French Drains: How Do They Work?
French drains, though their name indicates differently, were developed in the United States. They primarily operate by supplying invasive groundwater with a path of least resistance by which it can be redirected away from a basement or low-lying section of lawn. They are named after a new Hampshire man, Henry Flagg French, who, in 1860, published a volume with the inviting title: "Farm Drainage - The Principles, Processes, and Effects of Draining Land with Stones, Wood, Plows, and Open Ditches, and Especially with Tiles."
These days, French drains are normally used to fight flooding issues caused by surface and/or groundwater that a home owner may be having, particularly influencing their lawn, foundation or basement. They are additionally sometimes employed to drain off liquid effluent from septic tanks.
The elementary design, a gravel-filled trench, is basic but for it to continue operating indefinitely, it's crucial that it be effectively executed.
Flooding difficulties are normally linked with sloping ground, non-porous clayey soil, or a mixture of the two. For instance, if your home is built on a slope with your neighbors' residence occupying a lot further up the slope, heavy rainfall can precipitate a build up of groundwater speeding down from their property and onto your own. If your soil is not able to absorb all that water, you just might suffer damage to your residence's foundation, or leakage into a crawlspace or basement beneath the 1st floor of the home.
A linear French drain is a straightforward, cost-effective answer to such a problem. In this scenario, it functions as a moat that protects your residence by intercepting the groundwater rushing down the slope and redirecting it around and away from your house's foundation.
A linear French drain is a doable D.I.Y. job, if you are up to doing some backbreaking work (this does include digging a trench, which is after all a lot like a ditch) and you have the right tools and materials (1" round washed gravel, 4" PVC pipes with drainage holes, a trenching spade or power trencher and a builder's level)
Right, let's get down to brass tacks. It's time for a detailed explanation of how to construct a French drain, and how they work. First of all, you'll have to dig an L-shaped or U-shaped trench system, 6 inches wide and 24 inches deep, 4 to 6 ft from the home. It's crucial not to build the drain too near the house simply because, if you do, you'll be bringing groundwater up against the foundation, which is exactly what you don't want to do.
The principal leg of the trench system should be dug uphill from the residence. For a U-shaped French drain, it should be level and linked to two pipes on both sides of the home with 90 degree PVC elbow joints. For an L-shaped drain, the main leg has to slope down, at a minimum pitch of 1/8 inch per foot of fall, to the second leg which will run next to the property, also connected by means of a 90 degree PVC elbow joint.
When you are laying out your French drain, you want to make gravity work in your favor. Just as is the case with an above-ground river, groundwater flows downhill, so you'll have to work with the natural slope of your property and, if you can, have the exit PVC pipe come out above ground to give the groundwater an simple exit point.
As soon as you've worked out the layout of the system and accomplished the major work of digging the trenches, it's time to assemble the working parts of the drainage system: the gravel and pipes. First, tamp down any free soil in the bottom of the trench and line it with 1 to 2 inches of gravel, lay the PVC pipes on top of this first layer of gravel, with the holes pointing straight down, and then fill in the trench with more gravel, to one inch under ground level. Then all that's left to do is disguise the trench with sod or another ornamental touch of your own choosing. And you're done. The next time you experience a downpour, any excess ground water will enter your recently installed French drain and be diverted around your house and discharged at the end of the exit conduit.
You often hear that a French drain should be lined with geotech fabric and the piping be wrapped in a geotech sock to stop it from becoming clogged with silt. I don't advise doing either. If you were going to use geotech fabric anywhere, the place to put it would be on top of the trench to decrease the chance of silt and sediment from filtering down from above and filling in the air spaces in between the gravel. Most of the water that enters a French drain is groundwater flowing sideways underground, not downwards from the surface. Groundwater is not silty, it has already had the silt and sediment filtered out of it as it trickled straight down through the topsoil. If that seems doubtful to you, just ask your self if underground spring water and well water are clear or muddy. Each of them are of course generally crystal clear due to the fact that earth is a natural water purifier.
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