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www.aesgeo.com
May 26, 2016
Revised June 7, 2016
Project No. KE160152A
iCap Rhody Ridge, LLC
3535 Factoria Boulevard SE, Suite 500
Bellevue, Washington 98006
Attention: Mr. Levi Rowse
Subject: Geologic Document Review and
Site Reconnaissance
Rhododendron Ridge
11230 SE 80th Street
Newcastle, Washington
Dear Mr. Rowse:
This letter presents the results of Associated Earth Sciences, Inc.’s (AESI’s) site reconnaissance
for the property referenced above at 11230 SE 80th Street in Newcastle, Washington. The site
reconnaissance was performed in general conformance with AESI’s scope of work outlined in
our proposal dated March 30, 2016. Our study was intended to provide information regarding
the proposed residential development, particularly the proposed storm drainage outfall to be
constructed as a surface-mounted HDPE pipe leading down a steeply sloping, unimproved
portion of the City of Renton right-of-way for SE 80th Street.
PROJECT DESCRIPTION
The subject site is the existing residential property located at 11230 SE 80th Street in Newcastle,
Washington (King County Parcel Nos. 334330048-0, -2, -3). We understand that the proposed
project includes 15 residential lots, with associated grading, access, and utilities, including a
proposed storm drainage outfall to be constructed as a surface-mounted HDPE pipe leading
Rhododendron Ridge Geologic Document Review
Newcastle, Washington and Site Reconnaissance
May 26, 2016; Revised June 7, 2016 ASSOCIATE EARTH SCIENCES, INC.
JPL/ld – KE160152A3 – Projects\20160152\KE\WP Page 2
down a steeply sloping, unimproved portion of the City of Renton right-of-way for SE 80th
Street. We understand that the City of Renton has required a geologic reconnaissance of this
slope to provide an opinion regarding the suitability of the proposed outfall configuration. The
purpose of AESI’s study is to perform a site reconnaissance down the sloping portion of the SE
80th Street right-of-way to provide our opinion regarding stability or erosion issues along the
drainage alignment under current conditions, given the proposed surface-mounted drain
system. Also, our study includes review of available information, including past explorations,
wetland delineations by others, and Light Detection and Ranging (LiDAR) imagery, to provide an
opinion regarding the overall stability of the subject site and the slopes leading down to the
west, given the proposed improvements.
As part of a previous study, AESI observed an exploration boring advanced in the area of the
northwest corner of the subject site. Also, for our use in preparing this letter, we have been
provided with “Geotechnical Engineering Study – Rhododendron Ridge,” prepared by Earth
Solutions NW, LLC (Earth Solutions) and dated January 29, 2015, which documented
explorations completed by others at the subject site.
RESEARCH RESULTS
In addition to performing our site reconnaissance, AESI researched available published geologic,
soils, and ground water information about the site and area immediately adjacent to the
property. The resources that we consulted were:
• King County Website (iMAP)
• Previous geotechnical reports by AESI and others (see below)
• U.S. Geological Survey (USGS) Geologic Maps
Site geology and geomorphology were formed from glacial depositional processes and
subsequent erosion to form the current topographic landforms. The results of our research are
presented below.
Aerial Photograph Review
Non-stereographic aerial photographs available from iMAP were reviewed for the site. We
reviewed selected photographs taken from 1936 to 2013. The objective for the aerial
photograph review was to determine the general history of development in the vicinity of and
within the site.
Rhododendron Ridge Geologic Document Review
Newcastle, Washington and Site Reconnaissance
May 26, 2016; Revised June 7, 2016 ASSOCIATE EARTH SCIENCES, INC.
JPL/ld – KE160152A3 – Projects\20160152\KE\WP Page 3
• 1936 - This black and white photograph indicates that the subject site, along with the SE
80th Street right-of-way and the slopes to the west of the subject site, is vegetated with
what appear to be deciduous trees and shrubs. Several rows of orchard trees are
located to the east of the subject site. There are no obvious indications of slope
instability on the site. However, the resolution of the photograph is low.
• 1998 - This black and white photograph indicates that the current residence exists,
along with vegetated garden areas (rhododendrons?), and the slope to the west is
vegetated with what appear to be deciduous trees and shrubs. A multifamily residential
development is located below the slope to the west. There are no obvious indications
of slope instability on the site. However, the resolution of the photograph is low.
