Assessment of rock mass quality significantly impacts the design and construction of underground and open-pit mines from the point of stability and economy.This study develops the novel Gromov-Hausdorff distance for r...Assessment of rock mass quality significantly impacts the design and construction of underground and open-pit mines from the point of stability and economy.This study develops the novel Gromov-Hausdorff distance for rock quality(GHDQR)methodology for rock mass quality rating based on multi-criteria grey metric space.It usually presents the quality of surrounding rock by classes(metric spaces)with specified properties and adequate interval-grey numbers.Measuring the distance between surrounding rock sample characteristics and existing classes represents the core of this study.The Gromov-Hausdorff distance is an especially useful discriminant function,i.e.,a classifier to calculate these distances,and assess the quality of the surrounding rock.The efficiency of the developed methodology is analyzed using the Mean Absolute Percentage Error(MAPE)technique.Seven existing methods,such as the Gaussian cloud method,Discriminant method,Mutation series method,Artificial neural network(ANN),Support vector machine(SVM),Grey wolf optimizer and Support vector classification method(GWO-SVC)and Rock mass rating method(RMR)are used for comparison with the proposed GHDQR method.The share of the highly accurate category of 85.71%clearly indicates compliance with actual values obtained by the compared methods.The results of comparisons showed that the model enables objective,efficient,and reliable assessment of rock mass quality.展开更多
The strength of rock materials is largely affected by water and loading conditions, but there are few studies on mechanical properties of saturated rocks at high strain rates. Through compressive tests on dry and satu...The strength of rock materials is largely affected by water and loading conditions, but there are few studies on mechanical properties of saturated rocks at high strain rates. Through compressive tests on dry and saturated sandstone specimens, it was found that the dynamic compressive strength of both dry and saturated sandstone specimens increased with the increase of strain rates. The saturated rock specimens showed stronger rate dependence than the dry ones. The water affecting factor (WAF), as the ratio of the strength under dry state to that under saturated state, was introduced to describe the influence of water on the compressive strength at different strain rates. The WAF under static load was close to 1.38, and decreased with the increase of strain rate. When the strain rate reached 190 s^-1, the WAF reduced to 0.98. It indicates that the compressive strength of saturated specimens can be higher than that of dry ones when the strain rate is high enough. Furthermore, the dual effects of water and strain rate on the strength of rock were discussed based on sliding crack model, which provided a good explanation for the experimental results.展开更多
A reliable and accurate prediction of the tunnel boring machine(TBM)performance can assist in minimizing the relevant risks of high capital costs and in scheduling tunneling projects.This research aims to develop six ...A reliable and accurate prediction of the tunnel boring machine(TBM)performance can assist in minimizing the relevant risks of high capital costs and in scheduling tunneling projects.This research aims to develop six hybrid models of extreme gradient boosting(XGB)which are optimized by gray wolf optimization(GWO),particle swarm optimization(PSO),social spider optimization(SSO),sine cosine algorithm(SCA),multi verse optimization(MVO)and moth flame optimization(MFO),for estimation of the TBM penetration rate(PR).To do this,a comprehensive database with 1286 data samples was established where seven parameters including the rock quality designation,the rock mass rating,Brazilian tensile strength(BTS),rock mass weathering,the uniaxial compressive strength(UCS),revolution per minute and trust force per cutter(TFC),were set as inputs and TBM PR was selected as model output.Together with the mentioned six hybrid models,four single models i.e.,artificial neural network,random forest regression,XGB and support vector regression were also built to estimate TBM PR for comparison purposes.These models were designed conducting several parametric studies on their most important parameters and then,their performance capacities were assessed through the use of root mean square error,coefficient of determination,mean absolute percentage error,and a10-index.Results of this study confirmed that the best predictive model of PR goes to the PSO-XGB technique with system error of(0.1453,and 0.1325),R^(2) of(0.951,and 0.951),mean absolute percentage error(4.0689,and 3.8115),and a10-index of(0.9348,and 0.9496)in training and testing phases,respectively.The developed hybrid PSO-XGB can be introduced as an accurate,powerful and applicable technique in the field of TBM performance prediction.By conducting sensitivity analysis,it was found that UCS,BTS and TFC have the deepest impacts on the TBM PR.展开更多
In order to determine the relationship among energy consumption of rock and its fragmentation, dynamic strength and strain rate, granite, sandstone and limestone specimens were chosen and tested on large-diameter spli...In order to determine the relationship among energy consumption of rock and its fragmentation, dynamic strength and strain rate, granite, sandstone and limestone specimens were chosen and tested on large-diameter split Hopkinson pressure bar (SHPB) equipment with half-sine waveform loading at the strain rates ranging from 40 to 150 s- 1. With recorded signals, the energy consumption, strain rate and dynamic strength were analyzed. And the fragmentation behaviors of specimens were investigated. The experimental results show that the energy consumption density of rock increases linearly with the total incident energy. The energy consumption density is of an exponent relationship with the average size of rock fragments. The higher the energy consumption density, the more serious the fragmentation, and the better the gradation of fragments. The energy consumption density takes a good logarithm relationship with the dynamic strength of rock. The dynamic strength of rock increases with the increase of strain rate, indicating higher strain rate sensitivity.展开更多
