HomeMy WebLinkAboutAttachment_C_Geotechnical_Report
GEOTECHNICAL INVESTIGATION
Vantage Glen
Prepared for
Confluence Environmental Company
146 N Canal Street, Suite 111
Seattle, Washington 98103
Prepared by
Geosyntec Consultants, Inc.
710 2nd Avenue, Suite 550
Seattle, Washington 98104
Project: NWG0043
July 23, 2025
FINAL
Geotechnical Investigation July 2025
Geotechnical Investigation
Vantage Glen
Prepared for
Confluence Environmental Company
J. Bodie McCosby, LG.Henry H. Haselton, PE, PMP.
Project Professional Geologist Senior Principal Engineer
Project Number: NWG0043
July 23, 2025
7/23/2025 7/23/2025
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Geotechnical Investigation i July 2025
TABLE OF CONTENTS
1. INTRODUCTION AND PROJECT DESCRIPTION ...........................................................1
2. CITY OF RENTON GEOLOGICALLY HAZARDOUS AREAS .......................................2
2.1 Steep Slopes .................................................................................................................2
2.2 Landslide Hazards ........................................................................................................2
2.3 Erosion Hazards ...........................................................................................................3
2.4 Seismic Hazards ...........................................................................................................3
2.5 Coal Mine Hazards .......................................................................................................3
3. SUMMARY OF FINDINGS .................................................................................................4
3.1 Site Conditions .............................................................................................................4
3.1.1 Topography and Vegetation .............................................................................4
3.1.2 Geologic Setting ...............................................................................................4
3.2 Specific Forest Stand Conditions and Observations ....................................................5
3.2.1 Forest Stand 1 ...................................................................................................5
3.2.2 Forest Stand 2 ...................................................................................................7
3.2.3 Forest Stand 3 ...................................................................................................9
3.2.4 Forest Stand 4 .................................................................................................11
3.2.5 Forest Stand 5 .................................................................................................13
3.2.6 Forest Stand 6 .................................................................................................14
3.2.7 Forest Stand 7 .................................................................................................16
4. CONCLUSIONS AND RECOMMENDATIONS ..............................................................18
5. REFERENCES ....................................................................................................................19
6. LIMITATIONS ....................................................................................................................20
LIST OF TABLES
Table 1: Geologically Hazardous Areas
LIST OF FIGURES
Figure 1: Site Location Map
Figure 2: Critical Areas Map
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Geotechncial Investigation 1 July 2025
1. INTRODUCTION AND PROJECT DESCRIPTION
Geosyntec Consultants, Inc. (Geosyntec), conducted this geotechnical investigation for
Confluence Environmental Company (Confluence) to assist King Conservation District (KCD)
with permitting a forest restoration project (Project) at Vantage Glen (a King County Housing
Authority property) at 18100 107th Place SE, Renton (Site; Figure 1) also known asking County
parcels 3223059363, 3223059364, and 3223059070. Vantage Glen has 13 acres of forested land
divided into six forest management areas or “stands” ranging in size from approximately 1 to 3.5
acres and are shown in Figure 2 (Photograph 1). Geosyntec conducted a geotechnical
reconnaissance on March 12, 2025, to determine the presence and evaluate the condition of
geologically critical areas, as defined by Renton Municipal Code (RMC) 4-3-050G5 (RMC,
2025), including steep slopes, landslide hazard areas, erosion hazard areas, seismic hazard areas,
and coal mine hazard areas. The presence of streams and wetlands on and adjacent to the Site
was evaluated by Confluence (Confluence, 2025).
We understand that the Project will involve removal of invasive species and installation of
assorted native trees, using manual methods to promote the establishment of a healthy forested
habitat as part of KCD’s Forest Stewardship Program, which will restore approximately 13 acres
of forestland at the Site (KCD, 2024). In this report we provide an evaluation of the Site
geologically critical areas as they relate to the proposed Project. This report has been prepared to
support KCD’s critical areas exemption request to the City of Renton (COR).
Photograph 1: Showing forested area in Forest Stand 4.
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Geotechncial Investigation 2 July 2025
2. CITY OF RENTON GEOLOGICALLY HAZARDOUS AREAS
Development activities and alterations within geologically hazardous areas and associated
buffers in the COR are regulated under RMC 4-3-050 (RMC, 2025). The key regulations that
apply to the Project are the designation criteria and development standards for geologically
hazardous areas in RMC 4-3-050G5 (RMC, 2025). This report serves as a critical areas
assessment to support KCD’s critical areas review exemption request for the Project under RMC
4-3-050C(2) through identifying and evaluating geological risks and ensure the Project will not
adversely impact other critical areas, adjacent properties, or pose an unreasonable threat to public
health and safety. In the following sections, we outline and define the relevant geologically
hazardous areas, per RMC (RMC, 2025).
2.1 Steep Slopes
The COR defines steep slope critical area based on two definitions: sensitive slopes and
protected slopes.
