24-100334-Geotech Report-01.31.24Cobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
www.cobaltgeo.com (206) 331-1097
November 9, 2023
Muhammad Saad Mohyuddin
saadmohi@googlemail.com
RE: Geotechnical Evaluation
Proposed Residence
1013 S. 295th Place
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 1013 S. 295th Place in Federal Way, Washington. The site consists of one
nearly rectangular shaped parcel (No. 5151800035) with a total area of about 23,753 square feet.
The northern portion of the property is developed with a single family residence with partial
basement and driveway. The remainder of the property is undeveloped and vegetated with
grasses, bushes, ferns, ivy, and spares trees.
The developed portion of the site is nearly level to slightly sloping downward to the northwest and
southwest with low relief. The southern half of the site slopes downward to the north and south
into a shallow ravine-like feature. Slope areas have been previously graded and have magnitudes
of about 15 to 50 percent and local relief of 10 feet within the property. There is a taller slope that
is locally faced with walls south of the subject parcel sloping downward to the north into the
ravine. The ravine is very shallow and does not have evidence of seasonal streams or surface
waters.
The property is bordered to the east, west, and south by residential properties and to the north by
S. 295th Place.
The project includes subdivision of the parcel and construction of a residence in the southern half
of the site. A driveway will extend to the area along the west property line. Figure 1 shows the
current conditions and planned layout.
Grading could include cuts of 8 feet or less if a daylight basement is utilized, and foundation loads
will generally be light. Stormwater management may include dispersion, detention, or infiltration
facilities depending on feasibility. We should be provided with the plans when they become
available so that we may determine if additional recommendations are warranted.
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Area Geology
The Geologic Map of the Poverty Bay Quadrangle, indicates that the site is underlain by Vashon
Glacial Till and near the contacts with Vashon Advance Outwash.
Vashon Glacial Till consists of dense mixtures of silt, sand, clay, and gravel. These deposits are
typically impermeable below a weathered zone. Vashon Advance Outwash includes fine to
medium grained sand trace gravel with local interbeds of silt and clay. These deposits become
denser below a weathered zone.
The till and outwash can locally be overlain by Vashon Recessional Outwash. These deposits
include normally consolidated sand and gravel with lesser silt and clay. These deposits can be
highly permeable.
Soil & Groundwater Conditions
As part of our evaluation, we excavated two test pits within the property to determine the shallow
soil and groundwater conditions, where accessible.
The test pits encountered about 6 inches of topsoil and grass underlain by approximately 2.5 to 3
feet of loose to medium dense, silty-fine to fine grained sand trace to with gravel (Weathered
Recessional Outwash). This layer was underlain by medium to locally dense, fine to medium
grained sand with gravel trace cobbles (Recessional Outwash?), which continued to the
termination depths of the explorations.
Groundwater was not observed in the test pits during our evaluation. Based on the soil
conditions, it appears that perched groundwater would likely be 8 or more feet below grade
during later winter and spring months.
A review of available well logs (from the DOE well log site) within 2,000 feet of the site indicates
groundwater at more than 77 feet below grade.
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.
Steep Slope Hazard
Most 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).
The developed portion of the site is nearly level to slightly sloping downward to the northwest and
southwest with low relief. The southern half of the site slopes downward to the north and south
into a shallow ravine-like feature. Slope areas have been previously graded and have magnitudes
of about 15 to 50 percent and local relief of 10 feet within the property.
There is a taller slope that is locally faced with walls south of the subject parcel sloping downward
to the north into the ravine. The ravine is very shallow and does not have evidence of seasonal
streams or surface waters. Slope areas to the south are well vegetated.
It is our opinion that the modified and natural slopes with magnitudes over 40 percent within and
near the property are stable at this time. We did not observe evidence of erosion, landslide
activity or soil creep. These areas are not consistent with landslide hazard areas.
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Provided all stormwater runoff during construction is fully managed, and stormwater is
infiltrated or dispersed in areas with magnitudes of about 20 percent or more, the proposed
development should not increase the risk of soil movements on the subject property and adjacent
areas.
Any new foundation systems should be embedded an adequate depth in order to create a
minimum 7 foot effective setback from adjacent slope systems (downslope areas) where slope
magnitudes are 30 percent or more. This is the horizontal distance from the lower outside face of
the footing to the face of the adjacent slope.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Arents, Alderwood material (6 to 15 percent slopes). These soils would have a
slight to moderate erosion potential in a disturbed state depending on the slope magnitude.
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
medium dense to very 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 7-16)
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 1.343 0.461 1.0 Null 0.895 Null 0.568
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. “Null” indicates see Section
11.4.8 of the ASCE.
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Conclusions and Recommendations
General
The site is underlain by sand and gravel generally consistent with Vashon Recessional Outwash.
The proposed residence may be supported on a shallow foundation system bearing on medium
dense or firmer native soils or on structural fill placed on the native soils. Local overexcavation
or recompaction of loose weathered native soils may be necessary depending on the proposed
elevations and location of the new residence.
