REVIEWED-RESUB 1 BLD2023-0474+GEOTECH_REPORT+7.2.2023_11.16.21_PM+3644738BLD2023-0474 RESUB
Sanchez & McPhillips Addition Jul 03 2023 June 25, 2023
MONDS
1231 12th Avenue N (Tax Account #00606600002000) DEVELOPMENT CITY OF SERVCES Project #23015
Edmonds, Washington DEPARTMENT
The Leren Company Inc. Reviewed by
PO Box 12863
Mill Creek, WA 98082 � City of Edmonds ;
Planning Division
Attention: Tom Leren-------- - - - - - -
Subject: Geologic Hazards Assessment and Soil Design Recommendations
Mr. Leren:
The purpose of this letter is to address the geologic hazard areas associated with the subject
property and to provide some soil -related recommendations for the proposed addition. It is
our understanding that the proposed project consists of two small additions along the north
and east sides of the existing single-family residence. For the sake of brevity, this letter will
assume that readers are highly familiar with the subject property, the proposed construction,
and the referenced critical areas determination (CAD) (City of Edmonds, April 27, 2023).
Geologic Hazards Assessment
As indicated in the referenced CAD, there are mapped landslide hazard areas and erosion
hazard areas on and adjacent to the subject property. We have reviewed the proposed site
plan, the referenced critical areas determination, the available geologic and topographic
information, and we observed the site conditions on June 6, 2023. The following subsections
will summarize our conclusions with respect to the potential geologic hazards.
Subsurface Exploration/Geologic Setting
During our site visit, we advanced one hand auger boring near the northwest corner of the
north addition area. The location and a detailed soil log of the boring is attached to this report.
We interpret the native soils to be Vashon advance outwash sediments and our interpretation
is consistent with the referenced geologic mapping (Minard, 1983). Vashon advance outwash
sediments were deposited in fluvial and lacustrine environments as the Vashon glacial ice sheet
advanced. These sediments can vary in grain size depending on the depositional environment
and they were overridden and over consolidated by the Vashon glacial ice sheet. These
sediments are typically dense, moderately to highly permeable, and suitable for foundation
support.
Landslide Hazard Areas
During our site visit we did not observe any signs past or recent slope movement, accelerated
erosion, or adverse groundwater or seepage on the subject property. The observed slopes and
topographic features, such as graded slopes, rockeries, and retaining walls that "trigger" the
slope related critical areas mapping were obviously created by previous legal grading activities
associated with construction of the subject and adjacent properties, the existing home, the
driveway, and construction of 12th Avenue North. Based on these factors and in consideration
of the intent of the geologically hazardous areas code, it is our opinion that the subject
property should not be encumbered by any landslide hazard area development standards.
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Sanchez & McPhillips Addition June 25, 2023
1231 12t" Avenue N (Tax Account #00606600002000) Project #23015
Edmonds, Washington
Erosion Hazard Areas
The ECDC defines Erosion hazard areas are defined as soils mapped with "moderate" to "very
severe" erosion hazard, as determined by the USDA-NRCS soil mapping. In this situation, due to
the past site development and the current vegetation and landscaping present on the property,
the "erosion hazard areas" mapping is irrelevant and the potential for severe or accelerated
erosion does not exist. For a small project such as this, it is our opinion that construction
related erosion can be mitigated by implementing some basic temporary erosion and sediment
control best management practices (BMPs) during construction. The following is a list of BMPs
that should be implemented during construction:
• Prevention and maintenance of vehicle sediment track -out
• Temporary soil cover, as appropriate
• Perimeter controls such as silt fencing, straw wattles, or compost berms, as appropriate
• Establish permanent vegetation or landscaping soon after grading is completed
It should also be noted that the foundation excavation required for this project is somewhat
"self -containing". The excavation itself provides some perimeter control for erosion and since
there is not extensive soil disturbance, the TESC measures can be very basic and still adequately
mitigate accelerated erosion and runoff from the project earthwork.
Critical Areas Summary
It is our opinion that the mapped potential landslide hazard areas and erosion hazard areas do
not exist, and construction related erosion can be mitigated with basic TESC measures that are
consistent with the standard of practice for earthwork. As such, the proposed development
should not be encumbered by any geologic hazard development standards (ECDC 23.80.060
and 23.80.070).
Statement of Risk
ECDC 23.80.060 sets forth some specific language with respect to alterations of geologically
hazardous areas. We have reviewed the referenced plans for the proposed addition and to
address the intent of this code section, we provide the following statement with respect to the
proposed development:
It is the opinion of Andrew L. Glandon (State of Washington Licensed Engineering
Geologist #2689) that the proposed development will not increase the threat of the
geological hazard to adjacent properties beyond predevelopment conditions, will not
adversely impact other critical areas, are designed so that the hazard to the project is
eliminated or mitigated to a level equal to or less than predevelopment conditions;
and are certified as safe as designed and under anticipated conditions.
Soil Design Recommendations
Summary
For ease of reference, the site -specific soil design values we have determined are shown below:
• Allowable soil bearing capacity of native soils or
structural fill over native soils = 2,000 pounds per square foot
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PO Box 1 275
NORTH BEND, WA 98045
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Sanchez & McPhillips Addition June 25, 2023
1231 12t" Avenue N (Tax Account #00606600002000) Project #23015
Edmonds, Washington
• Coefficient of Friction = 0.35
• Passive Equivalent Fluid Pressure = 250 pounds per cubic foot
• Seismic Site Class = C
The above values are allowable and include appropriate factors of safety. This information is
presented in more detail in the following sections and these sections should be read to
understand the appropriate context.
