french drain with sand?? 7

[jdharris]

When my home was constructed in 2002, the local health department dicatated that our septic system needed a "perimeter drain", which is basically a french drain. Instead of using rock like every other one I've ever heard of, ours was constructed using sand. It works great (runs water constantly 7 months out of the year), but is there a reason to NOT use sand? I need to know, because I'm installing a similar drain around my home soon, and I'm unsure whether to use rock or sand. Thanks

 

[VAD]

Hello jdHarris

Sand acts as a better filter than rock in the case of removing impurities from effluent so that resulting effluent is cleaner etc. The use of rock or sand for other drain would depend on how fast you need water to be removed. If it is not septic effluent then rock drain may be preferred.

The above is my quick opinion. Others may have more elaborate or other explanations

 

[LHA]

I am not a geotechnical engineer, but here is my take:

I've never spec'd a sand perimeter drain and never seen one. As VAD eluded, the only reason you use sand in situations where you need a permeable subbase is to filter solids...BUT sand has very minimal effect on Dissolved Solids, and there should be no particulates by the time water reaches the perimeter drain, unless fines are migrating out of the fill. If this is the case (and it should not be the case), the sand should have been wrapped in geotextile, or it will be clogged within very little time. Clogging the sand will, of course render the perimeter drain useless. Then migrating water will pipe along beside the drain and eventually lead to the very problems it was supposed to prevent.

Unless a geotech expert tells you there is a reason to use sand, I would only use Clean, Graded stone, about 1.5" - 2.5" D50 for a perimeter drain. Don't forget the 3" perforated flexible plastic footer drain pipe w/ polyester knitted pipe envelope in 6" crushed stone bed and fitted end sections where pipe daylights (if daylighting is at all possible)...the little added cost will ensure the drain actually does what it was supposed to do.

 

[gandersen]

jdharris:

If the soils underneath your foundation are fine-grained (silts and silty clays), another reason for using sand is to prevent the migration of the fine soil particles (known as piping) from under your foundation into the drain. If your foundation soils pipe, then you might experience damaging settlements. We will routinely use sand in this case. If the volume of flow is going to be high, then it is possible to use a double filter (sand against the natural soils and then gravel against the sand and the perforated pipe). There are very good design procedures for sizing the appropriate filtering material that are available through the Corps of Engineers. I would also recommend that you contact a local geotechnical engineer to assess whether or not piping would be a problem for your particular application.

Good Luck

 

[Focht3]

Seepage, Drainage & Flow Nets by Harry R. Cedergren. This is the definitive text on the design of drains for soil and rock. Cedergren does an excellent job of describing and detailing the current body of knowledge on the design and performance of french drains, seepage blankets, filter gradation, etc. This book should be in the library of anyone involved in specifying granular or synthetic drains -

[blue]jdharris[/blue]:

Where is your home located? Do you have clay or silt soils in your yard? (The answer is likely 'Yes' since these are the predominant soil types in most of the U.S.)

Sand-filled french drains will last longer and are more resistant to clogging from soil in comparison to gravel/rock-filled french drains. A very common misconception is that a gravel-filled trench is "better" than a rock-filled one. This isn't true over the long haul - say, 10 to 20 years. The sand-filled french drains will outperform the gravel/rock-filled french drains every time.

Your health department knew what they were doing. Follow their lead -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[jdharris]

Fohct3-
My home is located in extreme southern Indiana. To quote the soil analysis that was done on my lot before building: :"..they exist on bottomlands and formed in mixed alluvium and material weathered from shale and sandstone. A fragipan layer does not exist in the profile. Permeability is moderate, and surface runoff is slow. A seasonal high water table exists in the subsoil."
These descriptions do not mean a lot to me, as this is not my profession.
Also, I should have clarified myself in the original post: The perimeter drain is not to handle any sewage material, it is only to remove water before it can infiltrate the septic system.
Thank you all for the interesting feedback

 

[Focht3]

...mixed alluvium and material weathered from shale...

Some of the soils are very likely silts and clays. Use the sand backfill.

Your posts were quite clear to me - septic systems create real problems when they interact with a shallow, moving groundwater "table." For me, the perimeter drain requirement was an obvious attempt to intercept any "perched" groundwater and direct it away from the septic system.




Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[utvols99]

For the sand backfilled french drain, what should the clay soil to sand interface "look" like ?

In a disturbed soil situation, what is the relative importance of compaction of existing clay soil at the sand interface point in order to maintain the sand filtering function?

Is a geotextile wrapped pipe necessary when using the sand backfill?

Interest piqued by the sand design due to clay infiltration in previous gravel/rock design (maybe lack of compaction?)

Any input appreciated.

 

[cvg]

wrapping a sand filled trench with geotextile to prevent migration of fines into the sand could cause the geotextile to plug and is not recommended. The migration of fines into the sand should not occur unless very course sand is used. A geotextile wrapped perf pipe embedded in the sand might be used to prevent the sand from moving through the perforations.

 

[uncivilized]

I am currently designing an underdrain system (subdivision street edge drain). The native soil is a very fine sand. Should I use a coarse sand or would the fine sand clog the coarse grains. Should I use a filter sock around the perf. pipe? Or should I use a pipe at all? Thanks.