Any new foundation systems should be embedded an adequate depth in order to create a
minimum 7 foot effective setback from adjacent slope systems (downslope areas) where slope
magnitudes are 30 percent or more. This is the horizontal distance from the lower outside face of
the footing to the face of the adjacent slope.
Infiltration is generally feasible in the soils consistent with the coarser outwash soils at depth.
Trenches or drywells should be located in areas with slope magnitudes of 20 percent or less.
Dispersion devices and permeable pavements are also generally feasible depending on their
elevations and locations. We can provide additional recommendations once a site plan has been
prepared.
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 12 inches. Deeper excavations will be necessary below large trees
where root systems can extend to greater depths, in areas of existing foundation systems, and in
any areas underlain by undocumented fill.
The native soils consist of silty-sand with gravel and poorly graded sand/gravel with silt and sand.
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.
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.
Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts
on the order of approximately 8 feet or less for foundation and utility placement. Any deeper
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.
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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.
Foundation Design
The proposed residence may be supported on a shallow spread footing foundation system bearing
on undisturbed medium 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.
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 1,500
pounds per square foot (psf) may be used for design.
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
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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
0.40 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
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 proposed in yard areas.
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)
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)
0.40
*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
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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.
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 18 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). Any fill
should be fully removed and replaced with imported fill.
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 150 pounds per cubic
inch (pci) assuming the slab-on-grade base course is underlain by structural fill placed and
compacted as outlined above. A minimum 4 inch thick capillary break should be placed over the
prepared subgrade. These materials should consist of 5/8 inch clean angular rock or pea gravel.
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 building.
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Stormwater Management Feasibility
The site is underlain by Vashon Recessional Outwash. Infiltration is suitable in these soil
deposits. Groundwater was not encountered in the explorations. Groundwater is likely more than
8 and possibly more than 50 feet below site elevations based on a review of nearby boring logs.
Because the recessional deposits have not been overridden by glacial ice, this soil unit is
considered normally-consolidated. The Washington State Department of Ecology Stormwater
Management Manual for Western Washington allows determination of infiltration rates of this
soil unit by Soil Particle Size Distribution testing. This method involves using a logarithmic
equation and grain size values along with correction factors for testing type, soil homogeneity,
and influent control.
The equation in conjunction with sieve analysis results yields design infiltration rates of between
2.5 to 3.5 inches per hour at depths of 3 to 6 feet below grade. These rates reflect application of
correction factors for variability (0.6 used), influent control (0.9), and testing analysis type (0.4).
We also performed an infiltration test in TP-1 at 4 feet below grade. Following saturation, testing,
and application of the correction factors noted above, the design rate was determined to be 2.5
inches per hour. We recommend using this rate for system design.
We note that the subsurface soils vary from Medium Sand to Coarse Sand/Gravel if sizing is
determined using the Textural Triangle information in the King County Surface Water Design
Manual (SWDM). We recommend using Medium Sand for any systems.
Infiltration systems should have a depth of at least 3 feet below existing grades and located at
least 10 feet apart. Any fine grained soils or interbeds of fine grained soils must be removed prior
to rock placement.
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. We should
observe the soil conditions in infiltration systems during construction to confirm soils are
consistent with our investigation.
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, sandy and gravelly 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.
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
Observe bearing capacity at foundation locations
Observe slab-on-grade preparation
Monitor subgrade preparation of roadways
Observe excavation stability
Verify soil conditions at any stormwater management systems
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.
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CLOSURE
This report was prepared for the exclusive use of Muhammad Saad Mohyuddin 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 Muhammad Saad Mohyuddin 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
11/9/2023
Phil Haberman, PE, LG, LEG
Principal
November 9, 2023
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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.
N
SITE PLAN
FIGURE 1
TP-1
132nd Street SE
Proposed Residence
295th Place 1013 S.
Federal Way, Washington
Cobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
(206) 331-1097
www.cobaltgeo.com
phil@cobaltgeo.com
Attachment
Cobalt Geosciences, LLC
PO Box 1792
North Bend, WA 98045
(206) 331-1097
www.cobaltgeo.com
phil@cobaltgeo.com
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 Residence
1013 S. 295th Place
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: October 2023
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
yellowish brown, . ( moist Weathered Recessional Outwash)
SM/
SP
End of Test Pit ’8
Medium dense to locally dense, fine to medium grained sand trace
gravel to gravel with sand, grayish brown, moist.
(Recessional Outwash?)
Local caving limits test pit depth
SP
Topsoil/Grass
Proposed Residence
1013 S. 295th Place
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: October 2023
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
yellowish brown, . (moist Weathered Recessional Outwash)
Local yard waste at ground surface
SM/
SP
End of Test Pit ’8
Medium dense to locally dense, fine to medium grained sand trace
gravel to gravel with sand, grayish brown, moist.
(Recessional Outwash?)
Local caving limits test pit depth
SP
Topsoil/Grass