Structural Evolution Of Strata And Mechanism Of Synergistic Actions Of Support And Surrounding Rock For Ultra-thick Longwall Top Coal Caving Panel In Laminated Igneous Intrusion Area

In recent years,longwall top coal caving method has been widely applied and great achievements have been made in mining the ultra-thick coal seam in Shanxi,Shaanxi and Inner Mongolia.However,there are still many practical problems in mining the ultra-thick coal seams under some special geological conditions.Dynamic disasters of working face and roadway seriously threaten the safe and efficient production in mining the 20m thick coal seam in Carboniferous Permian system of Datong coal field.The ultra-thick coal seam was widely invaded by magma in the region,and the cooled magma developed to hard igneous sill with different thickness on the top of the coal bed.At present,the studies on the behaviors of overburden strata in fully mechanized mining stope and the interactions between the support and surrounding rock have not been carried out.These studies have great practical significance to reveal the mechanism of dynamic disasters in longwall top coal caving face and roadway,put forward effective preventive measures,and ensure safe and efficient mining.The physical and mechanical properties of igneous rock and the influence of its thickness on the structural evolution of overburden strata in stope and the synergistic actions of support and surrounding rock for behavior of roof strata was studied based on the engineering background of Tashan coal mine in Datong coal field through theoretical analysis,physical,numerical modeling and field observation.The main conclusions are as follows:（1）The range of intrusive igneous rocks in no.3-5 ultra-thick coal seam of Datong coal field reached 30.5km²,accounts for 17.15%of the total area.The igneous sill buried on the top of coal seam and its thickness decreased from 8m to 0m from east to west.The strong mining pressure result in frequent support crash and roadway burst accidents.In field,the support with resistance of15000kN is still impossible to avoid the occurrence of the crashing accident.The accidents positions are not only occurred in the stage of main roof failure,but also in initial and subcritical mining stage.（2）Laboratory tests show that the igneous rock have been strong carbonated,which is mainly composed of orthoclase（31.4%）,dolomite（19.63%）,quartz（9.23%）and some clay minerals,such as chlorite,kaolinite,illite and montmorillonite.The uniaxial compressive strength is 128.36 MPa,the tensile strength is 8.6 MPa,and the cohesion and internal friction angle are 24.6MPa and 54°,respectively.The strength of the dry lamprophyre sample reached163MPa under the axial cyclic loading and unloading,and strong rock burst occurred in the tests.The uniaxial compressive strength of igneous rock reduced to 78MPa and 39MPa under water-rock interaction and acidification,respectively.Water-rock interactions improved the igneous rock changed from strong brittle failure to an elastoplastic failure,which was verified by the evolution of AE counts,AE accumulative energy and Felicity ratio in cyclic loading and unloading tests.Dolomite and orthoclase are the main minerals involved in the acidification reaction,in which dolomite plays a leading role.H₂SiO₃ colloids was formed by the reaction of orthoclase and acid solution,which inhibit the acid reaction of orthoclase.Taking effective measures to prevent the generation of H₂SiO₃ are of great significance to improve the effect of acidification and reduce the fracture pressure of the hard rock.（3）Results of the physical modeling with isopachous igneous rock strata,thickened igneous rock strata,and upper goaf showed that the roof strata can form“cantilever beam”and“voussoir beam”structure.The“combined cantilever”structure that developed from the igneous rock strata and thick sand stone stratum in immediate roof is periodic occurred in mining process.The motion of the broken“combined cantilever”structure is described as“Turn-Separation-Reverse turn–Slip”.The broken feature of the“combined cantilever”structure is shifting form layered failure to integral failure with increasing of thickness of igneous rock strata.When the lower longwall top coal caving face moved to the boundary of the upper goaf,the structural instability of the overburden strata caused by the ahead broken of the interlayer strata is the main reason for the support cashing accident.（4）The movement of overburden strata is controlled by the synergistic actions from the caved roof strata,support and the coal seam In front of the working face.The broken expansion of the caved roof strata have influence on the allowable subsidence space of the main roof strata and determine the stability of its structure.The mechanical properties of the coal seam in front of the working face can directly affect the fracture characteristics of overburden strata and determine the load on hydraulic support.The hydraulic supports of working face play a decisive role in controlling the movement of immediate roof strata and the stability of coal wall.The thickness of the igneous rock strata directly affects the expansion coefficient of the caved roof strata,the behavior of the immediate roof and the mechanical properties of coal seam.（5）Based on the results of physical modeling,the structural model of“combined simply supported beam-voussoir beam”was established,and its mechanical analysis was carried out.A method for calculating the working resistance of hydraulic support in the ultra-thick longwall top coal caving face in igneous intrusion area is presented and verified by case studies.The load on support is directly influenced by the structural feature of the“combined simply supported beam”in immediate roof.The structural feature is determined by the broken length,broken angle and ultimate rotation angel.The broken angle of the K3 sand stone strata,fine sand stone strata in immediate roof and the fine sand stone strata in main roof are 64°,65°和68°,respectively.The broken angle of igneous rock strata decreased from 62°to 60°with the increasing of its thickness.The thickened igneous rock increased the broken length of the hard rock strata in immediate roof,the broken length increased from 14m to 19m.When the thickness of igneous rock is less than 5m,the load on the hydraulic support does not exceed 15000kN,the load reaches 17708kN when the thickness of igneous rock strata reaches 8m.