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GEOTECHNICAL ENGINEERING REPORT - DRAFT
Proposed Mixed-use Development
South 348th Street & 1st Avenue South
Federal Way, Washington
Project No. 1536.01
4 December 2015
Prepared for:
FNW, Inc.
Prepared by:
Zipper Geo Associates, LLC
Geotechnical and Environmental Consultants
19023 36th Avenue W., Suite D
Lynnwood, WA 9803
ZGA
Zipper Geo Associates, LLC
Geotechnical and Environmental Consulting
19023 36th Avenue West, Suite D Lynnwood, WA 98036 (425) 582-9928
Project No. 1536.01
4 December 2015
FNW, Inc.
2711 West Valley Highway North, Suite 200
Auburn, Washington 98001
Attention: Mr. Brett Jacobsen
Subject: Geotechnical Engineering Report - DRAFT
Proposed Mixed-use Development
South 348th Street & 1st Avenue South
Federal Way, Washington
Dear Brett:
In accordance with your request and written authorization, Zipper Geo Associates, LLC (ZGA) has
completed the geotechnical engineering evaluation for the proposed multi-use development in Federal
Way, Washington. This report presents the findings of the subsurface exploration and laboratory testing
and presents our geotechnical recommendations for the project. Our services have been completed in
general accordance with our Scope of Services and Fee Estimate (Proposal No. P15250) dated 16
September 2015. Written authorization to proceed was provided by FNW, Inc. on 4 November 2015.
We appreciate the opportunity to be of service to you on this project. If you have any questions concerning
this report, or if we may be of further service, please contact us.
Sincerely,
Zipper Geo Associates, LLC
David C. Williams, LG, LEG Thomas A. Jones, PE
Principal Engineering Geologist Managing Principal
Copies: Addressee (1 electronic)
TABLE OF CONTENTS
Page
INTRODUCTION ........................................................................................................................................... 1
SITE DESCRIPTION ..................................................................................................................................... 1
PROJECT UNDERSTANDING..................................................................................................................... 1
SURFACE CONDITIONS ............................................................................................................................. 1
SUBSURFACE CONDITIONS...................................................................................................................... 2
Regional Geology ............................................................................................................................................. 2
Soil Conditions ................................................................................................................................................. 2
Groundwater Conditions ................................................................................................................................. 4
CONCLUSIONS AND RECOMMENDATIONS ............................................................................................ 4
General .......................................................................................................................................................... 4
Regulated Geologic Hazard Environmentally Critical Areas ............................................................................ 4
Seismic Design Considerations ........................................................................................................................ 6
Site Preparation ............................................................................................................................................... 6
Structural Fill ................................................................................................................................................... 9
Utility Trenches ............................................................................................................................................. 11
Temporary and Permanent Slopes ................................................................................................................ 12
Shallow Foundations ..................................................................................................................................... 13
Backfilled Permanent Retaining Walls .......................................................................................................... 14
On-Grade Concrete Slabs .............................................................................................................................. 15
Drainage Considerations ............................................................................................................................... 15
Asphalt Pavements ........................................................................................................................................ 16
Stormwater Management Considerations .................................................................................................... 17
CLOSURE ................................................................................................................................................... 19
FIGURES
Figure 1 – Site and Exploration Plan
Figure 2 – Building Location Plan
APPENDICES
Appendix A – Subsurface Exploration Procedures and Logs
Appendix B – Laboratory Testing Procedures and Results
Page 1
GEOTECHNICAL ENGINEERING REPORT - DRAFT
PROPOSED MIXED-USE DEVELOPMENT
SOUTH 348TH STREET & 1ST AVENUE SOUTH
FEDERAL WAY, WASHINGTON
Project No. 1536.01
4 December 2015
INTRODUCTION
This report documents the surface and subsurface conditions encountered at the site and our
geotechnical engineering recommendations for the proposed Federal Way mixed-use development. The
project description, site conditions, and our geotechnical conclusions and design recommendations are
presented in the text of this report. Supporting data including detailed exploration logs and field
exploration procedures, and results of laboratory testing are presented as appendices.
SITE DESCRIPTION
The project site is a largely undeveloped parcel located in the southeast quadrant of the 1st Avenue South
and South 348th Street intersection and occupies approximately 8.3 acres. Undeveloped properties
border the site to the east and south. The adjacent undeveloped properties have been mapped as
wetlands on the City of Federal Way Critical Areas Map (September 2012). The site is illustrated on the
Site and Exploration Plan, Figure 1.
PROJECT UNDERSTANDING
We understand that the proposed site improvements will include constructing one single-story wood-
framed retail building (Building A) in the northwest portion of the site and two four-story wood-framed
apartment buildings to the east and south (Buildings B and C) as illustrated on the Building Location Plan,
Figure 2. The buildings will be serviced by asphalt paved parking and access drives, and Buildings B and C
will include partial below-grade parking. Stormwater management is expected to include use of the existing
detention ponds and likely some form of LID management elements, although these have yet to be selected.
Moderate cuts and fills will be required to achieve site grades.
SURFACE CONDITIONS
The property is partially developed in that three detention ponds occupying approximately the
southeastern 1.5 acres were constructed a few years ago. The detention pond area is bordered by chain-
link fence and is accessed by about a 275-foot long gravel access road extending east from 1st Avenue
South. The site gradually slopes downward from the northwest corner to the south and west at an
approximate overall 10H:1V gradient. However, the western border of the site and approximately the
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Proposed Mixed-use Development – DRAFT
Federal Way, Washington
Project No. 1536.01
4 December 2015
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northwestern 275 feet of the site slopes down to 1st Avenue South and 348th Street with approximately 8
to 12-foot tall slopes inclined as steeply as approximately 45 to 50 percent. The slopes appear to be cut
slopes, probably graded in association with construction of South 348th Street and 1st Avenue South. Total
relief on the site is about 50 feet from north to south. Vegetation is varied and includes stands of mature
trees, blackberry bushes, brush, and grass. Scattered debris (pieces of pipe, ecology blocks, and small
piles of gravel), the obvious cut made for the existing access road, and slightly irregular topography
indicate that some limited grading occurred in portions of the site other than the detention ponds.
SUBSURFACE CONDITIONS
Regional Geology
We assessed the geologic setting of site and the surrounding vicinity by reviewing the following
publication:
Booth, DB, Waldron, HH, and Troost, KG, Geologic map of the Poverty Bay 7.5’ Quadrangle, King
and Pierce Counties, Washington, US Geological Survey Scientific Investigations Map 2854,
1:24,000, 2004.
The published geologic mapping indicates the site and vicinity are underlain by Vashon glacial till (Qvt).
Glacial till is typically composed of silt, sand, gravel, cobbles, and boulders. The till is glacially consolidated
and when intact is characterized by a loose to medium dense weathered horizon on the order of about 2
to 4 feet thick underlain by denser unweathered material. Unweathered till typically has a relatively high
density, relatively low permeability, and is generally well-suited for support of shallow foundations. The
partially developed nature of the site suggests that fill material may be present as well. Both native till
and some fill material were encountered in our test pits.
Soil Conditions
The subsurface evaluation for this project included excavating twelve test pits located approximately as
shown on the Site and Exploration Plan, Figure 1. Descriptive logs of the subsurface explorations and the
procedures utilized in the subsurface exploration program are presented in Appendix A. A generalized
description of soil conditions encountered at the exploration locations is presented below. Detailed
descriptions of soils encountered are provided on the descriptive logs in Appendix A.
