for predicting the mechanical performance of fully-grouted rockbolts.Finally,a series of parametric studies are conducted to explore the effect of model parameters on the mechanical properties of fully-grouted rockbolts.The results indicate that compared with the anchor length,the bolt diameter and the bond strength of the bolt-grout interface have a significance influence on the ultimate load of bolt,especially for the elastic and softening stage.Moreover,it can be found that using the linear softening model maybe overestimates the supporting performance of grouted bolt,resulting in an unsafe design for bolt.
基金Financial support for this work provided by the National Basic Research Program of China (No.2007CB209400)the 111 Project of China (No.B07028)+2 种基金the Key Program of National Natural Science Foundation of China(No.50834004)the National Natural Science Foundation of China (No.50874104)the Natural Science Foundation of Jiangsu Province(No.BK2006040)
文摘Harmonic wavelets not only possess the traditional advantages of a wavelet function,they also have other merits such as clear expressions,more flexible time-frequency divisions,a simple transformation algorithm,a finer box-like frequency spectrum and others.Given the frequency distribution characteristics of the nondestructive testing signals from a rockbolt support system and based on the discrete harmonic wavelet transformation theory,we have effectively abstracted signals from frequency ranges concerned by removing useless high and low frequency signals from the testing signals of the rockbolt support system and obtained filtered signals with a reconstruction algorithm of harmonic wavelets.Finally,we applied the harmonic wavelet transformation in filtering analog signals and measured response signals of rockbolts.The results indicate that harmonic wavelets also have excellent filtering characteristics.
基金CCTEG Coal Mining Research Institute(No.TDKC-2022-MS-01)the National Natural Science Foundation of China(No.52274123)the Mining Education Australia(MEA),Collaborative Research Grant Scheme(No.RS-59041).
文摘Energy-absorbing rockbolts have been widely adopted in burst-prone excavation support, and their serviceability is closely related to the frequency and magnitude of seismic events. In this research, the splittube drop test with varying impact energy was conducted to reproduce the dynamic performance of MP1rockbolts under a wide range of seismic event magnitudes. The test results showed that the impact process could be subdivided into four distinct stages, i.e. mobilization, strain hardening, plastic flow(ductile), and rebound stage, of which strain hardening and plastic flow are the primary energy absorbing stages. As the impact energy per drop increases from 8.1 to 46.7 k J, the strain rate of the shank varies between 1.20 and 2.70 s^(-1), and the average impact load is between 240 and 270kN, which may be considered as constant. The MP1 rockbolt has a cumulative maximum energy absorption(CMEA) of 31.9–40.0 k J/m, with an average of 35.0 k J/m, and the elongation rate is 11.4%–14.7%, with an average of 12.7%, both of which are negatively correlated with the impact energy per drop. Regression analysis shows that energy absorption and shank elongation, as well as momentum input and impact duration,conform to the linear relationship. The complete dynamic capacity envelope of MP1 rockbolts is proposed, which reflects the dynamic bearing capacity, elongation, and distinct stages. This study is helpful to better understand the dynamic characteristics of energy-absorbing rockbolts and assist design engineers in robust reinforcement systems design to mitigate rockburst damage in seismically active underground excavations.
基金Support from China Scholarship Council(funding number:201808370185)is also gratefully acknowledged.
文摘Rockburst;Rockburst damage;Yielding rockbolt;Numerical modeling;UDEC;Underground miningThe assessment of yielding rockbolt performance during rockbursts with actual seismic loading is essential for rock-burst supporting designs.In this paper,two types of yielding rockbolts(D-bolt and Roofex)and the fully resin-grouted rebar bolt are modeled via the"rockbolt"element in universal distinct element code(UDEC)after an exact calibration procedure.A two-dimensional(2D)model of a deep tunnel is built to fully evaluate the performance(e.g.,capacity of energy-absorption and control of rock damage)of yielding and traditional rockbolts based on the simulated rockbursts.The influence of different rockburst magnitudes is also studied.The results suggest that the D-bolt can effectively control and mitigate rockburst damage during a weak rockburst because of its high strength and deformation capacity.The Roofex is too"soft"or"smooth"to limit the movement of ejected rocks and restrain the large deformation,although it has an excellent deformation capacity.The resin-grouted rebar bolt can maintain a high axial force level during rockbursts but is easy to break during dynamic shocks,which fails to control rapid rock bulking or ejection.Three types of rockbolts cannot control the large deformation and mitigate rockburst damage effectively during violent rockbursts.The rockburst damage severity can be significantly reduced by additional support with cable bolts.This study highlights the effectiveness of numerical modeling methods in assessing the complex performance of yielding rockbolts during rockbursts,which can provide some references to improve and optimize the design of rock supporting in burst-prone grounds.
基金financially supported by the National Natural Science Fund of China (Nos. 51304067 and 51104057)the Fund of Opening Laboratory for Deep Mine Construction, Henan Polytechnic University (No. 2012KF-01)the Education Department of Henan Province (No. 13A440311)
文摘This paper presents a numerical study on the pullout behavior of the rockholt grouted system. Among the complicated failure modes of the rockbolt grouted system, the crack of the grout is concerned here. A tri- linear cohesive zone model （CZM） is used to simulate the inteffacial behavior of rockbolt-grout interface： and a plastic damaged model is adopted for the grout materials. The feasibility of the numerical method is verified by comparing the calculated results with the test observations. The numerical results indicate that two types of cracks of the grout materials can be identified as the inclined crack and the horizontal crack. The inclined crack forms firstly and then the horizontal crack. Both cracks can reduce the interracial shear stress and thus reduce the load transfer efficiency. Further analysis indicates that the crack of the gout material can induce the obvious drops of load capacity, which is not a safe failure mode. This study leads to a better understanding of the mechanism for rockbolt grouted system.
