Study On A Continuum-discontinuum Method Considering Element Cracking And Its Application

It is of great significance to study the failure of the rock-like material for expounding mechanisms of geological disasters and for predicting and preventing these disasters on theoretical and practical aspects.Actually,the failure of the rock-like material is a process of damage evolution,fracture propagations and penetration.This process is not only related to solid,but also to the action of fluid.The numerical simulation method has become an important research tool because mechanisms and processes of complex phenomena can be expounded by use of this method.Continuum-discontinuum methods have great advantages in simulating the transition of the rock-like material from continua to discontinua.This study was carried out based on the combined Lagrangian-discrete element method(the original method).(1)Based on the original method,a continuum-discontinuum method(the improved method)was developed through considering the cracking of the element to reduce the mesh dependence.Experiments of the three-point bending beam,complete Brazilian disc,Brazilian disc with a notch and uniaxial compression rock specimen were simulated.The improved method was validated through comparing numerical results with theoretical or experimental results.The effect of the angle of the pre-existing notch on fracture propagations in Brazilian disc was studied.The effect of the internal friction angle on fracture propagations in uniaxial compression rock specimens was studied.Comparing with the original method,the number of fractures is less,the bifurcation of fractures is less,and patterns of fractures are smoother in the improved method,which is more consistent with experimental results.(2)Based on the improved method,a continuum-discontinuum method was developed to consider the effect of fluid through considering the fracture seepage.In this method,fluid can only flow in fractures and the flow of fluid satisfies the cubic law.Seepage processes of the single fracture model and KGD model,and hydraulic fracturing processes of discs with holes were simulated.This method was validated through comparing numerical results with theoretical or experimental results.The effect of the inner diameter of the hole on fracture propagations during the hydraulic fracturing process was studied.(3)A method was proposed to simulate the interface coupling problem between solid and fluid through combining the continuum-discontinuum method and the SPH method.In this method,the continuum-discontinuum method was used to simulate the deformation,cracking,and movement of solid;the SPH method was used to simulate the flow of fluid.Processes of the hydrostatic equilibrium,breaking dam through the elastic gate,and gas compression were simulated.This method was validated through comparing numerical results with theoretical or experimental results.(4)Under the displacement controlled loading,propagation processes of initial tensile fractures,promoted tensile fractures and shear fractures in surrounding rock under different confining pressures were simulated using the improved method.Under the hydrostatic pressure,propagation processes of fractures in surrounding rock for different internal friction angles were simulated.Formation processes of V-shaped notches in surrounding rock under the hydrostatic pressure were simulated.(5)For rock specimens during hydraulic fracturing processes with directional perforations,with the increase of the distance far away from the perforation,fluid pressures decrease.With the increase of the perforation angle,fluid pressures corresponding to the fracture initiation increase,fluid pressures during processes of fracture propagations increase,and turning distances of fractures increase.The larger the difference between two horizontal stresses,the smaller the turning distance of the fracture.With the increase of time,the number of fracture segments increases,but increasing speeds decrease,which is related to the increase of the fracture volume.(6)For the simplified model of the coal and gas outburst,the intermittent increase occurs for fractures.With the increase of the initial gas pressure,the number of fracture increasing stages increases and the interval between two increasing stages tends to decrease.There are 105 figures,17 tables and 169 references in this thesis.