The soil descriptions presented below have been generalized for ease of report interpretation. Please
refer to the test pit logs for more detailed soil descriptions. Variations in subsurface conditions may exist
in proximity to exploration locations and the nature and extent of such variation may not become evident
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Proposed Mixed-use Development – DRAFT
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Project No. 1536.01
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until construction. If variations then appear, it may be necessary to reevaluate the recommendations of
this report.
The test pits disclosed subsurface conditions that were generally consistent with the published mapping.
Subsurface conditions at the exploration locations are summarized below.
Undocumented Fill
Probable fill material was observed at six of the exploration locations (test pits 3, 4, 6, 7, 8, and 12). The
fill depth in five of the test pits generally ranged from approximately 1 to 2 feet and was located either
near the detention ponds or the adjacent roadways. The fill composition varied, but generally consisted
of sandy silt with gravel with a varied organic content. The fill was underlain by a relic topsoil horizon at
the locations of test pits TP-3 and TP-12. It should be noted that the character and depth of
undocumented fill may vary over relatively short distances.
In addition to the fill observed at and below grade as described above, a 6 to 10-foot tall stockpile is
present to the northwest of the detention pond and at the location of TP-7. The material in the stockpile
consisted of brown silty sand with some gravel. Based upon the observed composition and location it is
likely that the stockpile was generated by stripping topsoil during construction of the detention pond area,
in our opinion. The fill observed at the test pit locations and in the stockpile is considered undocumented
in that it appears to have been placed in a non-engineered condition and due to its composition and
density is considered inadequate for support of structures, pavements, and utilities without mitigation.
Topsoil / Forest Duff
All of the explorations disclosed a surficial horizon of organic topsoil that ranged in thickness from
approximately 6 inches to 1 foot. In general the topsoil horizon consisted of brown silty sand with gravel,
and contained fine to medium roots throughout. In some locations roots were observed to extend below
the organic-rich topsoil horizon.
Glacial Till
The topsoil was underlain by glacial till that typically consisted of medium dense to very dense silty gravelly
sand with occasional cobbles up to approximately 12 inches in diameter. Although not observed at the
test pit locations, boulders are commonly present within the till. The upper 1 to 3 feet of the glacial till
consisted of a medium dense to dense, light orange to brown weathered zone. Below the weathered till
we generally observed dense to very dense, gray, silty gravelly sand with some sandy silt horizons with
scattered cobbles.
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Groundwater Conditions
Groundwater was not observed in any of the twelve test pits at the time of excavation. It should be noted
that groundwater conditions may fluctuate seasonally due to variations in precipitation, land use,
irrigation, or other factors.
CONCLUSIONS AND RECOMMENDATIONS
General
The proposed mixed-use development will include the construction of single-story retail building (Building
A) in the northwest portion of the site as well as two multi-story retail/apartment buildings (Buildings B
and C) to the east and south. Building A will be at grade, while Buildings B and C will include partial below-
grade parking. A grading plan had not been provided for our review at the time this report was written,
but from preliminary building grades it appears that overall grading will primarily involve cutting the
higher northern and western portions of the site to better match the grades of the adjacent streets to the
north and west. Tiered retaining walls will be constructed along the southern portions of Buildings B and
C. The proposed building layout is illustrated on Figure 2, the Building Location Plan.
Based on the results of the subsurface exploration program and our analysis, we have concluded that the
proposed development is feasible from the geotechnical perspective, contingent on proper design and
construction practices and implementation of the recommendations presented in this report.
Geotechnical engineering recommendations for foundation systems and other earthwork related phases
of the project are outlined below. The recommendations contained in this report are based upon the
results of the field exploration and laboratory testing, engineering analyses, and our current
understanding of the proposed project. ASTM and Washington State Department of Transportation
(WSDOT) specification codes cited herein respectively refer to the current manual published by the
American Society for Testing & Materials and the current edition of the Standard Specifications for Road,
Bridge, and Municipal Construction, (Publication M41-10).
Regulated Geologic Hazard Environmentally Critical Areas
The City of Federal Way regulates certain geologic hazards including areas susceptible to erosion,
landsliding, seismic, or other geological events, as well as defined steep slopes. These areas are defined
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in Section 15.05.030 of the Federal Way Revised Code (FWRC). A summary of the regulated geologically
hazardous critical areas is presented below.
Erosion Hazard
The FWRC defines erosion hazard areas as having a severe to very severe erosion hazard due to natural
agents such as wind, rain, splash, frost action, or stream flow. Mapping obtained from the USDA Natural
Resources Conservation Service (NRCS) indicates that the site has been characterized by the Everett-
Alderwood gravelly sand loam soils, 6 to 15 percent (EwC). These soils are formed in glacial till parent
material and are described as presenting a slight to moderate erosion hazard. Consequently, the site does
not meet the SMP criteria as an erosion hazard. Provided that site grading and construction occur in
accordance with a Temporary Erosion and Sedimentation Control (TESC) plan approved by the City of
Federal Way, and provided that TESC BMPs are adequately maintained during construction, it is our
opinion that the risk of significant sediment generation and off-site sediment transport is low. The site is
not designated as an erosion hazard area on the City’s September 2012 Critical Areas Map.
Steep Slope Hazard
A regulated steep slope is one with 10 or more feet of relief and an inclination of 40 percent or steeper.
Based upon review of the topographic survey of the site provide to us, and our site observations, a slope
segment meeting this definition is limited to a small section of the graded slope along the west side of the
site bordering 1st Avenue South. The cut slopes along the west and north sides of the site are inclined at
about 45 to 50 percent, and a segment of the west slope with approximately 10 to 12 feet of relief meets
the criteria for a regulated steep slope. The southern end of this slope segment, which is approximately
120 feet long, is approximately 80 feet north of the site access drive. The approximate location of the
steep slope is highlighted on Figure 1. Regrading of this slope is feasible from the geotechnical perspective.
Landslide Hazard
The FWRC defines landslide hazard areas as those areas potentially subject to episodic downslope
movement of a mass of soil or rock with including a combination of slopes greater than 15 percent,
permeable sediment, or with springs or groundwater seepage. The site is characterized by dense to very
dense glacial till, and within the depths of the test pits or along the existing graded slopes along South
348th Street and 1st Avenue South and the detention ponds in the southeastern portion of the site, we did
not observe permeable granular soils and groundwater seepage. The site does not meet the Code
definition of a landslide hazard.
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Proposed Mixed-use Development – DRAFT
Federal Way, Washington
Project No. 1536.01
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Seismic Hazard
The FWRC defines a seismic hazard as an area subject to severe risk of earthquake damage as a result of
soil liquefaction in areas underlain by cohesionless soils of low density and usually in association with a
shallow groundwater table or of other seismically-induced settlement. The site is underlain by glacial till,
a glacially consolidated soil characterized by a high density, and shallow groundwater is not present. As
such, the risk of damage due to a seismic event is low, in our opinion. The site does not meet the Code
definition of a seismic hazard. The site is not designated as a seismic hazard area on the City’s September
2012 Critical Areas Map.
Seismic Design Considerations
IBC Seismic Design Parameters: IBC Seismic Design parameters are summarized in the table below.
