Numerical modeling on strain energy evolution in rock system interaction with energy-absorbing prop and rock bolt

The interaction mechanism between coal and rock masses with supporting materials is significant in roadway control, especially in deep underground mining situations where dynamic hazards frequently happened due to high geo-stress and strong disturbed effects. This paper is to investigate the strain energy evolution in the interaction between coal and rock masses with self-designed energy-absorbing props and rock bolts by numerical modeling with the finite difference method. The interaction between rock and rock bolt/prop is accomplished by the cables element and the interface between the inner and outer props. Roadway excavation and coal extraction conditions in deep mining are numerically employed to investigate deformation, plastic zone ranges, strain energy input, accumulation, dissipation,and release. The effect on strain energy input, accumulation, dissipation, and release with rock deformation, and the plastic zone is addressed. A ratio of strain energy accumulation, dissipation, and release with energy input a, β, γ is to assess the dynamic hazards. The effects on roadway excavation and coal extraction steps of a, β, γ are discussed. The results show that:(1) In deep high geo-stress roadways, the energyabsorbing support system plays a dual role in resisting deformation and reducing the scope of plastic zones in surrounding rock, as well as absorbing energy release in the surrounding rock, especially in the coal extraction state to mitigate disturbed effects.(2) The strain energy input, accumulation is dependent on roadway deformation, the strain energy dissipation is relied on plastic zone area and disturbed effects, and strain energy release density is the difference among the three. The function of energyabsorbing rock bolts and props play a key role to mitigate strain energy release density and amount, especially in coal extraction condition, with a peak density value from 4×10^(4) to 1×10^(4)J/m^(3), and amount value from 3.57×10^(8) to 1.90×10^(6)J.(3) When mining is advanced in small steps, the strain energy accumulation is dominated. While in a large step, the released energy is dominant, thus a more dynamic hazards proneness. The energy-absorbing rock bolt and prop can reduce three times strain energy release amount, thus reducing the dynamic hazards. The results suggest that energy-absorbing props and rock bolts can effectively reduce the strain energy in the coal and rock masses, and prevent rock bursts and other hazards.The numerical model developed in this study can also be used to optimize the design of energyabsorbing props and rock bolts for specific mining conditions.

Effect of dynamic loading conditions on the dynamic performance of MP1 energy-absorbing rockbolts:Insight from laboratory drop test

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.

Coalbursts in China: Theory, practice and management

Coalburst is one of the most serious disasters that threaten the safe production of coal mines, and this disaster is particularly serious in China. This paper presents an overview of coalbursts in China since 1980s. From the "stress and energy" and "regional and local" perspectives, the achievements in the theory, practice and management of coalbursts in China are systematically summarized. A theoretical system of coalbursts has been formed to reveal the deformational behavior of coalbursts and explain the mechanism of coalbursts. The occurrence conditions of coalbursts are put forward and the critical stress is obtained. The stress index method for risk evaluation of coalbursts before mining is proposed, and the deformation localization prediction method of coalbursts is put forward. The relationship between energy release and absorption in the process of coalbursts is found, and the prevention and control methods of coalbursts, including the regional method, the local method and support, are presented. The safety evaluation index of coalburst prevention and control is put forward. The integrated prevention and control method for coal and gas outbursts is proposed. The prevention and control technology and equipment of coalbursts have also been developed. Amongst them, the distribution law of the critical stress in China coalburst mines is discovered. The technology and equipment for monitoring, prevention and control of coalbursts, as well as for integrated prevention and control of combined coalbursts and other disasters, have been developed. The energy-absorbing and coalburst-preventing support technology for roadways is invented, and key engineering parameters of coalburst prevention and control are pointed out. In China, coalburst prevention and control laws and standards have been developed. Technical standards for coalbursts are formulated, statute and regulations for coal mines are established, and regulatory documents are promoted.

Dynamic mechanical characteristics and application of constant resistance energy-absorbing supporting material

In deep underground engineering,rock burst and other dynamic disasters are prone to occur due to stress concentration and energy accumulation in surrounding rock.The control of dynamic disasters requires bolts and cables with high strength,high elongation,and high energy-absorbing capacity.Therefore,a constant resistance energy-absorbing(CREA)material is developed.In this study,the dynamic characteristics of the new material are obtained via the drop hammer tests and the Split Hopkinson Pressure Bar(SHPB)tests of the new material and two common bolt(CB)materials widely used in the field.The test results of drop hammer test and SHPB test show that the percentage elongation of CREA material is more than 2.64 and 3.22 times those of the CB material,and the total impact energy acting on CREA material is more than 18.50 and 21.84 times,respectively,indicating that the new material has high elongation and high energy-absorbing capacity.Subsequently,the CREA bolts and cables using the new material are developed,which are applied in roadways with high stress and strong dynamic disturbance.The field monitoring results show that CREA bolts and cables can effectively control the surrounding rock deformation and ensure engineering safety.

A new energy-absorbing bolt used for large deformation control of tunnel surrounding rock

In order to control the large deformation of tunnel surrounding rock,a new energy-absorbing bolt is developed.This bolt can be transformed into a rigid support when the deformation of the surrounding rock reaches the length of the sleeve tube,thus preventing the surrounding rock from continuing to deform.Moreover,this bolt has a simple structure and is easy to manufacture and assemble.Then the static tensile test is conducted on the bolt specimen to test its working performance.The test results show that when the cone angle of the cone block is small,the load–displacement curve of the bolt contains three stages;when the cone angle is large,the load–displacement curve contains only two stages.Meanwhile,both the average constant resistance and the maximum absorbed energy increase linearly with the increase of cone angle.On this basis,ignoring the influence of shear stress,and it is supposed that the thickness of the sleeve tube is constant,then the theoretical calculation formula of constant resistance for the new bolt is derived,and the rationality of the formula is verified using the static tensile test results.It is found that the error of the calculated result is less than 15%when the cone angle does not exceed 15.At last,the numerical simulation method is used to analyze the performance of the new bolt.The simulation results indicate that the generation of shear stress and the change of tube thickness during the movement of the cone block are two important factors that cause theoretical errors.

