Laboratory Investigation and Discrete Element Analysis in Open-graded Friction Course and Railway Crosstie-ballast Interaction

Granular materials, such as ballast and aggregate, are widely used nowadays in civil and transportation engineering. Discrete element method has been extensively used to simulate the behavior of granular materials. In this study, properties of granular materials used in pavement and railway were investigated by laboratory tests and discrete element modeling.

Firstly, the bonding performance between pavement layers was evaluated. Open-graded friction course pavement and traditional dense asphalt mixture pavement were both explored. Two dense asphalt mixtures (D, BM) and one open-graded friction course (OGFC) mixture were selected for the comparison. The laboratory test results show that, for traditional dense samples, the interlock effect between layers played an important role in pavement layer bonding. For specimens composed of OGFC and a dense mixture (D or BM), OGFC-BM showed a better shear performance than OGFC-D, due to the double effects of a larger interface contact area and a larger interface roughness than OGFC-D. The DEM modeling was focused on the interlock effect between pavement layers by conducting the direct shear box test. Results from DEM modeling show that D-BM gave a higher shear strength, which agreed with the laboratory test.

Secondly, laboratory tests were conducted to investigate the shear fatigue performance between OGFC and underlying layer. Results indicate that contact area between OGFC and underlying layer play the critically important role. The larger the contact area, the better the shear fatigue performance.

Thirdly, a full scale laboratory test was conducted to investigate the pressure distributions under a single steel or timber crosstie. It is found that pressure distribution was different for steel and timber crossties. Cyclic loading could change the pressure distribution under both steel and timber crossties, but the effect of cyclic loading was more obvious on steel crosstie than on timber crosstie.

Last, one coupled framework between discrete element method (DEM) and finite element method (FEM) was developed to investigate the ballast-tie interaction. The normal contact condition and the stress distribution beneath the steel crosstie and timber crosstie were obtained from the simulation. Stress distribution obtained by DEM-FEM simulation was consistent with the findings from the laboratory test.