Drainage - Earthwork_grading Engineering - French Drains

Earthwork/grading engineering - French Drains http://www.eng-tips.com/viewthread.cfm?qid=162089&page=4

1 of 16 24/03/2007 9:10 AM

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beryl10 (Chemical) 8

Aug

06

22:52

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 (Civil/Environme) 9

Aug

06

8:10

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 (Civil/Environme) 9

Aug

06

12:04


2 of 16 24/03/2007 9:10 AM

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 (Geotechnical) 9

Aug

06

21:54

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


3 of 16 24/03/2007 9:10 AM

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.


Aug

06

11:36

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.


Aug

06

12:58

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.


4 of 16 24/03/2007 9:10 AM

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.


Aug

06

16:58


5 of 16 24/03/2007 9:10 AM

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.


Aug

06

12:31

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 (Geotechnical) 22

Aug

06

16:49

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


6 of 16 24/03/2007 9:10 AM

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.


Aug

06

12:05

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


7 of 16 24/03/2007 9:10 AM

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.


Aug

06

12:41

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…


Aug

06

13:01

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.


Aug

06

13:20

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.


8 of 16 24/03/2007 9:10 AM

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.


Aug

06

13:22

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

thanks.


Aug

06

16:27

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


Aug

06

16:38

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.


Aug

06

17:49


9 of 16 24/03/2007 9:10 AM

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.


Aug

06

21:22

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.


Aug

06

23:21

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


10 of 16 24/03/2007 9:10 AM

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 (Civil/Environme) 24

Aug

06

23:59

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.


Aug

06

12:17

I'm rereading this whole thread and see that I asked before why one wouldn't want surface runoff going into a french drain, but I

realize I still don't quite understand why. Is the simple answer that not enough of it gets transported away in the slotted pipe? I

understand not feeding a gutter leader directly into the slotted pipe, but why not the runoff from the surrounding lawn into the gravel

trench with the slotted pipe?

I'm still questioning this b/c this will be alot of work to change, and wonder if its all that bad to leave it as a french drain.


Aug

06

13:06

french drains are ideal for intercepting and draining subsurface water. However, surface drainage is most efficiently removed using

surface drainage methods such as a swale or grated inlets into a pipe. Intercepting surface drainage with a french drain will increase

the amount of water that infiltrates into the ground at that location. You never said that you had water coming into the basement

through the foundation walls, however, by putting a french drain near your foundation and allowing it to also intercept surface water,

you could possibly create another problem rather than solve one.


11 of 16 24/03/2007 9:10 AM


Aug

06

13:25

thank you. That was the direct, to-the-point kind of reply I needed to hear. Really, it was my question that needed to be more direct.

With all my convoluted calculating I didn't ask the direct question.

Anyway, no, there is no water coming into the basement through walls. That was taken care of ages ago with, I guess, perimeter

drains (french drains?) in the basement leading to 2 sump pumps. That was before my time. As long as the pumps work, all is well,

although damp.

BigH (Geotechnical) 26

Aug

06

5:42

blueoak - see Terzaghi Peck and Mesri. Oldest guy is correct that for any fine grained soil, the use of concrete sand is "fine". This

was first told to me by Charles Ripley of the old Ripley Klohn Leonoff of Vancouver - and one of the pricipal sources for that given

in TP&M. For coarser soils, you will design the filter - but remember that a lot of work has been done over the years with respect to

the original equations and filter criteria. I suggest too that interested members read the few pages in Conduto's book on Soil

Mechanics and they will see a good summary of "filter" criteria for finer grained soils. This is also give in one of the US military

manuals.

With respect to cvg, if the soil in which the french drain is placed is clayey or low permeability soil and the material in the french

drain is sufficiently permeable, I doubt any of the water will enter a pipe anyway and he suggests but apparently - cvg must be

assuming that the holes are pointed up when he indicated that 'some' water would enter the holes - although it is more conventional

and in line with AASTHO recommendations to place the holes downward. A pipe, in my view, is really only necessary if you have a

large volume flow of water - or, since the pipe is not very expensive you put it in for redundancy. Very few early-on french drains

ever used pipes. I would put them in only if I believe that it is necessary to do so in order to ensure tha the french drain doesn't build

up an appreciable head of water.


