REVIEWED PLN RESUB1 BLD2024-0549+GEO REPORT+8.20.2024_1.13.16_PM+4451792BLD2024-0549
COBALT
GE0SCIENCES
September 13, 2023
Updated July 29, 2024
Kevin Reid
Ckreid93 p comcast.net
RESUB
Aug 20 2024
CITY OF EDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
RE: Geotechnical Evaluation
Proposed Residence
326 Sunset Avenue
Edmonds, Washington
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Reviewed by
,
City of Edmonds ;
Planning Division
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Cobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this report to
discuss the results of our geotechnical evaluation at the referenced site.
The purpose of our evaluation was to provide recommendations for foundation design, grading,
and earthwork.
Site and Project Description
The site is located at 326 Sunset Avenue North in Edmonds, Washington. The site consists of one
rectangular shaped parcel (No. 27032300101100) with a total area of o.18 acres.
The western portion of the site is developed with a single-family residence and driveway access at
the east property line.
Site vegetation includes grasses, bushes, shrubs, and small diameter trees. The site is nearly level
to very slightly sloping in multiple directions.
The proposed development includes a new residence in the area of the existing residence.
Foundation loads will generally be light and grading may include cuts of 2 to 3 feet.
Area Geology
The Geologic Map of the Edmonds East and West Quadrangle, indicates that the site is underlain
by Whidbey Formation.
Whidbey Formation includes till like deposits, fine grained silts with sand and clay, and local
lacustrine silts. These materials are usually impermeable and are typically dense to very dense
below a weathered zone.
Soil & Groundwater Conditions
As part of our evaluation, we excavated two hand borings within the property, where accessible.
The explorations encountered approximately 6 inches of grass and topsoil underlain by
approximately 3 to 4 feet of loose to medium dense, silty -fine to medium grained sand trace
gravel (Weathered Whidbey Formation). These materials were underlain by dense to very dense,
silty -fine to fine grained sand trace gravel (Whidbey Formation), which continued to the
termination depths of the explorations.
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Geotechnical Evaluation
Groundwater was not encountered in the explorations; however, the shallow soils were locally
mottled. Perched groundwater could be present at shallow depths below the site during the
winter months. We anticipate that groundwater could be within 4 feet of the ground surface
during the winter months.
Water table elevations often fluctuate over time. The groundwater level will depend on a variety
of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and
soil permeability. Water levels at the time of the field investigation may be different from those
encountered during the construction phase of the project.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that
the site is underlain by Alderwood-Urban land complex (2 to 8 percent slopes). These soils would
have a slight erosion potential in a disturbed state depending on the slope magnitude.
It is our opinion that soil erosion potential at this project site can be reduced through landscaping
and surface water runoff control. Typically, erosion of exposed soils will be most noticeable
during periods of rainfall and may be controlled by the use of normal temporary erosion control
measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The
typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion
control measures should be in place before the onset of wet weather.
Seismic Hazard
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the
International Building Code (IBC). A Site Class D applies to an overall profile consisting of
medium dense to very dense soils within the upper too feet.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to
obtain values for Ss, Sl, FQ, and F,,. The USGS website includes the most updated published data
on seismic conditions. The following tables provide seismic parameters from the USGS web site
with referenced parameters from ASCE 7-10 and 7-16.
Seismic Design Parameters (ASCE 7-16)
Site
Spectral
Spectral
Site
Design Spectral
Design
Class
Acceleration
Acceleration
Coefficients
Response Parameters
PGA
at 0.2 sec. (g)
at 1.o sec. (g)
Fa
Fv
SDs
SDl
D
1.288
0.454
1.0
Null
1.031
Null
0.548
Additional seismic considerations include liquefaction potential and amplification of ground
motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a
high groundwater table. The site has a low likelihood of liquefaction. For items listed as "Null'
see Section 11.4.8 of the ASCE.
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Geotechnical Evaluation
Critical Area Information
The following are discussion of the Edmonds code as referenced in the comment provided to us
by the project architect. These include ECDC 23.40.ogo and 23-70.020, 23.80.05o, and
23•90.020.
We note that 23.90.020 is for fish and wildlife habitat areas and 23-70.020 is for frequently
flooded areas. The fish and wildlife habitat section is outside of our scope of services and
expertise. A wetland or other qualified consultant may be required to comment on this section.
Similarly, frequently flooded areas are not within our typical geotechnical purview. We note that
the high tide levels remain below site elevations and that specific mitigation such as a higher
foundation elevation and/or ballast to reduce erosion potential within the property do not appear
to be warranted.
23.40.090
Critical areas report — Requirements.
A. Preparation by Qualified Professional. The applicant shall submit a critical areas report
prepared by a qualified professional as defined herein. For wetlands, frequently flooded areas and
fish and wildlife habitat conservation areas, an applicant may choose one of the qualified
technical consultants on the city's approved list to prepare critical areas reports per the
requirements of this title or may apply to utilize an alternative consultant. Critical areas studies
and reports developed by an alternative consultant may be subject to independent review
pursuant to subsection (B) of this section. All costs associated with the critical areas study shall be
borne by the applicant.
