My french drain in progress: will it work? 2

[barasingha]

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

 

[barasingha]

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

 

[oldestguy]

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.

 

[Okiryu]

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?

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.

 

[barasingha]

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

 

[oldestguy]

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.