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G EOTECHNICAL R EPORT
WOLDE RESIDENTIAL
13701 SE MAY VALLEY ROAD, RENTON, WASHINGTON 98059
The Concept Group
4701 SW Admiral Way, Ste 353 ● Seattle, WA 98116 ● (206) 581-0853
Civil & Geotechnical
Engineering Consultants
January 18, 2023
Elias Wolde
13701 SE May Valley Road
Renton, WA 98058
Subject: Wolde Residential Geotechnical Report
13701 SE May Valley Road, Renton, WA 98059
Dear Mr. Wolde,
As requested, The Concept Group is pleased to present this geotechnical report to assist with our
review and evaluation of the stairway and paver patio located at 13701 SE May Valley Road in
the City of Renton, Washington.
In preparing this report, we observed existing site conditions, researched the soil and geology of
the region, reviewed reports and site plans along with conducting an evaluation of the relative
slope stability on the north valley wall of the May Creek Drainage. In summary, the site is
underlain by Everett very gravelly sandy loam.
The site improvements include a masonry stairway down the native steep slope and patio area
constructed on the inside meander of May Creek.
The opinions presented in this evaluation are based, in part, on our observations, explorations,
interpretations and assumptions regarding subsurface conditions; therefore, if variations in the
site conditions are observed later, we may need to modify this letter to reflect those changes.
We appreciate the opportunity to be of service. Should you have any questions, please do not
hesitate to contact us.
Sincerely,
Anna-Trang Nguyen, P.E.
Project Engineer
Rex Humphrey, L.E.G
Engineering Geologist
Table of Contents
1.0 GENERAL ................................................................................................................................ 1
1.1 INFORMATION SOURCES .............................................................................................................................. 1
1.2 PUBLISHED INFORMATION ACCURACY ........................................................................................................ 1
2.0 SITE AND PROJECT DESCRIPTION ..................................................................................................... 2
3.0 SITE GEOLOGY .............................................................................. ERROR! BOOKMARK NOT DEFINED.
3.1 SITE GEOLOGY ............................................................................................................................................. 4
3.2 SOIL CONDITIONS ........................................................................................................................................ 4
4.0 GEOTECHNCAL FINDINGS ........................................................................................................ 5
4.1 SLOPES......................................................................................................................................................... 5
4.2 STAIRWAY .................................................................................................................................................... 5
4.3 PATIO ........................................................................................................................................................... 5
4.4 RETAINING WALLS ....................................................................................................................................... 6
5.0 SLOPE STABILITY MODELING ................................................................................................... 6
6.0 CONCLUSIONS .............................................................................. ERROR! BOOKMARK NOT DEFINED.
7.0 RECOMMENDATIONS ................................................................... ERROR! BOOKMARK NOT DEFINED.
8.0 CLOSURE .............................................................................................................................. 15
9.0 REFERENCES ......................................................................................................................... 14
Appendices
• Appendix A: USGS / IBC-2015 Seismic Design Sheet
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1.0 GENERAL
On December 9th, 2022, The Concept Group visited the residential lot located north of 13701 SE May
Valley Road in Renton, WA. The residential lot is approximately 22,771 sf (0.52 ac) and is situated on the
south side of SE May Valley Road, east of SE Coalfield Way and north of May Creek. The property contains
a single residential structure and is identified as parcel 342405-9113.
The property is nearly rectangular, approximately 80.4 ft. wide (east-west) and 283.16 ft long (north-
south). The existing structure is located on the north side of the parcel. The south side of the parcel
contains the ravine and drainage channel of May Creek.
The purpose of our evaluation is to examine the site improvements on the south half of the parcel
consisting of a masonry stairway across the steep northern ravine slope and a patio area created on the
inside meander of May Creek at the toe of the slope. These site improvements were constructed within
a City of Renton defined hazard area.
This evaluation is intended for the exclusive use of Elias Wolde, his consultants and contractors for the
intended purpose described. Site investigations and observations described in this evaluation represent
the standard of practice for the industry. Sources of information cited are uniformly accepted resources
when utilized in conjunction with field reconnaissance as confirmation. Opinions are based on using
these standardized practices to adequately characterize the local surficial geology and general conditions
at the site.
1.1 INFORMATION SOURCES
Soil classification and mapping is supported by research of the Natural Resource Conservations Service
(NRCS) and on-site soil evaluation performed on the above-mentioned dates. Geologic information for
this assessment is supported by information from the Geologic Map of King County. Our understanding
of the site geology is supported by the review of geologic mapping, published topographic and relief map
layers from the King County Geographical Information System (GIS), and site observations. Opinions are
based on our interpretation of the cumulative information and the contemporary conditions of the
geologic setting.