Sensitive slopes are defined as “a hillside, or portion thereof, characterized by:
1. an average slope of twenty five percent (25%) to less than forty percent (40%) as
identified in the COR Steep Slope Atlas or in a method approved by the City; or
2. an average slope of forty percent (40%) or greater with a vertical rise of less than
fifteen feet (15') as identified in the COR Steep Slope Atlas or in a method approved
by the City;
3. abutting an average slope of twenty five percent (25%) to forty percent (40%) as
identified in the COR Steep Slope Atlas or in a method approved by the City. This
definition excludes engineered retaining walls.” (RMC, 2024).”
Protected slopes are defined as “A hillside, or portion thereof, characterized by an average slope
of forty percent (40%) or greater grade and having a minimum vertical rise of fifteen feet (15') as
identified in the COR Steep Slope Atlas or in a method approved by the City.” (RMC, 2024).
Steep slope hazard areas were identified and are discussed in detail in Section 3.2.
2.2 Landslide Hazards
The COR defines landslide hazard areas based on four relative hazard levels outlined as:
1. Low Landslide Hazard (LL): Areas with slopes less than fifteen percent (15%).
2. Medium Landslide Hazard (LM): Areas with slopes between fifteen percent (15%)
and forty percent (40%) and underlain by soils that consist largely of sand, gravel, or
glacial till.
3. High Landslide Hazards (LH): Areas with slopes greater than forty percent (40%),
and areas with slopes between fifteen percent (15%) and forty percent (40%) and
underlain by soils consisting largely of silt and clay.
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Geotechncial Investigation 3 July 2025
4. Very High Landslide Hazards (LV): Areas of known mapped or identified landslide
deposits.
Landslide hazard areas were identified and are discussed in detail in Section 3.2.
2.3 Erosion Hazards
The COR defines erosion hazard areas based on two relative hazard levels outlined as:
1. Low Erosion Hazard (EL): Areas with soils characterized by the Natural Resource
Conservation Service (NRCS; formerly U.S. Soil Conservation Service) as having
slight or moderate erosion potential, and a slope less than fifteen percent (15%).
2. High Erosion Hazard (EH): Areas with soils characterized by the Natural Resource
Conservation Service (formerly U.S. Soil Conservation Service) as having severe or
very severe erosion potential, and a slope more than fifteen percent (15%).
Erosion hazard areas were identified and are discussed in detail section 3.2 of this report.
2.4 Seismic Hazards
The COR defines seismic hazard areas based on two relative hazard levels outlined as:
1. Low Seismic Hazard (SL): Areas underlain by dense soils or bedrock. These soils
generally have site classifications of A through D, as defined in the International
Building Code, 2012.
2. High Seismic Hazard (SH): Areas underlain by soft or loose, saturated soils. These
soils generally have site classifications E or F, as defined in the International Building
Code, 2012.
No significant seismic hazard areas were mapped or observed at the Site or adjacent areas.
2.5 Coal Mine Hazards
The COR defines coal mine hazards based on three relative hazard levels outlined as:
1. Low Coal Mine Hazards (CL): Areas with no known mine workings and no predicted
subsidence. While no mines are known in these areas, undocumented mining is
known to have occurred.
2. Medium Coal Mine Hazards (CM): Areas where mine workings are deeper than two
hundred feet (200') for steeply dipping seams, or deeper than fifteen (15) times the
thickness of the seam or workings for gently dipping seams. These areas may be
affected by subsidence.
3. High Coal Mine Hazard (CH): Areas with abandoned and improperly sealed mine
openings and areas underlain by mine workings shallower than two hundred feet
(200') in depth for steeply dipping seams, or shallower than fifteen (15) times the
thickness of the seam or workings for gently dipping seams. These areas may be
affected by collapse or other subsidence.
No coal mine hazard areas were mapped or observed at the Site or adjacent areas.
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Geotechncial Investigation 4 July 2025
3. SUMMARY OF FINDINGS
While Project plans are preliminary, it is our opinion that the proposed forest restoration Project
can be implemented in a way that will not adversely impact the geologically hazardous areas
within or in the vicinity of the Project. In fact, it is our opinion that the implementation of the
Project (replanting native trees and removal of invasive species) will serve to improve the
condition of the geologically hazardous areas at and adjacent to the Site.
A detailed discussion of our observations, conclusions, and recommendations is presented below.
3.1 Site Conditions
The Site consists of forest stands that flank the Vantage Glen neighborhood area to the north,
east, south, and west. The central portion of the Site contains residential multifamily and mobile
homes, along with a community center, and stormwater retention pond. The six forest stands
total approximately 13 acres of forestland ranging in size from 1 to 3.5 acres (Figure 2).
3.1.1 Topography and Vegetation
The central portion of the Site slopes gently to the west at approximately 4 degrees (7% grade)
with 22 degree (40% grade) slopes ranging from 20 to 105 feet tall sloping into the Site on all
sides of the Site (Figure 2). There is a ravine located at the southwest corner of the Site with
slopes ranging from 30 to 35 degrees (60% to 70% grade) that are up to 150 feet tall. The Site is
vegetated with various coniferous and deciduous trees, as well as native and invasive
groundcover.
