Table 1.

Definition of HSR Subgrade Defect.

Type of defect	Definition	Type of defect	Definition
Subgrade settlement	Settlement refers to the vertical deformation that occurs over a small or extensive area due to inadequate compaction of subgrade soil, insufficient depth of foundation treatment, damage between piles, creep of underlying soil layers, or regional settling.	Frost damage	In cold regions, subgrade and its protective structures experience uneven frost heave under low temperature conditions, leading to issues like tilting and cracking of protective structures.
Mud pumping	This defect occurs in areas with poor drainage. Repeated vibrations from train traffic cause softening or thixotropic liquefaction of the sub-ballast, leading to the formation of mud slurry.	Uplift deformation	This happens when expansive soils or rocks within the subgrade or its base react with external moisture, causing the subgrade to arch upwards.
Soil extrusion	This deformation occurs when sub-ballast soil undergoes shear failure under the load of the train, leading to lateral squeezing.	Slope failure	Slope slipping and collapsing refer to the shallow sliding or collapse of soil slopes.
Deterioration of protection structures	These defects in retaining structures occur due to natural environmental factors, uneven settling of foundations, and issues in construction and design.	Poor drainage	Poor or insufficient drainage leads to rainwater penetrating into or eroding the subgrade, deteriorating the quality of the subgrade or its stability.