Study on the Energy Dissipated Mechanism of Large-diameter Borehole for Coalburst Control
Supervisor:jiang jin quan ning jian guo
In china since the shallow coal resource is exhausted at some coalfields with each passing day,deep coal resources developing is extremely urgent to meet energy demand.In deep coal mining,the frequency and intensity of the coalburst is more intense,which brings great threat to the safety of coal mine production.Therefore,it is necessary to make a comprehensive study on the prevention and control of rockburst.At present,owing to the advantages of low disturbance,shortcut,convenience and so on,the largar-diameter borehole is used to prevent the harzard of coalburst in most coalburst-threaten coal mines.Actually,the academic community has reached a consensus on the energy relief effect of largar-diameter borehole.However,the energy dissipation mechanism of largar-diameter borehole for coalburst mitigation have not been studied,and it is still based on numerical simulation and engineering analogy to design layout parameter of largar-diameter borehole(borehole length,diameter and spacing etc.).Aa a result,some coalburst-threaten coal mine still has the coalburst hazard after the implementation of the large diameter borehole.For this reason,laboratory test,theoretical analysis,numerical simulation,theoretical analysis and field moitoring methods are used to research the failure mechanism of largar-diameter borehole,and study its energy dissipation evolution law,and then reveal the mechanism of largar-diameter borehole for coalburst mitigation.Finally,quantitative design method of largar-diameter borehole parameter for coalburst mitigation is introduced.The main research contents and achievements are as follows:(1)Conventional and cyclic loading-unloading triaxial compression tests and acoustic-emission experiments were carried out for coal specimens to study the influence of confining pressure on the stress-strain relations,volume strain-axial strain curve,peak axial strain and Maximum volume strain.On the basis of understanding of AE features,and energy evolution characteristics,the early-warning critical energy for coalburst assessment was introduced.The point at which the axial stress 86.23%of the peak strength indicates an increase in the risk of coal instability.The corresponding elastic strain energy accumulated within the coal can be regarded as a precursor to coalburst.Accordingly,a fitting formula is presented to predict the early-warning critical energy for brittle coal subject to different minimum principal stress.(2)Considering the goaf compression and stress recovery,a numerical model was built and verified using a simulation case study.According to the mining and geological conditions of longwall face 5302 in Xinhe coal mine,the redistribution of mining-induced energy during the advancing of longwall face was study,and revealed the dynamic evolution law of the mining-induced energy.Combinated the early-warning critical energy and redistribution of mining-induced energy,the distribution of coalburst-prone zone around longwall face 5302 was discussed.(3)In order to investigate the failure process of largar-diameter borehole,the particle simulation method was used in numerical simulations to discuss crack propagation,microcrack distribution.Then,dynamic failure process of largar-diameter borehole and time-space evolution of energy around longface face induced by borehole failure.The influence of coal mass strength,borehole diameter,side pressure coefficient and pressure on the borehole failure and stress adjustment was discussed,and the effect of borehole spacing and length on the distribution of energy was analysed.(4)A single large-diameter borehole was divided into two parts:borehole wall and borehole bottom.Using fracture mechanics and elastoplastic mechanics,etc.,the borehole collapse zone and plastic zone radiu of the borehole wall and borehole bottom were analytical obtained and verified with simulation study.Based on the theory of energy dissipation,dissipative energy of the borehole collapse zone and plastic zone were also analytical obtained,and energy dissipation efficiency was introduced to discuss the influence of coal mass strength,borehole diameter,pressure,borehole spacing and length on the energy dissipation efficiency.Finally,the mechanism of largar-diameter borehole for coalburst mitigation was revealed.(5)On the basis of understanding of the mechanism of largar-diameter borehole for coalburst mitigation,the redistribution of mining-induced energy and the distribution of coalburst-prone zone,quantitative design method of largar-diameter borehole parameter for coalburst mitigation was introduced,which was used to design largar-diameter borehole layout parameters(borehole length,diameter and spacing etc.)for mitigation of coalburst in longwall 5302 of xinhe coal mine.Field practice showed that the designed largar-diameter borehole reduced the stress or energy concentration,and mitigation of coalburst reached a certain effect.