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Filter fabric in french drains

TomH_'s picture

Has anyone had much experience using filter fabric (weed control fabric) in a french drain before filling with drain pipe and stone. Won't this just eventually clog up and keep the water from entering the trench. My soil is mostly clay. Thanks

(post #172673, reply #1 of 18)

The idea of the filter that it allows the moisture through the fabric to the pipe, without allowing silt / mud in to foul the pipe. It is very effective in doing this....
We use a filter sock...which goes around the pipe...made of the same material as the fabric. The sock..makes it run a bed of gravel...slope to drain...cover the drain pipe with the sock.install the pipe and fittings....cover all with gravel...and top off with soil. If you want to run a prophylactic layer of the top of the gravel, that will help with silting.

(post #172673, reply #2 of 18)

We have been using filter fabric for a few years and has been code. However some inspectors are backing off on checking for the fabric. The reason...silt plugging the path of water. Our main excavation sub also says he has seen it plugging and we are in a high clay area also. I hope it is not true. I have two many out there including my own home.Yet I havent seen or heard any problems with our installs and I know mine is draining. However the silt is going to travel down and we use plenty of rock and just before the soil layer or bedding material we also run another sheet of I hope the silt does not get down to the pipe and if so would be a small amount I hope I hope.
But silt will plug it. We had some overlap the end drain one time and plug from the silt that got in while we were assembling the pipe.

(post #172673, reply #5 of 18)

clay and silt will eventually clog both geotextiles and stone trenches. One method to slow this down is to provide several inches of sand around the filter stone, between the stone and insitu material. This is easier to draw on paper than to construct in the field, but it works.

Water moves extremely slowly though clay, which is why clay is considered impervious. If the clay in your area runs deep, the french drain may have very little effect on reducing the water table. That is, may only be effective very local to french drain. What is drain for??

(post #172673, reply #6 of 18)

Filter fabric came into use in the early 1970's because of strong marketing by the fabric manufactures. It will help prevent the filling of the gaps in gravel with silt and sand sized material in sandy soils, but filling of the gaps in the gravel will not materially effects the operation of the sub-drain (it will slightly reduce the permeability but not significiantly, go to an engineering library and look up an author called H.R. Cedergren). In heavy clays, there is generally not enough flow (Q) to mobilize alot of the fines (if there was, you would see voids developing adjacent to subdrains, caused by the removal and movement of the soil particles, similar to voids seen around long term sewer line breaks). Very few engineers have any real idea how sub drains work. Ask one how to design a subdrain, they will give you rules of thumb and general sizes, but none will give you any design theory.

The subdrains used around houses developed from the subdrains used by farmers to drain their fields. They are designed to drawdown a water table. If there is no water table, a subdrain actually acts as a dam (unsaturated conditions). The excavation is the subdrain, not the pipe within the trench (the pipe is an overflow device). Full scale testing in the 1950's found out that if you dig a trench and backfill it with the same soil, it will act as a good sub-drain for about 10 years (until the soil compacts, reducing its permeability). There was little difference between a trench and a trench with a pipe in the bottom (even over time, the water had to flow through the soil to get to the pipe). Work by Cedergen in the late 1940's in california showed the large increase in trench permeability if the trench was filled with gravel (this was known earlier by at least 1938). He was primarily concerned with controling runoff from roads. His concepts were then moved straight to subdrains around houses, but the engineers forgot that instead of having to deal with concentrated surface runoff, they were dealing with groundwater. ( I think I have gotten off the subject.)

It takes a long time (if ever) for filter cloth to become clogged. It cannot become more clogged (less permeability) than the soil surround it (especially in clays).

(post #172673, reply #8 of 18)


foundation drainage (behind walls) has a very specific purpose, to prevent the build up of free water behind the wall. Free water can cause an increase of 25 to 30 percent of the loads on a wall and can cause it to fail. That type of drainage must be done. this is where the concept of a filter drain came from. A filter drain is a filter cloth about 1/2 inch thick placed behind a wall. it is supposed to remove free water from directly behind the wall. The problem is that water builds up in the soil just behind the filter drain and still applies additional loads to the wall.

