Research On Scaled Boundary Finite Discrete Element Method For Modelling Particle Breakage Of Rock-fill Materials

The particle breakage is affected by internal factors such as particle strength,size,roundness,roughness,weathering degree,relative density,particle size distribution and fabric.It is also affected by external factors such as confining pressure,stress level,loading path,loading rate,moisture content et al.The particle breakage can be revealed by indoor test,numerical simulation,in situ test and the combination of these methods.With the development of computer,mathematics and mechanics in recent years,numerical simulation is becoming more and more popularly used for modelling the mechanical resposes of rock-fill materials.Due to the complexity and uncertenty of rock-fill materials,there is no efficient way for caputuring the characteristics of rock-fill materials.And also there are many key scientific and technological problems need to be solved.Numerical method counting for the complex shape of rock-fill particles is studied in this thesis.With an aim to develop a novel mthod to interprate the macro-scopic mechanical responses from particle-scopic.The outcomes of this study will be usefull in analysis,design,construction and maintenance for structures made by rock-fill materials,with great theoretical and practical value.This manuscript includes the following three main parts:(1)Every single particle is simulated by a polygon discrete element in order to capture the realistic shape for rock-fill materials.A polygon discrete element method is established and a program called PDEM is developed to model rock-fill material.A biaxial test of rock-fill sample is simulated by PDEM and PFC under different confining pressures.By comparing the results from PDEM with biaxial test and numerical results from PFC,the feasibility of the developed technique is demonstrated.This part includes the following aspects:(a)The moment of inertia of polygon with arbitrary number of sides is formulated by a semi-analytical method,which enables the calculation for the rotation of polygon to be convenient.(b)A linear search algorithm is adopted to obtain the coordinates for the intersection points,the total cost for searching the intersection points between two polygons is only 2(n + m)times,and easy to program.The contact detection for polygons is developed by a two-step process which includes approximation detection first and real contact detection after.(c)The normal contact force is calculated by a potential energy based polygon/polygon normal contact model,the tangential force is coupled by the sliding frictional model.This contact model solves both the normal contact forces and tangential contact forces for polygons results an theoretical foundation for the interaction of polygons.(d)A polygon discrete element program called PDEM is developed based on the theories mentioned above.A biaxial test of rock-fill sample which was conducted by Jiang et al.is simulated by PDEM and PFC under different confining pressures.By comparing the results from PDEM with biaxial test and numerical results from PFC,the feasibility of the developed technique is demonstrated.(2)In order to capture the particle breakage phenomenon in sharp-edged rock-fill materials,a novel numerical method called SBFDEM is developed by combine scaled boundary finite element method(SBFEM)and discrete element method(DEM).The developed technique makes the best use of the salient features of both the DEM and SBFEM.The particle breakage mechanism and its implication on macro-mechanical responses can by studied from particle scale simulation.This part includes the following aspects:(a)The sharp-edged particles are modelled by polygons,the motion and interaction of particles are solved by discrete element method,a full stress-strain analysis of each particle is obtained by using scaled boundary finite element method.Each particle is a discrete element in DEM and also a single polygon in SBFEM.The force calculation and transmission between these two modulus are demonstrated in this part in detail.(b)Hoek-Brown criterion is used to determine the 'plastic' points,breakage is triggered if the ratio of 'plastic' points reaches the critic value.The breakage path is assumed to be a straight line for simplicity and is determined by weighted least square approximation for all'plastic' points.Once breakage happens,the broken particle is replaced by two smaller particles,and the new generated particles will join the next step DEM and SBFEM calculation without any change.(c)A novel SBFDEM is established based on the theories mentioned above,and a program called SBFDEM is developed by using MATLAB.(3)Two rock-fill samples are modelled under biaxial compression condition with different confining pressures considering particle breakage.The force chain evolution,deformation of samples and macro stress-strain relationship are studied.The implication of different initial particle size distribution on particle breakage under biaxial compression is also evaluated.This work enriches and develops both the discrete element method and scaled boundary finite element method.The novel Scaled Boundary Finite Discrete Element Method has advantages in modelling particle shape and particle breakage.