A septic tank soakaway is a system that is used to dispose of sewage and wastewater from a property. The wastewater is stored in a tank, and then it is slowly released into a designated area, where it is allowed to percolate into the ground. There are a few different materials that can be used for a septic tank soakaway, including gravel, sand, rocks or even plastic connecting crates. The type of material that you use will depend on the location of your soakaway and the amount of water that needs to be drained.