The other use is to lower the water table beneath a house to prevent/reduce standing water in a crawlspace or water transmission through a slab. The problem is that a trench or a sub pump has a limited area of effectivness. Draw a 45 degree line up from the bottom of the trench, and that is the approximate distance that the trench/sump pump will effect the water table level. there will be no real drawdown further away from that (in clay soils, use a 30 degree angle from the vertical). This is why the installation of a single sump pump in a basement or crawlspace to control ground water is a complete waste of time and money.

(post #172673, reply #9 of 18)

Thanks everyone for the input.This was my first time posting a question to "breaktime" and it was a great help to get the benifits of everyones experiences.

(post #172673, reply #10 of 18)

It is generally correct to say that the flow thru clay will be slow enough to not cause the fines to wash out. However, here in the Northeast we have alot of glacial till which is sort of a mixture of gritty, silt, clay and rocks. In our area, underdrain (french drain) on highway projects between 25 and 35 years old have some (not all)underdrain that is clogged. We also have clogged filter fabric due to silt intrusion. Your right about many Engineers not understanding surface drainage. Groundwater hydraulics is a very complex 3 dimensional problem not easily solved due to the variations in soil properties and intensive calculations. But you don't have to understand all the theory to apply the technology. Just remember to backfill the bottom of the foundation excavation with stone and a pipe. If the house sits on a silt or clay soil, then place a drainage layer (permeable soil) under the slab also. Since the drainage layers will move more water than the soil surounding it, the water will drain away faster than can accumulate (assuming the water has an outlet-daylight or sump pump). That way, the foundation will stay dry.

The problem with backfilling the excavation with the same material is that after 10 years...Then what??? Not to mention that uncompacted fill, when settled will place an even larger load on the walls than one that is carefully placed and compacted in lifts. Check that Engineering library, there will be Chapter in any Soil Mechanics text on lateral loads, Rankine and Coulomb's theory.

(post #172673, reply #11 of 18)

MDuval - You are absolutly correct about the necessity of filter fabric in silty soils (especially glacial tills). It can also be needed in sandy soils, but sandy soils generally have enough permeability that water will flow quickly away (the exception is if there is a hard pan layer that causes a temporary perched water table after heavy rains).

You are also correct about including a gravel/drainage layer beneath concrete slabs (especially basements) that drain to a sump. I made my comment because too often I see contractors talk homeowners with wet asements/ crawlspaces /etc. into constructing a single sump, thinking that it will solve the problem. There is one homeowner in Berkeley CA who recently spent $200,000 building a basement and could not understand why water kept flowing into it. The engineers did a wonderful job on the structure, but had no concept of water control (roof downspouts drained adjacent to the foundation, no gravel layer below the basement slab, etc. They did put in a vapor barrier, but they do not work under water).

Water control is a large subject that we could talk on for hours. It is not new. In 1920, highway engineers noted that the old timers said that there were three important factors in building good roads: drainage, drainage, and drainage. The same goes for houses.

Actually, the settlement of a loosely compacted backfill will either slightly increase the load on a retaining wall or will slightly reduce it. If you assume Rankine solution (no wall friction), then settlement of the soil will reduce the soil height, decrease the soil volume, and increase the unit weight. However, the horizontal load on the wall will remain the same (there is no change in the overall weight of the soil). Where it acts (the resultant of the soil load) will be lowered, theoretically reducing the load on the wall. If you assume Coulomb (there is wall friction) there will be the same horizontal soil load (again acting at a lower point), but there will be an increase in the vertical load on the back of the wall (from the wall friction) which can do nasty things to your design. (I assume no change in soil friction angle -but one could argue this.). Putting in a compacted backfill will increase the loads on a wall (over loosely placed backfill). The soil is more dense, therefore greater loads on the wall. You also have to take into account the additional temporary loads from the compacting equipement, etc.

The point I was trying to make was that it is the trench that is the subdrain, not the pipe that is put at the bottom of the trench. This is why you rarely see water flowing from the pipe outlets from subdrains. If you are on the side of a hill, you should be aware of where the outlet of the trench is and take suitable precautions to control the water outflow for the trench itself (as well as the pipe). Put in clay plugs to force the water into the pipe, etc. (I have seen landslides caused by concentration of water from subdrain trenchs around the side of a house). To get an idea of why this occurs, calculate the volume of the trench that is below the drain pipe (reduce it by 75% to take into account porosity). This is the volume of water that is required to build up in the trench before any water will enter the drain pipe. In most cases, water will never enter the pipe. It will either flow out the ends of the trench or through the bottom of the trench. The trench will actually reduce the time it takes water to reach a basement floor. You commonly see a compacted clay cap built over the trench. These are useless. The first thing homeowners generally do it plant bushes next to the house, creating 1 foot in diameter holes through the cap (this is called a sieve). The cap also has no effect on horizontal (ground water flow).

