Investigating the effect of using softer rail-pads on ground-borne vibration from underground railways

In this study, a series of vibration measurements were performed on a section of one of the lines of the Doha Metro to explore the effect of changing rail-pads from conventional higher stiffness under-rail pads to lower stiffness web-supporting pads on the vibration levels at the free surface. Vibration levels were measured for the two cases of lower and higher stiffness' pads at the free surface directly above the railway track. Additionally, vibration levels were measured in the railway tunnel to investigate the effect of changing rail-pad's stiffness on the vibration levels at different locations inside the tunnel. Vibration measurements due to moving trains from the two cases were processed and analysed to calculate the Insertion Gain (IG) of vibration at the free surface and at the other locations. It was found that the use of softer rail-pads has resulted in reduction in the vibration levels at the free surface by about 10 dB for most frequencies in the range 40-200 Hz. Further numerical investigation was conducted to simulate the railway section under consideration. To simulate the track fastening system accurately, the dynamic stiffness of the rail-pads for the two cases was estimated using the measurement of vibration of rails due to impact hammer excitation and measurement of axle-box accelerations of a moving trains. The parameters of problem at-hand were used as inputs to the numerical Pipe-in-Pipe (PiP) model to calculate the insertion gain numerically at the free surface and this showed good match with the results from the measurements.