3.1.2 Geologic Setting
The Site is located south of Lake Washington in the uplands within the Puget Lowland, a broad
area of tectonic subsidence flanked by two mountain ranges: the Cascades to the east and the
Olympics to the west. The sediments within the Puget Lowland are the result of repeated cycles
of glacial and nonglacial deposition and erosion. The most recent cycle, the Vashon Stade of the
Fraser Glaciation (about 13,000 to 16,000 years ago), is responsible for most of the present day
geologic and topographic conditions.
Most of the Site is mapped as Vashon till (Qgt), which is underlain by Pre-Vashon continental
drift and nonglacial deposits (Qgpc) that are exposed in the ravine in the southwest corner of the
Site (Schuster et al., 2015). While not mapped, we observed evidence of advance outwash
deposits (Qga) throughout the Site, consisting of sand with silt and gravel (Photograph 2).
Advance outwash is a geologic unit known to occur stratigraphically between Qvt and Qgpc
(Schuster et al., 2015). This in conjunction with the Site being mapped as a “gravel pit”
(Schuster et al., 2015) suggest that, in contradiction to the mapping, Qga overlying Qgpc is the
dominant geologic sequence at the Site.
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Geotechncial Investigation 5 July 2025
Photograph 2: Showing advance outwash deposits, sand with silt and gravel.
3.2 Specific Forest Stand Conditions and Observations
Geosyntec assessed the surface conditions of the Site through literature review and geotechnical
field reconnaissance. On March 12, 2025, Geosyntec conducted a reconnaissance focused on
observing surface conditions and evaluating geotechnical risks at each of the six forest stands at
the Site (Figure 2). To supplement field observations, we reviewed historical aerial photos,
historical topographic maps, geologic maps, geohazard maps, and nearby subsurface
investigations. The specific site conditions, our observations in geologically hazardous areas, as
well as our assessments of the mapped geologically hazardous areas at each forest stand are
outlined in detail in the following sections.
3.2.1 Forest Stand 1
3.2.1.1 Observations – Forest Stand 1
Forest Stand 1 is located on the eastern side of the Site just south of the Community Center.
Forest Stand 1 is located on a slope up to 50 feet tall that is uniformly sloped 22 degrees (40%
grade) to the north (Figure 2). The slope was constructed as a result of the Site’s history as a
gravel pit with approximately 10 foot wide benches every 20 vertical feet (Schuster et al., 2015).
The slope is vegetated with coniferous and deciduous trees up to 1.5 feet diameter at breast
height (dbh) and sparse ground cover. Conifers were generally tall and straight with some trees
exhibiting slight “pistol butting” and/or slight downslope tilt (Photograph 3).
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand-tool excavations. We observed approximately 1 foot of topsoil underlain by soil we
determined to be advance outwash consisting of dense, brown to gray, moist sand with silt and
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Geotechncial Investigation 6 July 2025
gravel. We did not observe rills or other signs of surface erosion; however, we did observe
localized areas of hydrophilic plants indicating periodic seeps on the slope.
Photograph 3: Showing slight pistol butting and downslope tilt in conifers at Forest Stand 1.
Photogrpah looking east.
3.2.1.2 Geologically Hazardous Areas – Forest Stand 1
The geologically hazardous areas that affect Forest Stand 1 are discussed in greater detail in the
following sections.
Steep Slope Hazard Area – Forest Stand 1
The COR maps both sensitive and protected slopes in Forest Stand 1, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers), but observed no evidence of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, persistent groundwater seepage, or
excessive coniferous trunk curvature.
The slopes in Forest Stand 1 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the steep slope hazard area.
Landslide Hazard Area – Forest Stand 1
While not mapped by COR, slopes meeting the conditions for LM exist at Forest Stand 1 (Figure
2). The slope shows no significant indication of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, or excessive coniferous trunk
curvature. We observed minor trunk curvature in coniferous trees, and some localized periodic
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Geotechncial Investigation 7 July 2025
groundwater seepage on the slope that indicate minor surficial slope instability. We did not
observe evidence or indication of the presence of deep-seated landslides on the slope.
The slopes in Forest Stand 1 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the landslide hazard area.
Erosion Hazard Area – Forest Stand 1
While not mapped by COR, Forest Stand 1 meets the criteria of an erosion hazard area. The NRCS
maps the soils here as Alderwood gravelly sandy loam, which have a “slight” erosion risk (NRCS,
2025). Forest Stand 1 shows no indication of recent or active erosion, such as gullying, rills, or
exposed soil. However, due to the sloped nature of the Site, and the nature of the slopes soils, the
potential for erosion is high if appropriate best management practices (BMPs) are not
implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 1 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
3.2.2 Forest Stand 2
3.2.2.1 Observations – Forest Stand 2
Forest Stand 2 is located in the central portion of the Site. Forest Stand 2 is located on a slope up
to 70 feet tall that is uniformly sloped 22 degrees (40% grade) to the west (Figure 2). The slope
was constructed as a result of the Site’s history as a gravel pit with approximately 10-foot-wide
benches every 20 vertical feet (Schuster et al., 2015). The slope is sparsely vegetated with
coniferous and deciduous trees up to 1.5 feet dbh and sparse ground cover (Photograph 4).
