Study of the shadow effect caused by a railway tunnel

When a train runs in a tunnel the largest vibration on the ground surface may not occur directly above the tunnel but at some lateral distance away from the tunnel alignment. This has been called the ‘shadow effect’. The characteristics of this shadow effect can help in understanding the distribution of vibration on the ground surface. For the current study it is first shown, using an analytical ground model, that a shadow region may occur for a force at some depth in the ground even in the absence of a tunnel; the extent of this effect depends on the Poisson’s ratio of the soil. To introduce the tunnel a 2.5D finite element/boundary element model has been used to represent the coupled tunnel-ground situation. When the tunnel is present the vibration caused by excitation at the tunnel base shares many of the features found in the absence of the tunnel. However, the existence of the tunnel structure also influences these features, especially at high frequencies. It is found that, rather than the tunnel structure shielding the vibration from reaching the ground surface, its dominant effect is to transmit vibration from the tunnel base to the crown at high frequencies. The dependence of these effects on various parameters is studied, in particular the tunnel diameter, wall thickness and depth