How do seismic refraction surveys contribute to the understanding of subsurface conditions for tunneling?

Seismic refraction surveys provide valuable information about subsurface conditions for tunneling by determining the velocities of seismic waves traveling through different layers of soil and rock. These velocities are directly related to the density and stiffness of the materials, allowing for the identification of different geological layers and the estimation of their properties. By analyzing the travel times of seismic waves, the depth and thickness of different layers can be determined. This information is crucial for creating a subsurface profile along the tunnel alignment. Seismic refraction can identify the presence of bedrock and determine its depth. This helps in selecting the appropriate TBM type and excavation methods. It can also detect the presence of faults, fractures, and other geological structures that may affect tunnel stability. By mapping the subsurface conditions, seismic refraction surveys help to reduce the risk of encountering unexpected ground conditions during tunneling. This allows for better planning and cost estimation. The term 'seismic refraction' refers to a geophysical method that uses the travel times of seismic waves to image the subsurface. 'Seismic waves' are vibrations that travel through the earth. 'Tunnel alignment' refers to the planned route of the tunnel.