 

[cvg]

To answer your question, you should perform a filter design. It isn't difficult, if you have the gradation of your soil and of your proposed sand or gravel filter material.

One reference for a good method is in the US Army Corps of Engineers Manual EM 1110-2-1901 - Seepage Analysis and Control for Dams, Appendix D - Filter Design


check out the following threads for much discussion on the design of french drains.
thread162-48077
thread256-102747
thread274-92587

 

[utvols99]

From an application standpoint, the drain I'm designing is a subsurface drain, used to carry water away from the structure foundation.

The water inputs are both seepage and above-ground rainwater flow due to negative grade toward structure. Using the trench-bottom bedded perforated pipe wrapped in geotextile (with adequate AOS (non-woven needle-punched?)) filter matched to allow water passage, what would the downside of using the sand only back-filled drain be ?

The multi-filter design mentioned in Cedergren and the COE documents seem to be challenging to implement, unless there is something I'm missing...?

 

[cvg]

I would stick with a single filter material i.e. concrete sand or similar materail and make sure you have a good filter match with your native clay. Multi layer filter is probably not necessary unless you insist on using gravel fill.

 

[Focht3]

Agreed.



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?"

 

[woodleigh]

As a PE working the constructon side of house, I have installed numerous underdrains, french drains, etc. Every plan I have seen requires filter fabric to wrap the drain rock. I have seen alot of previously installed systems fail, and also have had to replace those same systems. What I still don't understand is what type of engineering logic is being used which makes one believe that the filter fabric wrapping the drain rock is not going to plug up. If it is believed that so many fines are going to flow in ........and plug up the drain rock (usually pea gravel), what type of logic makes one reason that that same fine material is not going to make an 1/16" to 1/8" thick layer of impermeable goo over the fabric.

To avoid settlement etc. issues caused by fines filling up the voids, or loss by running out the perf pipe, I believe the best method is a compromise. Use fabric, but cut some slits in the higher regions of the fabric, so that if the lower regions get plugged up, at least some what can get it.

 

[cvg]

I think the problem is lack of engineering logic - or inexperience with design using geotextiles.

regarding your compromise, use sand and then you can eliminate the filter fabric. the side benefit is that sand doesn't plug up nearly as easily as fabric

 

[woodleigh]

Sand also plugs fairly easily. I was just involved with some test sand filters in the Tahoe basin which I also saw get completely plugged after two seasons. The test filters included 6" of sand (super clean sand spec'd for water filtration purposes, AWWA Standard B-100-00), sitting ontop of a layer of over activated alumina wrapped in a burrito of filter fabric, over a perforated drain rock system. The runoff had to flow through the sand, through a layer of fabric, through the activated alumina, then into the underdrain/perforated pipe system before it could flow out of basin (unless it built up enough to flow over the basin spillway). The site was designed to drain only the roadway surface, but because of a heavy winter and a highground water table (and ground water seeping into the drainage inlets), there was a considerable amount of groundwater pouring into this basin/filter media system. After the second Spring season, the ground water kept flowing in rather slowly, and had alot of suspended fines, and eventually just plugged up the filter (it took almost a foot of head to push it through the sand layer). By mid summer, the ground water dried up, and the top of the filter sand looked like an old dry lake bed (hexogonal shaped cracks on top of the sand layer). I picked up some of this material and carefully examined it. It had amount 1/16" to an 1/8" of the fines completely plugging off the surface of the sand, and also some fines in the top 1" of 6" of sand (enough fines in the top 1" to keep the top inch of sand stuck together in a clodded fashion).

To make keep the filters perking water this winter season, they had to scoop of the top 1 to 2" of sand.

If this can happens on the roadway surface where you can see what I saw (and with relatively clean runoff), you can only guess how easily things are plugging up under ground (be it sand or filter fabric). If you use filter fabric around drain rock, then cut some small slits, in my personal opinion you are better off because you are preventing piping of material, but still keeping the water table from building up.

 

[Focht3]

You aren't preventing piping - in fact, you are encouraging it to form through the slits. That's a very bad idea!

It sounds to me as though the drain is simply too small to act as a proper filter. And the gradation of your "super clean" sand may not be appropriate for a sand filter. Also be aware that the activated alumina will alter the pH of the water that it encounters - which can drastically affect the clays' behavior if the water encounters the high-pH water. Conventional filter design assumes that you are dealing with inorganic clays and a pH-neutral pore fluid.

What are you using as a filter fabric? Please provide details.



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?"

 

[doc11]

The amount of fines that water can carry is determined by the speed that it is flowing. Why not make the trench a little wider or less of a slope to reduce piping and plugging of the filters.

 

[cvg]

If you are talking about clays, the speed of the water is not all that important in the carrying capacity. In fact, it can take days or weeks for suspended fine sediment to come out of suspension and settle on the bottom of a pond with no velocity at all. Even wind / wave action can resuspend clay fines. The sand however, settles out immediately unless you have sufficient velocity.

I believe focht3 has hit upon the problem, the size of the filter may be too small for the large amount of sediment which is coming in. It sounds as if the test filter / basins didn't pass the test... Most sediment basins cannot handle large flows and are better able to handle sediment removal during smaller flows. A suitable diversion should be provided to allow higher flows to bypass the filter to avoid plugging it. Alternatively, the basin could be made much larger to handle the larger flows.