Geotech Memo 20191115_Resub.pdfNELSON GEOTECHNICAL
ASSOCIATES, INC.
NGA GEOTECHNICAL ENGINEERS St GEOLOGISTS
Main Office Engineering -Geology Branch
17311 — 135`h Ave NE, A-500 55261ndustry Lane, #2
Woodinville, WA 98072 East Wenatchee, WA 98802
(425) 486-1669 • FAX (425) 481-2510 (509) 665-7696 • FAX (509) 665-7692
MEMORANDUM RECEIVED
DATE: October 24, 2019
NOV 15 2019
TO: Mr. David Muresan DEVELOPMENT SERVICES
Mr. Timothy Sarkela, PE - Western Engineers and Surveyors, Inc. COUNTER
FROM: Khaled M. Shawish, PE
Katelyn Brower, GIT
Carston T. Curd, GIT
RE: Comment Response Memorandum
Muresan Short Plat
15809 — 70" Avenue West
Edmonds, Washington
NGA File No. 10487B 19
Introduction
This memorandum documents our response to City of Edmonds review comments regarding the Muresan
Short Plat located at 15809 — 70' Avenue West in Edmonds, Washington. We have referenced the most
recent preliminary grading plans titled "Grading and SWPPP Plan For: Muresan Short Plat," dated June
19, 2019 and prepared by Western Engineers & Surveyors. We have also reviewed preliminary drainage
plans titled " Drainage Plan for Muresan Short Plat," dated December 12, 2018, and prepared by Western
Engineers and Surveyors.
We previously prepared a geotechnical report for the site updated on April 1, 2019. We have been
requested to provide geotechnical responses to comments furnished in an email to Timothy Sarkela, PE
from the City of Edmonds Development Services Department, Planning Division, dated August 29, 2019.
In the following section, we summarize the relevant geotechnical concerns raised by the City, followed by
our response and supplemental recommendations.
Comment 1:
" ...The geotechnical report prepared by Nelson Geotechnical Associates, Inc. dated April 1, 2019
indicated that while slopes on the property exceeded 40% they did not maintain that grade over a
horizontal distance of 25 feet or a vertical rise of 10 feet... ...based on the topographical survey
submitted with the application it is clear the slopes on the property meet the definition of potential
landslide hazards in ECDC 23.80.020.B4... ...as such, the geotechnical report must address the
development standards in ECDC 23.80.060 and 23.80.070. Additionally, the geotechnical should address
the review criteria for subdivisions in ECDC 20.75.085.A.1 and A.3. Please submit a revised geotechnical
report addressing the specific criteria noted above. "
Memorandum — Comment Response
Muresan Short Plat
Edmonds, Washington
NGA File No. 10487B 19
October 24, 2019
Page 2
Response
The site meets the criteria for landslide hazard areas and erosion hazard areas, as outlined in the Edmonds
Municipal code (ECDC 23.80.020). Pursuant to ECDC 23.80.070(4), the planned project meets the
requirements of design standards for development within Erosion and Landslide Hazard Areas. Our
responses to each item of the code are included below:
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.
Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical
areas.
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.
Although specific grading and residence plans have not been developed for the final site
development, we expect that development would be designed to limit impacts to overall topography
of the steep slopes on the site and be founded on the native soils that are interpreted to form the core
of the steep slopes at depth. Therefore, the factor of safety for landslide occurrences should not
decrease below 1.5 for static conditions and 1.2 for dynamic conditions. Grading plans submitted for
the short plat infrastructure will not affect the existing topography within the steeply sloping areas of
the site which meet the criteria for designation as a landslide hazard area.
Structures and improvements shall be located to preserve the most critical portion of the site and its
natural landforms and vegetation.
According to preliminary plans, the most gentle slopes in the property will be occupied by proposed
residences in order to preserve the critical areas within the vicinity of the site and reduce impact on
geological and other critical areas in the vicinity, however, we reviewed grading and drainage plans
for the development of the short plat infrastructure, for which the critical areas are preserved from
substantial alteration.
The proposed development shall not result in greater risk or a need for increased buffers on neighboring
properties.
Increased buffers are unnecessary for adjacent property due to the extent of the steep slopes on the
site and proposed development as part of this project.
The use of retaining walls that allow the maintenance of existing natural slope area is preferred over
graded artificial slopes.
Development shall be designed to minimize impervious lot coverage.
Final development -specific grading plans were not available at the time this memorandum was
prepared.
NELSON GEOTECHN/CAL ASSOCIATES, INC.
Memorandum— Comment Response
Muresan Short Plat
Edmonds, Washington
NGA File No. 10487B 19
October 24, 2019
Page 3
ECDC 20.75.085.A1 and A3
Where environmental resources exist, such as trees, streams, ravines or wildlife habitats, the proposal
shall be designed to minimize significant adverse impacts to the resources. Permanent restrictions may be
imposed on the proposal to avoid impact.
Where conditions exist which could be hazardous to the future residents of the land to be divided, or to
nearby residents or property, such as floodplains, steep slopes or unstable soil or geologic conditions, a
subdivision of the hazardous land shall be denied unless the condition can be permanently corrected,
consistent with ... ... this section.
We anticipate that steep slopes within the site will be permanently stabilized as part of construction
of the specific development on each of the affected lots of the short plat. However, it is our opinion
that the core of the steep slopes within the site is composed of competent, glacially consolidated
materials, and that the slopes are currently stable in respect to deep-seated instability. As mentioned
in the geotechnical report, erosion is likely to occur on steep slopes, especially within the loose
surficial soils and during inclement weather conditions. Permanent stabilization of the steep slopes
from erosion and the landslide hazard due to slope morphology would require grading or retaining
structures which would impact future construction plans within the vacant lots of the short plat.
