French Drains 5

[beryl10]

Hi,

I have a couple questions about french drains. Our property slopes from the street down towards the house and the house sits quite low in the gound, so, not surprisingly, water has always been an issue. We're putting in some french drains in the front of the house to intercept the water and move it off to drain pipes running out the back to a stream. These are being placed about 4 feet away from the foundation. I'm happy with this placement. I think it makes sense.

However, in another area, around the corner on this L-shaped ranch, the contractor is suggesting putting the drains right up next to the foundation of the house. The house has a basement, and I am not talking about drains at the floor of the basement, but at the level of the ground outside. I am wondering if this is a good idea. Isn't the french drain actually drawing water towards it, therefore pulling more water towards the house foundation if there is a 2' deep x 2' wide trench filled w/ gravel right up to it? Maybe all is fine if the drain works properly, but from what I understand, french drains are not the longest lived drain. They do clog up over time.

Next question, he also said that putting geotextile fabric on top of the drain slows down the percolation of water from the top. So, he just cut the fabric right off to the surface, which I don't think was very smart. I'd like to landscape right up to the edge of this trench (make the trench look like a gravel path with stepping stones), and now there's nothing keeping the soil from spilling right in. He just wants to dump river stone on top w/o the fabric barrier. Any thoughts on not completely wrapping these drains? Our soil tends to be on the clayey side, but this drain also abuts quite a bit of amended soil (more loamy and organic)

thanks in advance for comments, expertise, etc.

 

[LHA]

You are correct on both concerns.

Never put a french drain against a foundation with a basement. However, along the footings is OK, if you put some 3" dia perforated flexible HDPE, with a filter sock over it. This site will have several products:

http://www.ads-pipe.com/en/index.asp

Can't you grade away from the wall? Even the 4 feet you mentioned in the front is very close. If you can only get several inches of fall, over about 6-8 feet will help.

Always completely wrap the clean stone in the trench, and overlap a foot or more...for exactly the reason you've mentioned.

Engineering is the practice of the art of science - Steve

 

[cvg]

your contractor is partially correct - the fabric could slow down percolation, especially if specified incorrectly. It would need to be specified with sufficient hydraulic conductivity to pass the water. In addition, the fabric is designed to prevent the fine soil particles from contaminating the gravel. By doing that, eventually the fabric could get clogged with silt. Perhaps a better option is to provide a granualar sand and gravel mixture which will provide a natural filter to your local soil. This can be rejuvenated from time to time by scraping off the accumulated soil from the surface. If not, you may need to perform the same type of maintenance on your filter fabric.

Regarding the distance from the foundation, sloping the ground away from the foundation is recommended. A small swale should be graded over the top of the french drain to allow the water to collect and percolate properly. Recommend at least 2% slope or greater away from your foundation. With only 4', this provides less than 1 inch of depth in your swale. Would be better if this was 2 or 3 inches.

 

[oldestguy]

The message you are getting is that you need to filter any water going into a pipe, single size gravel or other drain system. If you don't use a filter, they will plug up and the whole thing will fail. I've seen gravel backfilled perforated "tile" plug up in one year.

For many years now (since the '30's Corps of Engineers study) it has been known that one of the best filter backfill materials to any drain is ASTM C-33 fine aggregate for concrete, known as "concrete sand". If your drain pipe has slots in it, you don'teven need to put a sock on it with this backfll.

I definitely would not set up the situation for surface water to enter the trench backfill, as with a "gravel path". Isn't there any way to divert this water away, even if you have to install an inlet or two and a separate "storm drain" system?

A clay layer on top of these drain (filtered) systems can be used to keep the surface water out.

However, if you are careful this system described below works well: You may even find a landscaper that has done it. I have never seen one that knew of this before I taught him.

You mix into the top 2 to 3 inches of soil (any kind) two pounds per square foot of powdered (not granulated) bentonite. It is known as "driller's mud", avaliable at plumbing supply houses. A roto-tiller works good for this. Don't use an excessive amount or this "water loving" material will swell and turn the place to grease. The principle here is that this amterial takes on some water and swells and fills the soil voids. A little does a lot of sealing. It is a natural volcanic clay.

To be effective, this procedure has to treat the whole area of house backfill, not just a few feet out. In most cases, you first strip the sod off all that backfill. Later roll the sod back and that lawn will stay quite green. Bushes can be left, but work closely around them.

If you wish, work the worst areas first and see how it works.

Of course you also do all that you can to shed off that surface water anyhow. Don't intentionally try to have surface water soak into some form of gravel drain. You will regret it.

