Question about foundation drains 3

[cliff234]

I am a structural engineer and I have a question about foundation drains behind basement walls. Where does the water in those drains flow to? As a structural engineer we require that the soil behind basement walls be drained, but I am not sure who designs and documents the drainage systems behind basement walls - and where does that water flow to?

Are there any good articles or information on the internet that would be useful in further understanding the fundamentals of basement wall foundation drains?

Thank you in advance for any insight you can share on this.

 

[cvg]

foundation drains are in a gray area. you need knowledge of soils and hydraulics to design. since you are preparing foundation drawings, you need to do it or have someone with experience do it for you.

drainage outfall is
a) to sump pump
b) to sewer
c) to low outfall away from the bldg

 

[cliff234]

Thanks for the response. I am not designing the foundation drains. I am a structural engineer and all I care about is that the soil be drained so that I don't have to design the wall for a saturated earth pressure. I am just curious as to where the water in these drains goes to. (Someone in my office asked and I had no idea.) I am particularly interested in commercial and institutional type buildings (not houses). If my building has a basement on all sides, then where does the water go? I have always assumed that ground water is not permitted to be discharged into sewers. If the ground does not slope away from the building then it sounds like the water has to flow to an underground tank of some sort where it gets pumped to the surface?

 

[JedClampett]

You are correct. If you can't daylight the drainage system, you need to sump it and pump it. Residential sometimes has the sumps inside the basement and I'm sure it's been done that way for commercial and industrial. Or the sump can be exterior to the building.
As far as where to pump it to, if you can't send it to storm sewers ask if it can be pumped to the retention ponds. And sizing the system is a grey area, like cvg says. It's hard to get anyone to put anumber on the quantity of water coming out of a groundwater system. Luckily, the pumps are usually sized close enough that all you do is run them more if the design guessed wrong.

 

[ACtrafficengr]

I assume you mean sanitary sewers? They are sometimes connected to storm sewers.

The county I work for does not allow new foundation drain connections to our storm sewers. One of our townships requires them. It has caused a few conflicts along our county routes in that town. It's best to check with the municipality(ies) involved.

Otherwise, you have to pump it to an infiltration drain, or to a rain garden, where some will infiltrate and some will transvaporate. I suppose you could use it for landscape irrigation or water features.

 

[cvg]

in some areas, drains connect to either sanitary or storm drain system. in other areas, you must dispose of the water by surface discharge.

and by gray areas, I also meant that most structural / foundation engineers really do not care or know what to do. Cliff, you confirmed my suspicion in your reply. As a result, sometimes foundation drainage is not allways adequately designed. It is often left up to a contractor to design build and the results are often not adequate.

 

[bimr]

Actually, a geotechnical engineer should design a residential foundation drain, although most people do not want to spend the money to employ one. Usually, an architect or structural engineer puts out a standard drawing on the foundation drain.

I have never come across a situation where the foundation drain is routed to the sanitary sewer.

The foundation drain is usually routed to the surface runoff, it can be routed to the storm drain if the municipality finds that solution acceptable.

 

[hokie66]

Whoever designs these drains, my assumption is always that they will fail at some stage. Therefore, I design basement walls with that assumption.

 

[cliff234]

As the structural engineer we will not take responsibility for design of foundation drains. We tell the architect that the soil behind the wall must be drained. If it is not then any substantial water build-up would probably result in water in the basement. Likewise, not only will saturated soil greatly increase the moment in the walls, it would also significantly increase the reaction at the bottom of the wall. We often have trouble getting adequate resistance to sliding at the base of drained walls. It would be a significantly greater problem dealing with a reaction due to saturated soil pressure. From the responses so far it sounds like foundation drains often get lost in the shuffle.

 

[dhengr]

Properly done, the drain tile should be near the bot. of ftg. elev., in gravel, and it should be on the outside of the bldg. It should not be on the inside, under the bsmt. slab, which is a poor substitute for not having done it right, in the first place. The idea is to keep the water out of the bldg. and out from under the bsmt. slab. So, then what’s the lateral resistance issue? The entire wall water proofing, not damp proofing; and the backfill detail should be done properly to drain down to the drain tile. The grade around the bldg. should be sloped to drain away from the bldg. The highest 6-8" of soil might be planting soil; the next 6-8" of soil should be compacted and impervious, sloped to drain away from the bldg. and extend out to meet undisturbed soil beyond the excavation; below this should be a geo-fabric which wraps the draining fill around the found. wall and on down to the drain tile. I agree with Hokie, and would not design my walls assuming the drain tile could never fail. And, I don’t see anything wrong with showing this general arrangement on my found. x-section, since it is what I want and expect, even if the drain tile is not my responsibility.

 

[hokie66]

I don't agree that the loading drained/undrained is a big issue. The construction is the same regardless of the loading, just with variation in the reinforcement and possibly the width. No place to skimp when there are so many unknowns, variables, and guesses involved. Resistance to sliding in a basement wall is normally available from the internal slab, so I don't think that is typically an issue either.

