French drain line, drainage: advice, tips
Copyright 2004 - 2009 by Mario D. Vaden. Header context: Largest and Tallest Coast Redwoods
These are tips and advice for French drains and drainage near western Oregon and Washington. Applicable in many other areas too. The most important part of drainage, is not knowing how to install it, but understanding when something like a French drain is the solution. That's why some soil knowledge and experience is essential.
Below, is DIY drainage and French drain advice that may help you avoid installing drainage that you don't need. Certainly you want to get rid of your problems, but sometimes a french drain is not the solution. Drainage advice here is based on experience at several country clubs, university campuses and many residential properties. Don't miss the soil page link - it's very relevant.
Heavy rain can trigger material shortages in a matter of hours. Postponing drain line projects until the rainy season can leave people with plenty of water and no pipe to move it. Don't wait until heavy rain storms to install your drainage project. This is especially true for cities like Portland in Oregon. And using Medford, Oregon, for an example: during winter 2005 - 2006, heavy rain depleted many drain line supplies in that city. Medford's normal yearly rain average is 19" - half that of Portland. Don't postpone drainage.
Image at right: Preparing to route non-perforated pipe segment under lawn for discharge line. Discharge trenches need no sand or gravel. In this yard, water was collected elsewhere in low spots using catch basins, open trenches and perforated pipe. But the discharge under a raised area of lawn required no open trench. With careful diggging, sod can be replaced.
Typically, French drains will include pipe near the bottom of a trench, with gravel or sand on top. If sand is used, a fabric cover for the pipe will be essential. The pipe is normally perforated to collect or release water, or solid to transfer water. Drain lines can be built with a combination of perforated drain pipe and solid drain pipe.
Use perforated pipe where you plan to collect water, and use solid pipe where you want water to flow away without being released yet. Sections of solid pipe can be preferred where trenches for discharge water pass near tree roots. If perforated pipe is located near trees or big shrubs, roots can grow into the gravel and drain line through the perforations.
Success depends on a variety of solutions and materials. Several of these images are from one back yard drainage project where several techniques were used. Perforated pipe was used to collect water. Pea gravel was used to fill most of the trench, but some areas near the surface were back-filled with sand to spare damage to lawn maintenance equipment like core airification units.
Where sand was added over the drain line or pea gravel, fabric was added to keep sand from washing into the drain line and plugging it.
Drainage can be improved in some areas, by using a small auger to bore holes past compacted layers of surface soil - shown in the image to the left. The holes are filled with sand for filling. In other areas, miniature trenches were dug, filled with sand and routed over to the main drain line.
Sometimes, site conditions, or the homeowner's budget, don't allow for stipping sod, completely improving soil conditions or transferring the water horizontally to another area. In situations like this, boring with an auger may help significantly, providing holes which are basically tiny circular drywells - small or large, depending on the auger bit size.
The same basic concept can be done by digging a big submerged cavity to be filled with gravel - commonly called a drywell. This option depends on whether or not the soil beneath the surface will allow water to naturally drain-away or seep out into the sublayers of soil.
The boring method shown in the image with the small auger is sometimes used near trees and has been called "vertical mulching" since holes may be filled with amendments like compost. For trees, the goal is often aeration, but occassionally drainage or penetration of compacted soil layers.
One method that has proved useful in established lawns, is what I call "micro trenching" and is shown in an image to the right. A sharpened shovel with a straight edge, can be used to dig tiny trenches which are only 2" to 3" deep, and maybe 1" to 2" wide. The miniature trenches are filled with sand, and these trenches can extend to another drain line, like a plastic perforated pipe in a main trench. In this case, I use a fabric over the drainage pipe, well-fitted to prevent sand from washing into the drain line from the micro trenches.
That system allows water to drain down and into sand, where it seeps away. In one respect, this is similar to how round plugs of soil are removed from golf greens, and the holes filled with sand, enabling water and air to pass through the upper sod, thatch and soil layer. With my method of micro trenching for drainage, the goal is not aeration as much as it is moving some water in selected areas without digging big trenches
Many readers can benefit from reading our soil care page and soil compaction page, to under basics about soil care and compaction.
