HomeMy WebLinkAboutRS_Infiltration_Report_180315_v1.pdfAges Engineering, LLC
A Geotechnical & Environmental Services LLC
Lakh Sandhu
9329 S. 2001h Street
Kent, WA. 98031
P.O. Box 935
Puyallup, WA. 98371
Main (253) 845-7000
www.agesengmeerng.com
October 16, 2017
Project No. A-1362
Subject: Infiltration Testing
Sandhu Residential
18809 — 116t1, Avenue SE
Renton, Washington
Parcel Number: 3223059075
Reference: 1. Preliminary Geotechnical Report, Sandhu Residence, prepared by Ages Engineering,
LLC and dated August 16, 2017
Dear Mr. Sandhu,
As requested, we performed two Small Pilot Infiltration Test (Small PIT) on the subject site
located at 18809 — 116t1i Avenue SE in Renton, Washington. We previously prepared a geotechnical
study for the subject site and presented our findings in the referenced report. In the report, due to the site
being underlain with dense glacial till, it was determined infiltration would not be feasible. According to
Section C.1.3 in the City of Renton Municipal Code, onsite infiltration testing should be performed to aid
in the determination of infiltration feasibility. We discussed these requirements with the City of Renton.
Based on these discussions, we performed two "Small PIT" infiltration tests on the site. One test was
performed in the upper native, medium dense, weathered glacial till soils, and one test was performed in
the deeper native, dense, unweathered glacial till soils.
INFILTRATION TESTING
We visited the site on October 11, 2017 to perform two Small PITS in the approximate center of
the subject site. The infiltration tests were performed in accordance with the specifications provided in
the 2014 Stormwater Management Manual for Western Washington and Reference 6-A of the 2017 City
of Renton Storm Water Design manual.
To prepare the infiltration test areas, we utilized an excavator to excavate the area around the
locations. The two locations were prepared differently since the tests were performed at different
elevations. Location 1 was towards the eastern side of the site center and Location 2 was towards the
western side of the site center. Location 1 was excavated to a depth of 3.0 feet below surface grades and
the size was approximately 4.0 feet by 4.0 feet equating to a surface area of 16.0 square feet. The test in
location 1 was performed in the native, medium dense, weathered glacial till soils. Location 2 was
Mr. Lakh Sandhu
October 16, 2017
initially excavated to a depth of 5.0 feet below surface grades. The surface area was approximately 5.0
feet by 5.0 feet equating to a surface area of 25.0 feet. Within this area, the actual infiltration test area
was excavated to a depth of 7.0 feet below current grades in the area (an additional 2.0 feet). The side
slopes were laid back at an approximate 0.75:1 inclination to avoid caving during the testing procedure.
The bottom of the infiltration test pit in Location 2 measured approximately 4.0 feet by 3.0 feet equating
to a surface area of 12.0 feet.
We performed the infiltration testing on the site during a period of dry weather at the end of a
relatively dry summer season. The actual infiltration rate of Location 1 was 3.25 inches per hour. The
actual infiltration rate of Location 2 was 0.45 inches per hour. Due to the test being performed in the dry
season, we recommend a Correction Factor equal to 0.33 be used in design.
CONCLUSIONS AND RECOMMENDATIONS
Based on our infiltration testing on the site, it is our opinion full infiltration of the development
storm water is not feasible due to the relatively shallow restrictive glacial till layer beneath the site.
However, small scale BMP's such as permeable pavement and/or bioretention facilities will be feasible.
The deep dense glacial till observed below 6.0 feet has an actual infiltration rate of 0.45 inches
per hour. With the correction factor applied, the rate would be 0.15 inches per hour. We did not perform
a mounding analysis of the site, and based on our understanding of the site history and geology, expect
ground water seepage along the eastern end of the site may occur if full infiltration is utilized.
The relatively shallow native medium dense glacial rill observed above 6.0 feet has an actual
infiltration rate of 3.25 inches per hour. With the correction factor applied, the rate would be 1.07 inches
per hour. Shallow facilities such as permeable pavement, bioretention planters and rain gardens would be
suitable for the storm water collected on each individual lot. Storm water runoff collected in roadway or
driveway catch basins should discharge off site.
We trust this information is sufficient for your current needs. If you have any questions, or
require additional information, pleas c
Respectfully Submitted,
Ages Engineering, LL rn
Bernard P. Knoll, Il, P.E.
Principal
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