REVIEWED BLD2022-0287+Geotechnical_Report+3.7.2022_2.24.55_PM+2722004BLD2022-0287
COBALT
GE0SCIENCES
March 2, 2022
Todd Echelbarger
toddoechelbarger.com
RECEIVED
May 03 2022
CITY OF EDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
RE: Limited Geotechnical Evaluation
Proposed Development
21322 92nd Place West
Lynnwood, Washington
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
REVIEWED -
CITY OF EDMONDS
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 21322 92nd Place West in Edmonds, Washington. The site consists of one
irregularly shaped parcel (No. 00479300000600) with a total area of o.19 acres.
The property is undeveloped and vegetated with grasses, bushes, understory, and sparse trees.
The site slopes downward from east to west at magnitudes of 5 to 12 percent and relief of about 15
feet. There is a short fill slope near the west property line. This slope is 40 percent in magnitude
and about 6 feet in height. The site contains erosion hazard areas per City of Edmonds mapping.
The site is bordered to the north, south, and west by residences, and to the east by 92nd Place
West. There is a sewer line near the west property line.
The proposed development includes construction of a new residence with likely daylight
basement and driveway. Foundation loads are expected to be light and grading may include cuts
of 8 feet or less. We have reviewed the civil plans that show that stormwater will be routed to City
infrastructure.
Area Geology
The Geologic Map of the Edmonds East 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 are typically dense below a weathered zone. These materials are typically
impermeable.
Soil & Groundwater Conditions
As part of our evaluation, we excavated two test pits within the property, where accessible.
The explorations encountered approximately 6 inches of grass and topsoil underlain by
approximately 2.5 to 5 feet of loose to medium dense, silty -fine to medium grained sand trace to
with gravel (Fill). This layer was underlain by 1.5 to 4.5 feet of loose to medium dense, silty -fine
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Geotechnical Evaluation
to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by
dense to very dense, silty -fine to medium grained sand trace to with gravel (Vachon Glacial Till),
which continued to the termination depths of the explorations.
Groundwater was not encountered in the test pits during the exploration work. Cemented soils
were observed at depth in TP-2 and we anticipate that perched groundwater will develop on the
denser till at shallow depths during the wet season.
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 Snohomish County indicate that
the property is underlain by Alderwood-Urban land complex (8 to 15 percent slopes). These soils
can have a slight to moderate erosion potential when exposed.
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.
The site may be mapped within an erosion hazard area per City of Edmonds GIS. Relevant code
items with discussion for erosion hazards are as follows:
23.80.o6o Development standards — General requirements.
A. Alterations of geologically hazardous areas or associated buffers may only occur for activities
that:
1. Will not increase the threat of the geological hazard to adjacent properties beyond
predevelopment conditions;
2. Will not adversely impact other critical areas;
3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or
less than predevelopment conditions; and
4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or
geologist, licensed in the state of Washington.
The proposed residence will not increase the threat of geologic hazards on adjacent properties,
will not impact other critical areas, and are safe as designed under anticipated conditions. The
level of risk due to the proposed development will be the same or lower than what is currently
present provided temporary and permanent runoff is fully controlled.
23.80.07o Development standards — Specific hazards.
A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide
hazards shall meet the requirements of ECDC 23.80.o6o, Development standards — General
requirements, and the specific following requirements:
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1. Minimum Building Setback. The minimum setback shall be the distance required to ensure the
proposed structure will not be at risk from landslides for the life of the structure, considered to be
120 years, and will not cause an increased risk of landslides taking place on or off the site. A
setback shall be established from all edges of landslide hazard areas. The size of the setback shall
be determined by the director consistent with recommendations provided in the geotechnical
report to eliminate or minimize the risk of property damage, death, or injury resulting from
landslides caused in whole or part by the development, based upon review of and concurrence
with a critical areas report prepared by a qualified professional;
No building setback is warranted from the erosion hazard areas. These areas have very low risk
and can be managed with proper TESC and BMPs.
2. Buffer Requirements. A buffer may be established with specific requirements and limitations,
including but not limited to, drainage, grading, irrigation, and vegetation. Buffer requirements
shall be determined by the director consistent with recommendations provided in the
geotechnical report to eliminate or minimize the risk of property damage, death, or injury
resulting from landslides caused in whole or part by activities within the buffer area, based upon
review of and concurrence with a critical areas report prepared by a qualified professional;
No buffer is required from the erosion hazard areas.
We recommend that all site soils be vegetated or protected from erosion through placement of
surface landscaping materials/hardscapes.