Conifers were generally tall and straight with some trees exhibiting slight pistol butting and/or
slight downslope tilt.
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand-tool excavations. We observed approximately 1 foot of topsoil underlain by soil we
determined to be advance outwash consisting of dense, brown to gray, moist sand with silt and
gravel. We observed some rills and exposed soil on the slope, indicating localized surface
erosion is occurring on the slope.
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Geotechncial Investigation 8 July 2025
Photograph 4: Sparse groundcover and bench on slope at Forest Stand 2. Photograph looking
south.
3.2.2.2 Geologically Hazardous Areas – Forest Stand 2
The geologically hazardous areas that affect Forest Stand 2 are discussed in greater detail in the
following sections.
Steep Slope Hazard Area – Forest Stand 2
The COR maps both sensitive and protected slopes in Forest Stand 2, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers) and surficial soil erosion (rills and exposed soil), but observed no evidence of
recent or incipient slope instability, such as bowl-shaped topography, tension cracks along the top
of the slope, groundwater seepage, or excessive coniferous trunk curvature.
The slopes in Forest Stand 2 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the steep slope hazard area by
stabilizing the surficial soils.
Landslide Hazard Area – Forest Stand 2
While not mapped by COR, slopes meeting the conditions for LM exist at Forest Stand 2 (Figure
2). The slope shows no significant indication of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, or excessive coniferous trunk
curvature. We observed minor trunk curvature in coniferous trees, some localized surficial erosion
features (rills and channels), and exposed soil on the slope that indicate minor surficial slope
instability. We did not observe evidence or indication of the presence of deep-seated landslides on
the slope.
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Geotechncial Investigation 9 July 2025
The slopes in Forest Stand 2 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the landslide hazard area.
Erosion Hazard Area – Forest Stand 2
While not mapped by COR, Forest Stand 2 meets the criteria of an erosion hazard area. The NRCS
maps the soils here as Alderwood gravelly sandy loam which have a “slight” erosion risk (NRCS,
2025). Forest Stand 2 shows indication of recent or active erosion, such as rills and exposed soil,
on the slope. Due to indications of recent erosion and the steep slope of Forest Stand 2, the
potential for erosion is high if appropriate BMPs are not implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 2 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
3.2.3 Forest Stand 3
3.2.3.1 Observations – Forest Stand 3
Forest Stand 3 is located in the southern portion of the Site. Forest Stand 3 is located on a slope
up to 100 feet tall that is sloped 22 degrees (40% grade) to the north and west (Figure 2). Most of
the north facing slope was constructed as a result of the Site’s history as a gravel pit with
approximately 10-foot-wide benches every 20 vertical feet (Schuster et al., 2015). The eastern
portion of the slope has a rockery at the toe that is up to 8 feet tall and appears to be in good
condition (Photograph 5). The western portion of the slope is a natural slope that is part of the
ravine formed by Panther Creek. The slope is vegetated with coniferous and deciduous trees up
to 1.5 feet dbh and ground cover. Conifers were generally tall and straight with some trees
exhibiting slight pistol butting and/or slight downslope tilt.
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand-tool excavations. We observed approximately 1 foot of topsoil underlain by soil we
determined to be advance outwash consisting of by dense, brown to gray, moist sand with silt
and gravel. We did not observe rills, exposed soil, or other signs of surficial erosion.
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Geotechncial Investigation 10 July 2025
Photograph 5: Showing mature conifers and rockery at Forest Stand 3. Photograph looking west.
3.2.3.2 Geologically Hazardous Areas – Forest Stand 3
The geologically hazardous areas that affect Forest Stand 3 are discussed in greater detail in the
following sections.
Steep Slope Hazard Area – Forest Stand 3
The COR maps both sensitive and protected slopes in Forest Stand 3, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers), but observed no evidence of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, groundwater seepage, or excessive
coniferous trunk curvature.
The slopes in Forest Stand 3 appear stable and based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the steep slope hazard area by
stabilizing the surficial soils.
Landslide Hazard Area – Forest Stand 3
The western portion of Forest Stand 3 is mapped by COR as a landslide hazard area and, while not
mapped by COR, the eastern portion of Forest Stand 3 contains slopes meeting the conditions for
LM at Forest Stand 3 (Figure 2). The slope shows no significant indication of recent or incipient
slope instability, such as bowl-shaped topography, tension cracks along the top of the slope, or
excessive coniferous trunk curvature. We did observe minor trunk curvature in coniferous trees
that indicate minor surficial slope instability. We did not observe evidence or indication of the
presence of deep-seated landslides on the slope.
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Geotechncial Investigation 11 July 2025
The slopes in Forest Stand 3 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the landslide hazard area.