 

[bimr]

You may want to post this question in the foundation engineering forum:

http://www.eng-tips.com/threadminder.cfm?pid=256

This issue has been discussed there.

 

[dhengr]

Hokie:
In an effort to solve a wet bsmt. problem around here, when they hadn’t bother to install drain tile on the outside in the first place, some folks do the following: They break out about 18" of the bsmt. slab all around, dig a trench down to the bottom of the ftg., place some gravel and a perforated drain tile in a geo-fabric sock, fill with gravel to the top of the ftg., tap all of the conc. blk. cores, install plastic tubes to drain over the tile, and finally replace the slab. This invariably happens during a wet period, when people have had it with the wet bsmt. Then the found. walls may sit in this laterally unsupported condition for at least 3-4 days, or longer, during the process. I’ve heard of instances where there were lateral found. movement problems with this. And, fly-by-night contractors who claim to know how to do this come out of the woodwork from time to time.

 

[brandonbw]

We are working on a 5 story building with 2 levels underground. The plumbing plan will put everything into a sump pump and outlet into our drainage plan. Normally the mechanical connects into our system which we always try to outlet onto the street through the curbface, that eventually ends up in the storm drain. As for the drain that seems to be a grey area for who wants to show it on their plans. When we show them, I have never seen them built to what we want. A soils report says what they want regarding the drain and foundation.

Without a soils report you are then designing to the max for everything structurally touching the ground, which seems like a huge waste of money.

B+W Engineering and Design
Los Angeles Civil Engineer and Structural Engineer

http://bwengr.com

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http://bwstructuralengineer.com

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http://bwcivilengineer.com

 

[darthsoilsguy2]

To add more complexity to the "nobody wants to own the foundation drain".... I work regularly with inspections of below grade waterproofing systems. These systems usually are "design-build" because you want to use entirely manufacturer recommended details so the warranty is valid. The contractor will typically submit all these details at the beginning of the job with the shop drawings. Most waterproofing systems use a very good drainage mat between the membrane and the excavation/backfill and this drainage mat is in the waterproofing details and is supplied by the waterproofer. I raised the concern on a transition from blind-side to formed&direct-applied that occurred halfway down the wall where there was a 12" to 36" cast concrete ledge that would inhibit drainage. This ledge was formed because the shotcrete excavation was all over the place. I have a letter from the waterproofing manufacturer stating something to the effect of.... The drainage mat we have provided is suitable for the protection of the waterproofing materials from damage by the soils. It is the responsibility of the foundation drain designer of record to determine the suitability of this product for drainage purposes. We offer no additional opinion or warranty of this product apart from suitability to protect the waterproofing.... BTW don't ask me who this "foundation drain designer of record" was....

If you ask my opinion, i think the architect should own the drainage around the building and the Civil site egr should own it as soon as it starts getting piped away from the building.

 

[hokie66]

I hear what brandonbw and darthsoilsguy2 are saying, and those situations are unfortunately the rule rather than the exception. That is why I always assume that for structural loading, the drainage will not work.

 

[LincolnPE]

Typical practice now for existing structures is to daylight the pipe to the side-lot swale to protect the foundation.

If the site is producing a lot of pumped water this can become a nuisance, especially in the cold climates due to ice buildup.

Municipalities are encouraging civil engineers on new projects that will be pumping significant drain water (based on geotech soil report) to create a pipe network that connects to the storm sewer system.

Never connect these systems to the sanitary system, always the storm system.

These foundation sump flows are insignificant when compared to the storm sewer capacities. Therefore typically not even asked to document any flow rates, plan checkers just want to see the pipe network to check for typical interference issues. These municipalities field a lot of calls and complaints about the nuisance water, especially the ice buildup in the streets and sidewalks in the winter. They do no want to deal with this anymore so encourage developers to connect to the storm sewer.

 

[darthsoilsguy2]

Also if it is an urban site (which a lot of the big-dig sump-pump jobs are), there is a decent chance groundwater contamination may require treatment on-site before it is discharged anywhere.

 

[DaMeRy]

in the fact i agree with bimr a geotechnical engineer should design a residential foundation drain, although most people do not want to spend the money to employ one. Usually, an architect or structural engineer puts out a standard drawing on the foundation drain.

 

[fattdad]

The calculation for the amount of water that may issue from a foundation drain is not that complicated, once you have a fix on the soil layering, horizontal hycraulic conductivity and position of the water table. If the basement is 60 by 80 ft, just calculate the square footage, convert it to an equivalent circle and use flow-to-well equations for gravity flow.

If pumping is necessary for the structural integrity of the design, you must have backup electricity! Ground water seems to be the highest when the power is out. Blame Murphey.

I'd never use a foundation drain just as a supplement to bearing strength though. To limit demand on damp/water proofing, to reduce horizontal earth pressurs and to optimize the structural design of below-grade elements, sure.

f-d

¡papá gordo ain’t no madre flaca!