Subsurface water flowing can cause water intrusion in below-grade living areas and can cause heave or settlement of foundations. To prevent this water intrusion into living areas or crawl spaces, subsurface drains, also known as French drains, can be installed. A subdrain generally consists of a trench lined with a perforated pipe and then backfilled with gravel. These drains collect subsurface water and convey it to a suitable discharge point. Subdrains can also be used to control groundwater on hillsides to maintain slope stability.