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My french drain in progress: will it work?

My french drain in progress: will it work?

(OP)
After above average rain last winter the site of my future home seeped for months. The site is carved into a hillside of highly decomposed limestone/shale/mudstone/etc. Basically a yellow/gray clay bed with caliche/adobe layers above.

The seep stopped this summer and I am now trying to beat the fall rains with a plan of attack.

First: Dig a trench along the cut all the way to competent stone and bring the trench to daylight on both ends.


Second: Sand was put into the trench to level the bottom and allow for a 1/8" per foot drop out both ends. Sand depth varied from 6 to 12". A 4" perferated drain pipe was centered along the bottom of the trench and the sand was flooded to aid in compaction. sand was placed along the sides of the pipe and compacted with a size 12 boot.


Third: 6" sdr-35 will be placed further up in the trench for surface drainage with a slope of 1/4" per foot. Two separate pipes, one flowing each way.


I am between steps 2 and 3 now and have stopped for football.

As the thread title asks: will it work?

Barasingha

RE: My french drain in progress: will it work?

You have a good start. Backfill with permeable material, and put a couple of clean outs at the3 high point, one to each direction.

Mike McCann, PE, SE (WA)


RE: My french drain in progress: will it work?

Wow someone doing it right. The sand filter is something many don't understand and, instead use open graded rock, figuring those open voids will carry water. As you have figured out those open graded stone "drains' also carry mud. The only question I have is what hole size for pipe perforations? Too large and maybe loss of too much sand into the pipe. However a coarse graded sand, such as concrete fine aggregate will bridge pretty well. ASTM-C-33

RE: My french drain in progress: will it work?

I usually suggest either a filter fabric 'sock' around the perforated pipe (assuming the perfs are large enough to allow sand entry) or solid pipe is slotted with a saw. Personally, I prefer using a hack saw, placing the slots on 3 sides of the pipe.
"compacted with a size 12 boot" - knobby or smooth sole?
Be sure to compact your foundation backfill, with special care between the foundation and your drain.

RE: My french drain in progress: will it work?

(OP)

Quote (msquared48)


Backfill with permeable material, and put a couple of clean outs at the3 high point, one to each direction.

The complete trench will be filled to the surface with the sand. The high point is at a 90^ turn in the drain--the only hard turn--and I wanted to have a "cross" fitting with a clean out each way but the supply house did not have a 4" cross so clean outs were omitted.

Quote (oldestguy)


The only question I have is what hole size for pipe perforations? Too large and maybe loss of too much sand into the pipe. However a coarse graded sand, such as concrete fine aggregate will bridge pretty well. ASTM-C-33

The perforations are 1/2" in size and all sand is sourced from a river and meets ASTM C-33 spec. The perfs are located at 7 and 5 O'Clock. Why should hole size matter if they're at the bottom of the pipe? I feel like even a half pipe would work well as long as the wall thickness is supported by the fill without settling or washing out from moving water. If I assume the sand pours into the pipe like an hour glass wouldn't the pipe still be empty above a line from 5 to 7 O'clock? The water can only enter the pipe as fast it can filter through the sand X the area of the holes; with or without sand in the bottom of the pipe. These conclusions/presumptions are based on nothing but the countless hours I spent thinking while shoveling sand so please correct me if I'm wrong.

Quote (emmgjld)


knobby or smooth sole?

Compaction was accomplished with a smooth sole. While not as good as a sheep's foot boot, smooth soles keep dirt out of the house and the wife off my back.

Thanks for the support so far. I planned it out and feel good about it but after buying and burying the pipe with expensive sand, I couldn't keep little whispers of doubt from questioning the plan--probably also the result of too much shoveling, uh... thinking, no.. shoveling. Thus, my post here for votes of nodding or head shaking,
Barasingha

RE: My french drain in progress: will it work?

Years ago Armco Steel sold under drain pipe with holes in the lower quarter. These were 5/16" as I recall. I ran a two year research study (sponsored by Armco) for my Master's using that pipe at five highway water problem sites. I never noticed any sand in the pipes. Since then when "working", for house footing drains and jobs like this one, the slotted pipe was used, slots all the way around and as early jobs with Armco pipes. Minor sand loss noted. Of the filtered with sand under-drains I know about (since about 1958), I don't recall any failing or that the clean outs were ever needed. I know of none with 1/2" holes, but that is not something I'd get worried about.

