Study On Mechanism And Control Of Floor Heave In Pre-driven Recovery Room With Soft Surrounding Rock

This thesis aims at solving the problem of significant floor deformation of the pre-driven recovery room of the 31109 panel of Lijiahao Coal Mine in the final mining stage.Further study of the stress distribution and deformation failure characteristics of the surrounding rocks of pre-driven recovery room in the final mining stage and calculation of the limit cantilever length of main roof when the working face retreats into the pre-driven recovery room can guide roof pressure control,reduce the amount of floor heave supplemented with control measures and avoid the problems of low efficiency and even difficulty of the fully mechanized mining equipment withdrawal in the final mining stage.In this thesis,theoretical analysis,similarity simulation and numerical simulation are used to analyze the stress of the surrounding rock and calculate the damage range of pre-driven recovery room floor in the final mining stage.Distribution law of front abutment pressure and its effect on the stress distribution of the surrounding rock of pre-driven recovery room with longwall mining and comparison of the stress distribution law of the surrounding rock of pre-driven recovery room with different sizes of inter pillars are further studied through numerical simulation.At the same time,the stability and failure mechanical of main roof is analyzed,then,the calculation of the limit cantilever length of main roof is obtained when the working face retreats into the pre-driven recovery room.The main results and conclusions obtained are as follows:(1)The results of similar simulation and numerical simulation experiments of the pre-driven recovery room shows that the influence of the front abutment pressure is between 40 and 50 m.In the final mining stage,front abutment pressure increases first and then decreases with the advancement of the working face,indicating that the front abutment pressure is gradually changed from the fender to the fender-inter pillar.After the working face retreats into pre-driven recovery room,the front abutment pressure is completely transferred to the inter pillar.The front abutment pressure reaches the peak before the working face retreats into pre-driven recovery room in the final mining stage which indicates that the surrounding rock of the pre-driven recovery room is more likely to be damaged in this stage.In the final mining stage,both the main and submain predriven recovery room are in a non-uniform stress environment,and the corresponding asymmetric floor heave is also prone to occur,and the peak stress of the surrounding rock of the submain pre-driven recovery room is always lower than the main pre-driven recovery room,therefore,the stress environment of the main pre-driven recovery room is worse during the advancement of the working face.(2)The calculation models of stress distribution and failure range of the predriven recovery room floor in the final mining stage are established,respectively,and the calculation equations of vertical stress,horizontal stress,shear stress and floor failure range at any point of the pre-driven recovery room floor are obtained.Combining with the engineering geological conditions of the working face of31109,it is pointed out that the value of vertical and horizontal stress of the predriven recovery room floor gradually decrease with depth,and the decreasing range and influence range of the vertical stress are higher than the horizontal stress.The floor of pre-driven recovery room has a pair of positive and negative "spiral" shear stresses and its direction change surface are not in the middle of the predriven recovery room,therefore,it is theoretically proved that the pre-driven recovery room floor is prone to asymmetric floor heave.The calculation result of the failure range of the pre-driven recovery room floor shows that the failure range of floor heave is in an inverse proportion with the cohesion and the friction angle.(3)The numerical calculation model of different inter pillars was established to study the influence of the inter pillar size on the stress distribution of the surrounding rock of the pre-driven recovery room.The results show that the change of the width of inter pillar has little effect on the stress distribution of the surrounding rock of the pre-driven recovery room during the tunneling stage.When the width of the inter pillar is small,the peak value of front abutment pressure and the stress of pre-driven recovery room surrounding rock will rise sharply in the final mining stage.At the same time,when the size of the inter pillar is greater than 15 m,the effect by increasing the coal pillar size for lowering the peak surrounding rock stress of the main recovery room is limited.Combined with the calculation equation of coal pillar bearing capacity,it is concluded that the reasonable width of inter pillar is 20 m.Under the condition of reasonable inter pillar width,the deformation law of the pre-driven recovery room floor shows that the displacement of the floor of the pre-driven recovery room increases first and then decreases with the depth,and reaches the maximum value at a depth of about5 m.It shows that the depth of 5 m is the most prone to damage and easy to be the initial depth of the floor heave.The distribution laws of horizontal stress and displacement of the pre-driven recovery room floor are the same as the stress growth laws of the two sides of the pre-driven recovery room under the influence of the front abutment pressure,which indicates that front abutment pressure is the main reason for the floor heave of the pre-driven recovery room.(4)The stability and failure mechanical analysis of main roof of pre-driven recovery room in the final mining stage is established.And when the working face retreats into the pre-driven recovery room,the limit cantilever length of main roof is obtained combined with the material mechanics.Combining with the engineering geological conditions of 31109 working face,it is pointed out that with the advancement of the working face,the effect of reducing the value of hydraulic powered support by increasing the strength of the fender and the support in the pre-driven recovery room is relatively limited.the length of key block has a crucial effect on the value of the working resistance of the hydraulic powered support.In front of the working face,the value of the bending moment of main roof increases first and then decreases and gradually approaches 0 with the increase of the length of the cantilever beam.The length of the key block generally increases with the increase of the length of the cantilever beam and the peak position of bending moment of main roof moves forward as the working face advances.The ultimate length of cantilever beam is 11 m,which can help to get the location where the brief pause of mining or the roof pre-fracturing should be applied.(5)The observation of mining pressure should be strengthened with the advancement of the working face.When the observation predicts that main roof will break in an unfavorable position before the working face retreats into the main recovery room,the mining rate of the working face should be adjusted in advance(when the mining rate is zero,it is the brief pause of mining)or the roof pre-fracturing measure should be took to keep the main roof from weighting when the working face retreats into the pre-driven recovery room.The comprehensive analysis shows that the pressure control of the pre-driven recovery room in the final mining stage combined with the over-excavation of the floor to fill the concrete cushion or grouting is a reasonable measure for the control of the floor heave.