French drain design - interfering house sewer lines 2

[drothe]

As a follow up question, I'd like your opinions on my idea to deal with the points where the F.D. interferes with sewer lines. I will not know exactly the position of the sewers until I do some digging.

Have a look at the diagram at

http://tinyurl.com/yg72tkc

If I discover that the sewer & F.D. pipe interfere (and it will), could I just simply continue the F.D. trench filled with 3/4" stone directly across the sewer drain to the other side, but without the F.D. pipe? The pipe would have a perforated cap at the ends either side of the point where it meets the sewer drain. The stone fill in the trench above and below the sewer drain would conduct water across the gap. Granted, not as well as the pipe, but if the gap is short then it shouldn't be too bad? Any other suggestions how to deal with this?

 

[Ron]

I see numerous problems with your design scheme. First, why are you putting a poorly protected french drain within the confines of a foundation. You're asking for long term settlement and raveling issues, not to mention that fact that your french drain probably won't actually work very long and it's not maintainable.

What is the purpose of the french drain? It appears that you are trying to intercept groundwater in a basement. That won't work! First of all it's in the wrong location. Secondly, it's not deep enough to be effective.

You need to re-think this whole design. Get a local geotechnical engineer who knows the soil and groundwater conditions to help or do this for you.

Also...your slab isn't thick enough.

 

[drothe]

Thanks for your response. You raise some good questions. I'm not sure what you mean by "poorly protected". Can you elaborate? Protected from what?

If it's sediment you're concerned about: Placed within the foundation it is protected more from sediment than outside the perimeter of the foundation. Most of what I've read claims that sediment travels vertically rather than horizontally, and there is far less soil above the pipe inside the foundation than outside.

Besides, I am not really concerned about sediment. The sump pit is actually the house sewer line access hole, dug out a bit to fit a main pump and backup. It has not shown any intrusion of sediment. Even so, I do intend to include clean out ports as the diagram indicates.

The existing pump setup, such as it is, works fine during normal rainfalls. The house is 60 yrs old with a poured concrete foundation in good condition. Seepage is through the floor/foundation wall interface due to high ground water levels during periods of extended extreme rainfalls. The pit does not overflow during these times. I take this to indicate that water cannot move through the soil to the pit quickly enough during extreme rains.

Regarding the slab not being thick enough: I intend to match the existing floor when I cover the drain. My 3 inch estimate was just a guess. I'll know when I break concrete. If I find it's only 3 inches, I'll have an extra inch or two poured on top

That brings to mind a question: When I put concrete over the drain, do I have to worry about its intrusion into the stone plugging up some of the channel? If so, what can I do to prevent this?

 

[Ron]

What I mean by "unprotected" is that you imply that all you are going to do is dig a trench, drop in a little gravel, drop in a perforated pvc pipe and fill the trench with more gravel. That process creates an unprotected underdrain.

You should dig the trench (our general recommendation is a trench that is at least 18 inches wide and at least 24 inches deep or at least 6 inches below the bottom of the pipe, whichever is deeper). The trench should then be lined with a filter fabric (geotextile with capability to filter fine soils). The trench should then be filled with 6 inches of No. 57 stone. A perforated, socked pipe should then be placed in the center of the trench. The trench is then backfilled with No. 57 stone to its top, and the filter fabric folded over the top of the gravel. This prevents lateral or vertical migration of fines into the trench, that would eventually fill up the void spaces and make the underdrain ineffective. Keep in mind that when fine soils migrate into an underdrain, they must come from somewhere which can leave looser soil or voids behind...that can cause settlement.

Putting the underdrains under a basement is not a good idea. I'll reiterate...you need some local help. Get it. This is not something you can just read about and get it right.

 

[drothe]

Well, enough said then. Thanks for the clarification Ron.

 

[oldestguy]

Pay attention to Ron. What he didn't say is that gravel, by itself, is not a filter and sediment can be moved into it by water flow, and, if you have enough silt and fine sand movement, it can cause undermining of the building. If that didn't happen, then the system you construct will likely fill with mud and plug.

While Ron's recommendation is a common one, I usually recommend an easier way that takes less work and is less likely to be done wrong. Instead of using the surrounding filter fabric and a gravel interior, I just place the perforated pipe and fill the trench with ASTM C-33 concrete fine aggregate (concrete sand). That is a perfect filter and I have never seen it fail. Sure it is less porous than gravel, but your flows are generally slow anyhow and the sand is more porous that the stuff the water comes from. The sock on the pipe can be used, but here again, I have never seen one fail without the sock. A slight bit of fine sand comes in, but soon the slots are bridged over by coarser particles.

I did my Master's work on this very subject some 55 years ago. I have preached this filter thing ever since and builders and architects continue to use just gravel, probably by force of habit. Through the years I have managed to convince many a person, sometimes after they wonder why their system failed.

 

[Ron]

oldestguy..thanks. I agree that concrete sand is an excellent filter. The reason we don't typically use it around here is that our concrete sand does not meet the gradation requirements of C33 for concrete sand...the fineness modulus is too low. The reason we can use it in concrete is because of a good record of service.

I'm guilty of not universalizing my answer...thinking only of the materials we have at hand here locally (Florida).

 

[concretemasonry]

I agree with oldest guy. Gravel is not a filter. Using coarse sand expands the porous volume/area surrounding the the filtered area and reduces the velocity that moves the finer particles.

The best interior french filters I have seen are a continuous drain tile loop with a clean-out that permits flushing in case of any future fines migrating.

The old french drains that were put in golf courses decades ago are still functioning with a mixed gradation of sand and fine rock and no filter cloth, plus they were just topped with a minimal anount of soil in the lower areas.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.