Is oldestguy's concrete sand best for my french drain? 1

[notfunny]

The area under my deck (8'X50', disassembled) is sloped 8" over 8' away from the house and toward a drain (as pictured). I have a heavy plastic (30 mil) which will cover the area under the deck and terminate over the drain, so fines will not migrate into the drain or fabric. I have the drain set up with 4" PVC (with holes) surrounded by gravel and fabric, but before I rebuild the deck I thought I might change the drain aggregate to concrete sand and no fabric.

I have seen oldestguy say the following...
You can't beat using ASTM C-33 fine aggregation (concrete sand) as a filter and drainage medium and you can't do it wrong. Can't say the same for gravel and filter fabric....The beauty of the sand fill is it is a good filter, you need no fabric....Leaving the top of a sand filled trench open won't hurt anything, but the top will be plugged in time with dirt. Of course gravel backfill will fill up with dirt unless that is surrounded all sides with a filter fabric - means more work and more cost.

Thank you in advance for any input

 

[oldestguy]

OG here. For the concrete sand usage, the holes in the pipe need to be small enough to prevent significant sand getting in. In the old days, with corrugated galavnized pipe, ARMCO steel made a pipe with 3/16" holes only in the lower 1/3. That worked well. Now with PVC, the slotted pipe will work about the same. If you drill the holes yourself, of course you can duplicate what Armco sold back in the 50's. What you have will work, but for a big installation and in trenches, it is almost impossible to get it right. My way is much easier. Cost of material is about the same, either way. If you can find an old handbook sold by Armco, it will detail this. It refers to a study by the Corps of Engineers, Vicksburg, MS, regarding the "universal filter" material.

 

[notfunny]

OG...what did you mean by, "but for a big installation and in trenches, it is almost impossible to get it right?" I have a sock on the pvc (which has holes on the lower third), will that clog or let sand through? Do you believe I would get better performance to remove the gravel and replace it with the sand? It seems like sand would also displace more water into the pipe, whereas water can occupy the spaces between the gravel and just sit there until it percolates downward. Also, I could merely add sand to the gravel and let it fill the spaces, or place a layer of sand over the gravel separated by fabric but not place fabric over the top of the sand to let water enter more freely.

 

[oldestguy]

For your very small installation what you have will work fine. What I mean by a large installation is where human access in limited for safety reason and where caving onto the pipe before it is properly protected is possible. Take a job that I was on in eastern Iowa a while back. On a hill side there was seepage in ground that was about 20 feet thick over bedrock, sloping toward a lake. On that hillside sat a house with slippage was going on because of the shallow water situation. To drain that water before it got to the sliding slope, a trench located up slope and some 15 feet deep was dug into which there would be a pipe and backfill. In seeping ground and trenching you have to work quickly to avoid cave in, etc. No way do you go in that trench. Here the backhoe started at the discharge end and wrinkled plastic pipe was laid from a coil and backfilled immediately with the concrete sand before any cave in took place. The job was done gradually working across the slope. Now show me how your wrapped gravel and piped system would work then.

As to pipe enclosed in a sock and then gravel or sand, the sock will hold the sand back. Your discussion about void volume differences is not important, since all the voids are saturated and roughly similar. If you want to mix sand with gravel, that is OK, since it is the fine fraction of that mix that does the filtering of mud. One fallacy of using just a sock and no special backflll is the fact the flow path of water is only through the sock at the holes in the pipe. You want that interface between filter and outside mud area, per foot of pipe to be as large as possible. A common problem with wrapped drainage pipe and no special filter backfill (such as gravel), the mud comes to the sock at the pipe slots or holes and stops there. That plugs that section of pipe and the drainage there stops. Many a sock covered pipe system with no added filter, such as a zone of sand, fails because of that (typically footing drains).

Your system works fine if it done properly. but the labor and care involved is much more a risk than my method. With mine, you lay the pipe and backfill with the filter material and you don't have to lay a hand on anything there. Foolproof. So you get a little sand in the pipe before it bridges over the openings. So what?

 

[notfunny]

OG,

You are helping me along...thank you for your patience.

Since I will be rebuilding the deck over this area and it will not be accessible to maintain, though I do have a pipe clean out port on one end, I want this drain to last and be trouble free.

I could not go too deep with the trench without compromising the footing of the retaining wall, so as I am picturing this project, I could remove the pipe and line it underneath with sand or I could merely remove some/most of the gravel from the top and sides, leaving enough gravel under the pipe to maintain the slope. If I leave the bottom inch under the pipe gravel, I will add sand up the sides of the pipe and 4" on top of the pipe. I am envisioning that I would just pour the sand on top of the gravel with no fabric separating the two aggregates. I also would not cover the sand with fabric. The drain mat would terminate over the center of the trench rather than running down the side of the trench, so the runoff would not stir up fines in the clay. I am inclined to leave the fabric under the pipe and on the sides of the trench as I would not mind if this clogs a bit and sends more water into the drain. Would you replace all gravel with sand? How is my use of fabric to your liking?

I have included a picture of the local sands the I have found available. From left, play sand is a no as it is round and does not interlock and I understand lime base sand is not good. The middle product is rated c-33, but seems very fine if not powdery. On the right is paver sand, which is very course. I have looked at each under 100X. Are you recommending the ASTM C-33 even though it seems silty? (package claims it is washed but under 100X it seems to have what look like salts attached to the jagged bits of rock).

Sincere thanks.

 

[oldestguy]

If you will look at page 271 of the NAVFAC Manual DM 7.1, you will see the gradation of the ASTM -33 filter. I'd copy this whole manual in PDF format, since it is a copy of a very useful soils manual.

http://web.mst.edu/~rogersda/umrcourses/ge441/dm7_01.pdf

Luckily the Vulcan folks have saved this which is no longer in print. In the next few pages you wuill see even a use of gravel and sand combination drain, but that is very difficult to practically construct.

I don't think that you need to gt too fancy here, since yur job is very small and very litttle water will be collected. Your fabric surrounded grfave and pipe shouldwork fine as long as it is carefully done.

As to your samples, the right hand one may be OK, but without really sieving the samples I can't say much. If the sources say their sand meets ASTM C-33 fine aggregate, that should be sufficient. It runs between 3/8"-#4 sieve and just above the #200 sieve.

The figures in that part of the publication have the general rules for particle sizes for graded filters that can be applied to many situations, such as drainage from within earth dams to pavement base drainage.