Occurrence Mechanism Of Rock Burst And Its Prevention Methods Under Complicated Geological Conditions

In recent years,with the economic transformation,industrial structure adjustment,natural gas and other clean energy consumption increased and the coal overcapacity,resulting in China's coal consumption accounted showed a trend of decrease,but the overall proportion remains above 60%.Coal in our country has been in a state of high intensity mining,the mine mining depth and strength increasing continually,the phenomenon that some mining area resources reserve insufficient has appeared,many mines had to turn to the coal seam in the mining that geological conditions are relatively complex.However,when the coal seams in the complex geological structures such as fold and fault are mined,it is very easy to be induced rock-burst by mining disturbance and strong tectonic stress disturbance.In this paper,during the working face mining and excavation of rock-burst appearance and destruction features in two kinds of typical geological structure condition as foothold,around the distribution regularity of mining shock in typical faults structure zone and typical folds structure area of mining activities has carried on the detailed analysis,and for the mining research of mining zone No.7 of Baojishan coal mine that is located in the wing of syncline structure and near F46 multiple tectonic area provided pointcuts.Comprehensive use of field investigation,theoretical modeling analysis,laboratory test,numerical simulation calculation and field engineering practice,The relevance of dynamic load or static load dominate the action of load superposition induced by rock-burst,the evolution regulation between the working face and bearing stress along the roadway on both sides of the gob in coal and the change of coal seam dip angle,the fault slip activation law and overburden fracture characteristics under irregular working face mining distribution,the prevention and control mechanism and industrial test for the multiple tectonic zone of rock-burst and other issues are studied systematically.The main conclusions are as follows:(1)The production geological data of Yuejin coal mine in Yima and Yanbei coal mine in Huating were collected.The temporal and spatial evolution of microseismic events during No.25110 working face mining and excavation which affected by the adjacent F16 thrust fault and the small fault structure in the working face,and working face of mining zone No.2520 that affected by syncline-anticlinal structure across the working surface.It is indicated that the serious rock-burst in the mining and excavation space induced by the dynamic load caused by fault and high concentration static load caused by convoluted geological structure.(2)Coal and rock under lower static load,the elastic strain energy of rock-burst caused by buckling failure takes place mainly supplied by the external dynamic load and dynamic load at this time in dynamic and static load superposition play a leading role;coal and rock under high concentration static load,because of accumulating a large amount of elastic strain energy,the elastic strain energy of rock-burst caused by buckling failure takes place mainly supplied by the internal static load,and now static load plays a leading role in dynamic and static load superposition,dynamic load plays a auxiliary role in that.(3)The bursting liability test results of 1# coal seam in mining zone No.7: dynamic failure time DT=19.2,elastic energy index WET=4.26,impact energy index KE=10.43 and uniaxial compressive strength Rc=27.35 MPa,the comprehensive evaluation has class III strong bursting liability;test results of roof bursting liability on the top of coal seam: recombination roof curve energy index UWQ=65.61 k J,has class II weak bursting liability.(4)Mechanical model of working face along seam tendency is established based on Winkler elastic groundwork assumption,and the vertical compressive displacement of coal in the upper and lower sides of the gob is calculated respectively.The results show: as the dip angle of coal seam from 45° to 0° degressive changes,the vertical compression displacement peak increases gradually in the entity coal that up and down on both sides of gob,the peak position also gradually transfer to the deeper coal;under the same coal seam dip,the peak displacement of the coal in the lower side of the gob is significantly larger than that of the upper body coal,but the peak position is slightly less than the entity coal of the upper side;the effect that the vertical compressive displacement greater-than 0.1 m in the body coal upper side of the gob is larger than that of the lower ones;the vertical compressive displacement in the entity coal that up and down on both sides of gob are positively correlated to the vertical support stress of the overlying strata.Therefore,the distribution law of the vertical compression displacement can directly reflect the distribution of the corresponding supporting stress.(5)The numerical simulation results show that the evolution law of the stress field during the working face of this section is obtained with the decrease of the coal seam inclination: the peak stress of the working face end increases from 43.67 MPa to 136.59 MPa,dip angle of coal seam at 30° started to L area of stress concentration,and the stress concentration degree is higher and higher,formed the high concentration dead load;the peak stress in coal pillar from 86.18 MPa reduce to 57.33 MPa,the peak stress in coal pillar damping is less-than L in peak stress,on the whole present a high concentration static load transfered from the coal pillar to the entity coal;the position of the peak point of the body coal side increased to the front of the coal wall in the working face,and increased from 8.49 m to 12.03 m,and the distance from the coal wall increased from 3.79 m to 8.05 m;the position of the peak point of the coal pillar is also increasing to the front of the coal wall,which increases from 4.51 m to 9.58 m,which is roughly the same distance as the roadway,is kept within 4 m of the coal pillar.(6)Based on Winkler elastic groundwork assumption,the vertical compressive displacement of the two coal bodies in the roadway is calculated respectively.The results show: as the dip angle of coal seam from 45° to 0° degressive changes,the position of the vertical compression displacement peak in the coal pillar keep in the 4 m from the coal wall basically,the peak displacement is also diminishing;the vertical compression displacement in the both sides of coal roadway and bearing stress of overlying strata of coal is positively correlated relationship,so the distribution of the vertical compression displacement can intuitively reflect the distribution law of the corresponding bearing stress.It is indicated that with the decrease of dip angle of coal seam,the narrow coal pillar along gob-side entry present a tendency of weakened capacity and lessened stress concentration due to damage,the entity coal presents the tendency of increasing with the stress concentration because of the stress transfer.