Design Of Dry Wells

[Lake06]

There does not seem to be a lot of guidance for the design of dry wells in New York State or in general. Two questions I have which I hope someone can give me some guidance for are;

1. It appears regulations require 2-3 feet of separation from the bottom of dry well to a water table or rock. What is the reason for the 2-3 feet? Treatment? It appears regulations require a greater distance in aquifers.
2. When figuring the area for infiltration, is the surface elevation at the bottom of the dry well the only area to consider? How about the vertical portion from the existing ground to the bottom of the drywell? (the perimeter of the soil wall around the drywell)

 

[cvg]

do you mean soakage pit?

of course you cant place the bottom of your sump on rock or at the water table if you expect to have any percolation through the bottom. maintaining separation from the water table will reduce contamination of groundwater with dirty storm water. recommend more than just 2 feet. if you have shallow groundwater, a drywell may not be an ideal disposal method for your storm water.


use the entire wetted surface for percolation, just be aware that for maximum percolation, you need pressure and so you will get less percolation near the top and more at the bottom. sediment will settle on the bottom, so it may clog over time

http://lmgtfy.com/?q=soakage+pit

 

[Lake06]

Thanks CVG. I don't believe this would be a soakage pit. The intended use would be for stormwater and the drywell would be empty inside with stone surrounding the exterior.

Do you know of any design guidlines that relate to utilizing the whole wetted surface?

When conducting a per test for a drywell design at what depth should the test be conducted in relation to the depth of the proposed drywell?

 

[cvg]

well, according to google, what you are building is a soakage pit for stormwater. around here a drywell would be drilled until hitting a permeable porous strata at least 10-feet above saturated soils and 100-feet away from any water supply well. typically this would be to 100 feet or greater depths.

here is some information on perc testing and drywell design.

Percolation Test Requirement for Retention Basins

Field investigations shall be performed and shall include soil borings and percolation tests taken at the bottom of the proposed basin to obtain percolation rates for use in the design of the stormwater storage facility. The standard procedure to be used is ASTM D 3385-03, Double Ring Infiltrometer. If the soils present are outside the accepted range for application of ASTM D 3385-03, then the use of drywells or shallow retention basins is recommended. Soils outside the acceptable range for ASTM D3385-03 are typically very pervious or very impervious with a saturated hydraulic conductivity greater than about 14 inches/hour or less than about 0.0014 inches/hour. Very impervious soils that are outside the range of applicability for ASTM D3385-03 are not suitable for stormwater percolation disposal system applications. Dry wells may be a better choice for these conditions. If there is a question regarding the applicability of this method for the soils at a particular site, ASTM D 3385-03 should be applied and the results checked against the acceptable range of values of hydrologic conductivity. ASTM D 3385-03 may also not be applicable for dry or stiff soils that will fracture when the rings are installed, or gravels that do not allow penetration by the rings The EPA Method may be used to test soils that are unsuitable for the ASTM D 3385-03 test method with prior approval by the County/District.
Field percolation test values should be reduced by a safety factor when designing any percolation facility (Stahre and Urbonas, 1990). This is necessary because soils will tend to clog with time, which has proven to be a significant cause for basin failure to drain within 36-hours. The design factors for the ASTM method are de-rating safety factors. The design factor to be applied shall be selected from Table 6.16 for the subsurface conditions identified by the soil boring holes. The measured percolation rate shall then be adjusted for design purposes using equation (6.5). The tests shall be performed by a testing laboratory, and the results sealed by a civil engineer, licensed to practice in the State of Arizona. Stormwater disposal by percolation is not allowable if the percolation rate, after application of the design factor, is less than 0.5 inches per hour. Stormwater disposal by percolation is not allowable if groundwater or an impermeable layer is encountered within 4-feet below the bottom of the basin.

Drywells

Drywells shall be designed, operated, and maintained in conformance with the most current __ guidelines. EPA (1980) procedures may be used for estimating initial design percolation rates. The final design rate shall be based on a constant-head percolation test performed on each completed well at the site. The test results for each well shall be de-rated based on the in-situ soil conditions. A de-rating factor of 2 shall be applied for coarse-grained soils (cobbles, gravels and sands). A de-rating factor of 3 shall be applied for fine grained soils (silts and loams). A de-rating factor of 5 shall be applied for clay soils. These de-rating factors are required to compensate for deterioration of the percolation capacity over time in addition to providing a factor of safety for silting and grate obstruction. The accepted design disposal rate for a dry well, after application of the de-rating factor, shall not be less than 0.1 cfs per well. The maximum allowable rate, after application of the de-rating factor, shall not exceed 0.5 cfs per drywell in any case for design purposes.

 

[bimr]

The NY guidelines state that only the bottom area should be used for infiltration.

http://www.dec.ny.gov/docs/water_pdf/swdm2010entire.pdf

Infiltration Testing Requirements
a. Install casing (solid 4-6 inch diameter, 30” length) to 24” below proposed SMP bottom
(see Figure D-1).

 

[psmart]

Like all stormwater management, everyone handles "dry wells" differently. Even the naming is inconsistent. You really need to get local guidance.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Lake06]

bimr, I looking for the section that relates the use of the bottom area only. Are you refering to section 6.3.4 of the NYS guideline?

 

[bimr]

"Infiltration practices shall be designed to exfiltrate the entire WQ/v through the floor of each practice (sides are not considered in sizing)."

Yes, that is my interpretation.