Intact rock-like specimens and specimens that include a single, smooth planar joint at various angles are prepared for split Hopkinson pressure bar(SHPB) testing. A buffer pad between the striker bar and the inciden...Intact rock-like specimens and specimens that include a single, smooth planar joint at various angles are prepared for split Hopkinson pressure bar(SHPB) testing. A buffer pad between the striker bar and the incident bar of an SHPB apparatus is used to absorb some of the shock energy. This can generate loading rates of 20.2-4627.3 GPa/s, enabling dynamic peak stresses/strengths and associated failure patterns of the specimens to be investigated. The effects of the loading rate and angle of load applied on the dynamic peak stresses/strengths of the specimens are examined. Relevant experimental results demonstrate that the failure pattern of each specimen can be classified as four types: Type A, integrated with or without tiny flake-off; Type B, slide failure; Type C, fracture failure; and Type D, crushing failure. The dynamic peak stresses/strengths of the specimens that have similar failure patterns increase linearly with the loading rate, yielding high correlations that are evident on semi-logarithmic plots. The slope of the failure envelope is the smallest for slide failure, followed by crushing failure, and that of fracture failure is the largest. The magnitude of the plot slope of the dynamic peak stress against the loading rate for the specimens that are still integrated after testing is between that of slide failure and crushing failure. The angle of application has a limited effect on the dynamic peak stresses/strengths of the specimens regardless of the failure pattern, but it affects the bounds of the loading rates that yield each failure pattern, and thus influences the dynamic responses of the single jointed specimen. Slide failure occurs at the lowest loading rate of any failure, but can only occur in single jointed specimen that allows sliding.Crushing failure is typically associated with the largest loading rate, and fracture failure may occur when the loading rate is between the boundaries for slide failure and crushing failure.展开更多
Accurate prediction of compressive strength of rocks relies on the rate-dependent behaviors of rocks, and correlation among the geometrical, physical, and mechanical properties of rocks. However, these properties may ...Accurate prediction of compressive strength of rocks relies on the rate-dependent behaviors of rocks, and correlation among the geometrical, physical, and mechanical properties of rocks. However, these properties may not be easy to control in laboratory experiments, particularly in dynamic compression experiments. By training three machine learning models based on the support vector machine(SVM), backpropagation neural network(BPNN), and random forest(RF) algorithms, we isolated different input parameters, such as static compressive strength, P-wave velocity, specimen dimension, grain size, bulk density, and strain rate, to identify their importance in the strength prediction. Our results demonstrated that the RF algorithm shows a better performance than the other two algorithms. The strain rate is a key input parameter influencing the performance of these models, while the others(e.g. static compressive strength and P-wave velocity) are less important as their roles can be compensated by alternative parameters. The results also revealed that the effect of specimen dimension on the rock strength can be overshadowed at high strain rates, while the effect on the dynamic increase factor(i.e. the ratio of dynamic to static compressive strength) becomes significant. The dynamic increase factors for different specimen dimensions bifurcate when the strain rate reaches a relatively high value, a clue to improve our understanding of the transitional behaviors of rocks from low to high strain rates.展开更多
In order to investigate the relationship between bar diameter and loading rate of the split Hopkinson pressure bar(SHPB) setup under the failure of rock specimen and realize the medium strain rate loading of specimen,...In order to investigate the relationship between bar diameter and loading rate of the split Hopkinson pressure bar(SHPB) setup under the failure of rock specimen and realize the medium strain rate loading of specimen,new SHPB setups with different elastic bar's diameters of 22,36,50 and 75 mm were constructed.The tests were carried out on these setups at different loading rates,and the specimens had the same diameter of elastic bars and same ratio of length to diameter.The test results show that the larger the elastic bar's diameter is,the less the loading rate is needed to cause specimen failure,they show good power relationship,and that under the same strain rate loading,specimens are broken more seriously with larger diameter SHPB setup than with smaller one.展开更多
