Innovative Surface Wave Mitigation in Unsaturated Soils Using Periodic In-filled Trench Barriers

Ground vibrations from transportation systems, industrial activities, and natural events can travel as surface waves and can carry energy that cause risks to buildings and infrastructure. These waves which move along the surface, can impact vast areas, making them particularly challenging in urban environments. Here, dense construction and proximity to vibration sources worsen the problem. Traditionally, engineers have tackled these surface waves using wave barriers like open trenches, in-filled trenches, and piles. These methods well-studied and widely used work particularly well in saturated or single-phase soils, where the soil is either fully saturated with water or considered a uniform medium. The effectiveness of these barriers hinges on the contrast in impedance between the barrier material and the surrounding soil which helps in reflecting, refracting or reducing the waves’ energy. However, when it comes to more complex soil conditions, especially unsaturated soils, these traditional methods often fall short. Unsaturated soils which are common in many natural and urban settings, present a unique set of challenges. Unsaturated soils are a mix of solid particles, water, and air not like saturated soils which are straightforward in composition. This mix leads to non-linear and variable mechanical properties such as changes in strength, stiffness, and permeability, all of which influence how waves move through the soil. The presence of multiple phases solid, liquid, and gas means that unsaturated soils support different types of waves, including compression and shear waves with each has its own speed and damping characteristics. This complexity makes it tough to predict wave behavior using models designed for simpler soil types, thereby limiting the effectiveness of conventional wave barriers. The real challenge in using periodic wave barriers in unsaturated soils lies in accurately modeling and predicting how these surface waves will spread and diminish in such a complex environment. Traditional wave barriers when optimized for saturated soils might not provide enough vibration control in unsaturated soils because of the different ways waves travel in these soils. For example, the interaction between the solid framework of the soil and the pore fluids both water and air can significantly change the wave’s speed and damping which leads to unexpected results when trying to reduce vibrations.

To this account a new study published in Philosophical Transactions of the Royal Society A by Professor Zhifei Shi and his PhD candidate Liangliang Wu from Beijing Jiaotong University’s School of Civil Engineering, set out to explore a new approach. They focused on using periodic in-filled trench barriers specifically designed for unsaturated soils. Their goal was to develop a more accurate and effective way to control surface waves in these complex soil environments. By applying periodic theory, which helps analyze how waves behave in structured media, the researchers identified the mechanisms that control wave damping in unsaturated soils and used this knowledge to design barriers that could better mitigate vibrations. To test their ideas, the researchers created numerical models simulating surface waves moving through a two-dimensional unsaturated soil model embedded with periodic trenches. This model took into account the complex interactions between the solid soil matrix and the fluids within it both water and air. Using advanced computational techniques, including the Bloch-Floquet theory, they simulated how the surface waves would behave when encountering these periodic barriers. Moreover, they examined the dispersion of the waves and managed to pinpoint the frequency ranges where the barriers effectively dampened the surface waves. Their findings were promising. The periodic in-filled trench barriers created distinct zones where surface waves were strongly suppressed. These zones of suppression, or attenuation zones, were closely linked to the material properties of the unsaturated soil, such as its saturation level, friction angle, and intrinsic permeability. For instance, they found that when the soil had higher saturation and a lower friction angle, the barriers were better at dampening low-frequency waves. Conversely, when the soil had lower saturation and a higher friction angle, the barriers were more effective against high-frequency waves. This showed that by tailoring the design of the barriers to the specific soil conditions, engineers could optimize vibration control. To further validate their findings, the researchers conducted a numerical simulation in the time domain, where the inputted time history of ground vibrations are obtained along Beijing Metro Line 13. The team designed and implemented a periodic wave barrier aimed at reducing the vibrations based on their simulations. During the simulation in time domain, they picked the vertical ground acceleration up both with and without the barriers in the model. Additionally, they also confirmed with the simulation results the effectiveness of their approach. The periodic barriers significantly reduced ground vibrations, cutting vertical acceleration by over 90% at the target frequencies. This confirmed that the barriers were not only theoretically sound but also practically effective in reducing surface wave propagation in unsaturated soils. Moreover, the frequency ranges where the barriers were most effective matched the attenuation zones predicted by their simulations. This alignment between theory and simulation highlights the practical value of their research.

In addition, the researchers explored how changes in intrinsic permeability which is an important soil property affected the performance of the barriers. Through both theoretical and numerical analyses, they found that adjusting the soil’s intrinsic permeability could shift the attenuation zones, potentially broadening the range of frequencies where the barriers were effective. This was a particularly exciting discovery, as it suggested that by carefully selecting or modifying soil properties, engineers could make these barriers even more versatile, capable of dampening vibrations across a wider range of frequencies. This study is significant because it opens up new possibilities for understanding and controlling wave propagation in complex soil structures. The discovery that periodic in-filled trench barriers can create effective attenuation zones in unsaturated soils, influenced by factors like saturation level, friction angle, and intrinsic permeability, offers a new framework for designing vibration control systems. Furthermore, the work of Professor Shi and Mr. Wu highlights the importance of considering unsaturated soil conditions in engineering designs and the traditional methods based on saturated soil models may not be sufficient in many real-world scenarios. In conclusion, Professor Shi and Mr. Wu provided an excellent and powerful tool for engineers to protect infrastructure from ground vibrations and with such ability to customize wave barrier designs to fit specific soil conditions means engineers can now implement more effective, site-specific solutions. This is especially important in urban environments, where protecting buildings and other structures from vibrations caused by transportation systems and other sources is vital. The success of the simulation in time domain further reinforces the practical value of this research and hopefully this will show that the theoretical concepts can deliver tangible benefits in real-world applications.