HomeMy WebLinkAboutRS_ KC Solar Lights Geotechnical report_v121312 30th Dr. SE, STE. 110, Bothell, WA 98021 | 425.774.0106 | hwageo.com
May 14, 2025
HWA Project No. 2023-072-21
Concord Engineering
2285 116th Avenue NE
Bellevue, Washington 98004
Attention: Xiaoping Zhang, P.E.
Subject: GEOTECHNICAL REPORT
Engineering Services for Solar Lighting for Parks & Recreation Division
Eastrail Crossing Sites
King County, Washington
Ms. Zhang,
At your request, HWA GeoSciences Inc. (HWA) completed a site reconnaissance and review of
available geotechnical information at four proposed locations for new solar lighting at Eastrail
crossings in Renton, Washington. The four proposed lights are located at three Eastrail
crossings, as follows:
• N 43rd Street Crossing (2 lights)
• N 41st Street Crossing (1 light)
• Mountain View Avenue N Crossing (1 light)
The locations of the crossing sites are shown on Figure 1, Trail Lighting Plan. The purpose of
the work was to evaluate available surface and subsurface information to provide
recommendations for allowable lateral bearing pressures for new light foundations. At the
request of the client, no explorations were performed for this evaluation.
SURFICIAL GEOLOGY
Surficial Geology for all sites is mapped at the 1:100,000 scale in Geologic Map of Surficial
Deposits in the Seattle 30’ x 60’ Quadrangle, Washington (Yount, Minard, & Dembroff, 1993).
The crossings at N 41st Street and N 43rd Street are mapped as alluvium, likely consisting of
loose, unconsolidated clay, silt, sand, gravel and cobbles deposited by alluvial processes (creeks,
streams, rivers, etc.), sometimes containing interbedded peat or organics.
The Mountain View Avenue N crossing is mapped as recessional outwash, likely consisting of
stratified deposits of sand and gravel deposited during the retreat of the last glaciation. Pockets
of silt and clay with the possibility of organics may be encountered within this unit. Glacial till
is mapped about 100 linear feet away from the proposed solar light location. Glacial till
typically consists of very dense, unsorted mixture of clay, silt, sand, gravel, cobbles and boulders
May 14, 2025
HWA Project No. 2023-072-21
KC Solar Lighting - Eastrail 5-14-25 2 HWA GEOSCIENCES INC.
all socketed firmly together due to being previously overridden by an ice sheet over 3,000 feet
thick in this location (Thorson, 1980).
SUBSURFACE GEOLOGY
A review of available geotechnical explorations at the three sites resulted in the log of one
previous boring in the vicinity of the crossings, designated Boring 71. This boring, completed in
1963, is located approximately 160 feet south of the N 41st Street crossing. This boring was
advanced from a starting elevation of 28.4 feet to a termination depth of approximately 34 feet
below sea level. The boring log indicates an average SPT N-value of about 12 in the upper
approximately 20 feet, with soils described mostly as silty sand and sandy silt with peaty silt
layers indicated in the upper approximately 5 feet. Below a depth of about 20 feet, the N-values
increase to over 74, and the soils are described as gray sand and gravel with some silt, likely
indicating the presence of glacial till. The boring was allowed to sit overnight and fill with
water. The water level was indicated at a depth of about 5 feet.
TRAIL HISTORY
Prior to Eastrail’s conversion to a trail accommodating pedestrian and bike travel, it was a
railway known as “The Belt Line” railroad, connecting Black River Junction (located in modern
day Renton) to Woodinville, and which was completed in 1904 (Kirkland History website). Rail
service carried lumber, steel products, and coal, as well as passenger cars and other economic
goods along the eastern shore of Lake Washington. Rail service continued until the early 2000s
after which various jurisdictions along the railway acquired property to convert the alignment to
pedestrian trail.
CONCLUSIONS AND RECOMMENDATIONS
Based on visual observations made during our site reconnaissance on April 1, 2025, each of the
proposed solar light locations are within previous railroad, and currently trail, embankment fill.
Given that the fill has been in place over 120 years and serviced active trains for over 100 years,
we anticipate that the fill is densely consolidated. As such, we recommend an allowable lateral
bearing pressure of 1,500 pounds per square foot (psf) for light foundation locations founded
within embankment fill. The conditions assumed should be verified during construction and
changes made if differing conditions, or soft/organic soils, are encountered.
CONDITIONS & LIMITATIONS
We have prepared this report for Concord Engineering and King County for use in design of this
project. The conclusions and interpretations presented in this report should not be construed as
our warranty of the subsurface conditions. Experience has shown that soil and groundwater
conditions can vary significantly over small distances. As indicated previously, no explorations
were performed for this study.
ENGINEERING SERVICES FOR SOLAR LIGHTING
FOR PARKS & RECREATION DIVISION
KING COUNTY, WASHINGTON RM/BKH
CFTRAIL LIGHTING
PLAN
2023-072-21BASE MAP PROVIDED BY: KING COUNTY DEPARTMENT OF NATURAL RESOURCES AND PARKS, SHEET TL-110, 05.01.2025
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