Backfill for french drain in clay soil 2

[fsolomon]

I am building a french drain in a high clay soil at my house. I am planning on backfilling the trench with sand to filter the clay and using a drain sleeve filter on the perforated drainage pipe.

I am able to get a ASTM D6707 drain sleeve filter with an AOS of 0.6mm. What kind of sand should I use? I can easily get mason sand, general purpose sand, and play sand.

Thanks,
Fred

 

[GeoPaveTraffic]

I would use mason or concrete sand. Shouldn't have any problems, that is what I have always specified in a situation like the one you describe.

 

[DRC1]

I would also wrap the drain in filer fabric. You can buy"socks" that slide over the drain and fit well. I bouht somme for asimilar project at Home Depot.

 

[BigH]

According to Ripley (both in personal conversation and as noted in Terzaghi Peck and Mesri), as GeoPaveTraffice indicated, a concrete fine aggregate is suffient to filter clayey soils. Would the aggregate be sufficient to act as the drain material? Only your analysis on suspected flow capacity can tell. You can put in a drain pipe directly within the fine aggregate but you may need to have a stone layer of a bit more permeability in order to effectively drain the concrete sand. (idea of graded filter) Thus, a 10 minus coarse aggregate drain (with the sand between the CA and clay) with/without a drain pipe would be likely.

 

[fsolomon]

GeoPaveTraffic and BigH thanks for your help. It sounds like mason sand is the correct filter media. I think it would be hard to construct vertical sand, gravel, and sand layers over the drain pipe. I would need to build vertical dividers between the different layers, and do 3 pours of material. Is creating a gravel/sand mix an acceptable alternative to increase permeability? Is pea gravel the correct size?

Thanks,
Fred

 

[GeoPaveTraffic]

fsolomon,

the idea with the graded filter is to place the gravel next to the pipe then the sand around the gravel. This means putting some sand in the bottom of the trench then some gravel then the pipe followed by some gravel and lastly filling the remainder of the trench with sand. However, you need to be carefull to keep the sand between the gravel and the clay walls in all cases. Also, the gravel must be properly graded to keep the sand from washing into the voids in the gravel.

Mixing the gravel with the sand will not change the permeability of the sand, so that will not work.

Unless you have or expect a lot of water flow, you probably do not need the gravel. If you post some information about the problem you are trying to fix with the drain, we can offer some advise.

 

[oldestguy]

Sorry guys, the use of gravel around the pipe may look nice on paper, but out on the job it is almost impossible to get it right. The slots in present day "wrinkled" plastic pipe will let a small amount of fines in, but then it stops, bridging over the slot.

The same negative comment goes for lining a trench with fabric and using gravel inside. A labor intensive thing which easily can be goofed up. Take it from this old guy, never have I had a job go bad using concrete sand directly against the pipe, including the old Armco "wrinkled tin" subdrain pipes with 3/16 " holes. The first such job (in clay) was done in 1955 and it still works fine.

 

[blueoak]

I am a fan of the gravel. For drain design, I like the gravel to be designed to carry the flow and throw a pipe in as bonus capacity. It is amazing what will crawl up a pipe to die. The sand is good for your filter design, but the gravel is really a good long term drainage path. Even in old orchards I have done work in it seems the old tile systems outperform the drain pipe installed 20 years later.

 

[cvg]

for a residential project, go with the kiss concept. keep it simple... Concrete sand, no fabric needed - slotted pipe is ok and will reduce the amount of sand needed (might be cheaper). backfill with impermeable material and slope your grades away from the house. If you feel the need for a more detailed design, you should hire a local geotechnical engineer.

 

[oldestguy]

cvg ya got it right.

Let's take the case of providing a cut off drain for a failing slope. You want to set the pipe as deep as possible, say 15 feet. No way should any one go in that trench. So you dig the trench, follow behind the machine by rolling in the plastic slotted pipe and dump in that sand quickly. It works and is safe.