STUDY ON INTERFACE STRESS BETWEEN BALLAST AND SUBGRADE FOR TRADITIONAL RAILWAY BASED ON COUPLED DEM-FDM

The discrete element method (DEM) and finite difference method (FDM) coupling algorithm were introduced into the analysis of interface stress between railway ballast bed and soil subgrade. The crushed ballast was simulated by the refined DEM modelling, combined with laser scanning and triaxial testing. Then, an interface element was established between interlayer structure boundaries of ballast bed and subgrade, and consequently the DEM and FDM were dynamically coupled through exchanging the data of forces and velocities between two layers within each time step. Thus, the coupling simulation model was established, which had been validated with the field measured results. On this basis, the characteristics of the dynamic stress distribution of subgrade under the load of one train passing were calculated by the coupling model, and the effect of tie-ballast bed contact state was investigated. The calculated results were compared with the ones derived from the finite element method model of tie-ballast bed-subgrade. The results show that:the force chain in ballast bed is discrete in the coupling model if tie-ballast bed not fully close contact, and on the contrary, the force chain is vertical diffusion; the distributions of subgrade surface stress for the two contact conditions of tie-ballast bed are significantly different, but the stress peak is close for each other. Comparing the stress peak derived from a coupling model and a FEM model, the former is significantly higher than the latter. The influence depth of local stress concentration on subgrade surface caused by irregular ballast was about 8 times as the maximum particle size of ballast (i.e.63 mm), approximating the depth of a subgrade surface layer.