B. Independent Review of Critical Areas Reports. Critical areas studies and reports on
geologically hazardous areas and those developed by an applicant representative or consultant not
as part of a three -party contract may, at the discretion of the director, be subject to independent
review. This independent review shall be performed by a qualified technical consultant selected
by the city with all costs borne by the applicant. The purpose of such independent review is to
provide the city with objective technical assistance in evaluating the accuracy of submitted reports
and/or the effects on critical areas which may be caused by a development proposal and to
facilitate the decision -making process. The director may also have technical assistance provided
by appropriate resource agency staff if such assistance is available in a timely manner.
C. Best Available Science. The critical areas report shall use scientifically valid methods and
studies in the analysis of critical areas data and field reconnaissance and reference the source of
science used. The critical areas report shall evaluate the proposal and all probable impacts to
critical areas in accordance with the provisions of this title.
D. Minimum Report Contents. At a minimum, the report shall contain the following:
1. The name and contact information of the applicant, a description of the proposal, and
identification of the permit requested;
Kevin Reid. Ckreidg.,iPcomcast.net
Project representative is Erik Lund with Tyler Engle Architects. 2126 Westlake Avenue, Seattle
WA 98121. 2o6 621-715o. erik(&tylerengle.com
2. A copy of the site plan for the development proposal including:
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Geotechnical Evaluation
a. A map to scale depicting critical areas, buffers, the development proposal, and any areas to be
cleared; and
Attached with this report.
b. A description of the proposed stormwater management plan for the development and
consideration of impacts to drainage alterations;
Connection to City infrastructure.
c. The site plan shall identify the location of all native and nonnative vegetation of six inches dbh
or larger;
We have included a site plan provided by the architect. The project designer can provide
additional plan sheets if required.
3. The dates, names, and qualifications of the persons preparing the report and documentation of
any fieldwork performed on the site;
Phil Haberman, PE, LG, LEG, principal with Cobalt Geosciences. We performed the work in
August and September of 2023. Documentation is present in the report and attached logs.
4. Identification and characterization of all critical areas, wetlands, water bodies, shorelines, and
buffers adjacent to the proposed project area;
City maps indicate that there is a seismic hazard in the northwest quarter of the site, extending to
the northwest and toward Puget Sound. Erosion and landslide hazard areas are noted northwest
of Sunset Avenue North, consistent with short man-made created slopes near the BNSF rail lines.
Wetlands and shorelines are present northwest of the site near Puget Sound.
5. A description of reasonable efforts made to apply mitigation sequencing pursuant to ECDC
23•40.120, Mitigation sequencing, to avoid, minimize, and mitigate impacts to critical areas;
Mitigation sequencing is not warranted from a geotechnical standpoint. The work includes
normal shallow perimeter foundation areas. Mitigation of seismic hazards is not warranted based
on our subsurface analyses. Other hazard areas are northwest of the property and separated by
developed roadway areas and right of way.
6. Report requirements specific to each critical area type as indicated in the corresponding
chapters of this title;
No specific geotechnical requirements are warranted at this time. Typical TESC and BMPs should
be installed until the project is completed and fully landscaped. The development area is outside
of the hazard areas and located a suitable distance and/or separated by developed right of way.
7. A statement specifying the accuracy of the report and all assumptions made and relied upon;
The report is based on site specific explorations and a review of geologic mapping, GIS maps, and
other published information (NRCS, USGS).
8. A description of the methodologies used to conduct the critical areas study, including
references; and
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We conducted subsurface analyses with auger, probes, and visual assessments. We verified soil
conditions using USCS and USDA soil classifications. We observed nearby areas, reviewed online
GIS and topographic maps, and historic explorations as noted on DNR Boring Database site.
9. Plans for adequate mitigation, as needed to offset any critical areas impacts, in accordance
with the mitigation plan requirements in ECDC 23.40.130.
None are required.
23.80.o5o
Special study and report requirements — Geologically hazardous areas.
Critical area report requirements for geologically hazardous areas are generally met through
submission to the director of one or more geotechnical reports. In addition to the general critical
areas report requirements of ECDC 23.40.090, critical areas reports for geologically hazardous
areas must meet the requirements of this section and Chapters 18.3o and 19.10 ECDC as
applicable. Critical areas reports for two or more types of critical areas must meet the report
requirements for each relevant type of critical area. Geotechnical report(s) submitted for the
purpose of critical areas review are required as necessary in addition to reports, data and other
information mandated per ECDC Titles 18 and 19. Geotechnical report(s) shall be required:
whenever a potential erosion hazard area or potential landslide hazard area, as mapped by
Edmonds critical areas inventory or shown on other information consistent with ECDC
23.80.030, is located within 50 feet of the proposed development site; whenever a development
site is located within a seismic hazard area; or when otherwise determined warranted by the
director (e.g., a distance equal to the height of the slope).
A. Preparation by a Qualified Professional. A critical areas report for assessing a potential
geologically hazardous area shall be prepared by a geologist licensed in the state of Washington,
with experience analyzing geologic, hydrologic, and ground water flow systems, and who has
experience preparing reports for the relevant type of hazard. If mitigation measures are
necessary, the report detailing the mitigation measures and design of the mitigation shall be
prepared by an engineer licensed in the state of Washington, with experience stabilizing slopes
with similar geotechnical properties. Critical areas studies and reports on geologically hazardous
areas shall be subject to independent review pursuant to ECDC 23.40.ogo(B).
