22-105973-Geotech Report Addendum-01-12-23
October 23, 2020
Amalani, LLC
105 South Main Street, Suite 230
Seattle, WA 98104
(206) 910-0278
Geotechnical Engineering Report Addendum
Revised Ravine Crossing and Comment
Response Letter
Creekwood Residential Plat
31000 block of 22nd Avenue SW
Federal Way, Washington
PN: 1822059132
Doc ID: SubdivisionsWest.94thAveSE.CRL2
INTRODUCTION
This letter summarizes changes to the proposed ravine crossing as well as response to
comments from the City of Federal Way from their December 19, 2019 letter. The primary
modification to the proposed plans is that the proposed Road A ravine crossing, originally proposed
to be supported by a Mechanically Stabilized Earth (MSE) fill wall has been redesigned to be
supported by a solider pile wall. We have previously prepared and/or reviewed the following
documents as part of the current revision and combined technical review:
Updated Geotechnical Engineering Report, Creekwood Residential Plat dated April 28, 2017;
Geotechnical Third Party Review of 2017 Creekwood Development Submittal Geotechnical
Engineering Report and Plan Review File #17-103948.00-SE by GeoDesign, Inc.;
Combined Technical Review letter from the City of Federal Way dated December 19, 2019; and
Revised Preliminary Plat, Creekwood P-Plat plan prepared by Core Design on September 28,
2020;
Soldier Pile Retaining Wall Design, Creekwood Plat, Federal Way calculation package by Lucia
Engineering, Inc dated June 8, 2020;
Soldier Pile Retaining Wall Design, Creekwood Plat, Federal Way plan and cross section sheet SH-
2.0 by Lucia Engineering, Inc dated June 8, 2020; and
Cast-in-Place Retaining Wall Design, Creekwood Plat, Federal Way plan and cross section sheets
SH-3.0 and SH-4.0 by Lucia Engineering, Inc dated October 7, 2020.
COMMENT RESPONSE
The following comments are from the December 19, 2019 Combined Technical Review letter
from the City of Federal Way. Not all the comments required geotechnical engineering review
and/or response. As described in our 2017 report, the proposed Creekwood Residential plat will
consist of 20 new residential lots with associated open space, stormwater tracts, and utilities. The
lots will be accessed off of a new road that extends west from 21st Place SW. A secondary
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October 23, 2020
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connection will be made to the current terminus of 22nd Avenue SW. The new road will generally be
at grade, but some minor cuts will be required along the north side of the new road across from Lots
16 through 20. The number of lots, location of lots, and grading on the lots is still generally the same
as the configuration described in our 2017 report.
The original plans called for a reinforced soils embankment to be used where the new road
crosses the ravine in the north-center portion of the parcel, but this has been changed to a solider
pile wall. The stormwater detention pond, with tiered gravity (large modular concrete block)
retaining walls, has been relocated to the north side of the Road A, west of the connection with 22 nd
Avenue SW. This was the original location of the proposed pond and was previously evaluated in the
July 2016 version of our report.
As with previous versions of the proposed development and our report, no grading or
development will occur within the identified steep slope or landslide hazardou s areas at the rear of
the proposed building Lots, although some minor grading will occur in the more gently sloping rear
yard areas. We still anticipate the proposed residences will consist of conventional one or two -story
wood-framed structures supported on shallow foundations and associated driveways. An updated
copy of the Preliminary Plat (Core Design, September 2020) is shown on the attached Site and
Exploration Plan, Figure 1.
The following section address the various comments pertaining to the geotechnical aspects
of the review comments provided by the City:
Comment 1 – Road Ravine Crossing: We have addressed the geotechnical engineering aspects from
the Public Works Division comments below. It is our opinion that that the conclusions in our 2017
Geotechnical Report that the proposed project is feasible from a geotechnical engineering
standpoint is feasible provided the recommendations contained in the report are included in the
plans and specifications for design and construction of the plat. As further stated below, the
proposed modification of the Road A ravine crossing and stormwater pond relocation provide
additional protection and result in a more stable site than the proposed configuration described in
our 2017 report. Other disciplines, included the soldier pile wall design and calculation by Lucia
Engineering and revised pond location by Core Design, will also address this comment.
