Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

French Drain question

French Drain question

French Drain question

I live in California where the soil is mostly clay. I'm having a French drain put around the house to keep the water out. Got two bids...with each contractor using a different technique.

Both are recommending to dig 12"-16" trench, 6" below the foundation, put in a 4" perforated PVC pipe, place a 10mil plastic liner on the foundation side, fill trench with crushed rock. The differences are:

Contractor #1 will do the above and also wrap the gravel with a geo Fabric. Slope of the pipe will be 1/8" for every foot. (pretty standard)

Contractor #2 will do the above but does not want to wrap the gravel or pipe with geo fabric. He will slope the pipe 1/4" for every foot and have several clean out along the system.

Contractor #2s reasoning is that the geo fabric will eventually clog up and reduce water flow and eventually fail. He allows some of the dirt to flow into the pipe and the steeper slope helps the dirt to flow out. Plus he recommends flushing the pipe with a garden hose every couple of years to wash way any soil build up.

Anyone have any experience of not using fabric to wrap the gravel? Has anyone experienced geo fabric clogging with Clay soil?



PS- I read some of the other informative threads that recommend using sand vs rock as the filler.

RE: French Drain question

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.

RE: French Drain question

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 http://bwengr.com

RE: French Drain question

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.


ípapß gordo ainÆt no madre flaca!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close