Urgent - Need help sizing a dry well 1

[SteveInVt]

Hi folks,

I'm somewhat out of my element and need help. I'm a small contractor doing residential work here in Vermont. Currently working on removal and replacement of a failing foundation on an addition. Roof area of the addition is 400sq ft. Soil is sand and it drains very nicely. The main house has a full basement and it is always dry. The addition had some old interior floor drains leading to a sump which appears to have been abandoned. Since we're opening the outside anyway, I was planning to run perimeter drains to a dry well and use a gravity approach instead of a sump pump. The dry well is to be sited at the far end of the lot and away from any structures of abutters.

I was just about to open the ground and have just been asked by the local building official to submit a plan for the drainage and dry well. I don't believe he's expecting anything formally engineered, given the small size of the project. I understand the construction part of the drainage system. So, using some conservative assumptions on rainfall (such as 1.25" per 2hr storm) and soil absorption rates, I'm looking for some general calculations that will give me the cubic feet requirements of the well.

Many thanks in advance.

-Steve

 

[DMcGrath]

I am assuming a hole in the ground filled with aggregate. The important thing to remember is to keep the well from clogging. Good selection of clean (washed) aggregate and a filter geotextile will help a lot. A way to clean it would be handy as well, to keep the infiltration up and keep from clogging the surrounding soil.

Now to the design - it is pretty simple. If you assume an infiltration rate for sand (say 4 inches per hour, which is probably half of the actual), this equates to 1/3 cubic foot per hour per square foot of bottom area. 1/3 cubic foot of water = 2.5 gallons. So if the bottom area of your well was 10 square feet, you could drain 25 gallons per hour, or .42 GPM. Without getting too technical, if you conservatively assume you will A) only infiltrate through the bottom and B) your infiltration rate will remain constant over a varying depth, you can use this as a basis.

First figure out the total volume of runoff (you mention 1.25" of rain), how much you will infiltrate per hour, what you don't infiltrate you will have to store. Your volume of storage will be the hole volume - volume of aggregate (assume 50% porosity of your aggregate). The total volume of the hole is a function of area x depth - the reason I mention this is the soils may change at depth (sand goes away?) Check this before you get too far. Also to consider is the water table - kind of hard to infiltrate water into a hole if it is already saturated.

Again I stress the importance of keeping the well clean so it continues to function as long as possible.

Hope this helps.

 

[SteveInVt]

Thanks DMcGrath!

So the distilled version of what I have arrived at is:

dry well dimensions = 7’length, 4’width, 4’depth
dry well volume = 112 cu ft
dry well storage = 56 cu ft
dry well soil infiltration rate = 70 gal/hr

Can someone sanity check this, is it reasonable?

Thanks,
Steve

 

[CarlB]

Seems over large to me - so probably fine if you're not trying to pinch every penny. But then I don't know what all is draining into it. From your explanation I'm guessing perimeter/footing drains that will never carry any water because you have good sandy soils. In general I agreed with McGrath's approach, but the 50% porosity is a littly higher than I'd use (maybe 30%). That still gives 250 gals of storage.