Comment 6 – Geologically Hazardous Areas (GHA) Review: The proposed soldier pile wall alternative to
the Road A road crossing is a more structural solution than the originally proposed MSE wall and
slope alternative. While both system are approved by the Washington State Department of
Transportation Geotechnical Design Manual, we understand that the City Public Work standards did
not allow for the MSE wall. The manual also required access to the bottom of the wall for
maintenance. The proposed soldier pile wall has a smaller footprint and allows the wall to placed
further upslope (away from the wetland and buffer area), thereby addressing the City’s concerns.
The wall was designed and engineered by Lucia Engineering, a licensed structural engineer.
We provided copies of our previous reports that included soil properties for the design completed
by Lucia Engineering. We performed a global stability analysis of the slope and wall where the road
crossing will occur using the same soil parameters used in the various slope stability analyses from
our 2017 report.
We used the computer program SLIDE 8.0, from Rocscience, to perform the slope stability
analyses on the existing and proposed site conditions. The computer program SLIDE uses methods
to estimate the factor of safety (FS) of the stability of a slope by analyzing the shear and normal
Amalani.Creekwood.AR1
October 23, 2020
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forces acting on a series of vertical “slices” that comprise a failure surface. Each vertical slice is
treated as a rigid body; therefore, the forces and/or moments acting on each slice are assumed to
satisfy static equilibrium (i.e., a limit equilibrium analysis). The FS is defined as the ratio of the forces
available to resist movement to the forces of the driving mass. A FS of 1.0 means that the driving
and resisting forces are equal; and a FS less than 1.0 indicates that the driving forces are greater
than the resisting forces (indicating failure).
To analyze the global and internal stability of the site and surrounding slopes, we performed
our analysis based on a cross section drawn through the tallest section of the Road A ravine
crossing, in the area of the proposed main building, access road, and retaining walls. The cross-
section location is shown on the attached Figure 1. Based on our site observations, the encountered
subsurface conditions, the existing site topography, and our laboratory results, we assumed dry unit
weights, friction angles (Phi), and cohesion values for the soils encountered in our explorations. Soil
unit weight and strength parameters were assigned based on our experience and our laboratory
testing of representative soil samples. The assigned soil strength properties are summarized on
slope stability analysis print out in Appendix A, and are consistent with the soil strength used in our
2017 and accepted by the City’s third party geotechnical engineer review .
The interpretation of the stratigraphy of the slopes modeled in cross section A-A’ is based on
our previous subsurface explorations. We used the Generalized Limit Equilibrium method using the
Morgenstern-Price analysis, which satisfies both moment and force equilibrium, to search for the
location of the most critical failure surfaces and their corresponding FS. The most critical surfaces
are those with the lowest FS for a given loading condition, and are therefore the most likely to move.
The site seismic stability conditions were analyzed by applying a horizontal acceleration
equal to one-half of the peak ground acceleration obtained from the ATC Hazards by Location Map
using 2015 IBC-2015, Risk Category III, Seismic Site Class C. We used a design peak ground
acceleration of 0.25g for the site. The cross section and slope stability results using both static and
seismic conditions for the existing and proposed development is included as Appendix A. Our
analysis shows that the resulting factors of safety with the proposed soldier pile wall are 2.35 and
1.43 for the static and seismic condition with a 3H:1V slope. The height of the wall can be reduced by
steepening the slope above the soldier pile. Using reinforced soils as labeled on the plans by Lucia
Engineering, the factors of safety are 2.16 and 1.31 for the static and seismic condition, respectively.
In this configuration, the reinforcing within the embankment fill does not extend into the public
right-of-way.
Comment 7 – Lots in Geologically Hazardous Areas: As stated in our 2017 report, the FWRC 19.145.230
requires a 50 foot setback from geologically hazardous areas. The code allows for reduction of the
buffers and setbacks, and even for improvements within the landslide hazard area, when a qualified
professional demonstrates that the improvements will not lead to or create any increased landslide
hazard or be at risk of damage by the landslide hazard. The proposed lot layout was created using
a buffer/setback of 25 feet, supported by the slope stability analysis contained in our 2017 report.