In summary, protect the ground from water entering, but once it gets in, use a filtered drain system to remove it.

If you find you are getting water in the filtered drain system all year, that outlet should be under the water at the discharge area or it will freeze and then nothing works.


I've preached this "serman" maybe a hundred times and still find that gravel seems to be in the minds of people as the required backfill to drains. However, the first underdrain I had installed was in 1954 as a grad student studying them under highways and last time I visited there it still works. I have never found a failure of sub-drains that have been backfilled with concrete sand in all my working life.

If I was writing the specs for this job I would say "No Gravel Allowed on the Job". The idea seems good (good percolation), but it is not a filter and it is difficult to keep it protected. You can't goof up the job using concrete sand.

 

[beryl10]

Thanks for the replies.

We are fairly limited in how much we can grade away from the wall since there are some big trees about 10 - 12’ in front of the house, and getting too close to them would cause a lot of root damage. We had already created a bit of a swale about 5 feet in front of the house, and picked that for the location where the pipe was installed. The ditch was dug out 2 feet wide, so the trench begins 4’ from the wall.

I dug out the gravel enough to put in new pieces of filter fabric along the sides so I can wrap the top. In looking at the dimensions of this trench, I was wondering how this really works. It seems to me the trench would have to fill up with quite a lot of water before it could even enter the pipe. To be exact, it would have to fill with at least 127 gallons of water before the level is at the pipe (25’ long x 2’ wide x 4” of gravel between the bottom of the pipe and the bottom of trench). So, isn't a french drain just creating a big basin for water to collect, only 4 feet from the basement wall foundation, and that the water would probably seep into the soil faster than it would ever build up to find its way into the pipe, except perhaps during a very big rain. But, then mightn’t a surface drain be more effective? The direct area of lawn between the house and the street is about 1300 sq. ft., which could, during a 100yr rainfall, produce 14 gal./min. runoff (based on 3”/hour, and grass surface runoff coefficent of.35), so, yes, it would fill to the height of the pipe in that situation. I guess my question is this. What happens to all the water that doesn’t make it up into the pipe? Would it be better to sit the pipe closer to the bottom of the trench? Most diagrams I see of french drains do set the pipes on about 2-4" of gravel. Or, wouldn't a narrower trench be better. Again, I see alot of them are 2' wide, but if it were only 12" or even just 8", then the water level would reach the pipe much less water in there.

On paper, the idea of a French drain was great. But, as I look at it now, trench dug out, I do question whether or not this is the solution. My initial thought on the French drain in this location is that it would act as a curtain or interceptor drain. However, I was just reading on the NDS website that a French drain can provide this function only if the downhill side is lined with a polyethylene film.

I have a question to oldestguy: Why don’t I want surface water to enter the gravel trench? I thought with gravel brought up to the surface, french drains can support the function of removing both subsurface and surface water.

Also, does anyone have any thoughts on some of the prefabricated composite drain systems?

Thanks again to everyone.

 

[oldestguy]

My intent in my first comment was to get you to change the whole system to collect surface water, separately from ground water. Also, you may see that I recommended two things more: Seal the ground surface and install a subdrain.

However, let's look at what you have and see if that can be fixed.

The term "French drain" in my view is an old fashoned way to collect ground water and divert it from an area, such as an orchard or farm area in low ground. It is not a filter and in time can plug up. It is not a system for collecting surface water specifically.

What you have built is not such a ground water collector. It appears to be a collector of surface water and designed so that the water collected also can soak into the ground and then affect your basement. I suspect your "contractor" is some sort of impractical dreamer, certainly not using common sense.

My recommendation is that you get rid of the "contractor" and get someone on the job with common sense, if you need help, to correct things.

Since you have a trench filled with gravel, you might change this to a surface collector only and perhaps that would do the job. But I doubt it. If you want to stick with the trench OK, but it would not be as permanent as if you had a shallow "ditch" or swale, lined or made water proof, possibly with the bentonite treatment of earth. Any place you have water in contact with earth is a place for water to infiltrate. House backfill is usually loose and water easily enters.

Sticking with the current trench:

To make this a surface water collector, this needs to be a waterproof container. Lining it, sides and bottom, with plastic is a thought, but I have never seen plastic to be totally waterproof, unless you seal all the seams. Concrete is better, but not perfect either.

Sealing the lining to a drain pipe also is needed and that drain pipe should be at the lowest elevation in the trench, and sloped down from there. The pipe should be solid walls, not slotted.

