REVIEWED PLN_BLD2022-0094+Geotechnical_Report+1.24.2022_11.08.17_PM+2638863COBALT
G E 0 S C I E N C E S
Limited Geotechnical Investigation
Proposed Residence
17123 — 691h Place West
Edmonds, Washington
March 10, 2020
GEOTECHNICAL INVESTIGATION
EDMONDS, WASHINGTON
March 10, 2020
Table of Contents
1.o INTRODUCTION............................................................................................................. 1
2.0 PROJECT DESCRIPTION..............................................................................................
1
3.0 SITE DESCRIPTION.......................................................................................................
1
4.o FIELD INVESTIGATION...............................................................................................
2
4.1.1 Site Investigation Program...................................................................................
2
5.0 SOIL AND GROUNDWATER CONDITIONS..............................................................
2
5.1.1 Area Geology........................................................................................................
2
5.1.2 Groundwater........................................................................................................
2
6.o GEOLOGIC HAZARDS...................................................................................................
3
6.1 Steep Slope Hazard..............................................................................................
3
6.2 Erosion Hazard....................................................................................................
3
6.3 Seismic Hazard....................................................................................................
3
6.4 Code Items...........................................................................................................
3
7.o DISCUSSION...................................................................................................................
7
7.1.1 General.................................................................................................................7
8.o RECOMMENDATIONS..................................................................................................
7
8.1.1 Site Preparation...................................................................................................
7
8.1.2 Temporary Excavations........................................................................................
7
8.1.3 Erosion and Sediment Control..............................................................................
8
8.1.4 Foundation Design...............................................................................................
9
8.1.5 Reinforced Concrete Retaining Walls..................................................................10
8.1.6 Stormwater Recommendations...........................................................................11
8.1.7 Rockery Recommendations.................................................................................11
8.1.8 Slab-on-Grade.....................................................................................................12
8.1.9 Utilities...............................................................................................................12
9.o CONSTRUCTION FIELD REVIEWS...........................................................................13
io.o CLOSURE...................................................................................................................13
GEOTECHNICAL INVESTIGATION
EDMONDS, WASHINGTON
March 10, 2020
Table of Contents (Continued)
LIST OF APPENDICES
Appendix A — Statement of General Conditions
Appendix B — Figures
Appendix C — Exploration Logs
LIMITED GEOTECHNICAL INVESTIGATION
EDMONDS, WASHINGTON
March 10, 2020
1.o Introduction
COBALT
GEOSCIENCES
In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical
investigation for the proposed single-family residence located at 17125 — 69th Place West in Edmonds,
Washington (Figure 1).
The purpose of the geotechnical investigation was to identify subsurface conditions and to provide
preliminary geotechnical recommendations for foundation design, retaining walls, earthwork, soil
compaction, utilities, general pavement guidelines, stormwater management, and suitability of the on -site
soils for use as fill.
The scope of work for the geotechnical investigation consisted of a site investigation followed by
engineering analyses to prepare this report. Recommendations presented herein pertain to various
geotechnical aspects of the proposed development, including foundation and wall design, drainage, and
earthwork.
2.0 Project Description
The project includes construction of a new single-family residence with daylight basement level. The
residence will be located in the western portion of the property with asphalt parking and drive areas
located near the west property line. A small yard area will likely be created east of the residence. This will
require construction of a new retaining wall or rockery to face cut native soils.
We anticipate that the structure will be supported on shallow foundation systems and site grading will
include cuts and fills on the order of 10 feet or less. We should be provided with the final plans to
determine if our recommendations have been implemented.
3.0 Site Description
The site is located at 17125 — 69th Place West in Edmonds, Washington (Figure 1). The property consists
of one rectangular parcel (No. 01017300001200) with a total area of 0.28 acres.
The site slopes downward toward the west at magnitudes ranging from 10 to 6o percent with overall
topographic relief of about 50 feet (within the property). The west portion slopes downward toward the
west at 10 to 20 percent with the remainder sloping downward at 45 to 6o percent magnitude. The site is
vegetated with small diameter evergreen and deciduous trees, Scotch broom and grasses.
The site is bordered to the north and east by single-family residences, to the south by 172nd Street SW, and
to the west by 69th Place West.
