Instability mechanism and control of section coal pillar in fully mechanized mining face with super thick roof and extra thick seam

Aiming at the slicing fully mechanized mining face in extra-thick coal seams,a combination of theoretical analysis and field test is used to study the instability mechanism and control technology of coal pillars under super thick direct roof. Firstly,analyze the movement law of the overburden along goaf of each slicing faces,and obtain the calculation formula for the overburden load along goaf of the middle and bottom slicing faces by establishing the mechanical model of the overburden structure. Secondly,based on the theory of elastic-plastic,an analytical solution of the stress at any point in the narrow coal pillar and wide coal pillar is obtained,and the actual engineering parameters are substituted into it to analyze the stress distribution law of coal pillars of different sizes. Finally,the key technology of coal pillar surrounding rock control in the middle and bottom slicing face sections is given,and field industrial tests are carried out. Research shows：During the middle and bottom slicing mining,the overburden with huge thickness will form a structure of " low-level short cantilever beam + masonry beam + high-level bending subsidence zone ". The stress state of the coal pillar in slicing mining is divided into two types："the stress superposition in elastic zone of wide coal pillar" and "the peak stress superposition in narrow coal pillar". The analytical formulas for the three-dimensional stress at any point in the wide coal pillar and the vertical stress at any point in the narrow coal pillar are respectively obtained. The overlying rock stress concentration factor,the height of the coal pillar and the width of the coal pillar are the main influencing factors. The extra-thick coal seam less than 15 m is still suitable for setting narrow coal pillars,and its internal vertical stress peak value decreases as the height of the coal pillar increases. Affected by multiple mining of slicing mining,the actual residual strength in the narrow coal pillar is lower,and it is more prone to instability. However,it is not easy to set narrow coal pillars in extra-thick and huge-thick coal seams greater than 15 m. The method of setting reasonable section coal pillar size is given,and the surrounding rock control scheme of " timely active support + secondary passive support + three times grouting of anchor cable in key parts to strengthen support" is put forward.