HomeMy WebLinkAboutMEMO 971516 Aberdeen Ave Short Plat Infiltration Comment Renton_RESUBMITTAL M E M O R A N D U M
DATE: February 10, 2017
TO: Mr. Curran Kwan
CC: Mr. Greg Guillen – CG Engineering
FROM: Khaled M. Shawish, PE Lee S. Bellah, LG
RE: Aberdeen Avenue Short Plat Supplemental Memorandum 1824 Aberdeen Avenue
Renton, Washington NGA File No. 971516
INTRODUCTION
This memo presents our responses to review comments raised by the City of Renton regarding the
proposed residence project located at 1824 Aberdeen Avenue in Renton, Washington.
We have previously prepared a geotechnical engineering evaluation for the project site titled “Aberdeen
Avenue Short Plat and Infiltration,” dated October 17, 2016. We understand that concerns have been
raised by the City of Renton Department of Community and Economic Development regarding
geotechnical aspects of the on-site infiltration testing. Their comments were presented in a comment letter
titled “Utility and Transportation Comments for Aberdeen Avenue Short Plat,” dated January 31, 2017,
prepared by Mr. Ian Fitz-James. For our use in preparing this memo, we were provided with Plan Sheets
C1.1 through C3.1 titled “Aberdeen Avenue Subdivision,” dated December 16, 2016, prepared by CG
Engineering. In the following sections, we summarize the geotechnical concern raised by the City of
Renton, followed by our responses to each concern.
Aberdeen Avenue Short Plat Supplemental Memorandum NGA File No. 971516
1824 Aberdeen Avenue February 10, 2017 Renton, Washington Page 2
_____________________________________________________________________________________________
NELSON GEOTECHNICAL ASSOCIATES, INC.
CITY OF RENTON REVIEW CONCERNS AND OUR RESPONSES
Review Comment Surface Water 3.a.:
Section 5.2.1 of the RSWDM requires that infiltration tests be performed at the elevation of bottom of the
proposed infiltration facility (Elev. 293 as shown on plans). Provide verification of the depth of the
infiltration tests. The geotechnical report states that the infiltration tests were performed at approximately
two feet below existing grade which is several feet above bottom of proposed facility
Response:
Based on our review of the provided plans, we understand that stormwater generated within the site will
be directed to a 5-foot wide by 80-foot long by 2.0 foot thick gravel-filled infiltration trench located
within the driveway area along the southern portion of the site. The base of the infiltration trench is
proposed to be at an approximate elevation of 293 feet. At the time our infiltration testing was performed
within the site, the exact location, and overall size and depth of the proposed infiltration system was
unknown. As a part of our previous evaluation of the property, we performed several additional deeper
test pit explorations throughout the property that extended up to 10.0 feet below the existing ground
surface. All of our test pits performed within the site encountered and were terminated within native
glacial recessional outwash soils consisting of fine to medium sand with gravel and silt. Test Pit 1 and
Infiltration Pit 1 located immediately north of the proposed infiltration area were located at a ground
surface elevation of 298 feet. Test Pit 1 extended down to 10.0 feet below the existing ground surface
(approximate elevation of 288 feet) and encountered native glacial recessional outwash soils to the depth
explored. Our on-site infiltration test performed within Infiltration Pit 1 was conducted in the native
recessional outwash soils at a depth of 2.0 feet below the existing ground surface (approximate elevation
of 296 feet). After the infiltration testing was concluded in Infiltration Pit 1, this pit was further
excavated an additional six feet (approximate elevation of 290 feet) and encountered the same granular
native recessional outwash soils to the depth explored. Based on this, it is our opinion that the proposed
infiltration trench located at an elevation of 293 feet should encounter native glacial recessional outwash
soils similar to the soils that we conducted our on-site infiltration testing into, and these soils appear to
extend at least five feet below the proposed infiltration facility. It is also our opinion, that the
recommended long-term design infiltration rate of 7.35 inches per hour should be valid at the planned
infiltration trench depth. We recommend that we be retained during construction to verify the soils
exposed in the entire infiltration area are similar to the soils encountered during our explorations.
Aberdeen Avenue Short Plat Supplemental Memorandum NGA File No. 971516
1824 Aberdeen Avenue February 10, 2017 Renton, Washington Page 3
_____________________________________________________________________________________________
NELSON GEOTECHNICAL ASSOCIATES, INC.
CLOSURE
We trust this memorandum should satisfy your needs at this time. Please contact us if you have any
questions or require additional services.
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