Larsen Residence 3rd Peer Review_tm.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: September 14, 2018
rd
RE: 3 Geotechnical Peer Review
Submittal Package for Structural Improvements in a Critical Area
th
Larsen Residence 15729 75 Place West
Edmonds, Washington
rd
This technical memorandum summarizes the results of 3 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. 18-04 -Call Geotechnical and Environmental
Engineering Services Agreement with the City.
LAI previously completed a geotechnical peer review and a supplemental geotechnical peer review for
this project and our initial and supplemental review comments are contained in technical memoranda
dated April 30, 2018 and August 10, 2018, respectivelyam has addressed
rd
these comments and prepared a revised permit application submittal. This 3 geotechnical peer
review focuses on the submittal, which includes the following information:
Cover letter prepared by CG Engineering, dated August 24, 2018
Supplemental Geotechnical Plan Review and Comment Response Letter prepared by Nelson
Geotechnical Associates, Inc. (NGA), dated August 23, 2018.
The following briefly reiterates review comments that are contained in our August 10, 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 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 and it is our opinion
stability model should consider the effect of groundwater on the stability of the slope.
Response: NGA did not encounter groundwater in their explorations. It is their opinion that while
perched groundwater could occur within the upper slope, it is likely minor and would not significantly
impact the existing global stability of the slope.
Review Comment: The slope stability analysis performed by NGA uses a seismic coefficient (k) equal to
h
0.20. Standard of practice in limit equilibrium slope stability modeling is to use a kh value equal to 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.
Response: NGA calculated seismic slope stability using a k equal to half of the PGA (0.27) resulting in
h
a critical slip surface with a factor of safety equal to 1.028, indicating marginally stable conditions
during a seismic event. It is their opinion that the proposed stabilization measures for the residence
do not reduce overall factor of safety from the existing site conditions. Additionally, the critical failure
surface would need to pass through the proposed deep foundation elements, therefore the deep
foundations should enhance global stability of the slope.
Review Comment: NGA states that
systems and protection of steep slopes, we would estimate that the probability of failure on the site is
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.
Response: NGA recognizes that this was a typographical error and has updated the probability of
failure to less than 10 percent in 25 years.
Based on LAIresponses to the peer review comments that are presented in
August 10, 2018 technical memorandum, it is our opinion that comments have been
adequately addressed and that the submittal package includes the information in Sections 19.10 and
23.80 of the ECDC that is identified as being required.
It should be noted that it is opinion that there are several ongoing landslide mechanisms within
the project area beyond the deep seated slide mechanism that is discussed in the NGA letter dated
August 23, 2018. These other landslide mechanisms could include groundwater blowout, high bluff
peeloff, and sliding block failure, all of which are heavily affected by groundwater and surface water.
Because the proposed residential improvements lie within the runout zone of landslides that may
emanate from the adjacent slope, the existing residence could be impacted by landslides caused by
Geotechnical Peer Review
th
Larsen Residence 15729 75 Place West 2 September 14, 2018
Landau Associates
one or more of these slide mechanisms, especially during a seismic event. However,
that the proposed stabilization measures for the residence will not increase the threat of the
geological hazard to the property or adjacent properties beyond predevelopment (i.e., before the
proposed stabilization measures are implemented) conditions.
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
th
15729 75 Place West in Edmonds, Washington. 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 re
professionals to prepare an appropriate design for the site conditions.
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
\[\\\\EDMDATA01\\PROJECTS\\074\\196\\R\\LARSEN RESIDENCE 3RD PEER REVIEW_TM.DOCX\]
Geotechnical Peer Review
th
Larsen Residence 15729 75 Place West 3 September 14, 2018