21-102624_ Geotechnical (Soils) Report_06-29-2021_V1Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
www.cobaltgeo.com (206) 331-1097
June 9, 2021
Muhammad Saad Mohyuddin
saadmohi@googlemail.com
RE: Geotechnical Evaluation
Proposed Commercial Development
Parcel No. 0421049257
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 determine the feasibility of utilizing infiltration devices for
stormwater runoff management along with providing recommendations for foundation and
retaining wall design.
Site and Project Description
The site is located at 30200 Pacific Highway South in Federal Way, Washington. The site consists
of one rectangular shaped parcel (No. 0421049257) with a total area of about 185,708 square feet.
The property is undeveloped and is heavily vegetated with ferns, grasses, blackberry vines, ivy,
and variable diameter evergreen and deciduous trees. There is some evidence of historic grading
in the upper elevation areas.
The site slopes downward from west to east at variable magnitudes and total relief of about 55
feet. Most areas have slopes with magnitudes of 5 to 20 percent. There is an area with steeper
slopes (75 percent with relief of about 25 feet) in the northern half of the property. There is a 3 to
4 feet tall modular block wall along the east property line, near Pacific Highway South.
The property is bordered to the north and south by undeveloped land, to the west by 16th Avenue
South, and to the east by Pacific Highway South.
The project includes construction of multiple new commercial buildings, pavement areas, drive
lanes, utilities, and landscaped areas. The locations, elevations, and sizes of the buildings have
not been finalized. Foundation loads are expected to be light to moderate and site grading could
include cuts of several feet for new foundations up to 15 feet or more if mass grading (fills) is
proposed/required. Stormwater management may include dispersion, detention, or infiltration
facilities depending on feasibility. We should be provided with a site plan, grading plan, and
civil plans when they become available.
Area Geology
The Geologic Map of the Poverty Bay Quadrangle indicates that the site is underlain by Vashon
Glacial Till.
Vashon Glacial Till consists of dense mixtures of silt, sand, clay, and gravel. These deposits are
typically impermeable below a weathered zone.
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Soil & Groundwater Conditions
As part of our evaluation, we excavated three test pits within the property to determine the
shallow soil and groundwater conditions, where accessible.
The explorations encountered about 12 inches of topsoil and grass/vegetation underlain by
approximately 1.5 to 2 feet of loose to medium dense, silty-fine to fine grained sand trace to with
gravel (Weathered Glacial Till). This layer was underlain by dense to very dense, silty-fine to fine
grained sand with gravel trace cobbles (Glacial Till), which continued to the termination depths of
the explorations.
Groundwater was not encountered in the explorations; however, the shallow soils were mottled.
This indicates that groundwater could developed on the dense till during the wet season.
Steep Slope Hazards
Critical area ordinances designate slopes with magnitudes greater than about 40 percent and
vertical relief of at least 10 feet as potentially geologically hazardous (steep slope/landslide
hazards). Additional criteria include areas where landslide activity has taken place historically or
where there is evidence of slope movements. Slope areas underlain by permeable soils overlying
impermeable soils often exhibit landslide activity.
The site slopes downward from west to east at variable magnitudes and total relief of about 55
feet. Most areas have slopes with magnitudes of 5 to 20 percent. There is an area with steeper
slopes (75 percent with relief of about 25 feet) in the northern half of the property. There is a 3 to
4 feet tall modular block wall along the east property line, near Pacific Highway South.
During our field assessment, we traversed slope areas within the site, where accessible. We also
observed off site steep slopes from within the property. As we conducted the traverses, we looked
for any signs that would indicate past slope failures or features indicating possible future
instability.
Overall, the steep slope areas within the site appear stable at this time with no evidence of severe
erosion, exposed soils, hummocky terrain, or other signs of landslide activity. The geologic units
that underlie the slope system are generally dense and resistant to deep seated slide activity.
It is our opinion that the site does not contain landslide hazards and that local steep slopes can be
safely and effectively re-graded with cuts and fills to create a more stable topography for site
development. We should be provided with grading and development plans with elevations to
confirm suitability and to provide specific benching and slope requirements. Additional
foundation recommendations and potentially, setback recommendations may also be warranted
at that time.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Alderwood gravelly sandy loam (0 to 15 percent slopes). These soils would have a
slight to moderate potential in a disturbed state depending on the slope magnitude.