• 2000 - This color photograph indicates no noticeable changes from the 1998
photograph. The coloring suggests both deciduous and evergreen trees are present on
the slope. There are no obvious indications of ground water seepage, erosion, or slope
instability on the site. However, as with the 1998 photograph, the resolution of the
photograph is relatively low.
• 2002 - The resolution of this photograph is good in comparison with the 1998 and
2000 photographs. This color photograph indicates no obviously noticeable changes
from the 2000 photograph. Both evergreen and deciduous trees are present on the
western slope. There are no obvious indications of ground water seepage, erosion, or
slope instability on the site.
• 2005, 2007, and 2009 - The resolution of these photographs is good in comparison with
the previous photographs. These color photographs indicate no obviously noticeable
changes at the subject site from the 2000 or 2002 photographs. The residential plat to
the east of the subject site was constructed during this time, along with an improved SE
80th Street segment leading to the east of the site. There are no obvious indications of
ground water seepage, erosion, or slope instability on the site.
• 2012 and 2013 - The 2012 photograph appears to have been taken in early spring, and
many of the deciduous trees are bare, allowing for a somewhat better view to the forest
floor along the slopes to the west of the subject site. The 2013 photograph includes
fully leafed deciduous trees. There are no obvious indications of ground water seepage,
erosion, or slope instability on the site.
Rhododendron Ridge Geologic Document Review
Newcastle, Washington and Site Reconnaissance
May 26, 2016; Revised June 7, 2016 ASSOCIATE EARTH SCIENCES, INC.
JPL/ld – KE160152A3 – Projects\20160152\KE\WP Page 4
Topography/Geomorphology
The topographic maps and LIDAR imagery available on iMAP indicate gently to moderately
sloping terrain within the subject site, and moderately to steeply sloping terrain to the west.
Based on our review of the LIDAR image, the slopes leading upward from the area of Lake
Washington to the upland encompassing the subject site include several bowl-shaped slide
features, including to the immediate north and south of the subject site. These features are
shown in Figure 1. However, based on our review of the above-mentioned aerial photography
and the conditions observed during our site reconnaissance, it is our opinion that the landslide
scarps suggested by the LIDAR image near to the subject site likely originated in the ancient
past, possibly during the lowering of lake levels at the conclusion of the Vashon Stade of the
Fraser Glaciation.
Geology/Ground Water
The published geologic and soils literature reviewed during this phase of our study includes the
following:
• Geologic Map of King County, Washington, by Derek B. Booth, Kathy A. Troost, and
Aaron P. Wisher (2006).
• “Geotechnical Engineering Study – Rhododendron Ridge”, Earth Solutions NW, LLC
(Earth Solutions), January 29, 2015.
• “Preliminary Subsurface Findings” memorandum, Associated Earth Sciences, Inc. (AESI),
August 6, 2015.
According to published geologic map and literature of the area, the site is underlain by Vashon
lodgement till (Qvt). The Qvt was deposited directly from basal, debris-laden glacial ice during
the Vashon Stade of the Fraser Glaciation approximately 12,500 to 15,000 years ago. The high
relative density of the unweathered till is due to its consolidation by the massive weight of the
glacial ice from which it was deposited. This very dense material is generally considered
suitable for support of light to heavily loaded foundations when in an intact, undisturbed
condition. Exposures of pre-Fraser sediments (Qpf) are mapped at the downslope, western
portion of the SE 80th Street right-of-way.
The 2015 Earth Solutions report included exploration test pits completed at the subject site. In
summary, the Earth Solutions test pits encountered soils described as glacial till at the southern
portion of the site, and sandier soils, described as glacial outwash in the report text, at the
northern portion of the site. An exploration boring advanced to a depth of 81.5 feet was
Rhododendron Ridge Geologic Document Review
Newcastle, Washington and Site Reconnaissance
May 26, 2016; Revised June 7, 2016 ASSOCIATE EARTH SCIENCES, INC.
JPL/ld – KE160152A3 – Projects\20160152\KE\WP Page 5
observed by AESI at the northwest corner of the site. The exploration boring described in the
2015 AESI memorandum encountered materials similar to that described in the Earth Solutions
report to roughly 13 feet, overlying very stiff to hard silt and clay. These silts were interpreted
as pre-Vashon glaciomarine silts, possibly of Possession age, similar to the pre-Fraser sediments
shown on the referenced geologic map. The silt displayed a few fractures, although the degree
of fracturing can be variable.
