How does improved drainage mitigate settlement?

Improved drainage mitigates settlement, which is the vertical sinking of the track structure, by preventing the saturation of the subgrade (the soil layer beneath the ballast) and reducing the pore water pressure within the soil. When the subgrade is saturated with water, the soil particles lose their strength and become more susceptible to deformation under load. This is because water fills the spaces between the soil particles, reducing the effective stress (the force transmitted between soil particles). High pore water pressure also reduces the shear strength of the soil, making it more likely to undergo plastic deformation, which is permanent deformation that contributes to settlement. Improved drainage, such as installing drainage ditches, subsurface drains, or using free-draining ballast, removes excess water from the subgrade, reducing the pore water pressure and increasing the effective stress. This strengthens the soil and makes it more resistant to deformation under load. By preventing saturation and reducing pore water pressure, improved drainage helps to maintain the stability of the track structure and minimize settlement. For example, if a section of track is located in an area with poor drainage, rainwater can accumulate in the subgrade, causing it to become saturated and soft. This can lead to uneven settlement of the track, resulting in poor ride quality and increased maintenance costs. By installing drainage ditches to remove the excess water, the subgrade can be kept dry and stable, minimizing settlement and improving track performance.