A new approach for prediction of face advance rete (FAR) prior to mining operation and determination of the operation efficiency after mining operation in retreat longwall mining panel is presented based upon the conc...A new approach for prediction of face advance rete (FAR) prior to mining operation and determination of the operation efficiency after mining operation in retreat longwall mining panel is presented based upon the concepts of rock engineering system (RES). For this purpose, six longwall panels considered in Parvadeh-I coal mine. Seven major effective parameters on FAR was selected including coal mine roof rating, gas propagation, safety factor of longwall face, ratio of joint spacing to cutting depth at longwall face, longwall face inclination, panel width, floor rock mass rating. To performance evaluation of the presented model, the relationship between the average vulnerability indexes of advance operation with FAR was determined in considered panels with coefficient of determination (R2) equal to 0.884 that indicate relatively acceptable correlation and compatibility. Investigations of the research indicated that it is possible to determine the actual operation efficiency under fair conditions by a RES-based model. The inevitable reduction of FAR for each longwall panel was determined by presented model that the difference amount between the maximum possible practical face advance rate (FARmpp) and recorded actual face advance rate (FARa) indicate the operation efficiency. Applied approach in this paper can be used to prediction of FAR in retreat longwall mining panel for same conditions that can have many benefits, including better and more accurate planning for the sales market and mine operation. Also, presented method in this paper can be applied as a useful tool to determination of actual operation efficiency for other sections and extraction methods in coal mines.展开更多
A new electrical method of conductive carbon-film(with waterproof and anticorrosion ability)was proposed to continuously measure crack propagation rate of brittle rock under THMC coupling condition.A self-designed cou...A new electrical method of conductive carbon-film(with waterproof and anticorrosion ability)was proposed to continuously measure crack propagation rate of brittle rock under THMC coupling condition.A self-designed coupling testing system was used to conduct THMC coupling fracture tests of the pre-cracked red sandstone specimens(where the temperature is only changed)by this new electrical method of conductive carbon-film.Calculation results obtained by the energy method coincide well with the test results.And the higher the temperature is,the earlier the crack is initiated and the larger the crack propagation rate and accelerated velocity are,which can prove the validity of the new electrical method.This new electrical method has advantages of continuously measuring crack propagation rate over the conventional electrical,optical and acoustic methods,and can provide important basis for safety assessment and cracking-arrest design of deep rock mass engineering.展开更多
The three-point bending experiments were applied to investigating effects of loading rates on fracture toughness of Huanglong limestone. The fracture toughness of Huanglong limestone was measured over a wide range of ...The three-point bending experiments were applied to investigating effects of loading rates on fracture toughness of Huanglong limestone. The fracture toughness of Huanglong limestone was measured over a wide range of loading rates from 9 × 10-4 to 1.537 MPa.m1/2/s. According to the approximate relationship between static and dynamic fracture toughness of Huanglong limestone, relationship between the growth velocity of crack and dynamic fracture toughness was obtained. The main conclusions are summarized as follows. (1) When the loading rate is higher than 0.027 MPa-ml/2/s, the fracture toughness of Huanglong limestone increases markedly with increasing loading rate. However, when loading rate is lower than 0.027 MPa-ml/2/s, fracture toughness slightly increases with an increase in loading rate. (2) It is found from experimental results that fracture toughness is linearly proportional to the logarithmic expression of loading rate. (3) For Huanglong limestone, when the growth velocity of crack is lower than 100 m/s, the energy release rate slightly decreases with increasing the growth velocity of crack. However, when the growth velocity of crack is higher than 1 000 m/s, the energy release rate dramatically decreases with an increase in the crack growth velocity.展开更多
Transition in the rate-dependent mechanical response of rock was investigated due to the presence of impersistent joint with different infill conditions.Four types of samples,i.e.intact,jointed with no grouting,jointe...Transition in the rate-dependent mechanical response of rock was investigated due to the presence of impersistent joint with different infill conditions.Four types of samples,i.e.intact,jointed with no grouting,jointed and grouted with cement,and jointed and grouted with epoxy,were fabricated using model material.A series of dynamic split Hopkinson pressure bar(SHPB) tests was conducted on prepared samples with strain rates varying between 53-130 salong with static uniaxial compression tests(10s).Progression of fracture/failure along samples was monitored using high-speed imaging and digital image correlation(DIC).Strength was observed to be significantly lower for jointed samples as compared to intact samples.However,the increasing trend of strength with strain rates remained similar for all types of samples.Epoxy was observed to be a better grout due to greater improvement in the strength of epoxy grouted jointed samples than cement grouted samples under both static and dynamic conditions.Significant changes were observed in fracture behavior(initiation,pattern and mechanism)with strain rate for intact and jointed unfilled/grouted samples.Fracturing was dominated by shear and tensile cracks at high strain rates compared to tensile cracks at low strain rates in all samples.Unlike static loading conditions,the location of cracks initiation shifts away from joint tips with increasing strain rate and depending upon existing infill conditions(unfilled/grouted).展开更多