Lots that are located above the delineated landslide hazard areas provide building pad are as
with factors of safety greater 1.5 and 1.1 for the static and seismic conditions, respectfully, while
maintaining the required 25 foot setback from the determined geologically hazardous areas. The
building envelope for each lot is setback 25 feet from the top of slope/landslide hazard area line.
This comment also indicates that no determination has been made in regard to this request
for reduction of the slope setback, or any other GHA intrusion request. However, the December 12,
Amalani.Creekwood.AR1
October 23, 2020
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2017 letter from GeoDesign, Inc, the City’s third party geotechnical engineer reviewer states that
with regard to FWRC 19.145.230, that the request for a reduced buffer appears to be supported by
the existing conditions and stability analysis provided in our 2017 report.
Comment 9 – Retaining Walls: Our 2017 report provided recommendations and a global stability
analysis for the gravity style (large modular concrete block) walls to be constructed in the detention
pond. The new pond location north of Road A still requires gravity walls to provide grade separation.
As shown on the wall plan and sections prepared by Lucia Engineering, the walls will have up to 6
foot exposed face and will require 1 foot of embedment. Tiered walls will be setback more than the
height of the lower wall, maintaining a separation between the two tiers that exceed the standard
1H:1V surcharge line. Our global stability analysis shown on section ND -ND’ is still appropriate for
the proposed new pond location and design and shows the wall configuration to exceed the
minimum required factors of safety. Our original report should and this addendum should be
sufficient to support the request for modification per FWRC 19.120.050 for the soldier pile wall
exceeding 6 feet in height.
Public Work Comments
The City of Federal Way, Public Works Department had several comments. We have
addressed the comments below as they pertained to several specific topics:
Erosion Control and Wet Weather:
Public Works has indicated that the construction guidelines in both our April 23, 2017
Geotechnical Engineering Report and the supplemental Associated Earth Science report appeared to
be general in nature. The 2017 report is both a project and site specific report that addresses and
satisfies the requirements of the International Building Code, as well as the City of Federal Way
critical areas ordinance and King County Surface Water Design Manual (KCSWDM). The construction
sequence is usually completed and submitted as part of the Construction Stormwater Pollution
Prevention Plan (CSWPPP) required by Core Requirement #5 of the 2016 KCSWD. The CSWPPP
incorporates measures from the Erosion and Sediment Control (ESC) plan and Stormwater Pollution
Prevention and Spill Control (SWPPS). Because the Creekwood Development is still in the
Preliminary Plat review and approval process, the above documents have not been prepared at this
time, minimum.
We did provide specific recommendations for wet weather and wet conditions on this site in
Section 7.5 of our 2017 report. The recommendations in section 7.5 are in addition to standard Best
Management Practices (BMPs) contained in the KCSWDM. In addition to our recommendation and
the recommendation of the ESC, SWPPS, and CSWPPP that will be prepared as part of the final site
design, the project will also need to obtain a National Pollutant Discharge Elimination System
(NPDES) Construction Stormwater permit from the Washington State Department of Ecology. In
addition, the site will require a Certified Erosion and Sediment Control Lead (CESCL) inspector be
assigned other and provide weekly inspections and monthly reports to DOE. Depending on rainfall
and if discharges occur, additional inspections may be required.
Our experience on other similar sized projects within both urban and critical areas, the
proper implementation and inspection of the ESC plan, CSWPPP and SWPPPS does significantly
reduce the potential for erosion and allows for any discharge to be properly document, assessed,
and corrected.
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October 23, 2020
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Detention Pond Design:
As discussed above, the stormwater detention pond was relocated back to the north side of
Road A, further away from the top of the steep slope/landslide hazard. This location was previous ly
addressed in a prior version of our report. In our 2017 report, we performed a sl ope stability
analysis through the pond and down the adjacent slope as shown on section ND-ND’ of our 2017
report. Factors of safety for the pond near the top of the steep slope area were on the 1.67 and 1.13
for the static and seismic condition respectively, which exceeded the required factors of safety for
pond location.
