HomeMy WebLinkAboutEx_05_Geotech_ReportCobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
www.cobaltgeo.com (206) 331-1097
February 4, 2022
Bob Wenzl
bob@tuscanywa.com
RE: Geotechnical Evaluation
Proposed Residential Development
18600 116th Avenue SE
Renton, Washington
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss the results of our geotechnical evaluation at the referenced site.
The purpose of our evaluation was to provide recommendations for foundation design, grading,
and earthwork.
Site Description
The site is located at 18600 116th Avenue SE in Renton, Washington. The site consists of three
rectangular parcels (No.’s 6197800080, 6197800263, and 6197800265) with an area of about
125,000 square feet.
The western portion of the site is developed with a residential structure and driveway. The
remainder of the site is undeveloped and vegetated with ferns, ivy, blackberry vines, understory,
and variable diameter evergreen and deciduous trees.
The site slopes gently to moderately downward from west to east at magnitudes of 5 to 40 percent
and relief of about 75 feet. Most of the site is level to slightly sloping. The steeper slopes have
limited relief and are located near lot 7. The site is bordered to the north, east, and south by
residential properties and to the west by 116th Avenue SE. There is an access road (SE 186th
Street) along the south side of the two eastern parcels.
The proposed development includes a new seven lot plat, detention systems, open spaces, utilities
and roadways. We understand that SE 186th Street will be continued through the southern
portion of the site, connecting to 116th Avenue SE.
Stormwater will include infiltration or other systems depending on feasibility. Site grading may
include cuts and fills of 3 feet or less for foundation systems and up to 15 feet for mass grading
and vault excavations. 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 King County, indicates that the site is underlain by Vashon Glacial Till.
Vashon Glacial Till consists of mixtures of silt, sand, gravel, and clay in a nonsorted matrix or
diamict. These deposits become denser with depth below a locally loose weathered zone.
Exhibit 5
RECEIVED
05/26/2023
AMorganroth
PLANNING DIVISION
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Soil & Groundwater Conditions
As part of our evaluation, we excavated five test pits where accessible. Test Pits 1 and 2 were
excavated on the eastern parcels in 2020. Three additional test pits were excavated in the
western portion in February 2022.
The recent explorations encountered approximately 12 inches of forest duff underlain by
approximately 2 to 4 feet of loose to medium dense, silty-fine to medium grained sand with gravel
(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 depths of the
explorations.
The previous test pits (TP-1 and TP-2) encountered approximately 12 inches of topsoil and
vegetation underlain by approximately 6.5 to 7 feet of loose to medium dense, silty-fine to
medium grained sand with gravel (Weathered Recessional Outwash?). These materials were
underlain by dense, silty-fine to medium grained sand with gravel (Glacial Till).
Groundwater was not encountered in the explorations during our work. We do not anticipate
significant volumes of groundwater to be present at shallow depths 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. It would be necessary to install a
piezometer to determine groundwater depths over a typical year.
Steep Slope Hazard
There may be short slope areas with magnitudes of 40 percent or more in the eastern third of the
site. These areas have relief of less than 10 feet and may be the result of prior excavation work.
The site is underlain by dense till and is not prone to landslide activity. The proposed
development will modify moderately steep slopes to create less steep topography that will be more
stable than what is currently present. No specific buffers or setbacks appear warranted at this
time.
Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site
is underlain by Alderwood gravelly sandy loam (0 to 15 percent slopes). These soils would have a
slight to moderate 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
stiff/medium dense soils within the upper 100 feet.
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We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to
obtain values for SS, S1, Fa, and Fv. The USGS website includes the most updated published data
on seismic conditions. The following tables provide seismic parameters from the USGS web site
with referenced parameters from ASCE 7-10 and 7-16.
Seismic Design Parameters (ASCE 7-10)
Site
Class
Spectral
Acceleration
at 0.2 sec. (g)
Spectral
Acceleration
at 1.0 sec. (g)
Site
Coefficients
Design Spectral
Response Parameters
Design
PGA
Fa Fv SDS SD1
D 1.374 0.513 1.0 1.5 0.916 0.513 0.561
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 1.375 0.468 1.0 Null 0.916 Null 0.584
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 relatively 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 soils consistent with weathered and unweathered Vashon Glacial Till.
