22-103898-Geo Report-9-6-22Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
www.cobaltgeo.com (206) 331-1097
February 16, 2022
Brian Le
brian@beehivefoodpark.com
RE: Geotechnical Evaluation
Proposed Parking Lot Expansion and Surface Structures
30600-30640 Pacific Highway South
Federal Way, Washington
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss the results of our geotechnical evaluation at the referenced site.
The purpose of our evaluation was to provide recommendations for stormwater management and
pavements.
Site Description
The site is located at 30600 Pacific Highway South in Federal Way, Washington. The site consists
of one irregularly shaped parcel (No. 0921049121) with a total area of 33,909 square feet.
The northern portion of the site is a paved parking lot for the commercial building located at
30640 Pacific Highway South. The southern portion of the site is a large nearly level area
vegetated with grasses.
The site is bordered to the north, east, and south by commercial properties and to the west by
Pacific Highway South.
The proposed development includes expanded parking lot areas in the current grass areas.
Surface materials and equipment will likely be placed on existing and new pavements. We
anticipate that these structures are temporary and do not require foundation systems. Please
notify us if foundation systems are proposed.
Stormwater management may include permeable pavements or other systems depending on
feasibility. Site grading may include cuts and fills of 2 feet or less. We should be provided with
the final plans to verify that our recommendations remain valid and do not require updating.
Area Geology
The Geologic Map of King County, indicates that the site is underlain by Vashon Glacial Till.
Vashon Glacial Till includes dense mixtures of silt, sand, gravel, and clay. These deposits are
typically impermeable below a weathered zone. The Vashon Recessional Outwash includes fine to
medium grained sand with gravel. These deposits are typically permeable.
Soil & Groundwater Conditions
As part of our evaluation, we excavated two test pits where accessible.
The explorations encountered approximately 6 inches of grass and topsoil underlain by
approximately 6 to 6.5 feet of loose to medium dense, silty-fine to medium grained sand with
February 16, 2022 Page 2 of 8 Geotechnical Evaluation
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gravel (Fill). These materials were underlain by medium dense, silty-fine to medium grained
sand with gravel (Weathered Glacial Till), which continued to the termination depths of the
explorations.
Groundwater was not encountered in the explorations during our work. We do not anticipate
significant volumes of groundwater to be present at this site.
Water table elevations often fluctuate over time. The groundwater level will depend on a variety
of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and
soil permeability. Water levels at the time of the field investigation may be different from those
encountered during the construction phase of the project. It would be necessary to install a
piezometer to determine groundwater depths over a typical year.
Conclusions and Recommendations
General
The site is underlain by thick areas of fill and at depth by Vashon Glacial Till. The proposed
construction consists of new parking lot areas and surface structures with no permanent
foundations. In general, surface pavements and temporary/moveable materials do not require
full removal of undocumented fill. The fill at the site was relatively dense and suitable to support
surface materials. If any poured in place foundations are proposed, please notify us as pin piles
would be required for foundation support.
Widespread infiltration is not feasible due to the thick zones of undocumented fill. Permeable
pavements for parking stalls may be considered; however, overexcavation of fill with
implementation of a stabilizing geotextile will be required. The fill soils have variable
composition and density and may degrade over time when saturated.
Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic-rich
soil and fill. Based on observations from the site investigation program, it is anticipated that the
stripping depth will be 6 to 18 inches. Note that the site is underlain by thick areas of fill that
underlie the topsoil and grasses.
The fill and native soils consist of silty-sand with gravel. Most of the native soils may be used as
structural fill provided they achieve compaction requirements and are within 3 percent of the
optimum moisture. Some of these soils may only be suitable for use as fill during the summer
months, as they will be above the optimum moisture levels in their current state. These soils are
variably moisture sensitive and may degrade during periods of wet weather and under equipment
traffic. The on site fill should not be used as structural fill. Some areas may be suitable for re-use;
however, this would likely only be possible during the summer months.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of
3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve).
Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted
to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the
ASTM D 1557 test method.
