REVIEWED PLN ENG BLD Geotech ReportSullivan Deck
1128 Sea Vista Place
Edmonds, Washington
Tom Sullivan
c/o Tom Leren, The Leren Company Inc.
1128 Sea Vista PI
Edmonds, WA 98020-4653
Attention: Mr. Tom Leren
RECEIVED
April 2, 2021 Jul 162021
ciry of EOMONos
Project #21021 "VEO AENT ERV'CEs
Subject: Geologic Hazards Assessment and Soil Design Recommendations
Sullivan Deck — (CRA2021-0007)
Mr. Leren:
The purpose of this letter is to provide and assessment of geologic hazard areas on the subject
property in response to the referenced critical areas determination (CAD) (City of Edmonds,
February 2, 2021), their implications to the proposed development, and to provide soil -related
design information for the proposed project. Based on our correspondence, it is our
understanding that you intend to design an elevated deck servicing the existing home that will
replace and be larger than the existing deck. The deck footprint expansion will be to the west
of the existing home. For the sake of brevity, this letter will focus on the area to the west of the
existing home, and it will assume that readers have general knowledge of the subject property
and the proposed deck replacement/addition.
Site and Geologic Conditions
Our assessment of the subject property is based on our site reconnaissance performed on
March 30, 2021, review of the referenced geologic map (Minard, 1983), and review of past
nearby project experience (SFG, June 19, 2017).
Site Conditions
The area immediately to the west of the existing home was a grass lawn that sloped very gently
down to the west. At the edge of the maintained yard there was a fence and then the slope
was steeper. The slope descended at approximately 30 to 40 percent, and this steeper portion
had an overall height of approximately 8 feet, based on our visual estimates. The steeper
portion of the slope was covered with dense juniper and laurel shrubs, blackberry vines, and
typical ground cover vegetation and associated organic detritus. There were some small wood
cribbing retaining walls near the toe of the slope on the northern portion of the slope. These
wood cribbing walls were on the order of 3 feet in height. We did not observe any signs of
springs or groundwater seepage, accelerated erosion, soil movement, or soil settlement during
our site reconnaissance.
Geologic Setting
Based on surface exposures, our previous nearby work, and the geologic mapping, the native
soils underlying the subject property are Vashon lodgement till sediments. Lodgement till is a
dense, poorly sorted (well graded) mixture of clay, silt, sand, and gravel that was deposited at
the sole of the Vashon glacial ice. Vashon lodgement till sediments, due to their glacially
SOUTH FORK GEOSCIENCES, PLLC Page 1
PO Box 1 275
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Sullivan Deck
1128 Sea Vista Place
Edmonds, Washington
April 2, 2021
Project #21021
consolidated nature, are relatively dense in place, have low permeability, and are moderately
to highly resistant to erosion.
Geologic Hazards Assessment
Summary
The City of Edmonds has mapped geologic hazard areas on and adjacent to the subject
property. There is a mapped erosion hazard area on the subject property and a mapped
landslide hazard area on the adjacent property to the south. These mapped geologic hazard
areas are shown in the attached figure from the referenced City of Edmonds CAD. However, it
is our opinion that the erosion hazard area is a stable erosion hazard area, and no mitigation is
necessary. It is our opinion that the proposed development does not adversely impact or is not
adversely impacted by the potential mapped landslide hazard area to the south due to the
orientation of the slope and the low overall total relief. The details or our conclusions are set
forth in the following sections.
Erosion Hazard Areas
The City of Edmonds Community Development Code (ECDC) defines an erosion hazard area as
follows:
23.80.020 Designation of specific hazard areas.
A. Erosion Hazard Areas. Erosion hazard areas are at least those areas identified by the
U.S. Department of Agriculture's Natural Resources Conservation Service as having a
"moderate to severe," "severe," or "very severe" rill and inter -rill erosion hazard.
Erosion hazard areas are also those areas impacted by shoreland and/or stream bank
erosion. Within the city of Edmonds erosion hazard areas include:
1. Those areas of the city of Edmonds containing soils that may experience severe to
very severe erosion hazard. This group of soils includes, but is not limited to, the
following when they occur on slopes of 15 percent or greater:
a. Alderwood soils (15 to 25 percent slopes);
b. Alderwood/Everett series (25 to 70 percent slopes);
c. Everett series (15 to 25 percent slopes);
2. Coastal and stream erosion areas which are subject to the impacts from lateral
erosion related to moving water such as stream channel migration and shoreline
retreat;
3. Any area with slopes of 15 percent or greater and impermeable soils interbedded
with granular soils and springs or ground water seepage; and
4. Areas with significant visible evidence of ground water seepage, and which also
include existing landslide deposits regardless of slope.
Other than the "Al" condition (USDA-NRCS mapping), none of the other conditions are present
on the subject property. The native soils map as "Alderwood series", but we presume that this
slope was regraded in association with the existing residential development, so the natural soil
pedology has likely been disturbed. Also, due to the organic detritus and vegetation cover of
the steeper portions of the slope, there are no conditions present that would warrant a
description of "moderate to severe, severe, or very severe rill and inter -rill erosion hazard". As
such, it is our opinion that this area is a stable erosion hazard area and should not be
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NORTH BEND, WA 98045?