Our review of the above-mentioned documents suggests that ground water seepage across
much of the site is anticipated to be limited to interflow. Interflow occurs when surface water
percolates down through the surficial weathered or higher-permeability sediments and
becomes perched atop underlying, lower-permeability sediments.
The site is located near the southern extent of the mapped Seattle Fault Zone. Recent studies
by the USGS (USGS; e.g., Johnson et al., 1994, Origin and Evolution of the Seattle Fault and
Seattle Basin, Washington, Geology, v. 22, pp. 71-74; and Johnson et al., 1999, Active Tectonics
of the Seattle Fault and Central Puget Sound Washington - Implications for Earthquake Hazards,
Geological Society of America Bulletin, July 1999, v. 111, n. 7, pp. 1042-1053) have provided
evidence of surficial ground rupture along a northern splay of the Seattle Fault. The recognition
of this fault splay is relatively new, and data pertaining to it are limited, with the studies still
ongoing. According to the USGS studies, the latest movement of this fault was about
1,100 years ago when about 20 feet of surficial displacement took place. This displacement can
presently be seen in the form of raised, wave-cut beach terraces along Alki Point in West
Seattle and Restoration Point at the south end of Bainbridge Island. The recurrence interval of
movement along this fault system is still unknown, although it is hypothesized to be in excess of
several thousand years. Due to the suspected long recurrence interval, the potential for
surficial ground rupture along the Seattle Fault Zone is considered to be low during the
expected life of the proposed structures.
RESULTS OF SITE RECONNAISSANCE
An AESI engineering geologist performed a reconnaissance of the site on May 3, 2016. The
purpose of our reconnaissance visit was to provide a visual evaluation of the site conditions,
including topography, drainage, geology, hydrology, and vegetation. Our observations of the
site conditions during our site reconnaissance are in general agreement with site conditions
summarized from our literature research presented above. Site topography varies from low
relief in the area of the existing house on the subject site to moderately sloping, with limited
areas of steeply (greater than 40 percent) sloping ground, in the areas of the SE 80th Street
right-of-way and the property to the west of the subject site. Topographic relief along the
portion of the SE 80th Street right-of-way from the subject site to 110th Avenue NE is about
Rhododendron Ridge Geologic Document Review
Newcastle, Washington and Site Reconnaissance
May 26, 2016; Revised June 7, 2016 ASSOCIATE EARTH SCIENCES, INC.
JPL/ld – KE160152A3 – Projects\20160152\KE\WP Page 6
160 feet, sloping down to the west. The area of the site surrounding the existing residence is
vegetated with landscaping shrubbery, including many rhododendrons, and small- to
medium-sized trees, and large (up to 42 inches in diameter) evergreen trees. The sloping area
to the west of the subject site is vegetated with abundant ferns, brush, and mixed deciduous
and evergreen trees up to 3 feet in diameter. The size and nature of these trees suggests that
the slope has been in its current configuration for some time. A series of manhole covers were
observed at the ground surface along the SE 80th Street right-of-way, suggesting that utility
installation activities have previously been completed down this slope.
No ground water seepage was observed on the slope at the time of our site visit.
CONCLUSIONS AND RECOMMENDATIONS
The sloping area present to the west of the subject site and the SE 80th Street right-of-way do
not show evidence of recent deep-seated landsliding. In our opinion, the slope should not be
adversely affected by the proposed improvements, including the proposed surface-mounted
HDPE storm water pipe leading down the SE 80th Street right-of-way, using current engineering,
architectural, and planning standards. Based on review of available published geologic
information, it appears that site soils should be suitable to support the anticipated HDPE storm
water pipe. In addition, the proposed storm water detention and discharge system will in
effect remove a portion of storm water volume from an area above the top of the sloping
ground, reducing the risk of saturation of the slope due to runoff from upgradient locations
during or subsequent to significant rainfall events. It is our opinion that the risk of landslides or
slope erosion posed by the proposed improvements is low for the site provided current
engineering standards and the recommendations contained this letter are followed.
The steep slopes at the area of the site were predominantly vegetated with ferns, other brush
and trees. This vegetation serves to protect the face of the slopes from soil erosion. We
recommend that this vegetation remain in place to provide root support for the near-surface
soils along the slopes.
Storm water runoff from impermeable surfaces should be collected, routed, and discharged
through a properly designed storm water system. Uncontrolled discharge from the
impermeable surfaces should not be allowed to flow towards or onto the steep slopes.
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