20-104332-UP-Geotechnical (Soils) Report-11-06-2020-V2GEORESOURCES
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Mr. Stan Afichuk
37237 42nd Avenue Southwest
Auburn, WA, 98001
(206) 708-0065
Stan@greenbuilddev.com
September 14, 2020
Geologic Hazard Assessment
Proposed Single -Family Residence
xxx South 293rd Street
Federal Way, Washington
PN: 720250-0080
Doc ID: AfichuckS.S293rdSt.GHA
INTRODUCTION
This updated report summarizes our recent site visit and observations, and updates our
November 2017 report. We understand that you purchased the property from the previous owner
but our November 2017 Geologic Hazard Assessment had not been submitted to the City. The 2017
report addressed potential geologic hazards for the proposed single-family residence to be
constructed on the above referenced vacant parcel. The location of the site is shown on the attached
Site Vicinity Map, Figure 1.
Our understanding of the project is based on our discussions with you; our previous work on
the adjacent parcel; our previous October 13, 2017 and November 28, 2017 site visits and our more
recent August 14, 2020 site visit; and our understanding of the City of Federal Way critical areas
ordinance (FWRC Chapters 19.05.070.G and 19.145). We understand that you propose to construct a
new single-family residence in the upper portion of the subject parcel as shown on the attached Site
Plan, Figure 2. According to the provided architectural elevation drawings, the garage floor elevation
will generally be about the same elevation as S 293rd Street. The main floor will be half a story below
the garage, while the upper floor will be half a story above the garage. There will be a daylight
basement level below the main floor.
We understand that the residence will be a three-story, wood -framed structure supported
on conventional spread footings. However, because of the site slopes, deep foundations such as
small diameter pin or needle piles, or drill piers, may be required in order to meet building setback
requirements. The daylight basement retaining wall will be a cast -in -place concrete wall. A sewer
lines is located in the central portion of the site and extends from a man -hole on the neighboring
parcel to the south down to the mobile park on the north. The sanitary sewer easement is not
shown on the site plan, but we understand that the easement is located entirely within the buffer.
East of the sewer is a stream channel that also flows down to the north. We understand from our
previous involvement on the adjacent parcel (including a Critical Areas Assessment and Stream
Corridor Buffer report prepared by Habitat Technologies dated April 5, 2016). The Habitat
Technologies report described the stream as an intermittent surface water drainage. The attached
Afi c h u kS. 5293 rd St. G HA
September 14, 2020
page 12
site plan indicates that there is a 50-foot stream buffer associated with the drainage. The buffer is
shown on the attached Site & Exploration Plan, Figure 2.
SCOPE
The purpose of our services was to update our November 2017 report. Specifically, our
scope of services for the project included the following:
1. Visiting the site and conducting a geologic reconnaissance to assess the site's soil,
groundwater and slope conditions; and
2. Preparing this Updated Geologic Hazard Assessment per the City of Federal Way Critical
Areas Ordinance for the proposed site development.
We were verbally authorized by you to begin this update during our August 14, 2020 site
visit. No new subsurface explorations were completed as part of this update. Since our initial site
visits, the existing conditions appears relatively unchanged, with the vegetation becoming more
established. Additionally, the house on the adjacent parcel to the south and east has been
completed, whereas it was under construction during our last site visits.
We previously prepared a similar Geologic Hazard Assessment for the adjacent residence
(parcel 7202500070) to the south. The City had GeoDesign, Inc perform a third party review on the
assessment and a revised assessment addressing the third party review was prepared on May 4,
2014. We understand that the revised assessment adequately addressed the review comments and
was ultimately approved by the City.
SITE CONDITIONS
The subject parcel is located in the Redondo area of Federal Way, Washington. According to
the provided site plan (included as our Site & Exploration Plan, Figure 2), the parcel is irregular in
shape, generally measures about 167 to 140 wide (east to west) by about 130 to 216 feet long (north
to south) and encompasses approximately 18,791 square feet. The parcel is bounded by South 293rd
Street and an older residence on the west, by the aforementioned new residence on the south by a
wooded, undeveloped area to the east, and by a mobile -home residential development on the
north.
