Re: Drywell Design

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Re: Drywell Design

From: info@builderswebsource.com
Category: Drainage/Waterproofing
Remote Name: 63.198.181.43
Date: 28 Jul 2001
Time: 06:25 PM

Comments

Proper drywell design requires knowledge of several factors, including the soil percolation rate as well as expected discharge volume over a certain period of time. Heavy clay soil has poor permeability, slowing the discharge process, whereas sandy soils drains rapidly.

In addition, you never want the bottom of your drywell to be less then at least two feet above the peak water table level, which is usually at its highest in Spring. Otherwise, the drywell will simply become a repository for ground water.

The top of the dry well should be well recessed below grade so it doesn't freeze or back up into the house or foundation. This may depend on where you live. If you have underground utilities nearby, the top of the drywell must be below this level (at least three feet below grade, typically).

If you're using a plastic drum, we recommend drilling 1" holes in random patterns all around the side walls and bottom. Be careful not to weaken the container. Wrap the bottom, sides and top with drainage filter fabric to prevent fine soil from clogging the holes. Use at least 1-1/2" diameter washed gravel around the base and outside of the drywell. At least 6" or more of drain rock is recommended. DO NOT fill the plastic drywell with rock. This only reduces your available discharge volume. (Note, in cases where the drywell is simply a hole in the ground with no container, then 1-1/2" to 3" diameter washed rock is recommended to fill the drywell. Filter fabric should be pinned to the bare earth to prevent silt from clogging the rock)

Given these considerations, what follows are guidelines for a typical drywell installation. However, keep in mind that if you are discharging greywater, such as from a washing machine, it's best to use a two-phase system to avoid discharging chemicals and muck into the soil. See the drawing from Architerra Enterprises as an example of a two-part drywell system:

The following drywell design criteria is adapted and reprinted from the City of Eugene, Oregon:

1. The required storage capacity of the drywell shall be determined by subtracting the volume of water that can exfiltrate out of the facility within a 24-hour period from the volume runoff from the contributing basin during a 24-hour, 5-year storm event.

2. The top of the drywell shall be minimum of 36 inches below the ground surface.

3. If the top of the drywell must be closer than 36 inches to the ground surface, special design considerations should be given at property corners (or where utilities must make property connections). A gap in the drywell design or utilities sleeves going through the drywell should be included in the design as needed for utility crossings.

4. The bottom of the drywell shall be no less than two (2) feet above the seasonal high groundwater elevation. (Note: Water quality infiltration systems require a five-foot separation.)

5. Drywells should be placed completely inside of the public right-of-way or within a drainage easement that provides maintenance access and does not conflict with public utility easements.

6. PUE's adjacent to drywells should be increased to nine (9) feet instead of the standard seven (7) feet.

7. Stormwater shall be directed through an approved City sedimentation or pollution control manhole before entering a drywell.

8. A standard cleanout or manhole is required at the end of each drywell.

9. The number and distribution of the holes in the dispersion pipe shall adequately allow the stormwater runoff to exit the pipe while maintaining a maximum hydraulic head elevation six (6) inches below the gutter elevation. Designers using pre-fabricated perforated pipe shall follow the manufacturer's recommendations and provide the sufficient length of pipe to disperse the water adequately. All design calculations and/or manufacturer's recommendations shall be submitted with the design application.

10. Approved drainage fabric shall be placed on the sides and top of the drywell.

11. Drywell shall be backfilled with 1½" to ¾" coarse aggregate for concrete.

12. Provide clear notes and/or graphics stating drywell dimensions for each drywell.

13. Where feasible designers should avoid locating drywells in the following locations:

a) on slopes greater than or equal to 10% and 10 feet in height;

b) within 50 feet of 10% slopes; add 5 feet of setback for each additional percent of slope up to 40% (maximum setback 175 feet);

c) on fill soils;

d) within 20 feet of building foundations on level lots; add 5 feet of setback for each percent of slope;

e) within conservation areas;

f) within wellhead protection areas;

g) where runoff has the potential for contact with hazardous materials or high concentrations of pollutants including, but not limited to, uncovered areas used for industrial activities, for vehicle fueling or maintenance, for storage of toxic or hazardous materials and high vehicle traffic areas (i.e. commercial sites with an ADT > 100 vehicles per 1000 sq.ft. gross building area; a road intersection with ADT > 25,000 on the main roadway and > 15,000 on any intersecting roadway.)

14. Utility concurrences are required for all drywell locations prior to final approval.

In addition to the above, Builders Websource recommends the following additional procautions:

- Drywells should be at least 30 feet from a septic drainfield and 5-10 feet or more from the foundation. The further away the better. Furthermore, drywells should be "downhill" from the house where possible to prevent discharge back towards the foundation.

- A removable screen or filter should be provided at a suitable location to prevent debris from entering the drywell. Periodic maintenance will be required to clean the filter or screen.

- If you do fill the drywell to prevent collapse of the plastic container, use 1-1/2" to 3" dia. washed drain rock. Keep in mind this will reduce your available discharge volume.

We hope these tips help. Keep in mind that these are just examples of techniques which may or may not apply to your particular requirements and soil conditions.

Builders Websource

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