If it was me, I'd bite the bullet and do this minimum step: But think about the affect of later doing the sub-drain as if affects this work.

Dig out the gravel and fill the trench with earth, preferably silty clay or a bentonite treated sand. Compact it if you can.

Waterproof the whole house backfill area at least on the uphill side with the bentonite treatment and slope everything to the filled trench area, which is then shaped like a swale and sloped to inlets. This is a surface water collector only.

These inlets can be purchased at plumbing houses and sealed to plastic pipe to carry off the water.

Lacking the inlets and drain pipe, continue the swale around the house and off from the house area.

If you really want to use "suspenders and belt", you first dig down alongside the upper foundation walls and install the slotted plastic sub-drain that is totally separate from the surface drain work. Use the concrete sand as the backfill up at least a few feet above the basement floor. The slotted pipe should be a low as possible, along side the footing if possible. Give it some slope if possible, but not mandatory. Use excavated soil for the remaining backfill.

Then do the surface waterproofing and swale as described above.

With what you now have I think you will see more water in the house than before. I am sorry I did not explain this before, and, I probably did not clearly explain that it is the wrong thing to do, in my opinion.

You should know that I have fixed many a site such as yours and that it is not always possible to stop all seepage. Sometimes water enters the house backfill far from where it then gets into the basement. Therefore you usually have to work in steps, get the most obvious done first and observe.

 

[beryl10]

Thanks for the response. You were clear the first time, but that drain was already in. And, the contractor already fired. Though, that was due to other sloppy work. If I were doing this project again, I would first hire an engineer to design the plan, and not just hire the guy who calls himself a drainage contractor. Live and learn.

After the whirlwind of activity around here, and several problems with the contractor, I starting thinking more and more about this design. Though I initially thought a french drain in front of the house to collect and remove surface and subsuface water flowing towards the house made perfect sense, I later starting seeing flaws with this as a solution. However, I've read about french drains being used in this manner - wrapping the house on the uphill side, a few feet away from it, and I initially thought it seemed logical.

Anyway, what I'm now thinking I could easily do is to just dig out the gravel, and replace the slotted pipe with a solid pipe and connect several catch basins along the length. This pipe would be 5' from the house (b/c thats where the slotted pipe is and the connection to the drainpipe going back to the stream), and the catch basins would sit in a bit of a swale.

Any comments on this idea are welcome.

thanks.

 

[oldestguy]

OK a good explanation.

Well, as I uderstand it, the sides and bottom of this filled trench still are in contact with the soil and not sealed, as with a plastic sheet. Thus, before the water is in the pipe and flowing away, it has the chance to infiltrate soil backfill to house and affect the basement. Usually backfll was shoved in with sloping "layers", that promote the ability of the water to seep toward the basement.

So, If you are going to replace the pipe anyhow, why not at least seal the bottom and sides of the tremch.

If gravel is to be used again and some inlets are to be installed, I am assuming you will have them in the base of the trench, right?

In that way you may not be able to correct possible plugging of the gravel with sllt in time.

I know it is more work, but why not forget about this trench thing and fill it with soil, placing your inlets where you can see them? You still can use the trench for the discharge line.

And if you wish to waterproof the ground surface with the bentonite treatment, it will tie in more easily with your surface water collection system.

The gravel thing is likely to be a maintenance head ache for years to come.

If you gave thought to use a bentonite treatment of the gravel for salvaging it, yet making it water tight, I think that will take some experimenting with varying precentage of bentonite, soak it and see what happens. This can be done in a 5 gallon bucket with bottom perorated. Too much bentonite and swelling may amaze you.

 

[geosavvy]

Bentonite is nasty stuff. Make sure you get your blend right before you spread it willy nilly.

Also, I personally would recommend against using any type of soil filter material. The gradation of the filter material has to be customized for the type of material you are filtering, else the particle arching required to filter will not happen. Its easier to go with the local tried and true geosynthetic fabric that works for the soils in your geologic area.

 

[beryl10]

Sorry, this is kinda long, but I'm just trying to understand a couple things.

It seems as though a lot of people on this forum aren’t so in favor of using a shallow French drain to collect, move and divert water from running towards a house where the property slopes towards it, yet, a lot of other people (landscapers, drainage parts stores, gardening forums) seem to think they’re great. I am no longer very convinced that they are a good solution to what is probably mostly a surface water issue, however, I’ve become obsessed with thinking about how water moves through the landscape, and soil, so, I have a couple questions.