Assessment of drillability of rocks is vital in the selection,operation,and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling.The commonly used rock drillabi...Assessment of drillability of rocks is vital in the selection,operation,and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling.The commonly used rock drillability prediction methods,namely,drilling rate index(DRI)and Cerchar hardness index(CHI)have limitations in predicting the penetration rate due to differential wear of the cutting tool in rocks with varied hardness and abrasivity.Since cutting tools get blunt differently in different rocks,the stress beneath the tip of the bit decreases until it reaches a threshold value beyond which the penetration rate becomes constant.In this research,a new composite penetration rate index(CPRI)is suggested based on the investigations on four metamorphic rocks viz.quartzite,gneiss,schist and phyllite with varied hardness-abrasivity values.The penetration-time behavior was classified into active,moderate,passive,and dormant phases based on the reduction in penetration rate at different stages of drilling.A comparison of predicted penetration rate values using DRI and CPRI with actual penetration rate values clearly establishes the supremacy of CPRI.Micro-structure and hardness-based index was also developed and correlated with CPRI.The new indices can help predict cutting tool penetration and its consumption more accurately.展开更多
A calculation formula of thermal-hydro-mechanical(THM)coupling crack initiation rate for brittle rock was derived based on the energy conservation law.The self-designed THM coupling fracture test with conductive adhe...A calculation formula of thermal-hydro-mechanical(THM)coupling crack initiation rate for brittle rock was derived based on the energy conservation law.The self-designed THM coupling fracture test with conductive adhesive electrical measurement method was applied to measuring the THM coupling crack propagation rate of brittle rock continuously.Research results show that both calculation and test results of crack initiation rate increased with increase of the temperature and the hydraulic pressure.They are almost in good agreement,which can prove validity of the calculation formula of THM coupling crack initiation rate.展开更多
Prediction of the drilling penetration rate is one of the important parameters in mining operations. This parameter has a direct impact on the mine planning and cost of mining operations, Generally, effective paramete...Prediction of the drilling penetration rate is one of the important parameters in mining operations. This parameter has a direct impact on the mine planning and cost of mining operations, Generally, effective parameters on the penetration rate is divided into two classes: rock mass properties and specifications of the machine, The chemical components of intact rock have a direct effect in determining rock mechan- ical properties, Theses parameters usually have not been investigated in any research on the rock drill- ability, In this study, physical and mechanical properties of iron ore were studied based on the amount of magnetite percent, According to the results of the tests, the effective parameters on the pen- etration rate of the rotary drilling machines were divided into three classes: specifications of the machi- nes, rock mass properties and chemical component of intact rock, Then, the rock drillahility was studied using rock engineering systems, The results showed that feed, rotation, rock mass index and iron oxide percent have important effect on penetration rate, Then a quadratic equation with 0,896 determination coefficient has been obtained, Also, the results showed that chemical components can he described as new parameters in rotary drill penetration rate,展开更多
Tackling the problems of underground water storage in collieries in arid regions requires knowledge of the effect of water intrusion and loading rate on the mechanical properties of and crack development in coal–rock...Tackling the problems of underground water storage in collieries in arid regions requires knowledge of the effect of water intrusion and loading rate on the mechanical properties of and crack development in coal–rock combinations. Fifty-four coal–rock combinations were prepared and split equally into groups containing different moisture contents(dry, natural moisture and saturated) to conduct acoustic emission testing under uniaxial compression with loading rates ranging from 0.1 mm/min to 0.6 mm/min. The results show that the peak stress and strength-softening modulus, elastic modulus, strain-softening modulus, and post-peak modulus partly decrease with increasing moisture content and loading rate. In contrast, peak strain increases with increasing moisture content and fluctuates with rising loading rate. More significantly, the relationship between stiffness and stress, combined with accumulated counts of acoustic emission, can be used to precisely predict all phases of crack propagation. This is helpful in studying the impact of moisture content and loading rate on crack propagation and accurately calculating mechanical properties. We also determined that the stress thresholds of crack closure, crack initiation, and crack damage do not vary with changes of moisture content and loading rate, constituting 15.22%, 32.20%, and 80.98% of peak stress, respectively. These outcomes assist in developing approaches to water storage in coal mines, determining the necessary width of waterproof coal–rock pillars, and methods of supporting water-enriched roadways, while also advances understanding the mechanical properties of coal–rock combinations and laws of crack propagation.展开更多
Nowadays,the development of novel metallic materials for rock support have attracted research interests since they can significantly improve the deformation and energy absorption capacities of rock bolts.Although prev...Nowadays,the development of novel metallic materials for rock support have attracted research interests since they can significantly improve the deformation and energy absorption capacities of rock bolts.Although previous studies proved the importance and mechanical advantages of utilizing high-strength and high-toughness(HSHT)steels in rock support,there is no systematic analysis to reveal the essential energy absorption parameter and the guidelines for further development of metallic rock support materials.This paper analyzes the energy absorption characteristics of novel HSHT steels(negative Poisson’s ratio(NPR)and twinning-induced plasticity(TWIP)steels)in comparison with conventional rock support materials.A physically based