The influence of the appropriate spacing between reinforcement and ballast on the performance of pullout resistance in multi-layer reinforced soil structures were explored.By improving the conventional single-layer geogrid-ballast pullout numerical test model,discrete element simulations of synchronized pullout under equivalent displacement amplitudes for multi-layer geogrids were achieved.The rigid clump particles were used in the numerical model to mimic the non-spherical characteristics of the ballast particles,and bonded particle strings were incorporated to simulate the uniformly loaded triaxial geogrid,by considering both particle interlocking effects within the ballast and the tensile behavior of the geogrid.Using criteria such as average pull-out force,interfacial strength at specific layer positions,and strain in the reinforcement,the interfacial coupled behavior between multi-layer geogrid-reinforced ballast was scientifically determined,and the interlayer interference mechanism within the system was investigated at the particle scale.The results show that the pull-out force and its rate of increase in multi-layer geogrids are lower than those in single-layer geogrids,especially at high normal stresses,and there is non-uniformity.As the number of reinforcement layers increases,significant differences in the resistance of the reinforcement are observed,along with an increase in the frictional energy dissipation.Overlapping interlayer particle displacements weaken the macroscopic mechanical response of the reinforcement because of insufficient development of particle force chains.Subsequent investigation of normal contact forces indicates that inter-layer interference in multi-layer geogrid-reinforced structures may lead to instability in the distribution of normal contact forces.
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