Risk Assessment Of Groundwater Inrush From The Floor Based On Roof-floor Collaborative Evolution

With the continuous increase of mining depth,the safety of coal mine roof and floor is facing great challenges under the environment of high ground stress and high water pressure.Due to the complexity of the mine environment and the particularity of the mined rock mass,coal seam mining breaks the stress balance state of the original rock mass,produces stress redistribution,causes the damage of the roof and floor,makes the cracks of the roof and floor develop,the cracks lead to aquifers,causes water inrush accidents,and puts life safety and safe production at risk.This thesis summarizes the previous research experience,combined with the engineering geological background of the first panel of the Wanfu Coal Mine,Shandong province.Using the methods of theoretical analysis,physical model test,numerical simulation,analytic hierarchy process and information entropy,aiming at the current situation that the synergy of roof and floor is less considered in the previous floor water inrush evaluation,this thesis studies the coevolution of roof and floor stress,displacement and fracture in coal seam mining,and applies it to the risk evaluation of floor water inrush.It provides a theoretical basis for practical engineering.The research results of this thesis are as follows:(1)According to the engineering geological conditions of the first panel in the Wanfu Coal Mine,the engineering geological model is generalized,and two groups of large-scale physical similarity model tests and two groups of 3DEC numerical simulation are carried out.One group is the longitudinal section model along the dip direction of the first panel,and the other group is the transverse section model along the strike direction of the first panel,which is used to study the coevolution of roof and floor stress,displacement and fracture in the process of coal seam mining.(2)According to the model test and numerical simulation results,the coevolution of top and bottom plate stress is obtained.For the cross-section coal pillar,the stress change trend of the top and bottom coal pillars is basically the same,and the stress of the coal pillars on both sides presents a symmetrical distribution.The closer to the mining area,the greater the stress change.For the coal pillar in the longitudinal section,the stress change curve of the top and bottom plate is also basically the same.The whole process can be roughly divided into two stages.First,before the panel advancing 800 m(240 cm in the model),the stress of the coal pillar near the open cut hole increases step by step with the advance of the panel.After the panel is advanced to 800 m(240 cm in the model),the coal pillar stress near the open-off cut basically remains unchanged,but the coal pillar stress near the stope line increases step by step with the advancement of mining.For the longitudinal section mining area,the stress variation trend of the roof and floor mining area is also basically the same.The whole stress change process can be roughly divided into five stages: the slow rise stage of stress,the step rise stage of stress,the stress release stage,the step rise stage of stress again and the gradual stability stage.(3)According to the model test and numerical simulation results,the co-evolution of roof and floor displacement is obtained.The model test shows that for the cross-section floor mining area,the overall displacement of the floor is upward,and the closer to the middle of the floor,the greater the upward displacement of the floor.For the longitudinal section floor mining area,the displacement change can be roughly divided into three stages: the stage of slow rise in displacement,the stage of sharp rise in displacement and the stage of gradual stability.For the longitudinal section roof mining area,the change of roof displacement can be roughly divided into five stages: the slow growth stage of displacement,the stage of stepwise growth of displacement,the stage of sharp growth of displacement,the stage of stepwise growth of displacement again and the stage of gradual stability.Numerical simulations show that for the cross-section roof and floor coal pillars,the displacement trend of the roof and floor on the same vertical survey line is basically the same;and the closer to the mining area,the larger the displacement change;the displacement of the coal pillars on both sides presents a symmetrical distribution.For the cross-section floor mining area,the entire displacement change process can be roughly divided into three stages: the upward displacement stage,the downward displacement stage,and the gradual stabilization stage.For the cross-section roof mining area,the closer to the middle of the mining area,the shorter the time required for the roof displacement to stabilize.The entire displacement change process can be roughly divided into three stages: the stage of rapid displacement increase,the stage of slow increase of displacement and the stage of gradual stability.For the bottom plate of the longitudinal section mining area,the vertical displacement change of the bottom plate can be roughly divided into five stages: the stage of slow downward displacement,the stage of downward growth of displacement steps,the stage of sharp upward growth of displacement,the stage of downward growth of displacement again,and the stage of gradual stabilization.For the roof of the longitudinal section mining area,the change of roof displacement can be roughly divided into four stages: the stage of slow downward displacement,the stage of step downward displacement,the stage of sharp downward displacement and the stage of gradual stability.(4)According to the results of model test and numerical simulation,the coevolution of roof and floor fracture development is obtained.For the model test,in the process of advancing the panel,vertical cracks and oblique cracks will be generated in the roof and floor.The vertical cracks of the roof and floor are mainly developed in the middle of the non-contact area of the roof and floor of the goaf in each advancing process of the panel,and the oblique cracks of the roof and floor are mainly developed directly behind each advancing of the panel.During the advancement of the panel,the maximum depth of floor crack development is 17 cm(equivalent to 51 m of the prototype),and the maximum height of roof crack development is 50 cm(equivalent to 150 m of the prototype).For numerical simulation,in the process of advancing,the maximum depth of floor crack development is 45 m and the maximum height of roof crack development is 90 m.(5)In view of the lack of less consideration of roof in the previous floor water inrush evaluation,a floor water inrush risk evaluation method considering the coevolution of roof and floor in the Wanfu Coal Mine is established,and the floor water inrush risk of the first panel is zoned.Seven factors including the thickness of the water resisting layer,the thickness of the aquifer,the brittle plastic ratio of the rock on the aquifer,the dip angle of the coal seam,the thickness of the coal seam,the bearing pressure of the panel and the buried depth of the coal seam are taken as the main controlling factors for the risk evaluation of water inrush from the floor.The combined weights of seven influencing factors are obtained by using analytic hierarchy process and information entropy method.Finally,the water inrush risk zoning map of the floor of the first panel of the Wanfu Coal Mine is obtained by using the layer superposition of surfer software.According to the risk zoning results,it is found that the coal seam floor is affected by the Taiyuan Formation’s limstone aquifer,and the areas with high water inrush risk are mainly located in the middle of the first panel,near the open-off cut and near the stope line.The results provide a scientific basis for the prevention and control of floor water in the first panel.The thesis has 68 figures,31 tables,and 104 references.