Description Value
2012 IBC Site Classification C 1,2
SS Spectral Acceleration for a Short Period 1.286g (Site Class B)
S! Spectral Acceleration for a 1-Second Period 0.495g (Site Class B)
SMS Spectral Acceleration for a Short Period 1.286g (Site Class C)
SM! Spectral Acceleration for a 1-Second Period 0.646g (Site Class C)
1. In general accordance with the 2012 International Building Code, Table 1613.5.2.
2. The 2012 International Building Code (IBC) requires a site soil profile determination extending a
depth of 100 feet for seismic site classification. The approved scope did not include the complete
100 foot soil profile determination. The test pits performed for this evaluation extended to a
maximum depth of approximately 15 feet, and this seismic Site Class C assignment considers that at
least dense soils continue below the maximum depth of the subsurface exploration based upon
published geologic mapping.
Site Preparation
Erosion Control Measures: The site has variable topography and significant grading is proposed.
Consequently, the potential for construction phase erosion may be considered relatively high unless City-
approved TESC BMPs are adequately designed, installed, and maintained. We recommend that silt fences,
berms, and/or swales be installed around stripped areas and stockpiles in order to capture runoff water
and sediment. If earthwork occurs during wet weather, we recommend covering stripped surfaces with
straw and protecting soil stockpiles with anchored plastic sheeting when areas are not being worked for
long periods.
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Temporary Drainage: Stripping, excavation, grading, and subgrade preparation should be performed in a
manner and sequence that will provide drainage at all times and provide proper control of erosion. The
site should be graded to prevent water from ponding in construction areas and/or flowing into
excavations. Exposed grades should be crowned, sloped, and smooth-drum rolled at the end of each day
to facilitate drainage if inclement weather is forecasted. Accumulated water must be removed from
subgrades and work areas immediately and prior to performing further work in the area. The site soils
have a relatively high fines content and should be considered highly moisture-sensitive. As such,
equipment access may be limited and the amount of soil rendered unfit for use as structural fill may be
greatly increased if drainage efforts are not accomplished in a timely manner.
Stripping: In preparation for grading we recommend removal of all existing vegetation, root grubbing, and
removal of existing fill material containing organic or deleterious material. This would include the
stockpile located northwest of the detention ponds. Organic-rich topsoil (soils containing more than 4
percent organic material by weight) will need to be stripped from structure and pavement locations, as
well as those areas to receive structural fill. The thickness of organic duff and topsoil observed at the test
pit locations ranged from about 6 to 12 inches, and roots extended to depths of about 18 inches.
However, variation in the organic material thickness should be expected; deeper accumulations of
organics may be encountered in depressions and around root masses. Duff and topsoil should be removed
and should not be reused as structural fill. Organic materials may be used in landscaping.
Stripping is recommended to include removal of undocumented fill material and any relic organic topsoil
below the fill due to the risk of future settlement if these materials are left in place. The undocumented
fill we observed at the test pit locations was typically in a loose condition and in some locations contained
organic material as well as debris. The depth of the fill ranged from approximately 1.5 to 2 feet (not
including the stockpile of strippings at the test pit TP-7 location) and relic topsoil was observed to about
a foot below the fill at the locations of test pits TP-3 and TP-12. Variation in the fill depth and composition,
and the depth of organics below the fill, should be expected. These materials should be removed under
the observation of a ZGA representative. Our representative will identify unsuitable materials that should
be removed and those that may be re-used as structural fill. The resultant excavations should be backfilled
in accordance with the subsequent recommendations for structural fill placement and compaction.
We recommend that site preparation activities take place in the drier summer months. Operating
wheeled and tracked equipment when the weathered glacial till soils are wet will result in significant
disturbance of the non-organic weathered glacial till soils and likely requiring its removal. This will
increase construction costs. Completion of logging and stripping under dry site and weather conditions
will reduce the potential for disturbance of the weathered till soils and reduce the likelihood of subgrade
disturbance and the need to replace disturbed soils with imported granular fill.
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Subgrade Preparation: Once stripping has been completed, all areas that are at design subgrade elevation
or areas that will receive new structural fill should be compacted to a firm and unyielding condition and
to a compaction level of at least 95 percent of the maximum laboratory density (per ASTM D 1557) within
the upper 12 inches. Some moisture conditioning of site soils may be required to achieve an appropriate
moisture content for compaction within ±2 percent of the soils laboratory optimum moisture content,
particularly during the warmer summer months when the soils will tend to dry relatively quickly when
exposed to sun and wind.
Subgrades should be evaluated through density testing and proof rolling with a loaded dump truck or
heavy rubber-tired construction equipment in order to detect soft and/or yielding soils. In the event that
soft or yielding areas are detected during proof rolling, the upper 12 inches of subgrade should be
scarified, moisture conditioned and re-compacted as necessary to obtain at least 95 percent of the
maximum laboratory density (per ASTM D 1557) and to a firm, non-yielding condition. Those soils which
are soft/loose, yielding, or unable to be compacted to the specified criteria should be over-excavated and
replaced with suitable material as recommended in the Structural Fill section of this report. If subgrade
compaction during wet site conditions or wet weather cannot be achieved, a minimum of 12 inches of
subgrade should be over-excavated and backfilled with compacted imported structural fill consisting of
free-draining Gravel Borrow or crushed rock. A stabilization geotextile could be used in unstable areas to
reduce the depth of over-excavation.
We recommend completing earthwork during drier periods of the year when the soil moisture content
can be controlled by aeration and drying, if necessary. If earthwork or construction activities take place
during extended periods of wet weather, the site-characteristic glacial till may become unstable or not be
compactable. In the event the exposed subgrade becomes unstable, yielding, or unable to be compacted
due to high moisture conditions, we recommend that the affected material be removed to a sufficient
depth in order to develop a stable subgrade that can be compacted to the minimum recommended levels.
The severity of construction problems will be dependent, in part, on the precautions that are taken by the
contractor to protect the subgrade soils.
Once subgrades are compacted, it may be desirable to protect prepared subgrades such as building pads
or haul roads. To protect stable subgrades, we recommend using crushed rock. The thickness of the
protective layer should be determined at the time of construction and be based on the moisture condition
of the soil and the amount of anticipated traffic.
Freezing Conditions: If earthwork takes place during freezing conditions, all exposed subgrades should be
allowed to thaw and then be compacted prior to placing subsequent lifts of structural fill. Alternatively,
the frozen material could be stripped from the subgrade to expose unfrozen soil prior to placing
subsequent lifts of fill or foundation components. The frozen soil should not be reused as structural fill
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until allowed to thaw and adjusted to the proper moisture content, which may not be possible during
winter months.
Structural Fill
Structural fill includes any material placed below foundations and pavement sections, within utility
trenches, and behind retaining walls. Prior to the placement of structural fill, all surfaces to receive fill
should be prepared as previously recommended in the Site Preparation section of this report.
Laboratory Testing: Representative samples of imported soil to be used as structural fill should be
submitted for laboratory testing at least four days in advance of its intended use in order to complete the
necessary Proctor tests.
Re-use of Site Soils as Structural Fill: The non-organic native soil encountered on the site is adequate for
use as general structural fill from a compositional standpoint provided the soil is placed and compacted
in accordance with the compaction recommendations presented in this report. Soil will need to be near
the optimum moisture content in order to compact it to the recommended density. Drying of over-
optimum moisture soils may be achieved by scarifying or windrowing surficial materials during extended
periods of dry weather. If encountered, soils which are dry of optimum may be moistened through the
application of water and thorough blending to facilitate a uniform moisture distribution in the soil prior
to compaction. Simply moistening the upper surface of a loose lift of the site soils will not allow adequate
distribution of the water in the lift; blending will be necessary to achieve an adequate moisture
distribution in the lift.
