Supplemental Peer Review_tm 8-10-18.pdf
Technical Memorandum
TO: Kristin Johns, Senior Permit Coordinator
City of Edmonds Development Services Department, Building Division
FROM: Sean Gertz, EIT and Steven Wright, PE
DATE: August 10, 2018
RE: Supplemental Geotechnical Peer Review
Submittal Package for Structural Improvements in a Critical Area
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Larsen Residence 15729 75 Place West
Edmonds, Washington
This technical memorandum summarizes the results of Landau Associates (LAI) supplemental
geotechnical peer review of the permit submittal package that was submitted to the City of Edmonds
(City) for proposed structural improvements to the above-referenced residence, which is located
within a critical area in Edmonds, Washington. The purpose of our geotechnical peer review is to
review portions of the submittal package and assess its compliance with the City development and
building permit requirements as contained in Edmonds Community Development Code (ECDC)
Chapter 19.10 for sites designated within or near the North Edmonds Earth Subsidence and Landslide
Hazard Area (ESLHA). This review also assesses the submittals compliance with the requirements of
Chapter 23.80 of the ECDC as required or noted in Chapter 19.10. This geotechnical peer review was
conducted in accordance with Task Order No. 18-03 -Call Geotechnical and
Environmental Engineering Services Agreement with the City.
LAI previously completed a geotechnical peer review for this project and our initial review comments
addressed these comments (and other review comments by others) and prepared a revised permit
application submittal. This supplemental geotechnical peer review focuses on the revised permit
application submittal.
LAI reviewed the following information forwarded by the City:
Cover letter prepared by CG Engineering, dated June 26, 2018
Building Construction Plans (stamped by Engineer) prepared by CG Engineering on February
21, 2018, revised on June 26, 2018
Geotechnical Plan Review and Comment Response Letter prepared by Nelson Geotechnical
Associates, Inc. (NGA), dated July 25, 2018.
The following briefly reiterates our review comments that are contained in our April 30, 2018
technical memorandum and states how each comment was addressed in the most recent submittal.
130 2nd Avenue South Edmonds, Washington 98020 (425) 778-0907
Landau Associates
Review Comment: The vicinity map does not identify existing conditions such as known landslide
masses, debris flows, or mud flows on or near the site.
Response: The vicinity map has been updated on Sheet S0.1 of the Building Construction Plans to
show the known slide area. NGA was unable to find documentation on any known debris flows or mud
flows within the immediate vicinity of the property.
Review Comment: The topographic survey does not show distances to existing habitable structures on
or near the site, as required in ECDC 19.10.030(D).
Response: Distances have been added to the site plan presented on Sheet S0.1 of the Building
Construction Plans.
Review Comment: The applicant did not submit a separate topographical drawing showing proposed
grade contours at 2-foot intervals, as required in ECDC 19.10.030(E.7).
Response: All work is within the footprint of an existing building and site grades will not change. As a
result, there are no proposed grade contours for the applicant to present.
Review Comment: The geotechnical report, dated February 28, 2018 by NGA does not provide analysis
of the rate of retreat of the bluff nor 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, as required in ECDC 19.10.030(H.6) and ECDC 23.80.050(F.2.e).
Response: The supplemental geotechnical response by NGA provides results of this analysis.
Review Comment: The geotechnical report, dated February 28, 2018 by NGA does not describe public
and private sewage disposal systems, as required in ECDC 23.80.050(F.2.c).
Response: The supplemental geotechnical response by NGA provides this information.
Review Comment: The geotechnical report, dated February 28, 2018 by NGA does not consider the
run-out hazard of landslide debris and/or the impacts of landslide run-out on downslope properties, as
required in ECDC 23.80.050(F.2.f).
Response: The supplemental geotechnical response by NGA addresses run-out hazard and potential
impacts on downslope properties.
Review Comment: The geotechnical report, dated February 28, 2018 by NGA does not adequately
assess the stability of the slope, as required in ECDC 23.80.050(F.2.g). The geotechnical engineer
should confirm the stability of the slope through modeling or numerical analysis and present the
results in the geotechnical report.
Geotechnical Peer Review
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Larsen Residence 15729 75 Place West 2 August 10, 2018
Landau Associates
Response: The supplemental geotechnical response by NGA provides results of this analysis.
Review Comment: The applicant did not submit documentation of required bonds, frozen funds, or
adequate instrument of credit,
insurance, as required by ECDC 19.10.030(J).
Response: It is our understanding that the City will collect the above listed items prior to permit
issuance.
The submittal package that LAI reviewed generally addresses the comments in our April 30, 2018
technical memorandum and includes the majority of the information in Sections 19.10 and 23.80 of
the ECDC that is identified as being required. However, LAI found the following deficiencies in the
information that was submitted:
The slope stability analysis performed by NGA does not appear to factor in the effect of
groundwater on the stability of the slope. Our experience suggests that groundwater can
impact the stability of slopes within the North Edmonds ESLHA
slope stability model should consider the effect of groundwater on the stability of the slope.
The slope stability analysis performed by NGA uses a seismic coefficient (k) equal to 0.20.
h
Standard of practice in limit equilibrium slope stability modeling is to use a k value equal to
h
one half of the peak ground acceleration (PGA). Per the online USGS Seismic Hazard Design
Tool, the PGA at the site is equal to 0.543g.
NGA states that nage and maintenance of the drainage systems and
protection of steep slopes, we would estimate that the probability of failure on the site is likely
If an event will never occur, its probability of
occurrence is 0 (or 0 percent). If an event is absolutely certain to occur, its probability of
occurrence is 1 (or 100 percent). Therefore, we do not understand what is meant by a
probability of less than 310 percent.
This technical memorandum has been prepared for use by the City of Edmonds in evaluating the
adequacy of permit submittal documents related to the proposed residential structure located at
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15729 75 Place West in Edmonds, Washington. The focus of this review was the geotechnical aspects
of the resubmitted application. The purpose of the review was to assess the adequacy of the
application documents for compliance with City requirements contained in ECDC 19.10 and
conformance with conventionally accepted geotechnical engineering practices. This geotechnical peer
prepare an appropriate design for the site conditions.
Geotechnical Peer Review
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Larsen Residence 15729 75 Place West 3 August 10, 2018
Landau Associates
We appreciate this opportunity to be of service to the City. Please contact us if you have any
questions or if we may be of further service.
LANDAU ASSOCIATES, INC.
Sean M. Gertz, EIT
Project EIT
Steven R. Wright, PE
Principal
SMG/SZW/rgm
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Geotechnical Peer Review
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Larsen Residence 15729 75 Place West 4 August 10, 2018