Study On Deformation And Fracture Law Of Overlying Strata In Deep Large Mining Height Working Face

In this thesis,taking the mining action of 12110 deep large mining height working face in Shoushan No.1 Coal Mine as the main line,the deformation field characteristics and breaking process of overlying strata under the mining action of deep large mining height working face were studied by theoretical calculation,similar model test and numerical simulation method.The deformation and fracture of overlying strata and the distribution of stress field in 12110 working face were obtained,and the influence of buried depth and mining height on the deformation and fracture evolution of overlying strata was analyzed based on 3D block discrete element software 3DEC.The main research contents and conclusions are as follows:(1)The overlying strata of 12110 working face were sampled and prepared,and the basic mechanical parameters and mineral composition of the overlying strata were obtained through laboratory tests.Based on the key strata theory,the key stratum of the overlying strata on the 12110 large mining height working face were determined,the initial caving step distance and periodic caving step distance of the strata were estimated,and the height distribution of the “three zones” considering the compaction effect was calculated.(2)The physical similarity model test was conducted on coal seam mining in deep large mining height working face.Based on the similarity theory,the similarity ratio of the physical similarity simulation experiments was determined.The similar materials with different ratios were prepared by orthogonal test,and the optimal similarity ratio of each rock layer was determined by comparing the physical and mechanical parameter indicators.The response characteristics of the stress of overlying rock at the measuring point under the mining action of the deep large mining height working face were analyzed,the stress evolution process of the overburden rock under the mining action was revealed,and the characteristics of the overlying rock fracture and structural evolution were discussed.Coal seam mining activities have disrupted the equilibrium state of the initial stress field of the surrounding rock near the working face,causing stress redistribution in the surrounding rock mass: with the continuous advancement of the lower coal seam mining,the stress of the overlying rock mass on the working face spreads and evolves around,and then evolves the abutment pressure zone in the coal wall and surrounding rock around the working face.The distribution of abutment stress zone: it increases rapidly from the exposed coal wall of the working face to the forward rock mass,forms stress concentration at the front end of the coal wall,and then gradually decreases to the undisturbed area to the original rock stress.The overlying rock roof of 12110 deep large mining height working face presents the structural characteristics of “composite cantilever beam structure,non-articulated rock plate structure,and articulated rock plate structure”.The immediate roof exists in the form of composite cantilever beam structure,and the periodic breaking of the cantilever beam structure evolves into non-articulated rock plate structure and articulated rock plate structure in the lower part,weakening the supporting effect of the lower gangue on the upper overlying rock and aggravates the breaking of the basic roof strata.(3)During the experiment,high-precision cameras were used to synchronously capture the settlement and deformation process of the overlying rock under the excavation of the working face.Based on the digital image correlation(DIC)method,the settlement of the overlying rock under mining action was quantitatively analyzed,and the deformation and fracture process of the overlying rock layer in the goaf was revealed through the evolution of the vertical displacement field cloud map.Under the action of mining,the hard rock in the upper strata of the working face and the soft rock in the lower strata show obvious deformation disharmony and separation,and the deformation intensifies as the advance of the working face and the increase of the span of the goaf.And pointed out the difference between the direct roof and the basic roof breaking form: the direct roof breaking occurs at the exposed roof of the overlying rock.First,a large bending and sinking deformation occurs,and then a sudden collapse occurs after a certain degree of development,and then a periodic breaking occurs in the form of a cantilever beam.The basic roof breaking: the deformation of the upper and lower strata is not coordinated along the weak surface.When the tensile strength of the upper rock mass exceeds the bearing capacity,the lower rock mass breaks.In addition,the separeation layer around the advancing side is prone to sprouting shear cracks,which connect with the caving fracture line under the mining of coal seam,aggravating the fracture of overlying strata.(4)The deformation,fracture,migration of overlying strata and the stress evolution characteristics and laws in surrounding rock of 12110 deep large mining height working face under mining were studied using the block discrete element numerical simulation software(3DEC).The collapse of overlying strata caused by coal seam mining makes the overall shape of the stope present a trapezoidal distribution.And a stress rising zone is formed on both sides of the working face,and a stress relaxation zone is formed on the upper rock layer of the working face.The stress state of the roof mass in the stress relaxation zone gradually turns into tension in the vertical direction,exacerbating the loosening and collapse of the overlying rock.The collapsed overburden is gradually compacted to replace the original coal seam under pressure,forming a new support point in the stope,and separating the upper “pyramid” shaped stress relaxation zone into a more complex stress environment.The main form of overburden failure was discussed: shear failure.And pointed out the distribution and evolution law of shear stress,maximum shear stress and maximum principal stress in overlying strata of working face under mining.(5)Based on 3DEC numerical simulation,the influence of burial depth and mining height on the deformation and fracture characteristics of overlying rock in the mining area was discussed.The burial depth directly affects the stope pressure,and changes the direction of the principal stress,which increases the tensile stress zone of the coal wall and decreases the shear stress zone.However,it has little effect on the mine pressure behavior,that is,it has little effect on the overburden pressure step,the height of the caving zone and the concentration coefficient of the abutment pressure.The increase of the mining height not only affects the height of the caving zone,but also makes it difficult to form a “masonry beam” structure,making it easy to form a “cantilever beam”structure,which may cause strong mining pressure.In practical engineering,it is manifested as an increase in the dynamic load and dynamic load coefficient of the support after structural instability.The thesis has 93 figures,12 tables and 103 references.