| With the deep mining,the coal mine rock burst presents new features.As the depth of mining increases,the deep rock mass tends to be broken into block media.Under the conditions of high ground stress and excavation disturbance,the anomalously low friction effect of block rock is easy to occur,and then induce the anomalously low friction rock burst.Therefore,a new concept of anomalously low friction rock burst was proposed.Taking the block rock as the research object,the anomalously low friction effect of the deep block rock was studied through theoretical analysis,experimental research and numerical simulation.The dynamic model of rock mass with anomalously low friction effect under vertical impact and overburden pressure was established,and the displacement and normal force criteria for the occurrence of anomalously low friction effect were proposed.A bidirectional disturbance and anomalously low friction effect test device was innovatively developed.The critical lateral pressure ratio was defined as a quantitative index for the deep and shallow mining.The range of lateral pressure ratio of the anomalously low friction rock burst with high ground pressure and frequency induced by coal and rock was given.The energy release characteristics of the dynamic response of anomalously low friction rock burst were analyzed.The conclusions provide important references for deep understanding the mechanism of deep rock burst in coal mines.(1)A new concept of anomalously low friction rock burst was proposed,and a block dynamic model of anomalously low friction effect under vertical impact and overburden pressure was established.The displacement and normal force criteria for the occurrence of anomalously low friction effect were proposed.Considering the characteristics of deep rock masses,a block dynamic model of anomalously low friction effect under vertical impact and overburden pressure was established.The displacement and normal force criteria for the occurrence of anomalously low friction effect were given.The influence of vertical impact energy,block mass,stiffness of structural plane between blocks and damp on block acceleration and normal force between blocks were analyzed.The results show that the acceleration amplitude of the working block increases with the increase of vertical impact energy and the stiffness of structural plane,decreases with the increase of block mass and damp between the blocks.The anomalously low friction effect occurs during the phase of detachment and weak extrusion between blocks.With the increase of the stiffness of structural plane,the normal force of block decreases with a quadratic function,and the vibration period decreases exponentially.The smaller the mass of the block,the closer the impact frequency is to the low frequency.The normal force reduction of the block is larger,the vibration period of the block is smaller,the anomalously low friction rock burst is more easily to occur.(2)The variation of anomalously low friction effect in the block rock mass structure under the unidirectional impact disturbance was analyzed.Comparing the experimental results with theoretical models and numerical simulations,the correctness of the theoretical model and the feasibility of numerical simulation were verified.The dominant direction and key structural plane of the anomalously low friction effect were determined.Considering disturbance effects such as roof fracture,engineering blasting,and mechanical vibration,the anomalously low friction effect experiment of block rock mass under unidirectional impact disturbance was independently designed.The dominant direction and key structural plane of the occurrence of anomalously low friction effect were obtained by experiments,and the influence of the characteristics of weak structural plane on the anomalously low friction rock burst was proposed.The numerical simulation method was used to analyze the influence of block size,lithology,coal-rock combination and coal seam inclination on anomalously low friction rock burst.The results show that the length direction of working block is the dominant direction of the anomalously low friction effect,and the interface at the bottom of the working block is the key structural plane of the anomalously low friction effect.The smaller the elastic modulus of the weak structural plane between the blocks,the thinner the structural surface thickness and the smaller the rock mass size,the smaller the elastic modulus of the rock mass,the greater the impact disturbance intensity,and the larger the difference between the elastic modulus of the coal and rock,the easier the anomalously low friction rock burst occurrence.Coal seam inclination has a significant effect on anomalously low friction rock burst.When the coal seam inclination angle ?<45°,the anomalously low friction rock burst is easy to occur at the bottom of the coal seam.When coal seam inclination angle ??45°,the friction self-locking occurs at the bottom of the coal seam,and anomalously low friction rock burst caused by shear force is easily generated at the top of the coal seam.When the coal seam inclination angle ?<60°,only the vertical impact disturbance has a significant effect on the anomalously low friction rock burst.When the coal seam inclination angle ?>60°,the vertical impact disturbance and the horizontal impact disturbance are both the main influencing factors for the occurrence of anomalously low friction rock burst.