The thickness of the ballast layer can be as low as 150 mm on little traveled lines and as thick as 400 mm on more frequented lines. When using concrete sleepers, the ballast thickness should be raised by at least 50 mm. The minimum thickness of the blanketing sand layer is generally 150 mm. However, for very soft surfaces it may be necessary to use a thicker layer. Ballast is used to stabilize, protect, and provide traction for rails. It also acts as a cushion between the rail and any vehicles that travel over it. Ballast is usually made up of gravel or crushed rock that has been mixed with some type of binder (such as cement) and shaped into balls or chunks. The size of the ballast will determine how much traffic can pass over it without causing damage to the rail. The thickness of the ballast layer depends on what type of track you are building. On lightly used branches off of main lines, the ballast layer can be as thin as 150 mm. But on heavily trafficked routes through cities, the ballast layer needs to be at least 300 mm thick to avoid damage from road vehicles. Concrete sleepers are used under roads and railroad tracks where there is a risk of soil movement due to heavy loading or seismic activity. They provide stable support for the rail line above them.