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Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts
on the order of approximately 3 feet or less for parking lot construction and most of the utility
placement. Temporary excavations should be sloped no steeper than 1.5H:1V
(Horizontal:Vertical) in loose native soils and fill and 1H:1V in medium dense native soils. If an
excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations
be sloped no steeper than 2H:1V, where room permits.
Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part
N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a
qualified person during construction activities and the inspections should be documented in daily
reports. The contractor is responsible for maintaining the stability of the temporary cut slopes
and reducing slope erosion during construction.
Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather,
and the slopes should be closely monitored until the permanent retaining systems or slope
configurations are complete. Materials should not be stored or equipment operated within 10 feet
of the top of any temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In the case of
temporary cuts, the existing soil conditions may not be completely revealed until the excavation
work exposes the soil. Typically, as excavation work progresses the maximum inclination of
temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental
recommendations can be made. Soil and groundwater conditions can be highly variable.
Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that
the project can proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, we should be
notified so that supplemental recommendations can be made. If room constraints or
groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed
by the WAC, temporary shoring systems may be required. The contractor should be responsible
for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences
and the project structural engineer review temporary shoring designs prior to installation, to
verify the suitability of the proposed systems.
Stormwater Management Feasibility
The site is underlain by thick fill and at depth by glacial till. We performed a small scale pilot
infiltration test (PIT) in TP-1 in the native soils below the fill. The test was performed in general
accordance with the Washington State Department of Ecology stormwater manual.
The area was excavated to a testing depth of approximately 7 feet below the ground surface. The
design infiltration rate was determined by applying correction factors to the measured infiltration
rate as prescribed in Volume III, Section 3.3.6 of the DOE. The measured rate must be reduced
through appropriate correction factors for site variability (CFV), uncertainty of test method (CFT),
and degree of influent control (CFM) to prevent siltation and bio-buildup.
It should be noted that construction traffic or other disturbance to the target infiltration area
could compact the soil, which may decrease the effective infiltration rates. The correction factors
and resulting design infiltration rate are also shown in the table below.
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Test
Number
Test
Depth (ft)
Measured
Infiltration
Rate (in/hr)
Correction Factors Design
Infiltration
Rate
(in/hr) CFV CFT CFM
TP-1 7.0 0.8 0.7 0.5 0.9 0.25
Due to the presence of thick areas of fill, widespread infiltration is not feasible or recommended.
The near surface fills and underlying native soils are consistent with glacial till (as fill or in situ
native soils). These soils have limited capacity for infiltration. Note that most of the fill would
have an infiltration rate similar to the rate above.
We estimate that permeable pavements could be considered; however, there are several issues to
consider with these systems. Since the site is underlain by significant depths of variable-
composition fill, permeable pavements would have to be in low loading areas only (parking stalls).
The subgrade would require a base rock thickness of at least 24 inches placed on a geotextile to
maintain some level of stability of the underlying fills. Specifically, we recommend a minimum 12
inch thickness of 1-1/4 inch clean angular rock placed over Tensar TX160 geogrid on the prepared
subgrade. An additional 12 inches of 5/8 inch clean angular rock should be placed over the 1-1/4
inch rock. A minimum of 5 inches of permeable asphalt or concrete may be placed over the rock
section. We should verify soil stability and conditions prior to rock placement.
Interceptor drains may be necessary to convey excess runoff to City stormwater infrastructure
depending on the location of the permeable pavement areas relative to other site features and off-
site materials/buildings. We can provide specific drain details once a site plan with sections has
been prepared.
We should be provided with final plans for review to determine if the intent of our
recommendations has been incorporated or if additional modifications are needed.
Erosion and Sediment Control
Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to
wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment
control measures should be implemented, and these measures should be in general accordance
with local regulations. At a minimum, the following basic recommendations should be
incorporated into the design of the erosion and sediment control features for the site:
Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance
of the site soils, to take place during the dry season (generally May through September).
However, provided precautions are taken using Best Management Practices (BMP’s), grading
activities can be completed during the wet season (generally October through April).
All site work should be completed and stabilized as quickly as possible.
