MS19.1 - Traffic Induced Vibrations

Among modes of transportation, railway transport has received increasing attention lately especially due to its capability of running fully on electricity, which can be generated from green sources. With the increasing demand on railway transportation, the previously acceptable issues caused by railway transport are quickly turning into challenging problems causing disruptions to the normal operation of traffic. One such an issue is the ground-borne vibration that is changing from a previously localised annoyance to a presently serious societal and economic challenge. Mitigation measures range from interventions at the source level (i.e., vehicle-structure interaction), at the receiver location (e.g., vibration isolating foundations), and on the transmission path. This work is concerned with the last category. More specifically, this study investigates the capabilities of a novel mitigation measure, a so-called metawedge, in reducing the ground-borne vibration at the receiver end. A metawedge is series of barriers (i.e., resonators) arranged periodically in the longitudinal direction and each one is offset with respect to the others in depth direction (i.e., while the first barrier is completely on the surface, the last barrier can be completely embedded). The slight depth difference of each resonator means that each representative cell has slightly different natural frequencies. Consequently, this countermeasure can convert the incoming Rayleigh (surface) waves into body ones, redirecting the energy content deep into the ground. Modelling results show that the metawedge is capable of significantly reducing the vibration levels with as few as five resonators. Furthermore, while conventional single trenches are efficient as mitigation measures only at a certain angle of the incoming waves (outside the critical cone), the metawedge is efficient inside this cone. Although it shows good potential, the results were obtained with unrealistic properties of the individual barriers (heavy steel blocks); therefore, future studies are necessary to design the individual resonator such that it complies with the low frequency requirements of ground-borne vibration of railway transportation. Nonetheless, this study serves as a proof of concept that metamaterials can potentially play an important role in addressing present and future challenges of the railway transportation.

In this paper, a meshless method for the simulation of dynamic soil-structure interaction prob-lems is verified in a three-dimensional context. The proposed meshless method considers a set of collocation points at the ground surface portions where the structure’s foundations will be located, whilst the virtual sources are placed in the soil in a hemisphere configuration, sur-rounding those foundations. The method is based on three steps: incident wave field evaluation, incident wave field virtualization and structure response determination. In the first stage, the free field response due to the incident wave field to be considered is evaluated at the collocation points. In this framework, without the structure, a set of virtual forces that equivalently repre-sent the free field response induced by that incident wave field is calculated. Finally, this set of virtual sources is applied to a structure-soil model to determine the response of the structure. The evaluation of the incident wave field at the first stage can be carried out by a simulation model of the incident wave field or by measurements on the real site, if possible. The proposed meshless method is verified in the context of the problem of a rectangular foundation embedded in a homogeneous half-space subjected to both non-moving and moving harmonic forces. Tips for the effective usage of the proposed method are outlined and the application of the method for the assessment of ground vibration in new buildings to be constructed close to operative railway lines is also discussed.

Base isolation is an established method of limiting the transmission of traffic-induced vibration into buildings. Despite this, the available design guidance is limited, and significant questions remain concerning how the vibration isolation performance may be optimised for a particular building. Recent research has sought to develop simplified models that enable the prediction of building vibration levels without resorting to complex numerical methods. This paper considers the example of a multi-storey building founded on piles adjacent an underground railway, and compares some simplified models against a comprehensive numerical model with the aim of establishing which aspects of the system behaviour must be accounted for when predicting isolation performance. Aspects considered include the various components of soil-structure interaction and the nature of the vibration transmission into the building. It is shown that simplified models that account for the fundamental dynamics at the building-foundation interface can provide an effective basis for predicting the overall performance of a base-isolation system.

Controlling the train-induced vibrations of environmental infrastructures, especially the buildings along the high-speed railway, has become an essential demand and hard challenge. In attempts to accurately predict the dynamic responses of ground and buildings adjacent to the railway, based on an existing well-characterized in-situ model test in Portugal, the corresponding numerical validation under moving axle loads is conducted by the spectral element numerical code SPEED, developed at Politecnico di Milano, where a fully coupled 3D model including both the ground and the building is considered. The track structure parameters of beam on elastic foundation (BOEF) are obtained by calibrating iteratively the analytical dynamic acceptance curve according to the experimental ones. Sweep frequency response analysis is used to determine the material properties of the building and footings. Under the excitations of 219 km/h moving trains, the recorded dynamic vertical response of nearby ground and the building slab center is compared with the numerical one. The accuracy of results is discussed with specific attention to the frequency range that dominates the dynamic response of the building.