The upland portion of the parcel, near the existing 293rd Street right of way, is flat to gently
sloping. Approximately 6 to 8 feet north of the curb line, the site slopes steeply down (about 50 to
100 percent) over a vertical height of about 6 to 8 feet. We interpret this upper slope to be fill placed
as part of the S 293rd Street road prism. The site flattens across a bench that slopes down at about
20 to 33 percent. Below the proposed house footprint, the site slopes steepen down to both the
north and east to about 50 to 75 percent. The lower steep slope section extends down to an
intermittent stream channel on the east and towards the mobile home park on the north. The
slopes end in the stream channel and then slope back up to the east across a wooded slope. Total
vertical relief across the site is on the order of 66 feet. The proposed location of the residence and
site topography are shown on the attached Site & Exploration Plan, Figure 2. An aerial photograph of
the site with topography obtained from the King County iMap website is included as the Site Vicinity
Map, Figure 3.
C,�0RrtsoL ROES
Afi c h u kS. S293 rd St. G HA
September 14, 2020
page 13
Vegetation on the site consists of tall grasses, blackberries, and both native and invasive
brush. Some scattered young deciduous trees and occasional older fir trees are located along the
north and east sides of the parcel. As discussed, shallow seasonal drainage course is located along
the eastern property line. No springs or seepage was observed at the time of our site visit. No
evidence of erosion, soil movement, landslide activity or deep-seated slope instability was observed
at the site or the adjacent areas at the time of our site visit.
During our previous site visits, we observed what was described by others to be tension
cracks in the lower portion of the site. We had interpreted these cracks to be desiccation cracking
that would be mitigated by the site development excavation. These tension cracks were not
observed during our recent visit.
Site Soils
The Natural Resource Conservation Service (NRCS) Web Soil Survey maps the soils as
Alderwood and Kitsap soils (AkF). The Alderwood and Kitsap soils are derived from glacial till, that
form on 25 to 70 percent slopes. These soils are listed as having a "moderate to severe" erosion
hazard a severe building limitation for slopes. These soils are listed in hydrologic soils group B/C. A
copy of the SCS map for the site vicinity is attached as Figure 4.
Site Geology
The Geologic Map of the Poverty Bay 7.5-minute Quadrangle, Washington, Derek B. Booth,
Howard H. Waldron, and Kathy G. Troost (2003) indicates the site underlain by coarse -grained older
glacial deposits (Qpogc). These soils were generally deposited before the Vashon stade of the Fraser
Glaciation, some 12,000 to 15,000 years ago. The glacial deposits typically consist of a heterogeneous
mixture of clay, silt, sand and gravel that has been over ridden by the continental ice mass associated
with the Vashon stade of Fraser glaciation. The glacial deposits are considered over -consolidated, are
typically in a very dense condition, and exhibit high strength and low compressibility characteristics
where undisturbed. Surficial soils are typically weathered to a loose to medium dense condition. No
areas of landslides or mass wasting or noted on the map within the immediate vicinity of the site,
however, a mass wasting deposit is shown on the map south and east of the parcel by about 500 to
600 feet. The mapped mass wasting deposit is on the east side of the stream channel valley from the
site and immediately below an area of existing residential development. An excerpt of the above
referenced map is included as Figure 5.
The Washington State Department of Ecology Coastal Atlas for the site area indicates the
flatter upland area to be "stable", while the slopes on the east side of the stream channel are
mapped as "unstable". The unstable designation is likely attributed to the height and inclination of
the slopes east of the streams. No areas of unstable recent or unstable historic landslides are
shown on the Coastal Atlas within the site area. A copy of the Coastal Atlas map for the site area is
attached as Figure 6.
A series of oblique aerial shoreline photos obtained from the Coastal Atlas website are
attached as Figures 7a through 7e. The photographs, taken between 1977 and 2016 show the site
and adjacent areas. The three more recent photographs (1996, 2000, 2006) show the existing
residential developments and site to be well vegetated. The 1977 shows the residential
development below the site to be existing and the neighborhood that the site is located in to be
under construction.
