Purpose for footing drains?
Purpose for footing drains?
(OP)
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.





RE: Purpose for footing drains?
RE: Purpose for footing drains?
Another reason for installing foundation drains may be to decrease the potential for damage due to frost heave. In otherwords, they may be in place to keep the water table below the frost depth for your area. Where is the project located?
RE: Purpose for footing drains?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Purpose for footing drains?
RE: Purpose for footing drains?
I agree that 2 inch sounds too small. I would go with 4 or 6 inch. The cost of the pipe itself is nominal in regard to the whole budget, and if you are going to put in a pipe, you might as well be sure it will work.
RE: Purpose for footing drains?
Is it possible the designer could be making some allowances for gas buildups here? The fact that the purpose would be for trickle type ventilation may explain the small drain diameter. Has the designer specified any fill materials likely to facilitate gas percolation? What is the gravel pit filled with?
RE: Purpose for footing drains?
I'm not a fan of wrapping a drainage pipe with a geotextile filter, as the filter will most likely eventually clog. What about using graded soil filters and sizing the slots or perforations to protect the aggregte from entering the pipe? Also, I have always specified that the slots/perforations belong on the bottom side of the pipe as placed. Whaddya think?
RE: Purpose for footing drains?
I think you're touching on an important point. I believe that any drainage system of this general type will eventually clog. It's just a matter of time and drainage flowrate. "Dirt" gets entrained in any sort of water flow, and deposits at the low point or when the particle flow is blocked by previous dirt. Even if you backfill with 100% gravel, there is dirt deposited on top of that whenever the wind blows. Next time it rains the dirt flows down to the point where it settles out. It could be at the geotextile filter, or at the slots whether they are up or down. None of the standard footing drain details I've ever seen would be a permanent solution.
RE: Purpose for footing drains?
RE: Purpose for footing drains?
RE: Purpose for footing drains?
Ihave always put the slots up. I have dicussed this once a long time ago with a manufacturer's rep and was told slots up is how they are designed to work.
If you have fairly clean backfill and approriately sized filter cloth, the fabric should not clog. In theroy. So they say. A little stone over the top helps just in case theroy isn't always perfect.
RE: Purpose for footing drains?
Interesting.
Since 1980 most of my work has been in designing, building, operating and closing solid waste landfills in the northeast USA. I cannot remeber seeing any design by firms in this area where the slots or perforations were placed on top. I do remember one event when I observed the contractor place the slots up (and of course I told them they must turn the pipe over). In an active landfilll, and I suspect many subsurface drains, flow in the pipe is well below its capacity, and placing the slots down will tend to result in lower heads in the surrounding backfill. What is the purpose/intent of placing the slots up?
In a solid waste landfill, using a geotextile filter is a definate no-no due to the documented biological and particulate clogging in this environment. Graded soil filters are used to develop a stable and more open granular transition/bridge between the different sized aggregates in the filter.
I agree that where clean granular backfill is used, the clogging potential for a filter wrap around a pipe is reduced. If a properly designed gravel backfill is used around the pipe, I prefer sizing the slots or perforations to 1 or .5 the D85 of the backfill. I have also seen designs where a geotextile filter is used to protect a pipe placed in erodible silts, where a graded soil filter design seems much more appropriate.
RE: Purpose for footing drains?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Purpose for footing drains?
Just wanted to add my two cents for what its worth.
RE: Purpose for footing drains?
If the intent is to reduce moisture infiltration into the building, do not depend on a "vapor barrier" to stop moisture. Only a true waterproofing material can stop moisture on a slab-on-grade. A 6 mil vapor barrier will allow almost as much moisture as a poorly constructed 4" slab. A properly constructed and cured slab will permit less moisture migration than the poly. Better to put your vapor barrier to work on the top of the slab and use it to cure.
You were told not to talk to the geotech??? Find out who this person is and take them out for a beer. Its better to have a network than to work in isolation. They can learn from you and vice-versa.
RE: Purpose for footing drains?
I disagree. Practically speaking, it takes a combination of the sand layer to break capillary rise, a good waterproofing on top of the sand, and a good floor slab to offer the best barrier against moisture infiltration.
I've seen "crack free" slabs that were built without a vapor barrier system that passed a lot of moisture (as evidenced by the presence of efflorescence and mildew), and badly cracked slabs that remained reasonably dry. No one element is sufficient of and by itself -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Purpose for footing drains?
It would make sense to put the slots down if you were trying to prevent an overall increase in water for the entire site. That way if the water level rises, it will flow down the drain pipe and into a sump or outfall area. It seems like this is the prevailing method in this area anyway.
Secondly, if the groundwater level isn't high and probably would never increase to the foundation level, it would make more sense to put the slots up than down. For structures in this type of environment and especially clayey, swelling soils you essentially create a bathtub where you excavate the foundation. The excavation is backfilled, but still the backfill area will have a higher porosity than the very stiff unexcavated clays thus creating a bathtub for the structure. Assuming that the groundwater level will not increase to the foundation level the major concern would be surface water getting into the backfill area. Usually this surface water will not inundate the whole building at once, unless you have a big flood, then it's a different beast all together.
So if you get isolated areas of the drain with water, you would want the drain to catch the water and "drain" it away from the rest of the structure. If the slots are put facing down, the drain would still catch the water, but then whisk the water along the foundation and then the water would leak out of the slots, thus saturating a perfectly dry portion of the foundation. If the slots are put facing up, they will catch the water to, but the water could flow all the way down the drain pipe to the outfall without ever leaking out.
So anyway, I think the best advice for the question is to talk with the geo engineer and see what his thinking was for the drain. He's really the only one who understands the project, the soil conditions at the site and most of all his reasoning.
RE: Purpose for footing drains?
I agree with Focht3. In addition, the vapor barrier actually serves two purposes (1) as an active barrier against moisture and (2) to protect the concrete from chemical attack due to sulfates that may be present in the soil (which can be devastating to concrete if sufficient moisture is present, reducing concrete to sand and gravel).
The sand serves two purposes as well (1) as Focht3 has stated, breaks capillary rise and (2) serves as an insulator to keep moisture from condensating on the underside of the concrete slab.
This system is in use in millions of residential and commercial slabs and is very effective against moisture intrusion from soil even though 100% coverage is not possible due to penetrations from pipes, etc. I have never observed sulfate attack on any slab that had a vapor barrier and sand fill, so in this respect I would say it's 100% effective.
RE: Purpose for footing drains?
I also agree with the efficacy of a sand layer to reduce capillary action.
It has been my experience (warehouses for paper storage for example) that the best defence against water is a combination of a waterproof paper or fabric under a 1" sand layer, and a drainage system around the footing of the bldg.
I admit that my experience with sulfate attack is limited to seawater, not groundwater, and we deal with that by adjusting the C3A content of the cement, and with good construction and curing practices.
RE: Purpose for footing drains?
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, BigH or jheidt2543 - 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...
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Purpose for footing drains?
As for the Research Council of Canada report - I haven't read it but heard of it second hand and understand same as laser28.
RE: Purpose for footing drains?
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.
RE: Purpose for footing drains?
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
RE: Purpose for footing drains?
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.
RE: Purpose for footing drains?
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
RE: Purpose for footing drains?
RE: Purpose for footing drains?
http://www.seao.org/sog.pdf
RE: Purpose for footing drains?
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.
RE: Purpose for footing drains?
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 -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Purpose for footing drains?
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.
RE: Purpose for footing drains?
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.