APPROVED Koh Residence Supplemental Geotechnical Peer Review 12.28.2023LALSANDAU
December 28, 2023
City of Edmonds
121 5th Avenue North
Edmonds, WA 98020
Attn: Ms. Denise Nelson, CPT 1 Senior Permit Coordinator, City of Edmonds Building Division
Transmitted via email to: denise.nelson@edmondswa.gov
Re: Supplemental Geotechnical Peer Review
Revised Permit Submittal Package - Koh Residence
Building Permit No. BLD2023-0640
Edmonds, Washington
Landau Associates Project No. 0074219.030.031
Dear Ms. Nelson:
An applicant proposes to construct a single-family residence at 15604 75th Place West in Edmonds,
Washington (site). The site is located in the North Edmonds Earth Subsidence and Landslide Hazard
Area.
Landau Associates, Inc. (Landau) previously reviewed the applicant's permit submittal package and
summarized its findings in a letter dated July 28, 2023. The applicant's design consultants have
prepared a revised permit submittal to address Landau's review comments.
At the City of Edmonds' (City) request, Landau has reviewed the revised permit submittal. The review
was completed in accordance with the scope outlined in Task Order No. 23-14, issued under the
amended 2021-2023 Professional Services Agreement between the City and Landau.
Landau reviewed geotechnical portions of the following documents for compliance with Chapters
19.10 and 23.80 of the Edmonds Community Development Code (ECDC):
• Bush, Roed & Hitchings, Inc. 2023. Revised Civil Plans. November 20.
• Motionspace Architecture + Design, PLLC. 2023. Revised Architectural Plans. November 20.
• Motionspace Architecture + Design, PLLC. 2023. Permit Corrections Review Cycle 1 (RC1) for
BLD2023-0640. December 18.
• Nelson Geotechnical Associates, Inc. 2023. Geotechnical Comment Response. December 8.
• Nelson Geotechnical Associates, Inc. 2023. Geotechnical Engineering Evaluation. May 23.
Comments from Landau's July 2023 review are reproduced below in italics. The comments are
followed by a brief assessment of the applicant's response:
155 NE 100th St, Ste 302 • Seattle, WA 98125 • 206.631.8680 • www.landauinc.com
City of Edmonds - Revised Permit Submittal Package for Koh Residence Landau Associates
Review Comment: Per Section 19.010.030(D) of the ECDC, vicinity maps shall identify all known
landslide masses, debris flows, or mud flows on or near the site which could threaten proposed
structures within 100 feet (ft), as referenced in the geotechnical evaluation. To be in
accordance with the ECDC, these items should be added to the vicinity maps prepared by Bush,
Roed & Hitchings, Inc. and Motionspace Architecture + Design, PLLC.
Response: The Geotechnical Comment Response letter by Nelson Geotechnical Associates
(NGA) indicates that there are no known landslide masses, debris flows, or mud flows near the
site, other than the Earth Subsidence and Landslide Hazard Area. In Landau's opinion, this
comment has been adequately addressed.
Review Comment: Per Section 19.010.030(F)8 of the ECDC, civil plans shall identify grading
plans, temporary and permanent shoring plans, and top and toe of slope setbacks. To be in
accordance with the requirements of the ECDC, temporary and permanent shoring plans as
well as top and toe of slope setbacks should be added to the civil plans prepared by Bush, Roed
& Hitchings, Inc.
Response: The Geotechnical Comment Response letter by NGA indicates that no temporary or
shoring systems are anticipated to be required. The Civil Plans by Bush, Roed & Hitchings, Inc.
have been updated to depict the recommended 50-foot buffer. In Landau's opinion, this
comment has been adequately addressed.
Review Comment: Per Section 19.010.030(H)3 of the ECDC, the geotechnical evaluation shall
include an analysis of material gathered through appropriate explorations, such as borings or
test pits, extending to a minimum depth of six ft below the proposed lowest footing or pile. The
two exploratory borings completed by Nelson Geotechnical Associates, Inc. did not extend to 6
ft below the tip elevations of the proposed augercast piles. To be in accordance with the ECDC,
additional explorations should be completed that extend to a minimum of 6 ft beyond the
lowest proposed augercast pile tip elevation.
Response: In its Geotechnical Comment Response letter, NGA has reduced the depths of
proposed deep foundation elements to a minimum of 30 ft below ground surface, which is at
least 6 ft shallower than the explorations completed at the site. Updated pile capacity
recommendations have been provided for this new deep foundation configuration. In
Landau's opinion, this comment has been adequately addressed.
• Review Comment: Per Section 19.010.030(H)6 of the ECDC, the geotechnical engineer shall, as
a part of the building permit process, provide analysis of the rate of retreat of the bluff
prepared by a geologist and estimate the bluff retreat amount and regression rate for periods
of 25 and 125 years. To be in accordance with the ECDC, this information should be added to
the geotechnical evaluation by Nelson Geotechnical Associates, Inc.
Response: The Geotechnical Comment Response letter by NGA presents estimates of bluff
retreat amount over periods of 25- and 127-years. In Landau's opinion, this comment has
been adequately addressed.
