Damage Mechanism And Water Inrush Risk Evaluation Of Mining Floor Layered Rock Mass Under Fluid-structure Coupling Effect

As the mining depth continues to extend to the deep,the upper coal mining turns to the lower coal mining.In the process of coal mining,the floor rock mass is often threatened by high ground stress and high confined aquifer of floor limestone,which is easy to induce the water inrush accident of mining floor.Considering that rock mass in coal measure strata is mostly stratified medium,and floor stratified rock mass is affected by complex stress path of stress concentration(pressure)-stress unloading(tension)-stress recovery(pressure)and water pressure of high confined aquifer during mining,It is of great theoretical and practical significance to study the damage mechanism of mining floor layered rock mass under the fluid-structure coupling effect for elucidating the formation of water inrush channel in mining floor and predicting and evaluating the risk of water inrush.Therefore,this paper takes mining floor layered rock mass as the research object,comprehensively considers the disturbance stress path,water pressure,rock mass bedding Angle and other factors,adopts cyclic loading and unloading triaxial test,theoretical analysis,numerical simulation,mathematical statistics and quantitative recursion theory to study the damage evolution law of mining floor layered rock mass under the fluid-structure coupling effect.A fluid-structure coupling damage model considering the Angle of strata bedding was established,and the formation process of water inrush channel in stratified rock mass of mining floor was simulated.A risk assessment model of water inrush in mining floor based on quantitative recursion theory was established.The following main achievements have been achieved:(1)Considering factors such as rock bedding Angle,permeability pressure,confining pressure and original rock stress(including unloading stress coefficient caused by coal mining disturbance),an orthogonal test scheme with four factors and five levels is established.Uniaxial test and triaxial test of compression-tension-compression cyclic load of layered rock mass are carried out,and damage variables of layered rock mass are defined based on elastic modulus and bedding Angle.The results show that the damage variable is negatively correlated with the elastic modulus.When confining and osmotic pressures remain unchanged,the elastic modulus of the underlying sandstone decreases with the increase of the bedding Angle,and the damage variable increases with the increase of the bedding Angle.When the bedding Angle is 0°,the elastic modulus of sandstone samples is the maximum,and when the bedding Angle is 90°,the elastic modulus of sandstone samples is the minimum.The damage variable D appears "negative damage" when the osmotic pressure is 3.5MPa and the Angle is 0°,the osmotic pressure is 4MPa and the Angle is 0°,the osmotic pressure is 4.5MPa and the Angle is 0°,the osmotic pressure is 5MPa and the Angle is 0°,the osmotic pressure is 5MPa and the Angle is 30°.This is because with the increase of osmotic pressure and confining pressure,micro cracks or open structural planes inside laminated sandstone are constantly closed and compacted under the action of external forces,resulting in the increase of stiffness of sandstone and the increase of elastic modulus,which is negatively correlated with the definition of damage variables,so as to result in the situation of"negative damage".(2)On the basis of assuming that the strength of internal rock particles follows Weibull distribution,the strength criterion of layered rock mass is introduced as the failure criterion of rock particles,and the damage evolution equation of understory sandstone is established considering the effects of permeability pressure and bedding Angle.According to the equivalent strain theory and effective stress principle,the seepage stress-damage coupling constitutive model considering bedding Angle was established by introducing the damage variable correction coefficient.Combined with COMSOL Multiphysics software,the simulation results were compared with the triaxial test results to verify the accuracy of the model.(3)Combined with the percolation-stress-damage coupling constitutive model considering the bedding Angle,taking the coal face of a mine in Jiaozuo mining area as the engineering background,based on COMSOL Multiphysics multi-field coupling numerical simulation software,the damage of floor rock mass and the formation process of water inrush channel in the process of coal mining were simulated.The closer the monitoring point is to the coal seam floor,the greater the displacement and stress of the monitoring point;conversely,the farther the monitoring point is from the coal seam floor,the smaller the displacement and stress of the monitoring point.The closer the monitoring point is to the coal seam floor,the smaller the pore water pressure it receives;conversely,the farther the monitoring point is from the coal seam floor,the greater the pore water pressure it receives.When the working face advances to 30m,40m,50m,60m and 70m,the failure depth of goaf floor is about 6m,10m,12m,13m and 13m respectively,indicating that the maximum failure depth of goaf floor has reached 13m when the working face advances to 60m.When the stope face advances to 50m,60m and 70m,plastic zones in the range of 2m,5m and 10m appear respectively at the bottom of the effective water barrier,and the plastic zone at the bottom of the effective water barrier shows a rapid expansion trend.When the stope face advances to 80m,the failure zone of aquifer has been connected with the upper limestone aquifer of Taiyuan Formation,forming a water inrush channel.(4)Combined with the time series data of monitoring points in the coal seam mining process of the above numerical simulation,based on the quantitative recursion theory,the evaluation method of water inrush risk of coal seam floor under mining was established.Firstly,the index parameters of recursion rate,certainty rate and stratification rate are selected for recursive quantitative analysis,and the optimal monitoring line is determined to be in the floor water-proof layer 28m away from the coal seam bottom interface.Secondly,based on the numerical simulation results of mining water inrush channel formation,the threshold values of maximum principal stress,displacement and pore water pressure are determined to be 51.5MPa,52.27mm and 1.93MPa,respectively.Finally,the evaluation model of water inrush risk of mining floor is established.When the output value of the model is 0,there is no water inrush risk of mining floor.When the model output is 1,there is moderate risk of water inrush in mining floor.When the model output is 2,there is a high risk of water inrush in mining floor.When the model output is 3,there is serious water inrush risk in mining floor.