We recommend that site soils used as structural fill have less than 4 percent organics by weight as
determined by the ASTM D 2974 and have no woody debris greater than ½ inch in diameter. We
recommend that all pieces of organic material greater than ½ inch in diameter be picked out of the fill
before it is compacted. Any organic-rich soil derived from earthwork activities should be utilized in
landscape areas or wasted from the site.
Imported Structural Fill: In the event that imported structural fill is required, the appropriate type of
imported structural fill will depend on weather conditions. During extended periods of dry weather, we
recommend that imported fill, at a minimum, meet the requirements of Common Borrow as specified in
Section 9-03.14(3) of the 2014 Washington State Department of Transportation, Standard Specifications
for Road, Bridge, and Municipal Construction (Publication M41-10). During wet weather, higher-quality
structural fill might be required, as Common Borrow may contain sufficient fines to be moisture-sensitive.
During wet weather we recommend that imported structural fill meet the requirements of Gravel Borrow
as specified in Section 9-03.14(1) of the WSDOT Standard Specifications.
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Retaining Wall Backfill: Retaining walls should include a drainage fill zone extending at least 2 feet back
from the back face of wall for the entire wall height. The drainage fill should meet the requirements of
Gravel Backfill for Walls as specified in Section 9-03.12(2) of the WSDOT Standard Specifications.
Pavement Subgrades: Any structural fill used within the upper one foot of pavement subgrades should
have a minimum California Bearing Ratio (CBR) of at least 15 when compacted to 95 percent of the
modified Proctor maximum dry density. Based on our experience and correlations between soil type and
CBR values, we have considered that a CBR value of 15 is representative of the native soil and has been
used to develop our pavement section recommendations. Our design recommendations assume that
imported fill types as recommended above (Common Borrow or Gravel Borrow) will meet the minimum
CBR requirement. However, samples of proposed imported fill should be submitted for laboratory testing
and approval prior to use.
Moisture Content: The suitability of soil for use as structural fill will depend on the time of year, the
moisture content of the soil, and the fines content (that portion passing the US No. 200 sieve) of the soil.
As the amount of fines increases, the soil becomes increasingly sensitive to small changes in moisture
content. Soils containing more than about 5 percent fines cannot be consistently compacted to the
appropriate levels when the moisture content is more than approximately 2 percent above or below the
optimum moisture content (per ASTM D1557). The optimum moisture content is that moisture content
which results in the greatest compacted dry density with a specified compactive effort. The fines content
of the samples tested in our laboratory ranged from approximately 18 to 24 percent. Consequently, the
soils should be considered highly moisture-sensitive. The moisture content of the samples of native soil
that we tested indicated that the soils were generally in a moist to wet condition relative to an anticipated
modified Proctor maximum dry density. However, soil moisture contents at the time of construction
should be expected to vary from our test results.
Fill Placement: Structural fill should be placed in horizontal lifts not exceeding 12 inches in loose thickness.
Thinner lifts may be required depending up the soil conditions and the type of compaction equipment in
use. Each lift of fill should be compacted using compaction equipment suitable for the soil type and lift
thickness. Each lift of fill should be compacted to the minimum levels recommended below based on the
maximum laboratory dry density as determined by the ASTM D 1557 testing procedure (modified Proctor).
The moisture content of fill at the time of placement should be within plus or minus 2 percent of optimum
moisture content for compaction as determined by the ASTM D 1557 test method.
Compaction Criteria: Our recommendations for soil compaction are summarized in the following table.
Structural fill for roadways and utility trenches in municipal rights-of-way should be placed and compacted
in accordance with the City of Federal Way standards. We recommend that a ZGA representative be
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present during grading so that an adequate number of density tests may be conducted as structural fill
placement occurs. In this way, the adequacy of the earthwork may be evaluated as it proceeds.
RECOMMENDED SOIL COMPACTION LEVELS
Location Minimum Percent Compaction*
Stripped native subgrade soils, prior to fill placement (upper 12
inches) 95
All fill below building floor slabs and foundations 95
Upper 2 feet of fill below pavements 95
Conventional pavement fill below two feet 90
Retaining wall backfill less than 3 feet from wall 90
Retaining wall backfill more than 3 feet from wall 95
Utility trench backfill 95
Landscape Areas 88 - 90
* ASTM D 1557 Modified Proctor Maximum Dry Density
Utility Trenches
We recommend that utility trenching conform to all applicable regulations, such as OSHA, for open
excavations. Trench excavation safety guidelines are presented in 29 CFR 1926.650, 1926.651, and
1926.652.
Trench Dewatering: Groundwater was not observed at the test pit locations, and the observed soil
conditions did not suggest the presence of groundwater within the anticipated excavation depths. We do
not anticipate that trench dewatering will be required.
Utility Subgrade Preparation: We recommend that all utility subgrades be firm and unyielding and free of
all soils that are loose, disturbed, or pumping. Such soils should be removed and replaced, if necessary.
All structural fill used to replace over-excavated soils should be compacted as recommended in the
Structural Fill section of this report. If utility foundation soils are soft, we recommend that they be over-
excavated 12 inches and replaced with compacted crushed rock.
Bedding: We recommend that a minimum of 4 inches of bedding material be placed above and below all
utilities or in general accordance with the utility manufacturer’s recommendations and local
requirements. We recommend that pipe bedding consist of Gravel Backfill for Pipe Zone Bedding as
described in Section 9-03.12(3) of the WSDOT Standard Specifications. All trenches should be wide
enough to allow for compaction around the haunches of the pipe, or material such as pea gravel should
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be used below the spring line of the pipes to eliminate the need for mechanical compaction in this portion
of the trenches. If water is encountered in the excavations, it should be removed prior to fill placement.
Trench Backfill: Materials, placement and compaction of utility trench backfill should be in accordance
with the recommendations presented in the Structural Fill section of this report. We recommend that the
initial lift thickness not exceed one foot unless recommended by the manufacturer to protect utilities from
damage by compacting equipment. Light hand operated compaction equipment may be utilized directly
above utilities if damage resulting from heavier compaction equipment is of concern.
Temporary and Permanent Slopes
Temporary excavation slope stability is a function of many factors, including:
The presence and abundance of groundwater;
The type and density of the various soil strata;
The depth of cut;
Surcharge loadings adjacent to the excavation; and
The length of time the excavation remains open.
As a cut is deepened, or as the length of time an excavation is open, the likelihood of bank failure increases;
therefore, maintenance of safe slopes and worker safety should remain the responsibility of the contractor,
who is present at the site, able to observe changes in the soil conditions, and monitor the performance of
the excavation.
It is exceedingly difficult under the variable circumstances to pre-establish a safe and “maintenance-free”
temporary cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe
temporary slope configurations since the contractor is continuously at the job site, able to observe the
nature and condition of the cut slopes, and able to monitor the subsurface materials and groundwater
conditions encountered. Unsupported vertical slopes or cuts deeper than 4 feet are not recommended if
worker access is necessary. The cuts should be adequately sloped, shored, or supported to prevent injury
to personnel from local sloughing and spalling. The excavation should conform to applicable Federal,
State, and Local regulations.
According to OSHA regulations, the contractor should make a determination of excavation side slopes
based on classification of soils encountered at the time of excavation. Temporary cuts may need to be
constructed at flatter angles based upon the soil moisture and groundwater conditions at the time of
construction. Adjustments to the slope angles should be determined by the contractor at that time. It
should be noted that much of the native soil encountered in excavations is expected to consist of relatively
clean sand and gravel and may be susceptible to rapid collapse in unsupported conditions.