crystal plasticity(CP)model was set up and calibrated to study the effect of strain hardening rate(SHR).Meanwhile,the roles of underlying physical mechanisms,i.e.the dislocation density and twin volume fraction,were studied.The results show that the improvement of energy absorption density(EAD)is essential for further development of rock support materials,besides the increase of energy absorption rate(EAR)for previous development of conventional rock support materials.The increase of EAD requires increases of both strength and deformation capacity of materials.For HSHT steels,the decrease of SHR has a positive effect on the improvement of EAD.In addition,the increase of EAD is followed by the increase of twin volume fraction and the decrease of plastic Poisson’s ratio which can promote deformation plasticity of materials.Meanwhile,the increase of EAR is correlated with the accumulation of dislocation density,which can increase the strength of materials.This paper provides the theoretical basis and guidelines for developing rock support materials in deep underground engineering and other related fields.展开更多
文摘Assessment of rock mass quality significantly impacts the design and construction of underground and open-pit mines from the point of stability and economy.This study develops the novel Gromov-Hausdorff distance for rock quality(GHDQR)methodology for rock mass quality rating based on multi-criteria grey metric space.It usually presents the quality of surrounding rock by classes(metric spaces)with specified properties and adequate interval-grey numbers.Measuring the distance between surrounding rock sample characteristics and existing classes represents the core of this study.The Gromov-Hausdorff distance is an especially useful discriminant function,i.e.,a classifier to calculate these distances,and assess the quality of the surrounding rock.The efficiency of the developed methodology is analyzed using the Mean Absolute Percentage Error(MAPE)technique.Seven existing methods,such as the Gaussian cloud method,Discriminant method,Mutation series method,Artificial neural network(ANN),Support vector machine(SVM),Grey wolf optimizer and Support vector classification method(GWO-SVC)and Rock mass rating method(RMR)are used for comparison with the proposed GHDQR method.The share of the highly accurate category of 85.71%clearly indicates compliance with actual values obtained by the compared methods.The results of comparisons showed that the model enables objective,efficient,and reliable assessment of rock mass quality.
基金Project(2015CB060200)supported by the National Basic Research Program of ChinaProjects(51322403,51274254)supported by the National Natural Science Foundation of ChinaProjects(2015cx005,2016cx017)supported by Innovation Plan of Central South University
文摘The strength of rock materials is largely affected by water and loading conditions, but there are few studies on mechanical properties of saturated rocks at high strain rates. Through compressive tests on dry and saturated sandstone specimens, it was found that the dynamic compressive strength of both dry and saturated sandstone specimens increased with the increase of strain rates. The saturated rock specimens showed stronger rate dependence than the dry ones. The water affecting factor (WAF), as the ratio of the strength under dry state to that under saturated state, was introduced to describe the influence of water on the compressive strength at different strain rates. The WAF under static load was close to 1.38, and decreased with the increase of strain rate. When the strain rate reached 190 s^-1, the WAF reduced to 0.98. It indicates that the compressive strength of saturated specimens can be higher than that of dry ones when the strain rate is high enough. Furthermore, the dual effects of water and strain rate on the strength of rock were discussed based on sliding crack model, which provided a good explanation for the experimental results.
基金funded by the National Science Foundation of China(41807259)the Innovation-Driven Project of Central South University(No.2020CX040)the Shenghua Lieying Program of Central South University(Principle Investigator:Dr.Jian Zhou)。
文摘A reliable and accurate prediction of the tunnel boring machine(TBM)performance can assist in minimizing the relevant risks of high capital costs and in scheduling tunneling projects.This research aims to develop six hybrid models of extreme gradient boosting(XGB)which are optimized by gray wolf optimization(GWO),particle swarm optimization(PSO),social spider optimization(SSO),sine cosine algorithm(SCA),multi verse optimization(MVO)and moth flame optimization(MFO),for estimation of the TBM penetration rate(PR).To do this,a comprehensive database with 1286 data samples was established where seven parameters including the rock quality designation,the rock mass rating,Brazilian tensile strength(BTS),rock mass weathering,the uniaxial compressive strength(UCS),revolution per minute and trust force per cutter(TFC),were set as inputs and TBM PR was selected as model output.Together with the mentioned six hybrid models,four single models i.e.,artificial neural network,random forest regression,XGB and support vector regression were also built to estimate TBM PR for comparison purposes.These models were designed conducting several parametric studies on their most important parameters and then,their performance capacities were assessed through the use of root mean square error,coefficient of determination,mean absolute percentage error,and a10-index.Results of this study confirmed that the best predictive model of PR goes to the PSO-XGB technique with system error of(0.1453,and 0.1325),R^(2) of(0.951,and 0.951),mean absolute percentage error(4.0689,and 3.8115),and a10-index of(0.9348,and 0.9496)in training and testing phases,respectively.The developed hybrid PSO-XGB can be introduced as an accurate,powerful and applicable technique in the field of TBM performance prediction.By conducting sensitivity analysis,it was found that UCS,BTS and TFC have the deepest impacts on the TBM PR.
