LANDAU COMMENTS 2 - Larsen Residential Peer Review Summary Letter 9.30.2020LANDAU
AssociATEs
September 30, 2020
City of Edmonds
121 5th Avenue North
Edmonds, WA 98020
Attn: Ms. Kristin Johns, Senior Permit Coordinator, City of Edmonds, Building Division
Transmitted via email to: kristin.johns@edmondswa.gov
Re: Supplemental Geotechnical Peer Review
Submittal Package for Larsen Residential Addition
Building Permit No. BLD2020-0564
Edmonds, Washington
LAI Project No. 0074207.020.021
Dear Ms. Johns:
An applicant proposes to construct a residential development at 7329 158th Street Southwest in
Edmonds, Washington (site). Landau Associates, Inc. (LAI) previously reviewed the applicant's permit
submittal package and summarized its review comments in a technical memorandum, dated August 6,
2020. The applicant's design consultants has addressed the comments and prepared a revised
submittal.
At the City of Edmonds' (City) request, LAI has completed a supplemental geotechnical peer review of
the applicant's revised permit submittal package. LAI's peer review was completed in accordance with
the terms presented in Task Order No. 20-08, issued under the On -Call Geotechnical and
Environmental Engineering Services Agreement between LAI and the City, effective January 23, 2020.
LAI reviewed geotechnical portions of the following documents for compliance with Chapters 19.10
and 23.80 of the Edmonds Community Development Code (ECDC):
• ASPI Land Surveying and Planning. 2018. Short Plat. April 23.
• CG Engineering. 2020. Civil Plan Set. Resubmitted September 14.
• CG Engineering. 2020. Structural Plan Set. March 23.
• H2D Architecture + Design. 2020. Architectural Permit Set. July 1.
• Nelson Geotechnical Associates, Inc. 2020. Geotechnical Plan Review Letter. Resubmitted
September 14.
• Nelson Geotechnical Associates, Inc. 2019. Preliminary Geotechnical Engineering Evaluation.
February 28.
The review comments from LAI's August 6, 2020 memorandum are reproduced in italics below. LAI
has included a description of how each comment was addressed in the applicant's resubmittal.
130 2nd Avenue South • Edmonds, Washington 98020 • (425) 778-0907 • www.landauinc.com
Geotechnical Review — Larsen Residential Addition
Landau Associates
Review Comment: In its preliminary evaluation, Nelson Geotechnical Associates, Inc. (NGA)
recommends constructing a debris catchment wall in, or adjacent to, the proposed addition;
however, the evaluation does not include lateral earth pressures that can be used to design the
wall. The evaluation should be updated to include lateral earth pressures, consistent with a
debris flow impact against the face of the wall.
Response: NGA's revised Geotechnical Plan Review Letter provides lateral earth pressures that
can be used to design the recommended debris catchment wall. NGA recommends a lateral
earth pressure value of 100 pounds per cubic foot for the active condition. Catchment
structures must be designed to withstand the impact of debris and to contain the anticipated
volume of debris. A site -specific study should be performed to determine the size of the
landslide, the zone of debris runout, and the velocity of the landslide debris at the proposed
catchment location. In addition, the height of the catchment wall must be sufficient to
prevent overtopping, and the wall must be capable of withstanding impact loads.
Based on the recommendations in the revised Geotechnical Plan Review Letter, it is not clear
that NGA considered impact loads or the anticipated volume of debris. NGA should confirm
that its design recommendations include impact loads and the anticipated volume of debris.
• Review Comment: The debris catchment wall recommended by NGA is not identified in the
plan sets reviewed by LAI. The applicant's permit submittal should be updated to include the
proposed location and configuration of the debris catchment wall.
Response: This comment has not been addressed. To maintain the effectiveness of the wall,
debris must be cleared following a landslide event. The location and configuration of the wall
should be included in the plan sets to verify that an excavator can access the catchment wall
and remove debris that has collected behind the wall. Additionally, a drainage layer should be
included on the upslope side of the wall to promote dewatering of landslide debris. Inclusion
of a drainage layer will reduce static loads on the wall and facilitate excavation of debris.
Review Comment: NGA's preliminary evaluation does not recommend a minimum building
setback from the hazard area, as required in Section 23.80.070.1 of the ECDC. The evaluation
should be updated accordingly.
Response: LAI recognizes that a debris catchment wall is proposed in lieu of a minimum
building setback from the hazard area. This approach does not accord with the exact
requirements of the ECDC; however, the concept is generally understood and should meet
development standards, provided the debris catchment wall is designed and constructed
adequately. If the wall is designed and constructed adequately, LAI will consider this review
comment resolved.
• Review Comment: NGA's preliminary evaluation does not recommend a buffer distance from
the hazard area, as required in Section 23.80.070.2 of the ECDC. The evaluation should be
updated accordingly.
Response: If the debris catchment wall is designed and constructed adequately, LAI will
consider this review comment resolved.
Review Comment: NGA's preliminary evaluation does not confirm that the proposed addition
meets the development standards in Section 23.80.060 of the ECDC. The evaluation should be
updated accordingly.
September 30, 2020
Geotechnical Review — Larsen Residential Addition
Landau Associates
Response: NGA's revised Geotechnical Plan Review Letter states that the proposed residential
addition meets the development standards described in Section 23.80.060 of the ECDC. In
LAI's opinion, this comment has been addressed.
This letter has been prepared to facilitate the City of Edmonds' evaluation of permit submittal
documents related to the proposed residential addition at 7329 158th Street Southwest in Edmonds,
Washington. Landau Associates reviewed geotechnical portions of the permit submittal 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. Any review of non-
geotechnical documents was completed for the sole purpose of verifying compliance with the
recommendations in the geotechnical report. Landau Associates' peer review does not diminish the
responsibility of the applicant's geotechnical and design consultants.
If you have questions or comments, please contact the undersigned at (425) 329-0251 or at
sgertz@landauinc.com.
LANDAU ASSOCIATES, INC.
Sean Gertz, PE
Senior Project Engineer
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Steven R. Wright, PE
Principal
BCS/SMG/SZW/mcs
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September 30, 2020