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.

 

[beryl10]

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.

 

[cvg]

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.

 

[beryl10]

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]

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.

 

[BigH]

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

 

[beryl10]

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?

 

[oldestguy]

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) .

 

[BigH]

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]

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.

 

[BigH]

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]

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]

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?

 

[oldestguy]

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]

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]

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

 

[oldestguy]

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]

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.

 

[oldestguy]

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 work until you see what outside work does.