GeoechResponseLetter_20170511_Resub.pdf
741 Marine Drive
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Bellingham, WA 98225
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RECEIVED
April 14, 2017
05/11/2017
Project No. 16-0234
Development Services Ctr.
Westgate Woods, LLC
City of Edmonds
C/O Bill Booth
th
2817 NW 94 Street
Seattle, Washington 98117
Re: Geotechnical Engineering Investigation
Edmonds Townhouses
9511 Edmonds Way
Edmonds, Washington
Dear Mr. Booth:
GeoTest Services, Inc. (GTS) is pleased to submit this response to the City of view
comments, provided to us by Harmsen and Associates. GeoTest Services (GTS) has
previously completed a geotechnical report for the above referenced project, titled Edmonds
Townhouses, 9511 Edmonds Way, Edmonds, Washington, dated June 10, 2016.
City Review Comments
Listed below
City Review Comment #3
GeoTest Determined an infiltration rate per the 2005 DOE Manual. The City now uses the 2014
manual.
GTS Response:
In our geotechnical report, GeoTest utilized a grain size approach to determine the feasibility of
infiltration on the property in general accordance with the 2005 Stormwater Manual for Western
Washington. The results of our grain size analysis indicated that an infiltration rate of 2 inches
per hour in predominately granular soil was feasible using this approach.
GTS understands that the City of Edmonds currently uses the 2012 Stormwater Manual for
Western Washington, amended in 2014, as a basis for determining infiltration feasibility. Using
a grain size approach under this manual, GTS determined a Saturated Hydraulic Conductivity
(Ksat) value using the sieve data from our original geotechnical report. The Ksat value was
then reduced through the application of correction factors based on the size of the facility, soil
variability, and the grain size approach in general accordance with the recommendations
presented in the Manual. GTS calculated a corrected, long-term design Ksat value of between
6 and 32 inches per hour using this approach.
It is our understanding that the current design infiltration rate for facilities on site is 2 inches an
hour. It is our opinion that the 2 inch per hour design rate is suitable for use on this project site.
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GeoTest Services, Inc.April 14, 2017
Edmonds Townhouses, Edmonds, WA Job No. 16-0234
City Review Comment #4
GeoTest recommends performing an in-situ infiltration test. No in-situ test results are included.
GTS Response:
The 2014 Manual grain size approach using our historic data produced an infiltration rate, with
built-in correction factors per the Manual, of 6 inches per hour. The infiltration rate used in the
design of the proposed bioretention facility is 2 inches per hour. GTS did not encounter
groundwater or restrictive layers during our explorations that would indicate a further reduction
would be needed to address mounding of groundwater.
It is our opinion that a design rate of 2 inches per hour is adequate for infiltration facilities on this
project site. GTS does not require additional confirmation of infiltration rates (i.e., Pilot
Infiltration Testing) provided that design infiltration rates are 2 inches per hour or less.
Closure
We appreciate the opportunity to provide geotechnical services on this project and look forward
to assisting you during the construction phase. If you have any questions regarding the
information contained in this report, or if we may be of further service, please contact the
undersigned.
Respectfully Submitted,
GeoTest Services, Inc.
Edwardo Garcia, P.E.
Geotechnical Department Manager
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