Sand as French Drain liner in Clay Soil

[bootspurs]

4 years ago, we installed a french drain that runs 50' parallel to the back of our house, then turns for another 44' to run down the side yard to the street. Our lot is lower than any of our neighbors, so we get all the runoff, and I imagine that we're that much closer to what seems like a high water table.

The drain we installed had 3" perforated corrugated pipe with filter fabric wrapped around only the pipe. The pipe was set in a bed of drain rock and covered, and I thought we sloped it at 1/8" per foot slope (which is the maximum slope I can get; literally a 12" drop over 94' total length). The pipe had a cap on the uphill side, and drained to the street on the downhill end.

This year we had flooding again, and last weekend I dug up the french drain. The pipe was literally packed with dirt for the first 50' of length, and was pretty muddy for the rest of it.

I now know now that the original design was wrong; with that minimal slope, we should have been using a smooth-wall pipe and made sure the slope was accurate for the entire drain. When we dug it up, there were sections that were not sloped at all, not good. Also, I have learned that common practice is to line the entire trench in filter fabric, not just the pipe.

Based on the amount of soil that infiltrated the pipe through the fabric, I'd say that our soil is more on the clay side of the spectrum than the sandy side, and the fine clay particulates are passing through the fabric.

Similar posts here have discussed using only sand as a filter, with no gravel or drain fabric, because the sand can filter the fine particulates that the fabric can not. This sounds reasonable to me, but when I tell my plan to landscaper friends, they say it's crazy to use only sand, but none have ever done it to know for sure. Just because most landscapers have been installing pipe/rock/fabric french drains everywhere doesn't make it right. Do folks still agree, sand would be the way to go? Any minimum depth around the pipe?


Another question: the fabric can get clogged by the clay, making the drain useless; can the sand can also get clogged, but just take much longer to do so?

The actual position of my triple-wall perforated drain pipe will be limited based on the start and end points and limited elevation delta.

My thought is to dig my trench as deep as I can swing it--maybe 30 or 36" at the lowest point--and start filling the trench with sand, no fabric or rock. I would consider wrapping the pipe in drain fabric to keep the sand from washing into the drain holes, but that would be the only point of the fabric.

Also, right now our gutter downspouts tie into the same single drain, but I plan to install parallel pipes in the trench; a solid pipe for surface collection and downspouts, and a perforated pipe for a dedicated french drain. The pipes would tie in just before the street outlet (I only have one outlet through my curb--I'm still kicking myself that I didn't drill two holes in the curb).

Thanks for any and all feedback about this.

 

[cvg]

first of all, how did the dirt get in the pipe? You said you wrapped it with fabric. Were there holes or gaps in the fabric wrapping? Did the dirt simply go through the fabric? Sounds like you might need to use a fabric with a smaller AOS. Or the dirt came from the down spouts?

Second, how big is your trench? You should try and have 12 inches of sand from each side of the pipe and also from the bottom. This would be a 2 1/2 foot wide trench. Even with gravel, this will allow a lot more flow into the pipe with less clay piping into the gravel.

Third, yes sand is a better filter than fabric is and doesn't suffer from problems like rips, tears or clogging on the surface.

It sounds like have way too much flow (for a french drain) and you may want to convey some runoff in an open channel instead. gutters do not need to tie into a french drain.

I would not tie the downspout into the perforated pipe. that will introduce a lot of sediment into the pipe and will possibly back up into the perf pipe making it ineffective. Drill another hole or discharge to a catch basin and then through the curb with one pipe.

 

[Ryb01]

A couple of questions for you that may be helpful to better answer your question;

1) Are you permitted to tie into the storm sewer system at the street and or connected already?
2) Is there a reason why you are you using a french drain and not a surface swale? You had mentioned that your property was lower than your neighbors but is there any grade from your property to the ROW?

3) Was there an outlet for the french drain initally? French drains are used for a variety of applications but mainly for infiltration purposes and or a degree of underground storage. You will not infiltrate much in clay soil.

4) Using sand as a fliter media will not do much good if there is no where for the water to go. If you properly wrap a geotextile around the trench and also have a perforated pipe with a filter sock you should be fine.

5) A solid pipe has no way to accept the surface runoof unless equipeed with inlets.

If you could drain your runoff through a swale to a catchbasin and further into the storm sewer (if connected) may be your best bet. Less headaches in the long run.


Hope this helps.

 

[beej67]

If you have the ability to tear the thing out and replace it with an open swale then that's a better solution. That will depend on your grades, though.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[concretemasonry]

The advantage of a filter blanket (sand or properly graded material and not clean rock) over a fabric around the pipe is that the interface area between theclay soil and the pipe is much larger and the velocity of the water from the soil is less, so less material will be flushed into the pipe.

