HomeMy WebLinkAboutPermit Correspondence 1997-8-5
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GEM CONSULTING, INC.
Geologic, Environmental, & Mineral Resource Consulting
P.O. Box 23635, Eugene, OR 97402-0430
(541) 686-1227, Fax 686-2411
August 5, 1997
Kevin Hare
5729 Main Street, #335
Springfield, Oregon 97478
Re: Drywell at 583 Cascade, Springfield, Oregon.
Dear Kevin,
This letter reports percolation rate testing results and provides a typical design for a drywell at 583
Cascade in Springfield. Based on a percolation rate of approximately 1.25 feet per hour (15 inches
per hour) for the native materials and a drainage area of 1125 square feet, a drywell approximately
7 feet by 7 feet and 5 feet deep is appropriate for this site.
The drywell should be lined with filter fabric and backfilled with clean rounded gravel (1/5 - 3
inches in size). The gravel backfill should be covered with filter fabric and covered with
approximately 1 foot of top soil. The drain pipe should consist of a 4-inch PVC pipe should be
placed near the top of the gravel. The pipe should have a screen and an accessible cleanout to
prevent organic debris from clogging the pipe or drywell.
On August 5, 1997, a permeability test was conducted at the proposed location of the drywell.
The test pit had been excavated with a backhoe and was approximately 3 by 7 feet horizontal and
3 feet deep. Groundwater was not encountered. To conduct the falling head perColation test,
water was added to the pit until the water surface had been raised approximately 8 inches.
Percolation of the water was then timed for increments of vertical drop. The results suggest that
the percolation rate ranges from 15 to 30 inches per hour. Therefore, 15 inches per hour was
used for sizing calculations.
Summarv of Calculations:
Tn"butary:
Roof Area of approximately 1125 square feet. One drywell is proposed to take all runoff from
this area.
Rainfall Intensity:
2 year frequency,S minute duration, 2.1 inches per hour as reported on the Rainfall Intensity and
Duration curve for Eugene (OTAK, Inc.).
Calculated Discharge (Q = CIA):
C = 1.0 (constant), I = intensity, A = area
2.1 in/hr x 1125 sq fi x .0833 fi/in x 7.48 gj cu fi = 1472 gph.
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Mr. Kevin Hare
August 5, 1997'
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Soil Type and Permeability:
From visual inspection of tt~st trench and percolation test. The soil consists of dark brown top
soil from the surface to a depth of approximately' 1 foot (SU A). The top soil is underlain by
brown soil consisting of subangular igneous boulders ranging in size form 6 inches to 3 feet, in a
sandy fine grained matrix (SU B). The calculated permeability of SU B is approximately 15 inches
per hour. '
Calculated Requir~ Contact Area:
15 in/hr x 0.0833 ft/in x 7.48 g/ cu ft = 9.3 gph/ sq ft
1472 gph divided by 9.3 gph/ sq ft = 158.3 sq ft
Therefore, required contact area with the soil is approximately .160 square feet.
Calculated Sizing:
Each wall will have 35 square feet of contact with the gravel and the floor will have 49 square feet
of contact area. (35 sq ft x 4)+(49 sq ft) = 161 sq ft. Therefore, dimensions of approximately 7
feet x 7 feet horizontal and 5 feet total depth (4 feet into SU B).
If you have any questions regarding these calculations, the design parameters, or site conditions,
please call us at 686-1227.
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Ir. Gunnar Schleider, CEG
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583 Cascade-Drywell
GEM Consulting, IDe
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