French Drain question 1

Assuming you are correct that the natural soil is clay. Even if it is not 100 percent clay and has a lot of silt in it, I have a question of you. These soils normally will be finer than the voids in the so called crushed rock. Thus there, is nothing in either plan that will keep the fines from entering the voids of the crushed rock and eventually plugging it, right? Placing a filter fabric around the perforated pipe will keep the fines from going farther, but that sort of a construction has been known to plug up by fines building up on the fabric at the openings in the pipe. I've seen that occur in one winter here in Wisconsin.

You can solve all these questions by specifying that all backfill around the pipe and maybe up a foot or so should be ASTM C-33 Fine aggregate (not coarse aggregate). This is commonly the fine aggregate used in concrete, sometimes called Torpedo sand in my area. Depending on the hole or slot sizes in the pipe, here may or may not be the need for a sock on the pipe. Concrete sand is a perfect all-around filter. This was proven in the 1930's by an extensive study by the US Corps of Engineers.

To be sure these well meaning, but ignorant in these things, contractors from still using crushed rock, specify that no crushed rock, single sized rock, open graded rock is allowed on the property.

I've been preaching this treatment of under drains ever since I received my Masters Degree studying these forms of drains in highway applications in 1956. Unfortunately for years folks have figured that rock with big voids will drain fast, but they forget about all the stuff that fills those voids. I've never had one of my installations plug up!!

If you want more proof look up US Navy NAFAC manuals on the internet,probably in DM-7. In my old version Figure 8-8 shows this subject deadline with filters of various types.

After doing this backfill properly it hardly makes any difference which slope, and other treatments, since you will prevent plugging of the system.

You may hear from guys who surround all that crushed rock with filter fabric. It works, but is labor intensive to do it right, especially in caving ground and is more costly than my simple method that really can't be done wrong.

Out of curiosity where does the drain daylight to? I have seen contractors do some interesting things.

B+W Engineering and Design | Los Angeles Civil Engineer and Structural Engineer

Never understood the need to slope a perforated pipe for drainage. I'd use separation geotextile against the, "Clay." I'd use open-graded aggregate to surround the pipe. Beyond the french drain, I'd go to a slopeing tight line to daylight.

f-d

ípapß gordo ainÆt no madre flaca!