Energy-absorbing porous materials:Bioinspired architecture and fabrication

Energy-absorbing materials are widely used in transportations,sports,and the military applications.Particularly,porous materials,including natural and artificial materials,have attracted tremendous attentions due to their light weight and excellent energy absorption capability.This review summarizes the recent progresses in the natural and artificial energy-absorbing porous materials.First,we review the typical natural porous materials including cuttlebone,bighorn sheep horn,pomelo peel,and sunflower stem pith.The architectures,energy absorption abilities,and mechanisms of these typical natural materials and their bioinspired materials are summarized.Then,we provide a review on the fabrication methods of artificial energy-absorbing porous materials,such as conventional foaming and three-dimensional(3D)printing.Finally,we address the challenges and prospects for the future development of energy-absorbing porous materials.More importantly,our review provides a direct guidance for the design and fabrication of energy-absorbing porous materials required for various engineering applications.

A cooperative model of photovoltaic and electricity-to-hydrogen including green certificate trading under the conditional value at risk

Cooperation in energy systems is no longer limited to the distribution of electricity,and more attention is paid to the trading of green certificates(GCs).This paper proposed a cooperative method for photovoltaic(PV)and electric-to-hydrogen(EH)trading,including GC trading under risk management.First,a novel PV and EH model is established and the cooperation mechanism is analyzed.Meanwhile,PV and EH models were risk-controlled using the conditional value at risk to reduce the impact of the uncertainty of PV electricity and EH loads.Then,the PV-EH cooperative model was established based on cooperative game theory;this was then divided into two subproblems of“cooperative benefit maximization”and“transaction payment negotiation,”and the above two subproblems were solved distributively by alternating direction multiplier method(ADMM).Only energy transactions and price negotiations were conducted between the PV and EH,which can protect the privacy and confidentiality of each entity.Finally,the effectiveness of the cooperation model was verified using a practical engineering case.The simulation results show that the cooperation of the PV-EH can significantly improve the operational efficiency of each entity and the overall efficiency of the cooperation and realize the efficient redistribution of electricity and GC.

Rockburst mechanism research and its control

A new rockburst classification, innovative works in developing a ‘‘strainburst test machine" and an‘‘impact-induced rockburst test machine" that can reproduce rockbursts in laboratory were researched.New concepts were proposed regarding the stress paths that take into account both the static and dynamic stresses analogous to that at excavation boundaries for generating artificially-induced strainburst and impact-induced rockburst. As an important method for rockburst control, a novel energyabsorbing bolt was developed, which has a constant-resistance under both static and impact loadings and a large-elongation capacity for containing large deformations of rock masses under burst-prone conditions.

A critical review on the developments of rock support systems in high stress ground conditions

Extreme ground behaviour in high-stress rock masses such as rockburst prone and squeezing ground conditions are encountered in a range of underground projects both in civil and mining applications.The occurrence of such ground behaviour types are difficult to predict and special design and construction measures must be taken to control them.Determining the most appropriate support system in such grounds is one of the major challenges for ground control engineers because there are many contributing factors to be considered,such as the rock mass parameters,the stress condition,the type and performance of the support systems,the condition of major geological structures and the size and geometry of the underground excavation.The main characteristics and support requirements of rockburst-prone and squeezing ground conditions are herein critically reviewed and characteristics of support functions are discussed.Different types of energy-absorbing rockbolts and other support elements applicable for ground support in burst-prone and squeezing grounds are introduced.Important differences in the choice and economics of ground support strategies in high-stress ground conditions between civil tunnels and mining excavations are discussed.Ground support benchmarking data and mitigation measures for mines and civil tunnels in burst-prone,squeezing and heavily swelling grounds conditions are briefly presented by some examples in practice.

Correlation study between the square-coneenergy-absorbing structure and the frontal collisionbehaviour of leading vehicles

In order to study the influence of square-cone energy-absorbing structures on the mechanical behaviour of the ollision performance of the leading vehicle,a parameterization method for rapidly changing the performance of energy-absorbing structures was proposed.Firstly,a finite element simulation model of the collision of the leading vehicle with a square-cone energy-absorbing structure was constructed.Then,the platform force,the slope of the platform force and the initial peak force of the force-displacement curve derived from the energy-absorbing structure were studied for the collision performance of the leading vehicle.Finally,the correlation model of the square-cone energy-absorbing structure and the mechanical behaviour of the collision performance of the leading vehicle was established by the response surface method.The results showed that the increase of the platform force of the energy-absorbing structure can effectively buffer the longitudinal impact of the train and reduce the nodding attitude of the train.The increase of the platform force slope can not only effectively buffer the longitudinal impact and vertical nodding of the train,but also reduce the lateral swing of the train.An increase in the initial peak force to a certain extent may lead to a change in the deformation mode,thereby reducing the energy-absorption fficiency.The correlation model can guide the design of the square-cone energy-absorbing structure and predict the deformation attitude of the leading vehicle.