1.2 PUBLISHED INFORMATION ACCURACY
It should be noted that the NRCS, the USGS geologic map of Seattle, and the King County GIS define
general areas of soil deposits, geology, and landforms. Given the large areas to identify and limited sample
points, the authors of the above sources had to infer boundaries, contacts, and other representations in
some areas. Only through on-site reconnaissance can we further detail and adjust information from the
maps as they relate to a specific site. They are not (from our experience) accurate on a lot-by-lot basis in
all cases. In this case, the Geologic unit identification is generally in concurrence with observed field
conditions, and the Survey identification appears to be in concurrence with observed conditions. Please
see the ‘Soil’ section below for further discussion.
A Critical Area Study was prepared by Confluence Environmental Company dated June, 2022 and a
Hydrologic and Hydraulic Evaluation was performed by Aspect Consulting dated July, 2022 to address
Flood Hazard Data based on predevelopment conditions. Copies of each document were provided to
Concept Engineering.
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2.0 SITE AND PROJECT DESCRIPTION
The nearly 22,771 sf residential lot is situated on the south side of SE May Valley Road, east of SE Coalfield
Way and north of May Creek in Renton, WA. The south side of the nearly 0.52-acre parcel has been
developed into a patio on the north side of May Creek with access down the steep north ravine slope by
a masonry stairway. The patio area is situated on a gently sloping meander on the north side of May
Creek which flows east to west across the southern property line.
Site Topography, Aerial Photography and Survey Site Plan(nts)
Topography and Aerial Photography obtained from King County GIS and Site plan from Terrane
Pre and Post construction conditions
Photos obtained from Confluence Environmental Company / Critical Area Study, June 2022
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Predevelopment photos show the north shoreline is a wide gently sloping meander approximately 5 to 7
percent extending north from the creek to the toe of the steep northern valley wall. The north face of
the valley is approximately 23 ft high with a slope grade of nearly 35 percent. A small area near the center
of the west side of the slope has grades of 40 to 50 percent.
A masonry and flagstone walk, and stairway have been constructed down the moderate to steep slope
facilitating the existing topography. A short block wall extends across the toe of the moderate to steep
slope. This wall is 4 ft tall at the west property line and 5 ft tall at the eastern line.
3.0 SITE GEOLOGY
Geologic Map of King County, Washington (excerpt)
Compiled by Booth, Troost and Wisher, March 2007
3.1 SITE GEOLOGY
The Geologic Map of King County shows the residential property is located on a broad glacial plain which
has been eroded and dominated by alluvial deposits from the May Creek. The geologic units are described
as Glacial till which has been incised by recessional outwash and alluvial deposits associated with the May
Creek drainage. May Creek flows to the west through a wide valley eroded into the glacial till by the
receding glacial ice. The valley contains sands and gravel outwash deposits from the melting glacier and
alluvial deposits from the more recent drainage.
The geologic description indicates the alluvial deposit is nearly unmodified channel deposits at the surface
and is probably 50 ft. or more in depth. The unweathered sand and gravel are suitable for construction
material.
deposited by streams issuing from the advancing ice sheet. In many areas the deposit may grade upward
into till. The deposit contains iron-oxide cemented layers, interbedded and intermixed fine- and coarse-
grained layers.
Project Location
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3.2 SOIL CONDITIONS
As discussed in the ‘Published Information Accuracy’ section above, on-site reconnaissance is necessary
to verify soil conditions on specific properties given the broad representation of soil units over large areas.
The Natural Resource Conservation Service (NRCS) identifies the soil texture or characteristics across the
lot as Everett very gravelly sandy loam with8 to 15 percent slopes. The Everett series soil consists of
gravelly sandy glacial outwash overlying the glacial till in the region. Our site observations concur with
the NRCS soil classification.
Wolde Property
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4.0 GEOTECHNCAL FINDINGS
4.1 SLOPES
Our site observations and review of the Confluence Critical Area Study concur that the development of
the stairway and patio north of May Creek were developed within defined critical areas. Our site visit
observed a thickly vegetated slope with a large cottonwood tree and extensive ground cover and ivy
across the slope.
The north ravine slope is approximately 23 ft high with a slope grade of nearly 35 percent. A small area
near the center of the west side of the slope has grades of 40 to 50 percent. The masonry and flagstone
walk, and stairway have been constructed down the moderate to steep slope facilitating a pre-existing
existing path and topography.