3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback
and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies
that:
a. The alteration will not increase surface water discharge or sedimentation to adjacent properties
beyond predevelopment conditions;
b. The alteration will not decrease slope stability on adjacent properties; and
c. Such alterations will not adversely impact other critical areas;
Provided earthwork activities are performed in accordance with the approved plans, all runoff is
fully control, and periodic geotechnical oversight is performed, the development will not decrease
slope stability on adjacent properties, will not increase surface water discharge or sedimentation
beyond current levels, and will not impact other critical areas. Temporary and permanent erosion
and sediment control devices should be in at all times during construction.
4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion
or landslide hazard area and/or buffer shall be designed to meet the following basic requirements
unless it can be demonstrated that an alternative design that deviates from one or more of these
standards provides greater long-term slope stability while meeting all other provisions of this
title. The requirement for long-term slope stability shall exclude designs that require regular and
periodic maintenance to maintain their level of function. The basic development design standards
are:
a. The proposed development shall not decrease the factor of safety for landslide occurrences
below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the
proposed development site is below these limits, the proposed development shall provide
practicable approaches to reduce risk to human safety and improve the factor of safety for
landsliding. In no case shall the existing factor of safety be reduced for the subject property or
adjacent properties;
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b. Structures and improvements shall be clustered to avoid geologically hazardous areas and
other critical areas;
c. Structures and improvements shall minimize alterations to the natural contour of the slope,
and foundations shall be tiered where possible to conform to existing topography;
d. Structures and improvements shall be located to preserve the most critical portion of the site
and its natural landforms and vegetation;
e. The proposed development shall not result in greater risk or a need for increased buffers on
neighboring properties;
f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred
over graded artificial slopes; and
g. Development shall be designed to minimize impervious lot coverage;
The construction will not require grading or disturbance of any slope/landslide hazard areas:
however, the work may occur within the locally mapped erosion hazard areas. This work can be
completed without affecting the hazard provided work is completed in a timely manner and
proper erosion control measures are in place during construction and permanent landscaping is
in place upon completion of the construction.
The construction does not result in greater risk to the property or adjacent areas.
Performed with proper erosion control and engineering controls, the proposed development does
not pose a risk to critical areas or the need to increase buffers on adjacent properties.
5. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal
of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited;
6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October ist of each year;
provided, that the director may extend or shorten the dry season on a case -by -case basis
depending on actual weather conditions, except that timber harvest, not including brush clearing
or stump removal, may be allowed pursuant to an approved forest practice permit issued by the
city of Edmonds or the Washington State Department of Natural Resources;
7. Point Discharges. Point discharges from surface water facilities and roof drains onto or
upstream from an erosion or landslide hazard area shall be prohibited except as follows:
a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard
areas downstream from the discharge;
b. Discharged at flow durations matching predeveloped conditions, with adequate energy
dissipation, into existing channels that previously conveyed storm water runoff in the
predeveloped state; or
c. Dispersed discharge upslope of the steep slope onto a low -gradient, undisturbed buffer
demonstrated to be adequate to infiltrate all surface and storm water runoff, and where it can be
demonstrated that such discharge will not increase the saturation of the slope; and
We concur with the above code items and have no additional comments at this time.
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Geotechnical Evaluation
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 ioo feet.
We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to
obtain values for Ss, S,, FQ, and F,,. 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-io and 7-16.
Seismic Design Parameters (ASCE 7-10)
Site
Spectral
Spectral
Site
Design Spectral
Design
Class
Acceleration
Acceleration
Coefficients
Response Parameters
PGA
at 0.2 sec. (g)
at i.o sec. (g)
Fa
Fv
SDS
SD,
D
1.269
0.496
1.0
1.504
o.846
0.498
0.514
Seismic Design Parameters (ASCE 7-16)
Site
Spectral
Spectral
Site
Design Spectral
Design
Class
Acceleration
Acceleration
Coefficients
Response Parameters
PGA
at 0.2 sec. (g)
at l.o sec. (g)
Fa
Fv
SDS
SD,
D
1.283
0.451
1.0
Null
o.856
Null
0.546
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 fill and weathered and unweathered Vashon Glacial Till. 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. Overexcavation of any fill is required. Note that the
fill is mostly present near the west property line and likely decreases to nil to the east, where the
residence will be located.
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Infiltration is not feasible due to the presence of fill, restrictive layers, and variable slopes that
would direct runoff to the west and potentially into the adjacent residences. We anticipate that
direct or perforated connection to City infrastructure will be utilized. We can provide additional
information if other systems are being considered or proposed. Note a drywell for footing drains
west of the residence is suitable.
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 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
variably moisture sensitive and may degrade during periods of wet weather and under equipment
traffic.
Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of
3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve).
Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted
to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the
ASTM D 1557 test method.
Temporary Excavations
Based on our understanding of the project, we anticipate that the grading could include local cuts
on the order of approximately 8 feet or less for foundation and utility placement. 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 in deeper excavations. 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.
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Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that
the project can proceed and required deadlines can be met.
