This system is viable for locations where
the underlying ground offers some level of permeability and infiltration is acceptable. Within the sub-base layer outlet pipes are installed that allow any excess water that cannot penetrate into the existing ground to be drained into other drainage devices such as watercourses, swales or sewers. This system is typically used where the sub-grade is not capable of draining all of the water as a means of reducing the volume of run-off from the site. Geotextiles may be incorporated into the system depending on the nature of
the project.