The existing slope does not show morphology or surface indications of soil or slope movement associated
with landslide or erosion of the near surface soil.
Predevelopment photographs show a well-established dirt trail estimated to be 36 to 48 in. wide,
following contour extending across the steep slope. The new stairway and masonry paths generally follow
the previously existing path down the slope.
4.2 STAIRWAY
The new stairway across and down the steep north ravine slope utilized an existing dirt and gravel trail
with wood stairs. The predevelopment trail / path consisted of a dirt walkway with wood stairs extending
across contour and down the steep ravine slope with no provision for storm water runoff or control.
The new stairway and path are masonry surfaces with side walls to contain and control surface water,
keeping the runoff away from the steep slope. Development of the stairway and path did not place fill
onto the slope and follows contour in areas where the stairs descend the slope.
4.3 PATIO
Predevelopment photos show the north shoreline is a wide gently sloping inside-meander north of May
Creek with grades generally less than 5 to 7 percent. Two large cottonwood trees are growing in the
meander near the north and south property lines.
The new patio is a circular structure near the center of the meander which extends north from the north
edge of the 100-year flood zone of May Creek. The leveled area extends from the creek to the toe of the
steep northern ravine wall. It appears the southern half of the new patio was leveled with a gravel mix to
better accommodate and level the patio. The two large cottonwood trees remain with masonry sitting
areas surrounding the trunk of each tree.
A 4 ft. tall masonry wall extends across the southern edge of the meander along the mapped 100-year
flood area. The Confluence report indicates a small portion of the flood zone near the southeast and
southwest corners of the lot. A flood event exceeding the 100-year flood zone would inundate the patio
area. The design and construction of the patio area, wall and small stairway at the south end of the patio
area to provide for water to flow through and out of the patio area with minimal erosion.
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4.4 RETAINING WALLS
A 4 ft tall masonry block walls extend across the toe of the moderate to steep ravine slope. The heal of
the wall is located at the toe of the slope and the wall is backfilled between the native slope and the wall.
The wall along the south side of the patio extends along the 100-year flood zone and is 3 ft tall.
5.0 SLOPE STABILITY MODELING
Computer Modeling Analysis
Computer modeling analysis was performed using GEO5 2020 modeling software. This modeling program
does not predict slope failure or run out but can assist the geotechnical professional by determining the
potential for circular or Translational stability of the site. The computer software provides analysis of the
site in a “Static” and “Dynamic” condition. The Static analysis is based on the existing surface, geology,
and existing site development. The Dynamic analysis is based on an applied earthquake ground
acceleration (S1). The ground acceleration value (S1) is obtained from the US Seismic Design Mapping
prepared by the USGS specific to the site address.
The GEO5 software evaluates 5 methods for modeling the slope: Bishop, Fellenius/Petterson, Jambu,
Spencer and Morgenstern-Price. The site’s existing condition and proposed developed conditions were
evaluated based on Static and Dynamic conditions. The analysis assumed a uniform loading of 500 psf for
the developed stairway.
ESTIMATED PROPERTIES OF ON-SITE SOILS FOR STABILITY ANALYSIS Soil Type Density
(pcf)
Cohesion
(psf)
Internal Friction
Angle
(degrees) Loose Silty SAND 105 0 28 Medium-Dense Silty SAND 115 0 30 Stiff Clayey SILT 95 300 26 Medium-Dense Silty SAND with some Gravel 120 0 32 Dense Gravelly Silty SAND 125 0 34 Glacial Till 140 1000 38
Based on our review of the slope through traditional methods (site reconnaissance, aerial photography,
topographical indicators, etc.) along with the modeling program; the undeveloped and developed slope
is indicated to be stable in a Static and Dynamic condition. Our evaluation assumes the stairway, path
and patio remain in place. Soil properties shown in the table below were utilized for each soil interface
in the slope evaluation.
Computer static and dynamic modeling of the pre-developed conditions along a cross sections for the
stairway indicates a Factor of Safety – Static conditions of 6.10 to 6.24 FS (1.5 FS Minimum) and Factor of
Safety – Dynamic conditions (0.5 g) of 2.07 to 2.13 FS (1.5 FS Minimum).
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Computer static and dynamic modeling of the developed conditions along a cross section for the stairway
indicate a Factor of Safety – Static conditions of 4.85 to 5.66 FS (1.5 FS Minimum) and Factor of Safety –
Dynamic conditions (0.5 g) of 1.86 to 2.02 FS (1.5 FS Minimum).