基金Projects(50674107, 10472134, 50490274) supported by the National Natural Science Foundation of China
文摘In order to determine the relationship among energy consumption of rock and its fragmentation, dynamic strength and strain rate, granite, sandstone and limestone specimens were chosen and tested on large-diameter split Hopkinson pressure bar (SHPB) equipment with half-sine waveform loading at the strain rates ranging from 40 to 150 s- 1. With recorded signals, the energy consumption, strain rate and dynamic strength were analyzed. And the fragmentation behaviors of specimens were investigated. The experimental results show that the energy consumption density of rock increases linearly with the total incident energy. The energy consumption density is of an exponent relationship with the average size of rock fragments. The higher the energy consumption density, the more serious the fragmentation, and the better the gradation of fragments. The energy consumption density takes a good logarithm relationship with the dynamic strength of rock. The dynamic strength of rock increases with the increase of strain rate, indicating higher strain rate sensitivity.
基金the Science and Technology authority of Taiwan, China, for financially supporting this research under Grant No.NSC 102-2221-E-027-071-MY3
文摘Intact rock-like specimens and specimens that include a single, smooth planar joint at various angles are prepared for split Hopkinson pressure bar(SHPB) testing. A buffer pad between the striker bar and the incident bar of an SHPB apparatus is used to absorb some of the shock energy. This can generate loading rates of 20.2-4627.3 GPa/s, enabling dynamic peak stresses/strengths and associated failure patterns of the specimens to be investigated. The effects of the loading rate and angle of load applied on the dynamic peak stresses/strengths of the specimens are examined. Relevant experimental results demonstrate that the failure pattern of each specimen can be classified as four types: Type A, integrated with or without tiny flake-off; Type B, slide failure; Type C, fracture failure; and Type D, crushing failure. The dynamic peak stresses/strengths of the specimens that have similar failure patterns increase linearly with the loading rate, yielding high correlations that are evident on semi-logarithmic plots. The slope of the failure envelope is the smallest for slide failure, followed by crushing failure, and that of fracture failure is the largest. The magnitude of the plot slope of the dynamic peak stress against the loading rate for the specimens that are still integrated after testing is between that of slide failure and crushing failure. The angle of application has a limited effect on the dynamic peak stresses/strengths of the specimens regardless of the failure pattern, but it affects the bounds of the loading rates that yield each failure pattern, and thus influences the dynamic responses of the single jointed specimen. Slide failure occurs at the lowest loading rate of any failure, but can only occur in single jointed specimen that allows sliding.Crushing failure is typically associated with the largest loading rate, and fracture failure may occur when the loading rate is between the boundaries for slide failure and crushing failure.
基金supported by National Research Foundation,Singapore under its Virtual Singapore R&D Programme (Award No.NRF2019VSG-GMS-001)。
文摘Accurate prediction of compressive strength of rocks relies on the rate-dependent behaviors of rocks, and correlation among the geometrical, physical, and mechanical properties of rocks. However, these properties may not be easy to control in laboratory experiments, particularly in dynamic compression experiments. By training three machine learning models based on the support vector machine(SVM), backpropagation neural network(BPNN), and random forest(RF) algorithms, we isolated different input parameters, such as static compressive strength, P-wave velocity, specimen dimension, grain size, bulk density, and strain rate, to identify their importance in the strength prediction. Our results demonstrated that the RF algorithm shows a better performance than the other two algorithms. The strain rate is a key input parameter influencing the performance of these models, while the others(e.g. static compressive strength and P-wave velocity) are less important as their roles can be compensated by alternative parameters. The results also revealed that the effect of specimen dimension on the rock strength can be overshadowed at high strain rates, while the effect on the dynamic increase factor(i.e. the ratio of dynamic to static compressive strength) becomes significant. The dynamic increase factors for different specimen dimensions bifurcate when the strain rate reaches a relatively high value, a clue to improve our understanding of the transitional behaviors of rocks from low to high strain rates.
基金Project(10472134) supported by the National Natural Science Foundation of China
文摘In order to investigate the relationship between bar diameter and loading rate of the split Hopkinson pressure bar(SHPB) setup under the failure of rock specimen and realize the medium strain rate loading of specimen,new SHPB setups with different elastic bar's diameters of 22,36,50 and 75 mm were constructed.The tests were carried out on these setups at different loading rates,and the specimens had the same diameter of elastic bars and same ratio of length to diameter.The test results show that the larger the elastic bar's diameter is,the less the loading rate is needed to cause specimen failure,they show good power relationship,and that under the same strain rate loading,specimens are broken more seriously with larger diameter SHPB setup than with smaller one.
文摘A new approach for prediction of face advance rete (FAR) prior to mining operation and determination of the operation efficiency after mining operation in retreat longwall mining panel is presented based upon the concepts of rock engineering system (RES). For this purpose, six longwall panels considered in Parvadeh-I coal mine. Seven major effective parameters on FAR was selected including coal mine roof rating, gas propagation, safety factor of longwall face, ratio of joint spacing to cutting depth at longwall face, longwall face inclination, panel width, floor rock mass rating. To performance evaluation of the presented model, the relationship between the average vulnerability indexes of advance operation with FAR was determined in considered panels with coefficient of determination (R2) equal to 0.884 that indicate relatively acceptable correlation and compatibility. Investigations of the research indicated that it is possible to determine the actual operation efficiency under fair conditions by a RES-based model. The inevitable reduction of FAR for each longwall panel was determined by presented model that the difference amount between the maximum possible practical face advance rate (FARmpp) and recorded actual face advance rate (FARa) indicate the operation efficiency. Applied approach in this paper can be used to prediction of FAR in retreat longwall mining panel for same conditions that can have many benefits, including better and more accurate planning for the sales market and mine operation. Also, presented method in this paper can be applied as a useful tool to determination of actual operation efficiency for other sections and extraction methods in coal mines.