Aug

06

5:45

oops - " . . . as he suggests . . .", not " . . . and he suggests . . ."


Aug

06

9:14

BigH - The french drain was built with a pipe, holes facing down, connecting to a solid pipe that the gutter leaders and sump pump

tie into and runs out to a stream at the back of the property. The french drain is only in the front of the house. It has gravel surfacing

to the top. No soil on top. Is meant to intercept surface water running down the sloped lawn in addition to subsurface water. So, why

do you say the water wouldn't enter the pipe? Or do you mean that subsurface water doesn't seep out of clay soil very well?


12 of 16 24/03/2007 9:10 AM


Aug

06

13:38

Hi All:

A fun topic, and I trust all is pretty well resolved at the house in question.

Now for blueoak and the "disagreement". Really there is no disagreement, since I refer to "subdrains" as being most suitably

backfilled with concrete sand. Let's define "subdrain". My use of that term "subdrains" started in 1954 (when I did my Master's

thesis on highway subdrainage at Cornell). The term comes from what Armco Steel put out years ago in their "Soils Manual', or

similar name. A subdrain,in my terminology, is mainly used for draining ground water. It is not generally a term used for toe

drains at earth dams or other important structures where the drainage is not generally taking ground water, but rather seepage water in

large quantities, etc. Yes, using the accepted ratios for filter design is a good idea for these jobs. But, for highway roadway frost

heave areas, base course drainage, house basements, etc. it generally is the case that designing a filter is not practical and usually not

needed.

Then comes what about the pipe and holes? Armco's original "wrinkled" corrugated steel subdrain pipe had its holes on the

underside at the quarter points, 3/16" diameter. Under heavy flow of water some of the finer grains got in, but soon the coarser

grains bridged over those holes.

For the more recent wrinkled plastic pipe with slots, maybe 1/16" wide, some sand also gets in and a bridging over then takes place

also. I have heard complaints by state code folks that they have seen the fabric sock on these pipes clog over at the slots, but I have

not seen this happen. Maybe these cases were backfilled with gravel? None of the installations I have been involved with used the

sock and they all seem to function fine. Since no excess sand gets into the pipes causing problems, we also no longer ask for

clean-outs to be installed, just in case.

The reason I am so against any gravel on the job is that we once called for using gravel directly around the pipe, with concrete sand

under, beside and over this gravel, something one would design with the filter ratios. A difficult thing to do, but it looks nice on a

drawing, to satisfy the architect who likes the term gravel for some reason.

Well I stopped by the school job where this was to be done and there the skid loader had dumped load after load of gravel over the

pipe two and three feet high, totally in "violation" of the nice looking drawing. After that, all gravel was removed from drawings and

the dumping of concrete sand was the case, and that has worked fine. As I have said before, it is difficult to goof up the job when

only concrete sand is the backfill, (at least around the pipe) .


Aug

06

9:45

beryl10 - it was more a general statement. The critical point in french drains is that the drain drains somewhere and the permeability

is sufficiently high (usual greater than 1 cm/sec) that water passes out quickly. Optimally, your french drain is connected by gravity

to an outlet. You may need some significant volume flow in order to fill up the drain rock below the pipe in order for the water to go

into the pipe. If you have the flow, the pipe will drain water; if you don't, the pipe will remain dry and the water will be passed by the

drainrock beneath the pipe. Now if your french drain is not connected in a positive fashion and you are relying only on the pipe to

drain water, then it really isn't a french drain but more like a "storage" pit.

blueoak (Civil/Environme) 31

Aug

06

14:09


13 of 16 24/03/2007 9:10 AM

Oldestguy,

Thanks for the definition and correction. I have been stuck with embankments lately and am a little shortsighted.