For the 1/2 inch holes as noted and for seepage flows from something other than clean gravel spring-fed seams, I suspect no problem.

RE: My french drain in progress: will it work?

(OP)
Oldestguy:

I have unsuccessfully tried to find the study about concrete sand being a good filter by the Corp of Engineers in 30's. Have any leads for me?

Barasingha

RE: My french drain in progress: will it work?

(OP)
emmgjld:

To get serious about compaction for a minute, I would like to explain myself.

I used this bulletin as my guide: http://www.jmeagle.com/pdfs/Technical%20Bulletins/...

My calculations resulted in a 6.5-8.5% deflection at the deepest point of my trench (7ft), a highway live load rating and no compaction of fill. Because it states that 7.5% deflection is the recommended limit for drain pipes, I determined my boot would keep me below the threshold.

Would you be so kind as to explain your comment/concern about compaction of fill between the foundation and trench?

Barasingha

RE: My french drain in progress: will it work?

Would you be so kind as to explain your comment/concern about compaction of fill between the foundation and trench?

To provide a 'semi-dam' between the water source & the foundation. Assuming the drain works, for the life of the foundation, the issue may be moot. If anything goes wrong, you have created a simple barrier as a backup.
Actually, If possible, I prefer constructing drains as the last resort & hope they never see water. Part of that is problems in a semi-arid to arid environment.

RE: My french drain in progress: will it work?

Does it have positive drainage?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: My french drain in progress: will it work?

OG here:

Back then I had a copy of the Armco manual and it probably was about the vintage of this one. In there the quote of the Corps of Engineer study explained the results.

It has been some years, but hopefully this answers the question.

http://www.worldcat.org/title/handbook-of-drainage...

RE: My french drain in progress: will it work?

(OP)

Quote (beej67)


Does it have positive drainage?

The site is dual sloped, down hill and parallel to the hill. The perf pipes are sloped for their entire length 1/8" per foot. The 6" drain tiles are sloped 1/4" per foot along their entire length. There are two exits to the trench and a single high point located at a 90^ turn in the trench. The 4" perf pipe is continuous--exits both ends--with a high point located at the 90^ turn. The 6" pipe is discontinuous; one 6" pipe runs along the future home site for roof drainage and another runs along the future garage site for the same purpose, they each drain out of their own end of the trench. At the ends of the trench the perf and 6" pipe exit to daylight at the same level but the 6" rises more upon entering the trench. I have concrete pipe that will be used at the daylight exits to prevent sun rot and traffic damage.

Quote (emmgjld)


To provide a 'semi-dam' between the water source & the foundation.

Thanks for the explanation. I plan to use the undisturbed clay bed to provide the semi-dam. The foundation excavation will vary from 5-15' away from the trench. I will take care to compact around conduits penetrating the semi-dam, although they will be much shallower than the trench. I plan to install another 4" perf around the foundation with another 6" drain tile for shedding the other half of the roofs water, these pipes exit straight off the hill (the other side of the house from the trench).

Oldestguy:
Thank you for the resources.

Barasingha

RE: My french drain in progress: will it work?

OG, I was reading some papers about filters in dams (Terzaghi-1940 and Sherard-1984). Maybe filter criteria for earth dams can be applicable for these cases?

RE: My french drain in progress: will it work?

I perhaps didn't so state, but here is my view and that of many. Concrete sand (ASTM-C33)is a great all-'round filter for most soils. When it comes to filtering cohesive clays it may not quite fit the general rules for filters. However, that material does not erode noticeably. My experience shows no problems in these cases with concrete sand as the filter. On the contrary, I have seen many failures where open graded gravel is the "filter". The worst ones are where that fails as perimeter footing drains the first year after construction.

RE: My french drain in progress: will it work?

I just ran across I had scanned for another purpose. Comes from an old version of NAVAC DM-7

The basic rules show up at the bottom.


RE: My french drain in progress: will it work?

(OP)

Quote (oldestguy)


The basic rules show up at the bottom.