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4.o Field Investigation
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4.1.1 Site Investigation Program
The geotechnical field investigation program was completed on March 3, 202o and included excavating
and sampling four hand borings within the property, where accessible.
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, classified the encountered soils,
kept a detailed log of each explorations, and observed and recorded pertinent site features.
The results of the hand borings explorations are presented 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 the Edmonds East Quadrangle, 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.
Hand Borings
The hand borings encountered approximately 2 to 6 inches of topsoil and vegetation underlain by 1.5 to
3.5 feet of medium dense (local loose soils), silty -fine to medium grained sand with gravel (Weathered
Glacial Till). This layer was underlain by dense to very dense, silty -fine to medium grained sand with
variable amounts of gravel and trace cobbles (Glacial Till), which continued to the termination depth of
the hand borings.
The soil conditions, topography, and lack of large vegetation indicate that the entire site has been
previously graded. The uniform nature of the topography coupled with a lack of topsoil development and
the presence of very dense soils at the ground surface indicate that the previous grade was likely several
feet or more above the existing elevations.
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5.1.2 Groundwater
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At the time of our investigation, groundwater was not encountered in any of the explorations. Based on
the soil conditions, we do not expect significant volumes of groundwater to be encountered during
construction. If groundwater is encountered, we anticipate that it would be perched between weathered
and unweathered glacial till.
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.
6.o Geologic Hazards
6.1 Steep Slope Hazard
Slopes with magnitudes greater than about 40 percent and vertical relief of at least io feet are typically
designated as potentially geologically hazardous (steep slope/landslide hazards). A majority of the site
consists of a steep slope area that was previously created through legal grading activities. The slope is
about 50 feet tall with an average magnitude of about 50 percent.
We did not observe evidence of shallow soil sloughing, spring activity, or any landslide activity. The
shallow soils consist of dense to very dense glacial till, which is not typically susceptible to global failure.
It is our opinion that the site slopes are stable in their current configuration.
6.2 Erosion Hazard
The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is
underlain by Alderwood-Urban land complex (2 to 8 percent slopes). We anticipate that the shallow soils
have a "Slight" to "Moderate" erosion potential in a disturbed state.
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 ist. Erosion control measures should be in place before
the onset of wet weather.
6.3 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 ioo feet.
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We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values
for Ss, S,, F,,, and F,,. 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 126.50% of g
S, 49.40% of g
FA 1.00
Fv i.5o6
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 potential for liquefaction.
6.4 Municipal Code Items
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
Project will not increase the threat of geologic hazards onto adjacent properties be,ond pre -developed
conditions, will not adversely affect other critical areas, and with proper implementation of BMPs and
design elements, will maintain the hazard level equal to or lower than what currently exists.
4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist,
licensed in the state of Washington.
Provided all work is performed in accordance with the approved plans and periodically monitored by the
geotechnical engineer, the development is considered to be safe.
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 ?�.80.o6o, Development standards — General requirements, and the
specific following requirements:
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
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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 specific setback is possible since the entire property is within a steep slope area. Location shown on
the site plan limits impact to critical areas to the minimal that is feasible. Footings should be embedded
at least 2 feet into dense to very dense soils and verified by the geotechnical engineer.
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 specific buffer is warranted due to the sloping nature of the entire property. Vegetation should not be
removed from non -development areas without a specific vegetation replacement plan.
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 all work is performed in accordance with the approved plans and periodically monitored by the
geotechnical engineer, the development will not increase discharge, decrease slope stability, or adversely
affect other critical areas.
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;
The site is stable in its current configuration with factors of safety above the minimum for static and
dynamic conditions.
b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical
areas;
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Proposed development located in least critical slope areas as feasible.
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;
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 ist 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
8. Prohibited Development. On -site sewage disposal systems, including drain fields, shall be prohibited
within erosion and landslide hazard areas and related buffers.
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7. o DISCUSSION
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7.1.1 General
It is our opinion that the proposed single-family residence may be supported on a shallow foundation
system bearing on medium dense or firmer native soils, re -compacted native soils, or on structural fill
placed on native soils per the recommendations in Section 8.1.1 and 8.1.4 of this report.
