Study On Mechanism And Control Technology Of Strong Ground Pressure In Mining Under Overlying Coal Pillar And Hard Roof

In multi coal seams mining,the problem of strong ground pressure in the lower working face or roadway caused by the coal pillar and interlayer hard roof is becoming more and more serious.It threatens to the mine safety.It is of great theoretical significance and engineering practical value to carry out research on the mechanism and control technology of strong mining ground pressure under the action of overlying coal pillars and hard roof.This paper takes the fully mechanized caving face of 4^-2coal seam in Buertai Coal Mine as the research object,the methods of field test,theoretical research,physical similar simulation and numerical calculation are adopted.By studying the strong ground pressure influence on fully mechanized caving face of 4^-2coal seam,which is caused by coal pillars of overlying 2^-2coal seam parallel coal pillar,vertical coal pillar and superimposed coal pillar and executed through interlayer thick hard roof.This paper proposed the mechanical models of"fan-shaped seesaw"and"inverted trapezoid"structures of the fully mechanized caving face over the overlying parallel coal pillars and the stress concentration area of the superimposed coal pillars,the disaster mechanism of strong ground pressure under the coupling action of the overlying coal pillar and the hard roof was revealed.Developed the technology of directional long borehole segmented hydraulic fracturing to weaken the hard roof and stress concentration area,established a strong rock pressure monitoring and early warning platform,verified it in practice and achieved remarkable results.The main research results as follows:（1）Through on-site borehole coring,uniaxial compression,Brazilian splitting and CT scanning experiments,the mechanical indexes and microstructural properties of cores at different depths were measured,and the original rock stress of 4^-2coal seam was actually measured,the main roof of 4^-2coal seam classification was carried out,and it was proposed that the dense siltstone with a thickness of 37.1m above the coal seam belongs to the I-level extremely hard roof,which support an important foundation for the research and control of the strong mine pressure mechanism..（2）The influence law of overlying parallel coal pillar,vertical coal pillar and end superimposed coal pillar on rock pressure behavior of fully mechanized top coal caving face through interlayer hard roof is measured.It is revealed that under the influence of parallel coal pillar,vertical coal pillar and end superimposed coal pillar,the mining of lower coal seam has the characteristics of strong rock pressure behavior such as support resistance rise,frame pressing,rib spalling and roof fall.The overlying coal pillar and hard roof are the main factors causing the strong ground pressure behavior of 4^-2coal seam.（3）Through theoretical analysis,similarity simulation and numerical calculation,the breaking movement“O-X”characteristics of hard roof in the process of entering and exiting the overlying parallel coal pillar in fully mechanized top coal caving face are studied.The structural mechanics model of roof"seesaw"during the period when the working face passes through coal pillars is established,the mechanism of strong ground pressure in fully mechanized top coal caving face under the influence of overlying coal pillars is revealed,and the engineering verification is carried out.The relationship between the seesaw spatial geometric structure movement and stress evolution of 4^-2coal overlying hard roof under the three states of fully mechanized top coal caving face,when entering coal pillar,under coal pillar and exiting coal pillar.It reveals the mechanism that the coal stress increases in front of the working face,the damage is obvious and it is easy to lead to the dynamic disaster of the working face when the coal pillar is 5-10m out.（4）The characteristics of overlying rock breaking,migration and instability of fully mechanized top coal caving face under the influence of vertical coal pillar are studied.The roof boundary condition and long side B of 4^-2coal seam are changed.The roof is transformed from four end fixed support condition to three side fixed support and one side simple condition,the broken rock mass in the goaf provides the main support.（5）The fracture and instability characteristics of overlying rock in fully mechanized top coal caving face under the influence of superimposed coal pillars are studied.The mechanism of strong ground pressure behavior in secondary mining roadway under the influence of the superposition of stress of overlying 2^-2coal along the trough pillar and 4^-2coal return air along the trough pillar is revealed.According to the O-X fracture characteristics of broken overburden in fully mechanized top coal caving face,the"inverted trapezoidal"structural mechanics model of roof fracture in fully mechanized top coal caving face under vertical coal pillar is constructed.Through the numerical simulation method,the spatial structure model of vertical coal pillar outside,directly above and inside the coal pillar at the end of fully mechanized top coal caving face is established.The research shows that the horizontal distance between the overlying vertical coal pillar and the coal pillar at the end of the fully mechanized top coal caving face has a significant impact on the rock pressure behavior.When it is located directly above the end,the surrounding rock stress and plastic zone are the largest,and the impact decreases gradually with the increase of the horizontal distance.（6）Based on the mechanism of strong ground pressure under the overlying parallel coal pillar,vertical coal pillar and superimposed coal pillar and hard roof in fully mechanized top coal caving face,the comprehensive treatment technologies of strong rock pressure,such as optimizing the mining layout,avoiding the direction of in-situ stress,using heavy high resistance hydraulic support,directional long borehole segmented hydraulic fracturing stope hard roof,conventional short borehole hydraulic fracturing trough roof and so on,are proposed.Comprehensive monitoring and early warning system for strong rock pressure in fully mechanized top coal caving face have been adopted,which realizes the intelligent monitoring and advance warning of the pressure from the roof of the working face.Through practical application,the influence of overlying coal pillar to the ground pressure in fully mechanized mining face is effectively reduced,and remarkable results are achieved.