Ground vibration from underground railways: how simplifying assumptions limit prediction accuracy
Thesis
Noise and vibration from underground railways is a documented disturbance to individuals living or working near subways. Much work has been done to understand and simulate the dynamic interactions between the train, track, tunnel and soil resulting in numerical models which can predict ground-borne vibration around the tunnels and at the soil surface. However, all such numerical models rely on simplifying assumptions to make the problems trackable: soil is assumed homogenous, tunnels are assumed long and straight, the soil is assumed to be in perfect contact with the tunnel, etc. This dissertation is concerned with quantifying the uncertainty associated with some of these simplifying assumptions to provide a better estimation of the prediction accuracy when numerical models are used for "real world" applications. The first section investigates the effect of voids at the tunnel-soil interface. The Pipe-in-Pipe model is extended to allow finite-sized voids at the interface by deriving the discrete transfer functions for the tunnel and soil from the continuous solution. The results suggest that relatively small voids can significantly affect the rms velocity predictions at higher frequencies (100-200Hz) and moderately effect predictions at lower frequencies (15-100Hz). The results are also found to be sensitive to void length and void sector angle. The second section investigates issues associated with assuming the soil is homogeneous: the effect of inclined soil layers; the effect of a subsiding soil layer; the effect of soil inhomogeneity. The thin-layer method approach is utilized as its semi-analytical formulation allows for accurate predictions with relatively short run times. The results from the three investigations suggest that slight inclination of soil layers and typical levels of soil inhomogeneity can result in significant variation in surface results compared to the homogeneous assumption. The geometric effect of a subsiding soil layer has a less significant effect on surface vibration. The findings from this study suggest that employing simplifying assumptions for the cases investigated can reasonably result in uncertainty bands of +/-5dB. Considering all the simplifying assumptions used in numerical models of ground vibration from underground railways it would not be unreasonable to conclude that the prediction accuracy for such a model may be limited to +/-10dB.