Infiltration of runoff from impervious surfaces is not feasible. The shallow soils consist of dense to very
dense glacial till, which is nearly impermeable in its unweathered state. We recommend direct
connection of runoff to the City infrastructure.
The proposed development will not adversely affect critical areas on or adjacent to the property provided
all work is performed in accordance with approved plans and the earthwork activities are monitored by
the geotechnical engineer during construction.
8.o Recommendations
8.1.1 Site Preparation
Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich soil.
Based on observations from the site investigation program, it is anticipated that the stripping depth will
range from 2 to 12 inches.
The excavated material is not suitable as fill material within the proposed building envelope but could be
used as fill material in non -settlement sensitive areas such as landscaping regions. In these non -
settlement sensitive areas, the fill should be placed in maximum 12 inch thick lifts that should be
compacted to at least 90 percent of the modified proctor (ASTM D 1557 Test Method) maximum dry
density.
The native soils below the vegetation and topsoil generally consist of glacial till. These materials are
generally considered suitable for use as structural fill provided they are within 3 percent of the optimum
moisture content. The native soils should be considered moisture sensitive and suitable for use as
structural fill only during the summer months.
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 10 feet or less for foundation (basement) placement and retaining wall or rockery
construction east of the residence. These excavations should be sloped no steeper than 3/4H:1V
(Horizontal:Vertical) in dense to very dense native soils. If an excavation is subject to heavy vibration or
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surcharge loads, we recommend that the excavations be sloped no steeper than 1.51-1:1V, where room
permits. If caving occurs, lower slope angles may be necessary. Near vertical excavations are feasible for
rockery or short modular block wall construction provided the cuts are short-term and periodically
monitored for stability by the geotechnical engineer.
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 io feet of the top of any
temporary cut slope.
Soil conditions may not be completely known from the geotechnical investigation. In the case of
temporary cuts, the existing soil conditions may not be completely revealed until the excavation work
exposes the soil. Typically, as excavation work progresses the maximum inclination of temporary slopes
will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be
made. Soil and groundwater conditions can be highly variable. Scheduling for soil work will need to be
adjustable, to deal with unanticipated conditions, so that the project can proceed and required deadlines
can be met.
If any variations or undesirable conditions are encountered during construction, we should be notified so
that supplemental recommendations can be made. If room constraints or groundwater conditions do not
permit temporary slopes to be cut to the maximum angles allowed by the WAC, temporary shoring
systems may be required. The contractor should be responsible for developing temporary shoring
systems, if needed. We recommend that Cobalt Geosciences and the project structural engineer review
temporary shoring designs prior to installation, to verify the suitability of the proposed systems.
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.
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• 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 single-family residence may be supported on a shallow spread footing foundation system
bearing on undisturbed medium dense or firmer native soils, re -compacted 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.
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 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 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 0.40
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 300 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 may be combined without reduction
in determining the total lateral resistance.
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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.
8.1.5 Reinforced 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.
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)
12H* (Uniform Distribution)
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 300 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 2,500 year seismic event (2 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 (if necessary for the vaults). The 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
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with at least 30 percent of the material being retained on the U.S. Standard No. 4 Sieve. The primary
purpose of the free -draining material is the reduction of hydrostatic pressure. Some potential for the
moisture to contact the back face of the wall may exist, even with treatment, which may require that more
extensive waterproofing be specified for walls, which require interior moisture sensitive finishes.
We recommend that the backfill be compacted to at least 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.
8.1.6 Stormwater Recommendations
The site is underlain by weathered and unweathered glacial till which is nearly impermeable and not
conducive to infiltration of stormwater runoff. Furthermore, the site is moderately to steep sloping
downward toward the west. The use of shallow infiltration or dispersion systems is not recommended for
sites with steep topography.
We recommend direct connection of site drainage systems to the City infrastructure.
8.1.7 Rockery Recommendations
Rockery walls are not considered as engineered gravity retaining walls mainly because the structure is not
an integral as the rocks are just siting and they are not structurally connected to each other. They
generally function as erosion protection for the materials they face that are themselves stable, which
should be dense to very dense soils with adequate fines, preferably glacially consolidated materials in the
region or controlled structural fill.
