Soils & Foundations: French Drain Aka Drain

So a French drain or a drain tile is usually used in the foundation behind retaining walls and the point is to drain the water to relieve any excessive hydraulic sorry hydrostatic pressure. So we see this in our drawings a lot. It's this, basically the French drain, and usually it's surrounded with maybe three-quarter inch gravel to protect it, to filter water and any sand in the foundation from entering that drain pipe. And it's good to know that the slope of this pipe is typically for gravity drainage. You need 1 to 2% of slope for the water to flow through that drain pipe. So here we can see there's fabric around the drain pipe to protect from dirt going in there and sand and residue. So it filters that and then it gets in this perforated pipe and it's sloped, like we said earlier, at a 1 to 2% slope. Here there is a membrane, waterproofing membrane, and then there's the French drain and it's perforated. And then they're going to put gravel on it to protect it and to enhance the drainage. So a French drain is a very important thing in relieving the hydrostatic pressure behind any kind of wall in the foundation, any kind of vertical surface in the foundation.