Flow into footing drains

[PEDARRIN2]

I am a plumbing/piping engineer trying to compile a good method to size interior sumps and the pumps to drain/pump water entering foundation drains to the storm water sewer system.

I have found a method that says to use a 10' wide area along the length of the foundation drain for the drainage area. It also says to use 1 gpm/100 sq ft for slow draining soils and twice that for quicker draining soils.

I cannot find the original source material for this method so I am a little leary to use it.

As stated, subsurface drainage is not my specialty so I figured I would ask the question to those who would know about it than I do.

 

[cvg]

percolation rate through soils is highly variable and depends on the type of soil and density. It is also difficult to obtain a reliable value from lab testing, however, you need somewhere to start and to be able to estimate a conservative number. First, I would contact a geotechnical engineer in your area and ask him for an approximate number. Then, you might have him take some samples and perform a test to determine the coefficient of permeability (k). The falling head test would be the quickest (cheaper).

 

[PEDARRIN2]

Thank you for your reply.

Our problem lies when there is no geotechnical or soils engineer on the project. What do we do then to come up with a reasonable flow into our sump inside the building?

 

[cvg]

Permeability of natural soils varies greatly as stated before.

To emphasize this, clean gravel which has very high permeability can transmit water at rates up to 10,000 cubic feet per square foot per day, fine sand might be about 1 cubic foot per square foot per day and clay which has very low permeability is as low as 0.00001 cubic foot per square foot per day flow rate.

Unless you adequately characterize your soil and in-place density, it would be hard to estimate the permeability.

 

[ndpw]

I think that in addition to soil properties you should also be considering the landscape surrounding the foundation. In general, groundwater tends to follow land topography. Therefore, if your foundation was at the bottom of hill or in some sort of valley, I would tend to think you'd want to account for this by upsizing the pump.

I tend to doubt that you can make an accurate generalization that will fit all or even most of your home sites. Using your formula with a large foundation footprint, say 100 x 50, would give you 300 feet of perimeter footing drain times 10 foot wide around it gives 3000 sf. To be conservative let's say a permeable soil at 2 gpm/100 sf requires you to have a 60 gpm pump. Most $100 1/2 hp sump pumps at Home Depot will give you this pumping capacity even with 8-10 feet head. Most footprints I call out in subdivision design are not this large, and therefore would likely only need only a 1/3 hp pump. I personally would not feel comfortable using your formula (and a 1/3 hp pump) for most foundations. I do not have a better formula for you to use, but I thought I'd throw out some potential limitations to your original formula. To me, there wouldn't be much difference in price between calling out a 1/3 hp or 3/4 hp sump pump (certainly less difference in price compared to soil borings and/or geotechnical engineer), and therefore I'd always opt for more pump capacity.

 

[cvg]

the sump design is also critical. the above example estimates about 2 gpm/sf or 60 gpm inflow into your sump. With a 60 gpm pump, you will be pumping continuously 24 hours per day. You need a large sump so you can cycle the pump on and off maybe 2 or 3 times per hour. You also need a pump with larger capacity.

If your sand is even more permeable, than your inflow could be 20 gpm/sf, or even 200 gpm/sf or if it is clay it could be .02 gpm/sf.

You need a basic characterization of your soil before you go any farther.