Controlling Subsurface Water Flow
The French drain is basically a trench filled with gravel. Strictly speaking, a French drain addresses sub-surface migratory water, water that flows beneath the surface. The concept is simple, the rock-filled trench creates a void in the soil, when the migrating water reaches the void its forward progress is stopped and the water percolates to the bottom of the trench.
Once the water is at the bottom of the trench it enters a perforated pipe that carries the unwanted water away. There are many variations to the French drain, which variation is used is dictated by your particular situation. The depth and the width of the trench will also vary.
The graphic shows the most common variation that Geo-Tech uses. The trench is filled with 1″-3″ washed river rock, the back and bottom of the trench is lined with 6 mil polyethylene sheeting so the water does not seep into the ground. A 4″ smooth walled, perforated PVC pipe is used to collect the unwanted water and re-direct it elsewhere. Geo-Tech does not use corrugated pipe for our French drains. Depending on the soil type, the PVC pipe is wrapped in a geo-fabric sock to prevent silt and sediment from entering the drain pipe.