Does this mean that concrete sand is good for perforations up to 2" in size?

Update: I have both pipes buried and most of the trench filled. I am out of sand and have 26 more tons on the way. I have used 195 tons so far.

Barasingha

RE: My french drain in progress: will it work?

OG, good reference. Thanks.

RE: My french drain in progress: will it work?

You are looking at particle size plots and a graded material with its 15 percent size at about 1/4". Thus the void spaces are likely about 1/5th of the 15 percent grain size.

One common way to save money with filters is to use the most correct grain sizes near the perforated pipe and then fill the rest of the trench with a clean sand, which is more permeable than the nearby trench walls. However, top off the backfill with clays to keep the surface water out of the trench. No point in collecting that if you can make it run off.

RE: My french drain in progress: will it work?

(OP)
I was looking at 'General requirements' number 3, the "basic rules" you pointed out at the bottom of your picture.

'3. To avoid movement of filter in drain pipe perforations or joints: D85 F/2 > 1'

The chart shows D85 of concrete sand to be 2 (looks like 2mm). According to the calculation the perforations need to be 2mm or smaller, but this is such a small hole (~1/16") that I am assuming the resulting units are in inches. Inches are the units of reference for "basic rule" #4.

Barasingha

RE: My french drain in progress: will it work?

I spent a few bucks to get a copy of the old Armco Handbook of Drainage and Construction Products. Years ago I received on gratis from them, but gave it away since. Anyhow there are three pages dealing with subdrains that I scanned, not the best, but perhaps of interest. I wanted to copy the info from the Corps of Engineer studies, shown in figure 210 showing quite a range of suitable filters.
The scanning missed the binding sections, but probably readable anyway.




Note the Armco pipe had 3/8" openings, so 1/2" is not all that far off.


RE: My french drain in progress: will it work?

Quote (barasingha)

The chart shows D85 of concrete sand to be 2 (looks like 2mm). According to the calculation the perforations need to be 2mm or smaller, but this is such a small hole (~1/16") that I am assuming the resulting units are in inches. Inches are the units of reference for "basic rule" #4.

I noted that also. The chart in the right top of that graph also indicates that the average D85 for concrete sand is 2mm, so I agree with barasingha that the holes will be very small and may not be commercially available. In that case it will be best to install clean gravel or a filter/geotextile around/close the pipe and then clean sand?

Quote (oldestguy)

One common way to save money with filters is to use the most correct grain sizes near the perforated pipe and then fill the rest of the trench with a clean sand, which is more permeable than the nearby trench walls. However, top off the backfill with clays to keep the surface water out of the trench. No point in collecting that if you can make it run off.

So, I think that OG suggestion may be the best approach.

RE: My french drain in progress: will it work?

(OP)
Too late for smaller holes, fabric, multiple filter materials, etc. The 4" pipe with 1/2" holes at 7 & 5 o'clock is 7' deep under ASTM C-33 concrete sand. It was already covered with sand before my original post. There is a more expensive 6" pipe above it and I will not be digging this out unless/until a failure.

Best case failure: the sand fills the perf pipe but the drain works through the sand filled trench alone.
Worst case failure: the sand continuously runs out of the ends of the pipe and I end up with piping around the 6" drain line and sink holes along one side of my yard.

Why wouldn't a half pipe work? (If it is assumed that the sides of the pipe cannot sink into the sand.) Trying to understand why holes on the bottom of a pipe are problematic with a sand or gravel fill material.

Oldestguy: Very interesting that holes up was used interchangeably with a solid pipe as a conductor in the information you posted. Makes sense from a fluids standpoint but not from a filter material/plant matter point of view.

barasingha

RE: My french drain in progress: will it work?

OG again. Hopefully the holes up item probably was a mistake. I would not recommend that. I've seen the Armco pipes carry a lot of water and a slight bit of sand. However, so even a small fraction of the open area of the pipe is pretty good, since that slow flow over a long period is a lot of water. Most of the drains I have observed carry only a little in the bottom (invert). These days most of the drains are corrugated plastic with slots and sometimes covered with a fabric sock. Very little sand comes along. I doubt that the completed installation will fail.

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