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Wraping gravel with geotextile in drainage 3

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MiddleCE

Civil/Environmental
Aug 28, 2008
10
I have a site that needs an interceptor drain to remove subsurface water from the soil above a proposed septic system leach field. This is to be installed upslope of the proposed leach field.
The soil is sandy loam over weathered sandstone. (About 4’ depth to the weathered sandstone)
Should a fabric be used to wrap the gravel in an attempt to keep fines out of the gravel? Could this plug up with fines and then prevent water movement into the gravel and pipe?
The plan had a filter fabric wrapping the gravel but the contractor did not install the fabric. Should this be replaced to match the plan?
3/4" clean drain gravel is used in the trench.
Thanks you for your comments.
 
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The contractor should follow the plan. There has to be a separator material. I see your concern with the fabric clogging, which is legitimate, but there is also concern with the stone clogging with fines. Based on the soil properties there is a way of choosing the right geotextile. The drain might work in the short-term, but I always think several years down-the-road over the life of the structure. By that time the contractor might be gone, and it would be someone else' responsibility.
Get the contractor to follow the plan.
 
Often use clean granular material wrapped in geotextile for drainage. It works well.
 
AcornOnSite - hoping the OP is from the infamous ACORN!! [bigsmile]; AcornOnSite is stuck with a dilemma that happens all the time - there is a design, the contractor gives the infamous salute and does what he wants to do - and the engineer is left holding the bag. It is quite easy to say - remove it and do it right. Politics, schedule and other associated factors will weigh in. I don't know how much "seepage" gradient is there; maybe not enough to move the soil particles into the soil, then again . . . If you want to play hardball, tell him to remove it and do it right or you will hold his money until the "proof in the pudding" occurs. If he wants his money, he'll redo it. If he tries legal means - you have a contract that says he is to do it . . . Put out a non-compliance (or non-conformance) on him and leave it up to the accountants and lawyers.
 
Not to throw too much dust in the air, but to voice my opinion.

In some 50 plus yeas of experience I have been involved with many a drain system. I did my grad research on highway sub-drains and learned then the beauty if using washed sand suitable for concrete (concrete sand) as the drainage medium and filter. Inside that trench fill of concrete snd sits a drainage pipe, these days slotted Plastic.

Never has one of these drains failed.

On the other hand I have seen engineers and contractors expounding the use of a filter fabric and gravel. I can't say that I have seen every one of these fail, but I have seen some that failed, generally I believe due to improper installation. It is difficult to do the job right, especially with no supervision on site and that there usually is a rush to get the job done. You already found that out.

If you look at the details in the specs for say a non woven fabric, the opening size still is larger than the "opening size" of concrete sand meeting the ASTM C-33 spec for fine aggregate. So, while you may have some transfer of fines via the fabric, that is not so with the concrete sand.

Finally, it is very difficult to improperly install a drain system done as I describe. Can't say the same for a gravel wrapped gravel system.

 
og... and in my limited years of experience, I've never had a problem with a graded trench using a granular material wrapped in geotextile. They work well.

Dik
 
oldestguy expouses, correctly, Charles Ripley's comments that concrete sand is a great filter material to fine-grained soils (Charles Ripley was the founder of Ripley-Klohn, then Kloen-Leonff, then . . . don't know anymore).
 
for mission critical subdrain applications such as dams and levees, we generally always use sand and rarely (almost never) use fabric. Sometimes we need to adjust the grading of the locally obtained C33 to get a good filter match with surrounding soil. And for increased flow we often need to use a second layer with uniformly graded gravel.
 
I won't repeat some of my info that is on several earlier posts on this subject, but I tried a Google search for "Corps of Engineer's research on concrete sand filter" and low and behold it brought up several of my old posts in other threads. I'm not a preacher, but I feel like one when I have to repeat this info.

One of the members in an old thread made reference to a book by Harry Cedergren which deals with filter design. I've used an old NAFAC manual with good info on filter criteria and design. That is available on the Internet.
 
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