Research On Mechanism Of Water Inrush From Floor In Deep Mining Induced By The Impact Load Due To Roof Collapse

With the increasing of mining depth in our country,the threaten of water inrush from mining floor is getting more serious,the frequency and intensity of the dynamic disaster is significantly increased,and the co-occurring hazards become more remarkably.It is accompanied by strong dynamic load disturbance when the dynamic disaster occurs such as roof collapse,rock burst and mine earthquake.And the influence scope of dynamic load can reach several hundred meters,so it will directly affect the confined aquifer in the mining floor.The disturbed effect will further aggravate the energy unevenly aggregated and released in time and space,especially for the rock masses is in a state of limiting equilibrium in deep mining.So the strong dynamic loading will very likely to cause water-resisting strata failure,and induce water inrush from seam floor in deep mining.Current research on the mechanism of water inrush major focus on the coupling action of seepage and mining-induced stress,and yet give less concern on the dynamic load and other factors.It can't fully explain the mechanism of water inrush only consider the static load,and not conducive to the water inrush prevention and control work when we face the complex mechanical environment in deep mining.Therefore,the impact of dynamic load cannot be ignored,and the research on the effect of dynamic load on floor water inrush will be an important supplement to the study of water inrush mechanism,and provides a new idea for water inrush risk prediction and prevention.Based on the background above,the main research of this thesis is the effect of dynamic load generated by roof collapse on the water inrush from floor.Firstly,the dynamic response characteristics of water-resisting strata and confined aquifer of mining floor have been investigated.Then the growth mechanism of water-bearing fracture due to the combination of dynamic and static loads has been researched based on the theory of dynamic fracture.And the manifestation process and rrising reasons of water inrush from intact floor and with fault structure floor have been studied by numerical simulation.Thereby,the mechanism of water inrush under the combined action of dynamic and static loads was revealed by the combined method of theoretical analysis,numerical simulation and engineering example.The main research contents and conclusions are as follows:?1?Based on the theory of elastic half-space dynamic foundation,the theoretical calculation model for the time history of impact dynamic loading stress generated by roof collapse is established.The dynamic load with extremely short duration and large stress amplitude,and stress-time curves manifest as pulse loading shape.The duration and stress amplitude are affected by the weight,height and loading area of the collapse rock mass,as well as elasticity modulus,Poisson ratio and density of floor rock mass.Then,the failure and damage mechanism of rock mass have been discussed,and the transfer rule of dynamic stress in the floor strata has also been analyzed.Thus,the water inrush hazard caused by dynamic load due to roof collapse was qualitative researched from the perspective of rock impact dynamics and damage mechanics.?2?In consideration of the fact that the floor rock masses are multilayered distribution due to deposition,the floor strata is equivalent to be the multilayered semi-infinite elastic plane,and a mathematical model for calculating stress field in the mining floor strata has been presented in this thesis.And the numerical solution of dynamic stress field based on the dynamic theory of elastic half-space.So the stress response rules of mining floor under the dynamic load induced by roof collapse were obtained by stress vector superposition.Based on this observation,the dynamic rupture process of mining floor has been discussed.It is obtained that that the stress field of mining floor dynamically increase and decrease,the degree and range of stress concentration are dynamically increase under dynamic loading.Therefore,it can be inferred that secondary failure of floor strata will occurs when roof collapse,and the floor failure depth further increased due to the impact load effect.?3?The dynamic stress-permeability equation has been established based on the groundwater hydraulics method.And the dynamic response characteristic of confined aquifer under the impact loading were studied.The pore water pressure increase sharply with the compressive impact loading.As pore water pressure increases,the permeability increase and strength reduction of porous rock mass,as well as the enhancement effect of hydraulic fracturing.And the high hydraulic pressure increase the effects of dissolution and destruction to the water-resisting floor rock mass,as well as the propagation and penetration of water-bearing fractures.Therefore,the height of water-conducting zone will increase further under impact loading.?4?In order to investigate the water inrush from intact floor strata caused by impact load,the water-resisting strata was simplified as rectangular thin plate with four edges clamped.So,the forced vibration model of water-resisting strata was established based on the theory of plate dynamics.And the failure criterion of intact floor strata under impact load was deduced by using the Mohr-Coulomb criterion,as well as the calculation formula for the critical maximum dynamic stress.?5?For the mining floor with fault structure,the mechanism of dynamic stress triggering fault activation was discussed through the analysis of stress state of fault plane under impact loading.The results show the range of fault activation critical dip was expanded,as well as the possibility of activation was increased for the fault with large shear strength due to the effect of impact loading.Water bearing structure crack propagation mechanism under impact load was studied based on the theory of fracture dynamics.Under the effect of impact load,the crack propagation models were influenced by increasing the pore water pressure.The dynamic load plays a great role in promoting water bearing structure crack initiation and growth,and it is beneficial for the tensile-shear failure to the crack which is more destructive.These show that the impact load increase the probability of fault activation,and the water inrush hazard are more easily happened than under static load.?6?The coupling model of seepage and dynamic stress was established with FLAC^3D software,and the model was used to simulate water inrush process from the intact mining floor under combined effect of dynamic load and static load.The dynamic response characteristics of stress field,failure zone and water pressure of confined aquifer were investigated with numerical simulation,as well as the influence of impact load on the failure depth of floor and the height of water-conducting zone were studied.The results show that the impact load lead to the improvement of stress concentration degree,thereby expand the range of failure zone of mining floor.And the results lead to the conclusion that excess pore water pressures will develop when confined aquifer subjected to impact load,thereby the thickness of water-resisting strata decrease due to the raising the height of water-conducting zone by water pressure.Thus,all of those result in the risk of floor water inrush increasing.So we get the conclusion that the simulation results reveal the mechanism of floor water inrush induced by impact load,and verify the theoretical analysis results.?7?The effects of different impact load characteristic parameters on the water inrush are considered in the numerical simulation.The results show that the impact load have a significant influence on the failure depth of mining floor and the height of water-conducting zone only when the stress amplitude exceeds the certain critical value,and the risk of water inrush increased with the increasing of dynamic stress amplitude.The results also show that the dynamic duration have a great influence on the height of water-conducting zone,and this was due to the longer time of duration for impact load,the longer time for the effects of excess pore water pressure,and the water-resisting strata are more likely to break.?8?According to the engineering geological conditions of a mine in Feicheng coalfield,the influence of impact load induced by first roof collapse on the mechanical attribute and stress state of F7 fault in the working face 8503 has been researched.The research reveal the mechanism of water-inrush induced by fault activation due to the combination of dynamic and static load.The results lead to the conclusion that F7 fault was less likely to activate under the abutment pressure when the working face reach to the location of initial collapse.And the normal stress and shear stress of fault plane dynamic changing in effect of the impact load due to roof collapse.The maximum value of fault activation index parameter T_s increases obviously,therefore the impact load can lead to the dynamic instability of fault.The maximum failure depth of mining floor change from 18m to 22m after the effect of impact load,as well as leading to the damage area in fault zone.And a penetrating water inrush pathway formed between the confine aquifer and working face 8503,the water inrush from F7 fault will happen after the roof collapse.?9?In order to avoid water inrush from mining floor induced by impact load,the corresponding prevention and control technology was studied.Based on the analysis of the water inrush pathway formation mechanism under the impact load,a rule-based intensity reducing and perturbation effect decreasing of dynamic load was proposed to prevent and control the water inrush hazard.Some specific advice and measures are as follows:forcing roof collapse,changing mining method or roof management mode,arranging work face reasonably,grouting reinforcement,and strengthening dynamic load monitoring.