Study Of Influence Of Ballast Orientation On Geogrid Reinforcement Performance By Discrete Element Method

As the main load-bearing material of ballast track,ballast has such advantages as high compressive strength,excellent drainage performance,good economic benefits,since the weak connection between ballast particles,in the process of heavy-load and high-speed train operation,ballast will rearranged,resulting in sleepers empty crane,which seriously threatens the safety of line operation.The introduction of geosynthetics limited lateral displacement of ballast and improved the integrity of ballast bed.However,due to the complexity of reinforced ballast interface,it is not clear enough to analyze the evolution law of the interface strength;at present,granular material was studied using two approaches: test and numerical analysis,traditional laboratory test is difficult to accurately derive microscopic contact forces between particles and their distribution;The study subjects of discrete element method(DEM)are particulate media,and it can well monitor the changes of the microscopic structure among the ballasts,such as the contact,separation,slip and rotation of particles.This dissertation presents a pullout test using PFC3 D to simulate pull-out behaviors of a triaxial geogrid embedded in ballast,under special consideration of the factors of ballast orientation,the effect of ballast orientation on the interfacial shear strength is investigated.In addition,the relevant microscopic parameters of geogrid and ballast are are obtained by calibration test.The main contents and the results obtained are summarized below:(1)In this study,a three-dimensional numerical model,which is based on the Discrete element method software PFC3 D,has been built to simulate the pull-out test of a triaxial geogrid embedded in ballast.Under constant velocity field,the correctness of the models was verified by analyzing the relationship of geogrid pullout force versus pullout displacement;Numerical results show that the pullout force of geogrid gradually increased with the development of pullout test under four kinds of normal stresses,and peaking pullout forces under high level normal stresses are larger than that under low level normal stresses.(2)The macroscopic and microscopic strength changes of interface of geogrid reinforced ballast are systematically analyzed,During the development of the pull-out test,the changes of geogrid normal stresses,axial force,local void ratio,coordination number and energy dissipation were monitored,and the normal contact force in shear band is calculated and plotted in polar coordinates.Number result show that ballasts with complex spatial structure result in the uneven normal stresses of geogrid within a 3D spatial model,and the first geogrid transverse rib bear the main pullout load;the most predominant force chains in the interface of reinforced ballast tend to rotate anticlockwise,which constitute the main load bearing structure.(3)The influence of ballast orientation on the pullout force and bond energy of the geogrid is systematically analyzed,It is found that ballast with 45° orientation shows the best mechanical properties,ballast with 0°,30°,45°,60° and 90° orientation are greatly disturbed under action of applied load and gravity;During the progress of pullout test,the angle of ballast orientation becomes 56.7°-57.3°.And it is found that there is a high correlation between the normal contact force,the main anisotropic direction and the pullout force of the geogrid.