Erosion Hazard Area – Forest Stand 3
The western portion of Forest Stand 3 is mapped by COR as an erosion hazard area and, while not
mapped, the eastern portion of Forest Stand 3 meets the criteria of an erosion hazard area. The
NRCS maps the soils here as Alderwood gravelly sandy loam, which have a “slight” erosion risk
(NRCS, 2025). While Forest Stand 3 shows no significant indication of recent or active erosion,
such as rills and exposed soil, due to the steep slope of Forest Stand 3, the potential for erosion is
high if appropriate BMPs are not implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 3 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
3.2.4 Forest Stand 4
3.2.4.1 Observations – Forest Stand 4
Forest Stand 4 is located on the southwestern side of the Site. Forest Stand 4 covers the ravine
and Panther Creek with slopes that are up to 100 feet tall sloped at 40 degrees (80% grade) with
localized near-vertical sections (Figure 2). The slope is vegetated with coniferous and deciduous
trees up to 2.5 feet dbh and thick ground cover. Conifers were generally tall and straight with
some trees exhibiting slight pistol butting and/or slight downslope tilt (Photograph 6).
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand tool excavations. We observed approximately 2.5 feet of topsoil and forest debris underlain
by soil we determined to be advance outwash consisting of dense, brown to gray, moist sand
with silt and gravel on the upper portions of the slope, and soil we determined to be Pre-Vashon
continental drift and nonglacial deposits consisting of dense, brown to gray, moist sand with silt
and gravel with trace organics and interbedded laminated clay in the creek cut banks and lower
portions of the slopes. We observed cut banks adjacent to Panther Creek, minor signs of surficial
erosion, and localized areas of hydrophilic plants indicating seeps on the slope.
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Geotechncial Investigation 12 July 2025
Photograph 6: Showing Panther Creek in Forest Stand 4. Photograph looking upstream to the
south.
3.2.4.2 Geologically Hazardous Areas – Forest Stand 4
The geologically hazardous areas that affect Forest Stand 4 are discussed in greater detail in the
following sections.
Steep Slope Hazard Area – Forest Stand 4
The COR maps both sensitive and protected slopes in Forest Stand 4, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers) and some evidence of recent or incipient surficial slope instability, such as
bowl-shaped topography and tilted and curved coniferous trees. We did not observe tension cracks
along the top of the slope, recent soil scarps, or excessive groundwater seepage.
The slopes in Forest Stand 4 appear to be exhibiting minor surficial instabilities common on
natural slopes of steep inclinations. Based on our understanding of the Project, these steep slope
hazard areas will not be adversely impacted by the proposed forest restoration. In fact, forest
restoration with native trees and plants will help to improve the condition and stability of the steep
slope hazard area.
Landslide Hazard Area – Forest Stand 4
The eastern portion of Forest Stand 4 is mapped by COR as a landslide hazard area and while not
mapped by COR, slopes meeting the conditions for LH and LV exist at Forest Stand 4 (Figure 2).
In addition, the Washington Geological Survey (WGS) landslide inventory database maps a
landslide scarp and deposit in the central portion of Stand 4 (WGS, 2024) (Figure 2). This
landslide feature is mapped as “low” confidence and prehistoric, or greater than 150 years old
(WGS, 2024). Based on the WGS mapping, our data review, and our site reconnaissance, it is our
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Geotechncial Investigation 13 July 2025
opinion that this mapped landslide is not currently active and has not been active for a significant
amount of time (prehistoric). The relative risk of this mapped landslide feature activating is similar
to that of the others slopes in Stand 4. While we observed evidence of surficial landslides on the
slopes including bowl-shaped topography and tilted and curved coniferous trees. We did not
observe tension cracks along the top of the slope, recent soil scarps, or excessive groundwater
seepage. We did not observe evidence or indication of the presence of deep-seated landslides on
the slope.
The slopes in Forest Stand 4 appear to be exhibiting natural normal surficial slope instabilities for
slopes of this steep inclination. Based on our understanding of the Project, the landslide hazard
area will not be adversely impacted by the proposed forest restoration. In fact, forest restoration
with native trees and plants will help to improve the condition and stability of the landslide hazard
area.
Erosion Hazard Area – Forest Stand 4
The central portion of Forest Stand 4 is mapped as an erosion hazard area by COR and, while not
mapped by COR, the rest of Forest Stand 4 meets the criteria of an erosion hazard area. The
NRCS maps the soils here as Alderwood gravelly sandy loam, which have a “slight” erosion risk
(NRCS, 2025). Forest Stand 4 shows indication of recent or active erosion in the creek area, such
as cut banks and exposed soil sections. However, the slopes were well vegetated and we did not
observe significant evidence of erosion, such as gullying, rills, or exposed soil. Due to the steep
slopes of Forest Stand 4, the potential for erosion is high if appropriate BMPs are not
implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 4 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
3.2.5 Forest Stand 5
3.2.5.1 Observations – Forest Stand 5
Forest Stand 5 is located in the western portion of the Site. Forest Stand 5 is located on a slope
up to 50 feet tall that is sloped 22 degrees (40% grade) to the east (Figure 2). The slope was
constructed as a result of the Site’s history as a gravel pit with approximately 10-foot-wide
benches every 20 vertical feet (Schuster et al., 2015). The slope is vegetated with coniferous and
deciduous trees up to 1.5 feet dbh and ground cover. Conifers were generally tall and straight
with some trees exhibiting slight pistol butting and/or slight downslope tilt.