G�C)R�sr�u��Es
Afi c h u kS. S293 rd St. G HA
September 14, 2020
page 14
Subsurface Conditions
Two hand augers were excavated on the slope on the parcel to the south. We were also able
to observed subsurface conditions as exposed on the active construction site to the south and as
exposed in a root -ball hole from a large fallen tree on the south side of the subject property. In our
opinion, the observed subsurface conditions generally confirmed the mapped stratigraphy. Similar
to the soils encountered in the upper hand auger on the adjacent parcel, we anticipate that upper
portion of the site soils has fill associated with construction of South 293rd Street and the sanitary
sewer that cross the parcel. The soils encountered in our hand augers were characterized in
accordance with the Unified Soil Classification System, a copy of which is attached as Figure 8. Logs
of the test pits are attached as Figure 9.
During construction of the house on the parcel to the south, old fill was encountered down
about 4 to 6 feet below grade in the upper portion of the site. This was also observed with the fill
encountered in the upper hand auger. Given the site grades, we anticipate the fill extends into the
south portion of the subject site.
GEOLOGIC HAZARDS
The City of Federal Way Critical Areas Ordinance for Geologically Hazardous Areas state
"geologically hazardous areas shall mean areas that, because of their susceptibility to erosion,
landsliding, seismic or other geological events, are not suited to siting commercial, residential or
industrial development consistent with public health or safety concerns." The Revised Code of
Federal Way is copied in italics, while our comments to the code are immediately following the code.
Erosion Hazard Areas per Federal Way Revised Code
The FWRC, Chapter 19.05.070.G(1) defines erosion hazard areas as "those areas identified by
the U.S. Department of Agriculture's (USDA) Natural Resource Conservation Service (NRCS) as having a
moderate to severe or severe to very severe rill and inter -rill erosion hazard due to natural agents such as
wind, rain, splash, frost action or stream flow; those areas containing the following group of soils when
they occur on slopes of 15 percent or greater: Alderwood-Kitsap ('AkF'), Alderwood gravelly sandy loam
('AgD'9, Kitsap silt loam ("KpD'), Everett ("EvD'), and Indianola ("InD'9, and those areas impacted by shore
land and/or stream bank erosion"
The site soils are mapped as AkF, which have a severe erosion hazard when exposed.
Conventional construction BMP's should be installed prior to beginning construction. This will
provide adequate erosion control for the disturbed areas of the site. It is critical that the installed
erosion control measures be monitored and maintained, and if necessary modified based on
changing site conditions. In the event that the site is not worked for 7 days or more, the disturbed
areas should be adequately erosion protected and maintained in the event of a significant storm
event. This may include the use of plastic sheeting or mulch.
Erosion control should specifically include the installation of silt fencing along the downslope
and side slopes of the active construction area. Straw waddles and berms may also be necessary.
We have not been provided with a copy of the proposed Temporary Erosion and Sediment Control
(TESC) plan at this time. However, provided standard BMP's are installed prior to beginning
construction, the potential for erosion or sediment leaving the site should be minimal.
G, ORE'-�Cl4JRCE5
Afi c h u kS. 5293 rd St. G HA
September 14, 2020
page 1 5
Landslide Hazard Areas per Federal Way Revised Code
The FWRC, Chapter 19.05.070.G(2) defines landslide hazard areas as "those areas potentially
subject to episodic downslope movement of a mass of soil or rock including but not limited to the
following areas."These are typically characterized as having the following indicators:
a. Any area with a combination of.•
i. Slopes greater than 15 percent,
ii. Permeable sediment overlying a relatively impermeable sediment or bedrock,•
iii. Springs orgroundwaterseeps.
b. Any area which has shown movement during the Holocene epoch, from 10,000 years ago to the
present, or which is underlain by mass wastage debris of that epoch.
c. Any area potentially unstable as a result of rapid stream incision, stream bank erosion or
undercutting by wave action.
d. Any area located in a ravine or on an active alluvial fan, presently or potentially subject to
inundation by debris flows or flooding.
e. Those areas mapped as Class U (unstable), UOS (unstable old slides), and URS (unstable recent
slides) by the Department of Ecology's Coastal Zone Atlas.
f. Areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps
published by the U.S. Geological Survey or Washington State Department of Natural Resources.
g. Slopes having gradients greater than 80 percent subject to rockfall during seismic shaking
h. Any area with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet except
areas composed of consolidated rock. A slope is delineated by establishing its toe and top and is
measured by averaging the inclination over at least 10 feet of vertical relief.