基金Projects(51474251,51874351) supported by the National Natural Science Foundation of China
文摘A new electrical method of conductive carbon-film(with waterproof and anticorrosion ability)was proposed to continuously measure crack propagation rate of brittle rock under THMC coupling condition.A self-designed coupling testing system was used to conduct THMC coupling fracture tests of the pre-cracked red sandstone specimens(where the temperature is only changed)by this new electrical method of conductive carbon-film.Calculation results obtained by the energy method coincide well with the test results.And the higher the temperature is,the earlier the crack is initiated and the larger the crack propagation rate and accelerated velocity are,which can prove the validity of the new electrical method.This new electrical method has advantages of continuously measuring crack propagation rate over the conventional electrical,optical and acoustic methods,and can provide important basis for safety assessment and cracking-arrest design of deep rock mass engineering.
基金Projects(50490275, 50621403, 50778184) supported by the National Natural Science Foundation of ChinaProject(NCET-07-0911) supported by Program of New Century Talents of Ministry of EducationProject(CSTC, 2009BA4046) supported by the Natural Science Foundation of CQ CSTC
文摘The three-point bending experiments were applied to investigating effects of loading rates on fracture toughness of Huanglong limestone. The fracture toughness of Huanglong limestone was measured over a wide range of loading rates from 9 × 10-4 to 1.537 MPa.m1/2/s. According to the approximate relationship between static and dynamic fracture toughness of Huanglong limestone, relationship between the growth velocity of crack and dynamic fracture toughness was obtained. The main conclusions are summarized as follows. (1) When the loading rate is higher than 0.027 MPa-ml/2/s, the fracture toughness of Huanglong limestone increases markedly with increasing loading rate. However, when loading rate is lower than 0.027 MPa-ml/2/s, fracture toughness slightly increases with an increase in loading rate. (2) It is found from experimental results that fracture toughness is linearly proportional to the logarithmic expression of loading rate. (3) For Huanglong limestone, when the growth velocity of crack is lower than 100 m/s, the energy release rate slightly decreases with increasing the growth velocity of crack. However, when the growth velocity of crack is higher than 1 000 m/s, the energy release rate dramatically decreases with an increase in the crack growth velocity.
文摘Transition in the rate-dependent mechanical response of rock was investigated due to the presence of impersistent joint with different infill conditions.Four types of samples,i.e.intact,jointed with no grouting,jointed and grouted with cement,and jointed and grouted with epoxy,were fabricated using model material.A series of dynamic split Hopkinson pressure bar(SHPB) tests was conducted on prepared samples with strain rates varying between 53-130 salong with static uniaxial compression tests(10s).Progression of fracture/failure along samples was monitored using high-speed imaging and digital image correlation(DIC).Strength was observed to be significantly lower for jointed samples as compared to intact samples.However,the increasing trend of strength with strain rates remained similar for all types of samples.Epoxy was observed to be a better grout due to greater improvement in the strength of epoxy grouted jointed samples than cement grouted samples under both static and dynamic conditions.Significant changes were observed in fracture behavior(initiation,pattern and mechanism)with strain rate for intact and jointed unfilled/grouted samples.Fracturing was dominated by shear and tensile cracks at high strain rates compared to tensile cracks at low strain rates in all samples.Unlike static loading conditions,the location of cracks initiation shifts away from joint tips with increasing strain rate and depending upon existing infill conditions(unfilled/grouted).
基金Authors thank the CPRI Project(NPP/2016/HY/1/13042016)for partially supporting the study.Support from NHPC Ltd.and NTPC Ltd.is also thankfully acknowledged.
文摘Assessment of drillability of rocks is vital in the selection,operation,and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling.The commonly used rock drillability prediction methods,namely,drilling rate index(DRI)and Cerchar hardness index(CHI)have limitations in predicting the penetration rate due to differential wear of the cutting tool in rocks with varied hardness and abrasivity.Since cutting tools get blunt differently in different rocks,the stress beneath the tip of the bit decreases until it reaches a threshold value beyond which the penetration rate becomes constant.In this research,a new composite penetration rate index(CPRI)is suggested based on the investigations on four metamorphic rocks viz.quartzite,gneiss,schist and phyllite with varied hardness-abrasivity values.The penetration-time behavior was classified into active,moderate,passive,and dormant phases based on the reduction in penetration rate at different stages of drilling.A comparison of predicted penetration rate values using DRI and CPRI with actual penetration rate values clearly establishes the supremacy of CPRI.Micro-structure and hardness-based index was also developed and correlated with CPRI.The new indices can help predict cutting tool penetration and its consumption more accurately.