(3)A bidirectional disturbance and anomalously low friction effect test device was developed,and the variation of anomalously low friction effect in block rock under bidirectional disturbance was studied,and the existing of critical stress in anomalously low friction rock burst was found.Using the self-developed bidirectional disturbance test device of anomalously low friction effect,anomalously low friction effect experiments were performed on 120 intact rock blocks,96 fractured rock blocks,and 90 coal blocks.It was found that the anomalously low friction rock burst has a critical stress state,and there is a critical value for the occurrence of anomalously low friction rock burst under confining pressure,axial compression,and horizontal impact.When the working blocks are fractured rock and coal,the anomalously low friction effect strength is greater than that of homogeneous and uniform size blocks.It is indicated that the anomalously low friction rock burst is more likely to occur in heterogeneous and non-uniform size blocks.The degree of fragmentation,that is,the block size impact on anomalously low friction rock burst is more significant than that of lithology,which also explains the reason that rock fractures such as fault and folds are more susceptible to rock burst.There is a significant influence range on occurrence of anomalously low friction effect.The effective stress range of the intact rock is 6MPa?10MPa,the coal is 4MPa?5MPa,and the broken rock is 6MPa?11MPa.(4)The influence of lateral pressure ratio on anomalously low friction effect was analyzed,and the critical lateral pressure ratio was proposed as the quantitative definition index of deep and shallow excavation.The range of lateral pressure ratio of rock and coal with anomalously low friction rock burst great strength and high frequency was presented.The lateral pressure ratio is the ratio of horizontal to vertical stress,which reflects the stress state of the deep rock mass,and its value decreases with depth.Taking the stacked rock,broken rock and coal blocks as the research object,the horizontal displacement amplitude of the working block was regarded as the characteristic parameter of the anomalously low friction effect,and the influence of the lateral pressure ratio was analyzed based on the experimental results.The results show that with the increase of the lateral pressure ratio,the amplitude of the horizontal displacement increases first and then decreases,indicating that the occurrence of anomalously low friction rock burst is intermittent.The lateral pressure ratio has an significant impact on anomalously low friction effect,and the lateral pressure ratio is between 0.4 and 1.33.The anomalously low friction effect of rock mass changes drastically,showing a strong influence on the interval effect.The horizontal displacement amplitude distribution has displacement-intensive high value areas and displacement sparse low value areas,and the displacement-intensive high value areas correspond to deep mining levels.The displacement sparse low value areas correspond to the shallow mining level,and the lateral pressure ratio range of the rock mass with anomalously low friction impact strength and high frequency is determined to be 0.4?1.33.For the broken rock,the strength of anomalously low friction rock burst is the highest when the lateral pressure ratio is 1.The coal anomalously low friction rock burst is 0.5 to 0.8.It is proposed that the critical lateral pressure ratio can be defined as a quantitative definition for the deep and shallow excavation,the critical lateral pressure ratio of rock is 1,and the critical pressure ratio of coal between the deep and the shallow is 0.8.Less than the critical lateral pressure ratio,the horizontal displacement curve shows a distinct nonlinear characteristic.Larger than the critical lateral pressure ratio,and the horizontal displacement curve shows an approximately linear characteristic.It is founded that when the lateral pressure ratio is less than 1,the non-coal mine will enter into deep mining,and coal mining will enter into deep mining when the lateral pressure ratio is less than 0.8.(5)The dynamic response characteristics of energy release in the process of anomalously low friction rock burst were analyzed.It is found that the kinetic energy of rock mass has agglomeration effect and short-time peak characteristic.A new method is proposed to invert the magnitude of rock burst by using the characteristics of kinetic energy change obtained through the anomalously low friction effect experiment.With the increase of axial pressure,that is,the mining depth increases,the block kinetic energy of the anomalously low friction rock burst has an agglomeration effect,and the main peak of kinetic energy appears rapidly after the block is disturbed.After being disturbance stimulated,the dynamic response time is significantly shortened,and the block kinetic energy concentration reaches a maximum in a very short time,that is,the short-term peak characteristics.The degree of fragmentation and lithology are the main controlling factors to the total kinetic energy for anomalously low friction rock burst.The results show that only when the axial pressure increases to a certain value,that is when the mining reaches a certain depth,the anomalously low friction effect between the blocks will induce and once the rock burst in the same mining area occurs,there is a danger of recurrence.It shows that the occurrence of anomalously low friction rock burst is intermittent.The experiment reproduced the rock burst process from the fore shock to the main shock.It is found that when the energy of anomalously low friction rock burst is released,the kinetic energy of coal and rock has the characteristics of shifting from high frequency to low frequency,and the main frequency is mainly in the low frequency within 20 Hz.The results are in accordance with the on-site rock burst monitoring data,which verifies the reliability of the experiments.There are 105 figures,27 tables,50 variables and 125 references in this paper. |
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