The site has slopes steeper than 15 percent, but no adverse or intersecting contacts are
mapped on the site, nor were any seeps or springs noted on the slope or along the seasonal stream
channel. No areas of mapped landslide debris or activity were noted on the published geologic map
or Coastal Atlas. The stream channel is not deeply incised, and we anticipate that runoff from the
developments along the west side of the stream channel have greatly decreased the amount of
season flow, thereby reducing the potential for rapid stream incision or stream bank erosion.
No areas of alluvial fans are mapped nor were any alluvial fans noted in the vicinity of the
site at the time of our past site visits. The site is listed as being stable by the Department of Ecology
Coastal Atlas, however the slopes on the east side of the stream are mapped as unstable because of
their height and inclination. No areas of historic or recent landslide activity were identified on the
Coastal Atlas. The site slopes are not steeper than 80 percent with and subject to rock fall during
seismic shaking.
Based on our observations and literature review, the site has one of the above indicators.
The area is mapped as unstable on the east side of the stream channel, about 50 feet away from the
proposed building site. While the site has one of the above indicators, it is our opinion that the
proposed site development will mitigate the typical risks associated with steep slope site; grading of
surficial disturbed/weathered soils, drainage and erosion control, therefore no additional buffer
other than the stream buffer should be imposed by the City of Federal Way.
As stated above, the steep slope below South 293rd Street appears to be part of the roadway
prism. The lower bench and steep slope appears partially attributed to construction and installation
of the adjacent sewer line installation. The flatter slope between these two steep slope areas does
G�C)R�sr�u��Es
Afi c h u kS. S293 rd St. G HA
September 14, 2020
page 16
appear to be natural. As shown on the site plan, the house will be setback more than 7 feet from
the north property and the steep slope that extends down to the adjacent mobile home park. This
area will ultimately be retained by the daylight basement wall.
Seismic Hazards per Federal Way Revised Code
Earthquake -induced geologic hazards per City of Federal Way Revised Code (2016 FWRC),
Chapter 19.05.070.G(3) may include liquefaction, lateral spreading, slope instability, and ground
surface fault rupture. In our opinion, the potential for liquefaction and lateral spreading is not
significant because of the dense nature of the on -site soils and the groundwater depth.
Given the mapped stratigraphy of Pre -Olympia age gravel, which was overridden by the
more recent Vashon stade of the Fraser Glaciation, it is our opinion that the potential for liquefaction
is minimal. We interpret the structural site conditions to correspond to a seismic Site Class "D" in
accordance with Chapter 20 of ASCE 20. This is based on the likely range of equivalent SPT
(Standard Penetration Test) blow counts for the soil types observed in the site area. These
conditions were assumed to be representative for the conditions based on our experience in the
vicinity of the site. These soils are typically not prone to liquefaction and do not constitute a seismic
hazard area.
CONCLUSIONS AND RECOMMENDATIONS
Based on our observations and site evaluations, it is our opinion as stated above, that the
site meets technical criteria of an erosion hazard areas, but not that of a landslide or seismic hazard
area. None -the -less, given the height and steepness of the site slopes, the Federal Way building
department will require building setbacks from slopes steeper than 3H:1V (Horizontal:Vertical) per
the International Building Code (IBC) section 1805.
Recommended Setback
The IBC requires a building setback from slopes that are steeper than 3H:1V (Horizontal:
Vertical) unless evaluated and reduced, and/or a structural setback is provided, by a licensed
geotechnical engineer. The setback distance is calculated based on the vertical height of the slope.
The typical IBC setback from the top of the slope equals one third the height of the slope.
