23-105386-SU-Geogrid-Reinforced Slope Stability Analysis-12-05-2315365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 • (425) 449-4704 • FAX (425) 449-4711
Earth Solutions NW LLC
Geotechnical Engineering, Construction
Observation/Testing and Environmental Services
October 30, 2023
ES-8771.01
Barghausen Consulting Engineers, Inc.
18215 – 72nd Avenue South
Kent, Washington 98032
Attention: Tom Barghausen, P.E.
Subject: Geogrid-Reinforced Slope Assessment & Global Stability Analysis
Creekwood Plat
South of 22nd Avenue Southwest and Southwest 309th Court
Federal Way, Washington
Greetings, Tom:
Earth Solutions NW, LLC (ESNW) has prepared this letter regarding the conceptual reinforced
slope design proposed for the subject project. Based on our preliminary assessment and the
results of a global stability analysis, permanent reinforcement of the slope to the north of Stream
Y (referred to as “the slope” herein) is feasible from a geotechnical standpoint using the concepts
and construction methods described in this letter.
Design Concept
ESNW understands that the following design concept is proposed to provide permanent
reinforcement and stabilization of the slope:
Welded wire baskets will be installed along the majority of the slope. The wire baskets will
measure 18 inches in both the vertical (height) and horizontal (depth) directions and will
be filled with clean crushed rock. Both vegetated face wrap and geogrid will wrap around
the wire baskets. The wire baskets will be constructed such that an effective one
horizontal to one vertical (1H:1V) slope is created.
Between the top of the welded wire baskets and the flatter area near the right-of-way, a
non-reinforced 2H:1V slope will be constructed.
As noted above, the design concept relies on both vegetated face wrap and geogrid to be
installed in a wrapped-face manner at each basket installation level. In general terms, the
baskets provide protection of the slope face against surficial or shallow instability (e.g., erosion
and weathering processes), and the geogrid affords the slope an acceptable level of protection
against deep-seated instability.
Barghausen Consulting Engineers, Inc. ES-8771.01
October 30, 2023 Page 2
Earth Solutions NW, LLC
Global Stability Analysis
Quantitative global stability analysis was completed using Morgenstern-Price methodology and
GeoStudio Slope/W modeling software. The primary analysis objective was to evaluate whether
the proposed design concept can be implemented to afford the slope an acceptable factor of
safety against failure in both the static and seismic (dynamic) conditions.
Soil strength parameters included in the analysis were generally consistent with the stability
analyses and geotechnical recommendations from prior geotechnical consultants (and their
respective reports) associated with the subject project. The phreatic surface (groundwater)
model was also based primarily on the work and modeling of prior geotechnical consultants. A
site-modified peak horizontal ground acceleration (PGAM) value of 0.688 g was considered for
site-specific modeling per the 2018 IBC. A pseudostatic coefficient of between one-third and
one-half of PGAM is generally considered acceptable per local standard of practice; accordingly,
a value of 0.3 was considered in the stability analysis.
Several geogrid strengths were evaluated as part of initial modeling. Different geogrid types
(such as Miragrid 5XT, 8XT, and 10XT) are a reflection of the difference in mechanical properties,
primarily the long-term design strength (LTDS). A higher geogrid type number (the “5” in “5XT”
or the “10” in “10XT”) typically indicates a higher LTDS and greater long-term strength. Different
vertical spacing distances between the geogrid layers were also evaluated. Closer spacing
between adjacent geogrid layers typically affords the slope a greater degree of reinforcement and
a higher overall factor of safety. For the final analysis (which is attached to this letter), Miragrid
10XT spaced at one-foot (vertical) intervals was considered.
The table below summarizes the soil modeling parameters used in our global stability analysis.
Soil Unit Unit Weight
(pcf)
Cohesion
(psf)
Internal
Friction Angle
(degrees)
Compacted structural fill
(clean crushed rock) 130 0 35
Recessional outwash 120 0 34
Advance outwash 135 350 38
Consistent with local standard of care, the global stability analysis indicates a minimum factor of
safety value of 1.2 and 1.5 for the dynamic and static conditions, respectively, are achieved. In
our opinion, the results of the global stability analysis indicate the proposed design concept is
feasible from a geotechnical standpoint and will afford the slope an acceptable level of protection
against both shallow instability and deep-seated instability.