(post #172673, reply #12 of 18)


(post #172673, reply #13 of 18)

Figuers: Excellent info! Would you mind detailing how you would recommend waterproofing a basement in new construction. How much/type of gravel under basement slab? How wide gravel filled trench? Where is drain pipe in relation to footing? Do you bury a portion of the drain pipe so it's slightly lower than the trench bottom to try to direct water into it? You mentioned needing more than one sump pump? How many and for a flat lot is there a better place to put them (ie corners)? How about a cap on the trench? You mentioned a clay cap being worthless. Is that always, or only in the situation you mentioned as a sieve with plant holes in it? With a poured basement wall and external insulation, do you recommend any particular type of waterproofing? Thanx.

Question 20 years later! (post #172673, reply #15 of 18)

As strange as this is, I came across this forum when searching for info on drainage for our yard and noticed that you seemed extremely knowledgable about this.  In fact you are the first person I've read or talked to who's answered a question that I've always wondered about, namely what is the function of the slotted pipe in French/interceptor drains in most situations given that the amount of water going into the drain would have to be so large that it filled the trench up to a level where it actually went into the pipe.  Everyone else just says that it has to be there to carry the water which never made sense to me.   I also noticed your comments about the outflow from the trench itself rather than just the pipe and ways to force water into the pipe using a clay plug.  This is also something I'd wondered about because where our proposed drain ends I need to somehow get the water from the trench/pipe fully into a non-slotted pipe to take it away to the street, and my concern is that most the water will just run out the end of the trench and saturate the surrounding ground.  Anyway, do you mind very much if I send you an email describing our general situation and get your advice?

Best regards




Attn. Figuers (post #172673, reply #16 of 18)

Figuers:  Apologies my last post was directed to you..

David (post #172673, reply #17 of 18)

No way you'll reach him via email.

this site has zip user info.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


The post youre responding to (post #172673, reply #18 of 18)

The post youre responding to is 18 years old.  Everyone is gone. There are a few of us here that sweep up and dust things off but basically it's an empty room.

Florida Licensed Building Contractor, 45 years experience in commercial remodeling, new homes, home remodeling and repairs and all types building maintenance.

(post #172673, reply #14 of 18)

Has anyone had much experience using filter fabric (weed control fabric) in a french drain before filling with drain pipe and stone. Won't this just eventually clog up and keep the water from entering the trench. My soil is mostly clay. Thanks

(post #172673, reply #3 of 18)

My old boss (a PE with a design/build firm) never specified it. His answer was that the fabric provided a one-dimensional barrier to silt, the gravel bed was three-dimensional. Also, if the pipe silts in it is possible (and easy) to flush it out with the cleanouts he provided in all underground piping systems. Just pull the plug and flush w/hose or maybe snake. Most the work we did was NOT connected to residential storm sewers. They were daylighted, or run to nearby creeks or ditches.

Whenever anyone gave him grief he always asked how it was that farmers could tile all their fields by direct burying perforated pipe with no stone or fabric! Some of those old clay tiles are solid pipe laid with 3/8 gaps between them, and they still work!


(post #172673, reply #4 of 18)

It depends on the soil, I think. If it is really silty (or whatever), it will eventually infiltrate and ruin your gravel bed, so why bother with the gravel if you don't protect it? The landscape fabric should envelop the gravel bed, none against the pipe. Use a high-quality fabric -- the stuff we bought from the masonry dealer comes in 12' rolls that they cut to the length you specify. It is so strong that it's VERY difficult to cut with a utility knife. The HD "weed control" stuff is not the best choice, it isn't even meant for this application.

Of course, some places you can do whatever you want and it doesn't matter. We have high clay which allows very little water penetration. The larger the fabric area, the less likely it will clog to the point of uselessness. I absolutely agree with providing cleanouts. A garden hose with a nozzle makes a great snake.

(post #172673, reply #7 of 18)


Very informative post. I have always questioned foundation drainage standard practices and how much the dozer owners really knew or cared.

Thanks for the enlightening,

Jack : )