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French drain - sloping pipe necessary?

French drain - sloping pipe necessary?

French drain - sloping pipe necessary?

I'm designing a French drain for underneath my basement floor and I have some questions about the elevations and sloping of the pipe.

Have a look at the diagram at http://tinyurl.com/yg72tkc ( opens to http://www.kodakgallery.com/gallery/creativeapps/slideShow/Main.jsp?token=476851392506%3A1438947685&sourceId=533754321803 )

I'm guessing the concrete floor is around 3" thick. Below that I'll have 3" of washed stone or concrete sand at the most distant point from the pit. Then from that same point a 4" dia. perforated PVC pipe sloping 8" down along 55 ft horizontally to the pit, based on 1.5" per 10 ft. The (constant) depth of the stone/sand filled trench will be such that at the pit there is 3" stone/sand under the pipe. The pump I'm using needs at least 18" depth below the pipe to operate properly. I'm pretty sure the floor does not deviate from level anywhere more than an inch or so.

If I add up all these depths. I get a pit depth of 3+3+8+4+18 = 36". I'm worried that this pit depth will be too deep and will degrade the pump performance from too great a lift. A 3ft pit will result in a 12 ft total lift. My pump is rated at 2200 gph @ 10 ft lift, but I consider that to be very optimistic with respect to actual performance due to losses from check valve, pipe bends, and overall pipe run losses - not the least of what I've actually observed of the pumps operation.

Right now I have a pit that is only around 20" deep without any under drain, and that has proved to be sorely inadequate with our recent heavy rain fall patterns. The pump operated for 30s every 15-30s during the height of the storms and that was not enough to prevent flooding. The water didn't overflow the pit. It just couldn't move through the soil to the pit quickly enough. I'm guessing that the actual pump performance is around 1800 gph or so because the water level in the 27x18" pit dropped 5-6 inches in each 30s interval that the pump was on (27x18x5/231/30*3600 giving 1260 for a lower bound estimate not taking into account the inflow).

To reduce the lift that the pump has to contend with, I am considering eliminating the slope of the PVC pipe. That way I will be able to make the trench and pit shallower, all be it only 8 inches. This would leave the bottom of the pipe 10 inches below the floor surface, 7 inches below the concrete if it's 3 inches thick. My reasoning is that as the water table rises, the stone/sand below the pipe will first conduct water to the pit as long as the pit liner is perforated to allow it. If the flow is great enough, the water level will rise to the pipe (perforated around the circumference at 90 deg intervals) and continue to drain in the same way that the stone drained - but very much better. After all, the trench itself will not be sloped so why does the pipe need to be?

I guess what I'm suggesting is that the F.D. is not a pipe in the usual sense. Rather due to the surrounding stone/sand and pipe perforations, it acts as a channel of rectangular cross section, whose flow capacity takes a sudden large jump when the water level rises to the pipe. So the normal slope recommendations for closed pipes may not apply, or at least not have the same level of importance.

Another argument for a level pipe is that if sloped, the water will not flow in the pipe at the most distant end until the water table is 8" higher than at the pit end. I consider the most distant points to be the most crucial areas to remove water from, so sloping the pipe seems to be a bad idea in this respect.

What does the community say about this issue? Any other comments about the design as shown in the diagram are welcome.

RE: French drain - sloping pipe necessary?

A fancy layout there.

I'd place the sump pit as near to the center of the layout as you can, so that it cuts down on the flow path from the far reaches.

Commonly plumbers do not have the luxury to place a decent slope on the pipes.  In most cases it works.  I have seen failures due to inability of the water to get to the pipes through find sand.  Therefore, I recommend never spacing pipes more than 15 feet apart.

RE: French drain - sloping pipe necessary?

You need to Slope the hard line, but not the perf pipe.   

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: French drain - sloping pipe necessary?

In a perforated pipe water will seek its level and automatically drain to the lowest point, which is usually a sump or the a"daylight drain".

Non-perforated pipe are meant to carry water away and not pick up any additional water.


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

RE: French drain - sloping pipe necessary?

Thank you for the responses. The layout is "fancy" because it's a finished basement & I want to minimize the number of walls I cross under. Lines run to the furnace because that is a local low spot in the floor. I understand the desire to pick a central position for the sump but I also need to pump it out the bottom right of the diagram to the storm sewers, and I don't want to run an over head line through the ceiling to that point. The buried line will be all perforated and I've decided to lay it level at the bottom of the trench on 2-3 inches of stone or sand. Stone or sand I have not decided on yet. I'm leaning toward stone because I do not expect to have a problem with silting and in any case will have clean outs and 90 deg bends each made with 2x45s for snake access.  

RE: French drain - sloping pipe necessary?

You do not need to slope a perforated pipe.


¡papá gordo ain't no madre flaca!

RE: French drain - sloping pipe necessary?

Thanks for your responses. I've decided not to slope the pipe significantly. But I will make sure it does not slope away from the pump any.

RE: French drain - sloping pipe necessary?

A couple of comments (similar to ones I've made in your other post)...

1.  Your concrete floor is not thick enough.
2.  Putting unprotected (unwrapped)underdrains under your basement floor provides an opportunity for significant raveling and potential settlement.
3.  Your pump is rated for 2200 gph at 10' TDH.  Pump performance is based on a curve, so if your total head is 15 feet, it will pump at something less than 2200 gph, but nevertheless will pump.  If you are getting 2200 gph infiltration, you have more problems than a pump will solve.
4.  You are trying to intercept groundwater.  If you want to attempt that, you should go outside your foundation and intercept a larger area...either way, you're not likely to be able to intercept it all.

Consider using a mudslab and waterproofing, then pour the floor slab over that.

Again, get a local geotechnical engineer involved.  They know the soil and groundwater conditions better than we, in our remote attempt, can know.

RE: French drain - sloping pipe necessary?

I think I understand now what you mean about an unwrapped trench causing settlement. Keeping fines out is not only to protect the F.D. from plugging, it's also to protect the integrity of the surrounding soil structure by keeping them in their place. Thanks for pointing that out, it's something I had not thought of.

Regarding the pump: I think it sinks to zero gpm at around 20ft or so and then stays there beyond that. As I said in an earlier post, I'm getting significantly less than 2200 performance, but even at that, it's been enough to keep the sump from overflowing. It is possible that I could overcome the pump during an extreme event by installing a F.D. but then the solution is a bigger or second pump. Keep in mind that this would be an extreme rainfall event, 1% chance or less, normally the sump pit is dry even when it's raining.

Mudslab, waterproofing, then pour the floor slab? This is a finished basement. Maybe that's all OK in theory but in practice far more expense/disruption than I'd want to do without trying a F.D. first.

French drains under basement slab are common in this area and typically operate 50y or more when done right, I'm told. I have spoken with people who have lived in 70y old houses with functional F.D.s for more than 25y with never having any trouble. Thing is, they don't know how they were constructed and prof. advice varies somewhat. Every professional installer has their own ways of doing things and it most always it always works out to varying degrees nonetheless. More than one way to skin a cat. I'm just making sure that I catch the big things that matter - like protecting the drain to avoid raveling and potential settlement. Thanks again for pointing that out.

I will investigate getting geotechnical engineering advice as well.


RE: French drain - sloping pipe necessary?


Water moves from a location of a higher energy state to a location of lower energy state along 'the path of least resistance'. This is the kind of stuff that soil and water focused agricultural engineers generally do - and do well...









tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

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