CANOD.pdfCity of Edmonds
Critical Area Notice of 'Decision
Applicant: J � roperty Owner:
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Critical Area File #:Permit Number:
C iZrE}- 20 i�� o 2� �LD Zo r7 o
Site Location: 2-210 1la� Parcel Number: G � 0 c ssy C 0,/,7e 0
Project Description: Pqrwle— Clu S Sf o o Nj
❑ Conditional Waiver. No critical area report is required for the project described above.
1. There will be no alteration of a Critical Area or its required buffer.
2. The proposal is an allowed activity pursuant to ECDC 23.40.220, 23.50.020, and/or
23.80.040.
3. The proposal is exempt pursuant to ECDC 23.40.230.
❑ Erosion Hazard. Project is within erosion hazard area. Applicant must prepare an erosion and
sediment control plan in compliance with ECDC 18.30.
critical Area Report Required. The proposed project is within a critical arca and/or a critical area
buffer and'a critical area report is required. A critical, area report has been suibmitted an, d evaluated
for compliance with the following criteria pursuant to ECDC 23.40.160:
1. / The proposal minimizes the impact on critical areas in accordance with ECDC 23.40.120,
Mitigation sequencing;
2. i The proposal does not pose an unreasonable threat to the public health, safety, or welfare
on or off the development proposal site;
3. The proposal is consistent with the general purposes of this title and the public interest;
4. i Any alterations permitted to the critical area are mitigated in accordance with ECDC
23.40.110, Mitigation requirements.
5. The proposal protects the critical area functions and values consistent with the best
available science and results in ruo net lose of critical functions and values; and
6. The proposal is consistent with other app]icable regulations and standards.
❑ Unfavorable Critical Area Decision. The proposed project is not exempt or does not adequately
mitigate its impacts on critical areas and/or does not comply with the criteria in ECDC 23.40.160 and
the provisions of the City of Edmonds critical area regulations. See attached findings of
noncompliance.
PI Favorable Critical Area Decision. The proposed project as described above and as shown on the
attached site plan meets or is exempt from the criteria in ECDC 23.40.160, Review Criteria, and
complies with the applicable provisions of the City of Edmonds critical area regulations. Any
subsequent changes to the proposal shall void this decision pending re -review of the proposal.
❑ Conditions. Critical Area specific condition(s) have been applied to the permit number referenced
above. See referenced permit number for specific condition(s).
Notice on Title. Critical area notice on title recorded under AFN za 17 OSI 10 W
Reviewer Si nYt uref Date
Appeals: Any decision to approve, condition, or deny a development proposal or other activity based on the
requirements of critical area regulations may be appealed according to, and as part of, the appeal procedure, if any,
for the permit or approval involved.
Revised 11/29/2016
May 16, 2017
Project No. 170214E001
Edmonds School District 15
20420 68th Avenue West
Lynnwood, Washington 98036
Attention: Mr. Will Thomsen
d
t
i n C o r p () r a t o, d
Subject: Limited Geotechnical Investigation and Slope Evaluation
New Portable Classroom
Sherwood Elementary School
22901 106th Avenue West
Edmonds, Washington
Reference: Surveyed Slope Profiles
Prepared by Core Design Inc.
Received April 26, 2017
Dear Mr. Thomsen:
i'F Y N "i,,k',;
Associated Earth Sciences, Inc. (AESI) has prepared this limited geotechnical investigation and
slope evaluation report to support the installation of a new portable classroom at the existing
Sherwood Elementary. This report is intended to address comment number 1, as presented in the
City of Edmonds plan review letter for permit #BLD20170250, dated April 3, 2017. Our work
included a site meeting, a review of published geologic mapping, drilling three hand -auger
explorations, computer slope stability modeling, and preparation of this report.
This report has been prepared for the exclusive use of the Edmonds School District 15, and their
agents, for specific application to this project. Our services have been performed within the
limitations of scope, schedule, budget, and generally accepted geotechnical engineering and
engineering geology practices in effect in this area at the time our report was prepared. No other
warranty, express or implied, is made. This report is intended to address all the applicable
geotechnical reporting guidelines for this project in accordance with the Edmonds City Code and
the Edmonds Community Development Code (ECDC) Section 23.80.050.
