French drain or drainage ditch? What is a good drainage trench surface

[kevone]

Our basement flooded multiple times during heavy rains the first year after we bought our house in western Davidson County (Nashville) Tennessee.

The house is roughly T shaped, the basement is under the horizontal cross of the T which is on the down-slope end. Water was ponding behind base of the T and in severe weather on one side of the cross.

In desperation while in the middle of a 2” soaker, my wife and I dug a shallow channel and were able get the ponding water to flow away from the house. So far, it has done the job –we have not had any more flooding, even after very heavy rain.

I would like a more permanent solution and based on the advice on most home improvement sites I was considering a French drain; however, after reading some threads on the subject in this forum I realized that a French drain might not be the best solution in my case. The soil here VERY clayey, I did a jar test on the soil and over 80% of it appears to be clay or fine silt –I cannot distinguish individual particles and it cracks on the surface when it dries. All the hills in the area are limestone. From what I have read in this forum, rainwater is most likely not permeating into the subsoil, but running over the top.

The property is at the base of a hill on a moderate slope. Behind the house, the slope is roughly 6’ per 100’, less in the front. There is a 3-4’ ridge about 15’ behind the house, beyond the ridge is a swale sloped to divert water from the house. The ponding occurs between the ridge and the house. My plan now is to dig a 6-8” deep V shaped trench from the problem site to lower ground on each side of the house.

Does this sound like a good assessment of the problem? Also, what would be an effective treatment for the surface of the trench. It is going to be over 100’ long and I would prefer not to use cement.

Thanks for any advise you can give me.

 

[msucog]

you could always try to hide the drainage trench as a landscaped dry creek bed. and i'd use perforated drain pipe (maybe not with the filter cloth because of the clays) in the bottom of the ditch to get the water away more efficiently. i think the pipe is only about $20 for 100 ft. i'm guessing the trench would need to be 1' deep min. to use 4" pipe so that there's enough gravel above it to hold it down. i guess you could always use something like a double piece of 6 mil poly sheeting to line the bottom of the ditch and then put gravel...would likely be more expensive for the poly but then you could make the ditch more shallow which would save on gravel. if there's one particular area that the water ponds, install a small area drain connected to the non-perforated pipe to carry the water away. then the pipe would be hid below the ground. i think those drains are about $20 plus the piping, so that's pretty cheap to solve your problem. you could even install multiple area drains and probably still be cheaper than buying enough gravel to put in the ditch. heck, if you have gutters and downspouts, you could connect those in to the system while you're at it.

 

[civilperson]

The method that succeeded, grading the flow away from your house is the solution to follow up on. Make sure that you have 3"-5" of fall away from the house in the first four feet all around the building. Extend downspouts at least six feet from the house. Keep shrubs and trees away from the base of your house since you will be watering them. Drain low spots that form ponds after heavy rains. Regrade the diversion swale between the ridge and the back of the house to maintain 2% slope.

 

[concretemasonry]

The excavation for your home was the same as for a swimming pool, except one has a house in it.

The footprint of your home is much smaller than the size of the construction excavation. There also may be trenches dug for construction access and utilities. Since you have clay soil, any disturbed soil will not be as "tight" as the virgin clay and will collect water unless it is shed from the suface quickly.

In many cases, very long dowspouts (10') are required to get beyond the excavation limits. If you use buried downspouts with pop-ups, make sure your trench has positive drainage or you will just be collecting water and carrying it back to the foundation.

If you put in a shallow surface system, if it is not far enough from the foundation, surface water in the fill may drain under the system and collect areound the foundation.

Do you know if the perimeter drain tile is functioning?

 

[kevone]

Thanks all for your replies.

Below is a rough cross-section of the site

First –sticking to KISS principles, I think I will dig the trench; line it with plastic sheeting and fill with rough gravel --making sure I slope it down as much as possible to lower ground.

I was also thinking of stabilizing the soil at the ponding site (please forgive me if my terms are not correct.) “oldestguy” suggested bentonite in another post. Given that I have clay soil, I wonder if lifting the sod and mixing in hydrated lime would be better in my situation. Possibly even using a earth/lime mix to line my trench. Does that sound practical?

I would only do this if there is still sogginess after the trench is complete.

[tt]
----------------|
|
|
house | /-------
| ---/----
| /-----\/---/
-------|--------|\------\/----/
base- | /|\
ment | | slight ridge swale hill slope
-------- | swale
| (trench site)
|
ponding
here
[/tt]

 

[msucog]

wow...how long did it take you to draw that up?

i'd stick with "simple"...get the water away as far from the house as you can...everything else should take care of itself. odds are that you'll end up doing that anyway if you try something more complicated. you might end up creating a scenario where grass doesn't want to grow or killing the sod. sometimes simple is better than swanky engineers' solutions...

 

[MissouriPE]

I answered so many of these types of questions as a public sector stormwater engineer and the answer for all localized drainage issues is most simply stated as "If you don't like where the water is going (or not going), give it somewhere else to go - it always flows downhill." You are already on the right track with what you've done. In general, if you want to catch sheet flow during a rain event, then a swale (shallow trench) is your best bet. If you want to take care of standing water that is there 12-24 hours after the rain stops, a french drain will work better. So quite possibly, the idea of some kind of combination is best depending on the specifics of the situation.

However, depending on the drainage area, what you have described is probably not quite big enough for a downpour event (we call them toad stranglers here). You might want it to be at least 12" total depth at the high point and more as it breaks away. Also, beware of the "V" shape for a swale - rounder, more parabolic will tend to erode less which is an issue in our midwestern silty clays.

For the french drain, perforated PVC is better than the HDPE flexible drain tile with ridges that they usually sell at Lowes and Home Depot. That flexible stuff doesn't always hold grade and critters sometimes like to build their nests up in there. Clean rock to the top or to within 4"-6" of the top (cap with topsoil with filter fabric between the soil and the rock will also work).

Wish I was as creative at drawing on here as you, but I'd probably come from the outside of the house, sloping down 6"-12", then an additional 6"-12" deep swale (as wide and parabolic as possible) with the bottom of the swale being a french drain (12" wide, 12" deep with 4" perf pvc at the bottom filled all the way to the top with clean rock and then line the swale with river rock to make it look like a landscape feature or cap the whole thing with topsoil and sow grass. I'd skip the plastic personally. Oh, and definitely deal with getting your downspout water redirected - definitely the most common self-inflicted wound when it comes to drainage problems.

HTH and feel free to ask more questions for clarifying. Hard to describe without being able to wave my hands at it LOL