• Review Comment: Per Section 23.80.050(F)1.d of the ECDC, the hazard analysis shall include an
estimate of slope stability as well as an estimate of the effect construction and placement of
structures will have on the slope over the estimated life of the structure. To be in accordance
with the ECDC, this information should be added to the geotechnical evaluation by Nelson
Geotechnical Associates, Inc.
December 28, 2023
City of Edmonds - Revised Permit Submittal Package for Koh Residence Landau Associates
Response: The Geotechnical Comment Response letter by NGA includes discussion regarding
the stability of the slopes and the effect of the construction of structures on the stability of
the slope over the estimated life of the structure. In Landau's opinion, this comment has been
adequately addressed.
Review Comment: Per Section 23.80.050(F)1.e of the ECDC, the hazard analysis shall include an
estimate of the bluff retreat rate or an estimate of the percent risk of landslide area expansion
that recognizes and reflects potential catastrophic events such as seismic activity or a 100-year
storm event. To be in accordance with the ECDC, this information should be added to the
geotechnical evaluation by Nelson Geotechnical Associates, Inc.
Response: The Geotechnical Comment Response letter by NGA includes estimates of bluff
retreat rate and an estimate of the percent risk of landslide area expansion. In Landau's
opinion, this comment has been adequately addressed.
Review Comment: Per Section 23.80.050(F)1.f of the ECDC, the hazard analysis shall include
consideration of the run -out hazard of landslide debris and/or the impacts of landslide run -out
on downslope properties. To be in accordance with the ECDC, this information should be added
to the geotechnical evaluation by Nelson Geotechnical Associates, Inc.
Response: The Geotechnical Comment Response letter by NGA includes discussion regarding
the run -out hazard of landslide debris and the impacts of landslide run -out on downslope
properties. In Landau's opinion, this comment has been adequately addressed.
Review Comment: The geotechnical report by Nelson Geotechnical Associates, Inc.
recommends a buffer of 50 ft from the top of the slope to the west of the project site. Based on
a review of the Civil Plans and Architectural Plans by Bush, Roed & Hitchings, Inc. and
Motionspace Architecture + Design, PLLC, the proposed structure will be situated
approximately 6 ft away from the identified landslide hazard area. This discrepancy should be
addressed in future permit submittals.
Response: The Geotechnical Comment Response letter by NGA provides clarification regarding
the extent of the recommended 50-foot buffer. The Civil Plans by Bush, Roed & Hitchings, Inc.
have been updated to depict the recommended 50-foot buffer. In Landau's opinion, this
comment has been adequately addressed.
• Review Comment: Sheet C3.0 of the Civil Plans by Bush, Roed & Hitchings, Inc. identifies
retaining walls designed by others on both sides of the proposed driveway. Design plans for
these retaining walls should be included in the permit submittal package for review.
Response: Sheet C3.0 of the Civil Plans by Bush, Roed & Hitchings, Inc. has been updated to
include top of wall and bottom of wall elevations. Sheet S1.5 of the Architectural Plans by
Motionspace Architecture + Design, PLLC has been updated to include a retaining wall
structural detail. In Landau's opinion, this comment has been adequately addressed.
• Review Comment: The structural calculations and architectural plans by Motionspace
Architecture + Design, PLLC indicate that the proposed structure will be supported on 16-inch
diameter augercast piles; however, the geotechnical engineering report by Nelson
Geotechnical Associates, Inc. only provides axial capacities for 18-inch diameter augercast
piles. The architectural plans and structural calculations should be updated to reference the
information provided in the geotechnical report. Alternatively, the geotechnical report may be
updated to provide axial capacities for 16-inch diameter augercast piles.
December 28, 2023
City of Edmonds - Revised Permit Submittal Package for Koh Residence Landau Associates
Response: The Architectural Plans by Motionspace Architecture + Design, PLLC have been
updated to depict the use of 18-inch diameter augercast piles. The Geotechnical Comment
Response letter by NGA includes updated axial capacity recommendations for 18-inch
diameter augercast piles with reduced tip elevations. In Landau's opinion, this comment has
been adequately addressed.
This letter has been prepared to facilitate the City of Edmonds' evaluation of permit submittal
documents for the proposed single-family residence at 15604 75th Place West in Edmonds,
Washington. Landau reviewed geotechnical portions of the documents for compliance with Chapters
19.10 and 23.80 of the Edmonds Community Development Code and for conformance with
conventionally accepted geotechnical engineering practices. Landau's peer review does not diminish
the responsibility of the applicant's geotechnical consultant to serve as the project's Geotechnical
Engineer of Record. Furthermore, the applicant's consultants are responsible for preparing a design
suitable for the site conditions.
Closing
If you have questions or comments, please contact Sean Gertz at 425.329.0251 or at
sgertz@landauinc.com.
LANDAU ASSOCIATES, INC.
Sean Gertz, PE
Associate
SMG
[P:\074\219.030\R\KOH RESIDENCE SUPPLEMENTAL GEOTECHNICAL PEER REVIEW 12.28.2023.DOCX
References
City of Edmonds. Edmonds Community Development Code. Title 19 Building Codes. Chapter 19.10
Building Permits — Earth Subsidence and Landslide Hazard Area.
City of Edmonds. Edmonds Community Development Code. Title 23 Natural Resources. Chapter 23.80
Geologically Hazardous Areas.
December 28, 2023