B. Area Addressed in Critical Areas Report. The following areas shall be addressed in a critical
areas report for geologically hazardous areas:
1. The project area of the proposed activity; and
2. All geologically hazardous areas within 200 feet of the project area or that have the potential to
be affected by the proposal.
Attached is the GIS image with hazards. All areas are northwest of the site or partially on site
(seismic hazards). These areas continue northeast and southwest along Sunset Avenue.
C. Geological Hazards Assessment. A geology hazard assessment report for a geologically
hazardous area shall include a field investigation and contain an assessment of whether or not
each type of geologic hazard identified in ECDC 2o.80.020 is present or not present and if
development of the site will increase the risk of landslides or erosion on or off the site.
Geotechnical reports shall be prepared, stamped and signed by a qualified professional. These
reports must:
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Geotechnical Evaluation
1. Be appropriate for the scale and scope of the project;
2. Include a discussion of all geologically hazardous areas on the site and any geologically
hazardous areas off site potentially impacted by the proposed project. If the affected area extends
beyond the subject property, the geology hazard assessment may utilize existing data sources
pertaining to that area;
Seismic hazards are noted in the northwest quarter of the site and further northwest. These are
likely limited in lateral and vertical extent based on our review of historic explorations and our
explorations.
3. Clearly state that the proposed project will not decrease slope stability or pose an unreasonable
threat to persons or property either on or off site and provide a rationale as to those conclusions
based on geologic conditions and interpretations specific to the project;
The project will not alter slope stability near the site. The site is outside of the zone of influence to
the very small and limited relief slopes northwest of the property.
4. Provide adequate information to determine compliance with the requirements of this chapter;
Herein.
5. Generally follow the guidelines set forth in the Washington State Department of Licensing
Guidelines for Preparing Engineering Geology Reports in Washington (20o6). In some cases,
such as when it is determined that no landslide or erosion risk is present, a full report may not be
necessary to determine compliance with this chapter, and in those cases a letter or abbreviated
report may be provided.
6. If a landslide or erosion hazard is identified, provide minimum setback recommendations for
avoiding the landslide or erosion hazard, other recommendations for site development so that the
frequency or magnitude of landsliding or erosion on or off the site is not altered, and
recommendations consistent with ECDC 23.80.o6o and 23.80.070.
Not required. Site is separated by these hazards by a developed roadway with adequate drainage
and stormwater conveyance. Typical TESC and BMPs are suitable for site development.
D. Incorporation of Previous Study. Where a valid critical areas report has been prepared within
the last five years for a specific site, and where the proposed land use activity and surrounding site
conditions are unchanged, said report may be incorporated into the required critical areas report.
The applicant shall submit a hazards assessment detailing any changed environmental conditions
associated with the site.
E. Mitigation of Long -Term Impacts. When hazard mitigation is required, the mitigation plan
shall specifically address how the activity maintains or reduces the preexisting level of risk to the
site and all other properties potentially impacted on a long-term basis (equal to or exceeding the
projected lifespan of the activity or occupation). Proposed mitigation techniques shall be
considered to provide long-term hazard reduction only if they do not require regular maintenance
or other actions to maintain their function. Mitigation may also be required to avoid any increase
in risk above the preexisting conditions following abandonment of the activity.
Not required.
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F. Additional Technical Information Requirements for Projects within Erosion and Landslide
Hazard Areas. In addition to the basic critical areas report requirements for geologically
hazardous areas provided in subsections (A) through (E) of this section, technical information for
any development within earth subsidence and landslide hazard areas shall meet the requirements
of Chapter 19.10 ECDC and include the following information at a minimum:
1. Site Plan. The critical areas report shall include a copy of the site plan for the proposal
showing:
a. The height of slope, slope gradient, and cross-section of the project area;
There is a short embankment slope northwest of the site. Slope is about 10 to 15 feet tall with
magnitudes of 30 to 50 percent. Area is likely manmade.
b. The location of springs, seeps, or other surface expressions of ground water on or within 200
feet of the project area or that have the potential to be affected by the proposal; and
None observed.
c. The location and description of surface water runoff features;
None observed. Stormwater infrastructure is present in Sunset Avenue.
2. Hazards Analysis. The hazards analysis component of the critical areas report shall specifically
include:
a. A description of the extent and type of vegetative cover;
Outside of the residence and development, site is vegetated with grasses, bushes, and sparse trees.
b. A description of subsurface conditions based on data from site -specific explorations;
We observed denser Whidbey Formation in the hand explorations.
c. Descriptions of surface and ground water conditions, public and private sewage disposal
systems, fills and excavations, and all structural improvements;
Surface and groundwater were not observed. Groundwater likely fluctuates with the tides.
