A layer of heavy stone, broken rock (rip-rap), gabions or pre-cast blocks is often used to provide permanent erosion protection for Submerged bridge piers and foundations.
Erosion protection structures or armor systems dissipate the hydraulic forces that cause erosion, and they preserve the subgrade soil or fill soil behind it. A typar is required between the subgrade soil and the rip-rap, gabions or pre-cast blocks to prevent piping and erosion of the soil while allowing the free passage of water.
The primary function of the Typar geotextile in erosion control is filtration to minimize erosion of the subgrade soil behind the armor system.