These soils become dense below a weathered zone. The proposed residential structure 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 locations of the new
footings. Note that the upper fine-grained soils were locally loose and may require removal in
foundation areas.
Widespread infiltration is not feasible due to the presence of a dense restrictive layer
(unweathered glacial till) approximately 4 feet below site elevations. We anticipate that a
detention system will be utilized in the eastern portion of the site with overflow to City
infrastructure or another improved conveyance. We can provide additional input upon request.
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Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic-rich
soil and fill. Based on observations from the site investigation program, it is anticipated that the
stripping depth will be 6 to 18 inches. Deeper excavations will be necessary below larger trees and
existing foundation elements.
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
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 7 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 encountered). 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
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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 structure 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 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. Note that the upper fine grained soils are locally
loose and some overexcavation should be anticipated to be necessary within 3 feet of the ground
surface.
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 6 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.
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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, if any are proposed.
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)
18H* (Uniform Distribution) 1 in 2,500 year
event
Seismic Increase for “At-rest” Conditions
(Lateral Earth Pressure)
12H* (Uniform Distribution) 1 in 500 year event
Seismic Increase for “Active” Conditions
(Lateral Earth Pressure)
6H* (Uniform Distribution)
Passive Earth Pressure on Low Side of Wall
(Allowable, includes F.S. = 1.5)
Neglect upper 2 feet, then 275 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.
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We recommend that the backfill be compacted to at least 90 percent of the maximum dry density
based on ASTM Test Method D1557. In place density tests should be performed to verify
adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently,
only light hand operated equipment is recommended within 3 feet of walls so that excessive stress
is not imposed on the walls.
Stormwater Management Feasibility
The site is underlain by weathered and unweathered glacial till. The dense glacial till acts as a
restrictive layer through which infiltration is nearly zero. Runoff tends to migrate laterally along
the dense glacial till, just below the ground surface.
We performed a small scale pilot infiltration test (PIT) in TP-1. The test was performed in general
accordance with the Washington State Department of Ecology stormwater manual.
The design infiltration rate was determined by applying correction factors to the measured
infiltration rate as prescribed in Volume III, Section 3.3.6 of the DOE. The measured rate must
be reduced through appropriate correction factors for site variability (CFV), uncertainty of test
method (CFT), and degree of influent control (CFM) to prevent siltation and bio-buildup.
It should be noted that construction traffic or other disturbance to the target infiltration area
could compact the soil, which may decrease the effective infiltration rates. The correction factors
and resulting design infiltration rate are also shown in the table below.
Test
Number
Test
Depth (ft)
Measured
Infiltration
Rate (in/hr)
Correction Factors Design
Infiltration
Rate
(in/hr) CFV CFT CFM
TP-3 4.0 0.32 0.7 0.5 0.9 0.10
Widespread infiltration is not feasible in the dense glacial till at depth. We recommend detention
systems with overflow to City infrastructure or an approved conveyance. We can provide
additional recommendations upon request.
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.
Slab-on-Grade
We recommend that the upper 12 inches of the existing native soils within slab areas be re-
compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method).
Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor
barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture
typically requires the usage of a vapor barrier. A materials or structural engineer should be
consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs
typically do not utilize vapor barriers.
The American Concrete Institutes ACI 360R-06 Design of Slabs on Grade and ACI 302.1R-04
Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier
selection and floor slab detailing.
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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.
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.
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All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility
trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at
least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5
feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent
of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench
backfill in pavement areas should be compacted to at least 90 percent of the maximum dry
density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe
manufacturer's recommendations.
The contractor is responsible for removing all water-sensitive soils from the trenches regardless of
the backfill location and compaction requirements. Depending on the depth and location of the
proposed utilities, we anticipate the need to re-compact existing fill soils below the utility
structures and pipes. The contractor should use appropriate equipment and methods to avoid
damage to the utilities and/or structures during fill placement and compaction procedures.
Pavements
The near surface subgrade soils generally consist of silty sand with gravel. These soils are rated as
good for pavement subgrade material (depending on silt content and moisture conditions). We
estimate that the subgrade will have a California Bearing Ratio (CBR) value of 10 and a modulus
of subgrade reaction value of k = 200 pci, provided the subgrade is prepared in general
accordance with our recommendations.
