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BigH (Geotechnical) (OP)
18 Dec 02 14:31
Hey all!  Am putting this out for your learned opinions.  I have mine, of course, but would like to see how others view the following.  We have had a lot of discussions on the site in the last while over this issue – some are strongly one way, others opposite.  Hope this piques some interest.

Scenario.  French drain installation in median of two lane each dual carriageway super-elevated section.  Approximately 1.8m deep, 600 wide, surrounded with geotextile.  We are installing a 150mm diameter perforated pipe within the French drain.  Perforations of 8mm diameter; 4 rows of holes all in same half section of pipe.  Infill is +10 to –20mm stone although contractor will ask soon to put in 20-40mm stone above the drainage pipe - availabiity of materials.  Pipe will tap into a manhole at 75m spacing which will also collect water from depressed median drain.  Then water goes out from manhole in a 200mm pvc pipe to daylight at embankment slope.  Opinions sought !!  Do you put the pipe perforations down, or up???  Up – reasons why!  Down – reasons why!  Look forward to some interesting dialogue.

Best regards.  And the best of the holiday seasons to all - somewhat belated to some, ahead for others!
doc11 (Geotechnical)
18 Dec 02 22:19
Ok, I'll bite on this one.  Why not rotate the pipe 90 degrees and place the holes on the side and alternate which side of the trench the holes are facing every 30 or so meters.  Otherwise I'd place the holes on the bottom, this will allow all the water to drain from the trench (providing it is properly sloped).
Helpful Member!  boo1 (Mechanical)
19 Dec 02 10:13
Drainage lines are often perforated and wrapped in geotextile or buried in a granular filter bed, and serve to carry water to the weepholes from areas deeper within the backfill.  Generally, the holes for French drains should be pointing down, or at least oriented in a downward direction. This is counter-intuitive, as people think this will cause the water to drain out of the pipe, rather than into it. However, in order for a French drain to function properly, you want hydrostatic pressure to force the water through the bottom of the pipe, which effectively lowers the water level in the trench to be consistent with the bottom of the pipe (known as the invert), as opposed to the top of the pipe.

Cheers
DRC1 (Civil/Environmental)
19 Dec 02 13:21
UP
You are carrying the water away. Invert should be beloww level desired to drain. Most black courragated pipe has drain holes in an array in the upper 270 degree quadrant.
However, when all else fails, check with the manufactuerer of the pipe.
Ron (Structural)
20 Dec 02 13:53
Assuming the groundwater mounding is above the top surface of the pipe, then placing the holes up will afford better drainage as you get help from gravity as the water goes through the hole, it will actually help to pull water from the interstitial soil spaces (voids)and into the pipe.  That's also assuming the pipe is not flowing full.

If your French drain is an interceptor and you want to limit the uptake of water from below, then the faster route would be with the holes down, but still requires hydraulic gradient to push water into the pipe.

In my humble opinion, having the holes on top is the better of the choices.
boo1 (Mechanical)
20 Dec 02 16:38
Ron, with the pipe perferation up only the area above the pipe holes will drain into pipe, until the flow level is the gravel bed is to the top of the pipe.  With the holes down the flow will fill the pipe quicker and drain faster.  A quick sketch of fluid levels in the pipe may get my point across.
Focht3 (Geotechnical)
20 Dec 02 20:16


This one makes me chuckle!  Good job, BigH!

I have two answers - one theoretical, the other practical.

Theoretical:
The holes are on the bottom - see boo1's excellent arguments.

Practical:
It doesn't really matter (unless BigH left out some critical concern.)  The 150 mm distance (about 6 inches) appears insignificant in the scale of the problem.  (Only about 8 percent of trench depth.)  I didn't run the calculations, but if the head in the trench between the manholes is above the perforated pipe, it will drain into the pipe either way.  You could get a slight increase in the phreatic surface at the mid-point, but I doubt that it is significant.  (This would only occur if the influx into the trench exceeded the perforated pipe's capacity.  Highly unlikely.)

A question for BigH:
Any problems with the holes clogging that might affect the choice of hole orientation?
BigH (Geotechnical) (OP)
20 Dec 02 20:25
This is the kind of discussion that I was looking for!!  Different engineers/different ideas - on something as 'simple' as hole orientation.  I will comment more later after we see if more put in their points of view.  For Focht3, I doubt that the holes will "clog" - the whole of the french drain is wrapped in suitable geotextile and the use of stone between 10 and 20mm dia size should preclude any stone getting through the 8mm perforation hole.  Might be some "dust" but this should be washed through pipe if water ever does get into the pipe!!  . . . oops.
Focht3 (Geotechnical)
23 Dec 02 13:18
Okay, the fabric will take care of the clogging.  (We have a lot of problems with clogging of old French drains - gravel backfills against clay soils.  It's now one of my "standard" concerns.)