Foundations
Spread footings founded on medium dense or denser native soils or structural fill (95 percent
maximum dry density as determined by Modified Proctor ASTM: D1557) placed atop the native
soils may be designed using an allowable bearing pressure of 2,000 pounds per square foot
(psf), including both dead and live loads. An increase of one-third may be used for short-term
wind or seismic loading. All footings must penetrate to the prescribed bearing stratum, and no
footing should be founded in or above loose, organic, or existing fill soils.
Anticipated settlements of spread footings designed as described above may be on the order of
-inch over an anticipated lifespan of the structure. Loose or disturbed surface soils, excessive
moisture present or poor foundation subgrade preparation could result in larger settlements.
Lateral Resistance
Lateral loads can be resisted by friction between the foundation and the supporting soils,
and/or by passive earth pressure acting on the buried portions of the foundations. The spread
footings must be backfilled with structural fill compacted to a dense, non -yielding condition to
achieve the passive resistance provided below. The structural fill must extend horizontally
outward from the embedded portion of the foundation a distance equal to at least three times
the embedment depth over which the passive resistance is applied. We recommend the
following design parameters:
• Passive equivalent fluid = 250 pounds per cubic foot
• Coefficient of friction = 0.35
The above values are allowable and include appropriate factors of safety.
Seismic Site Class
Based on our subsurface exploration and estimation of soil properties at depth utilizing
available geologic data, Site Class "C" may be used for the design of the project, as defined by
ASCE 7 "Minimum Design Loads for Buildings and Other Structures", Chapter 20.
Foundation Drainage
Foundation drainage for the new addition areas should match the foundation drainage of the
existing home. Due to the advance outwash sands encountered and the era of construction
(1958) it is likely that there are not any foundation drains on the existing house. If foundation
drainage is present, a typical perimeter foundation drain should be installed around the new
addition areas and tie into the existing foundation drain. If there are no existing drains,
foundation drains are not necessary for the addition areas.
SOUTH FORK GEOSCIENCES, PLLC Page 3
PO Box 1 275
NORTH BEND, WA 98045
425-890-4858 1 INFO@SFGEO.COM
Sanchez & McPhillips Addition
1231 121h Avenue N (Tax Account #00606600002000)
Edmonds, Washington
June 25, 2023
Project #23015
Stormwater Drainage
Based on the reference site plan, the new proposed roof area (impervious surface) increase due
to the addition is 188 square feet. In our opinion, this increase is insignificant with respect to
runoff flow volumes. Roof drains from the addition should connect to existing downspout drain
points that we presume infiltrate into the native soils via drywells or infiltration trenches.
Review Comment Response Letter
We reviewed the referenced plan review comments letter (City of Edmonds, May 18, 2023) and
it is out understanding that these comments have been addressed by The Leren Company in the
plans to be resubmitted. South Fork Geosciences is available for consultation throughout the
permitting process if necessary.
Closure
We have enjoyed working with you on this project and are confident that the information
provided will aid in the design and permitting of your project. If there are any questions, please
feel free tocaafiaet.us.
And
Engineering Geologist / Owner
South Fork Geosciences, PLLC
Attachments: Soil Exploration Location
Soil Exploration Log
References:
"Critical Area Determination (CRA2023-0064)",
Amber Brokenshire, Planning Division, City of
Edmonds, April 27, 2023
"SANCHEZ & McPHILLIPS RESIDENCE, SINGLE FAMILY
ADDITION & REMODEL, 1231 12T" AVE N EDMONDS,
WA 98020 — Site Plan Sheet AO", Puget Sound
Planning, April 3, 2023
"Plan Review Comments, Engineering Division - RE:
Application #: BLD2023-0474, Project:
Sanchez/McPhillips—Addition/Remodel, Project
Address: 1231 12th Ave N" FROM: Natalie Griggs, City
of Edmonds Engineering Division, May 18, 2023
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
Washington State Geologic Information Portal
Web Soil Survey - Home (usda.gov)
Edmonds City Code and Community Development
Code (codepublishing.com)
Snohomish County Property Account Summary
SOUTH FORK GEOSCIENCES, PLLC Page 4
PO Box 1 275
NORTH BEND, WA 98045
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Sanchez & McPhillips Addition
1231 12Ih Avenue N (Tax Account #00606600002000)
Edmonds, Washington
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June 25, 2023
Project #23015
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Source: Sanchez & McPhillips Residence — Site Plan
Sheet AO
Puget Sound Planning
Soil Exploration Location
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PO Box 1275
NORTH BEND, WA 98045
425-890-4858 1 INF09SFGEO.COM
Sanchez & McPhillips Addition
1231 12t" Avenue N (Tax Account #00606600002000)
Edmonds, Washington
H B-1
Soil Exploration Log
0-10in Bark mulch [Fill]
June 25, 2023
Project #23015
6-5-2023
10in-1.5ft medium dense, damp gray fine to medium SAND with gravel,
trace silt (SP) [Fill/Vashon Advance Outwash]
1. 5-3.5ft medium dense to dense, moist, gray fine to medium SAND with
gravel, trace silt (SP) [ Vashon Advance Outwash]
Total Depth = 3.5 feet No groundwater seepage observed. No caving observed. No
samples obtained.
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PO Box 1 275
NORTH BEND, WA 98045
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