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand-tool excavations. We observed approximately 1 foot of topsoil underlain by soil we
determined to be advance outwash consisting of dense, brown to gray, moist sand with silt and
gravel. We did not observe rills, exposed soil, or other signs of surficial erosion.
3.2.5.2 Geologically Hazardous Areas – Forest Stand 5
The geologically hazardous areas that affect Forest Stand 5 are discussed in greater detail in the
following sections.
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Geotechncial Investigation 14 July 2025
Steep Slope Hazard Area – Forest Stand 5
The COR maps both sensitive and protected slopes in Forest Stand 5, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers), but observed no evidence of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, groundwater seepage, or excessive
coniferous trunk curvature.
The slopes in Forest Stand 5 appear stable and based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the steep slope hazard area by
stabilizing the surficial soils.
Landslide Hazard Area – Forest Stand 5
While not mapped by COR, Forest Stand 5 contains slopes meeting the conditions for LM (Figure
2). The slope shows no significant indication of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, or excessive coniferous trunk
curvature. We did observe minor trunk curvature in coniferous trees that indicate minor surficial
slope instability. We did not observe evidence or indication of the presence of deep-seated
landslides on the slope.
The slopes in Forest Stand 5 appear stable, and based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the landslide hazard area.
Erosion Hazard Area – Forest Stand 5
The southern portion of Forest Stand 5 is mapped by COR as an erosion hazard area and, while
not mapped, the northern portion of Forest Stand 5 meets the criteria of an erosion hazard area.
The NRCS maps the soils here as Alderwood gravelly sandy loam, which have a “slight” erosion
risk (NRCS, 2025). While Forest Stand 5 shows no significant indication of recent or active
erosion, such as rills and exposed soil, on the slope, and due to the steep slope of Forest Stand 5,
the potential for erosion is high if appropriate BMPs are not implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 5 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
3.2.6 Forest Stand 6
3.2.6.1 Observations – Forest Stand 6
Forest Stand 6 is located in the northern portion of the Site. Forest Stand 6 is located on a slope
up to 40 feet tall that is uniformly sloped 22 degrees (40% grade) to the south and west (Figure
2). The slope was constructed as a result of the Site’s history as a gravel pit with approximately
10-foot-wide benches every 20 vertical feet (Schuster et al., 2015). The slope is vegetated with
coniferous and deciduous trees up to 1.5 feet dbh and heavy ground cover prominently consisting
of blackberry. Conifers were generally tall and straight with some trees exhibiting slight pistol
butting and/or slight downslope tilt.
FINAL
Geotechncial Investigation 15 July 2025
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand-tool excavations. We observed approximately 1 foot of topsoil underlain by soil we
determined to be advance outwash consisting of by dense, brown to gray, moist sand with silt
and gravel. We observed some rills and exposed soil on the slope, indicating localized surface
erosion is occurring on the slope.
3.2.6.2 Geologically Hazardous Areas – Forest Stand 6
The geologically hazardous areas that affect Forest Stand 6 are discussed in greater detail in the
following sections.
Steep Slope Hazard Area – Forest Stand 6
The COR maps both sensitive and protected slopes in Forest Stand 6, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers) and surficial soil erosion (rills and exposed soil), but observed no evidence of
recent or incipient slope instability, such as bowl-shaped topography, tension cracks along the top
of the slope, groundwater seepage, or excessive coniferous trunk curvature.
The slopes in Forest Stand 6 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the steep slope hazard area by
stabilizing the surficial soils.
Landslide Hazard Area – Forest Stand 6
While not mapped by COR, slopes meeting the conditions for LM exist at Forest Stand 6 (Figure
2). The slope shows no significant indication of recent or incipient slope instability, such as bowl-
shaped topography, tension cracks along the top of the slope, or excessive coniferous trunk
curvature. We observed minor trunk curvature in coniferous trees, some localized surficial erosion
features (rills and channels), and exposed soils on the slope that indicate minor surficial slope
instability. We did not observe evidence or indication of the presence of deep-seated landslides on
the slope.
The slopes in Forest Stand 6 appear stable and, based on our understanding of the Project, will not
be adversely impacted by the proposed forest restoration. In fact, forest restoration with native
trees and plants will help to improve the condition and stability of the landslide hazard area.
Erosion Hazard Area – Forest Stand 6
While not mapped by COR, Forest Stand 6 meets the criteria of an erosion hazard area. The NRCS
maps the soils here as Alderwood gravelly sandy loam which have a “slight” erosion risk (NRCS,
2025). Forest Stand 6 shows indication of recent or active erosion, such as rills and exposed soil,
on the slope. This and due to the steep slope of Forest Stand 2, the potential for erosion is high if
appropriate BMPs are not implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 6 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
FINAL
Geotechncial Investigation 16 July 2025
3.2.7 Forest Stand 7
3.2.7.1 Observations – Forest Stand 7
Forest Stand 7 is located on the southwestern portion of the Site. Forest Stand 7 covers the ravine
and Panther Creek with slopes up to 125 feet tall sloped 40 degrees (80% grade) with localized
near-vertical sections (Figure 2). The slope is vegetated with coniferous and deciduous trees up
to 2.5 feet dbh and thick ground cover. Conifers were generally tall and straight with some trees
exhibiting slight pistol butting and/or slight downslope tilt.