基金Project(51474251) supported by the National Natural Science Foundation of China
文摘A calculation formula of thermal-hydro-mechanical(THM)coupling crack initiation rate for brittle rock was derived based on the energy conservation law.The self-designed THM coupling fracture test with conductive adhesive electrical measurement method was applied to measuring the THM coupling crack propagation rate of brittle rock continuously.Research results show that both calculation and test results of crack initiation rate increased with increase of the temperature and the hydraulic pressure.They are almost in good agreement,which can prove validity of the calculation formula of THM coupling crack initiation rate.
文摘Prediction of the drilling penetration rate is one of the important parameters in mining operations. This parameter has a direct impact on the mine planning and cost of mining operations, Generally, effective parameters on the penetration rate is divided into two classes: rock mass properties and specifications of the machine, The chemical components of intact rock have a direct effect in determining rock mechan- ical properties, Theses parameters usually have not been investigated in any research on the rock drill- ability, In this study, physical and mechanical properties of iron ore were studied based on the amount of magnetite percent, According to the results of the tests, the effective parameters on the pen- etration rate of the rotary drilling machines were divided into three classes: specifications of the machi- nes, rock mass properties and chemical component of intact rock, Then, the rock drillahility was studied using rock engineering systems, The results showed that feed, rotation, rock mass index and iron oxide percent have important effect on penetration rate, Then a quadratic equation with 0,896 determination coefficient has been obtained, Also, the results showed that chemical components can he described as new parameters in rotary drill penetration rate,
基金Project(2014QNB31)supported by the Fundamental Research Funds for the Central Universities,ChinaProjects(51674248)supported by the National Natural Science Foundation of ChinaProject supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD),China
文摘Tackling the problems of underground water storage in collieries in arid regions requires knowledge of the effect of water intrusion and loading rate on the mechanical properties of and crack development in coal–rock combinations. Fifty-four coal–rock combinations were prepared and split equally into groups containing different moisture contents(dry, natural moisture and saturated) to conduct acoustic emission testing under uniaxial compression with loading rates ranging from 0.1 mm/min to 0.6 mm/min. The results show that the peak stress and strength-softening modulus, elastic modulus, strain-softening modulus, and post-peak modulus partly decrease with increasing moisture content and loading rate. In contrast, peak strain increases with increasing moisture content and fluctuates with rising loading rate. More significantly, the relationship between stiffness and stress, combined with accumulated counts of acoustic emission, can be used to precisely predict all phases of crack propagation. This is helpful in studying the impact of moisture content and loading rate on crack propagation and accurately calculating mechanical properties. We also determined that the stress thresholds of crack closure, crack initiation, and crack damage do not vary with changes of moisture content and loading rate, constituting 15.22%, 32.20%, and 80.98% of peak stress, respectively. These outcomes assist in developing approaches to water storage in coal mines, determining the necessary width of waterproof coal–rock pillars, and methods of supporting water-enriched roadways, while also advances understanding the mechanical properties of coal–rock combinations and laws of crack propagation.
基金supported by the National Natural Science Foundation of China(Grant Nos.52204115 and 41941018)the Foundation of Research Institute for Deep Underground Science and Engineering(Grant No.XD2021022).
文摘Nowadays,the development of novel metallic materials for rock support have attracted research interests since they can significantly improve the deformation and energy absorption capacities of rock bolts.Although previous studies proved the importance and mechanical advantages of utilizing high-strength and high-toughness(HSHT)steels in rock support,there is no systematic analysis to reveal the essential energy absorption parameter and the guidelines for further development of metallic rock support materials.This paper analyzes the energy absorption characteristics of novel HSHT steels(negative Poisson’s ratio(NPR)and twinning-induced plasticity(TWIP)steels)in comparison with conventional rock support materials.A physically based crystal plasticity(CP)model was set up and calibrated to study the effect of strain hardening rate(SHR).Meanwhile,the roles of underlying physical mechanisms,i.e.the dislocation density and twin volume fraction,were studied.The results show that the improvement of energy absorption density(EAD)is essential for further development of rock support materials,besides the increase of energy absorption rate(EAR)for previous development of conventional rock support materials.The increase of EAD requires increases of both strength and deformation capacity of materials.For HSHT steels,the decrease of SHR has a positive effect on the improvement of EAD.In addition,the increase of EAD is followed by the increase of twin volume fraction and the decrease of plastic Poisson’s ratio which can promote deformation plasticity of materials.Meanwhile,the increase of EAR is correlated with the accumulation of dislocation density,which can increase the strength of materials.This paper provides the theoretical basis and guidelines for developing rock support materials in deep underground engineering and other related fields.