Something gave me the impression that this was a "tempest in a teapot" question.  Please elaborate on the "if water ever does get into the pipe!!  . . . oops" comment at the end of your last posting.
Prudent (Structural)
2 Jan 03 10:39
I've had this 'footing drain' arguement over beer, with many of my engineering buddies, for a few years. We usually split 50/50 on the matter, depending on what side the other chap takes, or who's paying for the beer.  

However, on my home projects, with perimeter footing drains, I turn the drain holes upward, and than grab my chordless drill....and methodically punch some 3/8" holes at the mfr's req'ed spacing...about 1" above the bottom...on either side. I'm partial to believing that water runs downhill, and likes a little trough to travel in....even with the help of a hydraulic gradient/head.


Rjeffery (Civil/Environmental)
2 Jan 03 13:08
Why go through the expense of putting preferated pipe in at all?  just rap your 10-20mm crushed and washed stone in the filter fabric and lay it in the trench then back fill.  P-stone is free draining, isn't it?
boo1 (Mechanical)
2 Jan 03 15:10
Off course Rjeffery is right, but I have used pipe in french drain applications in conjuction with catch basions and might continue the pipe after the french drain stops.  The pipe increases the free area and drainage capabilty of the system. In my area the pipe is cheaper than stone.

Cheers
Rjeffery (Civil/Environmental)
2 Jan 03 15:15
Pipe cheaper than crushed stone?????  :)
Ron (Structural)
2 Jan 03 17:05
No...air is cheaper than crushed stone though and the pipe creates air space!

Pipe also conveys water faster than stone alone (there's a poem in here somewhere!)

The best way to solve this for all of us to meet, drink lots of beer and test the capacity of the pipe.
Rjeffery (Civil/Environmental)
2 Jan 03 19:16
I heerd tell thet beer is jest po' mens bread ina can!

(with all due respect to the staff spelling expert...  Just typing in the ver-nack-u-lar. :)

I'll start collecting the supplies for the experiment, someone will have to write the procedure and post it in the ASTM forum...  :0!
drt (Geotechnical)
2 Jan 03 23:16
Looks like I’m a little late on this one, but here's my two cents.

I trying to imagine a pipe sitting on top of a sloped ground with the perforations aligned perfectly UP.  If I take a hose and spray water along the length of the pipe, I can't imagine much water getting in.  Then imagine the contractor has perforations a few degrees off.  Even less water gets in the pipe.  Doesn't seem too effective.

I always viewed the pipe as a second line of defense (a preferential flow channel) - a redundancy if you will.  Thus, the rock carries all the flow until such time as the water level reaches the bottom of the pipe.  Therefore, for me its perforations DOWN.

DRT




MikeDB (Structural)
3 Jan 03 9:49
We have had the same debate in our company as well.  We use the perforated pipe to collect water or oil runoff in a secondary containment area around transformers in substations (pipe drains to an oil seperator).  We do not use a filter fabric around the pipe, just "clean" rock.  Our solution was to use 4 holes at 2,4,8 & 10 o'clock, but then we ususally only have 60-80' per project.    
boo1 (Mechanical)
3 Jan 03 10:44
"...so that the perforations are facing downward."
http://www.nrcs.usda.gov/feature/backyard/watercon.html

A nice link
http://www.ecy.wa.gov/programs/sea/pubs/95-107/groundwater.html

Pipe
4" = 0.087 sqin  area
100 lft 4"  = 8.7 cuft volume
100' = $33
1 cuft = $3.79

Stone
1.4 ton=1 cuyd = 27 cuft
$40 /ton
27cuft=$56
1 cuft= $2.07    *****cheaper******
Ron (Structural)
3 Jan 03 16:43
OK boo1...ya got me!

I still think the better solution is to drink the beer and then "performance test" the system.
Rjeffery (Civil/Environmental)
3 Jan 03 17:01
If the preferated pipe is placed holes up, won't it be bouyant and tend to displace itself?

If it intended to remove water from under a roadway build on, say 1.3 feet of OGB [Open Graded Base] wouldn't we want the water to infiltrate up into the pipe so as not to leave a 'wedge' of water 1/2 pipe dia X slope length?
boo1 (Mechanical)
3 Jan 03 17:09
Ron there is no got me.  I enjoy all you guys teaching me, a few simple things.   
   
But what an interesting topic, something simple but different opinions exist!
MikeDB (Structural)
7 Jan 03 11:27
I am a little confused by the pipe vs. rock comparison of 1--3-03.  Looks like one would want to compare the area of the pipe to an equivilent void area of the rock.  Wouldn't it take at least 2 or 3 times the volume of rock to get the same void area?  If so, the pipe would appear less expensive.
boo1 (Mechanical)
7 Jan 03 11:34
Mike should you compare the void areas?
Ron (Structural)
7 Jan 03 19:39
boo/Mike...no, the void area is already there.  The only difference is the exit velocity....will be higher with the pipe, but probably not a big deal since these are low volume, low flow propositions.

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