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Sullivan Deck
1128 Sea Vista Place
Edmonds, Washington
April 2, 2021
Project #21021
encumbered by any erosion hazard area development standards. Though no mitigation is
necessary, as a standard of practice, temporary erosion and sediment control best
management practices (BMPs) should be implemented during construction. For a project with
such limited earthwork, temporary soil cover and perimeter controls will be field fit on an as
needed basis during construction.
Adjacent Mapped Landslide Hazard Area
As previously stated, there is a mapped potential landslide hazard area on the adjacent
property to the south. We have reviewed the ECDC definition of a landslide hazard area (ECDC
23.80.020.B) and based on our field observations from afar (no legal access) and review of
available information, we presume that the only criteria that is met on this property is that the
slope is in excess of 40 percent and over 10 feet in vertical height. The nearest proposed deck
foundations are over 35 feet from the south property line. Due to the slope orientation and the
location of the proposed deck foundations, it is our opinion that the adjacent property is not
adversely affected by the proposed deck construction and the proposed deck is not adversely
affected by the mapped potential landslide hazard area. As such, it is our opinion that the
subject property and proposed deck project should not be encumbered by any landslide hazard
area related development standards.
Soil Design Recommendations
The following recommendations are for the soil -related elements of the structural design. The
deck may be founded on conventional pad footing bearing on medium dense native soils, or
structural fill soils placed atop medium dense native soils. South Fork Geosciences should verify
the soil suitability and soil bearing capacity at the time of construction. The following soil
design values should be used for structural design of the project:
• Allowable Soil Bearing Capacity = 2,000 pounds per square foot
• Coefficient of Friction = 0.3
• Passive Equivalent Fluid Pressure = 300 pounds per cubic foot
• Seismic Site Class = C
All the above values are allowable and include appropriate factors of safety. South Fork
Geosciences is available for additional consultation if needed.
Closure
Since finished plans were not available at the time this letter was prepared, South Fork
Geosciences should review architectural plans prior to construction to ensure that our
recommendations have been properly incorporated into the project design.
SOUTH FORK GEOSCIENCES, PLLC Page 3
PO Box 1 275
NORTH BEND, WA 98045
425-890-4858 1 INFO@SFGEO.COM
Sullivan Deck
1128 Sea Vista Place
Edmonds, Washington
April 2, 2021
Project #21021
We have enjoyed working with you on this project and are confident that the information
provided in this letter will aid in the design and completion of your project. If there are any
questions wetter, please feel free to contact us.
Sincer
'� „n91
I Andrew L. Glandon I
An ,
Engineering Geologist / Owner
South Fork Geosciences, PLLC
Attachment: City of Edmonds Critical Area Map
References:
Minard, James P., "Geologic map of the Edmonds East and Part of the Edmonds West Quadrangles,
Washington" U.S. Geological Survey, Miscellaneous Field Studies Map MF-1541, scale 1:24,000, 1983
"Subsurface Conditions, Geologic Hazards, and Preliminary Design Recommendations, Hachler
Residence Addition, 1140 Sea Vista Place, Edmonds, Washington", South Fork Geosciences, Project
#17056, June 19, 2017
"Subject: Critical Areas Determination (CRA2021-0007) Site Address: 1128 SEA VISTA PL", City of
Edmonds, Michele Szafran, February 2, 2021
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PO Box 1 275
NORTH BEND, WA 98045
425-890-4858 1 INFO@SFGEO.COM
Sullivan Deck
1128 Sea Vista Place
Edmonds, Washington
April 2, 2021
Project #21021
.' City of Edmonds
CRITICAL AREA MAP
j
Legend
Approximate deck
/
= Creeks
footprint (added)
`
p Seismic Hazard Areas
Q
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_ Earth Subsidence and Landslide l
rn
00
�! Minimum Buffer Adjacent to Haz
Wetlands
Wetlands Boundary
Wetland Bouclanes Not Complete!
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= Welland Knorm Extents
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Floodplains
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Contour Lines
50. 100
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Landslide Hazard Area 40%
,� M• ;fly
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❑ Severe Erosion Hazard 15Y 40%
Ll Erosion Hazard Areas 159'«401Y.
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Notes
0 6799 136.0 Peet
1,21 5 101.3 This map is a user generated static output from
an Internet mapping site and a for
CRA2021-0007
1128 SEA VISTA PL
reference only. Data layers that appear on this map may or may not be accurate.
WGS_1986_Wr!e_Mercator Auxiliary_Sphe•e
current, or otherwise reliable.
® City of Edmonds
THIS MAP IS NOT TO BE USED FOR DESIGN OR CONSTRUCTION
SOUTH FORK GEOSCIENCES, PLLC Page 5
PO Box 1 275
NORTH BEND, WA 98045
425-890-4858 1 INFO@SFGEO.COM