Dry Well Design

[ONENGINEER]

I have reviewed several publications for Dry well design and summarised as follow:

A perc test is carried out. If a 6" water column was used for perc test we multiply the field result by 2. (to represent a 12" water column).

Then we measure the volume of runoff. Divide the volume of runoff by rock drain porosity and that will give the volume of dry well.

This means that the volume of dry well is independent of the rte of percolation. Is that right! If so the perc result is only used to assess the feasibility of dry well installation.

It seems that I am missing something. I would appreciate if one could enlighten me how the perc test result could affect the dimensions (volume) of the dry well.

 

[cvg]

perc rate controls the rate of disposal. recommend you drain the well within about 36 hours so it can handle another storm

 

[cap4000]

The typical permeability rate for the soils around a drywell is 0.5 inches per hour. What type of soil do you have?

 

[ONENGINEER]

A perc test carried out showed 2.3"/hour under 6" hwad of water. Thank you for the follow up.

 

[fattdad]

(already posted in your "rock pit" thread. I'm not sure that a "perc" test provides all answers so I'll post in this thread too.)

To dig a pit, saturate it for a spell and then return to take time readings with respect to water level is akin to a "perc test." Perc tests are widely used to design drainfields for home septic systems.

Drainfields are shallow trenches that are narrow and filled with gravel. Tyipcally, trench separations are 3 times the trench width. The flow of water is gradual in time, i.e., it's rare that all trenches are filled up completly and time is allowed for the infiltration to occur.

If you are doing a pre-construction engineering assessment in a proposed area for stormwater infiltration, you may anticipate in advance that the bottom of the pit needs to be at 4 or 5 ft. You may drill a hole and find that the soil at that depth is sandy or otherwise favorable for infiltration. Heck you may find out that the hydraulic conductivity is some high value and that would seem to be all that's needed. Maybe not though. . .

Let's say at the depth of 4 ft you have a sand, but at the depth of 7 ft you have a clay. Let's also say that you want to make an infiltration pit that's 20 ft by 20 ft. Will you have the entire 400 sf surface area available for infiltration at that favorable "sand rate?" O.K. let's think about that.

If you have 3 ft of headroom between the bottom of the pond (i.e., 4 ft) and the clay layer, just how many feet of infiltraiton will take place prior to saturating the entire 3-ft interval? Well with a porosity value of 0.33 a one-foot water column would saturate the entire 3 ft sand layer. Then what happens? The water flow is governed by horizontal hydraulic conductivity and it's the horizontal flow through this 3-ft thick sand layer, which represents 240 sf (i.e., about 50 percent of the area in the base of the pond).

If you don't consider the overall flow regime of the problem, you'll run the risk of designing something that won't work.

Just some thoughts on this matter.

f-d

¡papá gordo ain’t no madre flaca!

 

[molerat2210]

Are you sure you have your terminology correct? A percolation test is usually performed by a County Sanitarian for septic field design. Geotechs don't usually perform this since it's a touchy legal issue for zoning.

If you are designing for SWM then we perform infiltration tests in my part of the world. However, there are many factors such as depth to bedrock, groundwater depth, limits on silt and clay content and nature of soil (no fill) to determine if infiltration is feasible. The infiltration test result can be used to size a dry well.

Here in MD this design process if very regimented by the State and local govt reviewers.