5 Apr 10 17:39
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.