HomeMy WebLinkAboutRS_Cobalt_Geosciences_Geotech_Report_250821_v1Cobalt Geosciences, LLC
P.O. Box 1792
North Bend, WA 98045
www.cobaltgeo.com (206) 331-1097
April 10, 2025
Rupinder Dulay
rupinder@dulayhomes.com
RE: Geotechnical Evaluation
Proposed Development
18855 and 18845 102nd Avenue SE
Renton, Washington
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this report to discuss
the results of our geotechnical evaluation at the referenced site.
The purpose of our evaluation was to provide recommendations for foundation design, grading,
and earthwork.
Site & Project Description
The site is located at 18845 and 18855 102nd Avenue SE in Renton, Washington. The site consists
of two parcels (No. 3223059285 and 3223059028) with a total area of 51,271 square feet.
The northeast portion of the property is developed with a residence and driveway. The remainder
of the property is vegetated with grasses, bushes, blackberry vines, ivy, ferns, other understory, and
variable diameter trees.
Overall, the site slopes downward to the west at low magnitudes and relief of about 15 feet. There
are steeper slopes on the parcel to the west. These slope areas have magnitudes of 15 to 35 percent
and relief of up to 40 feet.
The site is bordered to north, south, and west by residential properties and to the east by 102nd
Avenue SE.
The proposed development includes one or more new residential structures. The locations will
depend on final platting and land use regulations. Stormwater will include infiltration or other
systems depending on feasibility.
Site grading may include cuts and fills of 3 feet or less and foundation loads are expected to be light.
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 the Renton Quadrangle, indicates that the site is underlain by Vashon Glacial
Till.
Vashon Glacial Till includes mixtures of silt, sand, gravel, and clay in a nonsorted matrix or diamict.
These deposits become denser with depth.
Figure B1
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Soil & Groundwater Conditions
As part of our evaluation, we excavated three test pits, where accessible. The explorations
encountered approximately 6 inches of grass and topsoil underlain by approximately 1.5 to 2 feet
of loose to medium dense, silty-fine to medium grained sand trace gravel (Weathered Glacial Till).
These materials were underlain by dense to very dense, silty-fine to medium grained sand trace
gravel (Vashon Glacial Till), which continued to the termination depths of the explorations.
Groundwater was observed at very shallow depths in TP-1 and TP-2. No groundwater was present
in TP-3. Groundwater appears to become perched during the wet season on the denser glacial till,
which is relatively shallow in this area.
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.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Alderwood gravelly sandy loam (8 to 15 percent slopes). Based on our experience,
the site 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 Hazard
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 7-16.
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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 1.385 0.472 1.0 Null 0.924 Null 0.589
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. For items listed as “Null” see
Section 11.4.8 of the ASCE.
Conclusions and Recommendations
General
The site is underlain by glacial till which becomes denser with depth. There may be areas of fill
associated with prior grading.
The proposed residential 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. Local
overexcavation of loose weathered native soils may be necessary depending on the proposed
elevations and locations of the new footings.
The site is near locally steeper slopes (west of the site). While not a landslide hazard area, we
recommend a minimum building setback of 20 feet from slopes with magnitudes of 35 percent or
more and relief of at least 15 feet. We note that this setback is likely located either off site to the
west or near the west property line. Typical building setbacks will likely result in locations in excess
of this value.
Infiltration is not feasible due to the presence of shallow groundwater and a restrictive layer.
Dispersion devices, detention, and local permeable pavements in areas with minimal cuts and/or
fills are generally suitable. We can provide additional input once a site plan is 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 in areas of existing
foundation systems (where present), larger trees where roots persist, and in any areas underlain by
undocumented fill.
The 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 HIGHLY
moisture sensitive and may degrade during periods of wet weather and under equipment traffic.
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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 3 feet or less for foundation 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, 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.
