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Related Articles


Foundation Drains

Foundation Drains

Foundation Drains

I have a situation we have a long (8500') concrete foundation (20' wide) for a fish passage which includes 18" foundation drains on each side of it.  The foundation is sloped at 8%.  We want to divert water out of the foundation drain to a storm main running parallel to the fish passage at 2500' intervals.

Is it possible to have the foundation drains sitting below the bottom of the foundation in order to install fittings and divert the drain under the fish passage to the storm main on the opposite side?  If the top of pipe of the foundation drain is 6" below bottom of concrete foundation will it still adequately collect underground drainage?

RE: Foundation Drains

you should have manholes or cleanouts spaced at intervals of perhaps 500 feet. any more than that will make the drain difficult or impossible to inspect and clean. the manholes can be used to drop, turn the flow and divert under the foundation to the other side

RE: Foundation Drains

Manholes within within the foundation drain system?

RE: Foundation Drains

So if manholes are installed over the foundation drain system that would require the 18" perf piping to be moved 3'+ away from the foundation, does anyone see a problem with the drain system working properly?

RE: Foundation Drains

not necessarily that large of a structure, could be just a cleanout and not large enough for human entry. either way, they are necessary for maintenance and that is where you can make your grade and alignment changes.

RE: Foundation Drains

what is the purpose of the foundation drain?  It seems that a fish passage would run the risk of being wet all the time.  Maybe I'm missing something. . .


¡papá gordo ain't no madre flaca!

RE: Foundation Drains

concrete lined channel, kind of like a spillway, but designed for fish. a foundation drain will eliminate piping and uplift under the channel. foundation drains are commonly used in spillways and concrete lined channels.


RE: Foundation Drains

Thanks CVG, I got the picture and the nature of the engineering concern now. . .

I don't think I'd have a perforated pipe or open-graded gravel anywhere near the upstream 2/3rds of the fish ladder foundation.  I'd pour concrete directly against undisturbed foundation soil.  For the lower (downstream) 1/3rd of the fish ladder foundation I'd provide at least 18 inches of open-graded gravel below the foundation and adjacent to any/all vertical fish-ladder walls.  It'd be up to you whether to have a perforated pipe in this open-graded material.  I'd likely have some perforated pipe to allow a good daylight, not prone to vegetative clogging.

It's good practice to use separation geotextile between the open-graded aggregate and the foundation soil.

I just did this same approach to a newly-constructed concrete spillway and all is well.


¡papá gordo ain't no madre flaca!

RE: Foundation Drains

"If the top of pipe of the foundation drain is 6" below bottom of concrete foundation will it still adequately collect underground drainage?"

I might be concerned that you are collecting too much water.   If I understand the situation, you are proposing that the bottom of the perf pipe is 2' below the bottom of the foundation (18"+6").  When I've designed foundation drains, the structural/foundation designers require that the bottom of drain is NOT lower than the bottom of the foundation.  They are concerned with the potential of lowering the groundwater elevation, which could affect the stability of the foundation, particularly if it is an existing foundation.

But if that's actually a concern, I'm guessing it would have been mentioned by now?   

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