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It is our opinion that soil erosion potential at this project site can be reduced through landscaping
and surface water runoff control. Typically, erosion of exposed soils will be most noticeable
during periods of rainfall and may be controlled by the use of normal temporary erosion control
measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The
typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion
control measures should be in place before the onset of wet weather.
Seismic Parameters
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the
International Building Code (IBC). A Site Class D applies to an overall profile consisting of
stiff/medium dense soils within the upper 100 feet.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to
obtain values for SS, S1, Fa, and Fv. The USGS website includes the most updated published data
on seismic conditions. The following tables provide seismic parameters from the USGS web site
with referenced parameters from ASCE 7-10 and 7-16.
Seismic Design Parameters (ASCE 7-10)
Site
Class
Spectral
Acceleration
at 0.2 sec. (g)
Spectral
Acceleration
at 1.0 sec. (g)
Site
Coefficients
Design Spectral
Response Parameters
Design
PGA
Fa Fv SDS SD1
D 1.305 0.499 1.0 1.501 0.87 0.499 0.529
Seismic Design Parameters (ASCE 7-16)
Site
Class
Spectral
Acceleration
at 0.2 sec. (g)
Spectral
Acceleration
at 1.0 sec. (g)
Site
Coefficients
Design Spectral
Response Parameters
Design
PGA
Fa Fv SDS SD1
D 1.335 0.458 1.0 Null 0.89 Null 0.564
Additional seismic considerations include liquefaction potential and amplification of ground
motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a
high groundwater table. The site has a low likelihood of liquefaction.
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Conclusions and Recommendations
General
The site is underlain by variably thick forest duff along with weathered and unweathered Glacial
Till. The proposed structures may be supported on shallow foundation systems bearing on
medium dense or firmer native soils or on structural fill placed on the native soils. All
undocumented fill and any loose native soils must be removed below foundation elements
extending outward at a 1H:1V envelope to the base of the excavation.
Overall, the steeper site slopes are stable at this time. We anticipate that mass grading will
effectively remove the steep slope systems, thereby removing the need for building setbacks. We
can provide additional recommendations, including setbacks (if warranted), once a grading plan
has been prepared.
Infiltration is not feasible due to the likely presence of shallow perched groundwater during the
wet season along with a relatively shallow layer of very dense cemented till, which acts as an
aquitard. Preliminarily, we recommend using detention systems with overflow to City/County
infrastructure. Dispersion systems, rain gardens, ponds, permeable pavements, and other
shallow systems may be feasible depending on their location and elevations. We can provide
additional recommendations once civil plans become available.
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 24 inches. Deeper excavations will be necessary below large trees
where root systems can extend to greater depths and in any areas underlain by undocumented fill.
The on site soils consist of silty-sand with gravel. Some 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. These soils were very wet and will require drying during summer months if they will be
used as fill.
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.
If mass grading occurs and includes large cuts and fills, permanent fill slopes should have
maximum inclinations of 2H:1V and be constructed per the structural fill recommendations
above. Fill should be placed on level benches created in medium dense or firmer native soils.
These benches should be at least 5 feet wide with vertical cuts of 2 to 4 feet.
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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 foundation and utility placement. Deeper
excavations may be required as part of overall site mass grading.
Any deeper temporary excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical)
in loose native soils and fill, 1H:1V in medium dense native soils, and 3/4H:1V in dense to very
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 forest duff and at depth by weathered to unweathered glacial till. While
groundwater was not observed, the shallow soils are locally mottled, indicating that shallow
groundwater likely develops on the cemented till at shallow depths during the wet season.
We performed an infiltration test (PIT) in TP-2 at a depth of 3 feet below grade. Following testing
and application of correction factors for site variability (0.8), influent control (0.9) and testing
(0.5), the infiltration rate was 0.08 inches per hour. This is lower than what is considered to be
feasible.
Infiltration is not feasible due to the soil and anticipated shallow groundwater conditions.