Zipper Geo Associates, LLC
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Project No. 1536.01
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We recommend that all permanent cut or fill slopes constructed in native soils or with imported structural
fill be designed at a 2H:1V inclination or flatter. All permanent cut and fill slopes should be adequately
protected from erosion both temporarily and permanently. If the slopes are exposed to prolonged rainfall
before vegetation becomes established, the surficial soils will be prone to erosion and possible shallow
sloughing. We recommend covering permanent slopes with a rolled erosion protection product, such as
coir matting or Curlex II, if vegetation has not been established by the wet season (typically November
through May).
Shallow Foundations
Based on our analyses, conventional spread footings will provide adequate support for the proposed
buildings provided that the foundation subgrades are properly prepared. We anticipate that foundation
subgrade soils will generally consist of native glacial till or compacted structural fill.
Allowable Bearing Pressure: Continuous and isolated column footings bearing on undisturbed, dense to
very dense native glacial till may be designed for a maximum allowable net bearing capacity of 5,000 psf.
Foundations bearing on structural fill placed and compacted in accordance with the recommendations
presented herein may be designed for a maximum allowable net bearing capacity of 2,500 psf. A one-
third increase of these bearing pressures may be used for short-term transient loads such as wind and
seismic forces. The above-recommended allowable bearing pressures include a factor of safety of 3.
Shallow Foundation Depth and Width: For frost protection, the bottom of all exterior footings should
bear at least 18 inches below the lowest adjacent outside grade, whereas the bottoms of interior footings
should bear at least 12 inches below the surrounding slab surface level. We recommend that all
continuous wall and isolated column footings be at least 12 and 24 inches wide, respectively.
Lateral Resistance: Resistance to lateral loads can be calculated assuming an ultimate passive resistance
of 540 pcf equivalent fluid pressure (triangular distribution) and an ultimate base friction coefficient of
0.5. An appropriate safety factor (or load/resistance factors) should be included for calculating resistance
to lateral loads. For allowable stress design, we recommend a minimum 1.5 safety factor. We recommend
neglecting passive resistance in the upper 18 inches of embedment.
Estimated Static Settlement: Assuming the foundation subgrade soils are prepared in accordance with
recommendations presented herein, we estimate that total static settlement may approach 0.75 inches
and differential static settlement may approach half the total settlement over a distance of about 40 feet.
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Backfilled Permanent Retaining Walls
The project is expected to include backfilled cast-in-place (cip) concrete retaining walls, primarily along
the southern portions of Buildings B and C. For recommended bearing capacities and lateral resistance
parameters, refer to the Shallow Foundations section above. Additional recommendations for these
structures are provided below.
Lateral Earth Pressures: The lateral soil pressures acting on backfilled retaining walls will depend on the
nature and density of the soil behind the wall, and the ability of the wall to yield in response to the earth
loads. Yielding walls (i.e. walls that are free to translate or rotate) that are able to displace laterally at
least 0.001H, where H is the height of the wall, may be designed for active earth pressures. Non-yielding
walls (i.e. walls that are not free to translate or rotate) should be designed for at-rest earth pressures.
Non-yielding walls include walls that are braced to another wall or structure, and wall corners.
Assuming that walls are backfilled and drained as described in the following paragraphs, we recommend
that yielding walls supporting horizontal backfill be designed using an equivalent fluid density of 35 pcf
(active earth pressure). Non-yielding walls should be designed using an equivalent fluid density of 50 pcf
(at-rest earth pressure).
Design of permanent retaining walls should consider additional earth pressure resulting from the design
seismic event. For the seismic case, yielding walls should be designed for a uniform (rectangular), total
earth pressure distribution of 10H and non-yielding walls should be designed for a uniform, total earth
pressure distribution of 18H. The recommended total earth pressure distributions for the seismic case
include both the seismic and static components of earth pressures (i.e. the active or at-rest static
components of 35 pcf or 50 pcf should not be added to the total uniform pressure distribution). For
cantilever cast-in-place walls, the total earth pressure distributions for the seismic case should be applied
from finished grade at the bottom of the wall to the top of wall.
The above-recommended lateral earth pressures do not include the effects of sloping backfill surfaces,
surcharges such as traffic loads, other surface loading, or hydrostatic pressures. If such conditions exist,
we should be consulted to provide revised earth pressure recommendations.
Tiered Wall Considerations: Preliminary architectural drawings provided for our review indicated that
some of the retaining walls will be constructed in a tiered configuration. We recommend configuring the
walls such that the foundations of walls above a lower wall not intrude past a 1H:1V slope extended
upward from the base of the lower wall. Otherwise, it will be necessary to design successive lower walls
to accommodate the additional lateral earth pressures resultant from the loading of the upper wall(s).
Zipper Geo Associates, LLC
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Wall Drainage: Adequate drainage measures must be installed to collect and direct subsurface water
away from retaining walls. All backfilled walls should include a drainage aggregate zone extending at least
2 feet from the back of wall for the full height of the wall. The drainage aggregate should consist of
material meeting the requirements of WSDOT 9-03.12(2) Gravel Backfill for Walls. We did not observe at
the test pit locations any soils that would meet the gradational requirements for wall backfill, so it will be
necessary to import the wall drainage aggregate. A minimum 4-inch diameter perforated rigid
thermoplastic drain pipe should be provided at the base of backfilled walls to collect and direct subsurface
water to an appropriate discharge point. Drain pipe perforations should be protected using a non-woven
geotextile fabric such as Mirafi 140N in order to prevent soil particles from entering the pipe. Wall
drainage systems should be independent of other drainage systems such as roof drains. We recommend
incorporating cleanouts in wall drainage systems.
On-Grade Concrete Slabs
The following sections provide recommendations for on-grade floor slabs.
Subgrade Preparation: Subgrades for on-grade slabs should be prepared in accordance with the Site
Preparation and Structural Fill sections of this report.
Capillary Break: We recommend the on-grade slabs be underlain by a minimum 4-inch thick layer of
compacted granular fill consisting of coarse sand and fine gravel containing less than 5 percent fines,
based on that soil fraction passing the US No. 4 sieve. Alternatively, a clean angular gravel such as No. 7
Aggregate per WSDOT 9-03.1(4) C could be used for this purpose. Alternative capillary break materials
should be submitted to ZGA for review and approval before use.
Vapor Retarder: The use of a vapor retarder should be considered beneath concrete slabs on grade that
will be covered with wood, tile, carpet or other moisture sensitive or impervious coverings, or when the
slab will support equipment sensitive to moisture or is otherwise considered moisture-sensitive. When
conditions warrant the use of a vapor retarder, the slab designer and contractor should refer to ACI 302
and/or ACI 360 for procedures and cautions regarding the use and placement of a vapor retarder. We
generally recommend a minimum 10 mil vapor retarder.
Drainage Considerations
Surface Drainage: Final site grades should be sloped to carry surface water away from the buildings and
other drainage-sensitive areas. Additionally, site grades should be designed such that concentrated runoff
toward softscape surfaces is avoided. Any surface runoff directed towards softscape slopes should be
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collected at the top of the slope and routed to the bottom of the slope and discharged in a manner that
prevents erosion.
Building Foundation Drains: Similar to the retaining wall drains, building foundation drains are
recommended to consist of a minimum 4-inch diameter, Schedule 40, rigid, perforated thermoplastic pipe
placed at the base of the heel of the footing with the perforations facing down. The pipe should be
surrounded by a minimum of 6 inches of clean free-draining granular material conforming to WSDOT
Standard Specification 9-03.12(4), Gravel Backfill for Drains or similar gradation material. A non-woven
geotextile fabric such as Mirafi 140N, or equivalent, should envelope the free-draining granular material.