Groundwater is likely 20 or more feet below site elevations. No evidence of surface water is
noted. Perched interflow could develop on denser or finer grained soils.
d. An estimate of slope stability and the effect construction and placement of structures will have
on the slope over the estimated life of the structure;
Development will not impact slope stability of the hazard areas which are more than 6o feet from
the new residence (and existing residence). Slope is less than 15 feet tall.
e. An estimate of the bluff retreat rate or an estimate of the percent risk of landslide area
expansion that recognizes and reflects potential catastrophic events such as seismic activity or a
too -year storm event;
N/A. Slope areas are outside of the zone of influence to the site.
f. Consideration of the run -out hazard of landslide debris and/or the impacts of landslide run -out
on downslope properties;
N/A
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g. A study of slope stability including an analysis of proposed cuts, fills, and other site grading;
Not required. Minimal grading is proposed.
h. Recommendations for building siting limitations; and
None noted.
i. An analysis of proposed surface and subsurface drainage, and the vulnerability of the site to
erosion;
Site is nearly level and not susceptible to severe erosion. Normal development and foundation
drains are proposed and are suitable.
3. Geotechnical Engineering Report. The technical information for a project within a landslide
hazard area shall include a geotechnical engineering report prepared by a licensed engineer that
presents engineering recommendations for the following:
a. Parameters for design of site improvements including appropriate foundations and retaining
structures. These should include allowable load and resistance capacities for bearing and lateral
loads, installation considerations, and estimates of settlement performance;
Included herein.
b. Recommendations for drainage and subdrainage improvements;
Provided herein. Foundation drains are recommended.
c. Earthwork recommendations including clearing and site preparation criteria, fill placement
and compaction criteria, temporary and permanent slope inclinations and protection, and
temporary excavation support, if necessary; and
Provided herein. Typical recommendations apply to this project and property.
d. Mitigation of adverse site conditions including slope stabilization measures and seismically
unstable soils, if appropriate;
Not applicable.
4. Erosion and Sediment Control Plan. For any development proposal on a site containing an
erosion hazard area, an erosion and sediment control plan shall be required. The erosion and
sediment control plan shall be prepared in compliance with requirements set forth in Chapter
18.3o ECDC.
By others.
G. Limited Report Requirements for Stable Erosion Hazard Areas. At the director's discretion,
detailed critical areas report requirements may be waived for erosion hazard areas with suitable
slope stability. Report requirements for stable erosion hazard areas may be met through
construction documents that shall include at a minimum an erosion and sediment control plan
prepared in compliance with requirements set forth in Chapter 18.3o ECDC.
Provided proper TESC is utilized until completion, the risk to off site erosion hazards will be very
low to nil. The site is separated by the hazard area by developed right of way with functioning
stormwater systems and drainage conveyance.
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H. Seismic Hazard Areas. In addition to the basic critical areas report requirements for
geologically hazardous areas provided in subsections (A) through (E) of this section, a critical
areas report for a seismic hazard area shall also meet the following requirements:
1. The hazards analysis shall include a complete discussion of the potential impacts of seismic
activity on the site (for example, forces generated and fault displacement).
N/A. Development is outside of the mapped hazard and the mapped hazard is very limited in
lateral and vertical extent. The risk to the property is essentially nil.
2. A geotechnical engineering report shall evaluate the physical properties of the subsurface soils,
especially the thickness of unconsolidated deposits and their liquefaction potential. If it is
determined that the site is subject to liquefaction, mitigation measures appropriate to the scale of
the development shall be recommended and implemented. [Ord. 4026 § 1 (Att. A), 2o16; Ord.
3527 § 2, 20041.
Site does not contain liquefiable sediments. Soils become dense at shallow depths and have a low
risk of seismically inducted movements.
Article IV. Development Standards — Geologically Hazardous Areas
23.80.060
Development standards — General requirements.
A. Alterations of geologically hazardous areas or associated buffers may only occur for activities
that:
Alterations are not proposed.
1. Will not increase the threat of the geological hazard to adjacent properties beyond
predevelopment conditions;
2. Will not adversely impact other critical areas;
3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or
less than predevelopment conditions; and
N/A. Hazards are outside of the development area. Mitigation is not required.
4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or
geologist, licensed in the state of Washington.
Are certified as safe provided we verify soil bearing, foundation drainage, fill compaction, and
final erosion control. This assumes all TESC is installed and maintained until project completion.
B. Critical Facilities Prohibited. Critical facilities shall not be sited within geologically hazardous
areas unless there is no other practical alternative. [Ord. 4026 § 1 (Att. A), 2016; Ord. 3527 § 2,
2004].
N/A
23.80.070
Development standards — Specific hazards.
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A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide
hazards shall meet the requirements of ECDC 23.80.o6o, Development standards — General
requirements, and the specific following requirements:
1. Minimum Building Setback. The minimum setback shall be the distance required to ensure the
proposed structure will not be at risk from landslides for the life of the structure, considered to be
120 years, and will not cause an increased risk of landslides taking place on or off the site. A
setback shall be established from all edges of landslide hazard areas. The size of the setback shall
be determined by the director consistent with recommendations provided in the geotechnical
report to eliminate or minimize the risk of property damage, death, or injury resulting from
landslides caused in whole or part by the development, based upon review of and concurrence
with a critical areas report prepared by a qualified professional;
Not applicable due to the current location of the site relative to short hazard areas. Site is outside
of the influence of these slopes.
