Investigation of Ground Borne Noise Transmission by Numerical Simulation

Abstract

Ground borne noise and vibrations from underground railway tunnels can pose significant challenges in urban environments, negatively affecting the quality of life of nearby residents. This thesis investigates the transmission of ground-borne vibration from underground railway tunnels in bedrock, with a focus on the influence of fracture zones, tunnel geometry, and model dimensionality. The aim is to improve understanding of how vibrations propagate through stiff geological media and how the resulting vibrations at the surface are affected by subsurface inhomogeneities. A series of numerical simulations were carried out using COMSOL Multiphysics in both 2D and 3D, analyzing single and twin tunnel configurations under various conditions. Special attention was given to the presence of vertical fracture zones, which were modeled as weakened regions within the rock mass. The effect of tunnel depth and structural layout was also examined. In addition, a parametric study was conducted to determine which material properties of the fracture zones affect the most the wave propagation. Simulation results showed that when vibration travels from the source through the fracture zone, attenuation increases significantly at the ground surface beyond the fractures, especially at mid-to-high frequencies (250-800 Hz). Tunnel depth and geometry were also found to influence surface vibration levels, with deeper tunnels and certain twin tunnel configurations reducing amplitudes. Both 2D and 3D simulations exhibited similar trends in how frequency influenced the results. However, the 3D model, by incorporating the third spatial dimension, provided a more realistic representation and captured greater spatial detail in how the effects varied across the environment. The findings were compared with site measurements conducted above an operational railway tunnel. The measurements confirmed key simulation trends, including the dominant frequency range and the general decay of vibration levels with distance.