I live in a rural area that has a septic tank with a clay overflow pipe that daylight outlets out the side of a gently sloping "ridge" away from the house. A hand dug trench carries the water from that point to a level area. I would like to install a french drain where the existing trench is. I know the basic design criteria and I have a nice gentle slope from the existing outlet.
My questions are:
Is a french drain what I need for this situation?
What size pipe and trench?
What kind of pipe?
Should I daylight outlet the french drain or can it filter underground?
Should I backfill to the elevation of the bottom of the existing clay pipe outlet?
Should I leave a shallow trench for the water to collect and then filter down through the pipe (the reason I ask is because if I fill to existing ground level, the "stuff" will run straight down the small ridge and not have time to filter into the drain)?
Sorry if this is all very convoluted, maybe someone can help.
Thanks to anyone that can help.

Whoa! Back the truck up! Are you saying t6hat in place of a drainfield you're dispersing the liquid effluent from the septic tank out on the open ground? Before we go any further, where do you live? If that's so what you need isb't a french drain but a drainfield. Can you tell me why one wasn't installed along with the septic tank? Inquiring minds want to know. Cheers, Tom

Yeah, I figured that would be the first point someone would make. The house is 60+ yrs old on a farmstead in SE Missouri. It was my grandma and grandpa's. I don't know if septic systems were around back then or if it was put in later but that's just how it was done. You'll find that enforcement in SE MO is lacking... there isn't a county electrical code either. I'm sure if I piped the effluent into a drainage ditch DNR would be all over that but nothing has ever been said by EPA or anything, and it's highly visible from the road. I want to fix the problem correctly but funds are limited. I figured if I couldn't install a leachfield the french drain would at least be better for my purposes as well as the environment that what is currently here.

You may install a french drain or install a mini drainfield using
Perforated drain pipe as described at the end of this post.
The French drain is a time-honored system for eliminating excess water from low points and other areas prone to saturated soil. French drains are quite simple: in essence they are just trenches filled with gravel, with sand on top of that. Often, you'll see French drains defined to include a drain pipe as well, though the traditional design is simply the gravel-filled trench. The advantages of French drains are low cost and easy installation. In addition, they can be covered over with turf after installation, making them less conspicuous.

Dig the trench

A French drain starts with digging a trench. The depth and width of the trench can vary, but 5 to 6 inches wide and 8 to 12 inches deep are common sizes and usually satisfy most needs.

Ensure proper slope


Grading is a critical consideration — you must ensure that enough slope exists for the water to actually flow, and flow in the right direction. It might be adequate to check very short stretches of drain with a level to ensure that a slope exists to carry water in the desired direction. However, you should take whatever measures are necessary, including a survey and grading, if needed, to ensure that you have at least a 0.5 percent slope. A 1 or 2 percent grade is better.

Fill with gravel

Add gravel to the trench to within a few inches of the surface. Gravel for this use is typically 0.5 to 1 inch in size.

Top off with sand

On top of the gravel, lay at least 3 or 4 inches of coarse sand. This provides a medium in which turf can grow so that the trench will not be visible. But remember that the sand must be coarse or it won't allow water to properly drain through.

Turf the trench

Turf may be seeded into the sand or simply allowed to grow in from the adjacent stand, if the turf is a spreading type. Or, you can lay sod over the sand. However, if you do this, be sure to wash the soil from the sod roots before laying it so that you don't contaminate the sand with finer soil.


Adding a drain pipe
If you wish to use drain pipe, the process is similar to that just described, except you will first lay an inch or two of gravel in the bottom of the trench, on which a perforated drain pipe will rest. The drain pipe should be wrapped in a filter fabric. Then, finish filling in with gravel around the pipe, ensuring at least 1 inch of gravel surrounds the pipe on all sides. Four- or 6-inch drain pipes are commonly used, and are easily accommodated by 6- to 8-inch trenches.
Be sure to run the upper end of the drain pipe to the surface and cap it so that you can, if necessary, access the pipe to unclog it.
Good luck, Tom