2. Buffer Requirements. A buffer may be established with specific requirements and limitations,
including but not limited to drainage, grading, irrigation, and vegetation. Buffer requirements
shall be determined by the director consistent with recommendations provided in the
geotechnical report to eliminate or minimize the risk of property damage, death, or injury
resulting from landslides caused in whole or part by activities within the buffer area, based upon
review of and concurrence with a critical areas report prepared by a qualified professional;
A geotechnical buffer is not required from our standpoint. Site is separated from hazards by
developed right of way with functioning drainage systems.
3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback
and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies
that:
Not proposed.
a. The alteration will not increase surface water discharge or sedimentation to adjacent
properties beyond predevelopment conditions;
b. The alteration will not decrease slope stability on adjacent properties; and
c. Such alterations will not adversely impact other critical areas;
4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion
or landslide hazard area and/or buffer shall be designed to meet the following basic requirements
unless it can be demonstrated that an alternative design that deviates from one or more of these
standards provides greater long-term slope stability while meeting all other provisions of this
title. The requirement for long-term slope stability shall exclude designs that require regular and
periodic maintenance to maintain their level of function. The basic development design standards
are:
N/A. Site is separated from these hazards by developed right of way.
a. The proposed development shall not decrease the factor of safety for landslide occurrences
below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the
proposed development site is below these limits, the proposed development shall provide
practicable approaches to reduce risk to human safety and improve the factor of safety for
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landsliding. In no case shall the existing factor of safety be reduced for the subject property or
adjacent properties;
N/A. Site is outside of the zone of influence of the short steep slopes near the BNSF lines.
b. Structures and improvements shall be clustered to avoid geologically hazardous areas and
other critical areas;
Currently proposed.
c. Structures and improvements shall minimize alterations to the natural contour of the slope,
and foundations shall be tiered where possible to conform to existing topography;
Site is relatively flat.
d. Structures and improvements shall be located to preserve the most critical portion of the site
and its natural landforms and vegetation;
Provided.
e. The proposed development shall not result in greater risk or a need for increased buffers on
neighboring properties;
Current development will not require a need of increased buffers and is similar to what is
currently present on the site and adjacent areas.
f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred
over graded artificial slopes; and
Minimal grading is required. Some short walls are proposed.
g. Development shall be designed to minimize impervious lot coverage;
By others.
5. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal
of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited;
N/A
6. Seasonal Restriction. Clearing shall be allowed only from May ist to October ist of each year;
provided, that the director may extend or shorten the dry season on a case -by -case basis
depending on actual weather conditions, except that timber harvest, not including brush clearing
or stump removal, may be allowed pursuant to an approved forest practice permit issued by the
city of Edmonds or the Washington State Department of Natural Resources;
If proper TESC are utilized, work could occur during any season from our perspective.
7. Point Discharges. Point discharges from surface water facilities and roof drains onto or
upstream from an erosion or landslide hazard area shall be prohibited except as follows:
a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard
areas downstream from the discharge;
Direct connection is proposed.
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b. Discharged at flow durations matching predeveloped conditions, with adequate energy
dissipation, into existing channels that previously conveyed stormwater runoff in the
predeveloped state; or
Per civil.
c. Dispersed discharge upslope of the steep slope onto a low -gradient, undisturbed buffer
demonstrated to be adequate to infiltrate all surface and stormwater runoff, and where it can be
demonstrated that such discharge will not increase the saturation of the slope; and
N/A
8. Prohibited Development. On -site sewage disposal systems, including drain fields, shall be
prohibited within erosion and landslide hazard areas and related buffers.
N/A
B. Earth Subsidence and Landslide Hazard Area. In addition to the requirements of this chapter,
development proposals for lands located within the earth subsidence and landslide hazard area as
indicated on the critical areas inventory shall be subject to the provisions of Chapter 19.10 ECDC.
N/A
C. Seismic Hazard Areas. Activities proposed to be located in seismic hazard areas shall meet the
standards of ECDC 23.80.o6o, Development standards — General requirements. [Ord. 4314 § 1o6
(Exh. A), 2023; Ord. 4026 § 1 (Att. A), 2o16; Ord. 3527 § 2, 20041.
Site is outside of seismic hazard areas. The risk of liquefaction is low based on a relatively high
soil density. Mitigation is not warranted.
Conclusions and Recommendations
General
The site is underlain by Whidbey Formation deposits which become denser with depth. There
may be areas of fill associated with development in this part of Edmonds. Fill depths would likely
be 3 feet or less.
The proposed residence may be supported on a shallow foundation system bearing on medium
dense or firmer native soils or on structural fill placed on the native soils. Local overexcavation
or recompaction of loose weathered native soils may be necessary depending on the proposed
elevations and locations of the new footings.
Stormwater collection systems should be connected to City infrastructure via tightline or
perforated connection.
Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich
soil and fill. Based on observations from the site investigation program, it is anticipated that the
stripping depth will be 6 to 12 inches. Deeper excavations will be necessary in areas of existing
foundation systems and in any areas underlain by undocumented fill.
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Geotechnical Evaluation
The native soils consist of silty -sand with gravel to sandy silt with gravel. Most of the native soils
may be used as structural fill provided they achieve compaction requirements and are within 3
percent of the optimum moisture. Some of these soils may only be suitable for use as fill during
the summer months, as they will be above the optimum moisture levels in their current state.
