How does soil type specifically affect the calculation of seismic base shear in tower foundation design?

Soil type significantly affects the calculation of seismic base shear because it influences how seismic waves propagate through the ground and how the foundation interacts with those waves. Seismic base shear is the total horizontal force that the foundation must resist during an earthquake. Different soil types have different stiffness and damping characteristics, which affect the amplitude and frequency of ground motion at the tower site. Softer soils, like clay or loose sand, tend to amplify ground motion, particularly at certain frequencies, leading to higher seismic forces on the structure. Stiffer soils, like bedrock or dense gravel, generally result in lower ground motion amplification. This amplification is accounted for through site coefficients, which are factors used to modify the spectral acceleration values obtained from seismic hazard maps. These site coefficients, denoted as Fa and Fv in seismic design codes, are directly dependent on the soil type at the site. In addition, the soil's shear wave velocity, a measure of its stiffness, is used to determine the site class, which then dictates the appropriate site coefficients to use. Therefore, a site with soft soil will have larger site coefficients, resulting in a higher calculated seismic base shear and a more robust foundation design, compared to a site with stiff soil.