At appropriate intervals such that water backup does not occur, the drainpipe should be connected to a
tightline system leading to a suitable discharge. Cleanouts should be provided for future maintenance.
The foundation drains should be separate from the roof drain system.
Asphalt Pavements
Pavement Life and Maintenance: It should be realized that asphaltic pavements are not maintenance-
free. The following pavement sections represent our minimum recommendations for an average level of
performance during a 20-year design life; therefore, an average level of maintenance will likely be
required. A 20-year pavement life typically assumes that an overlay will be placed after about 12 years.
Thicker asphalt, base, and subbase courses would offer better long-term performance, but would cost
more initially. Conversely, thinner courses would be more susceptible to “alligator” cracking and other
failure modes. As such, pavement design can be considered a compromise between a high initial cost and
low maintenance costs versus a low initial cost and higher maintenance costs. Please note that we made
assumptions regarding traffic type and frequency in the absence of specific traffic count data.
Soil Design Values: Pavement subgrade soils are anticipated to consist of the site-characteristic native
glacial till (silty gravelly sand). Our analysis assumes the pavement section subgrade will have a CBR value
of 15. This value is based upon published correlations between soil type and CBR values and our
experience.
Recommended Pavement Sections: For light duty pavements (parking stalls), we recommend 2.5 inches
of asphalt concrete over either 4 inches of crushed surfacing base course or 3 inches of Asphalt Treated
Base (ATB). For heavy duty pavements (main access routes, truck delivery routes), we recommend 3
inches of asphalt concrete over either 6 inches of crushed rock surfacing course or 4 inches of ATB. Areas
subject to heavy surface loading, such as dumpster approach slabs that experience short-term high wheel
loading, would benefit from either a thicker asphalt pavement section or the use of concrete pavement.
Please note that using ATB instead of crushed surfacing base course below the pavement will provide
limited opportunity for sub-pavement drainage and may shorten the pavement lifespan as the site soils
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Proposed Mixed-use Development – DRAFT
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have a relatively low permeability. Placing some clean crushed surfacing base course below the ATB (such
as 3 inches, for example) will improvement pavement section drainage. In the event that this is considered
favorably, we can provide a revised pavement section that considers the value (other than drainage)
contributed by free draining granular material below the ATB.
Materials and Construction: We recommend the following regarding asphalt pavement materials and
pavement construction.
Subgrade Preparation and Compaction: The upper 12 inches of native stripped subgrade should
be prepared in accordance with the recommendations presented in the Subgrade Preparation
section of this report, and all fill should be compacted in accordance with the recommendations
presented in the Structural Fill section of this report.
Asphalt Concrete: We recommend that the asphalt concrete conform to Section 9-02.1(4) for PG
58-22 or PG 64-22 Performance Graded Asphalt Binder as presented in the 2014 WSDOT Standard
Specifications. We also recommend that the gradation of the asphalt aggregate conform to the
aggregate gradation control points for ½-inch mixes as presented in Section 9-03.8(6) HMA
Proportions of Materials.
Base Course: We recommend that the crushed aggregate base course conform to Section
9-03.9(3) of the WSDOT Standard Specifications.
Compaction and Paving: All base material should be compacted to at least 95 percent of the
maximum dry density determined in accordance with ASTM D 1557. We recommend that asphalt
be compacted to a minimum of 92 percent of the Rice (theoretical maximum) density or 96
percent of Marshall (Maximum laboratory) density. Placement and compaction of asphalt should
conform to requirements of Section 5-04 of the 2012 WSDOT Standard Specifications.
Stormwater Management Considerations
The site contains stormwater detention ponds constructed as part of a previous abandoned development
effort. We understand that the City of Federal way will require some additional stormwater management
features and that stormwater management improvements will need to comply with the King County 2009
Surface Water Design Manual (2009 Manual).
Based on the findings of the field exploration, laboratory testing, and our analysis, conventional
stormwater infiltration does not appear feasible from the geotechnical perspective given the relatively
high fines content and the density of the unweathered glacial till that characterizes the site. The relatively
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low permeability of the soils is illustrated by the fact that the on-site ponds retain water all year long,
based upon our review of historic aerial photographs. We have concluded that limited stormwater
infiltration as defined by the 2009 Manual may be feasible, but that the long-term infiltration rate that
should be applied to the site soils will be relatively low.
Preliminary Infiltration Rate Determination
Our scope of services did not include field infiltration testing as required by the 2009 Manual as part of
designing an infiltration system. However, conclusions regarding stormwater infiltration can be drawn
from subsurface conditions disclosed by the subsurface explorations and laboratory testing completed to
date. For reference, per the USDA textural classification method allowed under earlier versions of some
stormwater manuals, the glacial till soils at the site are considered sandy loam and would under other
circumstances be assigned a short-term infiltration rate on the order of 1 inch per hour and a long-term
rate of 0.25 inches per hour using a previously recommended reduction factor of 4. A similar rate is
obtained when correlating ASTM grain size distribution test data to observed infiltration system
performance. However, these rates would be applicable to normally consolidated soils and do not
consider the influence of soil density. The unweathered soils observed at the test pit locations are
glacially consolidated, dense, and would be less permeable than the overlying weathered soils. Based
upon our experience with other projects of a similar nature, we would recommend applying a reduction
factor of at least 10 to the 1 inch per hour short-term rate to achieve a long-term infiltration rate of no
greater than 0.1 inches per hour. Additional reductions of this rate applicable to the type of infiltration
system that reflect the potential for lack of future maintenance and soil clogging used should be applied
as well.
Groundwater Considerations
Groundwater was not observed at the test pit locations. However, the possibility exists that a seasonal
perched groundwater condition may develop above the unweathered glacial till due to the relatively high
density and high fines content of the soils. This condition may adversely affect the performance of
systems that rely on infiltration, if on a limited basis.
General Stormwater Infiltration Considerations
One LID stormwater management technique that may be applicable to the site is the use of permeable
pavements/hardscape surfaces. Given the low permeability of the native site soils, little infiltration of
water passing through permeable surfaces should be expected. Consequently, it would likely be necessary
to include a section of permeable crushed reservoir rock below the pavement, and this would increase
the overall pavement section costs. The disposition of water in the reservoir rock section should be
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Proposed Mixed-use Development – DRAFT
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Project No. 1536.01
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considered and measures taken to present water flowing out from the rock section at pavement edges
into areas where the water could be detrimental, such as immediately next to the buildings or along
property boundaries. Installing perforated collection pipes in the reservoir rock section may be required
as well. On sloping sites it may be necessary to include dams in the rock section in order to achieve the
necessary storage capacity. It should be recognized that a conventional permeable pavement section on
the site may not provide the necessary treatment for water originating form pollution generating surfaces
and that other means of treatment will be necessary.
The use of rain gardens in low permeability soils is increasingly common. Rain gardens typically include a
zone of amended soil to provide for treatment of water originating from pollution generating surfaces, as
well as a collection zone below consisting of clean washed rock that allows for conveyance of treated
water to an appropriate discharge point. We recommend including overflow pipes in raingardens as well.
CLOSURE
The analysis and recommendations presented in this report are based, in part, on the explorations
completed for this study, review of referenced documents, and laboratory testing results. We recommend
ZGA be provided an opportunity to review the plans and specifications as the project progresses in order
to assess that the recommendations and design considerations presented in this report have been
properly interpreted and implemented into the project design.
