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Geotechnical Investigation
Proposed Residential Development
122xx SE Petrovitsky Road
Renton, Washington
June 17, 2019
EXHIBIT 7
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GEOTECHNICAL INVESTIGATION
RENTON, WASHINGTON
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Table of Contents
1.0 INTRODUCTION ............................................................................................................. 1
2.0 PROJECT DESCRIPTION .............................................................................................. 1
3.0 SITE DESCRIPTION ....................................................................................................... 1
4.0 FIELD INVESTIGATION ............................................................................................... 1
4.1.1 Site Investigation Program ................................................................................... 1
5.0 SOIL AND GROUNDWATER CONDITIONS .............................................................. 2
5.1.1 Area Geology ........................................................................................................ 2
5.1.2 Groundwater ........................................................................................................ 2
6.0 GEOLOGIC HAZARDS ................................................................................................... 3
6.1 Erosion Hazard .................................................................................................... 3
6.2 Seismic Hazard .................................................................................................... 3
7.0 DISCUSSION ................................................................................................................... 3
7.1.1 General................................................................................................................. 3
8.0 RECOMMENDATIONS .................................................................................................. 4
8.1.1 Site Preparation ................................................................................................... 4
8.1.2 Temporary Excavations ........................................................................................ 4
8.1.3 Erosion and Sediment Control.............................................................................. 5
8.1.4 Foundation Design ............................................................................................... 5
8.1.5 Stormwater Management ..................................................................................... 6
8.1.6 Slab-on-Grade ...................................................................................................... 7
8.1.7 Groundwater Influence on Construction .............................................................. 7
8.1.8 Utilities ................................................................................................................ 7
8.1.9 Pavements ............................................................................................................ 9
9.0 CONSTRUCTION FIELD REVIEWS ............................................................................ 9
10.0 CLOSURE ...................................................................................................................10
LIST OF APPENDICES
Appendix A — Statement of General Conditions
Appendix B — Figures
Appendix C — Test Pit Logs
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GEOTECHNICAL INVESTIGATION
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1.0 Introduction
In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical
investigation for the proposed residential development located at 122xx SE Petrovitsky Road in Renton,
Washington (Figure 1).
The purpose of the geotechnical investigation was to identify subsurface conditions and to provide
geotechnical recommendations for foundation design, stormwater management, earthwork, soil
compaction, and suitability of the on-site soils for use as fill.
The scope of work for the geotechnical evaluation consisted of a site investigation followed by engineering
analyses to prepare this report. Recommendations presented herein pertain to various geotechn ical
aspects of the proposed development, including foundation support of the new buildings and new
pavements.
2.0 Project Description
The project includes construction of two multi-story, multi-unit residential buildings in the southern
portion of the property. The buildings will have six units (north) and five units (north) for a total of 11
residential units. Parking areas will be located along the east property line and a drive lane will extend
into the property and between the buildings from the west.
Anticipated building loads are expected to be light to moderate and site grading will include cuts and fills
on the order of 4 feet or less. Stormwater management will include infiltration devices, if feasible. We
should be provided with the final plans when they become available.
3.0 Site Description
The site is located at 122xx SE Petrovitsky Road in Renton, Washington (Figure 1). The property consists
of one irregularly shaped parcel (No. 0739000050) with a total area of about 33,542 square feet.
The site is undeveloped and vegetated with grasses, ferns, ivy, blackberry vines, Scotch Broom, and
variable diameter deciduous trees. The site slopes gently downward from north to south at magnitudes of
less than 10 percent and relief of about 8 feet.
The site is bordered to the east, west, and north by commercial and residential developments and to the
south by SE Petrovitsky Road.
4.0 Field Investigation
4.1.1 Site Investigation Program
The geotechnical field investigation program was completed on May 30, 2019 and included excavating
and sampling four test pits within the property for subsurface analysis.
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The soils encountered were logged in the field and are described in accordance with the Unified Soil
Classification System (USCS).
A Cobalt Geosciences field representative conducted the explorations, collected disturbed soil samples,
classified the encountered soils, kept a detailed log of the explorations, and observed and recorded
pertinent site features.
The results of the sampling are presented on the exploration logs enclosed in Appendix C.
5.0 Soil and Groundwater Conditions
5.1.1 Area Geology
The site lies within the Puget Lowland. The lowland is part of a regional north-south trending trough that
extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington,
this lowland is glacially carved, with a depositional and erosional history including at least four separate
glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to
the east by the Cascade Range. The lowland is filled with glacial and non -glacial sediments consisting of
interbedded gravel, sand, silt, till, and peat lenses.