(7)According to "X" breaking theory,the stability of the curved triangular block in the roof of the roadway along gob is determined based on calculating the coefficient of rotary instability and sliding instability,that results show: the lower coal seam inclination can restrain the possibility of the sliding instability of the key block B,but it can effectively promote the possibility of the rotary instability of the key block B;the greater depth of coal seam can promote the possibility of rotary instability and sliding instability of the key block B;improve support strength of the side of coal pillar,roof and body coal can reduce rotary instability of the key block B,and support intensity change on the side of coal pillar and roof has a higher sensitivity for the rotary instability coefficient,the body coal has a lower sensitivity.(8)Numerical simulation results show that during the No.707 irregular mi ning face,the wide mining face(170 m)of the early stage,the shear displacement distribution of fault structure plane tend to 0 m,the fault structural stability is good,is difficult to happen the slip activation;the wide and narrow mining face(170 m and 85 m)of the medium transition stage,the maximum shear displacement on the fault structure on is 0.142 m,the stability of fault structural gets worse sharply,easy to induce fault structure to slip activation instability;the narrow mining face(85 m)of the latter stage,the maximum shear displacement on the fault structure is 0.105 m,the fault structural stability is better than last stage,easy to slip activation instability for fault structural.(9)Mechanical model of stress state of the fault tectonic zone is established.It is concluded that the conditions for the fault activation are as follows:For the same fault inclination ?,with the increase of lateral pressure coefficient ?,the corresponding function ?(?,?)curve is monotonically increasing(except for ?=0° and 90°).It is indicate that the fault of adjacent to the oblique geological structure axis,due to the affected by the strong tectonic stress field has biggish ?,so the fault is in a state of stable atresia and accumulated a large number of elastic strain energy.The large amount of elastic strain energy can be released when suffered by mining disturbance,which can form a strong dynamic load source.(10)The numerical simulation results of No.707 irregular working face shows: the maximum subsidence displacement of the key strata in the overlying strata of the gob during the stage of wide working face and the transition stage of wide and narrow working face is close to 0.6 m,which can be determined that the key layer is broken at this time;the maximum subsidence displacement of the key strata in the overlying strata of the gob during the stage of narrow working face is close to 0.16 m,which can be determined that the key layer sinks less and is not broken.Based on the theory of thin plate,the formula for the trapezoidal body breaking height of overlying strata is as follows:According to the geological bar chart of the mining location of No.707 irregular working face in the mining area and the above formula,the calculation results is same as the numerical simulation result.(11)Mechanical model is established to analyze the influence of breaking of key strata in the overlying strata for the supporting stress in the surrounding coal wall,computational analysis shows that the rupture of key strata in overlying strata in the gob can lead to the increase of peak stress and influence range of the forward supporting stress in the coal wall along the direction of the working face;and also known that the peak stress and the influence range of the inner side of the coal wall along the inclination of the working face are increased with the breaking of the key layer.It can be seen that the rupture of key strata in overlying strata will cause the high concentration stress and its influence range to be further increased in the surrounding coal mining area,thus forming a higher concentrated static load.(12)Through a comprehensive analysis for the mechanism of automatic unloading of solid coal and the mechanism of gangue-triangular pillar loading and unloading and the mechanism of unloading of the gangue roof,dislocation along gob-side entry can effectively reduce the occurrence of rock-burst,and its key technology lies in surrounding rock unloading pressure structure consist of crushing gangue roof,gangue-triangular pillar and unloading press entity coal.The mining engineering example in No.25110 working face near F16 thrust fault in Yuejing mine,Yima proves that the technology can effectively prevent and control the roadway subjected to strong dynamic load disturbance.(13)The high pressure water jet unloading technique can transfer and release stress where the coal body which is affected by the high concentration stress,so that the original stress state can be redistributed.The unloading at the centre of coal body forms "weak structure" elastic and plastic zone,which has significantly prevent effect to start impact energy transfer,and shallow coal around roadway as a protection zone of high pressure water jet drilling and supporting system constitutes the structure of "strong structure" area,its impact energy releases to the roadway space has the second stop.The mining engineering example in No.250203 working face located in the syncline-anticlinal fold structure in Yanbei mine,Huating proves that the technology can effectively prevent and control the roadway subjected to high concentration static load influence.(14)No.706 working face as for the following mining face No.705 which the rock-burst happened frequently,and has similar engineering geology conditions with No.705 working face.During the No.706 working face mining,bases on the dynamic and static load induced shock mechanism,carries out the roadway surrounding rock unloading and reinforcement synergy controlling technology.Electromagnetic radiation monitoring and mine pressure observation results show that the control effect of surrounding rock of roadway is good,especially the around coal pillar which is easy to transform affected by mining pressure also got great control,shows that the plan measures achieves the expected control effect.