Modified French Drain to reduce slope erosion

[fpbear]

I am building a French Drain running parallel and 10 feet from my home, on a gentle slope away from the foundation. Another 5 feet downslope there is a retaining wall and a small but steep hill. The retaining wall has some cracks forming over the years, which worsened during the heavy El Nino rainfall season. As a temporary measure (until I can afford a more permanent retaining wall reconstruction) I am hoping to reduce some of the pressure on the wall using this trench to draw subsurface water 100 feet away and then down into a concrete canal.

When I finish digging the trench will run from 12" to 24" deep, 7" wide. I have 4-inch pipe with drain holes and filter sock.

I planned to use concrete sand but did not find this at the store. Instead I purchased #16 silica sand. Is this a good material for the back fill? The soil is clay.

Would it be a good idea to line the bottom and downslope side of the trench with a material that slows water percolation toward the retaining wall side? If so would it be better to use plastic, landscape fabric, etc? The upslope side of the trench would be left uncovered to catch subsurface water coming from this direction.

Thanks ~ Dan

 

[emmgjld]

Several threads on this site go into drain design and construction. My typical recommendations for a shallow, typical 'barrier drain' include;
---Sand backfill to within 6 inches of the ground surface and then the normal 'topsoil'. A filter fabric between the sand and the topsoil is usually appropriate. Do not allow surface water to enter the drain.
---Do not attempt to carry surface water in the subsurface drain. Overloading and plugging of the sand will occur. Make an additional surface drain.
---Line the downhill side of the drain with 6 mil plastic, maybe even double layered, and continue the plastic down across the bottom and up the uphill side to at least the mid height of the drain pipe.
---Be sure the plastic is either continuous or properly 'shingled' at the overlaps. Be sure to contain any water in the drain.
---Use an appropriate size pipe. I find that a 2" pipe is often more appropriate for low water flows. Do you really expect to see much water discharging from the 4" pipe? If so, you need a real design to account for this much flow.
---If a smaller diameter pipe is used, slot the pipe with a hacksaw, cutting the slots at an angle to the pipe axis, at about 6" spacing, rotating the pipe. Don't cut the slots too deep, just enough to make a positive penetration.
---The filter sock is probably good but, may be optional if the slots are not too wide. If the water flows are high, plugging could occur.
---Place the pipe in the bottom of the carefully prepared and graded trench, on top of the plastic. As the sand is placed, the pipe will be surrounded by the sand filter/collector.
---Make sure the outlet is good, positive and working. Often some coarse screening is placed to keep the varmints out.

 

[fpbear]

Good tips emgjld, thank you. It would be ideal if I could use this same drain to also capture surface water runoff, as I don't have the resources to build a second surface drain. The clay soil is compacted pretty well so a lot of the water will just run along the top in heavy rainstorms. I thought the sand would not plug (wouldn't a landscape fabric plug just as easily)? I would be interested to learn more how this plugging occurs and if there is any way to catch some surface water with the existing drain while avoid plugging.

So far I couldn't find discussions of the use of silica sand in these types of drains. Grain 16 is coarse, just small enough so it is considered sand rather than gravel. Silica sand is used in water filtration devices so I figure it has good water permeability and would work well for a drain.

Thanks ~ Dan

 

[emmgjld]

Surface water should be captured and removed with a small ditch or swale. There have been times I have used a pipe within a swale, covered with filter fabric and landscaping gravel. This works but requires inspection and periodic cleaning. The 'mud' in the runoff collects in the gravel and plugs the fabric. If the pipe and fabric are at the ground surface, cleaning or replacing the fabric is not too difficult but, must be accomplished.

The use of a coarser sand may be acceptable. Remember the larger pore sizes can collect a lot of mud or silt, requiring cleaning or replacement. Good permeability is important but it is also important that high groundwater velocities be avoided as more silt and clay is brought into the sand filter/collector, causing plugging.

 

[blueoak]

Do you need to backfill? A V ditch wouldn't be much wider than 7 to 8' at 2:1 slopes (1 or 1.5:1 often flatten after awhile). This may be cheaper and more temporary until you fix the wall. You get surface and your 24" subsurface too.

With clay soils though is it a groundwater table more than storm influenced? You may easily overload the pipe capacity and only slightly decrease the pressure on the wall. Since this is your property I might dig the trench and see what happens. We would do this for sizing pumps as a phased project.
As for sand filters and plugging see:

http://www.info.usda.gov/CED/ftp/CED/neh633-ch26.pdf

The Corps has some very good information as well as books by Harry Cedergren (Seepage Drainage and Flow Nets).

 

[fpbear]

Thanks blueoak, the USDA handbook has interesting charts corresponding to the grain size. I'll take a look at these charts for #16 grain size with fine clay soil. The reason for a backfill rather than a V ditch or swale is because this is the back yard lawn with frequent use.

During an El Nino winter storm the water pools on the surface. I think it slowly percolates into the ground and then puts pressure on the lower parts of the retaining wall. The clay soil is faily compact so this could occur in a matter of hours. If I can prevent much of the upslope surface water from percolating into the soil I guess I can reduce the pressure on the wall.

 

[blueoak]

From my ag days, a typically cheap temporary solution is also a mole drain. Normally lasts 2-3 years depending on your site. You can run a few up the slope to provide extra drainage.

http://www.tgdrains.com.au/page4.htm

With a french drain I would only be worried that you will increase inflitration near your wall, unless you do good surface drainage also.

 

[fpbear]

"With a french drain I would only be worried that you will increase inflitration near your wall."

Good point.. I thought about this as well, and that is the reason for the modification to create a barrier drain, as emmgjd describes:

"---Line the downhill side of the drain with 6 mil plastic, maybe even double layered, and continue the plastic down across the bottom and up the uphill side to at least the mid height of the drain pipe."