Foundation Design
The proposed structures may be supported on shallow spread footing foundation systems bearing
on undisturbed medium dense or firmer native soils or on properly compacted structural fill placed
on the suitable native soils. Any undocumented fill and/or loose native soils should be removed
and replaced with structural fill below foundation elements.
Structural fill below footings should consist of clean angular rock 5/8 to 4 inches in size. We should
verify soil conditions during foundation excavation work.
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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 2,500
pounds per square foot (psf) may be used for design. Detention vaults set at least 5 feet below grade
may be designed with a bearing pressure of 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
0.40 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 250 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 for new cast in place walls.
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)
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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 12 inches, 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
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.
Stormwater Management Feasibility
Infiltration is not feasible due to the presence of shallow groundwater and a restrictive layer.
Dispersion devices, detention, and local permeable pavements in areas with minimal cuts and/or
fills are generally suitable. Deeper infiltration trenches and drywells are not feasible.
The design infiltration rate was determined by applying correction factors to the measured
infiltration rates 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.
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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.
Test
Number
Test
Depth (ft)
Measured
Infiltration
Rate (in/hr)
Correction Factors Design
Infiltration
Rate
(in/hr) CFV CFT CFM
TP-3 1.0 0.8 0.9 0.5 0.9 0.324
This rate is for permeable pavements and rain gardens only. Soils had Cation Exchange Capacity
of 8.2 meq and organic content of 3.5 percent by weight.
Slab-on-Grade
We recommend that the upper 12 inches of the 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 210 pounds per cubic
inch (pci) assuming the slab-on-grade base course is underlain by structural fill placed and
compacted as outlined above. A 4- to 6-inch-thick capillary break layer should be placed over the
prepared subgrade. This material should consist of pea gravel or 5/8 inch clean angular rock.
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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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.
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.
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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
Observe soil conditions at stormwater system locations
Monitor foundation drainage placement
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 Rupinder Dulay 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 Rupinder Dulay 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
4/10/2025
Phil Haberman, PE, LG, LEG
Principal
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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
Proposed Development
18845 & 18855 102nd Avenue SE
Renton, Washington
Site Image
Figure 1
Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Subject Property
TP-1
TP-1
TP-3
TP-2
Attachment
Cobalt Geosciences, LLCPO Box 1792North 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 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
Exploration
Logs
Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-1
Date: April 9, 2025
Contractor: Austin
Depth: 8’
Elevation: Logged By: PH Checked By: SC
Groundwater: 1.5’
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
Loose to medium dense, silty-fine to medium grained sand with gravel,yellowish brown to grayish brown, moist.
( Glacial Till)Weathered
SM
End of 8’Test Pit
Topsoil/Vegetation
SM Dense, silty-fine to medium grained sand with gravel
grayish brown, moist. (Glacial Till)
Proposed Development
18845 & 18855 102nd Avenue SE
Renton, Washington
Exploration
Logs
Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-2
Date: April 9, 2025
Contractor: Austin
Depth: 8’
Elevation: Logged By: PH Checked By: SC
Groundwater: 1’
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
Loose to medium dense, silty-fine to medium grained sand with gravel,yellowish brown to grayish brown, moist. ( Glacial Till)WeatheredSM
End of 8’Test Pit
Topsoil/Vegetation
SM Dense, silty-fine to medium grained sand with gravel
grayish brown, moist. (Glacial Till)
Proposed Development
18845 & 18855 102nd Avenue SE
Renton, Washington
Exploration
Logs
Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-3
Date: April 9, 2025
Contractor: Austin
Depth: 8’
Elevation: 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
Loose to medium dense, silty-fine to medium grained sand with gravel,yellowish brown to grayish brown, moist. ( Glacial Till)WeatheredSM
End of 8’Test Pit
Topsoil/Vegetation
SM Dense, silty-fine to medium grained sand with gravel
grayish brown, moist. (Glacial Till)
Proposed Development
18845 & 18855 102nd Avenue SE
Renton, Washington