Existing Conditions
The subject property is the existing Sherwood Elementary School located at 22901 106th Avenue
West in Edmonds, Washington. The school was constructed in 1967 and is situated at the toe of a
west -facing slope. The slope rises from about elevation of 287 feet near the proposed portable
location up to an elevation of 360 to 370 feet. The slope inclination ranges from 30 to as much as
75 percent. The lower portion of the slope face appears to have been cut (steepened) to the
Kirkland Office 1911 Fifth Avenue I Kirkland, WA 98033 P 1425.827.7701 F 1425.827.5424
Everett Office 12911 % Hewitt Avenue, Suite 2 1 Everett, WA 98201 P 1425.259-0522 F 1425.827.5424
Tacoma Office 1 1552 Commerce Street, Suite 102 1 Tacoma, WA 98402 P 1253.722.2992 F 1 253.722.2993
www.aesgeo.com
Sherwood Elementary School Limited Geotechnical Investigation and Slope Evaluation
Edmonds, Washington New Portable Classroom
current configuration to accommodate the school building and the emergency fire lane. Based on
the recent survey lines that were completed up the subject slope, the lower portion of the slope in
the vicinity of the proposed new portable ranges from 37 degrees (75 percent) to 42 degrees
(87 percent). The slope supports a mature growth of both coniferous and deciduous trees with a
typical native plant understory.
We understand that the proposed portable will be situated south of the covered basketball court,
just north of the northwest corner of the main building. The proposed portable is approximately
27 x 35 feet in plan dimension. The east wall of the portable will be located some 30 feet
horizontally from the toe of the existing slope.
Geologic Setting
The City of Edmonds has notified you of the presence of a landslide hazard area designation for
the site and vicinity. The site contains an Environmentally Critical Area (ECA) due to the presence
of slopes greater than 40 percent. Steep slopes are defined in Section 23.80.020, as "Anyslope of
40 percent or steeper that exceeds a vertical height of 10 feet over a 25- oot horizontal run." No
previous landslides are known to have occurred on or near the property.
We reviewed the Washington State Division of Geology and Earth Resources Geologic Map GM-14,
Preliminary Surficial Geologic Map of the Edmonds East and Edmonds West Quadrangles, prepared
by Mackey et al. The subject site is mapped as being underlain by Vashon advance outwash (Qva).
Advance outwash is deposited by the meltwater of an advancing glacier; it consists of stratified
compact sand and gravel. The slope to the east is mantled with lodgement glacial till (Qvt). Glacial
till is a heterogeneous mixture of silt, sand, and gravel deposited at the sole of the southward
advancing glaciers. The outwash and till were subsequently overridden and consolidated by the
weight of a thick glacial ice mass. Glacially consolidated soils typically have high shear strength,
and low-compressibility characteristics.
The steep, lower portion of the site slope has been subject to surface weathering and erosion,
resulting in an accumulation of sand and gravel at the slope toe. The erosion is part of a natural
process where the exposed surface of the glacially consolidated outwash soil is weathered and
loosened by precipitation, freeze/thaw, animal burrowing, and foot traffic. The loosened soils
then migrate down the steep slope via gravity forming an accumulation of colluvium at the slope
toe. This process has likely been ongoing since the slope was trimmed to the current
configuration.
We explored the subsurface conditions below the existing slope using hand-auger equipment. The
exploration logs are attached in Appendix A. Our explorations encountered Vashon advance
outwash (Qva) characterized as brown fine to coarse sand, trace silt and gravel, predominantly
medium sand. The unit contained a weathered horizon approximately 2 to 2.5 feet deep. The soil
graded with depth from medium dense to dense at a depth of 2 feet. HA-2 and HA-3 were
advanced on the slope face and encountered hand-auger refusal at a depth of 2.5 feet. We
advanced other explorations in the same area in an attempt to explore deeper than 2.5 feet but
were unsuccessful. Our exploration at the base of the slope, HA-1, extended to a depth of 8 feet.
May 16, 2017 ASSOCIATED EARTH SCIENCES, INC.
KOM/Id-170214EO01-3-Projec[sJ20170214JKEJWP Page 2
Sherwood Elementary School Limited Geotechnical Investigation and Slope Evaluation
Edmonds, Washington New Portable Classroom
HA-1 encountered loose grading to medium dense sands and gravels. This material is likely fill soils
placed during original site grading. We did not encounter groundwater in any of our explorations.
We did not observe any other indications of past or ongoing landslide activity on or around the
subject property. The slopes do support a mature growth of medium to large deciduous and
coniferous trees. Some of the deciduous trees on the steep, lower portion of the slope have
grown such that they are "hanging" down over the slope face. The growth pattern is consistent
with trees that grow on steep slopes.
