20-103186-Geotechnical (Soils) Report-08-20-2020-V1
DE-Civil, PLLC
P.O. Box 446
Tacoma, WA 98401
Ph (253) 228-0513
www.de-civil.com
March 16, 2020
Bill and Jenny Horner
35507 – 6th Avenue Southwest
Federal Way, Washington 98023
Geotechnical Report
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
PN: 3021049112
Job: 18-217 Horner.GR
INTRODUCTION
This geotechnical report summarizes our site observations and provides geotechnical recommendations
and design criteria for the proposed Short Plat to be located at 35507 – 6th Avenue Southwest in the City
of Federal Way, Washington. The site is currently developed and has been cleared of native vegetation
and is well vegetated grass and ornamental trees. The general 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, and our experience in the area.
We understand that the proposed development will include subdividing the property into a total of
three lots with the future construction a single-family residence on each lot. It is anticipated that the
buildings will consists of typical wood frame construction supported on conventional concrete spread
footings. The existing site configuration is illustrated on the attached Site Plan, Figure 2.
SCOPE
The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for
developing and providing geotechnical recommendations and design criteria for the proposed drainage
system. Specifically, our scope of services for the project included the following:
1. Conducting a geologic reconnaissance of the site area.
2. Exploring the subsurface conditions by monitoring the excavation of four track-hoe excavated
test pits at select locations across the site.
3. Addressing the appropriate geotechnical regulatory requirements for the proposed site
development.
4. Determining the local groundwater depth, for dewatering and drainage design purposes.
5. Provide our opinion with regard to the feasibility of on -site stormwater infiltration/dispersal and
provide an infiltration rate, if appropriate.
SITE CONDITIONS
Surface Conditions
The parcel is mostly rectangular in shape and measures approximately 150 feet in width, east to west,
by 290 feet in length, north to south, and encompasses 1.08 acres. The site is currently developed with a
single-family residence onsite. The site slopes down from south to north at grades of approximately 5
percent. Overall topographic relief across the site is approximately 12 feet. Off-site to the north is a
regional drainage facility that includes a 2:1 slope down to the facility with an overall elevation change
of approximately 30 feet. A site plan showing the existing contours is attached as Figure 2.
18-217 Horner.GR
March 16, 2020
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No deep-seated slopes instability was observed at the site or the adjacent areas. No erosion or other soil
movement was observed at the time of our site visit.
Site Soils
The USDA Natural Resource Conservation Service Web Soil Survey for King County maps the soils as
Everett gravelly sandy loam (Evb & Evd). The Eveerett soils consist of moderately deep, well drained
soils formed in glacial drift and outwash on slopes of 0 to 8 percent and 15 to 30 percent. Theses soils
are known to have a high rate of water transmission. An excerpt of the USDA Soil map is included as
Figure 3.
Site Geology
According to the 7.5 Minute Quadrangle for Poverty Bay, by Derek B. Booth, Howard H. Waldron and
Kathy Goetz Troost, 2003, shows the site as underlain by Vashon Ice Contact (Qvi). These glacial soils
were deposited during the Vashon Stade of the Fraser Glaciation, approximately 12,000 to 15,000 years
ago. The ice contact deposits consists of well to poorly sorted mixture of sand and gravel that was
partially overridden by glacial ice. Since they were partially overridden by the ice mass and can be
considered normally consolidated, are in a dense condition, and exhibit moderate strength and
compressibility characteristics where undisturbed. Based on our test pits the site soils are consistent
with the available mapping for the area. An excerpt of the Geological map is included as Figure 4.
Subsurface Explorations
On October 30, 2018 we explored subsurface conditions at the site by monitoring the excavation of two
test pits to depths varying between 4.5 - 5.5 feet. The test pits were approximately located in the field
by our representative by pacing from existing features such as roadways and lathe left by the previous
survey of the parcel. The approximate location of the explorations are indicated on the attached Site
Plan as Figure 2.
An engineer from our office maintained logs of the subsurface conditi ons encountered in each test pit,
obtained representative soil samples, and observed pertinent site features. Representative soil samples
obtained from the test pits were placed in sealed containers and taken to a laboratory for possible
further examination and testing.
Subsurface Conditions
Our explorations encountered consistent subsurface conditions. The test pits showed a silty sand and
gravel to a depth of approximately 4.5 feet. Soil logs are included as Figure 4.
Groundwater Conditions
No groundwater was encountered in either Test Pit. Based on our test pits, we do not anticipate that
local groundwater will occur at shallow elevations. However, groundwater levels will occur in response
to precipitation patterns, off-site construction activities, and site utilization. Perched groundwater
typically develops when the vertical infiltration of precipitation through a more permeable soil is slowed
at depth by a deeper, less permeable soil type.
