APPROVED PLN REVIEWED PLN RESUB 4-BLD2022-0926+Geotechnical_Report+1.16.2024_12.10.00_PM+4001742REVIEWED
BY
CITY OF EDMONDS
COBALT
G E 0 S C I E N C E S
July 26, 2023
Updated December 18, 2023
Landsverk Quality Homes
Attn: Duane Landsverk & Joseph Rowett
24113 56th Avenue West
Mountlake Terrace, Washington
RE: Plan Review & Addendum
Proposed Development
19128 94th Ave West
Edmonds, Washington
RESUB
Jan 16 2024
DEVELOPMENT SERVICES
DEPARTMENT
BLD2022-0926
Cobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
In accordance with your authorization, Cobalt Geosciences, LLC has prepared a plan review and
geotechnical addendum letter for the proposed development.
We have reviewed the provided civil plans by IECO dated May 3, 2022 with updates July 2023.
We have no specific comments at this time.
The proposed pipe anchors (concrete blocks keyed into native soils) appear to be suitable from a
geotechnical standpoint. We should verify final locations and subgrade soil conditions for these
features.
We should be on site during construction to verify soil bearing of foundation areas, stormwater
facility placement, temporary excavations, building locations, erosion control, any subsurface
drainage placement, and soil compaction.
We previously emailed the structural engineer regarding soil parameters for use in the concrete
detention vault design. These included an at -rest earth pressure of 55 pcf for walls, a 14H psf
seismic surcharge for at -rest conditions, and a soil bearing pressure of 5,000 psf for vaults set at
least 5 feet below grade. These values may be used for design.
We note that the site is underlain by dense to very dense glacially consolidated soil deposits.
These materials have a low risk of global instability; however, there can be a slight chance of
sloughing within the upper, weathered soil zones, particularly if stripped of topsoils and
vegetation through a wet season without ongoing grading taking place.
The risk of landslide activity is very low for the planned development and adjacent slope areas
provided proper TESC is in place during construction and grading activities are performed in
accordance with the approved plans, permit requirements, and under supervision by the
geotechnical engineer.
The rate of bluff retreat is estimated to be 0 to 2 centimeters annually. This is most likely to occur
where slope magnitudes exceed 70 percent. This would be more consistent with soil creep and
not actual bluff retreat since there is no bluff on site. The risk of landslide runout is essentially
zero since the risk of landslide activity is very low to low. If local sloughing occurred due to
improper clearing, etc., we would conservatively estimate small sloughs to have 3 to 7 feet of
runout.
www.cobaltgeo.com
M
(2o6) 331-1097
December 18, 2023
Page 2 of 2
Geotechnical Addendum
No specific mitigation or mitigation sequencing appears to be warranted based on the type of
construction in conjunction with the geologic conditions. We recommend that work progresses
continuously until completed once the site is prepared with erosion control measures and is
stripped of vegetation. Site grading should include benches along with temporary excavations as
needed and vegetation removal should occur in areas only where work will occur.
Our geotechnical report and recommendations are suitable and reliable for the currently known
soil conditions at the site and our knowledge of the project to date. We should be on site during
construction to verify aspects of construction and to provide any additional input as grading takes
place.
Please contact us if you have any questions.
Sincerely,
Cobalt Geosciences, LLC
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www.cobaltgeo.com (2o6) 331-1097