The current plans for common elements of the plat suggest minimal grading for installation.
Additional precautions for adjacent properties are not needed for the critical areas within the site in
response to planned grading and drainage improvements.
Comment 2:
" ... It is unclear that the undocumented fill area is entirely prohibited from infiltration, revise and
expand infiltration infeasibility to address:
i. Geotechnical report only goes so far as to state infiltration should terminate in native soils but
infiltrative soils were found beneath the fill in all but two test holes; it appears that
infiltration is feasible in much of the area currently shown as not feasible. "
Response
The entire site is feasible for infiltration based on soils information documented in our previous report,
including the undocumented fill area, as long as the base of any infiltration system advances through all
undocumented fill and terminates within the native soils. We recommend that the base of any proposed
infiltration systems terminate within native soils.
Additionally, we did not encounter any groundwater or impermeable horizon to the depths explored, as
documented in our previous report issued for the site. Based on our experience in the area, groundwater
and impermeable horizons are at significant depth and there should be no groundwater or impermeable
horizons within 5 feet of the base of proposed infiltration systems on this site.
NELSON GEOTECHN/CAL ASSOCIATES, INC.
Memorandum — Comment Response
Muresan Short Plat
Edmonds, Washington
Comment 3:
NGA File No. 10487B 19
October 24, 2019
Page 4
"... update MR #6 section to select and size an adequate BMP or include geotechnical information
needed to demonstrate treatment capacity of soils. "
Response
In accordance with the 2014 Department of Ecology (DOE) Stormwater Management in Western
Washington as Amended by the City of Edmonds, infiltration facilities that double as treatment facilities
will need to be tested for Cation Exchange Capacity (USEPA method 9081) and Organic Content (ASTM
D 2974) to determine if the soil is adequate for removing the target pollutants.
Cation Exchange Capacity (CEC) and Organic Content tests were conducted by AMTest Laboratories on
soil samples obtained from the site as shown on Tables 1 and 2 below, respectively. The 2014
Department of Ecology(DOE) Stonmwater Management in Western Washington as Amended by the City
of Edmonds requires soil to be used for treatment to have a CEC greater or equal to 5 milliequivalents
(meq) CEC per 100 grams (CEC/100g). The manual also specifies that filtration soils must have a
minimum of 1.0 percent organic content. Based on the test results, the upper native glacial site soils do
not meet the minimum CEC requirement but meets the organic content requirements. The sample was
taken of the near surface native soils in the south-central portion of the site. The test results are attached
to this memorandum as Appendix A.
Table 1. Cation Test Results
Test Pit Number
Depth
(Feet)
Cation Test Results
CEC/100)
Suitable for Filtration
es/No)
Muresan HA-301
1.5
4.8
No
Table 2.Organic Content Results
Test Pit Number
Depth
(Feet)
Organic Content
Results (Percent)
Suitable for Filtration
(Yes/No)
Muresan HA-301
1.5
3.0
Yes
It is our opinion that the sample is representative of the near -surface native soils on the site. The soils
meet the minimum organic content requirement but are just below the minimum required cation exchange
capacity for treatment of stormwater. We recommend the soils be amended through the use of mulch
within the infiltration system.
NELSON GEOTECHN/CAL ASSOC/A TES, INC
Memorandum — Comment Response NGA File No. 10487B 19
Muresan Short Plat October 24, 2019
Edmonds, Washington Page 5
Minimum Risk Statement
Pursuant to ECDC 23.80.060, it is our opinion that construction of the planned infrastructure as part of
this project should not increase the threat of erosion or landsliding to adjacent properties beyond
predevelopment conditions, should not adversely impact other critical areas, is designed so that the hazard
to the project is eliminated or mitigated to a level equal or less than predevelopment conditions, and is
safe as designed under anticipated conditions. Recommendations provided in our previous report as well
as this memorandum should be strictly followed. We should be retained to review final development
plans and monitor the earthwork phase of the prof ect.
We trust that this memorandum should satisfy your needs at this time. Please do not hesitate to contact us
should you require anything further.
Appendix A Attached
NELSON GEOTECHN/CAL ASSOCIATES, INC
Am Test Inc.
NE 126TH PL Suite AMMT
Suite C
Kirkland, WA 98034
(425) 885-1664 L A B O R A T o R I E s
www.amtestlab.com
ANALYSIS REPORT
NELSON GEOTECH
17311 135TH AVE NE
WOODINVILLE, WA 98072
Attention: CARSTON CURD
Project Name: MI IRFSAN
PO Number: 1048713-19
All results reported on an as received basis.
AMTEST Identification Number
Client Identification
Sampling Date
Conventionals
19-A015734
MURESAN HA-301
09/25/19, 11:30
Professional
Analytical
Services
Date Received: 09/25/19
Date Reported: 10/ 8/19
PARAMETER
RESULT
UNITS
IQ I'D.L.
METHOD
ANALYST
I DATE
Cation Exchange Capacity
14.8
meq/100g
1 10.5
1 SW-846 9081
JKF
10/08/19
Miscellaneous
PARAMETER
IRESULT
JUNITS
10
1D.L.
METHOD
JANLST
I DATE
Organic Matter
13.0
%
1
1
ISM 254OG
JKF
10/08/19
" n/w
Kathy ugiel
President