Messages and Online Forum Q&A

Re: barn drainage - filter fabric

From: info@builderswebsource.com
Category: Drainage/Waterproofing
Remote Name: 63.198.181.43
Date: 20 Apr 2002
Time: 10:53 AM

Comments

We apologize for the apparent conflict in advice between our earlier BuildersTALK reply and our drainage technote, Principles of Residential Drainage and Basement Waterproofing. However, either approach is suitable depending on your conditions. If you decide to use coarse washed sand as your backfill, or if your existing soils are sandy or silty, we recommend a pipe sock to prevent the sand from clogging up the holes in the French drain. Otherwise, the sand may just wash through the pipe, undermining your drainage trench over time.

However, if your soil is more clayey, a geotextile filter fabric should be used to line the trench, backfilling with 3/8" pea gravel or Type 6A stone (typically 1/2" - 7/8" in diameter). Sometimes, both a pipe sock and a trench liner (filter fabric) can be used in cases where there are extreme problems with varying types of soils or backfills. There are several manufacturers of filter fabrics. One reputable brand specified in many civil engineering projects is Amoco (see http://www.geotextile.com for more information). You can use a product such as Amoco 4545 or 4552, nonwoven polypropylene geotextile fabric. There are other choices as well -- each optimized for various flow rates and tensile strengths.

The aforementioned website also has valuable technical information regarding the use of geotextiles for a range of applications including separation barriers, landscaping and drainage. Be sure that your French drain slopes consistently at least 2" per 10 feet (about a 2% slope). This will help to ensure that water will flow towards the outlet, carrying along with it any fine particles that may enter the French drain. In areas where trees are present near the foundation, the use of solid pipe is recommended so as to avoid roots from penetrating into the pipe. Keep these solid pipe areas as short as possible since the holes are necessary to pick up subsurface water. We hope this clarifies our original response.

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