Geert Degrande is the head of the Division of Structural Mechanics, one of the five divisions within the Department of Civil Engineering. He also served as department chair for eight years. Geert emphasizes that the research discussed in this interview is the work of the entire team. He himself has extensive experience in the subject at hand, starting with his 1992 PhD on wave propagation in layered porous media, followed by a stay at École Centrale de Paris in 1994-1995, where he studied dynamic soil-structure interaction, or more specifically, the influence of the soil on the response (natural frequencies and damping) of structures near the source. He is, therefore, the ideal person to discuss vibration issues in the built environment, or the effect of road and rail traffic on nearby residential areas.
Railway traffic and residential comfort
The applied research is currently mainly funded by the SILVARSTAR project, under the Shift2Rail Open Call of the EU's Horizon 2020 framework program (which has since been replaced by the new Horizon Europe framework). The goal of SILVARSTAR is to develop software and methods to assess the noise and vibration impact of railway traffic at the system level, with two objectives: (i) to develop a generally accepted and practically validated methodology and a user-friendly tool to predict ground vibrations, and (ii) to develop a fully functional system for auralization* and visualization, based on physically correct synthesized railway noise, with an interface to VR (virtual reality) visualization software. Figure 1 clearly illustrates the objectives of SILVARSTAR.
*Auralization is a method of creating an acoustic environment based on measured or simulated data
The tool that Degrande and his team are developing to predict the effect of ground vibrations on buildings works in two steps. First, the free-field response to the moving train is calculated, followed by the calculation of the building’s response to this free-field wave. Numerical models for the first step are a coupling of sub-models of the train, the track, and the soil. Accurate prediction of ground vibrations requires detailed information about the soil parameters and the excitation. This can be done by exciting the track with an impact hammer equipped with a built-in force sensor and measuring the response at various nearby points, or by measuring the response as a train passes by.
Figure 2 shows several snapshots from recordings in the Czech Republic. The building's response to the incoming wave can be calculated using classical methods of dynamic analysis. In the numerical modeling, the track is modeled as a beam, while the ballast and soil are represented by finite element and boundary element models. The resulting waves are both longitudinal (dilatational) and transverse (shear) waves. The interaction between these wave types generates Rayleigh waves, which propagate along the surface. In most cases, railway-induced vibrations can be modeled by treating the ground as a layered half-space, with shear wave velocity depending on soil stiffness. Railway-induced vibrations are generated by quasi-static and dynamic axle loads, due to various mechanisms such as wheel or rail irregularities, impacts at rail segment joints, and flat wheel imperfections caused by wear during braking.
How to improve residential comfort?
Measures to reduce vibrations caused by railway traffic can be implemented at the source, along the transmission path, and at the receiver (Figure 3). Addressing the vibration source is the most logical approach. This can be achieved by reducing the irregularities in the rails and wheels through grinding or reprofiling. The stiffness of the primary suspension on the train can also be reduced. The most significant effect comes from incorporating flexible elements in the track, such as soft rail fastenings, ballast mats, or sleeper pads. Basic vibration theory states that if the natural frequency of the formed mass-spring-damper system is much lower than the excitation frequencies, then the force transfer is less than one.
Measures along the transmission path are aimed at preventing the waves from the source from propagating to the structure to be protected. This can be done by installing soft or hard vertical barriers in the ground, horizontal plates under the rails, or heavy masses (e.g., cages filled with stones) next to the tracks. Measures on the receiver side are also possible, but each building must be addressed individually.
In addition to developing these general methods for improving the residential comfort of surrounding buildings, the research group has also worked on solving specific problems, such as vibration analyses of the floors in clean rooms and other spaces near tram or railway lines, where sensitive equipment like wafer steppers, microscopes, and medical imaging devices must be placed. For instance, Degrande's team provided advice to the Technical Services of KU Leuven regarding the proposed foundation design for Corelab 1B, where equipment must operate vibration-free with nanometer accuracy.
The group collaborated with a French company to address a unique problem in the design of the high-speed railway line between London and Birmingham, where the bow wave problem arose. This phenomenon occurs when the train speed reaches the propagation speed of the ground vibration, similar to the sonic boom that occurs when an airplane breaks the sound barrier.
Seismic metamaterials and 'The curse of high dimensionality
As is fitting for good academic researchers, Degrande and his team's work also addresses more fundamental topics, but, as is appropriate for good engineers, always with an eye toward potential applications.
An example of this is their new FWO project that investigates how seismic metamaterials placed between railways and buildings can lead to efficient vibration reduction. Metamaterials are artificially manufactured structures that exhibit exceptional properties, such as sound dampening and making objects invisible by bending light. Seismic metamaterials consist of tuned resonators placed at the surface or within the ground, which influence surface waves, helping to protect infrastructure from earthquakes.
Degrande is examining whether these metamaterials can also be effectively utilized to reduce vibrations from railway traffic, with the aim of achieving better efficiency than traditional rigid barriers. One challenge in this endeavor is the high dimensionality of the models used for designing vibration-reducing measures. To enable rapid optimization with varying parameters, a drastic model order reduction is necessary, allowing users to visualize the effects of parameters in near real-time. The applied technique for model order reduction is Proper Generalized Decomposition (PGD), and a sensitivity analysis can study the influence of uncertain soil characteristics by defining the shear modulus of the soil as a stochastic variable.
Conclusion
We are all familiar with the sound barriers along our highways aimed at improving sound comfort near residential areas. Much less conspicuous but far more comprehensive are the measures described here that aim to mitigate the harmful effects of vibrations caused by road and rail traffic. While sound barriers are purely curative, several of the techniques discussed here are preventive; they require attention from the very early design stages of the transport infrastructure.
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