Additional perimeter erosion and sediment control features may be required to reduce the
possibility of sediment entering the surface water. This may include additional silt fences, silt
fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration
systems.
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Any runoff generated by dewatering discharge should be treated through construction of a
sediment trap if there is sufficient space. If space is limited other filtration methods will need
to be incorporated.
Utilities
Utility trenches should be excavated according to accepted engineering practices following OSHA
(Occupational Safety and Health Administration) standards, by a contractor experienced in such
work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent
to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be
avoided. Depending upon the location and depth of some utility trenches, groundwater flow into
open excavations could be experienced, especially during or shortly following periods of
precipitation.
In general, silty soils were encountered at shallow depths in the explorations at this site. These
soils have low cohesion and density and will have a tendency to cave or slough in excavations.
Shoring or sloping back trench sidewalls is required within these soils in excavations greater than
4 feet deep.
All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility
trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at
least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5
feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent
of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench
backfill in pavement areas should be compacted to at least 90 percent of the maximum dry
density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe
manufacturer's recommendations.
The contractor is responsible for removing all water-sensitive soils from the trenches regardless of
the backfill location and compaction requirements. Depending on the depth and location of the
proposed utilities, we anticipate the need to re-compact existing fill soils below the utility
structures and pipes. The contractor should use appropriate equipment and methods to avoid
damage to the utilities and/or structures during fill placement and compaction procedures.
Pavements
The near surface subgrade soils generally consist of silty sand with gravel. These soils are rated as
fair for pavement subgrade material (depending on silt content and moisture conditions). We
estimate that the subgrade will have a California Bearing Ratio (CBR) value of 8 and a modulus of
subgrade reaction value of k = 180 pci, provided the subgrade is prepared in general accordance
with our recommendations.
We recommend that at a minimum, 18 inches of the existing subgrade material be moisture
conditioned (as necessary) and re-compacted to prepare for the construction of pavement
sections. Deeper levels of recompaction or overexcavation and replacement may be necessary in
areas where fill and/or very poor (soft/loose) soils are present. Note that the soils at the site
mostly consist of fill.
The subgrade should be compacted to at least 95 percent of the maximum dry density as
determined by ASTM Test Method D1557. In place density tests should be performed to verify
proper moisture content and adequate compaction.
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The recommended flexible and rigid pavement sections are based on design CBR and modulus of
subgrade reaction (k) values that are achieved, only following proper subgrade preparation. It
should be noted that subgrade soils that have relatively high silt contents will likely be highly
sensitive to moisture conditions. The subgrade strength and performance characteristics of a silty
subgrade material may be dramatically reduced if this material becomes wet.
Based on our knowledge of the proposed project, we expect the traffic to range from light duty
(passenger automobiles) to heavy duty (delivery trucks). The following tables show the
recommended pavement sections for light duty and heavy duty use.
ASPHALTIC CONCRETE (FLEXIBLE) PAVEMENT
LIGHT DUTY
Asphaltic Concrete Aggregate Base* Compacted Subgrade* **
2.5 in. 6.0 in. 18.0 in.
HEAVY DUTY
Asphaltic Concrete Aggregate Base* Compacted Subgrade* **
3.5 in. 6.0 in. 18.0 in.
PORTLAND CEMENT CONCRETE (RIGID) PAVEMENT
Min. PCC Depth Aggregate Base* Compacted Subgrade* **
6.0 in. 6.0 in. 18.0 in.
* 95% compaction based on ASTM Test Method D1557
** A proof roll may be performed in lieu of in place density tests
The asphaltic concrete depth in the flexible pavement tables should be a surface course type
asphalt, such as Washington Department of Transportation (WSDOT) ½ inch HMA. The rigid
pavement design is based on a Portland Cement Concrete (PCC) mix that has a 28 day
compressive strength of 4,000 pounds per square inch (psi). The design is also based on a
concrete flexural strength or modulus of rupture of 550 psi.