1. If the coefficient of runoff for grass is .35, I assume that means that 35% of the rainfall flows over the surface and 65% percolates into the soil. So, where does that 65% of the water go once it percolates into the soil? Does it move horizontally below the surface until it finds its way to the bottom of the hill? Or is that majority of that 65% of the water getting used by the grass, trees and other vegetation before it has a chance to move down slope?

2. Why doesn’t the calculation for runoff take into consideration slope? I assume that slope must make a big difference, and on a steep grassed slope, more water is runoff than on a shallow slope where it has more time to percolate into the soil.

3. If a French drain (not lined with anything impermeable) daylights (to collect surface runoff in addition to subsurface water), how much of the total water actually makes its way into the slotted pipe versus percolating through the ground at the bottom? In a slow but steady rain, could the water actually penetrate the soil beneath the pipe faster than it builds up to a level high enough to enter the holes on the bottom of the pipe? I guess many factors affect this, including how saturated the soil is to begin with, the permeability of the soil, and even the dimensions of the French drain trench (width and depth below the pipe determines how much water must fill before it reaches the bottom of the pipe).

4. What system would collect more water? A series of surface drains or the French drain with gravel to the surface?

Here's my thinking on this:

If we can assume a surface drain collects 100% of surface runoff, which is 35% of total rainfall on this grass covered sloping surface, then 35% of the rainfall will be removed by the surface drains.

If the French drain trench captures 100% of the surface runoff (35%) + some fraction of subsurface water (the 65%), LESS the amount that percolates into the soil below the pipe, what total percent is entering the drainpipe?

Of course, I have no idea what percent of the subsurface water it will capture, and what the loss from percolation below will be. But, if the fraction of subsurface water that enters the trench is much larger than the total loss to percolation below the trench, then a French drain is the better system. But, if the percolation into the soil is high, and/or the amount of subsurface water entering very low, then surface drains are the better solution.

I don’t suppose there’s any rule of thumb for this, is there?

Am I over thinking this?

The trench is already there and I can finish it off as a french drain, or put in solid pipe with a few catch basins and inlets. I don't feel like experimenting with the bentonite or other methods of making the trench impermeable. If percolation is a big issue, esp. 4 ft. from the basement wall, I'd go with the solid pipe and inlets.

 

[cvg]

to answer some of your questions -

Percolating water generally moves vertically downward unless there is some driving force such as daylight, an embankment or impermeable layer that forces it to move horizontal. Once it leaves the french drain trench, it may not daylight.

The rate of water use by plants is slow and doesn't have a very large effect on the amount of storm runoff or soil percolation. The 65% is retained, soaks in, evaporates and some is transpirated by plants - but does not run off.

Runoff calculation does consider slope – it is a function of the time of concentration. For higher slopes or smoother surfaces, runoff velocity is higher and the time of concentration is smaller - consequently, the peak runoff is higher

Water will seek the path of least resistance – if a smooth pipe is there it will flow through the pipe much faster than it percolates into the ground. This is one reason to provide a pipe in a french drain (assuming the purpose of the french drain is for removing water rather than for allowing the water to soak into the ground)


What system would collect more water? Depends on the design, but surface drain is probably more efficient at removing stormwater, if the stormwater can be directed to the drain before it soaks into the ground…

 

[beryl10]

But, if the french drain pipe is on 4" of gravel above the bottom of the trench, doesn't the trench have to fill up with 4" (actually 5" since the holes are a little off the bottom) of water before it gets to the pipe? At the dimensions of the current trench (28" wide x 25 ft. long) it would fill in with 127+ gallons of water before reaching the pipe. A big storm would of course fill it at 14GPM which would reach the pipe quickly, but for the smaller rainfall amounts (lets say, 1GPM over the course of a day) it might never fill it enough to reach the pipe, right? then could percolation exceed the rate at which it fills?

i guess this is my fear. that we're putting more water into ground close to the foundation than w/o the trench.

 

[cvg]

some water will percolate down, hit the pipe and run into a hole without ever reaching the bottom of the trench. Also, you are vastly overestimating the amount of water necessary to fill your french drain trench. It is filled with gravel or sand which occupies most of the space. Water only fills the volume between the grains of sand. I would guess closer to 20 or 30 gallons of water maximum to fill the bottom 4 inches.

However, as recommended by oldestguy - to remove surface water, I would stick with a surface channel and grated inlets into your (non-perforated) pipe. If you are trying to lower water table next to the house, then use the french drain with the perforated pipe.

 

[beryl10]

ah, yes, I totally forgot about all the space taken up by the gravel.

thanks.

