Study On Mechanical Characteristic Of Geogrid-Reinforced Ballast Under Static And Cyclic Loads

As a reinforced material to improve the shear strength of granular materials,geogrid can effectively reduce the settlement of ballasted track,restrict the lateral displacement of track bed and enhance the stability of track bed.In this paper,a series of single-layer and double-layer geogrid pull-out tests under static load are carried out through a self-made large-scale pull-out test box.The effects of laying depth,laying scheme and boundary conditions on the reinforcement performance of geogrid are studied,DEM simulation and microscopic analysis are carried out for various test conditions.Then,the influence of boundary conditions on ballast settlement under dynamic load is studied by establishing the single rail sleeper ballast box model;Through the existing indoor test,the composite ballast model is established,the effects of subgrade stiffness,laying geogrid and boundary conditions on ballast settlement under dynamic load are studied.The main conclusions are as follows:(1)The pull-out test and simulation results of single and double-layer geogrids show that the optimal laying depth of single-layer geogrid is 300 mm;The optimum laying depth combination of double-layer geogrid is 200-300 mm,as the corresponding double-layer reinforcement efficiency coefficient reaches the maximum.During the pull-out process of geogrid,an arched stress chain will be formed between geogrid and particles,and the contact force between geogrid and particles will gradually concentrate in the pull-out direction.In the pull-out process,the volume variable of the model changes with the laying depth of the geogrid.The smaller the laying depth of the geogrid,the larger the volume variable of the model.In the test and simulation,after setting constraints at the side nodes of the geogrid,the peak value of the geogrid pullout resistance and the growth rate of the geogrid pull-out resistance increase slightly during the pull-out process,which shows that the reinforcement performance of the geogrid will be stronger under the side constraints in the actual project.(2)The test and simulation results of pull-out single-layer geogrid of doublelayer geogrid show that setting a static layer geogrid can not improve the pull-out resistance of the pull-out layer,and the static layer geogrid is in a failure state,indicating that the joint reinforcement effect of double-layer geogrid laying can take effect when the double-layer geogrids are used to strengthen the ballast particles.(3)The simulation results of ballast box model with different boundary conditions under cyclic load show that the restraint ability of the model to particles is different under different boundary conditions.Compared with the rigid boundary,the sleeper settlement under the periodic boundary and Winkler elastic subgrade boundary increases and the elastic modulus of the model decreases.Among them,the distribution range of lateral contact force vector under the periodic boundary increases and the lateral contact force decreases by 41%.The displacement of particles in the lateral area in the model is greater than that in the rigid boundary model,indicating that there exists interlocking between ballast particles at the periodic boundary and the movement direction is not limited;The distribution range of lateral contact force vector under Winkler boundary does not change obviously,and the lateral contact force increases by19%.Setting the geogrid in the ballast box model can effectively strengthen the geogrid-reinforced ballast,disperse the upper load and reduce the sleeper settlement.Moreover,the periodic geogrid under lateral constraints can reflect the membrane function of the geogrid,and the periodic geogrid is closer to the real stress state of the geogrid than the aperiodic geogrid.(4)Under the cyclic loading,the composite element test and simulation results show that the stiffer the subgrade stiffness,the greater the ballast force borne by the subgrade base.With the decrease of subgrade stiffness,the upper load can be transmitted to the bottom more uniformly,and the sleeper settlement decreases.Laying the geogrid can effectively restrain the ballast particles,disperse the upper load,so as to reduce the vertical contact force at the bottom of the model,so as to reduce the sleeper settlement,and the geogrid reinforcement performance under the soft subgrade is better than that under the stiff subgrade.In the composite element model of track bed with side confined,part of the upper load can be horizontally transmitted to both sides of the sleeper,so as to reduce the stress on the bottom of the model,and the particles on both sides of the sleeper are difficult to move due to the restriction of rigid wall.Therefore,the sleeper settlement of the composite element model with side confined is significantly lower than that of the composite element model without side confined.