CONSTRUCTION FIELD REVIEWS
Cobalt Geosciences should be retained to provide part time field review during construction in
order to verify that the soil conditions encountered are consistent with our design assumptions
and that the intent of our recommendations is being met. This will require field and engineering
review to:
Monitor and test structural fill placement and soil compaction
Verify soil conditions for pavements and permeable pavements or other infiltration
systems
February 16, 2022 Page 7 of 8 Geotechnical Evaluation
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Observe excavation stability
Geotechnical design services should also be anticipated during the subsequent final design phase
to support the structural design and address specific issues arising during this phase. Field and
engineering review services will also be required during the construction phase in order to
provide a Final Letter for the project.
CLOSURE
This report was prepared for the exclusive use of Brian Le and his appointed consultants. Any use
of this report or the material contained herein by third parties, or for other than the intended
purpose, should first be approved in writing by Cobalt Geosciences, LLC.
The recommendations contained in this report are based on assumed continuity of soils with
those of our test holes and assumed structural loads. Cobalt Geosciences should be provided with
final architectural and civil drawings when they become available in order that we may review our
design recommendations and advise of any revisions, if necessary.
Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is
the responsibility of Brian Le who is identified as “the Client” within the Statement of General
Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of
these not be satisfied.
Sincerely,
Cobalt Geosciences, LLC
2/16/2022
Phil Haberman, PE, LG, LEG
Principal
February 16, 2022 Page 8 of 8 Geotechnical Evaluation
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Statement of General Conditions
USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its
agent and may not be used by any third party without the express written consent of Cobalt
Geosciences and the Client. Any use which a third party makes of this report is the responsibility
of such third party.
BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this
report are in accordance with Cobalt Geosciences present understanding of the site specific
project as described by the Client. The applicability of these is restricted to the site conditions
encountered at the time of the investigation or study. If the proposed site specific project differs
or is modified from what is described in this report or if the site conditions are altered, this report
is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the
report to reflect the differing or modified project specifics and/or the altered site conditions.
STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in
accordance with the normally accepted standard of care in the state of execution for the specific
professional service provided to the Client. No other warranty is made.
INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and
statements regarding their condition, made in this report are based on site conditions
encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or
sampling locations. Classifications and statements of condition have been made in accordance
with normally accepted practices which are judgmental in nature; no specific description should
be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in
situ conditions can only be made to some limited extent beyond the sampling or test points. The
extent depends on variability of the soil, rock and groundwater conditions as influenced by
geological processes, construction activity, and site use.
VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be
encountered that are different from those described in this report or encountered at the test
locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected
conditions are substantial and if reassessments of the report conclusions or recommendations are
required. Cobalt Geosciences will not be responsible to any party for damages incurred as a result
of failing to notify Cobalt Geosciences that differing site or sub-surface conditions are present
upon becoming aware of such conditions.
PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and
specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next
project stage (property acquisition, tender, construction, etc), to confirm that this report
completely addresses the elaborated project specifics and that the contents of this report have
been properly interpreted. Specialty quality assurance services (field observations and testing)
during construction are a necessary part of the evaluation of sub-subsurface conditions and site
preparation works. Site work relating to the recommendations included in this report should only
be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be
responsible for site work carried out without being present.
Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
TOPO MAP
FIGURE 1
N
Proposed Parking Areas
30600-30640 Pac. Highway S.
