CANOD.pdfOctober 24, 2016
ES -3710.01
Leif Helleren Construction Inc.
PO Box 1764
Edmonds, Washington 98026
Attention: Mr. Leif Helleren
° B
t` ffY
ON Or,
Subject: Response to Retaining Wall Comments
707 and 711 Main Street
Edmonds, Washington
Earth
Solutions
NWLL(
Earth Solutions NW LLC
Reference: City of Edmonds
Plan Review Comments for Plan Check # 2016-1250 & -1251
Retaining Walls at 707 & 711 Main Street
October 11, 2016
Earth Solutions NW, LLC
GeoWall Max Fill Wall and Calculations
ES -3710.01, dated September 20, 2016
Edmonds City Code
Chapter 23.80 — Geologically Hazardous Areas
Dear Mr. Helleren,
• Geotechnical Engineering
• Construction Monitoring
• Environmental Sciences
As requested, Earth Solutions NW, LLC (ESNW) has prepared this letter responding to the
referenced City of Edmonds comments memo.
Geologically Hazardous Areas Assessment
As part of this geotechnical evaluation, the referenced chapters of the Edmonds City Code was
reviewed. Per the Edmonds City Code requirements, the following topics related to
development plans and site conditions are addressed.
1805 - 136th Place Nf, , Suite 201 ® Bellevue, WA 98005 0 (425) 449-4704 0 FAX (425) 449-4711
Leif Helleren Construction Inc.
October 24, 2016
Slope Reconnaissance
ES -3710.01
Page 2
During our fieldwork, we performed a visual slope reconnaissance across the steep slope area
to the northwest of the site and the minor steep slope areas throughout the site. The steep
slope to the northwest of the site appears to be partially manmade, likely created by a
combination of cut and fill grading activities associated with the alley and the house to the
north. The steep slope to the northwest of the site is vegetated with grass groundcover. The
steep slope areas throughout the site are comprised of rockeries and rubble walls near the site
boundaries to the north and south and were likely constructed as part of the grading activities
associated with the alley and Main Street.
The main focus of our reconnaissance was to identify signs of instability or erosion hazards
along the slopes, rockeries, and rubble walls. The typical instability indicators include features
such as head scarps, tension cracks, hummocky terrain, groundwater seeps along the surface,
erosion features such as gulleys and rills, and bulges in the rockeries or rubble wall. During the
slope reconnaissance, no signs of recent, large scale erosion or slope instability were observed
either on-site or adjacent to the site. In general, based on the slope reconnaissance, stability of
the steep slope to the northwest of the property and the rockeries and rubble walls on-site can
be characterized as good.
Landslide Hazard Areas
With respect to landslide hazard areas, section 23.80.020 of the Edmonds City Code defines
landslide hazard areas as "areas potentially subject to landslides based on a combination of
geologic, topographic, and hydrologic factors. They include areas susceptible because of any
combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other
factors. Within the city of Edmonds, landslide hazard areas specifically include:
1. Areas of ancient or historic failures in Edmonds which include all areas within the
earth subsidence and landslide hazard area as identified in the 1979 report of Robert
Lowe Associates and amended by the 1985 report of GeoEngineers, Inc., and further
discussed in the 2007 report by Landau Associates;
2. Coastal areas mapped as class u (unstable), uos (unstable old slides), and urs
(unstable recent slides) in the Department of Ecology Washington Coastal Atlas;
3. Areas designated as quaternary slumps, earthflows, mudflows, or landslides on maps
published by the United States Geological Survey or Washington State Department
of Natural Resources;
4. Any slope of 40 percent or steeper that exceeds a vertical height of 10 feet over a 25 -
foot horizontal run. Except for rockeries that have been engineered and approved by
the engineer as having been built according to the engineered design, all other
modified slopes (including slopes where there are breaks in slopes) meeting the
overall average steepness and height criteria should be considered potential
landslide hazard areas;
Earth Solutions NW, LLC
Leif Helleren Construction Inc. ES -3710.01
October 24, 2016 Page 3
5. Any slope with all three of the following characteristics:
a. Slopes steeper than 15 percent;
b. Hillsides intersecting geologic contacts with a relatively permeable sediment
overlying a relatively impermeable sediment; and
c. Springs or ground water seepage;
6. Any area potentially unstable as a result of rapid stream incision or stream bank
erosion;
7. Any area located on an alluvial fan, presently subject to, or potentially subject to,
inundation by debris flow or deposition of stream -transported sediments; and
8. Any slopes that have been modified by past development activity that still meet the
slope criteria."