These soils are variably moisture sensitive and may degrade during periods of wet weather and
under equipment traffic.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of
3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve).
Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted
to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the
ASTM D 1557 test method.
Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts
on the order of approximately 3 feet or less for foundation removal and utility placement.
Temporary excavations should be sloped no steeper than 1.511:1V (Horizontal:Vertical) in loose
native soils and fill, iH:iV in medium dense native soils and 3/4H:1V in dense to very dense
native soils, if encountered at depth. If an excavation is subject to heavy vibration or surcharge
loads, we recommend that the excavations be sloped no steeper than 2H:iV, where room permits.
Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part
N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a
qualified person during construction activities and the inspections should be documented in daily
reports. The contractor is responsible for maintaining the stability of the temporary cut slopes
and reducing slope erosion during construction.
Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather,
and the slopes should be closely monitored until the permanent retaining systems or slope
configurations are complete. Materials should not be stored or equipment operated within io feet
of the top of any temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In the case of
temporary cuts, the existing soil conditions may not be completely revealed until the excavation
work exposes the soil. Typically, as excavation work progresses the maximum inclination of
temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental
recommendations can be made. Soil and groundwater conditions can be highly variable.
Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that
the project can proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, we should be
notified so that supplemental recommendations can be made. If room constraints or
groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed
by the WAC, temporary shoring systems may be required. The contractor should be responsible
for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences
and the project structural engineer review temporary shoring designs prior to installation, to
verify the suitability of the proposed systems.
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Geotechnical Evaluation
Foundation Design
The proposed residence may be supported on a shallow spread footing foundation system bearing
on undisturbed medium dense or firmer native soils or on properly compacted structural fill
placed on the suitable native soils. Any undocumented fill and/or loose native soils should be
removed and replaced with structural fill below foundation elements. Structural fill below
footings should consist of clean angular rock 5/8 to 4 inches in size. We should verify soil
conditions during foundation excavation work.
For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively,
for continuous wall and isolated column footings supporting the proposed structure. Provided
that the footings are supported as recommended above, a net allowable bearing pressure of 2,500
pounds per square foot (psf) may be used for design.
A 1/3 increase in the above value may be used for short duration loads, such as those imposed by
wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted
to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing
excavations should be inspected to verify that the foundations will bear on suitable material.
Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or
adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12
inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower.
If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch.
Differential settlement, along a 25-foot exterior wall footing, or between adjoining column
footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most
settlement is expected to occur during construction, as the loads are applied. However, additional
post -construction settlement may occur if the foundation soils are flooded or saturated. All
footing excavations should be observed by a qualified geotechnical consultant.
Resistance to lateral footing displacement can be determined using an allowable friction factor of
0.40 acting between the base of foundations and the supporting subgrades. Lateral resistance for
footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds
per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12
inches below grade in exterior areas). The frictional and passive resistance of the soil may be
combined without reduction in determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction.
Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the
footing excavations, should be removed prior to placing concrete. The potential for wetting or
drying of the bearing materials can be reduced by pouring concrete as soon as possible after
completing the footing excavation and evaluating the bearing surface by the geotechnical engineer
or his representative.
Concrete Retaining Walls
The following table, titled Wall Design Criteria, presents the recommended soil related design
parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall
system is used. This has been included for new cast in place walls, if proposed.
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Geotechnical Evaluation
Wall Design Criteria
"At -rest" Conditions (Lateral Earth Pressure — EFD+)
55 pcf (Equivalent Fluid Density)
"Active" Conditions (Lateral Earth Pressure — EFD+)
35 pcf (Equivalent Fluid Density)
Seismic Increase for "At -rest" Conditions
(Lateral Earth Pressure)
14H* (Uniform Distribution)
Seismic Increase for "Active" Conditions
(Lateral Earth Pressure)
7H* (Uniform Distribution)
Passive Earth Pressure on Low Side of Wall
(Allowable, includes F.S. = 1.5)
Neglect upper 2 feet, then 250 pcf EFD+
Soil -Footing Coefficient of Sliding Friction (Allowable;
includes F.S. = 1.5)
0.40
"H is the height of the wall; Increase based on one in 500 year seismic event (10 percent probability of being exceeded in
50 years),
+EFD — Equivalent Fluid Density
The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by
water accumulation behind the retaining walls. Uniform horizontal lateral active and at -rest
pressures on the retaining walls from vertical surcharges behind the wall may be calculated using
active and at -rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight
Of 125 pcf may be used to calculate vertical earth surcharges.
To reduce the potential for the buildup of water pressure against the walls, continuous footing
drains (with cleanouts) should be provided at the bases of the walls. The footing drains should
consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed
down and enveloped by a minimum 6 inches of pea gravel in all directions.
The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should
consist of free -draining granular material. All free draining backfill should contain less than 3
percent fines (passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S.
Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard
No. 4 Sieve. The primary purpose of the free -draining material is the reduction of hydrostatic
pressure. Some potential for the moisture to contact the back face of the wall may exist, even with
treatment, which may require that more extensive waterproofing be specified for walls, which
require interior moisture sensitive finishes.