The principal of using properly graded material under and around a perforated pipe (or a rock filled area) has been used for decades where the goal is to eliminate wet areas with scarring the terrain. These hidden lines are common on golf courses and are frequently discovered while still functioning on 60 to 100 year old course.

Corrugated pipe is cheap, but pvc is much more reliable and is easy to switch from perforated to solid depending on water is to be collected or just transported through an area.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[bootspurs]

Thanks for all the feedback. I'll try to clarify things a bit more:

I assume the dirt got into the old corrugated perforated pipe by passing through the fabric and through the slits. The pipe had an end cap, and was solid with dirt even before the downspouts tied into the system. The downspouts might have added to it, but they didn't add 12' of soil up-grade from their tie-in point. My fear about using the fabric with a smaller AOS is that the fabric will clog, sooner than if it was just sand.

The open trench is now 12" wide, with a depth that goes between 12" and 24" (a 12" drop over 94' length). I can increase the trench size per the suggestions.

There is no grade from the yard to the ROW; max grade would be 1/8" per foot, but we don't want to grade our entire yard like this, so I think a swale is out. I can grade to three small areas for surface drains, including under the two downspouts, which would all go to a solid pipe.

The sand would act as a filter for the clay, but there would be a smooth-wall perforated pipe in the drain too, so the water would have a place to go.

Thanks again,

 

[concretemasonry]

Downspout discharge should never be run in a perforated pipe that is meant to pick up additional water. They should be separate.

The reason is that the runoff from the roof is immediate and the ground moisture pick-up lags behind. This is a very common problem in a home with perforated drain tile that can get a flooded basement caused by the downspout flow being greater than the ability of the discharge pipe (especially perforated, corrugated pipe). In the basement case, the water backs up in the drain tile because it is a relief path and can come through the drain tile perforations and the back-up from a plugged portion could also be carried back into the interior drain tile and plug that.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[oldestguy]

Hey friend: I see you are catching on to what is needed. I have been preaching this use of concrete sand (not just any old sand)for so called French drains. You will run into landscapers, plumbers, and code writers that call for rock backfill, but those guys don't go out and fix the jobs that fail.

It is hard to stop stupid practices.

Recently, I had to buy a house suitable for handicap use (for wife) and it was built by a local contractor for his brother 11 yeas ago. It is not into a ground water situation, but has flat grades and not good surface water drainage (along with dumb landscaping using decorative rock in a shallow trench alongside the house).

The house plans called for a 3" diameter corrugated plastic pipe covered with a sock fabric placed on the wall footings and run to a sump. Nothing was shown on the plans for footing drain backfill. So of course they dumped dirt (in this case a very fine silty sand.

The house perimeter drain functions when there is infiltrated water. Fortunately all downspouts are directed away from the house. But now and then one comes loose in a storm and the sump pump runs. There was an early period when the house had no gutters. Then the sump pump really ran.

I am amazed that the drain still works, since the sock has let silt through and the sump shows muddy water and mud in the bottom.

I recently had to cut out a basement concrete wall when installing a cellar door entry. I found that 3" pipe covered with mud, but amazingly still working. The inside was almost full of mud and barely open.

I am expecting that in a few years or less that drain will cease to work. So I will have the expense of a whole new drain system but will first use a waterproofing of the ground surface over all backfill. That treatment avoids having a perimeter drain (for surface water infiltration). It involves the use of bentonite.

I knew about this drain before I bought, but the house had the handi-cap features I needed. These type houses are scarce.

The story here is, I have seen many a job like this fail early on and here I have one in my own yard. It is well on its way to failure. I really am surprised it has not failed yet.

Replacement, if it comes to that, is use of 3" corrugated slotted plastic pipe and the whole zone around the pipe filled with ASTM C-33 concrete fine aggregate (concrete sand). No rock or fabric is to be used. I probably will fill up a few feet with concrete sand and then local soil as backfill. The pipe will have no slope, sitting on the footing. If I am able, the pipe actually will be beside the footing to have some slope.

Good luck to you.

 

[fattdad]

there is no good need to slope a perforated pipe. It'd make more sense to slope the bottom of the trench, but a perforated pipe only has flow when the gravel bedding is fully saturated (i.e., there's a hydraulic grade line above the invert of the pipe.

You should have trench wall, filter fabric, stone and then pipe.

I didn't read everything else.

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

my sense is that he doesn't really need a french drain at all, just a storm drain system to remove surface water...