Federal Way, Washington
Subject Property
TP-1
PT
Well-graded gravels, gravels, gravel-sand mixtures, little or no fines
Poorly graded gravels, gravel-sand mixtures, little or no fines
Silty gravels, gravel-sand-silt mixtures
Clayey gravels, gravel-sand-clay mixtures
Well-graded sands, gravelly sands, little or no fines
COARSE
GRAINED
SOILS
(more than 50%
retained on
No. 200 sieve)
Primarily organic matter, dark in color,
and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427)HIGHLY ORGANIC
SOILS
FINE GRAINED
SOILS
(50% or more
passes the
No. 200 sieve)
MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION
Gravels
(more than 50%
of coarse fraction
retained on No. 4
sieve)
Sands
(50% or more
of coarse fraction
passes the No. 4
sieve)
Silts and Clays(liquid limit lessthan 50)
Silts and Clays
(liquid limit 50 or
more)
Organic
Inorganic
Organic
Inorganic
Sands with
Fines(more than 12%fines)
Clean Sands
(less than 5%fines)
Gravels with
Fines
(more than 12%
fines)
Clean Gravels
(less than 5%
fines)
Unified Soil Classification System (USCS)
Poorly graded sand, gravelly sands, little or no fines
Silty sands, sand-silt mixtures
Clayey sands, sand-clay mixtures
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
or clayey silts with slight plasticity
Inorganic clays of low to medium plasticity, gravelly clays, sandy clays,
silty clays, lean clays
Organic silts and organic silty clays of low plasticity
Inorganic silts, micaceous or diatomaceous fine sands or silty soils,
elastic silt
Inorganic clays of medium to high plasticity, sandy fat clay,
or gravelly fat clay
Organic clays of medium to high plasticity, organic silts
Moisture Content Definitions
Grain Size Definitions
Dry Absence of moisture, dusty, dry to the touch
Moist Damp but no visible water
Wet Visible free water, from below water table
Grain Size Definitions
Description Sieve Number and/or Size
Fines <#200 (0.08 mm)
Sand
-Fine
-Medium
-Coarse
Gravel
-Fine
-Coarse
Cobbles
Boulders
#200 to #40 (0.08 to 0.4 mm)
#40 to #10 (0.4 to 2 mm)
#10 to #4 (2 to 5 mm)
#4 to 3/4 inch (5 to 19 mm)
3/4 to 3 inches (19 to 76 mm)
3 to 12 inches (75 to 305 mm)
>12 inches (305 mm)
Classification of Soil Constituents
MAJOR constituents compose more than 50 percent,
by weight, of the soil. Major constituents are capitalized
(i.e., SAND).
Minor constituents compose 12 to 50 percent of the soil
and precede the major constituents (i.e., silty SAND).
Minor constituents preceded by “slightly” compose
5 to 12 percent of the soil (i.e., slightly silty SAND).
Trace constituents compose 0 to 5 percent of the soil(i.e., slightly silty SAND, trace gravel).
Relative Density Consistency
(Coarse Grained Soils) (Fine Grained Soils)
N, SPT, Relative
Blows/FT Density
0 - 4 Very loose
4 - 10 Loose
10 - 30 Medium dense
30 - 50 Dense
Over 50 Very dense
N, SPT, Relative
Blows/FT Consistency
Under 2 Very soft
2 - 4 Soft4 - 8 Medium stiff8 - 15 Stiff15 - 30 Very stiff
Over 30 Hard
Cobalt Geosciences, LLCP.O. Box 82243Kenmore, WA 98028(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Soil Classification Chart Figure C1
Proposed Parking Lot
30600-30640 Pac. Highway S.
Federal Way, Washington
Test Pit
Logs
Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-1
Date: February 2022
Contractor: Jim
Depth: 10’
Elevation: N/A Logged By: PH Checked By: SC
Groundwater: None
Material Description
Moisture Content (%)PlasticLimit Liquid
Limit
10 20 30 400 50
1
2
3
4
5
6
DCP Equivalent N-Value
7
8
9
10
SM
End of Test Pit 10’
Test Pit TP-2
Date: February 2022
Contractor: Jim
Depth: 8’
Elevation: N/A Logged By: PH Checked By: SC
Groundwater: None
Material Description
Moisture Content (%)Plastic
Limit Liquid Limit
10 20 30 400 50
1
2
3
4
5
6
DCP Equivalent N-Value
7
8
9
10
Medium dense, silty-fine to fine grained sand with gravel,
yellowish brown to grayish brown, moist.
(Weathered Glacial Till)
SM
Grass and Topsoil
SM
End of Test Pit 10’
Grass and Topsoil
SM
Medium dense becoming loose, silty-fine to medium grained sand with gravel, dark yellowish brown, moist.
(Fill)
Medium dense becoming loose, silty-fine to medium grained sand
with gravel local debris, dark yellowish brown, moist. (Fill)
Medium dense, silty-fine to medium grained sand with gravel,
reddish brown to yellowish brown, moist.
(Weathered Glacial Till)