IECO Response dated 4.6.16.pdfVIII'
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April 6, 2016
JoAnne Zulauf, Engineering Technician
121 - 5th Avenue North
Edmonds, WA 98020
(425) 771-0220
Re:: Mueller 4 -lot Short Plat (PLN 20150007)
IECO No. 14-0675
Dear Ms. Zulauf,
This letter is to answer an email sent out on March 15, 2016. Within the email there were three
bullet point comments on the proposed storm system. We have listed the bullet points below
along with our response and the response from the geotechnical engineering. We hope that this
clears up any confusion about the design and allows the city and Herrera to continue with the full
review.
The site is classified as having Type D saturated soils, yet the geotech evaluation came up
with a 2.0 inch/hour infiltration rate which is inconsistent infiltration rates that we typically
see in saturated soils.
Geotechnical Response: Soils mapping of the area indicates Type D soils across the site,
however, our on-site subsurface exploration indicates the site is underlain by Type B
sand soils. A design infiltration rate of 2.0 inches per hour is representative and
appropriate for the Type B sand soils present throughout the subject site.
Civil Response: We have recalculated the rain gardens using a Type B soil. Please see the
revised report. '
The geotech report states that groundwater was encountered at 3 feet below existing grades;
however, upon reviewing the soil borings, groundwater was actually encountered as shallow
as F below grade
Geotechnical Response: The groundwater level significant to the proposed raingarden
systems was encountered at depths of three to four feet below existing grades at our test
locations. Shallow surface water influenced zones of seepage trapped within the topsoil
layer were also identified at depths of one to two and one-half feet at test pit locations
TP -103, TP -104, and TP -105. These zones of trapped surface water are not indicative of
the significant groundwater level. In this respect, the seasonal high groundwater level
should be defined at a depth of three feet below existing grades for design of the
proposed stormwater system.
Civil Response: No response needed..
Based on 6" of freeboard, 6" of ponding, and 18" of bioretention soil media (BSM), the
bottom of the rain garden would be 2.5' below grade. Recommendations from Ecology state
that groundwater should be an additional 1' below the bottom of the BSM (3.5' below
grade). Because groundwater was encountered at 1' at 2 locations, 2.5' at 1 location, and 3.5'
at 2 location, three of the rain gardens would be likely to see groundwater intrusion into the
BSM and two of the rain gardens are at the borderline for what is considered to be feasible in
terms of groundwater depths.
Geotechnical Response: It is our opinion that the proposed raingarden system is a good
application for the subject site and meets the objective of the drainage manual for
retaining and infiltrating to the extent practical runoff from the development. Based on
updated site plans, we understand the raingarden have been designed to maintain a
minimum one foot separation from the groundwater level at -depth within the sand.
Civil Response: The rain gardens have been design to keep a minimum of one foot from the
groundwater level. Please see the table below which indicated the groundwater elevation
with the proposed rain garden elevations. The proposed grade is different from the
natural grade, when the test pits were taken, Therefore the separation from groundwater
can be achieved.
Test
Pit
Proposed
Raingarden
Separation at
RainBottom
Surface
Groundwater
Top
Bottom of
Bottom of
of
Garden
Ponding
BSM
BSM and
Elevation
Elevation
Elevation
Elevation
Elevation
Groundwater
1 33.0
30.0
33.5
32.5
31.0
1
2 31.0
28.0
32.0
30.5
29.0
1
3 29.5
26.5
30.0
29
27.5
1
4 31.5
...........
28. 5
.. _. ......
33.0
... .. ... , .._.
32.0
30.5
_
2
5 _34
31.0
34.5
33.5
32.0
1
Please feel free to contact me with any questions or comments you may have.
Sincerely,
1010 -
RI KaIab, PE
PO Box 1478 • Everett, WA 98206 • Phone: (425) 303-9363
Fax: (425) 303-9362 • Email:info@insightengineering.net