We evaluated the soil with a 0.5-inch-diameter steel probe rod (T-probe), as well as with limited
hand-tool excavations. We observed approximately 2.5 feet of topsoil and forest debris underlain
by soil we determined to be advance outwash consisting of dense, brown to gray, moist sand
with silt and gravel on the upper portions of the slope, and soil we determined to be Pre-Vashon
continental drift and nonglacial deposits consisting of dense, brown to gray, moist sand with silt
and gravel with trace organics and interbedded laminated clay in the creek cut banks and lower
portions of the slopes. We observed cut banks adjacent to Panther Creek, minor signs of surficial
erosion, and localized areas of hydrophilic plants indicating seeps on the slope.
3.2.7.2 Geologically Hazardous Areas – Forest Stand 7
The geologically hazardous areas that affect Forest Stand 7 are discussed in greater detail in the
following sections.
Steep Slope Hazard Area – Forest Stand 7
The COR maps both sensitive and protected slopes in Forest Stand 7, which we confirmed during
our reconnaissance (Figure 2). We observed some evidence of surficial soil creep (pistol butted
and tilted conifers) and some evidence of recent or incipient surficial slope instability, such as
bowl-shaped topography, and tilted and curved coniferous trees. We did not observe tension
cracks along the top of the slope, recent soil scarps, or excessive groundwater seepage.
The slopes in Forest Stand 7 appear to be exhibiting minor surficial instabilities common on
natural slopes of steep inclinations. Based on our understanding of the Project, these steep slope
hazard areas will not be adversely impacted by the proposed forest restoration. In fact, forest
restoration with native trees and plants will help to improve the condition and stability of the steep
slope hazard area.
Landslide Hazard Area – Forest Stand 7
The eastern portion of Forest Stand 7 is mapped by COR as a landslide hazard area and, while not
mapped by COR, slopes meeting the conditions for LH and LV exist (Figure 2). We observed
evidence of surficial landslides on the slopes, including bowl-shaped topography and tilted and
curved coniferous trees. We did not observe tension cracks along the top of the slope, recent soil
scarps, or excessive groundwater seepage. We did not observe evidence or indication of the
presence of deep-seated landslides on the slope.
The slopes in Forest Stand 7 appear to be exhibiting natural normal surficial slope instabilities for
slopes of this steep inclination. Based on our understanding of the Project, the landslide hazard
area will not be adversely impacted by the proposed forest restoration. In fact, forest restoration
with native trees and plants will help to improve the condition and stability of the landslide hazard
area.
FINAL
Geotechncial Investigation 17 July 2025
Erosion Hazard Area – Forest Stand 7
The central portion of Forest Stand 7 is mapped as an erosion hazard area by COR and, while not
mapped by COR, the rest of Forest Stand 7 meets the criteria of an erosion hazard area. The
NRCS maps the soils here as Alderwood gravelly sandy loam, which have a “slight” erosion risk
(NRCS, 2025). Forest Stand 7 shows indication of recent or active erosion in the creek area, such
as cut banks and exposed soil sections. However, the slopes were well vegetated, and we did not
observe significant evidence of erosion, such as gullying, rills, or exposed soil. However, due to
the steep slopes of Forest Stand 7, the potential for erosion is high if appropriate BMPs are not
implemented.
It is our opinion that the forest restoration Project can be conducted in a way that will not
adversely impact the identified erosion hazard, provided appropriate mitigation measures,
including BMPs to reduce erosion, are used during project implementation. Specifically, the
management practice measures outlined in KCD’s Forest Stewardship Plan for Stand 7 are
appropriate and sufficient to reduce erosion risk (KCD, 2024).
FINAL
Geotechncial Investigation 18 July 2025
4. CONCLUSIONS AND RECOMMENDATIONS
Based upon our Site reconnaissance and review of publicly available information, it is our
opinion that the geologically hazardous areas at the Site defined by RMC 4-3-050G5 (RMC,
2025), including steep slope, landslide, and erosion hazard areas will not be adversely impacted
by the proposed forest restoration Project. We did not observe the presence of significant seismic
or coal mine hazard areas at the Site. We determined the man-made steep slopes to be relatively
stable with some evidence of surficial erosion, and the natural slopes in the ravine to be
relatively stable, although exhibiting some surficial instabilities associated with natural slope
processes. We observed no indicators of incipient or ongoing deep-seated rotational landslide
activity. Our assessments of the geologically hazardous areas for each forest stand are
summarized in Table 1. In our opinion, the proposed Project will not adversely impact the
critical areas, adjacent properties, or pose an unreasonable threat to public health and safety. It is
our opinion that the forest restoration Project can be implemented such that it will improve the
overall condition and stability of geologically hazardous areas at the Site. It is our opinion that
that Project should be exempt from further critical areas permitting per RMC 4-3-050C(3), which
states, “enhancement activities—which include removal of noxious or intrusive species,
plantings of appropriate native species and/or removal of diseased or decaying trees which pose a
clear and imminent threat to life or property—are an exempt activity in geologically hazardous
areas and their buffers.”
