Viaduct design for minimization of direct and structure-radiated train noise

The Kowloon-Canton Railway Corporation (KCRC) is constructing the West Rail extensions from Kowloon into the New Territories. The Hong Kong Noise Control Ordinance (NCO) stipulates that operational noise levels along parts of this alignment are not to exceed an L-eq(30 min) of 55 dB(A). Satisfying the criteria requires a decrease of roughly 24 dB in the wayside noise level characteristic of an unmitigated viaduct with compulsory reductions in both the direct train and structure-radiated noise. Given the required reduction in noise level, edge sound barrier walls alone will not adequately attenuate direct train noise. Other considerations also rule out extensive use of full enclosure. A mitigation design was developed which consists of augmenting the performance of absorptive parapet walls by creating noise plena beneath the cars and under adjacent walkways. These in-series plena provide source attenuation of undercar noise to supplement the edge walls. A mathematical model is described and results are presented demonstrating that the attenuation achieved with the in-series plena augmenting absorptive edge walls is 12-17 dB greater than the edge walls alone. A model of the structure, trackform and vehicle is developed to determine the structural vibration levels caused by wheel/rail interaction and to predict structure-radiated noise levels from the viaduct vibration. Model predictions are compared with measurement data: resilient baseplates, resiliently supported ties and floating slab trackform (FST) are evaluated with regard to wayside noise reduction. FST with soft resilient baseplates is found to be the only trackform which adequately reduces the structure-radiated noise; however, all trackforms provide less vibration reduction on the viaduct than in tunnels because of the relatively low impedance of the viaduct. Structural changes in the viaduct can affect the level of radiated noise, especially at low frequencies, but not sufficient to eliminate the floating slab. Projections of total wayside noise are obtained by combining the structure-radiated and direct train noise. Results confirm that both the direct and the structure-radiated noise must be adequately attenuated to conform to the NCO. Noise radiated from the floating slab is significant if the rails were attached with stiff fixation; otherwise, the train noise dominates. (C) 2000 Academic Press.