Response of Railroad Ballast to Vertical Vibration

The effect of vertical vibration on some railroad ballasts in laboratory tests is presented. The ballasts were found to behave in a similar manner to that previously reported for sands, i.e., peak acceleration was the main parameter affecting densification. For unsurcharged ballast, little increase in density occurred at peak accelerations below 1 g, a large increase in density occurred at peak accelerations between 1 g and 2 g, but little further increase in density occurred at higher peak accelerations. Surcharge pressures above 50 psi (345 kN/m²) led to an increase in the peak acceleration needed to produce a substantial increase in density. Gradings modeled on the Talbot grading equation with n values between 0.7 and 1.0 gave the highest densities under vibration. There was some downward migration of fine particles during vibration. The amount of segregation appeared to be independent of grading but dependent on the particular ballast tested.