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.

 

[ishvaaag]

Well, drainage may help to avoid some problems. The intent surely will be to keep the level of moisture under the building locked at some level; how effective the drainage who knows. Washing of fines or soluble particles under the buildings is one of the more frequent causes of bad structural behaviour; in fact in one of the towns I used to practice more recently a ground hole opened under a 6 stories or so building ... and it took over 300 m3 of concrete to just fill the hole. Even so, today the owners are asking a legal declaration of ruin.

 

[gandersen]

dirtsqueezer:

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?

 

[Focht3]

There are also some expansive soil problems that can be helped with subsurface drainage -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[dirtsqueezer]

Yeah, no permafrost climate here- I'm in the Pacific Northwest, though that would've spiced it up a little. I've heard one or two horror stories about 'blue clay', though North of here. The building is located right next to a gravel pit, and is built on the same stuff. I just don't like not being without a good explanation for soils work! Thanks, guys. Still scratching our heads about this one.

 

[DRC1]

Local building code proably requires footing drains. In periods of heavy rains, the backfill will might collect water faster than the native material can drain. I feel it is a good idea as it is cheap insurance against a water problem. The drain should be wrapped in filter cloth with slots up and have positve pitch to drain. It is very inexpensive to do durring constrution and very expensive afterwards if it is decided that it is needed.
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.

 

[ChrisMcLean]

Just a thought - I've been doing some drawings for a foundation design in a Radon action area and one of the things required by the standards in this country (BRE 212 - Buildings on Gas Contaminated Land) are drains in the foundations to prevent gas build-up - obviously more critical for landfill gas than Radon/CO2 which I also see from time to time.

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?

 

[jimbo2]

DRC1
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?

 

[DenverKev]

Jimbo2,

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.

 

[lschenk]

I wonder why nobody has suggested asking the Geotech why he has the drainline in his design?

 

[dirtsqueezer]

Lol. I'm actually not supposed to call the geotech! There has been a trend in our area of the testing companies and the geotech companies moving into their respective areas of business. I'm not sure if we've actually lost work because of it, but I was specifically told not to contact the geotech if at all possible. Not the kind of communication I'd expect if you were trying for the best possible outcome on a project, but, boss's orders. Yeah, that's the first thing I thought of....

 

[DRC1]

Jimbo
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.

 

[jimbo2]

DRC1

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.

 

[Focht3]

I've never had a sales rep tell me to put the slots up. (Not that I would have taken his advice...)




Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[ERV]

Slots down also aids to prevent clogging.

Just wanted to add my two cents for what its worth.

 

[Laser28]

Back to the question:

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.

 

[Focht3]

Hmmm,

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.

 

[jjesik]

I have a couple of different thoughts about slots and 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.

 

[ERV]

Laser28,

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.

 

[Laser28]

Erv and Focht3. Please reread my post. I agree with the idea of a water or vapour barrier, but research performed by the National Research Council in Canada about 25 years ago indicated that the use of 6 mil vapor barrier under slabs did not STOP the passage of water vapor, merely retarded it. the VP also contributed to slab curling in thin (4-6&quot slabs.

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.