We recommend that at a minimum, 12 inches of the existing subgrade material be moisture
conditioned (as necessary) and re-compacted to prepare for the construction of pavement
sections. Deeper levels of recompaction or overexcavation and replacement may be necessary in
areas where fill and/or very poor (soft/loose) soils are present.
The subgrade should be compacted to at least 95 percent of the maximum dry density as
determined by ASTM Test Method D1557. In place density tests should be performed to verify
proper moisture content and adequate compaction.
The recommended flexible and rigid pavement sections are based on design CBR and modulus of
subgrade reaction (k) values that are achieved, only following proper subgrade preparation. It
should be noted that subgrade soils that have relatively high silt contents will likely be highly
sensitive to moisture conditions. The subgrade strength and performance characteristics of a silty
subgrade material may be dramatically reduced if this material becomes wet.
Based on our knowledge of the proposed project, we expect the traffic to range from light duty
(passenger automobiles) to heavy duty (delivery trucks). The following tables show the
recommended pavement sections for light duty and heavy duty use.
ASPHALTIC CONCRETE (FLEXIBLE) PAVEMENT
LIGHT DUTY
Asphaltic Concrete Aggregate Base* Compacted Subgrade* **
2.5 in. 6.0 in. 12.0 in.
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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.
Note that the City of Renton will likely have a road section requirement for the extension of SE
186th Street. The above information is for site roads and driveways.
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 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 Bob Wenzl 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.
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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 Bob Wenzl who is identified as “the Client” within the Statement of General
Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of
these not be satisfied.
Sincerely,
Cobalt Geosciences, LLC
2/4/2022
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.
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Proposed Development
18600 116th Avenue SE
Renton, Washington
N
SITE PLAN
FIGURE 1
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Attachment
Cobalt Geosciences, LLC
PO Box 1792
North Bend, WA 98045
(206) 331-1097
www.cobaltgeo.com
phil@cobaltgeo.com
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
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: 8349A775-7CD5-48E8-9101-6CAB65879B26
Test Pit
Logs
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Proposed Development
18600 116th Avenue SE
Renton, Washington
Test Pit TP-1
Date: May 20, 2020
Contractor: Client provided
Depth: 8.5’
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,
reddish brown to yellowish brown, moist.
(Recessional Outwash?)
SP/
SM
End of Test Pit 8.5’
Dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till) -Locally cemented
SM
Topsoil/Grass
Test Pit TP-2
Date: May 20, 2020
Contractor: Client provided
Depth: 9’
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,
reddish brown to yellowish brown, moist.
(Recessional Outwash?)
SP/
SM
End of Test Pit 9’
Dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till) -Locally cemented
SM
Topsoil/Grass
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Test Pit
Log
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-3
Date: January 20, 2022
Contractor: Wenzl
Depth: 7.5’
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
SM
Dense to very dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM
End of Test Pit 7.5’
Topsoil/Grass
Loose to medium dense, silty-fine to medium grained sand with gravel,
yellowish brown, moist. (Weathered Glacial Till)
Proposed Development
18600 116th Avenue SE
Renton, Washington
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Test Pit
Log
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-4
Date: January 20, 2022
Contractor: Wenzl
Depth: 7.5’
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
SM
Dense to very dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM
End of Test Pit 7.5’
Topsoil/Grass
Loose to medium dense, silty-fine to medium grained sand with gravel,
yellowish brown, moist. (Weathered Glacial Till)
Proposed Development
18600 116th Avenue SE
Renton, Washington
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26
Test Pit
Log
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo@gmail.com
Test Pit TP-5
Date: January 20, 2022
Contractor: Wenzl
Depth: 7.5’
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
SM
Dense to very dense, silty-fine to medium grained sand with gravel,
grayish brown, moist. (Glacial Till)
SM
End of Test Pit 7.5’
Topsoil/Grass
Loose to medium dense, silty-fine to medium grained sand with gravel,
yellowish brown, moist. (Weathered Glacial Till)
Proposed Development
18600 116th Avenue SE
Renton, Washington
DocuSign Envelope ID: 8349A775-7CD5-48E8-9101-6CAB65879B26