Using the IBC Setback criteria and given the vertical height of the slope below the proposed
daylight basement residence has a vertical height of about 40 feet. This would require a setback
from the steep slope area of about 17 feet. As currently proposed, the residence will be setback
about 5 feet from the top of the steep slope area. Where the setback cannot be met, the IBC allows
for a "structural setback' where foundation elements are extended vertically and the setback
distance is measured from the base of the deepened foundation element to the face of the slope at
the corresponding elevation.
Given the slope below the house, we estimate foundation elements will need to be extended
a minimum of about 7 feet along the east side of the residence. A detail showing the structural
setback configuration is attached as Figure 10.
Storm Drainage
Based on the results of our shallow subsurface explorations and sloping nature of the site,
infiltration of collected stormwater runoff is not feasible. Based on our discussions, we understand
G�C)R�sr�u��Es
Afi c h u kS. S293 rd St. G HA
September 14, 2020
page 1 7
that roof and driveway runoff will be directed back to South 293rd Street, while the footing and
basement wall drains should be tightlined to discharge into the stream corridor below the proposed
residence. Since no seepage was noted on the upper of the site slopes, the amount of water
discharging from the footing and wall drains should be minimal.
LIMITATIONS
We have prepared this report for Mr. Stan Afichuk and other members of the permitting and
design team for use in evaluating a portion of this project. Subsurface conditions described herein
are based on our observations of exposed soils on the parcel. This report may be made available to
regulatory agencies or others, but this report and conclusions should not be construed as a
warranty of subsurface conditions. Subsurface conditions can vary over short distances and can
change with time.
Variations in subsurface conditions are possible between the explorations and may also occur
with time. A contingency for unanticipated conditions should be included in the budget and schedule.
Sufficient monitoring, testing and consultation should be provided by our firm during construction to
confirm that the conditions encountered are consistent with those indicated by the explorations, to
provide recommendations for design changes should the conditions revealed during the work differ
from those anticipated, and to evaluate whether earthwork and foundation installation activities
comply with contract plans and specifications.
The scope of our services does not include services related to environmental remediation and
construction safety precautions. Our recommendations are not intended to direct the contractor's
methods, techniques, sequences or procedures, except as specifically described in our report for
consideration in design.
If there are any changes in the loads, grades, locations, configurations or type of facilities to be
constructed, the conclusions and recommendations presented in this report may not be fully
applicable. If such changes are made, we should be given the opportunity to review our
recommendations and provide written modifications or verifications, as appropriate.
G�C)R�sr�u��Es
AfichukS.S293rdSt.GHA
September 14, 2020
page 1 8
We have appreciated working for you on this project. Please do not hesitate to call at your
earliest convenience if you have any questions or comments.
Respectfully submitted,
GeoResources, LL
Keith S. Schembs, LEG Eric W. Heller, PE, LG
Principal Senior Geotechnical Engineer
KSS:EWH:kss
Doc ID: AfichuckS.S293rdSt.GHA
Attachments: Figure 1: Site Location Map
Figure 2: Site & Exploration Plan
Figure 3: Site Vicinity Map
Figure 4: NRCS SCS Soils Map
Figure 5: Geologic Map
Figure 6: Coastal Atlas
Figures 7a through 7e: Oblique Shoreline Photographs
Figure 8: Soil Classification System
Figure 9: Hand Auger Logs
Figure 10: Structural Setback
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Approximate Site Location
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Soil
Soil Name
Parent Material
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Geologic Map
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Approximate Site Location
