Overstress Effect Of Rock And Coal And Study On Mechanism Of Overall Rock Burst Instability On Island Coal Panel In Underground Deep Mining

Rock burst is one of the typical mining dynamic disaters,with increasing mining depths,the frequency and intensity of rock burst hazards are becoming increasing in eastern coal mines(in-situ stress is more than 25 MPa),China,where rock burst in the island panel has become a common problem in deep coal mining.Due to the early strip pillar mining or unreasonable mining layout,rock burst control of the island panel is an urgent problem in need of solution.In this paper,the overall rock burst instability mechanism of the island panel,the burst failure characteristics and precursor information identification of coal samples under different loading and unloading stress paths,overlying strata structure and its evolution law,the theory and technology of the burst force and engeering practice of monitoring and prevention on the island panel were systematically studied,by combining in-situ investigation,theoritical analysis,laboratory experiment,numerical simulation and engeering practice.Rock burst characteristics in underground deep coal mining were investigated and analyzed.The results indicate that rock bursts have occured frequently during coal entry excavation,these areas are also those where rock bursts occur frequently during coal face mining.The coal seam is generally thick in the damage zone,where stress concentration and stress anomaly are present.The burst failure form changes from local rock burst to overall instability burst,which not only endangers the degree,but also extends the scope.Rock and coal mass in deep mining is significantly influenced by overstress effect,i.e.,the supervision of static and dynamic loads applied to the coal seam by surrounding rock under different loading and unloading stress paths is always greater than the strength of coal mass.The concept model of overstress effect was proposed,which can be divided into three types: rock burst induced by roadway excavation,rock burst induced by local loading and rock burst induced by overall instability.Experimental study was conducted on coal sample burst failure under overstress and trueaxial loading and unloading conditions,which revealed the overstress loading effect and model.Experimental results show that there exists significant time-delay effect under different loading and unloading stress paths.Compared to triaxial loading,coal samples are more easy to deform and failure under triaxial unoading.Under the constant confining pressure,burst failure occurs when the axial stress is more than 3 times uniaxial compressive strength of coal sample.The wave velocity inversion results show that the macrofractures position in coal samples after failure has a positive corresponding relation with the region of affluent wave velocity anomalies,and the low wave velocity zone corrects well with the areas of microfractures evolution.This paper puts forward the thory of overall rock burst instability on the island panel.It is pointed out that the overlying strata will move and break coordinately when the mine-out of adjacent multiple mining panel,which is the main inducing factor of the island pillar rock burst.The gravity stress and the transfeered stress from the overlying strata above the goaf are the main sources of static loads,further rupturing of the sub key stratum and its control overlying strata and the first weighting of the key stratum are the main sources of dynamic loads,rock burst will be easily induced by static and dynamic stresses superposition.Numerical simulation for the whole process of longwall coal mining was carried out.The soft-straining model was adopted to calculate the subsidence and deformation of roof,and the double-yield model was adopted to simulate consolidation of the cave-in rock materials.The critical plastic strain was used to identify the cave-in zone,fracture zone and bending zone.The evolution characteristics of the overlying strata is the main source of static and dynamic loads of the island panel.From the point of view of stress distribution and surface subsidence,energy released by roof rupture and fault slip,the possibility of overlying strata evolution monitored by microseismic method was revealed.Futher,the precursor information of the island pillar rock burst could be indentified.Based on the mechanism of overall rock burst instability,the pre-evaluation method of rock burst risk on the island coal panel was proposed,and the methods for monitoring and prevention of island pillar rock burst were summarized.Research achievements were applied to the island panels 3112 and 3108 in Chaoyang Coal Mine,Shandong Province.From the source of static and dynamic loads,the practice of monitoring and prevention of high stress and stress anomaly zones was conducted to eliminate the static stress concentration,and dynamic disturbance was weakened by taking break-tip blasting,field application results were remarkable.