Below, we provide some general recommendation for steep slope management that can be
implemented during and after Project completion to help maintain the stability of the
geologically hazardous areas at the Site.
• Care should be taken during on-Site or near-Site activities to limit the amount of bare
or exposed soil on or near the face of the slope. If landslide activity or erosion results
in bare, exposed soil, we recommend revegetating the affected area with native,
perennial growth or covering the affected area in a biodegradable erosion control
netting, such as jute mat. For new vegetation, we recommend well-rooted plants that
bolster the surficial slope stability.
• Cuttings, trimmings, other dead vegetation, and refuse should not be concentrated
into compost piles or dump sites on or near the slopes to avoid adding weight to the
slope. Instead, cuttings, trimmings, and other dead vegetation from forest
management activities should either be removed from the slope, or evenly dispersed
on the slope in a way that will not inhibit growth of new vegetation.
• Practical efforts should be made to divert runoff water away from the top of the slope.
Uncontrolled, concentrated runoff should not be allowed to flow over the top of the
slope.
• Stormwater tightlines and overall stormwater systems of upslope properties should be
inspected and tested for proper flow and function each winter and after any
significant erosion or landslide event. Stormwater systems should be repaired
immediately if deficiencies or damage are observed. All stormwater tightlines should
be repaired and/or extended to ensure that flows are conveyed to the base of the steep
slope.
FINAL
Geotechncial Investigation 19 July 2025
5. REFERENCES
Confluence (Confluence Environmental Company). 2025.Vantage Glen stream study - Draft.
Prepared for King Conservation District, Renton, Washington, by Confluence, Seattle,
Washington.
City of Renton (COR), 2025, Information Technology Maps and GIS Data – COR Maps,
https://maps.rentonwa.gov/Html5viewer/Index.html?viewer=cormaps, accessed March
20, 2025.
City of Renton, Washington, Municipal Code (RMC). 2025. Renton Municipal Code. Retrieved
March 20, 2025, from https://www.codepublishing.com/WA/Renton/
King Conservation District (KCD). April 2024.Vantage Glen Forest Stewardship Plan for King
County Housing Authority, Vantage Glen Property. Prepared by Jason Saura, Forest
Stewardship Program Coordinator. 800 SW 39th Street, Suite 150, Renton, Washington.
Natural Resource Conservation Service (NRCS). 2025. Web soil survey [online database]. U.S.
Department of Agriculture, NRCS, Soil Science Division, Washington D.C. Available at:
http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm accessed on April 1, 2025.
Schuster, J.E., Cabibbo, A.A., Schilter, J.F., and Hubert, I.J. (Schuster et al), 2015, Geologic
Map of the Tacoma 1:100,000-scale Quadrangle, Washington: Washington Division of
Geology and Earth Resources Map Series 2015-03, Scale, 1:100,000.
Washington Geological Survey (WGS), 2024, Washington State Landslide Inventory Database-
GIS data, November 2024: Washington Geological Survey Digital Data Series 29,
version 1.3. Previously released November 2023.
https://fortress.wa.gov/dnr/geologydata/publications/data_download/ger_portal_landslide
_database.zip
FINAL
Geotechncial Investigation 20 July 2025
6. LIMITATIONS
Work for this project was performed for Confluence Environmental Company and King
Conservation District (Client), and this letter was prepared in accordance with generally accepted
professional practices for the nature and conditions of work completed in the same or similar
localities, at the time the work was performed. This letter does not represent a legal opinion. No
other warranty, expressed or implied, is made.
All reports prepared by Geosyntec Consultants, Inc. (Geosyntec), for the Client apply only to the
services described in the Agreement(s) with the Client. Any use or reuse by any party other than
the Client is at the sole risk of that party, and without liability to Geosyntec. Geosyntec’s original
files/reports shall govern in the event of any dispute regarding the content of electronic
documents furnished to others.
FINAL
Geotechnical Investigation May 2025
TABLE
Table 1. Geologically Hazardous Areas
Project No. NWG0043, Vantage Glen, Washington
Geosyntec Consultants, Inc. Table 1
7/18/2025 Geotech investigation – Vantage Glen
V:\NWG0043 Vantage Glen Geotech Study\Deliverables\Geotechnical Investigation\Final\Table 1_Geologic Hazards.docx Page 1 of 1
Forest Stand Potential Geologic Hazards Level of Risk1
1
Steep Slopes Low
Landslide Low
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
2
Steep Slopes Low
Landslide Low
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
3
Steep Slopes Low
Landslide Low
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
4
Steep Slopes Moderate
Landslide Moderate
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
5
Steep Slopes Low
Landslide Low
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
6
Steep Slopes Low
Landslide Low
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
7
Steep Slopes Moderate
Landslide Moderate
Erosion High
Seismic Not Observed/Not Mapped2
Coal Mines Not Observed/Not Mapped2
Notes: (1) The “Level of Risk” is a qualitative assessment based on our engineering geology and geotechnical engineering judgment. (2) Indicates that the hazard was not
mapped or reported by City of Renton critical area mapping.
FINAL
Geotechnical Investigation May 2025
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