If any variations or undesirable conditions are encountered during construction, we should be
notified so that supplemental recommendations can be made. If room constraints or
groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed
by the WAC, temporary shoring systems may be required. The contractor should be responsible
for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences
and the project structural engineer review temporary shoring designs prior to installation, to
verify the suitability of the proposed systems.
Foundation Design
The proposed residence may be supported on a shallow spread footing foundation system bearing
on undisturbed 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. It may be feasible to recompact loose
outwash soils in lieu of overexcavation and replacement. We should verify soil conditions during
foundation excavation work.
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.
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 1/2 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
0.4o acting between the base of foundations and the supporting subgrades. Lateral resistance for
footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds
per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12
inches below grade in exterior areas). The frictional and passive resistance of the soil may be
combined without reduction in determining the total lateral resistance.
Care should be taken to prevent wetting or drying of the bearing materials during construction.
Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the
footing excavations, should be removed prior to placing concrete. The potential for wetting or
drying of the bearing materials can be reduced by pouring concrete as soon as possible after
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completing the footing excavation and evaluating the bearing surface by the geotechnical engineer
or his representative.
Concrete Retaining Walls
The following table, titled Wall Design Criteria, presents the recommended soil related design
parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall
system is used. This has been included for basement 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)
21H* (Uniform Distribution) 1 in 2,500 year
event
Seismic Increase for "At -rest" Conditions
(Lateral Earth Pressure)
14H* (Uniform Distribution) 1 in 500 year event
Seismic Increase for "Active" Conditions
(Lateral Earth Pressure)
7H* (Uniform Distribution)
Passive Earth Pressure on Low Side of Wall
(Allowable, includes F.S. = 1.5)
Neglect upper 2 feet, then 250 pcf EFD+
Soil -Footing Coefficient of Sliding Friction (Allowable;
includes F.S. = 1.5)
0.40
*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 go percent of the maximum dry density
based on ASTM Test Method D1557. In place density tests should be performed to verify
adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently,
only light hand operated equipment is recommended within 3 feet of walls so that excessive stress
is not imposed on the walls.
Slab -on -Grade
We recommend that the upper 18 inches of the existing native soils within slab areas be re -
compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method).
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 36oR-o6 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.
Stormwater Management Feasibility
The site is underlain by fill and thin weathered glacial till over dense glacial till. The dense glacial
till acts as an aquitard, reducing vertical infiltration of stormwater to near zero.
Infiltration is not feasible due to the presence of fill, restrictive layers, and variable slopes that
would direct runoff to the west and potentially into the adjacent residences. We understand that
roof runoff will be directed into City infrastructure. Note that a drywell for footing drains west of
the residence is suitable.
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
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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 and sandy 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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CLOSURE
This report was prepared for the exclusive use of Todd Echelbarger 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 Todd Echelbarger 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
54896
3/2/2022
Phil Haberman, PE, LG, LEG
Principal
`c 2513
Q
Sed Gea
PHIL P HABEIW,�J
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Geotechnical Evaluation
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 parry.
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.
www.cobaltgeo.com (2o6) 331-1097
Approximate
TP_ Test Pit Location
Sno. Co GIs Map
Proposed Residence
21322 92nd Place West
Edmonds, Washington
SITE
MAP
FIGURE i
N
A
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeoPgmail.com
Slab on Grade
Basement or Shallow Foundation Wall
12" Free Draining Backfill and/or Drainage Mat
Attached to Wall
Backfill Soils Compacted
per Geotechnical Report
4" Diameter Perforated Pipe
--�� H
Native Soils Benched
as Required
Filter Fabric Over Rock
(Mirafi 14oN)
3//4" Washed Rock or
Clean Angular Rock
Not to Scale
Cobalt Geosciences, LLC
PO Box 1792
Typical Foundation Drain Detail North Bend, WA 9So45
COBALTAttachment (2o6) 331-1097
GEOSCIENCLI-Swww.cobaltgeo.com
philocobaltgeo.com
Unified Soil Classification System (USCS)
MAJOR DIVISIONS SYMBOL
TYPICAL DESCRIPTION
GW
Clean Gravels
Well -graded gravels, gravels, gravel -sand mixtures, little or no fines
Gravels
(more than 50%
(less than 5% _ GP
fines)
Poorly graded gravels, gravel -sand mixtures, little or no fines
COARSE
GRAINED
SOILS
of coarse fraction
retained on No. 4
sieve)
GM
Gravels with
Fines
(more than 12% GC
fines)
Silty gravels, gravel -sand -silt mixtures
Clayey gravels, gravel -sand -clay mixtures
(more than 50%
retained on
;°e sw
Clean Sands
Well -graded sands, gravelly sands, little or no fines
No. 200 sieve)
Sands
(50% or more
of coarse fraction
(less than 5% SP
fines)
Poorly graded sand, gravelly sands, little or no fines
passes the No. 4
sieve)
sM
Sands with Fines
Silty sands, sand -silt mixtures
(more than 12% sc
fines)
Clayey sands, sand -clay mixtures
ML
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
FINE GRAINED
SOILS
(50% or more
Silts and Clays
(liquid limit less
than 50)
Inorganic cL
or clayey silts with slight plasticity
Inorganic clays of low to medium plasticity, gravelly clays, sandy clays
silty clays, lean clays
oL
Organic
Organic silts and organic silty clays of low plasticity
passes the
MH
Inorganic silts, micaceous or diatomaceous fine sands or silty soils,
No. 200 sieve)
Silts and Clays
(liquid limit 50 or
more)
Inorganic CH
elastic silt
Inorganic clays of medium to high plasticity, sandy fat clay,
or gravelly fat clay
OH
Organic
Organic clays of medium to high plasticity, organic silts
HIGHLY ORGANIC
SOILS
primarily organic matter, dark in color, PT
and organic odor
Peat, humus, swamp soils with high organic content (ASTM D4427)
1 Classification of Soil Constituents 1
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 o to 5 percent of the soil
(i.e., slightly silty SAND, trace gravel).