We recommend that the backfill be compacted to at least 90 percent of the maximum dry density
based on ASTM Test Method D1557. In place density tests should be performed to verify
adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently,
only light hand operated equipment is recommended within 3 feet of walls so that excessive stress
is not imposed on the walls.
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Geotechnical Evaluation
Stormwater Management Feasibility
The site is underlain by relatively dense Whidbey Formation. These soils are very fine grained
and represent an aquitard. The shallow soils were mostly mottled and the unweathered Whidbey
Deposits act as a restrictive layer. Infiltration is not feasible due to the presence of mottled fine
grained soils and a very shallow restrictive layer.
We recommend direct or perforated connection of new runoff devices to City infrastructure. We
should be provided with final plans for review to determine if the intent of our recommendations
has been incorporated or if additional modifications are needed.
Slab -on -Grade
We recommend that the upper 12 inches of the existing fill and/or native soils within slab areas be
re -compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). We
anticipate that new slab on grade will be supported on deeper fills due to basement removal. All
fill must be compacted to the above standard in lifts of 12 inches or less. Compaction equipment
should include a large ho-pack or 10 ton minimum drum roller. We should verify fill suitability
and compaction during construction.
Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor
barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture
typically requires the usage of a vapor barrier. A materials or structural engineer should be
consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs
typically do not utilize vapor barriers.
The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302.1R-04
Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier
selection and floor slab detailing.
Slabs on grade may be designed using a coefficient of subgrade reaction of 18o pounds per cubic
inch (pci) assuming the slab -on -grade base course is underlain by structural fill placed and
compacted as outlined in Section 8.1. A 4- to 6-inch-thick capillary break layer should be placed
over the prepared subgrade. This material should consist of pea gravel or 5/8 inch clean angular
rock.
A perimeter drainage system is recommended unless interior slab areas are elevated a minimum
Of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should
consist of a 4-inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain
rock wrapped in a non -woven geosynthetic filter fabric to reduce migration of soil particles into
the drainage system. The perimeter drainage system should discharge by gravity flow to a
suitable stormwater system.
Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate
surface water flow away from the building and preferably with a relatively impermeable surface
cover immediately adjacent to the building.
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Geotechnical Evaluation
Erosion and Sediment Control
Erosion and sediment control (ESQ is used to reduce the transportation of eroded sediment to
wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment
control measures should be implemented, and these measures should be in general accordance
with local regulations. At a minimum, the following basic recommendations should be
incorporated into the design of the erosion and sediment control features for the site:
• Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance
of the site soils, to take place during the dry season (generally May through September).
However, provided precautions are taken using Best Management Practices (BMP's), grading
activities can be completed during the wet season (generally October through April).
All site work should be completed and stabilized as quickly as possible.
• Additional perimeter erosion and sediment control features may be required to reduce the
possibility of sediment entering the surface water. This may include additional silt fences, silt
fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration
systems.
• Any runoff generated by dewatering discharge should be treated through construction of a
sediment trap if there is sufficient space. If space is limited other filtration methods will need
to be incorporated.
Groundwater Influence on Construction
Groundwater was not encountered in the explorations. Groundwater could be present at shallow
depths during the wet season, perched on the dense fine grained native soils.
We do not anticipate that significant volumes of groundwater will be encountered in shallow
excavations during the dry season.
If groundwater is encountered, we anticipate that sump excavations and small diameter pumps
systems will adequately de -water short-term excavations, if required. Any system should be
designed by the contractor. We can provide additional recommendations upon request.
Utilities
Utility trenches should be excavated according to accepted engineering practices following OSHA
(Occupational Safety and Health Administration) standards, by a contractor experienced in such
work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent
to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be
avoided. Depending upon the location and depth of some utility trenches, groundwater flow into
open excavations could be experienced, especially during or shortly following periods of
precipitation.
In general, silty soils were encountered at shallow depths in the explorations at this site. These
soils have low cohesion and density and will have a tendency to cave or slough in excavations.
Shoring or sloping back trench sidewalls is required within these soils in excavations greater than
4 feet deep.
All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility
trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at
least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5
feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent
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Updated July 29, 2024
Page IS of 20
Geotechnical Evaluation
of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench
backfill in pavement areas should be compacted to at least go percent of the maximum dry
density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe
manufacturer's recommendations.
The contractor is responsible for removing all water -sensitive soils from the trenches regardless of
the backfill location and compaction requirements. Depending on the depth and location of the
proposed utilities, we anticipate the need to re -compact existing fill soils below the utility
structures and pipes. The contractor should use appropriate equipment and methods to avoid
damage to the utilities and/or structures during fill placement and compaction procedures.
CONSTRUCTION FIELD REVIEWS
Cobalt Geosciences should be retained to provide part time field review during construction in
order to verify that the soil conditions encountered are consistent with our design assumptions
and that the intent of our recommendations is being met. This will require field and engineering
review to:
■ Monitor and test structural fill placement and soil compaction
■ Observe bearing capacity at foundation locations
■ Observe slab -on -grade preparation
■ Monitor foundation drainage placement
■ Observe excavation stability
Geotechnical design services should also be anticipated during the subsequent final design phase
to support the structural design and address specific issues arising during this phase. Field and
engineering review services will also be required during the construction phase in order to
provide a Final Letter for the project.
