Drain Pipe perforations orientation 6

[rollofox]

Conventional practice calls for placing retaining wall drain pipe with the perforations down. A contractor on one of my jobs was confused by the pipe he got that had perforations on opposite sides. If he put the perforations down, there would be perforations up. What to do? He compromised and put the perforations sideways. Should I make him rotate the pipe 90 degrees?

 

[haggis]

I think the contractor may be correct by installing the pipe with the holes at the side. The idea is that water percolating down through the backfill can enter the pipe and drain off to the storm sewer. Even with a single row of holes, the holes should be on top.

 

[cvg]

holes up or holes down depends if you are draining trying to dewater the soil, or allow the water to percolate into the soil

 

[GeoPaveTraffic]

For a drain pipe placed behind a retaining wall the purpose is to remove as much water as possible. Therefore the perforations should be placed down and the pipe sloped to drain. That said it is unlikely that having the perforations 90 degrees off will make a significant difference.

 

[BigH]

We had a discussion in another thread on this - consensus is that about 1/2 like it up; 1/2 like it down. I'm a down man. I really don't believe that in drain pipes inside french drains or drain rock you can really argue about the best orientation based on hydrostatic level - why - most drain pipes are only 100mm diameter or, perhaps 150mm diameter. Tell me what 50mm in the "water level" makes?? I think it should be down for water has to push fines up into the pipe; else the fines can wash/fall down into the pipe if up. But, consensus is - matter of personal preference.

 

[pmkPE]

I agree with BigH, the holes should be down to avoid soil settling into the pipe. Water entering such drain pipes does not have the velocity to scour away any residual sediments; gently rising water in the pipe will most likely not push soil up into the pipe. However, with the holes on top what is to keep the fines from getting into the pipe. Note that some geotextiles will trap larger particles but the typical geotextile used in such situations will not trap the fines.

 

[cvg]

I would hope that any design for a toe drain would consider either a filter fabric or gravel / sand filter layer capable of filtering the silts and to avoid piping. If so, doesn't much matter which way the holes go

 

[BigH]

Good point cvg - perforations (as per AASHTO) are 5mm to 9mm in diameter. So material surrounding the pipe should be such that it does not fall through. I typically specify 10-20 clear stone. There are, in various places, criteria for this but some I seem do seem a bit out fo whack. It is now most typical to use geotextile as a french drain surround so the fines is not that an important issue - but before, when natural filters were used, it likely was - even for a properly designed filter.

 

[pmkPE]

I would add that filter fabric and stone should be used on french drains because I would want to have a high confidence that after twenty years of service that fines would be inhibited from entering the system. With fluctuations in the water table and/or the natural drainage from the soil above fines will eventually migrate to the pipe. The inclusion of gravel and filter fabric will be the most effective means of interception and will prolong the effective service life of the drain.

 

[BigH]

pmkPE - agree; and it is also easier to construct! But a note - when the original Mirifi 140R (I believe it was an "R" at the time) was used in a dam back in the 70s - my boss showed me a sample they had just cut out from a dam that had failed (or else been severely stressed). The geotextile was totally clogged and was as impermeable as clay! Made me wonder then a bit - but since the 1970s confidence, understanding, etc. has improved significantly.
By the way - Yuck to the US Cricket Team. Lost to New Zealand by 251 runs - a record for a one-day over loss! Yuck!

 

[pmkPE]

Another thought is vegetation. Most people have french drains next to their houses along with various shrubs and small trees. I just helped my father-in-law dig up an old terra cotta drain pipe that runs next to his driveway down to the street. Tree roots had literally broken into the pipe at the joints which did not have O-ring gaskets. (I believe they started out very small diameter as we found inside the pipe and continued to thrive and grow on the moisture in the pipe before the pipe cracked/broke at the bells). The pipe was clogged for more than 12 feet solid with roots. Just one more reason to include stone and geotextile around the drain pipe. BTW the roots had all entered in from the top of the pipe which invert was no more than 18 inches deep. Perhaps this is another reason to place the perforations on the bottom of the pipe.