The performance of earthwork, structural fill, foundations, and pavements depend greatly on proper site
preparation and construction procedures. We recommend that ZGA be retained to provide geotechnical
engineering services during the earthwork-related construction phases of the project. If variations in
subsurface conditions are observed at that time, a qualified geotechnical engineer could provide
additional geotechnical recommendations to the contractor and design team in a timely manner as the
project construction progresses.
This report has been prepared for the exclusive use of FNW, Inc., and its agents, for specific application to
the project discussed and has been prepared in accordance with generally accepted geotechnical
engineering practices. No warranties, express or implied, are intended or made. Site safety, excavation
support, and dewatering requirements are the responsibility of others. In the event that changes in the
nature, design, or location of the project as outlined in this report are planned, the conclusions and
recommendations contained in this report shall not be considered valid unless ZGA reviews the changes
and either verifies or modifies the conclusions of this report in writing.
APPENDIX A
FIELD EXPLORATION PROCEDURES & LOGS
APPENDIX A
FIELD EXPLORATION PROCEDURES AND LOGS
Field Exploration Description
The field exploration included excavating 12 test pits (TP-1 through TP-12) at the approximate locations
shown on the Site and Exploration Plan, Figure 1. The locations of the test pits were determined by pacing
and taping from site features shown on a topographic site map, dated 21 September, prepared by Prizm
Surveying, Inc. Ground surface elevations at the test pit locations were interpolated from contours and
spot elevations on the referenced site plan. The locations and elevations of the explorations should be
considered as accurate as the methods used to determine them.
Test Pit Procedures
The test pits were excavated with a tracked excavator operated by an FNW, Inc. employee. A ZGA
engineering geologist observed the test pit excavations, logged the subsurface conditions, and obtained
representative soil samples. The samples were stored in moisture tight containers and transported to our
laboratory for further visual classification and testing.
The enclosed boring and test pit logs describe the vertical sequence of soils and materials encountered in
each exploration, based primarily upon our field classifications. Where a soil contact was observed to be
gradational, the logs indicate the average contact depth. Where a soil type changed between sample
intervals, the contact depth has been inferred. The logs also graphically indicate the blow count, sample
type, sample number, and approximate depth of each soil sample obtained from the boring. If
groundwater was encountered in a borehole, the approximate groundwater depth, and date of
observation, are depicted on the log.
Test Pit TP-1
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 243 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist to wet, dark brown, silty SAND, with
gravel, fine to medium roots throughout (Topsoil)
Medium dense to dense, moist, light orange to brown, silty
SAND, with gravel (Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND (Glacial Till)
Grades with scattered cobbles to 10-inch diameter
TP-1 completed at approximately 15 feet.
No groundwater seepage observed at time of excavation.
1 S-1 @ 0.5 ft. 17
2
S-2 @ 2 ft. 17
3
4
S-3 @ 4 ft. 14
5
6
7
8
9
S-4 @ 9 ft. 10
10
11
12
13
14 `
15 S-5 @ 14.5 ft.
7
Test Pit TP-2
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 244 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist to wet, brown, sandy SILT, with
gravel, fine to medium roots throughout (Topsoil)
Medium dense to dense, moist, light orange to brown, fine
sandy SILT, with gravel (Weathered Glacial Till)
Dense to very dense, moist, gray, silty gravelly SAND, trace
cobbles to 10-inch diameter (Glacial Till)
TP-2 completed at approximately 15 feet.
No groundwater seepage observed at time of excavation.
1 S-1 @ 0.5 ft. 9
2
S-2 @ 2 ft. 9
3
4
S-3 @ 4 ft. 10
5
6
7
S-4 @ 7 ft. 10
8
9
10
11
12
13
14 `
15 S-5 @ 14.5 ft.
6
Test Pit TP-3
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 239 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist, brown, silty SAND, with gravel, fine
roots throughout (Topsoil)
Medium dense, moist, gray, sandy SILT, with gravel, some
10-inch diameter quarry spalls (Fill)
Medium dense, moist, black, SILT, with sand, some gravel
(Probable Relic Topsoil)
Medium dense to dense, moist, light orange to brown, fine
sandy SILT, with gravel (Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND, trace cobbles
and boulders (Glacial Till)
TP-3 completed at approximately 10 feet.
No groundwater seepage observed at time of excavation.
1 S-1 @ 0.5 ft.
S-2 @ 1 ft.
2
3 S-3 @ 2.5 ft.
4
S-4 @ 4 ft. 6 GSA
5
6
7
8
9
10 S-5 @ 9.5 ft.
11
12
Test Pit TP-4
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 254 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist, brown, silty SAND, with gravel, fine
roots in upper 10 inches (Possible fill)
Medium dense, moist, light orange to brown, sandy SILT,
with gravel, trace cobbles (Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND, trace cobbles
and boulders to 1-foot diameter (Glacial Till)
TP-4 completed at approximately 8 feet.
No groundwater seepage observed at time of excavation.
1
S-1 @ 1 ft.
2
3 S-2 @ 2 ft.
4
S-3 @ 4 ft.
5
6
7
8 S-4 @ 8 ft.
9
10
11
12
Test Pit TP-5
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 251 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist, brown, sandy SILT, with gravel, fine
roots throughout (Topsoil)
Medium dense, moist, light orange to brown, silty gravelly
SAND (Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND, trace cobbles
and boulders (Glacial Till)
TP-5 completed at approximately 6 feet.
No groundwater seepage observed at time of excavation.
1 S-1 @ 0.5 ft.
S-2 @ 1 ft. 17 GSA
2
3
4 S-3 @ 3.5 ft.
5
6 S-4 @ 6 ft.
7
8
9
10
11
12
13
Test Pit TP-6
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 244 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist, brown, sandy SILT, with gravel, fine
roots throughout (Topsoil)
Medium dense, moist, brown, sandy SILT, with gravel, trace
fine roots (Possible Fill)
Medium dense, moist, light orange to brown, silty SAND,
with gravel (Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND, trace cobbles
and boulders (Glacial Till)
TP-6 completed at approximately 15 feet.
No groundwater seepage observed at time of excavation.
1
S-1 @ 1 ft. 7
2
3
S-2 @ 3 ft. 6
4
S-3 @ 4 ft. 6
5
6
7
8
S-4 @ 8 ft. 5
9
10
11
12
13
14
`
15 S-5 @ 15 ft. 8
Test Pit TP-7
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 257
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose to medium dense, moist, brown, sandy SILT
to silty SAND, with to some gravel (Strippings fill)
TP-7 completed at approximately 5.5 feet.
No groundwater seepage observed at time of excavation.
1
2
3
4
S-1 @ 4 ft.
5
6
7
8
9
10
11
12
Test Pit TP-8
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 253 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass over loose, moist, brown, sandy SILT, with gravel, fine
roots throughout (Topsoil)
Medium dense, moist, brown, sandy SILT, with gravel
(Possible Fill)
Medium dense, moist, light orange to brown, silty gravelly
SAND, trace cobbles (Weathered Glacial Till)
(3-inch diameter tree root at 3 feet)
Dense to very dense, moist, gray, silty gravelly SAND, trace
cobbles and boulders (Glacial Till)
TP-8 completed at approximately 6 feet.
No groundwater seepage observed at time of excavation.
1
S-1 @ 1 ft.