Based on site observation, a north -descending steep slope is located across the alley to the
northwest of the subject property. Per the above definition of landslide hazard areas, the steep
slope to the northwest of the subject property classifies as a landslide hazard area based on a
slope gradient of 40 percent or steeper with a vertical relief of 10 feet or more. The proposed
development will not alter the off-site landslide hazard area and will maintain sufficient setback
from the area.
Based on the City of Edmonds critical areas report for the property (August 25, 2015), landslide
hazard areas are located along the north and south property boundaries. The on-site landslide
hazard area at the north property boundary consisted of a rockery and rubble wall with
backslope up to approximately eight feet in total height. The on-site landslide hazard area at
the south boundary line consisted of a rockery with backslope up to approximately 15 feet in
total height. Based on the above definition, the rubble wall and rockery with backslope at the
north boundary line does not classify as a landslide hazard area as it is less than 10 feet in total
height and the rockery with backslope at the south boundary line classifies as a landslide
hazard area based on a slope gradient of greater than 40 percent or steeper with a vertical
relief of 10 feet or more. These rockeries have been removed as part of site grading activities
and the proposed engineered retaining wall will be constructed to reestablish proposed grades.
In this respect, given the largely "manmade" nature of the identified landslide hazard areas, the
site grading activities have effectively eliminated the on-site potential landslide areas and
construction of the proposed engineered retaining walls is acceptable from a geotechnical
standpoint and will not impact stability of the site or adjacent areas.
Earth Solutions NW, LLC
Leif Helleren Construction Inc.
October 24, 2016
Erosion Hazard Areas
ES -3710.01
Page 4
With respect to erosion hazard areas, section 23.80.020 of the Edmonds City Code defines
erosion hazards as "at least those areas identified by the U.S. Department of Agriculture's
Natural Resources Conservation Service as having a "moderate to severe", "severe", or "very
severe" rill and inter -rill erosion hazard. Erosion hazard areas are also those areas impacted by
shoreland and/or stream bank erosion. Within the city of Edmonds, erosion hazard areas
include:
1. Those areas of the city of Edmonds containing soils that may experience severe to
very severe erosion hazard. This group of soils includes, but is not limited to, the
following when they occur on slopes of 15 percent or greater:
a. Alderwood soils (15 to 25 percent slopes);
b. Alderwood/Everett series (25 to 70 percent slopes);
c. Everett series (15 to 25 percent slopes);
2. Coastal and stream erosion areas which are subject to the impacts from lateral
erosion related to moving water such as stream channel migration and shoreline
retreat;
3. Any area with slopes of 15 percent or greater and impermeable soils interbedded
with granular soils and springs or ground water seepage; and
4. Areas with significant visible evidence of ground water seepage, and which also
include existing landslide deposits regardless of slope."
The on-site soils are generally consistent with Alderwood urban land complex. Based on the
Edmonds City Code definition, the areas on-site and adjacent to the site which are steeper than
15 percent classify as erosion hazard areas. In our opinion, the soils observed throughout
generally exhibit a low to moderate erosion hazard potential. Typical temporary and permanent
erosion control features such as silt fencing along the site boundary, temporary surface water
control features (as necessary), and permanent landscaping have been incorporated into
project plans and construction. In our opinion, the proposed project will not increase the
erosion potential of the site or surrounding properties.
Analysis of Proposal
Based on the information provided to us, the proposed engineered retaining walls will not
impact the landslide hazard and erosion hazard area to the northwest of the subject site. The
on-site potential landslide areas have been removed during site grading activities and the
proposed retaining walls will be sufficiently setback and will not alter the off-site potential
landslide area. Based on the results of our investigation, in our opinion, the proposed project
will not increase the threat of the geological hazard to adjacent properties beyond
predevelopment conditions and will not adversely impact other critical areas.
Earth Solutions NW, LLC
Leif Helleren Construction Inc.
October 24, 2016
Minimum Buffer
ES -3710.01
Page 5
Based on the results of our study and our understanding of the proposed development, in our
opinion, the proposed development should incorporate a minimum no -disturbance buffer of 10
feet from the identified landslide hazard area located to the northwest of the subject site.
We trust this letter meets your current needs. Should you have questions, or if additional
information is required, please call.
Sincerely,
EARTH SOLUTIONS NW, LLC
Project Engineer
cc: RAM Engineering, Inc.
Attention: Mr. Rob Long (Email only)
i Raymond A. Coglas, P.E.
Principal
Earth Solutions NW, LLC
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