Quick question about filter fabric at footings 1

[xp1]

Should filter fabric be allowed to extend underneath the footing as shown in the first image (pink)? I feel like I would want to prevent that for some reason. So the second and third images show my own guesses. Although I'm not sure why; bearing pressure is what's important for a wall footing right, not so much shear friction on the underside or anything like that?

Structural Engineering Intern

 

[xp1]

This website seems to confirm my thoughts, all the filter fabrics are shown surrounding pipes and gravel but not being placed below footings: [URL unfurl="true"]http://www.apexhomeinspections.biz/articles/foundation_drainage.pdf[/url] anybody know why this is the case though?

Structural Engineering Intern

 

[BAretired]

There is no such thing as a quick question.

I don't agree with any of the three sections. Filter fabric belongs on the outside face of the wall and over the weeping tile.

What is the purpose in placing filter fabric between the granular fill and the natural soil on the right or under the footing or under the weeping tile?

BA

 

[xp1]

So in the original images, you would only agree with fabric being placed where the light green line is?

I'm not sure what the purpose is but the website I provided shows fabric all around the granular fill, even between the fill and natural soil. So you wouldn't agree with these images?

I've never seen these before and just started doing research because of an RFI.

Structural Engineering Intern

 

[BAretired]

I would place filter fabric on the exterior face of the wall and on top of the footing, extending it over the drainage tile to the natural grade beyond. See attached thick red line on upper section.

BA

 

[xp1]

Ah, I just read somewhere that the filter fabric should surround the gravel because otherwise water will mix with the soil and fill the spaces between the gravel. This makes sense right?

Structural Engineering Intern

 

[tolchijb]

If you are in an area where the natural soil is fairly permeable (sandy soils) I probably would not worry too much about separating the free draining fill from the native soils. However, where I am located the native soils are clayey and almost impermeable. If the free draining material is not separated from the native soil, the free draining material will become clogged by the fine clay and silt particles and the drain will no longer do its job, which is to mitigate hydrostatic pressure from building up behind the wall.

 

[tolchijb]

BA Retired,

Why would you place the filter fabric on the exterior face of wall?

 

[jdgengineer]

As the structural engineer, I would avoid showing too much about the filter fabric (personally) and would say waterproofing / drainage by others. With that said, it's my understanding that yes the filter fabric surrounds the gravel with the intent of keeping that drainage layer in tact. Typically, the geotechnical report would show a detail for how they would like the fabric installed (at least with geotechs we work with).

 

[BAretired]

@tolchijb,

If the virgin soil is relatively impervious clay, the majority of surface runoff should be removed by proper grading of the site. The small amount of weep water which finds its way through fissures in the soil to the wall should be led directly to the drainage tile using filter fabric.

If the virgin soil is a well draining granular material, I wouldn't use filter fabric at all.

BA

 

[oldestguy]

These pretty drawings look great. However, think about how things can get goofed up. For the example of setting the edge of the footing or slab on the fabric as an apparent anchorage point and think about site conditions. Rain, mud, guys slogging back and forth and forms. A nicely laid fabric sure has to stand a bunch of abuse before it is left maybe where it is supposed to be, maybe. Granular backfill is not placed by hand. Instead lazy operators dump it in piles, no neat separating planes like the drawing shows. With people in a hurry to backfill, who knows what is left down there. I won't preach my methods here. You can find them elsewhere, but the aim is to avoid situations that easily can be done wrong.

 

[thaidavid40]

In our area of the Tennessee Valley, we find that most existing subdrains of any real age (say, 15 years and up) get plugged with migration of soil fines into the drainage fill and/or pipes, as this area has a wide presence of silts and clays. For that reason, we pretty-much universally specify filter fabrics for use with subdrain fills and pipes. We would typically use a detail like your middle detail, though many will take the extra step of running the fabrick up the short face of the footing, and wrapping it over the top of the drain pipe fill, thus "boxing it in", with a full wrap of fabric all around. Off the top of my head I can't think of a reason why running it under the footing should cause any problems, though I wonder why you would want to. That just costs extra project money for fabric that isn't doing anything for you.

Thaidavid

 

[xp1]

Thanks for all the great answers everybody, I appreciate the input. Going to ask them to revise the filter fabric placement and that it shouldn't be below the footing (as you say Thaidavid, this could be more for their benefit than ours,) and I'll let them take it from there.

Structural Engineering Intern