French Drains?

[MasterPlumbersKeymaster]

I’m putting in a stairwell below ground that goes into a basement. At the bottom of the step I want to put a drain. Can I just drain the water directly in to a pile of crushed rocks under the concrete? I’ve already dug down 2 feet. Is that enough?

[robalaParticipant]

If you tie the drain into the sewer drain, you don’t need to worry about the amount of water that may accumulate at the bottom. I realize that sometimes this isn’t as easy, but it’s worth it.

[John Aldrich1Participant]

BORDER_MAGIC, robala is correct, it is better to drain the storm water from the bottom of the stairwell. However, if you are being served with an onsite sewage treatment system, the hydraulic overloading from a storm event will have a detrimental effect on the proper functioning of the sysem. If you are being served by a centralized sewage collection system, then the sewer authority probably prohibits the practice of storm water drainage into their system for the very same reason.

The best approach is to install a gravity drain pipe to an appropriate storm water drainage area. If this is not possible, then a sump pump controlled by a float switch could be installed, with the discharge pipe going to an appropriate storm water drainage area.

If you live in an arid area with little precipitation, then perhaps a French Drain is the best option. The size of the reservoir of the French Drain is determined by the volume of water to be drained, and the soil percolation rate of the soil below the stairwell.

You must calculate the maximum volume of storm water to be dispersed, then be sure that there is adequate permeability in the soil where it will be applied. Measure the dimensions of the gravel filled basin to determine the volume of the reservoir capacity. The VOID volume of UNIFORM sized gravel, say 3/4 inch washed gravel or 1 inch washed gravel, is approximately 45 percent. Multiply the cubic feet of gravel by .45 to determine the void volume, and then multiply the product of that calculation by 7.48, the number of gallons of water per cubic foot. Size the reservoir to store the volume of water from the maximum storm event. Be sure to grade the soil around the stairwell so that storm water will flow away.

To increase the reservoir capacity of the French Drain, install some plastic leach field chamber units, or possibly some “RainStore” units under the concrete in lieu of, or in addition to gravel. You can view the RainStore product on the web page of the manufacturer. The web page address is:
http://www.invisiblestructures.com

You should be able to design a French Drain with this information, or determine whether or not a French Drain is appropriate for your specific application. Good Luck. JWA

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