Creep Characteristics Of Backfill And Overlying Rock Mass In Goaf And Its Influence On Long-term Deformation Of Channel

The water conveyance channel crossing the coal mine goaf is a major engineering and technical problem of the Middle Route and East Route of the South-to-North Water Transfer Project.In order to ensure the safe operation of the water conveyance channel,the goaf under the channel is grouted and filled in the early stage of the project to avoid the surface deformation caused by the secondary activation of the goaf.However,in the process of grouting filling,some voids are formed in the goaf due to the complicated distribution of the roadway in the goaf,the irregular shape distribution and the consolidation and settlement of the slurry;at the same time,with the passage of time,the groundwater level in the engineering area will continue to rise,groundwater will form a certain degree of water pressure in the voids of the goaf through the naturally occurring fissures in the bedrock and the fissures caused by bedrock damage during the pre-mining process.During the operation of the channel,the creep of the goaf filling body and the overlying rock mass under different water pressures should also be taken seriously.According to the monitoring data of the goaf in the Guocun section of Yuzhou,the channel and rock mass have some degree of settlement within the initial 3 years of operation.Combined with the defects of the grouting filling project itself,this paper analyzes the reason for filling the goaf.The water pressure in the area caused the deterioration of the strength of the filling body and the overlying rock mass,which caused the residual settlement of the goaf during the channel operation period.Based on the above analysis,the grading loading creep test of the goaf filling body and the overlying rock mass under different water pressures is carried out,and the corresponding constitutive model is established for parameter identification.Finally,FLAC^3D is used to calculate the deformation analysis of the filling goaf considering the water pressure and the long-term deformation prediction of the channel.The main research contents are as follows:（1）Based on the analysis of geological and engineering related data,due to the consolidation and settlement of the slurry in the goaf,the complex distribution of the roadway in the goaf and the irregular shape,there may be voids in the goaf after filling.During the long-term operation of the channel,over time,the groundwater in the bedrock reaches the cavity through the fissure,which may form a certain degree of water pressure in the goaf and overlying rock.In the long-term operation of the channel,the creep of the filling body and the overlying rock mass under water pressure will also affect the surface deformation.（2）In order to get the mechanical characteristics of the rock mass in the goaf before the grouting treatment,the mechanical properties of the unloading rock in the goaf are carried out,and the uniaxial compression test is carried out on the overlying rock mass under the saturated water condition.The triaxial compression test and the triaxial unloading test obtained the rock mechanical properties and parameters of the goaf under different confining pressures and unloading rates,which provided the basis for subsequent tests and calculations.（3）During the creep test of the backfill and overlying rock mass samples,the instantaneous strain increases with the increase of the water pressure level under the same unit axial stress increment,which is due to the application of the initial test water pressure.The pore water pressure in the saturated packed body sample will rapidly increase from the outside to the inside to the level of water pressure.During this process,the pore water pressure inside the sample increases with the increase of water pressure,so that the pore tip stress increases.Large,causing fine cracks to expand from the outside to the inside,thereby deteriorating the mechanical properties of the sample;at the same time,the greater the water pressure,the greater the deterioration of the mechanical properties of the sample.（4）At lower water pressure,the constant creep time of the filling sample under the breaking stress is longer,while the time for attenuating creep and accelerating creep is shorter,and the sample has the characteristic of sudden failure;When the sample is increased,the uniform creep time of the sample becomes shorter,and the time for attenuating creep and accelerating creep becomes longer,and the burst damage characteristics of the sample become weaker.It indicates that the greater the water pressure,the greater the softening degree of the filling sample.Similarly,as the water pressure increases,the attenuation creep time of the last stage of the rock sample before the failure becomes longer,and the uniform creep time becomes shorter.The difference is that the curve of the accelerated acceleration stage of the rock sample is shorter and the burst is sudden.The characteristics of the damage are less affected by the water pressure,indicating that the rock sample is less softened by water pressure.（5）By analyzing the characteristics of the creeping curve of the packed body and the overlying rock mass under the action of water pressure,combined with the establishment principle of the nonlinear creep constitutive model and the actual stress conditions in the goaf.The Burgers model is used to simulate the creep properties of the overlying rock mass and the modified Nishihara model is used to simulate the creep properties of the backfill.The selected models are compared with the experimental data,and the fitting correlation is high,which indicates that the selected model can well describe the creep characteristics of the filling body and the overlying rock mass considering different water pressure effects.According to the creep parameters of the filling body and the overlying rock mass under different water pressures,the evolution equation of the parameters with the water pressure is established.（6）The mechanical model of coal pillar-filled composite creep is established.The structure of coal pillar and backing body as supporting overlying rock mass is analyzed from the perspective of mechanics.The cooperative creep deformation process under the uniform load of the roof and the curve of creep deformation with time is established,and the structural model and equilibrium equation of each stage of coal pillar-filled collaborative creep are established.The creep constitutive model of coal pillar and backfill is carried out in finite difference software FLAC^3D.The secondary development and the calculation model of coal pillar-filled collaborative creep is established,and the process of coal pillar-filled collaborative creep is described from the perspective of numerical calculation.（7）Taking the undercut filling goaf in the Guozhou section of the South-to-North Water Transfer Project as an example,a numerical model was established to calculate the surface deformation and internal deformation of the rock mass caused by the creep of the filling goaf under different water pressures.Calculation result is smaller than actual monitoring data,indicating that there is a certain degree of water pressure in the goaf.When considering different water pressures,the calculation results are compared with the settlement displacement of the actual monitoring points.It is estimated that the water pressure range in the goaf of the study area.In addition,taking the Guocun goaf and surface channels as an example,based on the geological data,a three-dimensional calculation model was established,and the influence of the creep characteristics of the filling goaf on the long-term deformation of the channel was studied.