Plumbing: How French Drains Work
French drains, which, despite their name, originated in the United States, essentially function by providing invasive groundwater with a path of least resistance by which it can be diverted away from a structure or low section of turf. They are named after a New Hampshire man, Henry Flagg French, who, in 1860, published a book with the intriguing title: Agricultural drainage: the principles, processes and effects of draining the land with stones, wood, plows and open ditches, and especially with tiles.
Today, French drains are generally used to combat flood problems caused by surface and / or groundwater that a homeowner may have, especially affecting their lawn, foundation, or basement. They are also sometimes used to drain liquid effluents from septic tanks.
The basic design, a gravel-filled trench, is simple but in order for it to continue to function in the long term, it is important that it is well executed.
Flood problems are often associated with sloping terrain, non-porous clay soils, or a combination of the two. For example, if your property is built on a slope and your neighbors’ house is much further up the slope, heavy rains can precipitate a pool of groundwater that runs from your property to yours. If your soil is not able to absorb all of that water, it is highly possible that you will experience foundation damage to your home or leak into a basement or basement below the ground floor of the home.
A linear French drain is a simple and cost effective solution to this problem. In this scenario, you act as a moat that protects your home by intercepting groundwater running down the slope and directing it around and away from the foundation of your home.
A linear french drain is a do-it-yourself DIY project, if you don’t mind doing backbreaking work (this does it involves digging a trench, which is a lot like a trench after all) and you have the right tools and materials (1 “round washed gravel, 4” PVC pipe with drainage holes, a shovel for digging or a electric trencher and a builder’s level)
So, let’s get to the nitty-gritty of how to build a French drain and how it works. First, you will need to dig an L or U-shaped trench system, 6 “wide and 24” deep, four to six feet from the house. It is important not to build the drain too close to the house because doing so will bring water against the foundation, which is exactly what you don’t want.
The main span of the trench system should be dug into the slope of the house. For a French U-shaped drain, it must be level and connected to two pipes on either side of the house with 90 degree PVC elbow joints. For an L-shaped drain, the main run should slope downward, at a step of at least 1/8 inch per foot of drop, to the second run that will run the length of the house, also connected by means of a 90 degree PVC elbow. attached.
When you design your drainage system, you want gravity to work for you. Like a river, groundwater flows downhill, so you will have to work with the natural slope of your property and, if possible, have the outlet pipe come out above the ground to give the groundwater a point. easy exit.
Once you’ve decided on the system design and done the heavy lifting of digging the trenches, it’s time to install the functional parts of the drainage system – the gravel and pipes. First, tamp down any loose soil at the bottom of the trench and cover it with 1 to 2 inches of gravel, place the PVC pipes on top of this first layer of gravel, with the holes pointing down, then fill in the raincoat with more gravel, one inch below ground level. Then all you have to do is cover the trench with grass or another decorative touch of your choice. And you are ready. The next time it rains heavily, the excess groundwater will enter your newly installed French drain and be diverted around your home and discharged to the end of the outlet pipe (s).
It is commonly recommended that a French drain be lined with geotech fabric and that the pipe be wrapped in a geotech sock to prevent it from becoming clogged with silt. I do not recommend doing either. If you were to use geotechnical fabric anywhere, the place to put it would be at the top of the trench to prevent silt and sediment from seeping in from above and filling the air spaces between the gravel. Most of the water that enters a French drain is groundwater that flows sideways, not downward from the surface. Groundwater is not silty, silt and sediment have already leaked out as it trickled through the topsoil. If in doubt, ask yourself if underground spring water and well water are clear or muddy. Both, of course, are usually crystalline because soil is a natural water purifier.