Computer Modeling
Slope stability modeling was performed on the slope above the new portable classroom location
using the computer program Slope/W. This program accepts inputs that we provide based on
subsurface explorations, recently surveyed slope profiles, laboratory data, and other sources. The
model identifies critical slope failure surfaces that are then analyzed to determine their factor of
safety against failure under the input conditions. The factor of safety is a ratio between forces
driving slope failure and forces resisting slope failure. A factor of safety of 1.0 is indicative of a
failure surface with driving and resisting forces that are equal, and a slope failure is predicted.
Factors of safety that exceed 1.0 indicate that resisting forces are greater than driving forces. For
new construction, a static factor of safety of 1.5 or more is considered typical. Under design
earthquake loads, pseudostatic (dynamic) factors of safety of 1.1 or more is considered typical. For
this project, we modeled the design earthquake using an acceleration of 0.25g, which is consistent
with local standards of practice for slope modeling applications, in our opinion.
Existing topography inputs for slope stability modeling were taken from the slope survey sections
prepared by Core Design. Slope stability modeling requires soil strength parameters. For this
project, the site sediments that were modeled included advance outwash sediments. We did not
complete laboratory direct shear tests to obtain soil properties. For modeling we .used estimated
soil strength parameters based on our previous experience with similar soils and published data.
For advance outwash, we assumed a moist unit weight of 125 pounds per cubic foot (pcf), and an
internal friction angle of 42 degrees. These parameters represent the dense, glacially consolidated
nature of the advance outwash soils.
We completed slope stability modeling at the cross-section location which represented the
steepest measured slope angle of 42 degrees. At this section, we modeled the slope stability and
completed a search for the most critical slope failure surface (with the lowest factor of safety).
This analysis was completed under pseudostatic conditions assuming a lateral ground surface
acceleration of 0.25g. Our analysis was completed to evaluate the factor of safety for a failure
affecting the steep, lower slope face. This modeling indicated that shallow failures affecting the
upper few feet of the slope face represented the critical or lowest factor of safety. The lowest
factor of safety was 1.3 static and 0.85 dynamic and is shown on the model printouts attached to
this letter. The lower than typical factor of safety for the near-surface slope soils reflects the
steepness of the lower portion of the slope and the assumption the slope soils are entirely
cohesionless (granular) in nature. The modeling indicated that deeper failure planes affecting
May 16, 2017 ASSOCIATED EARTH SCIENCES, INC.
KDM/Id-170214E001-3- ProjectsJ20170214kKE�WP Page 3
Sherwood Elementary School Limited Geotechnical Investigation and Slope Evaluation
Edmonds, Washington New Portable Classroom
more than the upper few feet of the slope face represented factors of safety that were greater
than 1.5 static and 1.1 dynamic.
Conclusions
The following summarizes our observations and opinions about the planned location of the subject
portable classroom:
The steeper lower portion of the site slope was created during past grading activities.
The slope appears to have been stable in its current orientation for 50 years.
Slope modeling indicates adequate factors of safety against deep-seated slope failure, but
lower than typical factors of safety for shallow slope face failures.
The proposed portable installation will not require the modification of the slope and
therefore will not affect the mapped steep slope as defined by the reviewed City code.
• The slope is comprised of dense glacially consolidated advance outwash soils.
• The slope vegetation has been effective as reducing face erosion and should be
maintained.
In our opinion, the new portable can be located as planned with the following project design
adjustments implemented. We recommend a combined buffer and building setback from the toe
of the slope should be a total of 15 feet. This buffer/setback is intended to provide a toe of slope
soil accumulation area to reflect the previously mentioned ongoing slope face erosion. The
existing "stay off the slope" warning signs should be maintained. We recommend that some of the
large deciduous trees on the slope above the portable be removed to reduce the tree fall hazard.
The trees can be cut, but the stump and root system should be left in place. We will work with the
District to identify which trees we feel should be removed from the slope.
If you should have any questions or require further assistance, please do not hesitate to call.
Sincerely,
ASSOCIATED EARTH SCIENCES, INC.
Kirkland, Washington
23580
QST,
NNJ A "
Kurt D. Merriman, P.E.
Senior Principal Engineer
Attachments: Appendix A. Exploration Logs
Appendix B. Slope Stability section
May 16, 2017 ASSOCIATED EARTH SCIENCES, INC.
KDM/ld-170214EWI-3- Pr0Jectst20170214JKEJWP Page 4
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.... . ... . ........... ............. . . . . . .................... ...... .. -
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