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Depending on the planned grading, elevations, and building locations, other shallow systems
could be feasible. These could include permeable pavements, dispersion, detention, and rain
gardens. We anticipate that detention with overflow will be the most likely system required;
however, we can provide additional input once grading and civil plans have been prepared or are
in preparation.
We should be provided with final plans for review to determine if the intent of our
recommendations have been incorporated or if additional modifications are needed.
Foundation Design
The proposed commercial buildings may be supported on shallow spread footing foundation
systems bearing on undisturbed dense or firmer native soils or on properly compacted structural
fill placed on the suitable native soils. Any undocumented fill should be removed and replaced
with structural fill below foundation elements. Structural fill below footings should consist of
clean angular rock 5/8 to 2 inches in size. Bearing soils were generally encountered between 2
and 3.5 feet below grade.
For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively,
for continuous wall and isolated column footings supporting the proposed structure. Provided
that the footings are supported as recommended above, a net allowable bearing pressure of 3,000
pounds per square foot (psf) may be used for design. Detention vaults set at least 6 feet below
grade may be designed at 5,000 psf.
A 1/3 increase in the above value may be used for short duration loads, such as those imposed by
wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted
to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing
excavations should be inspected to verify that the foundations will bear on suitable material.
Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or
adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12
inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower.
If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch.
Differential settlement, along a 25-foot exterior wall footing, or between adjoining column
footings, should be less than ½ inch. This translates to an angular distortion of 0.002. Most
settlement is expected to occur during construction, as the loads are applied. However, additional
post-construction settlement may occur if the foundation soils are flooded or saturated. All
footing excavations should be observed by a qualified geotechnical consultant.
Resistance to lateral footing displacement can be determined using an allowable friction factor of
0.35 acting between the base of foundations and the supporting subgrades. Lateral resistance for
footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds
per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12
inches below grade in exterior areas). The frictional and passive resistance of the soil may be
combined without reduction in determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction.
Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the
footing excavations, should be removed prior to placing concrete. The potential for wetting or
drying of the bearing materials can be reduced by pouring concrete as soon as possible after
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completing the footing excavation and evaluating the bearing surface by the geotechnical engineer
or his representative.
Concrete Retaining Walls
The following table, titled Wall Design Criteria, presents the recommended soil related design
parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall
system is used. This has been included if any concrete walls are utilized for basements or vaults.
Wall Design Criteria
“At-rest” Conditions (Lateral Earth Pressure – EFD+) 55 pcf (Equivalent Fluid Density)
“Active” Conditions (Lateral Earth Pressure – EFD+) 35 pcf (Equivalent Fluid Density)
Seismic Increase for “At-rest” Conditions
(Lateral Earth Pressure)
21H* (Uniform Distribution) 1 in 2,500 year
event
Seismic Increase for “At-rest” Conditions
(Lateral Earth Pressure)
14H* (Uniform Distribution) 1 in 500 year event
Seismic Increase for “Active” Conditions
(Lateral Earth Pressure)
7H* (Uniform Distribution)
Passive Earth Pressure on Low Side of Wall
(Allowable, includes F.S. = 1.5)
Neglect upper 2 feet, then 250 pcf EFD+
Soil-Footing Coefficient of Sliding Friction (Allowable;
includes F.S. = 1.5)
0.35
*H is the height of the wall; Increase based on one in 500 year seismic event (10 percent probability of being exceeded in
50 years),
+EFD – Equivalent Fluid Density
The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by
water accumulation behind the retaining walls. Uniform horizontal lateral active and at-rest
pressures on the retaining walls from vertical surcharges behind the wall may be calculated using
active and at-rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight
of 125 pcf may be used to calculate vertical earth surcharges.
To reduce the potential for the buildup of water pressure against the walls, continuous footing
drains (with cleanouts) should be provided at the bases of the walls. The footing drains should
consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed
down and enveloped by a minimum 6 inches of pea gravel in all directions.
The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should
consist of free-draining granular material. All free draining backfill should contain less than 3
percent fines (passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S.
Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard
No. 4 Sieve. The primary purpose of the free-draining material is the reduction of hydrostatic
pressure. Some potential for the moisture to contact the back face of the wall may exist, even with
treatment, which may require that more extensive waterproofing be specified for walls, which
require interior moisture sensitive finishes.
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We recommend that the backfill be compacted to at least 90 percent of the maximum dry density
based on ASTM Test Method D1557. In place density tests should be performed to verify
adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently,
only light hand operated equipment is recommended within 3 feet of walls so that excessive stress
is not imposed on the walls.
Slab on Grade
We recommend that the upper 12 inches of the existing native soils within slab areas be re-
compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method).
Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor
barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture
typically requires the usage of a vapor barrier. A materials or structural engineer should be
consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs
typically do not utilize vapor barriers.
The American Concrete Institutes ACI 360R-06 Design of Slabs on Grade and ACI 302.1R-04
Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier
selection and floor slab detailing.
Slabs on grade may be designed using a coefficient of subgrade reaction of 180 pounds per cubic
inch (pci) assuming the slab-on-grade base course is underlain by structural fill placed and
compacted as outlined in Section 8.1. A minimum 4-inch thick layer of clean angular rock (5/8
inch) or pea gravel should be placed over the subgrade as a capillary break material.
A perimeter drainage system is recommended unless interior slab areas are elevated a minimum
of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should
consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain
rock wrapped in a non-woven geosynthetic filter fabric to reduce migration of soil particles into
the drainage system. The perimeter drainage system should discharge by gravity flow to a
suitable stormwater system.
Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate
surface water flow away from the building and preferably with a relatively impermeable surface
cover immediately adjacent to the buildings.
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.
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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.
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.
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.
Pavements
The near surface subgrade soils generally consist of silty sand with gravel. These soils are rated as
good 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 10 and a modulus
of subgrade reaction value of k = 200 pci, provided the subgrade is prepared in general
accordance with our recommendations.
We recommend that at a minimum, 12 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.
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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.
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. 12.0 in.
HEAVY DUTY
Asphaltic Concrete Aggregate Base* Compacted Subgrade* **
3.5 in. 6.0 in. 12.0 in.
PORTLAND CEMENT CONCRETE (RIGID) PAVEMENT
Min. PCC Depth Aggregate Base* Compacted Subgrade* **
6.0 in. 6.0 in. 12.0 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.
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Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions and should not
be used for any other site.
Sincerely,
Cobalt Geosciences, LLC
6/9/2021
Phil Haberman, PE, LG, LEG
Principal
PH/sc
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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, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
SITE PLAN
FIGURE 1
TP-1
N
Proposed Comm. Development
30200 Pacific Highway South
Federal Way, Washington
TP-1
TP-3
TP-2
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 less
than 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 Soft
4 - 8 Medium stiff
8 - 15 Stiff
15 - 30 Very stiff
Over 30 Hard
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Soil Classification Chart Figure C1
Proposed Comm. Development
30200 Pacific Highway S.
Federal Way, Washington
Test Pit
Logs
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-1
Date: June, 2021
Contractor: Jim
Depth: 8’
Elevation: 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
Loose to medium dense, silty-fine to medium grained sand with gravel
mottled tan to yellowish brown, moist. (Weathered Glacial Till) SM
End of Test Pit 8’
Dense to very dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till)SM
Forest Duff
Proposed Comm. Development
30200 Pacific Highway S.
Federal Way, Washington
Test Pit
Logs
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-2
Date: June, 2021
Contractor: Jim
Depth: 8’
Elevation: 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
Loose to medium dense, silty-fine to medium grained sand with gravel
mottled tan to yellowish brown, moist. (Weathered Glacial Till) SM
End of Test Pit 8’
Dense to very dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM
Forest Duff
Proposed Comm. Development
30200 Pacific Highway S.
Federal Way, Washington
Test Pit
Logs
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-3
Date: June, 2021
Contractor: Jim
Depth: 8’
Elevation: 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
Loose to medium dense, silty-fine to medium grained sand with gravel
mottled tan to yellowish brown, moist. (Weathered Glacial Till) SM
End of Test Pit 8’
Dense to very dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till)SM
Forest Duff