The Geologic Map of King County, indicates that the site is underlain by Vashon Glacial Till.
Vashon Glacial Till is typically characterized by an unsorted, non -stratified mixture of clay, silt, sand,
gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively
impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden
and incorporated by the glacial ice.
Explorations
The test pits encountered 6 to 12 inches of topsoil and vegetation underlain by approximately 3.5 to 5.5
feet of loose to medium dense, silty-fine to medium grained sand with gravel (Fill and/or Weathered
Glacial Till). These materials were underlain by dense to very dense, silty-fine to medium grained sand
with gravel (Glacial Till), which continued to the termination depth of the explorations.
5.1.2 Groundwater
Groundwater was not encountered in any of the explorations. There is a chance that perched
groundwater may be encountered during late winter and early spring months. We anticipate that
groundwater would be perched between fill or weathered till and underlying unweathered glacial till.
Depths to groundwater would vary from about 3 to 6 feet below grades.
Water table elevations often fluctuate over time. The groundwat er 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.
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6.0 Geologic Hazards
6.1 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). 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 t ypical 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.
6.2 Seismic Hazard
The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015
International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of 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 site specific seismic design parameters and adjusted maximum spectral response acceleration
parameters are as follows:
PGA (Peak Ground Acceleration, in percent of g)
SS 138.10% of g
S1 51.50% of g
FA 1.00
FV 1.50
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 relatively dense soil deposits that underlie the site have a low liquefaction potential.
7.0 DISCUSSION
7.1.1 General
The site is underlain by areas of fill along with weathered and unweathered glacial till. The proposed
residential buildings may be supported on shallow foundation systems bearing on medium dense or
firmer native soils and/or structural fill placed on suitable native soils. Local overexcavation of fill and/or
loose soils may be necessary below proposed foundation elements.
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We recommend utilizing dispersion devices (if feasible based on space), detention, permeable pavements,
or rain gardens for stormwater management. The shallow soils are fine-grained and do not allow for
widespread infiltration.
8.0 Recommendations
8.1.1 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 pro gram, it is anticipated that the stripping depth
will be 6 to 18 inches. Deeper excavations will be necessary below large trees and in any areas underlain
by undocumented fill materials.
The native soils consist of silty-sand with gravel and sandy silt with gravel. These 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.
8.1.2 Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts on the
order of approximately 5 feet or less for foundation and utility placement. Excavations should be sloped
no steeper than 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 1.5H: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
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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 developin g 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.
8.1.3 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.
8.1.4 Foundation Design
The proposed residential buildings 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. If structural fill is used to support foundations, then the zone of
structural fill should extend beyond the faces of the footing a lateral distance at least equal to the
thickness of the structural fill.
Depending on the location and finish floor elevations of new residences, some overexcavation may be
required. Any fill will need to be removed below new footings and replaced with compacted structural fill
as discussed above.
For shallow foundation support, we recommend widths of at least 18 and 24 inches, respectively, for
continuous wall and isolated column footings supporting the proposed structure. Provided that the
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footings are supported as recommended above, a net allowable bearing pressure of 3,000 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 exca vations 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 fact or 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 275 pounds per cubic foot (pcf)
acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior
areas).
The allowable friction factor and allowable equivalent fluid passive pressure values include a factor of
safety of 1.5. The frictional and passive resistance of the soil m ay be combined without reduction in
determining the total lateral resistance. A 1/3 increase in the above values may be used for short duration
transient loads.
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 concret e as soon as possible after completing the footing
excavation and evaluating the bearing surface by the geotechnical engineer or his representative.
8.1.5 Stormwater Management
It is our opinion that infiltration of stormwater runoff utilizing trenches or drywells is not feasible due to
the underlying soil conditions and likely groundwater conditions during the wet season. We performed a
small-scale pilot infiltration test in TP-1 at a depth of 4 feet below grade. Following testing a nd
application of correction factors for testing (0.5), site variability (0.33), and influent control (0.9), the
infiltration rate was found to be 0.18 inches per hour. This is lower than what is considered feasible by
the Washington State Department of Ecology.
It may be feasible to utilize dispersion trenches with vegetated flowpaths for a portion of runoff from new
impervious surfaces. Additional options for management include detention vaults, rain gardens, and
permeable pavements for flow control.
