A Retention Wall is a vertical or diagonal wall structure used to retain soil or water at an elevated height on one side of the wall, keeping it above the natural level that the soil or water would achieve.  Retention walls are used to create relatively flat areas such as roads or gardens that are cut into a hillside. Retention walls are also used as levees to keep water out of an area.  The soil and moisture on one side of the wall exerts pressure (stress) on the wall.  This is why the materials used must be appropriate for the application in both density and porosity.