Can Filter fabric be clogged by surrounding fill comprised of CSA concrete sand? 2

[ajk1]

It is proposed to remove an area (about 10 feet x 35 feet plan area) of loose wet sand under an old existing 5" thick slab on grade floor of a below grade parking garage. The clay tile weepers run through this area so will replace a 35 foot length, the existing wet sand removed to a depth of 3 feet and replaced with 0.7 MPa controlled density cementitious fill to about 2" below the invert of the new weeper and then a CSA concrete sand placed to about 3" below the slab on grade, than a new slab on grade place. The traditional way of laying clay tile weepers is to leave 1/4" wide open joints between each tile (and that is how the old weepers here were laid in 1957).

QUESTION:
If we place a filter fabric around each open joint between clay weeper tiles, is there any danger that the new CSA concrete sand will clog the filter fabric? Since the CSA concrete sand has no fines, it is thought that the filter fabric will not be clogged by it, but just looking here for a confirmation.

 

[oldestguy]

That sounds correct. I'd instead of using the clay tile again (They might be misaligned during fill placement), I'd use a 4" D. corrugated plastic pipe slotted instead.
With a coarse graded concrete sand very little sand is lost into the pipe, due to bridging the slots by the coarser parts. If you are worried about that, these pipes also come with a filter sock over the pipe. This kind of drain pipe saves a lot of labor time and is pretty fool-proof.

Any danger of clogging is pretty well avoided if the fill itself is a good filter (which concrete sand is).

Why dig out 3 feet anyway?

 

[Ron]

Agree with OG with one exception....I'm not a fan of corrugated underdrain pipe. I prefer rigid wall PVC with slots or holes, socked with filter fabric. It is much easier to maintain slopes to drain and the fittings are rigid and attachable to inlet stub-outs or other connections. Yes, there are fittings for corrugated pipe; however, you will have to drag the contractor (kicking and screaming) to the supplier and make him buy them and install them .

I would not use individual clay drain tiles. They are serviceable; however, they are labor intensive to install, tough to maintain a slope line and relatively expensive as compared to PVC or ABS.

 

[ajk1]

To oldestguy - Thanks for your confirmation that the concrete sand will not clog filter fabric. Many other type "filters" get clogged and have to be changed (like oil filters or furnace filters), but clearly filter fabric around a drain pipe is a different thing. I wish they would rename it to not include the word "filter", but maybe there is no other appropriate word (permeable barrier? or is that an oxymoron?). Thanks, much appreciated.

To Ron - yes, as you suggest rigid PVC with holes and sock is likely what we will use, as that is what I see generally used in this area. Thanks. Much appreciated.

 

[oldestguy]

ajk: There is one place where "filter" fabric wrapped pipes do clog. The slotted corrugated plastic pipes are commonly used as footing drains to a sump and pump around a residence (mandated by some building codes). When just any old backfill soil is then placed and there is some water tending to get to that pipe, the filter catches the bulk of the fine fraction and there is build-up of that on the fabric right over the slots. I've seen these plug up and the system fail the first year after construction. My experience is that when concrete sand is the backfill surrounding the pipe (with slots or holes in the bottom), they do not fail.

If you wish to check out the filtering capability ASTM C-33 fine aggregate (concrete sand) the US Corps of Engineers published information on its use about 1937 from its Vicksburg, MS labs. It is a good all-purpose filter for most soil types.

 

[ajk1]

to Oldestguy - that is interesting information. Thank you.

 

[ajk1]

Would the attached be the right way to install the weeper (a rigid wall perforated PVC pipe)?
Can the controlled density cementitious fill come right to the bottom of the weeper, or should there be a few inches of CSA concrete sand under the bottom of the weeper?

 

[EngineeringEric]

Be sure to state the the holes in the pipe face down, not up.... or however it is supposed to be for your application. you'd be amazed...

 

[ajk1]

To Engineering Eric:
Right. I will add that to the drawing. Is it ok to bring the concrete sand to the underside of the perforated pipe, or shuld there be a few inches of the concrete sand below the pipe?

 

[oldestguy]

The drawing looks good, excepth at bottom of pipe there may be a problem on the earth there. The sand filter should be between any other earth and the pipe. Thus, perhaps leave a small "trench" into which a few inches of sand is placed, followed by the pipe. If your pipe is we rapped with filter cloth, you can have the full diameter of the pipe with holes, maybe 1/4" diam. The Corps of Engineers tests showed that (back then on filter cloth) holes in the pipe should be in the bottom half, more than 22 degrees below the horizontal axis. Armco Steel Co. pipes back then used 1/2" diameter holes, but when I did research with them on projects (1955) there was a little sand that got in the pipes, but no problem. I also found that when muddy water came to the concrete sand that the contact there was sharp, with not noticeable penetration of the "mud" into the sand.

 

[ajk1]

To oldestguy:

In my case the material immediately below the pipe will be a very low strength concrete (about 100 psi) that will be placed for about 2 feet below the pipe to replace the existing weak wet sandy material therethat has sttled; the pipe does not contact any soil under it. In that case, would I be correct in assuming that the no fines concrete sand fill need not be placed under the pipe,but only around it?

 

[Ron]

ajk1....I require 6 inches of filter media (in your case, sand) below the. Pipe and within the wrapped trench

 

[ajk1]

To Ron:

Ok, thanks.

 

[oldestguy]

My opinion is that with a weak concrete below no fines are expected therefrom, so Yes. Looks OK.

Guess my approach for drainage using perforated, slotted or wrapped pipe, generally in rough conditions that may occur is less stringent than Ron. I spec that you lay the pipe in on a sand layer, if you can (sometimes trenches are caving), but be sure you have the pipe surrounded with the filter sand. Sometimes the trench is filled most of the way with the sand, but at times you don't have that condition, but surrounding the pipe is mandatory. Since ASTM concrete sand many times is processed to get the correct gradation and thus is more x pensive than "bank-run" sand, so getting the filter around the pipe first is then followed by filling with less costly bank-run sand with low P-200 fraction.