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We should be provided with the final stormwater plans to verify suitability of the location, type, and depth
of any on-site management system. These recommendations should be considered preliminary and for
discussion until the final design has been determined. We can update these recommendations and/or
provide additional recommendations once an overall stormwater management plan has been developed.
8.1.6 Slab-on-Grade
We recommend that the upper 12 inches of the existing fill and/or 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. Floo r 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 6 inch thickness of pea gravel or 5/8 inch clean angular rock should be placed
over the subgrade as a capillary break.
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 perime ter
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.
8.1.7 Groundwater Influence on Construction
Groundwater was not encountered in any of the explorations. We anticipate that perched groundwater
could be encountered during construction if the work takes place during late winter to early spring. Any
groundwater would be light in volume and likely within 3 to 6 feet of the ground surface.
If groundwater is encountered, we anticipate that sump excavations and small diameter pumps systems
will adequately de-water short-term excavations, if required. Any system should be designed by the
contractor. We can provide additional recommendations upon request.
8.1.8 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
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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 lea st 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 th e 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.
8.1.9 Pavement Recommendations
The near surface subgrade soils generally consist of silty sand with gravel and areas of sandy silt 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 pav ement 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. Any soils that cannot be compacted to required levels and soils that have
more than 40 percent fines by weight should be removed and replaced with imported structural fill.
The subgrade should be compacted to at least 95 percent of the maximum dry density as determined by
ASTM Test Method D1557. In place density tests should be performed to verify proper moisture content
and adequate compaction.
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.
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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.
9.0 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
Monitor pavement subgrade conditions
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Observe slab-on-grade preparation
Observe subsurface drainage systems
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.
10.0 Closure
This report was prepared for the exclusive use of VY Properties, LLC and their appointed consultants. Any
use of this report or the material contained herei n 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 VY Properties, LLC 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.
Respectfully submitted,
Cobalt Geosciences, LLC
Original signed by:
DRAFT ONLY – NOT FOR PERMITTING
Phil Haberman, PE, LG, LEG
Principal
PH/sc
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APPENDIX A
Statement of General Conditions
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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.
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
10.2
PO Box 82243
Kenmore, WA 98028
cobaltgeo@gmail.com
206-331-1097
APPENDIX B
Figures: Vicinity Map, Site Plan
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
SITE
N
Project
Location
Renton
WASHINGTON
VICINITY
MAP
FIGURE 1
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Proposed Residential Development
122xx SE Petrovitsky Road
Renton, Washington
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
SITE PLAN
FIGURE 2
N
Proposed Residential Development
122xx SE Petrovitsky Road
Renton, Washington
TP-1
TP-4
TP-1
TP-2
TP-3
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
APPENDIX C
Test Pit Logs
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
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
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
Proposed Development
122xx SE Petrovitsky Road
Renton, 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: May 30, 2019
Contractor: Jim
Depth: 10’
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 yellowish brown to grayish brown, moist.
(Weathered Glacial Till)
SM/
ML
Dense to very dense, silty-fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM/
ML
End of Test Pit 10’
Topsoil/Grass
Test Pit TP-2
Date: May 30, 2019
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 yellowish brown, moist. (Weathered Glacial Till)
SM/
ML
Dense to very dense, silty-fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM/
ML
End of Test Pit 8’
Topsoil/Grass
SM Loose to medium dense, silty-fine to medium grained sand with
gravel, dark yellowish brown, moist. (Fill)
Loose to medium dense, silty-fine to medium grained sand with
gravel, dark yellowish brown, moist. (Fill)
SM
DocuSign Envelope ID: 6FEDE754-AC7F-4A9A-847E-EDB66D55B606
Proposed Development
122xx SE Petrovitsky Road
Renton, 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: May 30, 2019
Contractor: Jim
Depth: 10’
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 yellowish brown to grayish brown, moist.
(Weathered Glacial Till)
SM/
ML
Dense to very dense, silty-fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM/
ML
End of Test Pit 10’
Topsoil/Grass
Test Pit TP-4
Date: May 30, 2019
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 yellowish brown, moist. (Weathered Glacial Till)
SM/
ML Dense to very dense, silty-fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM/
ML
End of Test Pit 8’
Topsoil/Grass
Loose to medium dense, silty-fine to medium grained sand with
gravel, dark yellowish brown, moist. (Fill)
SM
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