HomeMy WebLinkAbout19223 68th Dr Ne_BLD2410- Inspection Reports Part 2_2025
M E M O R A N D U M
DATE: August 28, 2019
TO: Mr. Dave Prause
FROM: Khaled M. Shawish, PE
Katelyn S. Brower, GIT
RE: Supplemental Stormwater Infiltration Memorandum
R+L Carriers Truck Terminal Development
69th Drive NE
Arlington, Washington
NGA File No. 10572C19
This memo presents our supplemental geotechnical recommendations and opinions regarding the proposed
stormwater infiltration within the proposed R+L Carriers Truck Terminal development project located at
68th Drive NE in Arlington, Washington.
Introduction
We have previously prepared a geotechnical and stormwater evaluation report for the project titled
“Geotechnical Engineering Evaluation – R&L Truck Terminal Development – 68th Drive NE –
Arlington, Washington,†dated October 30, 2018. We concluded that stormwater infiltration was feasible
within the property based on our on-site infiltration testing performed within the site. We generally
encountered native fine to coarse sand and gravel with silt throughout the site at depth. Our infiltration
test within the central-western portion of the site obtained a field measured rate of 27.5 inches per hour
rate. A long-term design infiltration rate of 14.85 inches per hour for infiltration systems within the west-
central portion of the site was provided in our previous report, however, we understand you wish to
perform a supplemental test at the bottom of the proposed infiltration gallery onsite to verify the long-term
design rate.
Supplemental Stormwater Infiltration Memorandum NGA File No. 10572C19
R+L Carriers Truck Terminal Development August 28, 2019
Arlington, Washington Page 2
_______________________________________________________________________________________________
Stormwater Infiltration
We performed a supplemental infiltration evaluation on August 9th, 2019. The 2014 WSDOE Stormwater
Management Manual for Western Washington was utilized to determine the long-term design infiltration rate
of the site soils. At the request of the client and according to this manual, on-site infiltration testing
consisting of the Small-Scale Pilot Infiltration Test (PIT) was used to determine the long-term design
infiltration rates. We conducted a small PIT within the bottom of the proposed infiltration system within
Infiltration Pit 101, as shown on the attached Schematic Site Plan in Figure 21. The subsurface soils
generally consisted of gravel with medium to coarse sand and silt that we interpreted as native glacial soils.
The test was conducted within a pit that measured 4.0-feet long by 3.0-feet wide by 1.0-foot deep. As soon
as the infiltration pit was excavated, the pit was attempted to be filled with 12 inches of water and we began
the soaking period of the PIT. At this time, the water flow rate into the hole was monitored with a water
flow meter for the pre-soak period.
After the soaking period was completed, the water level was maintained at approximately 9-inches for one
hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 4.52 gallons per minute (271
gallons per hour). This equated to an approximate infiltration rate of 36.2 inches per hour. The water was
shut off after the steady-state period and monitored constantly. After approximately 20 minutes, the water
level within the pit had completely dissipated, resulting in an infiltration rate of 27.0 inches per hour.
In accordance with the Table 3.5 of the 2014 WSDOE Stormwater Management Manual for Western
Washington, correction factors of 0.8, 0.75, and 0.9 for CFv, CFt, and the long-term conductivity loss factor,
respectively were applied to the field measured infiltration rate. A total correction factor of 0.54 was applied
to the measured field infiltration rate obtained from the falling head portion of the test to determine the long-
term design infiltration rate. The 27.0 inches per hour rate obtained from the steady state period was utilized
as the overall measured field infiltration rate.
Using the above correction factor, we calculated a long-term design infiltration rate of approximately
14.58 inches per hour, which could be utilized to design the on-site infiltration systems within the gravel
and fine to coarse sand found on this site. We recommend that the base of any on-site infiltration systems
be terminated within the native clean granular soils. We should stress that this rate can be significantly
reduced through over-compaction of the native soils and/or contamination with less permeable materials.
We trust this memorandum should satisfy your needs at this time. Please contact us if you have any
questions or require additional services.
o-o-o
One Figure Attached
NELSON GEOTECHNICAL ASSOCIATES, INC.
Project Number
10572C19
Figure 1
R&L Truck Terminal
Development
Site Plan
(425) 486-1669 / Fax: 481-2510Geotechnical Engineers & Geologists
17311-135th Ave. NE, A-500
Woodinville, WA 98072N
Woodinville OfficeG
A
Nelson Geotechnical
www.nelsongeotech.comAssociates, Inc.
(509) 665-7696 / Fax: 665-7692
East Wenatchee, WA 98802 INF-101
5526 Industry Lane, #2East Wenatchee Office
No.
1
8/22/19Date
OriginalRevision
LEGEND
0 100 200
INF-101
DPN By
Property line Number and approximate
Scale: 1 inch = 100 feet
KSB CK location of infiltration test pit
Reference: Site plan based on a plan dated April 16, 2019 titled "New Buildings for R+L Carriers Arlington, WA," prepared by Group Four, Inc.
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