0584_001.pdfGEOTECH
CONSULTANTS, INC.
Silvia Heldridge
930 Glen Street
Edmonds, Washington 98020
via email: sheld6d- e a hotmail.com
2401 loth Ave E
Seattle, Washington 98102
(425) 747-5618
January 21, 2019
JN 18381
sioe YY\
Subject: Response to City of Edmonds Stormwater Review Comments
Proposed Single -Family Residence
923 Glen Street
Edmonds, Washington
Permit No. BLD20181405
Reference: Stormwater Review Comments, Heldridge SFR, City of Edmonds Engineering Division,
November 26, 2018,
Geotechnical Engineering Study, same site, Geotech Consultants, Inc., September 12,
2018.
Dear Ms. Greene:
We prepared a geotechnical engineering study for this project dated September 12, 2018 as
referenced above. In our study, we recommended that onsite infiltration of collected stormwater not
be used on the subject site. We recently received the above -referenced, November 26 review letter
from the City of Edmonds requesting clarification of our infiltration recommendations. Below we
have included the three comments that were included in the review letter (in bold and italics), and
directly thereafter we have given our response to each comment.
a. Update boring logs to better explain/Identify the separate soils layers encountered
and where soils become infeasible for infiltration; currently, the borings appear to
suggest a uniform prol`tle which does not match with the restrictive layers described
at various depths in the text.
Updated Test Pit Logs intended to better distinguish the separate soil layers are attached to
this letter. The portion where the soils became cemented, which is the significant soil
change in the test borings, is now more clearly delineated.
b. Revise text to clarify, explain, and/or describe the cementation observed and impact
on Infiltration (i.e.. Why is this not the material which is "typically suitable for
ingltration'g.
The soil that is noted as being cemented, as described in the text of the study the test pit
logs included in the study, has been glacially -compressed; this has caused the dense
condition of the underlying sand and the cementation. The glacial compression has resulted
in fewer and smaller void spaces between the sand particles, making the soil much less
permeable. As discussed in the General section of our study, we also observed mottling
above the dense underlying sand indicative of groundwater becoming perched on top of the
less permeable underlying soils during the wet winter months. As a result, water ihtrodupedi
Heldridge
January 21, 2019
JN 18381
Page 2
to the upper soils will likely percolated downwards, until it becomes perched on top of the
dense, cemented, soils below; the perched water will then migrate downgradient on top of
the glacially -compressed soils towards the existing basement beneath the northern
neighboring residence.
c. Update text to explain/describe why Test Pit 4 (i.e.. southern portion of lot) was not a
viable location for infiltration; current infeasibility justification focuses on impacts
specific to north portion.
The dense, cemented, glacially -compressed sand was encountered below a depth of 1.5
feet below the ground surface in Test Pit 4 on the southern portion of the subject property.
Furthermore, we understand that the new residence on the subject site will have a
basement. Stormwater infiltrated on the southern end of the site near Test Pit 4, upslope of
the proposed residence, will become perched on the shallow, cemented, glacially -
compressed sand and migrate towards the new basement foundation walls. Thus, we
believe that infiltrating in the area of Test Pit 4 is not appropriate.
If you have any questions this letter, or if we may be of further service, please do not hesitate to
contact us.
Respectfully submitted,
GEOTECH CONSULTANTS,. INC.
Adam S. Moyer
Gegtk�lj�ical Engineer
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01/21/19
D. Robert Ward, P.E.
Principal
Attached: Test Pit Logs
cc: Ryan Rhodes Architecture - Christine Greene
via email. christine()-ryanrhodesdesi ns.com
ASM/DRW:kg
GEOTECH CONSULTANTS, INC.
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TEST PIT 1
Description
over:
crown SAND with gravel, roots, and trace silt, fine- to medium -grained, dry,
loose
-reduced organics content
-reduced silt content, becomes gray -brown, moist, medium -dense
Gray SAND, fine to medium -grained, cemented, very moist, dense (glacially -compressed)
* Test Pit terminated at 5.5 feet on August 15, 2018.
* No groundwater seepage was observed during excavation.
* No caving observed during excavation.
TEST PIT 2
J50
Description
Grass and topsoil over:
FILL Dark -brown SAND with silt, roots, gravel, and plastic and wood debris, fine-grained, dry,
Rust -brown SAND with trace gravel, fine- to medium -grained, moist, loose
Sp 11 -becomes medium -dense
Gray with heavy rust -mottling, SAND with trace gravel, cemented, very moist, dense
(glacially -compressed)
* Test Pit terminated at 5.5 feet on August 15, 2018.
* No groundwater seepage was observed during excavation.
* No caving observed during excavation.
GEOTECH
CONSULTANTS, INC.
TEST PIT LOG
923 Glen Street
Edmonds, Washington
Job Date: Logged by: plate:
18381 Jan, 2019 1 ASM 1 3
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TEST PIT 3
Description
Grass and topsoil over:
Brown SAND with silt, gravel, and brick and concrete debris, fine-grained, dry, loose
(FILL)
Rust -brown SAND, fine-grained, moist to dry, loose
-increased gravel content, becomes brown, moist, medium -dense
* Test Pit terminated at 6.0 feet on August 15, 2018.
* No groundwater seepage was observed during excavation.
* Caving observed between 0 and 4 feet during excavation.
TEST PIT 4
Description
Topsoil over:
Rust -brown SAND with gravel, fine- to medium -grained, moist, loose
Rust -brown gravelly SAND, fine to medium -grained, cemented, medium -dense to dense
(glacially -compressed)
* Test Pit terminated at 4.5 feet on August 15, 2018.
* No groundwater seepage was observed during excavation.
* No caving observed during excavation.
GEOTECH
CONSULTANTS, INC.
TEST PIT LOG
923 Glen Street
Edmonds, Washington
Job Date: Logged by: Plate:
18381 1 Jan. 2019 1 ASM 1 4