Photo taken 7/29/2016 and obtained from the Washington State Department of Ecology Coastal Atlas Website
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Oblique Shoreline Photographs
Proposed Single -Family Residence
xxx South 293rd Place
Federal Way, Washington
Approximate Site Location
Photo taken 7/26/2006 and obtained from the Washington State Department of Ecology Coastal Atlas Website
(httpsJ/fortress. wa.gov/ecy/coastalatlas/tools/Map. asp)
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Oblique Shoreline Photographs
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Approximate Site Location
Photo taken 9/25/2006 and obtained from the Washington State Department of Ecology Coastal Atlas Website
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Oblique Shoreline Photographs
Proposed Single -Family Residence
xxx South 293rd Place
Federal Way, Washington
Approximate Site Location
Photo taken 5/19/1993 and obtained from the Washington State Department of Ecology Coastal Atlas Website
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SOIL CLASSIFICATION SYSTEM
MAJOR DIVISIONS
GROUP
GROUP NAME
SYMBOL
GRAVEL
CLEAN
GW
WELL -GRADED GRAVEL, FINE TO COARSE GRAVEL
GRAVEL
COARSE
GP
POORLY -GRADED GRAVEL
GRAINED
More than 50%
SOILS
Of Coarse Fraction
GRAVEL
GM
SILTY GRAVEL
Retained on
WITH FINES
GC
CLAYEY GRAVEL
No. 4 Sieve
SAND
CLEAN SAND
SW
WELL -GRADED SAND, FINE TO COARSE SAND
More than 50%
SIP
POORLY -GRADED SAND
Retained on
No. 200 Sieve
More than 50%
Of Coarse Fraction
SAND
SM
SILTY SAND
Passes
WITH FINES
No. 4 Sieve
SC
CLAYEY SAND
SILT AND CLAY
INORGANIC
ML
SILT
FINE
GRAINED
CL
CLAY
SOILS
Liquid Limit
Less than 50
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SILT AND CLAY
INORGANIC
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
More than 50%
Passes
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
No. 200 Sieve
Liquid Limit
50 or more
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES:
1. Field classification is based on visual examination of soil
in general accordance with ASTM D2488-90.
2. Soil classification using laboratory tests is based on
ASTM D2487-90.
3. Description of soil density or consistency are based on
interpretation of blow count data, visual appearance of
soils, and or test data.
EOI ESOU CES
earth science & geotccrrnlca %�ngirleeTing
LBO aacinr Fl%wr E. I Flrt_ WA N44 d I I I w w, SoOr9;9,rrac,eaek.:
SOIL MOISTURE MODIFIERS:
Dry- Absence of moisture, dry to the touch
Moist- Damp, but no visible water
Wet- Visible free water or saturated, usually soil is
obtained from below water table
Soil Classification System
Proposed Single -Family Residence
xxx South 293rd Place
Federal Way, Washington
Hand Auger HA-1
Location: Upper west central portion of site
Approximate Elevation: 276 feet
Depth
(inches) Soil Type Soil Description
0 - 6 - Topsoil
6 - 24 - Dark Orange -brown silty fine to medium SAND with gravel, some , asphalt (loose to
medium dense, mosit)(fill)
24 - 36 SM Orange -brown silty fine to medium SAND with gravel (medium dense, moist)(weathered
till)
Terminated at 36 inches
No caving observed.
No groundwater seepage observed.
Hand Auger HA-2
Location: East central portion of site, above stream channel
Approximate Elevation: 270 feet
Depth
(inches) Soil Type Soil Description
0 - 4 - Topsoil
4 - 22 - Dark Orange -brown silty fine to medium SAND with gravel, some wood (loose to
medium dense, moist)(weathered till/fill?)
22 - 42 SM Orange -brown silty fine to medium SAND with gravel (medium dense, moist)(weathered
till)
Terminated at 42 inches
No caving observed.
No groundwater seepage observed.
KSS Excavated on: June 15, 2012
Hand Auger Logs
Proposed Single -Family Residence
xxx South 293rd Place
earth scieriee & geotechnical eng:neerlf3 Federal Way, Washington
M" FaCific Fray F. i Mirk WA M4?d i i www, $tgrg;0�rraa,rarkc
Doc: AfichuckS.S293rdSt.F I September 2020 1 Figure 9
Conventional Footing
Slopes Greater
Than 30 Percent
Slopes Greater
Than 30 Percent
Setback Distance
Footing Extension
Setback Distance
GEORESOURCES
earth science & geotechnirai engineering
Loop Racinc wr E. I Flrr_ WA Mid?a I I wow. ZPor@iokdreiPs.F4[ka
Deck Post
Foundation
Foundation
Element
Foundation
Footing
Extension
Foundation
Element
Not to scale
Structural Setback
Proposed Single -Family Residence
xxx South 293rd Place
Federal Way, Washington