Study on the bearing mechanism of irregular pipes under the differential surrounding rock action of coal roadway adjacent to the goaf in extra-thick coal seams

• Proposing the no coal pillar mining technology by pipe ventilation in coal roadway adjacent to the goaf for the fully mechanized top coal caving face of thick coal seams. • Definiting the interaction mechanism between ventilation pipes and differential surrounding rock. • Discussing the failure behavior of irregular pipes and proposing optimization principles for pipeline contours. • Pinpointing the surrounding rock movement characteristics around the pipeline and the pipe bearing mechanism. Based on the difficult problem of controlling the surrounding rock of roadways in extra thick coal seams, the no coal pillar mining technology by pipe ventilation in coal roadway adjacent to the goaf for the top coal caving working face of extra thick coal seams was proposed. The interaction mechanism between ventilation pipes and differential surrounding rock, and the bearing performance of various irregular pipes under different load forms were studied using methods such as on-site investigation, numerical simulation, physical similarity simulation experiments, and on-site practice. The research conclusion was as follows: 1 The contact form between trapezoidal pipe and surrounding rock was mainly the near contact, and the contact form of curved pipes was mainly the sticking contact, with more uniform contact stress. 2. The equivalent stress, plastic strain and deformation of the top and coal wall side curved pipe were minimized under equal strength loading condition, which was the most ideal pipe shape. 3. Strengthening the pressure on the coal wall side of the pipe under non-uniform strength form was beneficial for maintaining pipe stability. 4. Physical similarity simulation experiment have shown the top and coal wall side curved pipe exhibited bending deformation towards the inside of the pipe to adapt to rock deformation, which could provide strong control over the surrounding rock. On site tests have shown that the pipeline can meet the ventilation requirements of the working face without significant deformation.