文摘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.
基金supported by 2016 Hongik University Research Fund and the Convergence R&D program of MSIP/NST[Convergence Research-14-2-ETRI,Development of Internet of Things(IoT)-based Urban Underground Utility Monitoring and Management System]
文摘This paper proposes an empirical formula to estimate the shear strength of hydraulic expansion rockbolts.The field experimental results were obtained from eleven pullout tests to evaluate the results computed by the proposed formula.It was found that shear resistance of hydraulic expansion rockbolts significantly depends on the uniaxial compressive strength and elastic modulus of rock,with high correlation coefficients of 0.7651 and0.8587,respectively.The developed formula enables estimation of the maximum pullout load in an analytical process without pullout tests in the field.Conversely,due to the poor interlocking at the initial pullout load,the measured displacements were higher than the estimated ones.To reduce the interlocking effects between bolt and rock,we recommend preloading of 29.4 kN.Preload allows reducing the distance between the measured and estimated displacement and making two load-displacement curves practically identical with marginal differences of 1.1 to 1.5 mm at the maximum pullout load.
基金This work is funded by the National Science Foundation of China(Grant Nos.U1906208,52104157,51904056 and 51874069)and the Fundamental Research Funds for the Central Universities of China(Grant Nos.N2101028 and N2101015).
文摘The axial loading in rockbolts changes due to stress redistribution and rheology in the country rock mass.Such a change may lead to debonding at rockbolt to grout interface or rupture of the rockbolt.In this study,based on laboratory experiments,ultrasonic guided wave propagation in fully grouted rockbolt under different pull-out loads was investigated in order to examine the resultant debonding of rockbolt.The signals obtained from the ultrasonic monitoring during the pull-out test were processed using wavelet multi-scale analysis and frequency spectrum analysis,the signal amplitude and the amplitude ratio(Q)of low frequency to high frequency were defined to quantify the debonding of rockbolt.In addition to the laboratory test,numerical simulation on the effect of the embedment lengths on ultrasonic guided wave propagation in rockbolt was conducted by using a damage-based model,and the debonding between rockbolt and cement mortar was numerically examined.It was confirmed that the ultrasonic guided wave propagation in rockbolt was very sensitive to the debonding because of pull-out load,therefore,the critical bond length could be calculated based on the propagation of guided wave in the grouted rockbolt.In time domain,the signal amplitude in rockbolt increased with pull-out load from 0 to 100 kN until the completely debonding,thus quantifying the debonding under the different pull-out loads.In the frequency domain,as the Q value increased,the debonding length of rockbolt decreased exponentially.The numerical results confirmed that the guided wave propagation in the fully grouted rockbolt was effective in detecting and quantifying the debonding of rockbolt under pull-out load.
文摘A rockbolt acting in the rock mass is subjected to the combined action of the pull-out load and confining pressure, and the bond quality of the rockbolt directly affects the stability of the roadway and cavern. Therefore, in this study, confining pressure and pull-out load are applied to grouted rockbolt systems with bond defects by a numerical simulation method, and the rockbolt is detected by ultrasonic guided waves to study the propagation law of ultrasonic guided waves in defective rockbolt systems and the bond quality of rockbolts under the combined action of pull-out load and confining pressure. The numerical simulation results show that the length and location of bond defects can be detected by ultrasonic guided waves under the combined action of pull-out load and confining pressure. Under no pull-out load, with increasing confining pressure, the low-frequency part of the guided wave frequency in the rockbolt increases, the high-frequency part decreases, the weakening effect of the confining pressure on the guided wave propagation law increases, and the bond quality of the rockbolt increases. The existence of defects cannot change the strengthening effect of the confining pressure on the guided wave propagation law under the same pull-out load or the weakening effect of the pull-out load on the guided wave propagation law under the same confining pressure.
基金the funding support from Alpha Foundation for the Improvement of Mine Safety and Health Inc.(ALPHAFOUNDATION,Grant No.AFC820-52)。
文摘Underground mine pillars provide natural stability to the mine area,allowing safe operations for workers and machinery.Extensive prior research has been conducted to understand pillar failure mechanics and design safe pillar layouts.However,limited studies(mostly based on empirical field observation and small-scale laboratory tests)have considered pillar-support interactions under monotonic loading conditions for the design of pillar-support systems.This study used a series of large-scale laboratory compression tests on porous limestone blocks to analyze rock and support behavior at a sufficiently large scale(specimens with edge length of 0.5 m)for incorporation of actual support elements,with consideration of different w/h ratios.Both unsupported and supported(grouted rebar rockbolt and wire mesh)tests were conducted,and the surface deformations of the specimens were monitored using three-dimensional(3D)digital image correlation(DIC).Rockbolts instrumented with distributed fiber optic strain sensors were used to study rockbolt strain distribution,load mobilization,and localized deformation at different w/h ratios.Both axial and bending strains were observed in the rockbolts,which became more prominent in the post-peak region of the stress-strain curve.
