Study On Overburden Mining Destruction Rules Of Mud-covered Coal Seam And Application Of Water-preserved Mining

Thick red loess mud cover occurs in the roof of shallow coal seam in yushanfu mining area,It has good cementation,high clay mineral and poor permeability.It makes the mining failure rules of overlying strata are changed,and the water-conducting fracture zones are mostly pinched out or suppressed in the mud cap.At present,many scholars do not have a deep understanding of this point,When carrying out the mining demonstration under water body and water conservation mining design,they still use the calculation of fracture mining ratio under the condition of thick bedrock column,and neglect the water bridging property of mud cover,so the result is often conservative.Therefore,it is very necessary to study the mining failure rule of shallow buried thin bedrock and thick mud cover coal overburden,which is of great significance for prevention and evaluation of roof water damage and water conservation mining practice.Based on the engineering background of the 2301 working face of haoliang coal mine in yushenfu mining area,this paper has carried out the related research on the mining failure rule of shallow buried thin bedrock mud-covered coal overburden,and applied it to the practice of water conservation mining.Firstly,this paper analyzes the stratum structure and its mechanical characteristics of yushenfu mining area,and puts forward the “mud cover” effect of overburden mining failure,Physical mechanics and hydrologic tests of clay cap type viscous red and loess samples were carried out.Secondly,the mining failure rules of mud-covered coal seam are studied by means of physical simulation test,numerical simulation,field measurement of mining-induced fractures in overburden and analysis of the mining pressure developing law of working face.Then the water isolation test of the red clay layer was carried out,and the mechanical mechanism of mud cover effect was expounded based on the random medium theory.Finally,a new method for optimizing the size design of coal pillar for waterproof protection of mud-capped coal seam is put forward and applied to the practice of waterproof protection of 2301 working face in haojialiang coal mine,the purpose of water conservation mining is realizedIt is helpful to reasonably determine the size parameters of protection pillar,so as to raise the mining limit and increase the resource recovery rate.The thesis mainly obtains the following research achievements:(1)It analyzes the stratigraphical structure and its mechanical characteristics ofyushanfu mining area,and puts forward the concept of "mud cover effect" of overburden mining failure,sets forth that the essence of mud cover effect is the effect of mud cover on the development of water-conducting fractures.And the overburden structure of the mud-capped coal seam is simplified into four geological models: thin bedrock thick mud cover,thin bedrock thin mud cover,thick bedrock thin mud cover and thick bedrock thick mud cover.(2)Through the physical mechanics and hydrologic tests on the samples of viscous red and loess,it can be seen that the samples of viscous red and loess contain more clay minerals,mainly chlorite,illite and montmorillonite,All of them have certain cohesion and volumetric expansibility,high shear strength,strong crack resistance,small coefficient of saturation permeability,it has the characteristics of weak water permeability and good water insulation,This makes it easy to produce mud cap effect when mining overburden rock under the cohesive soil layer.(3)The similar simulation test of mining-induced fractures in overlying rock shows:the mining-induced fractures in overlying rock will undergo the process of "opening and closing" under the mud cover to bridge repair,Moreover,the development of "two zones" is affected by the inhibition of the cohesive soil layer,and the development is incomplete and inadequate.There is no obvious zonation in vertical direction,and the mining-induced fracture development of overlying rocks along the horizontal direction is zonal with the change of bedrock thickness.The mining deformation failure of overlying rock is characterized by "integral subsidence".(4)It can be known from the numerical simulation results of overburden mining of mud-capped coal seam:Due to the inhibitive effect of mud-cap type clay,the mining deformation and damage of overlying rock are reduced,The plastic zone of overburden is prevented from further upward development,and the development height of "two zones" of overburden is reduced and the distribution pattern is changed from the saddle to the arch,moreover,the development height of the water-conducting fracture zone tends to increase gradually with the increase of the bedrock thickness,and decreases gradually with the increase of the thickness of the red soil layer.(5)The field measured data of mining-induced fractures in overlying rocks indicate that after the fractures develop upward into the laterite of jingle formation,the fractures gradually close and the upper part has weak conductivity under the influence of the characteristics of high expansibility and strong plasticity of thecohesive soil.The development height of "two zones" was greatly reduced,and the development degree of mining-induced fractures was also significantly reduced.Finally,the overburden cave-mining ratio was 2.66,and the mining-mining ratio was6.4~7.04.(6)Analysis indicates that the mining face pressure of mud-capped coal seam is strong and has the characteristics of short step spacing,small static pressure and large dynamic load,at the same time,the fracture mechanics model of near-field roof rock overburden is established,and it is pointed out that the near-field basic roof rock cannot form the articulated structure of "masonry beam",which is converted to the structure form of "short cantilever beam",and the mining pressure characteristics of working face are explained.(7)The water isolation test of the clay under mining shows that the soil in the initial undisturbed state is well,Mining-induced fractures undergo a process of opening and then closing,The change rule of mining water insulation is first strengthened and then weakened,and it changes with the thickness of soil layer.The smaller the thickness of soil layer is,the greater the mining water insulation is.At the same time,the soil permeability coefficient also increases first and then decreases,finally,the soil permeability coefficient formula is obtained by fitting.(8)The mechanical mechanism of mud cover effect is expounded,It is pointed out that the difference of vertical displacement of soil caused by mining is the main cause of vertical shear fracture.Based on the random medium theory,the vertical displacement of soil during mining is calculated.The calculated results show that the vertical displacement difference will occur in the clay during the mining process,which will lead to the vertical mining cracks,after mining,the vertical displacement difference of soil expansibility disappeared,and the cracks closed automatically.(9)Points out that the effect of mud cap strata under the action of mining damage forms "mud covered bridge belt",the existence of the bridge with makes face when making waterproof protective coal pillar design can reduce the thickness of this layer,appropriately applied to Hao Jialiang water of coal mine 2301 working face of mining practice,through the numerical simulation of GMS and mine water inflow measured data shows the reasonable optimization design method,improves the working face mining upper limit.