 

[beryl10]

How does one determine where the water table is? I assume to lower the water table, the french drain has to be very deep.

 

[beryl10]

I don't know if any of you are familiar with the "Ask the Builder" website. Here he explains water movement through soil and using the french drain to protect ones foundation.

"When it rains, water enters soil and pushes the air to the surface. Gravity then takes over. If your yard slopes, the water within the soil actually begins to flow downhill."

"A linear french drain is simply a "moat" that protects your yard or house from sub-surface or surface water. You construct it by digging a 6 inch wide trench approximately 24 inches deep. .... If your intent is to protect your house from water, you construct the trench approximately 4-6 feet away from the foundation. In many cases the trench system is U shaped as it passes around your house."

He extends the gravel to the surface to collect surface water.

http://www.askthebuilder.com/175_Drying_Soggy_Soil_-_A_Simple_Trench_Drain.shtml

All the explanations I've gotten through this forum make alot of sense, but then so does Ask-the-Builder, to some extent.

 

[cvg]

percolation is driven by gravity and as such, the only way it can "flow downhill" is if there is something blocking it from going straight down such as a layer of rock or clay or an easier path to follow such as through a pipe, through a crack etc.

"Bob the builder" has a degree in geology, but apparently according to his profile on the website, has earned his living flipping houses and never practiced engineering.

 

[oldestguy]

Hi again:

I think too much time and worry is being done about quantities and rates of water flow. Heck, storms are all different and that once in 10 year thing may be carried OK by the job built, but the once in 20 or more years won't. Intensity of each rainfall also is different.

So you really probably are not in a position to worry about which your system will take. You just build it as big as practical and take simple other precatuions so that any standing water won't run into window wells and other places of concern.

I take exception to a few statemsnts made above. The best all around filter for subdrains is concrete sand, for all soils. You don't need to worry about gradations of those materials either. Where there may theoretically be fine clays that theoretically will pass thru the voids of the concrete sand, well don't worry. The cohesion of that clay material keeps it in position pretty well. THAT CLAY IS UNLIKELUY TO SEEP ANY WATER ANYHOW. It is the sand seams that do the seepihg and they are held back by the concrete sand.

Another thing about concrete sand. It is darn difficult to foul up the the job. On too many jobs, asking for complicated procedures is asking too much of the usual contractor doing small jobs. Then too you get the guy that has been using questionable practices (such as using gravel around sub-drains), of recommending these "french drains" and he "knows better" and keeps doing it the same old dumb way.

Also, water seeping into basement backfill will follow the the path of least resistance and it usually is not straight down. It usually is slanted towards the wall, due to the usual way this backfilling is done. Thus water soaking in 4 feet from the basement wall will flow towards the wall on that slope.

Take to heart my philosophy about construction:

IF SOMETHING CAN GO WRONG ON CONSTRUCTION, IT WILL GO WRONG.

 

[beryl10]

Yes, I probably have been over calculating, but was just trying to quantitatively understand how well a French drain removes water, and where the water is coming from. When we talk about subsurface water, does one mean 1 foot down, or 5 or 10? With a ditch only 1.5 feet deep, how much subsurface water would a French drain even intercept? These questions are what led me down the quantitative road wondering, would this French drain put more surface water into the ground than the amount of subsurface water it removes?

So, anyway, I am going to install a solid pipe w/ 5 inlets (catch basins) along the 25’ length, and backfill with soil. Forget about this whole gravel pit. I hope this will prove to be the right decision, as moving tons of gravel is no small chore, and now I’m stuck with a pile of gravel (guess I can use it for a foundation for that garden shed I’ve been wanting to put up).

Why do you think everyone backfills with gravel, when concrete sand is superior?

Does the backfill against a basement foundation really go out 4 feet away from it?

Even though I’ve abandoned the French drain, I was wondering whether clogging really such a big an issue with the stiff HDPE pipe that has 2 rows of fairly large holes along the bottom compared with the corrugated pipe with slit type holes all the way around? Do roots tend to go into these holes? I was amazed how many fine roots have already grown through the geotextile fabric, though wondered if they would continue on through the gravel to find their way into those holes. It seems it would be difficult to plug them up with silts.

 

[blueoak]

Sorry oldestguy, but I disagree with your filter statement. I think for residential and other little jobs spec'ing concrete sand is fine and your advice for this job is excellent.

But if you need a filter on an important structure you need to do the work on designing a filter. I home isn't that big a deal to the neighbors, but a dike or dam with fines migrating downstream is a little more important. Concrete sand isn't always applicable and what about filter design below riprap or gabions.