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French Drain

French Drain

French Drain

I am considering installing a french drain.  I have clay soil.  I will use the recommended geotext pipe with a sleeve but is there a problem if I do not use gravel?  if I simply drop in the pipe & refill with clay will it affect the drain?

RE: French Drain

Yes, it won't work. Clays do not let water pass through very well. You first have to figure out the volume of water that will enter the drain. Then do a little test on the clay to get the speed that it will absorb water and see if the results are something you can live with. You may need a Geotech engineer if the water you are dealing with is damaging a bldg.
You may be constructing a larger problem than you have now.

RE: French Drain

Yes there will be a problem!

The purpose of a french drain is to hold runoff and allow it to percolate into the surrounding soil.  When sizing a french drain you calculate the available storage using both the pipe and the voids in the gravel.  

If you eliminate the gravel, the system will not hold or percolate that same amount of water. If you were in sandy soil the difference would not be so great, but clay has a very low percolation rate. In order for the system to work without gravel, you would have to significantly increase the length of perforated pipe, and even then it would not generally work as well.

Use the gravel, its worth the money and effort!

Good Luck

LauraP, P.E.

RE: French Drain

To ISP01

A french drain is not to hold water and let it percolate.
It is primarily used to lower the water table. It is to drain water, hence the name, from the surrounding soil such as from behind retaining walls, etc. It is also used as toe drains for earth fill dams, to stabilize (dry out) swampy/soggy areas in golf courses, prevent ground water from entering basements, etc. Also may be referred to as filter subdrain.

What you are talking about would be more commonly called a leaching line which works as you describe.

You have to have the gravel to allow the water into the pipe from the surrounding soil and the filter fabric around the gravel to eliminate filling the gravel voids and plugging the infiltration capabilities of the system.

RE: French Drain

Let's see if we understand each other.

If you are using a perforated drain pipe with a geotextile "sock" wrapped around the pipe, then placing rock and geotextile in the trench is not necessary.  You can simply drop in the wrapped pipe and the geotextile should prevent the pipe from getting clogged with clay.  The geotextile sock replaces the rock and geotextile trench.

However, depending on how clayey your soils are, this may not help you in any case, because the surrounding soils themselves may drain too slowly, even if you did backfill the trench with rock.

Consider how much clay is in your soil.  Dig a hole and see if the water percolates out of the soil and how quickly.

Also, the purpose of your french drain is not mentioned.  

RE: French Drain

snafuman -

The gravel may indeed be necessary, especially in clay soil.  It greatly increases the surface area and thus increases the capacity of the french drain.  


without the gravel, the surface area is 1 sq. ft. per ft. of pipe

2 foot by 2' trench with 4 inch diameter perforated drain pipe, backfilled with gravel and wrapped with geotextile (filter fabric) - surface area is 6 sq. ft. per ft. of french drain

In the example you could get as much as 6 times the amount of water draining to your pipe.

RE: French Drain


In the areas that I have practiced (California & Florida) the term "french drain" is usually used to describe a drainage system that has no discharge except through percolation.  We have also described them as exfiltration trenches.  The gravel is very useful in increasing the surface area (as CVG said) but in our case the water is moving from the pipe into the soil.  Locally, we also include the trench gravel void space (below the pipe only) and with FS factors to increase capacity.

The other systems you described, toe drains etc. sound like underdrains to me, since they would have to have a point of discharge for the pipe.  Those systems are used to lower the water table, but in these cases the perforated pipe is connected to a discharge system (eg. storm sewer or other down gradient system) In order to lower the water table the water has to have someplace to go.

RE: French Drain

We live in low terrain, about 5 to 10 FT MSL (Miami, FL).

Exfiltration trenches, or french drains, are the main type of storm drainage used.

Runoff water is collected in the drainage system, and percolates at a rate allowed by the soil.

The use of rock in the trench and geotextile wrapping is not only mandatory, but additionally, the height of all elements in the trench with relation to the water table level is standarized (depth of rock bottom, perforated pipe invert, and pipe depth).

I just happen to have some details form the City of Deerfield Beach, I can email it to yo for reference (they are quite simple, you may get better drawings from Miami Dade County (miamidade.gov))

RE: French Drain

Ramcode I would like the detail sheet.  Plz sent it to BsoLana@yahoo.com    

Ty in advanced

RE: French Drain

This is an interesting thread (because I have a similar problem), but when I look around for filter fabric I find that it's rated at apparent opening size (AOS) in equivalent seive as about 30 to 100. It takes a seive size of about 200 to stop my clay (in fact 85% passes number 200 seive). What filter fabric specs should I look for in order to filter out the clay and keep it from clogging my drain?

RE: French Drain

Check out Advanced Drainage System's (ADS) edge drains, they have some very informative literature

RE: French Drain


Due to the extremely small size of most clay particles it would be very difficult, if not impossible, to find a fabric that would filter clay.  Based strictly on AOS US sieve sizes there is a spun bond, non-woven geotextile that is used in the landscaping industry with an AOS of 170.  Please be warned though that with this small of an AOS there is very little water flow and would pretty much make your french drain useless.  My suggestion would be to use this type of fabric around the actual pipe (maybe with a slighty larger AOS) and then use a heavy weight needle punched non-woven around the gravel.

RE: French Drain


This thread is a great example of why I dislike the term, "french drain."  It's a very poorly defined term, and it is very frequently misused.  Check out this link: definitions of 'french drain'.  As you can see, even the language 'experts' can't agree!

Now, an answer to the original question:

Quote (amateurhandyman):

I am considering installing a french drain.  I have clay soil.  I will use the recommended geotext pipe with a sleeve but is there a problem if I do not use gravel?  if I simply drop in the pipe & refill with clay will it affect the drain?
Yes, you WILL have a problem if you simply put the filter fabric-wrapped pipe in a clay-filled trench.  It will clog and fail very quickly.
But don't use a gravel backfill in soils with any appreciable clays or silts.  The fine grained soils will migrate into the gravel and clog it, too.  Backfill the trench with clean sand, such as play sand or concrete sand.  The sand will help to filter the silt and clay, allowing a much larger surface area to intercept the water.

This topic was covered in the Geotechnical Engineering area: Thread 274-92587.

Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

RE: French Drain

Although this is a late response, I have had much success with subsurface drainage systems of the geocomposite types, such as Contech Stripdrain or GeoTechniques TerraFlow that have a 4-oz geotextile overwrap, using well-graded concrete sand meeting the requirements of ASTMC-33 fine aggregates with less than 5% fines. This sand is a natural soil filter very well suited to mitigate movement of fines from the clay towards the drain system.

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