Relative Density
(Coarse Grained Soils)
Consistency
(Fine Grained Soils)
N, SPT,
Relative
N, SPT,
Relative
Blows/FT
Density
Blows/FT
Consistency
0-4
Very loose
Under 2
Very soft
4 -10
Loose
2-4
Soft
10 - 30
Medium dense
4-8
Medium stiff
30 - 50
Dense
8 -15
Stiff
Over 50
Very dense
15 - 30
Very stiff
Over 3o
Hard
Grain Size Definitions
Description
Sieve Number and/or Size
Fines
<#200 (0.08 mm)
Sand
-Fine
#200 to #40 (0.08 to 0.4 mm)
-Medium
#40 to #10 (0.4 to 2 MM)
-Coarse
#10 to #4 (2 to 5 mm)
Gravel
-Fine
#4 to 3/4 inch (5 to 19 mm)
-Coarse
3/4 to 3 inches (19 to 76 mm)
Cobbles
3 to 12 inches (75 to 305 mm)
Boulders
>12 inches (305 mm)
Moisture Content Definitions
Dry Absence of moisture, dusty, dry to the touch
Moist Damp but no visible water
Wet Visible free water, from below water table
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA98028 Soil Classification Chart Figure C1
(206) 331-1097
_ www.cobalt eg o.com
cobaltgeo(d-) gmail.com
Test Pit TP-1
Date: November 3, 2021 Depth: 10' Groundwater: None
Contractor: Client provided Elevation: Logged By: PH Checked By: SC
-- o> o Moisture Content (%)
N JO -0 Plastic 1 Liquid
U E Limit Limit
L � L
Material Description
o ? o DCP Equivalent N-Value
G 0 10 20 30 40 50
R
__
To.P oilation ________________________________
1
SM
Loose to medium dense, silty -fine to medium grained sand with gravel
l
dark yellowish brown to grayish brown, moist.
2
(Fill)
3
4
5
------
6
----
X.
---
SM
---------------------------------------------
Medium dense to dense, silty -fine to medium grained sand with grave
,
grayish brown, moist.
(Glacial Till)
8
9
18
End of Test Pit 10'
Test Pit TP-2
Date: November 3, 2021
Depth: 8'
Groundwater: None
Contractor: Client provided
Elevation:
Logged By: PH Checked By: SC
—
0
0)
o
Moisture Content (%)
JO
U
-0
T
U
Plastic 1 Liquid
Limit Limit
Material Description
DCP Equivalent N-Value
o
6
?
o
G
0 10 20 30 40 5
--
Topsoilv_egetation --------------------------------
SM
1
Loose to medium dense, silty -fine to medium grained sand with gravel
dark yellowish brown to grayish brown, moist.
2
(Fill)
—.3 -----------
•:.'•
--
---------------------------------------------
SM
Medium dense, silty -fine to medium grained sand with gravel,
4
yellowish brown to grayish brown, moist.
(Weathered Glacial Till)
5
--6 -----------
---
---------------------------------------------
SM
Dense to very dense, silty -fine to medium grained sand with gravel,
7
grayish brown, moist.
(Glacial Till)
End of Test Pit 8'
9
10
Cobalt Geosciences, LLC
Proposed Residence
P.O. Box 82243
COBALT21322
92nd Place West
Test Pit
Kenmore, WA 98028
(2o6) 331-1097
F
Edmonds, Washington
Logs
www.cobaltgeo.com
cobaltgeoPgmail.com