The original pond designed called for three tiers of walls. The new configuration only has two
tiers of walls. Also, moving the pond further away from the slope will only increase the factors of
safety and provide additional protection to the slopes. The pond is still proposed to be lined. The
interior walls will be constructed using UltraBlocks instead of the original blocks in the original
design. The walls were designed by Lucia Engineering. The slope stability on the interior slopes of
the pond should be unchanged from the conditions analyzed using section ND -ND’ in our 2017
report as the height and spacing between the walls is consistent, one tier of walls have been
removed reducing the overall height, and the UltraBlocks and original blocks are similarly sized
units.
Soldier Pile:
Public Works made several other comments to be addressed by the Geotechnical Engineer
that pertain to the soldier pile wall that should actually be addressed by either the structural
engineer, civil engineer, or contractor, and should be addressed during building plan review and not
during preliminary plat review.
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October 23, 2020
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We trust that this letter addresses both the City and design team comments. We appreciate
the opportunity to be of continued service to you on this project. If you have any questions or
comments, please do not hesitate to call at your earliest convenience.
Respectfully submitted,
GeoResources, LLC
Keith S. Schembs, LEG Eric W. Heller, PE, LG
Principal Senior Geotechnical Engineer
KSS:EWH/kss
Doc ID: Amalani.Creekwood.AR1
Attachments: Figure 1 – Revised Site and Exploration Plan
Appendix A – Slope Stability Analysis
APPENDIX A
SLOPE STABILITY ANALYSIS
2.022.02
W
W
2.022.02
RuHu TypeWater
Surface
Phi
(deg)
Cohesion
(psf)
Strength
Type
Unit
Weight
(lbs/ft3)
ColorMaterial
Name
0None340Mohr-
Coulomb120Qvr
Automatically
Calculated
Water
Surface38350Mohr-
Coulomb135Qva
0None3050Mohr-
Coulomb90Forest
Duff300275250
2252001751500 20 40 60 80 100 120 140 160 180 200 220 240
Scenario StaticGroupExisting - Soldier Pile Wall
Company GeoResources, LLCDrawn By
File Name Amalani.Creekwood.soldierpile.stability.slmdDate10/23/2020
Project
Amalani.Creekwood
SLIDEINTERPRET 9.005
1.361.36
W
W
1.361.36
RuHu TypeWater
Surface
Phi
(deg)
Cohesion
(psf)
Strength
Type
Unit Weight
(lbs/ft3)ColorMaterial
Name
0None340Mohr-
Coulomb120Qvr
Automatically
Calculated
Water
Surface38350Mohr-
Coulomb135Qva
0None3050Mohr-
Coulomb90Forest
Duff
0.2
2752502252001751500 20 40 60 80 100 120 140 160 180 200 220 240
Scenario SeismicGroupExisting - Soldier Pile Wall
Company GeoResources, LLCDrawn By
File Name Amalani.Creekwood.soldierpile.stability.slmdDate10/23/2020
Project
Amalani.Creekwood
SLIDEINTERPRET 9.005
2.352.35
W
W
2.352.35
RuHu TypeWater
Surface
Cohesion
Type
Phi
(deg)
Cohesion
(psf)
Strength
Type
Unit Weight
(lbs/ft3)ColorMaterial
Name
0None340Mohr-
Coulomb120Qvr
Automatically
Calculated
Water
Surface38350Mohr-
Coulomb135Qva
0None3050Mohr-
Coulomb90Forest
Duff
0None350.02Mohr-
Coulomb130Structural
Fill
0NoneConstant14000Undrained150Concrete
Force
Direction
Pile Shear Strength
(lbs)
Failure
Mode
Out-Of-Plane
Spacing (ft)
Force
