Fracture Extension Law Of Sandstone Under The Case Of Time-dependent And Seepage Based On Granular Discrete Element And Its Application

With the increase of coal mining depth,the rock mass under complex geological conditions is significantly affected by the time effect,and the challenges faced by coal mine safety production are even more severe.For some coal mines that are in water-rich areas,the roof will produce water-conducting fracture channels under the influence of mining,which will penetrate the aquifer,and induce water inrush accidents.Therefore,the study of the formation law of the crack passage in the roof,which provides the theoretical basis for the prediction of the roof accident and the roof support plan,is one of the keys to the safe mining of the mine.In this paper,the PFC-CFD coupling method is used to establish a coal-series sandstone numerical model.This model considers the interaction of time effect and seepage,and analyzes the damage evolution law under different seepage pressures.Based on this,the seepage model was applied to the study of fractures in the roof.The main research contents and conclusions are as follows:(1)Based on the PFC-CFD coupled method,a seepage numerical model of the coal-series sandstone was established.The reliability of the simulation process was verified.The fracture development rules and seepage characteristics of the model were analyzed.The greater the seepage pressure of the model,the more internal microfractures,and the distribution of microfractures from a centralized form to a dispersive one.The floating range of the pressure that particles at different positions subjected to increases with the seepage pressure.The larger the seepage pressure,the larger the peak value,and the peak value appears in the lower part of the model.(2)A PSC model considering time effects was written using FISH language.The influence of initial fractures with different dip angles on the development of crack propagation in PSC model was analyzed.The damage evolution of PSC model can be divided into three stages: initial damage stage,damage evolution stage,and destruction stage.Compared with the conventional model,the number of micro-fractures after the final breaking of the PSC model is less,and the starting angle of the initial fracture tip is smaller.(3)This paper combining the PSC model with the seepage model,a coal-series sandstone seepage model with time effects was established.The number of fractures in the model gradually increased with the seepage pressure.When the seepage pressure is 0.5 MPa,2.5 MPa,and 3.5 MPa,the failure mode is mainly shear failure.When the permeation pressure is 1.5 MPa,the failure mode is the mode of tension and shear coexistence.The high hydraulic gradient of the model is concentrated on the top of the rock,which forms a "ring" structure.A low pressure gradient region appears at the bottom of the seepage field.(4)In order to study the law of lateral development of fractures in overlying strata,a seepage model of roof strata considering time effect was established.The effects of overburden pressure,initial fracture morphology and seepage pressure on the fracture morphology of the model were analyzed which was compared with the conventional roof rock formation model.The study found that the theory of particle discrete element can reveal the failure mechanism of the rock well.The fractures of the roof rock begin to develop at the tip of the initial fracture.The seepage model of the roof rock considering the time effect is complex in the development of fractures,and there are many macroscopic fractures.The angle of cracks developed at the tip of the fracture increases with the seepage pressure.