Purpose for footing drains? 3

[dirtsqueezer]

Not to appear subversive, but I've been contemplating for the better part of the day on why our geotech has recommended that 2" perf pipe be placed around our slab and footings. The building consists of a metal frame on spread footings and a slab with thickened edges, about 6K square feet, with a perimeter asphalt parking lot. It's possbile in my mind that he wanted draw the groundwater down and away from the foundation- but why? Practically speaking, the pipe isn't large enough to handle any significant flow, and I don't see any immediate danger to the footings in the way of infiltration that this pipe helps to prevent. It seems that the thickened edge, as well as the vapor barrier under the slab would be enough to keep water from coming up through the slab as they're designed. Can anyone suggest what my client could be paying for? Thanks.

 

[Focht3]

[blue]Laser28[/blue]:

I think we're on the same page; perhaps you can understand why I took issue with your first post by re-reading it. Your first post implied that vapor barriers are ineffective, and thus not needed. Your second post seems to clear up your position on this point -

I'm not surprised that the presence of a vapor barrier correlated with slab curling; I would have expected that anyway. Curling is likely to occur when the top surface of the slab dries too quickly causing the top surface to shrink before the slab has sufficient strength to resist these forces. You would get less overall difference in drying (between the top and bottom of the slab) with a sand layer in contact with the lower surface of the concrete slab as it cures. This would probably result in both sides of the slab drying at nearly the same rate. But the presence of curling is not an indictment of vapor barriers, nor is it an excuse to eliminate them. It does point to the need for better concrete mixes and curing techniques -


First, let me state that I have not read the National Research Council in Canada report that you have referenced. But I suspect that others in this forum have. (Hey, [blue]BigH[/blue] or [blue]jheidt2543[/blue] - do either of you have a copy?) The following comments may or may not apply to the referenced NRCC study -

In my view, it is very important to read all technical journals with something of a "jaundiced eye." Many writers get so wrapped up in the designing, collecting and presenting of technical data that the analysis gets rushed too much - and they don't take the time to really consider the data in a "big picture" context, so the really obvious answer is overlooked. I know of quite a few examples of this in the geotechnical engineering literature; and I'm willing to bet a cold pitcher of Bass Ale that the problem isn't limited to the geotechnical engineering community.

It's the reader's duty to evaluate the statements and claims made in any learned journal and decide for his/her self whether they are reasoned and appropriate. Remember that we all put our pants on one leg at a time...no one (or group) is infallible.



If you want to continue this discussion, why not start a new thread? We're a bit off-topic...



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[BigH]

Do a search on Perforated Pipes - Holes up or down. I had initiated a thread about 8 months ago on this - had a nice discussion. You may be able to add to it.
As for the Research Council of Canada report - I haven't read it but heard of it second hand and understand same as laser28.

 

[jheidt2543]

Sorry, I haven't read the Canadian report either. However, a recent issue of a trade magazine made mention of the practice of placing a sand layer between a vapor barrier and the slab-on-grade. The jist of the the article (which I will try to find and reference) is that the sand layer is no longer recommended. I've never been in favor of that detail and I finally felt vindicated after reading the article, although my reasons were more for constructablility.

As far as a 6 mil vapor barrier passing water vapor, I've never read a study on it and years ago the standard was to use 4 mil. However, any gym floor with either a wood or synthetic surface would require a heavier vapor barrier, one trade name was "Moist-Stop".

IMHO, the best slab curl prevention is to concentrate on proper slab curing, just as Focht3 noted.

 

[euygar]

Hi,

ChrisMcLean, can you please give us more information about what you've told in this thread. I see that you pointed out an interesting detail. Hope to see your reply on this matter.

Note: It would be great if you can give us some references so that we can find something to read also.

Eris Uygar

 

[ChrisMcLean]

It seems that the possibility of the pipes being included for gas percolation is unlikely based on the other responses to the thread but it happened to be something that we do quite a bit of here and that I was working on at the time. We don't normally include foundation drains in constructions we do.

In the UK, pressure for building land is fairly intense due to current legal positions over development of greenfield sites. Consequently, the number of new properties being constructed within the 250m action area of landfill sites that are potentially evolving either carbon dioxide or methane increases all the time. Also, further to some health scares back in the 80's Radon is quite a large issue.

Schemes incorporating slotted collection pipes have been used is landfill gas situations for some time, though on reflection they normally include vertical risers and are usually for areas with high gas concentrations. Use of this kind of arrangement enables the constructor to do away with the granular percolation trench around the edge of the building required by most other mitigation schemes and can be attarctive for that reason. It's normally only a major consideration to remove build-ups of explosive gas for under buildings - use of a gas-proof membrane should prevent ingress of asphyxiant gases into the structure and so risks from radon/co2 are reduced.

 

[euygar]

Hi ChrisMcLean,

thank you very much for the detailed information. Can you give me reference to a written document on this subject. I will be so glad having such an information.

Eris Uygar

 

[ChrisMcLean]

If you give me a postal address I will forward you a copy of the documents relating to this subject. Email me at chris.mclean@anglian-windows.com.

 

[RockEngineer]

Just for one more thought here is what the Structural Engineers Association of Oregon is proposing for slabs on grade with respect to sand, vapor barriers and drainage.

http://www.seao.org/sog.pdf

 

[emmgjld]

Good Discussion. The issue of vapor barriers, sand/aggregate layers and curling problems has been discussed many times in these forums. I have observed that local practice tends to vary, usually depending upon what usually works and apparently influenced by an extraordinary number of environmental conditions cited by the discussors.

In my arid to semi-arid environment in western Colorado, I have 5 specific drain design recommendations (and a large number of modifications, many of which deal with drainage/protection of slabs from intruding waters), depending upon the soil, groundwater, anticipated surface water and actual foundation type conditions. I see value in most of the discussion in this post, the usefullnes depending upon the actual conditions of the site and structure.

The placement of a sand layer immediately beneath the concrete has been recently discouraged by ACI, about time as far as I am concerned. I have usually (but not always) advocated the use of a COMPACTABLE, fairly well graded and slightly cohesive sandy gravel or gravely sand immediately beneath the slab, to aid in finishing and to help reduce curling. If a true drainage layer/capillary break layer is required, I usually (but not always) put it below the sandy gravel or gravely sand and the vapor barrier is usually (but not always) between the aggregates. However, Always open to ideas.

I agree that proper curing is an important factor in minimizing curling but, with the seasonal humidity variations experienced here, I have experienced slabs which curl and 'uncurl' on a seasonal basis. Each year the variations are a little less than previous and near stability has been reached in 4 to 10 years. An interesting problem. I have found that proper reinforcement (with bars, not ww fabric) in in the upper 1/3 of the slab helps a lot.

 

[Focht3]

Hmmm,

Seasonal slab curling? Never heard of it - and I don't see a mechanism within the slab itself for this to occur. I do see seasonal moisture variation as a possible cause, though, of the observed foundation movements -



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[emmgjld]

Good to see the post was read to the end.

Seasonal Slab Curling was found to be related to indoor enviromental conditions. In the winter, the heating system (generally forced air) dries the building (relative humidity 10-20%) and slab surface, resulting in curl. The summer air conditioning ('swamp coolers') increases the humidity to 80-90%, resulting in the slab surface being moistened and 'uncurling'. It took a few years and several different slabs to determine the problem was not the slab, foundation or subgrade conditions. Always be ready for a surprise.

 

[Focht3]

Someone needs to publish a paper on it -



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