Sump Pump & French Drain

[rlewistx]

I posted this in the "pump" forum but I thought I might repost it in this forum in case some of you don't monitor the other forum, since this is also a groundwater issue.

I'm helping someone with the design of a french drain system and sump pump. A basement is being put in about 6-12" below the water table. The contractor has installed a french drain system around the basement. It is about 20 ft. x 60 ft. in plan. The gravel is 4 feet below the floor. The perforated pipe is 3 feet below floor. The drain empties into a 4 ft. dia. manhole. The bottom of the manhole is about 40 inches below the french drain pipe. The invert of the french drain into the manhole is about 32" above the bottom.

The flow into the manhole is about 300 gal. per hour. My idea is to fill the manhole up to the french drain invert which is about 40 inches of storage in the manhole, then pump it out.

I have about 18 feet of head to overcome. I used the tables in Graingers for calculating the head based on vertical rise, horizontal run and fittings. I would like to pump out at 25 gal./min. Graingers catolog shows a 1/2 hp pump will do this. The problem I have is the standard pumps don't have the range of on and off that I woul like. I would like a 36 inch range and a teathered switch is only about 9 inches of range rise. I would like this to come on about once an hour to discharge, which is why I want the 36" range.

Any suggestions of how to best tackle this? Should I use a smaller range?

 

[dicksewerrat]

Use a transducer as the switch. They can be set for on/off at whatever level you want.

 

[Focht3]

Side comment: if the soils are clays or silts, be sure that the gravel is wrapped in Mirafi 160N or equivalent. Better yet, use concrete sand as your filter media. Otherwise the gravels will clog, the 'french drain' will fail, the basement will be wet, etc.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[cvg]

how about a float activited switch - similar to the one in your toilet tank?

 

[rlewistx]

This is what I thought would be readily available. It's not. The standard ones have only a short vertical distance to them. Apparently you have to put one of these together from scratch. This is beyond my electrical expertise, which is very little.