Where the subgrade consists of coarse soils e.g. sands and gravels, the geotextile acts as a separator between the subgrade and the ballast. The subgrade, under the effect of pressure and vibration from trains passing overhead, is prevented from working its way up into the large voids between the rail ballast stones and conversely, the ballast stones are prevented from working their way down into the soil subgrade. Without a geotextile in place, progressive settlement of the track could occur resulting in loss of track alignment and increased maintenance requirements. The pores in the geotextile are small enough to block the passage of fine sand and very coarse silt but large enough to allow the transmission of groundwater for pore water pressure relief.