Geologically Hazardous Areas – per City of Federal Way Municipal Code, Chapter 13.6.4.7
Geologically hazardous areas means areas which because of their susceptibility to erosion, landsliding,
seismic or other geological events require specific studies to determine appropriate buffers or property
use. Geologically hazardous areas include the following areas: erosion hazard areas, landslide hazard
areas, and seismic hazard areas.
18-217 Horner.GR
March 16, 2020
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No critical areas are present on or near this site.
CONCLUSIONS
Based on the results of our data review, site reconnaissance, subsurface explorations and our
experience in the area, it is our opinion that the site is suitable for development. Conventional footings
may be utilized in areas of medium dense to dense native soil. The infiltration of stormwater runoff is
feasible for the project. Pertinent conclusions and geotechnical recommendations regarding the design
and construction of the proposed development are presented below.
Site Preparation
Areas to be graded should be cleared of deleterious matter including any existing structures,
foundations, abandoned utility lines, debris and vegetation. The portions of the site covered with
vegetation should be stripped of any duff and organic-laden soils. These materials can be stockpiled and
later used for erosion control. Material that cannot be utilized on site should be removed from the site.
Where placement of fill material is required, the stripped/exposed subgrade areas should be compacted
to a firm and unyielding surface prior to placement of any fill. Excavations for debris removal should be
backfilled with structural fill compacted to the densities described in the “Structural Fill” section of this
report.
We recommend that a member of our staff evaluate the exposed subgrade conditions after removal of
vegetation and topsoil/fill stripping is completed and prior to placement of structural fill. The exposed
subgrade soil should be proof-rolled with heavy rubber-tired equipment during dry weather or probed
with a 1/2 inch-diameter steel rod during wet weather conditions.
Any soft, loose, or otherwise unsuitable areas delineated during proof-rolling or probing should be
recompacted, if practical, or over-excavated and replaced with structural fill, based on the
recommendations of our site representative. The areas of old fill material should be evaluated during
grading operations to determine if they need mitigation; recompaction or removal.
Structural Fill
All material placed as fill associ ated with mass grading, as utility trench backfill, under building areas, or
under roadways should be placed as structural fill. The structural fill should be placed in horizontal lifts
of appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be
compacted to at least 95 percent of MDD (maximum dry density as determined in a ccordance with
ASTM D 1557).
The appropriate lift thickness will depend on the fill characteristics and compaction equipment used.
We recommend that the appropriate lift thickness be evaluated by our field representative during
construction. We recommend that our representative be present during site grading activities to
observe the work and perform field density tests.
The suitability of material for use as structural fill will depend on the gradation and moisture content of
the soil. As the amount of fines (material passing US No. 200 sieve) increases, soil becomes increasingly
sensitive to small changes in moisture content and adequate compaction becomes more difficult to
achieve. During wet weather, we recommend use of well -graded sand and gravel with less than 5
percent (by weight) passing the US No. 200 sieve based on that fraction passing the 3/4-inch sieve, such
as Gravel Backfill for Walls (9-03.12(2)). If prolonged dry weather prevails during the earthwork and
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March 16, 2020
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foundation installation phase of construction, higher fines content (up to 10 to 12 percent) will be
acceptable.
Material placed for structural fill should be free of debris, organi c matter, trash and cobbles greater than
6-inches in diameter. The moisture content of the fill material should be adjusted as necessary for
proper compaction.
Suitability of On-Site Materials as Fill
During dry weather construction, any non-organic on-site soil may be considered for use as structural
fill; provided it meets the criteria described above in the structural fill section and can be compacted as
recommended. If the soil material is over-optimum in moisture content when excavated, it will be
necessary to aerate or dry the soil prior to placement as structural fill. We generally did not observe the
site soils to be excessively moist at the time of our su bsurface exploration program. The areas of native
sandy gravel material are comparable to sandy “pit run” and may be used as structural fill.
We recommend that completed graded-areas be restricted from traffic or protected prior to wet
weather conditions. The graded areas may be protected by paving, placing asphalt-treated base, a layer
of free-draining material such as pit run sand and gravel or clean crushed rock material containing less
than 5 percent fines, or some combination of the above.
Site Drainage
All ground surfaces, pavements and sidewalks at the site should be sloped away from structures. The lot
should also be carefully graded to ensure positive drainage away from all structures and property lines.
Surface water runoff from the roof area, driveways, perimeter footing drains, and wall drains, should be
collected, tightlined, and conveyed to an appropriate discharge point. We recommend that footing
drains are installed for the townhome(s). The roof drains should not be connected to the footing drain.