ApplicationTypeColorSupport
Name
Parallel to
surface1e+06Shear6Active
(Method A)
Pile/Micro
PileSoldier Pile275250225
2001751500 20 40 60 80 100 120 140 160 180 200 220 240
Scenario StaticGroupProposed - Soldier Pile Wall
Company GeoResources, LLCDrawn By
File Name Amalani.Creekwood.soldierpile.stability.slmdDate10/23/2020
Project
Amalani.Creekwood
SLIDEINTERPRET 9.005
1.431.43
W
W
1.431.43
RuHu TypeWater
Surface
Cohesion
Type
Phi
(deg)
Cohesion
(psf)
Strength
Type
Unit Weight (lbs/
ft3)ColorMaterial
Name
0None340Mohr-
Coulomb120Qvr
Automatically
Calculated
Water
Surface38350Mohr-
Coulomb135Qva
0None3050Mohr-
Coulomb90Forest Duff
0None350.02Mohr-
Coulomb130Structural Fill
0NoneConstant14000Undrained150Concrete
Force DirectionPile Shear Strength
(lbs)
Failure
Mode
Out-Of-Plane Spacing
(ft)
Force
ApplicationTypeColorSupport
Name
Parallel to
surface1e+06Shear6Active (Method
A)
Pile/Micro
PileSoldier Pile
0.2
2752502252001751500 20 40 60 80 100 120 140 160 180 200 220 240
Scenario SeismicGroupProposed - Soldier Pile Wall
Company GeoResources, LLCDrawn By
File Name Amalani.Creekwood.soldierpile.stability.slmdDate10/23/2020
Project
Amalani.Creekwood
SLIDEINTERPRET 9.005
2.162.16
W
W
2.162.16
Water
Surface
Cohesion
Type
Phi
(deg)
Cohesion
(psf)
Strength
Type
Unit Weight
(lbs/ft3)ColorMaterial
Name
None340Mohr-
Coulomb120Qvr
Water
Surface38350Mohr-
Coulomb135Qva
None3050Mohr-
Coulomb90Forest Duff
None350.02Mohr-
Coulomb130Structural
Fill
NoneConstant14000Undrained150Concrete
Tensile
Strength
(lbs/ft)
Strip
Coverage
(%)
AnchorageForce
Orientation
Force
Direction
Pile
Shear
Strength
(lbs)
Failure
Mode
Shear
Strength
Model
Friction
Angle
(deg)
Adhesion
(psf)
Material
Dependent
Out-Of-
Plane
Spacing
(ft)
Force
ApplicationTypeColorSupport
Name
Parallel
to
surface
1e+06Shear6Active
(Method A)
Pile/Micro
Pile
Soldier
Pile
1500100Slope Face
Bisector of
Parallel and
Tangent
Linear400NoActive
(Method A)GeosyntheticRSS
Primary300275
250225200175150-25 0 25 50 75 100 125 150 175 200 225 250
Scenario StaticGroupProposed - 1H:1V + 2H:1V Soldier Pile Wall
Company GeoResources, LLCDrawn By
File Name Amalani.Creekwood.soldierpile.stability.rev01.slmdDate10/23/2020
Project
Amalani.Creekwood
SLIDEINTERPRET 9.005
1.311.31
W
W
1.311.31
Water
Surface
Cohesion
Type
Phi
(deg)
Cohesion
(psf)
Strength
Type
Unit Weight (lbs/
ft3)ColorMaterial
Name
None340Mohr-
Coulomb120Qvr
Water
Surface38350Mohr-
Coulomb135Qva
None3050Mohr-
Coulomb90Forest Duff
None350.02Mohr-
Coulomb130Structural
Fill
NoneConstant14000Undrained150Concrete
Tensile
Strength
(lbs/ft)
Strip
Coverage
(%)
AnchorageForce
Orientation
Force
Direction
Pile Shear
Strength
(lbs)
Failure
Mode
Shear
Strength
Model
Friction
Angle
(deg)
Adhesion
(psf)
Material
Dependent
Out-Of-
Plane
Spacing (ft)
Force
ApplicationTypeColorSupport
Name
Parallel
to
surface
1e+06Shear6Active
(Method A)
Pile/Micro
Pile
Soldier
Pile
1500100Slope Face
Bisector of
Parallel and
Tangent
Linear400NoActive
(Method A)GeosyntheticRSS
Primary
0.2
300275250225200175150-25 0 25 50 75 100 125 150 175 200 225 250
Scenario SeismicGroupProposed - 1H:1V + 2H:1V Soldier Pile Wall
Company GeoResources, LLCDrawn By
File Name Amalani.Creekwood.soldierpile.stability.rev01.slmdDate10/23/2020
Project
Amalani.Creekwood
SLIDEINTERPRET 9.005