Research On The Evolution And Failure Of Coal Wall Cracks Induced By Mining In Steeply Inclined Coal Seam

Aiming at the problem of coal face failure and instability in steeply dipping coal seam mining,a combination of on-site investigation,coal sample mechanical experiment,numerical simulation,physical similarity simulation and theoretical analysis is comprehensively used.The roof structure,the mechanical response of coal face,the evolution of fissures and the laws of failure and instability are systematically studied in steeply dipping coal seam mining,and the stability of the steeply dipping coal face is comprehensively identified.The following main conclusions are drawn:(1)When the coal face is under unidirectional stress,the uniaxial mechanical experiments show that the fissures develop slowly in the early stage of loading.As the specimen is loaded to the middle and late stage,when the initial load of deformation localization and rapid evolution accounts for 66.08%-69.51% of the peak load of the specimen,the fissures of the specimen develop rapidly and fail.At this time,the horizontal displacement has obvious sub-regional characteristics bounded by fissures,and increases rapidly.It shows that the coal face loses stress and displacement boundary constraints and becomes a free surface in this state,and its bearing capacity is reduced,which can easily lead to rib spalling.(2)According to the similar simulation and numerical simulation,the main roof first weighting interval are 49 m and 50 m respectively,and the average periodic weighting interval are 17 m and 16.67 m respectively in the process of coal seam mining.The roof begins to collapse ahead of the middle and upper part of the working face along the inclination,and slides down and fills the lower gob,showing the characteristics of partitioned and asymmetric failure.The maximum principal strain value at the roof keeps increasing up to 1.39,and the strain concentration position continuously shifts to the overlying strata.Within a certain span of the overlying strata,before the roof periodic weighting,the horizontal displacement of the coal face increases continuously,but the increase process is slow and the displacement is small.However,after the roof periodic weighting,the low-level roof is pressed frequently,and the horizontal displacement of the coal face increases greatly in an instant.When the coal is cut along the inclined direction to 130 m,the displacement of the coal face is small before the overlying strata collapse in a large range.When the coal was cut to 130 m and the overlying strata collapsed in a large range,the displacement of the coal face along the inclination direction,vertical direction,trend direction and total displacement increased instantaneously,and the order was larger.At this time,the failure of coal face is mainly manifested as the protruding to the outside of the free surface and the accompanying sliding.(3)Under the influence of mining,a certain degree of shear failure occurs to the coal face,and the fractures and deformation of the coal face in the upper and middle of the working face are more frequent and serious,and continue to spread to the lower part of the working face.Its failure forms are mostly "pyramid","wedge","inverted step shape" and "step shape".Especially during the periodic weighting,the plastic failure zone and the degree of failure of the coal face increase obviously,and maintain a large horizontal displacement.Along the inclination,the stress release situation is different due to the different degree of coal face failure.The coal face stress in the middle and upper part of the working face is fully released,and the stress value is small;The stress of the coal face in the lower part of the working face is not fully released,and the stress value is relatively large.Finally,there are obvious asymmetric distribution characteristics of stress release zone and plastic failure zone along the inclination.(4)By establishing the mechanical model of the "coal face-roof-support" system with a steeply dipping,the failure mechanism of the stope surrounding rock instability and the vicious cycle of the mutual influence of the rib spalling,the end face roof fall,and the failure of the support system are clarified.By constructing a failure mechanics model of the coal face of the steeply dipping working face,it is further shown that the slip-body of the steeply dipping coal rib spalling is an irregular quadrangular pyramid shape.And the slip-body begins to fracture and failure before the upper layer,and finally forms a spalling.At the same time,the influencing factors of the rib spalling and the quantitative relationship between each influencing factor and the coal face stability are determined.And further through numerical analysis,the influence law of coal seam mining height,dip angle,cohesion,joint spacing on the failure and instability of steeply dipping coal seam and the coal face displacement prediction model are obtained.Based on this,according to cloud theory,a cloud model of coal face stability for steeply dipping working face is established,and the coal face stability is comprehensively identified and reasonable coal face prevention measures are put forward.