Knowledge of soil and compaction, may save needlessly installing a drain line - here is a story for an example...
Years ago, a friend asked about installing drainage at his home near Beaverton, Oregon. He recieved advice from 2 professional drain installers, to install a drainage system, saying that he had a "high water table".
Within 5 minutes of arriving at and checking my friend's home property, I told him that his problem - a mushy lawn with standing puddles after rain - had to be surface soil compaction; not a high water table. I pointed-out the old Oregon oak trees and ponderosa pine trees on his mid-size city lot. Since those trees would not flourish for one or two centuries in a to-the-surface high water table, mentioning their presence convinced him to correct poor soil conditions instead of wasting time and money on drain lines.
To fix the problem, we removed the old sod, ammended the soil with a little lime and an inch or two of compost, and rototilled the soil to a depth of 4". Then the area was seeded with a sun & shade mixure of grass seed. In a few weeks, the soil surface was firm, water did not puddle after rainfall, and the new lawn was much healthier. That's because he did not need drain lines.
That was one of many yards where I've seen this. And it was clear as day that the 2 separate "experts" providing estimates did not have a clue. Can you imagine how much money they have drained out of people with no need ?
Functional dry creekbeds: another technique for drainage, is putting a drain line underneath a dry creekbed - the entire dry creekbed. You can put several sections of perforated pipe next to each other - parallel - and even layered on top of one another. This turns a dry creekbed into a holding cistern to control runoff or allow rainw to seep into the property. See diagrams below.
A French drain is basically a drain line and ditch filled up to the ground surface with rock or gravel. This kind of drainage system may have perforated pipe under the gravel, or may lack drain pipe or tile and rely on water flowing through a trench of gravel alone. But there is a benefit in using a perforated pipe underneath.
If a drain line is completely covered with soil, it is not a French drain. A French drain trench is exposed (visible), enabling surface water to run into the trench. It may be concealed partially, such as when grass is allowed to grow into its surface gravel - rarely cloggin it, as long as thick thatch does not accumulate. Then the water flows through the trench and away to an opening or porous release area (maybe even a drywell). Sometimes, the French drain trench may be the temporary storage cistern.
In my book, I consider a sand filled trench to be a French drain too, but fabric should cover the pipe beneath if drain pipe is included..
Several suggestions on this page - below - can extend the life of French drains and drain lines. Each idea will require extra time or extra money. But these ideas and methods are inexpensive for the average residential drainage project.
Some French drains fail due to clogging. So, some of this information is geared for preventing clogging. In fact, if your trench will be deep (1.5 feet to 4 feet, or deeper), you should consider lining the wall of the trench with a fabric to prevent soil from dislodging into the rock fill and clogging the upper trench.
1. Many French drains can fail because the soil on the surface presses inward and seals the top of the trench. If the surrounding soil is clay, even a ¼” thick layer of soil would seal the French drain and prevent it from being functional if the soil erodes or moves on top of the trench. A way to avoid this is to slope the sides of the trench so that the top of the trench is wider than the bottom of the trench. Basically, this provides a “V” shape trench opening. When this is done, pressure on the top of the trench pushes energy down and outward on the sidewalls of the trench. If the trench walls are not sloped – thus vertical – then pressure at the surface can collapse the trench walls and press sidewall soil onto and into the gravel; squeezing the trench narrower. So widen the trench top a little bit, and slope the sidewalls.
2. Consider laying a landscape fabric in the trench before installing the drain line and gravel (or sand). This landscape fabric layer can provide a layer that also hinders trench wall soil from pressing inward into the gravel and drain line. Be sure the fabric is porous. This is not an essential technique. It is beneficial, but if you wanted to "cut-one-corner", this may be the corner to cut.
