Investigating the effects of in-track countermeasures to reduce ground-borne vibration from underground railways

In this research, the effects of two different types of in-track countermeasures on ground-borne vibration from underground railways are investigated. First, the influence of new type of supports is investigated, in which the rotation of the rail is partially or fully restrained. Second, a different simple local resonator is considered consisting of masses on springs placed at fixed spacing between the supports. To investigate the effects of these countermeasures, a numerical model is developed consisting of two sub-models: the first sub-model is an excitation model in which the track is modelled as an infinite beam on discrete supports connected to rigid foundation, for the calculations of forces transmitted to tunnel bed. The beam is analysed under the action of a number of axle masses with harmonic excitation induced through relative displacement between the un-sprung axle mass and the beam using a pull-through roughness. This first sub-model employs the dynamic stiffness method to perform the calculation. The second sub-model accounts for a tunnel embedded in a half space and is used to calculate the responses in the free surface with the input forces taken from the first sub-model after transformation to the wavenumber domain. The second sub-model is based on the well-known Pipe in Pipe (PiP) model. The discontinuously-supported model used in this work is verified using a continuously supported model. The effects of the suggested countermeasures on vibration mitigation are explored via parametric analysis that shows the impact of the added rotational stiffness, resonators' natural frequency and the resonators' mass. The results show for the parameters considered that using the rotation restrained support can effectively reduce the ground-borne vibration levels by up to 20 dB for low frequencies. Furthermore, the use of local resonators can reduce the ground-borne vibration by 4-10 dB for frequencies in the range 20-200 Hz.