Evaluation of Model
Copies of the stability analysis are attached showing the computer interpretations of the safety factor for
a static and dynamic condition circular failure. Computer modeling applied a safety factor of 1.5 as the
minimum Static acceptable results in the failure plane interface. The interface extends from the top of
the ravine to the toe of the slope through the existing stairway, path and retaining wall at the toe.
Stability Modeling Cross Section (nts)
Site plan from Terrane Topographic and Boundary Survey
The model was then re-run for the existing Dynamic factor of safety with a 1.5 FS minimum acceptable.
Earthquake (seismic) Dynamic loading of 0.5g was applied to the existing conditions and same interface.
Modeling was conducted for the existing conditions as shown on the site plan utilizing topography
provided by Terrane Topography and Boundary Survey.
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Predevelopment – Static Conditions
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Predevelopment – Dynamic Conditions
Developed Conditions – Static
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Developed Conditions – Dynamic
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6.0 CONCLUSIONS
Site observations found the existing slope and developed conditions to be stable with no visible
morphology of surface or landslide activity and no observed erosion of the ravine wall or north shoreline
of May Creek.
The existing stairway, landings and patio areas were observed to be stable and well built. The orientation
of the stairway, landings and patio area are consistent with the pre-existing trail which traversed the north
ravine slope and northern inside meander of May Creek.
The stairway is constructed on contour and on the surface of the slope with minimal excavation or
disturbance to the slope face.
The southern edge of the patio area is protected by a 3 ft. masonry wall which appears to have been
constructed north of and along the 100-year flood zone for the creek. The patio area is approximately 3
ft. above the creek and 2 ft above the naïve shoreline of the meander north of the creek. The native
predeveloped surface was observed around two large cottonwood trees growing on the meander north
of the creek.
In the event of a flood event greater than the 100-year elevation the patio area is constructed in a manner
which will allow water to pass through and over the area and not retain the water.
Computer modeling of the predevelopment and existing conditions associated with the stairway and patio
indicate the slope was stable in its predeveloped condition and is stable in the developed condition.
7.0 RECOMMENDATIONS
The existing stairway and patio on the north ravine slope are stable and provide erosion protection. The
native slope is vegetated and shows no indications of erosion or slope movement.
The development of the stairway and patio was completed over an existing trail which traversed the north
ravine slope. The new stairway and patio were constructed within a potentially critical area based on
slope grades and stream classification of May Creek.
We recommend the stairway, trail and patio remain in place. Removal of the structures will cause
significant disturbance to the slope and north shoreline of May Creek. Surface disturbance will require
one or more growing seasons to re-establish vegetation.
Stabilization and control of surface storm water runoff currently provided by the stairway on a portion of
the north ravine slope provides protection from surface erosion and surface translational soil movement.
Removal would create the potential for surface and slope deterioration.
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8.0 CLOSURE
This evaluation has been prepared for Elias Wolde and his project team. The conclusions and
recommendations presented are based, in part, on the exploration and testing performed for this study;
therefore, if variations in the subgrade conditions are observed at a later time, we may need to modify
this report to reflect those changes. The evaluation was performed in general accordance with the
agreed-upon scope of services.
The client is responsible for advising the project team, designers, contractors, subcontractors and
regulators of the content of this evaluation. Noncompliance with any of the recommendations presented
will release The Concept Group from any liability resulting for the use of this evaluation.
This evaluation has been prepared for planning and design purposes, specific to the existing stairway and
patio and has been prepared in accordance with generally accepted standards and practices at the time
of the evaluation writing. No warranty expressed or implied is made.
Submitted by
The Concept Group
Anna-Trang Nguyen, P.E. Rex Humphrey, L.E.G.
Project Engineer Engineering Geologist
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9.0 REFERENCES
Confluence Environmental Company, Kerrie McArthur, PWS and Natalie Dietsch, WPIT, June 2022
Aspect Consulting, Memorandum, John Knutson, PE, PMP and Erik Pruneda, PE, CPESC, CFM, July, 2022
Derek B. Booth, Kathy A. Troost & Aaron P. Wisher, March 2007, Geologic Map of King County, Scale
1:100,000
U.S. Seismic Design Mapping, SEAOC / OSHPD, IBC 2015, https://seismicmaps.org
Natural Resources Conservation Services, Web Soil Survey URL, King County Area, Washington, Survey
Area Data: Version 16, June 4, 2020
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Appendix A
USGS Seismic Design Sheet