BigH,

As always good references. I am a little worried though that "concrete sand" can mean too many things. I have seen "concrete sand"

tested that didn't meet filter requirements on a clay dam.


Sep

06

3:30

blueoak - you are correct in that the concrete sand may not meet the filter requirements against the clay in a dam - but one must

remember the underlying assumptions under which the filter criteria were based; and it is my recollection that the filter criteria were

developed with coarse grained soils in mind (sand, gravel) not with clay to sand. Clay's biggest problem in dams is with its

propensity towards dispersion and that is why they have developed the pin-hole test back in the early 70s. Concrete sand basically

means normal well graded coarse medium to fine sand in my view. Again, see Conduto, see Terzaghi Peck and Mesri - and, if my

memory serves me right, Milligan in one of the recent Terzaghi lectures (2002 to 2004) discusses this in his paper.

As well the infamous filter criteria, many love to use Hazen's Rule for the determination of "permeability" although one will have

problems whether to use 100 or 150 as the coefficient. But, the Hazaen's rule was developed for medium grained single sized sand -

yet, most texts do not point this out and engineer's over the years have used it irrespective of its assumption.

oldestguy - 1954, eh? Were you there when Cornell beat Ohio State two years running in football? Do you realize that Cornell

holds a 2-0 record agains that Big 10 powerhouse???

oldestguy (Geotechnical) 3 Sep

06

10:58

Big H

Nice to see another Cornellian. Can't recall. Undergrad time was '46 -51 (5 year course) then the 3 years in navy (you remember

NROTC?) before grad school there.. In undergrad days I was too poor, so I was an usher at the games. I only recall Army beating

the Big Red about '47.

That job was at Judd Falls road, near campus.

newoldguy (Geotechnical) 11

Sep

06

11:00

Some great information here.....Please help with a couple of questions... I am planning a curtain drain to be placed

about four feet out from the foundation of a house with a damp basement. I am planning to go down six to seven feet

and I like the discussions about concrete sand with drain pipe.Does the sand need to be "washed sand"? Any warnings about going

that deep with a "sand only" backfill?



14 of 16 24/03/2007 9:10 AM

Sep

06

15:52

OK to the Newoldguy: The concrete sand I refer to meets ASTM spec C-33 for fine aggregate. It very likely is washed to get to that

gradation. In this neck of the woods they call it "Torpedo Sand" for some reason.

The obvious thing you want to do is cut off the flow towards the wall. I don't like the idea of anyone in a trench that deep unless it is

sheeted and braced. With no one entering the trench, this works most times. You follow the back-hoe excavator and immediatelty

roll in the 4" corrugated slotted plastic pipe and immediately dump some concrete sand on it. With that you then are 90 percent

done. For cost saving you might then follow with a local bank run sand up to near the surface where some impervious fill would top

it off. Should some cave-in occur, usually this method will still do the job. If you wish to try to compact the backfill, it depends on

your site. For some reason I don't recall having any later settlement problems on the jobs with no concerted compaction effort. Per

your question, I don't see where any "risk" comes in with sand backfill. The concrete sand does an amazing job without much

work. A little enters the slots, but then it stops due to bridging by coarser grains.

I'd not use this system for surface water drainage, as this thread is involved with.

With this method you may find the bottom of the trench and the pipe may not stay on a nice grade line. Therefore, going deeper to

allow wome wiggle room might be in order. The deeper the better for protecting the structure from water, but it might undermine the

footings.

You can see that doing it this way, it is difficult to goof it up.

Next is where to drain it to. On some jobs we install a man-hole and an electric sump pump, or run it to daylight down the hill. In

any case, if you are in cold climate, that outlet needs protection from freezing. An outlet under a lake is ideal. Your local codes may

allow it to go into the basement with a sump there and discharge to where they allow.

