Is oldestguy's concrete sand best for my french drain?
Is oldestguy's concrete sand best for my french drain?
(OP)
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
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





RE: Is oldestguy's concrete sand best for my french drain?
RE: Is oldestguy's concrete sand best for my french drain?
RE: Is oldestguy's concrete sand best for my french drain?
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?
RE: Is oldestguy's concrete sand best for my french drain?
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.
RE: Is oldestguy's concrete sand best for my french drain?
http://web.mst.edu/~rogersda/umrcourses/ge441/dm7_...
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.