Pilot Infiltration Test
On December 1, 2018 we returned to the site to perform a full-scale infiltration test, also known as a
Pilot Infiltration Test (PIT). During the test we excavated a pit to the depth of approximately 3 feet
exposing the existing sand and gravel soils. The test was located in the north east area of the site (see
Figure 2 Site Plan) and was performed in general conformance with the City drainage manual
requirements and determined an average field measured rate of 24.1 inches per hour. The correction
factors should be 0.5 for testing, 0.65 for geometry and 0.9 for plugging, resulting in a long -term design
infiltration rate of 7 inches per hour. After the test we excavated the bottom of the pit an additional foot
and found no evidence of groundwater or mounding a fter the test was performed.
LID Feasibility
The following table presents our recommendations for various Best Management Practices (BMP’s) for
various Low Impact Development (LID) BMP’s.
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March 16, 2020
Page 5
BMP Viable Limitations or Infeasibility Criteria
Lawn and Landscape Areas
T5.13: Post Construction Soil Quality
and Depth
Yes None
Roofs
T5.30: Full Dispersion No There is insufficient native vegetation.
T5.10A: Downspout full infiltration
systems.
Yes Full infiltration can be accomplished if there is
sufficient room for the individual infiltration
trenches on each lot.
Bio Retention Yes This BMP is feasible for the same general reasons as
for full infiltration.
T5.10B Downspout Dispersion
systems
No For similar reasons given for full dispersion.
T5.10C Perforate stub connections No
Other Hard Surfaces
T5.30 Full Dispersion No There is insufficient native vegetation.
T5.15 Permeable Surfacing Yes The site infiltration rates are sufficient to allow for
permeable surfacing.
Bio Retention Yes This BMP is feasibile for the same general reasons
as for full infiltration.
T5.12: Sheet Flow Dispersion
T5.11 Concentrated flow dispersion
No For similar reasons given for full dispersion.
Site Buildable Area
Based on our test pits and correlation of site vegetation with an ticipated limits of the peat we have
delineated the approximate limits of the peat on the Site plan Figure 2. We recommend that any
proposed development occur outside of the peat areas. Should development be proposed within the
peat area special considerations will need to be employed which are beyond the scope of this report.
LIMITATIONS
We have prepared this report for Gian Singh and other members of the design team for use in
evaluating a portion of this project. Su bsurface 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.
Within the limitations of scope, schedule and budget, our services have been executed in accordance
with generally accepted practices in this area at the time this report was prepared. No warranty, express
or implied, should be understood.
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March 16, 2020
Page 6
We trust this is sufficient for your current needs. Should you have any questions, or require additional
information, please contact us at your earliest convenience.
Respectfully submitted,
Development Engineering, PLLC
Glen Coad, PE
Owner
WGC
Doc ID: 18-217 Horner.GR
Attachments: Figure 1: Vicinity Map
Figure 2: Site Plan
Figure 3: USDA Web Soil Survey
Figure 4: USGS Geological Map
Figure 5: Soil Classification Chart
Figure 6: Test Pit Logs
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
VICINITY MAP
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 1
Not to Scale Approximate site location
Not to Scale Approximate Test Pit location
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
Site Plan
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 2
TP-X
TP-1
TP-2
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
USDA WEB SOIL SURVEY
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 3a
Not to Scale Approximate site location
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
USDA WEB SOIL SURVEY LEGEND
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 3b
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
USGS GEOLOGICAL MAP – POVERTY BAY
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 4
Not to Scale Approximate site location
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
SOIL CLASSIFICATION CHART
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 5
Test Pit TP-1
Location: (See Figure 2)
Depth (ft) Soil Type Description
0.0 – 1.0 Topsoil
1.0 – 2.5 SM Brown silty SAND and gravel (loose, moist)
2.5 – 4.5 SM Tan silty SAND and gravel (med dense, moist)
4.5 – 5.5 SM Grey silty SAND w/gravel (dense, moist)
Terminated at 5.5 feet below ground surface.
No caving observed.
No Seepage observed.
Slightly cemented material present at depth.
Test Pit TP-2
Location: (See Figure 2)
Depth (ft) Soil Type Description
0.0 – 1.0 Topsoil
1.0 – 3.0 SM Brown silty SAND and gravel (loose, moist)
3.0 – 4.5 SM Tan silty SAND and gravel (loose, moist)
Terminated at 4.5 feet below ground surface.
No caving observed.
No Groundwater encountered.
No cemented material present.
Logged by WGC, 10/30/2018
DEVELOPMENT
ENGINEERING, PLLC
P.O. Box 446
Tacoma, Washington 98401
(253) 228-0513
www.de-civil.com
SOIL LOGS
Proposed Short Plat
35507 – 6th Avenue Southwest
Federal Way, Washington
Job No: 18-217 December 2018 Figure 6