As an altrnative to the trenching some contractors will talk the owner into allowing a trench to be cut thru the floor next to the wall

and install a drain pipe there. It may work, but I've see these with water then seeping up out in the middle of the basement

floor. Perhaps the drain was not deep enough then.

jtc500 (Civil/Environme) 11

Sep

06

20:45

I have recently become part owner of a small rancher on a poorly drained site.I have decided to install a perimeter drainage system

and after much research conclude that concrete sand surrounding slotted plastic pipe (no socks) will be my choice method. It also

seems to me that if this system were to fail some time down the road that it would be a lot easier to dig out and replace than a system

involving sone or gravel.I also think I saw research many years ago claiming that uniform sand at 2mm would resist the passage of

termites.This foundation is only four courses high. In most areas except on the driveway side I think a 30" trench would be about

right dug to bottom of footing (angling slightly deeper going out)Probably will use one slotted 6" pipe but two or three 4" pipes

sound right also. Will tie into outside sump crock and try to pump to street.Cannot wait to start when I get some free time.I am 10

year bulder turned 25 year arborist-treeguy who would appreciate comments.

fattdad (Geotechnical) 11

Sep

06

21:37

I liked oldestguy's reply regarding the definition of subdrain. Regarding the use of crushed stone, washed sand or something


15 of 16 24/03/2007 9:10 AM

in-between what's at issue is matching the grain size of the sand/gravel material to the slot size and the grain size of the water-bearing

formation. If I were at work right now, I'd give the basic guidlines, which relate to the D10 (or is it D20) and the D60 size. It's fairly

straighforward. That said, in this day and age, most just don't fuss with all this calculating 'cause there's filter fabric to rely on. In

some instances this can be false security.

Regarding the attraction of water to a subdrain from that basis avoiding the use against a below grade wall, I would not share that

concern. If you have positive drainage, it just would not be an issue.

fatt but-but-not-that-old-I-guess dad


Sep

06

20:12

ITC500 Nice to see you are planning a perimeter drain, doing something line a "buried moat around the building". The idea would

what I call a cut-off drain, cutting the flow toward the building. In my research I found it impractical to depend much on drawing

down the water table, by installing a low placed subdrain to hope the water will run to it. The main place you can hardly get away

from doing this is for agricultural drainage or athletic fields. For a football field I call for drains under the main yard markers, as

well as a perimeter drain. The aim is mainly to drain off rain water from in the soil, using the draw down effect. Not perfect, but it

works sufficient for play to go on.

So at a house, if possible, it also is a good idea to get some drains inside the building in case some water for some reason gets past

the outside drain. In sandy country these should be no farther apart than 15 feet, since most sands are not highly permeable and a

steady flow gradient of about 1 in 7 seems to be common.

Pipe size of 4 inch is plenty large enough for even the heaviest groundwater flow (usually). I do recall one 8 inch line flowing half

full for drainage of a road cut in gravelly sand.

jtc500 (Civil/Environme) 14

Sep

06

0:23

Thanks much oldestguy for your response.I am however dealing with a four block crawlspace with duct runs below making the

prospect of installing an interior drainage grid system scare me a bit.Of course down the road I might be forced on my belly to do just

that.It does occur to me also that since a dwelling routes the rain water outside the perimeter drain changes the problem at least a

little from the exposed football field example.Say the water table has risen to the bottom of the footing.If even more water was

introduced ouside the perimeter would that water not take that first and easy path to the drainage trenches.Of course I can also see

that the faster water is added would require wider and deeper trenches for a dry interior.


Sep

06

20:57

jtc500 The crawl space thing is different from what I had presumed. I was thinking of deep basement.

I suspect you are just trying to keep severe dampness from inside, as with "ponding in there". Your perimeter idea sounds good and

probably all that is needed in that case, assuming bottom of crawl space is a foot or more above your drain elevation. Hold off inside


16 of 16 24/03/2007 9:10 AM

work until you see what outside work does.

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