 

[bootspurs]

cvg, you may be right about needing a surface drain system only, but at this point my trench is dug, so I am going to install parallel drain systems (solid pipe for surface runoff w/ inlets, perf pipe for french drain). I think it's better to overdo it now, especially if it means even less of a chance of flooding 10 + years from now.

f-d, you don't mention sand in your note about the french drain makeup, but my plan was to include sand as a filter for the fine particulates.... what is your take on this? I have clay soil, and the last drain that was in filled up completely with dirt over 3 years, despite the last drain being wrapped in drain fabric.

 

[cvg]

suggest you put in surface inlets to pick up the surface water and connect those to the non-perforated pipe. Also, a 3 inch pipe seems a bit small for connecting to downspouts, especially with such a flat slope.

 

[oldestguy]

Bootspur:

There is continuing difference in recommendations between us "experts" as to what materials to use in these trenches.

While use of fabric around the gravel is expounded by some, and it should work if all is done exactly right and the fabric type match to the soil is right. However, it is labor intensive and can be done wrong. It is relatively new in use as compared to the concrete sand usage. Being new doesn't necessarily make it better. While pipe types have changed through the years, the backfill filter requirements have not.

In my case promoting concrete sand, this has been proven to be a good system since the US Corps of Engineers did extensive studies in the '30's to show that concrete sand has the correct gradations to filter most soils (clays usually are so sticky that they don't erode significantly). My Master's thesis was based on two years of field studies of under-drains under highways where I found many failures of drains with improper filters and also had installed several test sites with concrete sand filters.

As I have stated many times, of all the jobs I have been involved with, not one has ever failed with concrete sand as the backfill (to my knowledge).

The spec can be simple: Place in the slotted pipe and follow with the concrete sand.

 

[bootspurs]

Fair enough. Just before you posted I ordered 5 yards of concrete sand, so wish me luck!

 

[BigH]

For those who want a "learned" reference (although with the practice oldestguy has had over the years and the people with who he worked with, the learned reference author might take a back seat . . . - From Terzaghi, Peck and Mesri (1995), page 82:
"According to Ripley (1986), a clean cohesionless sad is not only capable of effectively filtering ever the finest silt and clay soils generally found in nature, but it also resists segregation during construction. this conclusion is supported by others (e.g., Lowe, 1988). Sands typically used for concrete, having a D15 of about 0.25 to 0.5 mm and a uniformity coefficient of about 1.5 to 2, can serve as a conservative filter to block even the finest clays."{/i] Charles Ripley advised me of this personally back in 1985 when I attended a seminar in which he was in attendence. Good man, he was . . .

 

[beej67]

So when you spec out concrete sand for a french drain application, you don't bother with filter fabric at all? Just a trench, a perforated pipe, and sand backfill?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[oldestguy]

For the currently available slotted stuff in the USA, there will be a slight infiltration of fine sand until the larger particles bridge over the slots. This has not caused any grief that I have heard of. We used to spec a clean out set-up, but they were never needed.

I would suppose no problem using the sock enclosure, which is like belt and suspender treatment. More expensive.

In the old days the metal pipe had 3/16" holes in the bottom third, which worked fine, since the upward flow of water didn't drag along any sand.

On this general subject, in the use of gravel and a filter fabric, on one school job where the architect had specked gravel and the trench lined with fabric, I happened by before the trench was filled. Being labor cost conscious, the contractor used a front end loader to fill the trench, spotted along the line, not uniformly filled.

The job was ready for laying the fabric over the backfill and then some earth. Some sections of fabric had fallen down due to the way rock was dumped. Some sections had no rock and others over filled. There was no plan for a laborer to smooth things between these dump zones or to correct the fabric situation. If I hadn't spoken up, a very lousy job would have been done.

On the other hand, even with this way of filling, as long as some sand was around all pipe sections, concrete sand would have worked fine. It is nice to show what is wanted on the plan, but you gotta sit there and make sure it does for the fabric alternative.

 

[fattdad]

I'd use a separation geotextile, open-graded aggregate and a perforated pipe. Yes, there are engineering methods to "design" a filter and they are widely used by the US Corps of Engineers and properly used in embankment dam construction, where the consequences of internal erosion can lead to dam failure. If you want to move water out of the soil and into a pipe, crushed stone is 100x times better than sand.

The use of these new-fangled separation geotextiles over the last 20-odd years is tried and true. Are the appropriate for all filter requiremetns? No. Are they appropriate to separation the formation soil from the aggregate in a french drain? Yes.

f-d

¡papá gordo ain’t no madre flaca!