CLOSURE
This report was prepared for the exclusive use of Kevin Reid and his appointed consultants. Any
use of this report or the material contained herein by third parties, or for other than the intended
purpose, should first be approved in writing by Cobalt Geosciences, LLC.
The recommendations contained in this report are based on assumed continuity of soils with
those of our test holes and assumed structural loads. Cobalt Geosciences should be provided with
final architectural and civil drawings when they become available in order that we may review our
design recommendations and advise of any revisions, if necessary.
Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is
the responsibility of Kevin Reid who is identified as "the Client" within the Statement of General
Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of
these not be satisfied.
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Geotechnical Evaluation
Statement of General Conditions
USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its
agent and may not be used by any third parry without the express written consent of Cobalt
Geosciences and the Client. Any use which a third party makes of this report is the responsibility
of such third party.
BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this
report are in accordance with Cobalt Geosciences present understanding of the site specific
project as described by the Client. The applicability of these is restricted to the site conditions
encountered at the time of the investigation or study. If the proposed site specific project differs
or is modified from what is described in this report or if the site conditions are altered, this report
is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the
report to reflect the differing or modified project specifics and/or the altered site conditions.
STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in
accordance with the normally accepted standard of care in the state of execution for the specific
professional service provided to the Client. No other warranty is made.
INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and
statements regarding their condition, made in this report are based on site conditions
encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or
sampling locations. Classifications and statements of condition have been made in accordance
with normally accepted practices which are judgmental in nature; no specific description should
be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in
situ conditions can only be made to some limited extent beyond the sampling or test points. The
extent depends on variability of the soil, rock and groundwater conditions as influenced by
geological processes, construction activity, and site use.
VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be
encountered that are different from those described in this report or encountered at the test
locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected
conditions are substantial and if reassessments of the report conclusions or recommendations are
required. Cobalt Geosciences will not be responsible to any parry for damages incurred as a result
of failing to notify Cobalt Geosciences that differing site or sub -surface conditions are present
upon becoming aware of such conditions.
PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and
specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next
project stage (property acquisition, tender, construction, etc), to confirm that this report
completely addresses the elaborated project specifics and that the contents of this report have
been properly interpreted. Specialty quality assurance services (field observations and testing)
during construction are a necessary part of the evaluation of sub -subsurface conditions and site
preparation works. Site work relating to the recommendations included in this report should only
be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be
responsible for site work carried out without being present.
www.cobaltgeo.com (2o6) 331-1097
- , v
'V o 1
Sno. Co. GIS Map
0
HB-1 Approximate N
Hand Boring
Location A
Not to Scale
Cobalt Geosciences, LLC
Proposed Residence SITE Al" P.O. Box 82243
COBALT326 Sunset Avenue North Kenmore, WA 98028
331-1097
Edmonds, Washington FIGURE 1 ( www.cobaltgeo.com
cobaltgeo(&gmail.com
_ J
r
Red and Yellow
Areas are
Landslide and
Erosion Hazards
tf;
Seismic Hazards
rPOEM
d
o
ii
Site Plan Excerpt
Proposed Residence
326 Sunset Avenue North
Edmonds, Washington
SITE PLAN
FIGURE 2
City GIS Map
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo(&gmail.com
Hand Boring HB-1
Date: September 2023
Depth: 6'
Groundwater: None
Contractor:
Elevation:
Logged By: PH Checked By: SC
N
0)
o
-0
Moisture Content (%)
Plastic I Liquid
u
L
N
Limit Limit
F
�
N
Material Description
o
?
o
DCP Equivalent N-Value
G
0 10 20 30 40 50
------
1
----
—
—
--
SM
Topsoil/Grass
--------------------------------------------
Loose to medium dense, silty -fine to medium grained sand with
gravel, dark yellowish brown, moist. (Weathered Whidbey Formation)
2
-Grades to sandy silt locally
3
4
-------
5
----
SM--
---------------------------------------------
Dense to very dense, silty -fine to medium grained sand with gravel,
mottled yellowish brown to grayish brown, moist. (Whidbey Formationl)
End of Hand Boring 6'
7
8
9
10
Hand Boring
HB-2
Date: September 2023
Depth: 4.5'
Groundwater: None
Contractor:
Elevation:
Logged By: PH Checked By: SC
0)
J
o
-0
Moisture Content (%)
Plastic Liquid
N15
U
E
3
Limit Limit
N
Material Description
?
o
DCP Equivalent N-Value
o
C
0 10 20 30 40 50
------ ---- --- ---------------------------------------------
1 SM Loose to medium dense, silty -fine to medium grained sand with
gravel, dark yellowish brown, moist. (Weathered Whidbey Formation)
2
3
------ ---- :.--- ---------------------------------------------
4 SM Dense to very dense, silty -fine to medium grained sand with gravel,
mottled yellowish brown to grayish brown, moist. (Whidbey Formation)
5 End of Hand Boring 4.5'
6
7
8
9
10
Proposed Residence Hand Boring
326 Sunset Avenue North Logs
Edmonds, Washington
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(2o6) 331-1097
www.cobaltgeo.com
cobaltgeoCdigmail.com