2
S-2 @ 2 ft. 11 GSA
3
4 S-3 @ 3.5 ft.
5
6 S-4 @ 6 ft.
7
8
9
10
11
12
13
Test Pit TP-9
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 260 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Ferns and grass over loose, moist, brown, sandy SILT, with
gravel, fine roots throughout (Topsoil)
Medium dense, moist, light orange to brown, silty gravelly
SAND, trace decaying tree roots
(Weathered Glacial Till)
Dense to very dense, moist, gray, silty gravelly SAND, trace
cobbles (Glacial Till)
TP-9 completed at approximately 6 feet.
No groundwater seepage observed at time of excavation.
1 S-1 @ 0.5 ft.
S-2 @ 1 ft.
2
3 S-3 @ 2.5 ft.
4
5
6 S-4 @ 6 ft.
7
8
9
10
11
12
Test Pit TP-10
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 232 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass and brush over loose, moist, brown, sandy SILT, with
gravel, fine roots throughout (Topsoil)
Medium dense, moist, light orange to brown, gravelly sandy
SILT, trace decaying tree limb
(Possible Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND (Glacial Till)
Very dense, moist, gray, silty SAND, with gravel, trace
cobbles and boulders (Glacial Till)
TP-10 completed at approximately 15 feet.
No groundwater seepage observed at time of excavation.
1 S-1 @ 0.5 ft. 12
S-2 @ 1 ft. 15
2
3
S-3 @ 3 ft. 7 GSA
4
5
6
S-4 @ 6 ft. 6
7
8
9
10
11
12
13
14 `
15 S-5 @ 15 ft. 6
Test Pit TP-11
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 227 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass and brush over loose, moist, brown, sandy SILT, with
gravel, fine roots throughout (Topsoil)
Medium dense, moist, light gray, silty gravelly SAND
(Weathered Glacial Till)
Very dense, moist, dark gray, silty gravelly SAND, trace
cobbles and boulders (Glacial Till)
Very dense, moist, gray, silty SAND to sandy SILT, with
gravel, trace cobbles and boulders (Glacial Till)
TP-11 completed at approximately 15 feet.
No groundwater seepage observed at time of excavation.
S-1 @ 0.3 ft. 9
1
S-2 @ 1 ft. 7
2
3
4 S-3 @ 3.5 ft. 5
5
6
7
8
9
10
S-4 @ 10 ft. 6
11
12
13
14 `
15 S-5 @ 15 ft. 7
Test Pit TP-12
Location: See Site and Exploration Plan, Figure 1
Approx. Ground Surface Elevation: 235 feet
Project: Federal Way Mixed-use
Project No: 1536.01
Date Excavated: 11/12/15
Depth
(ft)
Material Description
Sample
NC
%M
Testing
Grass and brush over loose, moist, brown, sandy SILT, with
gravel, fine roots throughout (Topsoil)
Loose to medium dense, moist, gray, silty SAND, with gravel,
trace roots (Fill)
Medium dense, to soft, moist, dark brown to black, SILT,
with sand (Probably Relic Topsoil)
Medium dense to dense, moist, light orange to brown, sandy
SILT, with gravel (Weathered Glacial Till)
Very dense, moist, gray, silty gravelly SAND, trace cobbles
and boulders (Glacial Till)
TP-12 completed at approximately 15 feet.
No groundwater seepage observed at time of excavation.
S-1 @ 0.3 ft.
1
S-2 @ 1 ft.
2 S-3 @ 1.5 ft.
S-4 @ 2 ft.
3
S-5 @ 3 ft.
4
5
6
7
8
9
10
11
12
13
14 `
15 S-6 @ 15 ft.
APPENDIX B
LABORATORY TESTING PROCEDURES & RESULTS
APPENDIX B
LABORATORY TESTING PROCEDURES AND RESULTS
Descriptions of the types of tests performed on selected soil samples by TCI are given below.
Visual Classification
Samples recovered from the exploration locations were visually classified in the field during the
exploration program. Representative portions of the samples were carefully packaged in moisture tight
containers and transported to our laboratory where the field classifications were verified or modified as
required. Visual classification was generally done in accordance with ASTM D 2488. Visual soil
classification includes evaluation of color, relative moisture content, soil type based upon grain size, and
accessory soil types included in the sample. Soil classifications are presented on the exploration logs in
Appendix A.
Moisture Content Determinations
Moisture content determinations were performed on representative samples obtained from the
explorations in order to aid in identification and correlation of soil types. The determinations were made
in general accordance with the test procedures described in ASTM D 2216. The results are shown on the
exploration logs in Appendix A.
Grain Size Analysis
A grain size analysis indicates the range in diameter of soil particles included in a particular sample. Grain
size analyses were performed on representative samples in general accordance with ASTM D 422. The
results of the grain size determinations for the samples were used in classification of the soils, and are
presented in this appendix.
0
10
20
30
40
50
60
70
80
90
100
0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS
GRAIN SIZE ANALYSIS
Comments:
36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200
Coarse Medium Fine Silt ClayFineCoarse
COBBLESBOULDERS GRAVEL SAND FINE GRAINED
SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER
Project No.:PROJECT NAME:
Federal Way Mixed-UseDATE OF TESTING:
Exploration Sample Depth (feet)Moisture (%)Fines (%)Description
TP-3 5.0 6.4 Silty gravelly
SANDS-4 23.7
1536.01
11/25/2015
ASTM D 422Test Results Summary
Zipper Geo Associates, LLC
Geotechnical and Environmental Consultants
0
10
20
30
40
50
60
70
80
90
100
0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS
GRAIN SIZE ANALYSIS
Comments:
36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200
Coarse Medium Fine Silt ClayFineCoarse
COBBLESBOULDERS GRAVEL SAND FINE GRAINED
SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER
Project No.:PROJECT NAME:
Federal Way Mixed-UseDATE OF TESTING:
Exploration Sample Depth (feet)Moisture (%)Fines (%)Description
TP-5 1.0 16.9 Silty gravelly
SANDS-2 22.8
1536.01
11/25/2015
ASTM D 422Test Results Summary
Zipper Geo Associates, LLC
Geotechnical and Environmental Consultants
0
10
20
30
40
50
60
70
80
90
100
0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS
GRAIN SIZE ANALYSIS
Comments:
36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200
Coarse Medium Fine Silt ClayFineCoarse
COBBLESBOULDERS GRAVEL SAND FINE GRAINED
SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER
Project No.:PROJECT NAME:
Federal Way Mixed-UseDATE OF TESTING:
Exploration Sample Depth (feet)Moisture (%)Fines (%)Description
TP-8 2.0 10.6 Silty gravelly
SAND S-2 18.3
1536.01
11/25/2015
ASTM D 422Test Results Summary
Zipper Geo Associates, LLC
Geotechnical and Environmental Consultants
0
10
20
30
40
50
60
70
80
90
100
0.0010.0100.1001.00010.000100.0001000.000PERCENT FINER BY WEIGHTPARTICLE SIZE IN MILLIMETERS
GRAIN SIZE ANALYSIS
Comments:
36"12"6"3"1 1/2"3/4"3/8"4 10 20 40 60 140 200
Coarse Medium Fine Silt ClayFineCoarse
COBBLESBOULDERS GRAVEL SAND FINE GRAINED
SIZE OF OPENING IN INCHES U.S. STANDARD SIEVE SIZE HYDROMETER
Project No.:PROJECT NAME:
Federal Way Mixed-UseDATE OF TESTING:
Exploration Sample Depth (feet)Moisture (%)Fines (%)Description
TP-10 3.0 7.4 Silty gravelly
SAND S-3 20.6
1536.01
11/25/2015
ASTM D 422Test Results Summary
Zipper Geo Associates, LLC
Geotechnical and Environmental Consultants
