Studying the Strengthening Effect of Railway Ballast in the ‎Direct Shear Test due to Insertion of Middle-size Ballast Particles

This paper summarized some common grading curves of ballast layers and found that the content of 16-32 mm ballast particles ("middle-size particles" in this paper) had a significant effect on the direct shear performance of the ballast layer. In this paper, the direct shear tests of the ballast layers with different contents of middle-size particles were reproduced using the discrete element method (DEM). Two different compactions of the ballast samples were used, and the reasons for the changes of shear strength of the ballast layers with different size distributions were analyzed from macroscopic and microscopic perspectives. The results showed that the strengthening effect of the ballast due insertion of middle-particles could only be observed for normally compacted ballast, whereas the same insertion with fully compacted ballast would decrease the shear strengths properties. The fully compacted ballast is subjected to the dilation. The reason of the strengthening effect for the normally compacted ballast were the contraction and dilation processes. Insertion of the middle-size particles up to 20-30% at most increase the dilation processes. Thus, the results show that the ballast layers with conventional narrow particle size distribution (narrow PSD) have higher shear strength than wide range particle size distribution (wide range PSD) if the ballast is good fully compacted. Additionally, it should be noted that the number of small particles will increase during the lifecycle of the ballast layer due to corner brakeage and the external contamination. Moreover, the drainage aspects of the wide range PSD should be considered. Therefore, the excessive insertion of middle-size particles is not justified.