3. Dig the trench a little deeper, and lay one, two or a few inches of gravel on the bottom underneath the drain pipe. No matter how well a drain line is built – a French drain – it probably will get clogged in the future whether 10 years down the road or 50 years from now. Every rain storm will move some silt into the drain line. If the drain pipe is laid on the bottom of the trench, it will start to get plugged with sediment from the first time it functions. But if a few inches of gravel are laid under the drain tile, this gravel base will be what starts to accumulate the initial sediments. This can extend the life of the French drain by years or decades. The deeper the gravel base under the drain line pipe, the longer the drain line can last. The drain line hardly collects sediment at all as long as there is a porous cavity below in the gravel base.
By the way, a smooth stiff drainline usually traps less sediment that the corrugated (ringed looking) flexible drain pipe. But the difference is minimal. The ribs of the corrugated line really don't hold that much. Also, the flexible pipe frequently has better spaced perforations. But if you don't use the stiff pipe, be sure the trench bottom is very even for the flexible pipe.
4. The finer the gravel used to fill the trench, the better for preventing collapse of the trench walls. If large rock is used to fill a French drain – like 2” to 3” diameter river rock – it’s easier for soil to move inward into the cavities between the rocks from the side walls of the French drain trench. Consider using pea gravel or fine gravel like ¼ - 10 crushed gravel. Sand can be used to fill the trench, but a fabric mesh must be used to keep sand from washing through the tiny drain tile openings and clogging the drain line. In many cases, I found that landscape cloth fabrics had smaller openings than drain line fabric. Be sure the fabric will not allow sand to wash through the fabric pores. It may be worth installing a double layer of fabric if sand will be used. But in general, fine gravel can be a better choice than sand. Usually, only a country club superintendent will need to use sand for a French drain when draining water from the sand bunkers.
I'd like to add that sand can add benefits in small areas. For some drains, I've cut "V" shapes slits a few inches deep in areas of water puddles. These slit trenches run from the trench to the perimeter of the puddle. I fill these slits with sand only. Where the slits meet the main trench; the trench is topped with sand, with fabric well-placed to prevent the sand from washing into the French drain. I do this often in lawns, and let the lawn grass spread into the slits. The sandy slits still allow water to move and grass to grow. These are what I refer to as "micro-drainage" or micro-managing a drain line.
5. Use washed gravel. Don’t get crushed gravel like ¼ minus that has a lot of dirt. Get gravel or rock like pea gravel or ¼ - 10 crushed rock that has been washed of most soil. These may have a little bit of a soil film, but in general, are relatively clean. You don’t need a bunch of dirt washing from trench gravel and filling your drain cavity with sediment. Don’t skimp on gravel cost. Get clean rock.
6. Lime has a tendency to chemically aggregate soil – that’s good. It can help open up the soil structure and make it more permeable. Lime is inexpensive. If lime is added, it can be sprinkled on top of the finished trench and gravel and allowed to leach down and chemically aggregate the bottom of the trench.
7. If the trench will go by tree and shrub roots, consider inserting segments of solid drain line in those areas (meaning pipe with no holes in it's sides). Tree roots rarely penetrate drain line that is not perforated. If perforated drain tile is near roots, those roots penetrate small openings and grow inside the drain line. The invading root mass can become so large that it becomes compressed looking – shaped with rib impressions from the sides of the drain line pipe. The root mass will block water and render the drainage system ineffective.
8. Root barriers are available. If you need a drain to draw water from near trees and can’t insert a segment of solid pipe, consider placing a layer of root barrier against the trench wall between the tree roots and the drain line. Root barriers can be available from bamboo supply businesses, some nurseries and garden centers. Another source is tree or arborist supply stores.
Adding root barrier may be good if only to keep the roots from expanding and crushing the drain pipe someday. Any, or all these ideas combined can extend the life of your French drain. The right procedures and proper planning will prevent or significantly postpone this kind of clogging.
If you slope the trench walls, pressure from above can be diverted downward and sideways. If trench walls are straight up and down, pressure from people walking or machines can squeeze soil into the trench. The slant helps to prevent soil from pushing into the drain line area.