Barghausen Consulting Engineers, Inc. ES-8771.01
October 30, 2023 Page 3
Earth Solutions NW, LLC
Construction Considerations & Long-term Outlook
ESNW has been involved in projects where the proposed design concept has been successfully
implemented and constructed. An advantage of the proposed design concept over traditional
retaining walls is that there is no maintenance aspect associated with the geogrid-reinforced
basket constructions. For example, WSDOT typically requires a design life of at least 75 years
for welded wire structures, and the geogrid-reinforced basket construction would be constructed
to meet or exceed the WSDOT-specified design life of 75 years. Provided the system is
constructed as designed, the slope will be permanently stabilized and will not require
maintenance once the geogrid-reinforced slope is installed and the baskets erected to form the
new slope face.
The design concept outlined in this letter is an industry-accepted means of providing retention
and permanent stabilization for slopes. Welded wire systems have been used successfully in
other jurisdictions (cities and counties) and are approved for use by WSDOT.
Per the data sheet provided by the manufacturer (Earth Wall Products) of the proposed vegetated
face wrap product, the face wrap is manufactured using polypropylene monofilament thread to
provide “a mesh structure for both erosion protection and space for vegetation to grow”. On
previous projects, ESNW has observed suitable vegetation regrowth accomplished by simply
hydroseeding the finished slope face. If desired, a few inches of topsoil could also be placed
within the wire baskets.
With respect to the global stability analysis and overall design concept, it should be noted that
longer geogrid lengths were used within roughly the lower half of the reinforced slope. This allows
the reinforced slope to be constructed while ensuring the geogrid does not encroach into the
right-of-way.
We trust this letter meets your current needs. Please call if you have any questions about this
letter or if we can be of further assistance.
Sincerely,
EARTH SOLUTIONS NW, LLC
Keven D. Hoffmann, P.E.
Associate Principal Engineer
Attachment: Slope/W Output
Distance (ft)
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 Elevation (ft)170
180
190
200
210
220
230
240
Elevation (ft)170
180
190
200
210
220
230
240
ES-8771.01
Creekwood Plat
Section A-A'
Proposed 2:1-to-1:1 Slope Transition w/
Wire Basket Facing for Lower Slope
Modeled Condition
Road A
Transition Point
Between SlopesEdge of ROW2H:1V Slope
1H:1V Slope w/
Wire Basket Facing
24' Geogrid
14' Geogrid
1.90
Distance (ft)
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 Elevation (ft)170
180
190
200
210
220
230
240
Elevation (ft)170
180
190
200
210
220
230
240
ES-8771.01
Creekwood Plat
Section A-A'
Proposed 2:1-to-1:1 Slope Transition w/
Wire Basket Facing for Lower Slope
Static Condition
Road A
Transition Point
Between SlopesEdge of ROW2H:1V Slope
1H:1V Slope w/
Wire Basket Facing
24' Geogrid
14' Geogrid
Color Name Unit
Weight
(pcf)
Effective
Cohesion
(psf)
Effective
Friction
Angle (°)
Piezometric
Surface
New
Structural
Fill
130 0 35 1
Qva 135 350 38 1
Qvr 120 0 34 1
Color Name Type
10XT Geogrid TenCate
1.23
Distance (ft)
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 Elevation (ft)170
180
190
200
210
220
230
240
Elevation (ft)170
180
190
200
210
220
230
240
ES-8771.01
Creekwood Plat
Section A-A'
Proposed 2:1-to-1:1 Slope Transition w/
Wire Basket Facing for Lower Slope
Seismic Condition
Horizontal Acceleration = 0.3
Road A
Transition Point
Between SlopesEdge of ROW2H:1V Slope
1H:1V Slope w/
Wire Basket Facing
24' Geogrid
14' Geogrid
Color Name Unit
Weight
(pcf)
Effective
Cohesion
(psf)
Effective
Friction
Angle (°)
Piezometric
Surface
New
Structural
Fill
130 0 35 1
Qva 135 350 38 1
Qvr 120 0 34 1
Color Name Type
10XT Geogrid TenCate