At this site, rockery walls up to 8 feet in exposed height may be considered. Alternatively, a multi -tiered
rockery system could be constructed. If a tiered system is used, we recommend a minimum lateral
distance of 6 feet between rockeries (back of lower wall to face of upper wall). Areas between and below
rockery walls should be level and we recommend a maximum cut slope magnitude of 50 percent (211:1V)
above the uppermost wall.
We recommend a minimum of 6 inches of embedment for all rockery walls. All walls should have a
minimum batter of 6V:111 (vertical to horizontal) and be backfilled with about 12 to 18 inches (width) of 2
to 4 inch sized angular quarry rock extending up the back of the walls.
Rockery drains should consist of a minimum 4-inch diameter perforated PVC pipe surrounded by washed
rock located at the toe of the wall and below the 2-4 inch quarry rock backfill. Pipes should daylight at
each end of the wall and a rock pad should be constructed below the outfalls. The areas below the outfalls
should be well vegetated and nearly level (maximum 15 percent slopes). We should review the final
grading and drainage plans to verify suitability from a geotechnical standpoint.
All rockeries should be constructed per the Associated Rockery Contractors (ARC) guidelines except as
noted above (exceptions).
11
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See(http://www.ceogeo.org/schedule/o9244404pm Current%202013%2oARC%2oRockery%2oConstru
ction%2oGuidelines.pdf ). A geotechnical firm should monitor keyway excavation, drainage, rock
placement, backfill, and excavation work by the geotechnical engineer.
Short concrete modular block walls are also feasible to face dense cut glacial till. We recommend a
maximum exposed wall height of 4 feet with a level backslope/fore slope, and 2.5 feet with a backslope of
Up to 2H:1V. We recommend a minimum block size of 8 inches tall by 14 inches wide by 12 inches deep
with ceramic pin connectors between block rows. Backfill should consist of clean angular rock up the
back of the wall and a drain system as described above should be installed at the base of the walls. If
tiered walls are utilized, we recommend a minimum 4 feet setback between walls.
8.1.8 Slab -on -Grade
We recommend that the upper 12 inches of the native soils within any proposed slab areas be re -
compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Any remaining
organic debris or fill should be removed prior to fill placement and compaction.
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 in
Section 8.1.
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 these buildings and preferably with a relatively impermeable surface cover
immediately adjacent to the buildings.
8.1.9 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.
12
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LIMITED GEOTECHNICAL INVESTIGATION
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March 10, 2020
COBALT
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In general, sandy and silty soils were encountered at shallow depths in the explorations at this site. These
soils have low cohesion and have a tendency to cave or slough in excavations. Shoring or sloping back
trench sidewalls is required within these soils.
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.
9. o 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
■ Verify the soil bearing at foundation locations for the building
■ Verify slab subgrade and capillary break material below slab -on -grade
■ Observe footing drainage placement
■ Observe proof rolls of roadway subgrade prior to asphalt placement
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.
io.o Closure
This report was prepared for the exclusive use of Richard Vaisvila 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.
13
PO Box 82243
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LIMITED GEOTECHNICAL INVESTIGATION
EDMONDS, WASHINGTON
March 10, 2020
COBALT
GEOSCIENCES
Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the respo
nsibility of Richard Vaisvila 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:
3/10/2020
Phil Haberman, PE, LG, LEG
Principal
PH/sc
14
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APPENDIX A
Statement of General Conditions
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 parry 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.
APPENDIX B
Figures: Vicinity Map, Site Plan
10.2
PO Box 82243
Kenmore, WA 98028
cobaltgeoogmail.com
2o6-331-1097
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Cobalt Geosciences, LLC
P.O. Box 82243 Proposed Single -Family Residence VICINITY MAP
Kenmore, WA 98028 1 12 6 th Place West
(2o6) 331-1097 % Jc 9
_ www.cobaltgeo.com Edmonds, Washington FIGURE 1
cobaltgeo(&gmail.com
HB-i
Approximate Hand Boring Location
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, WA 98028
(206) 331-1097
www.cobaltgeo.com
cobaltgeo(&gmail.com
Not to Scale
Proposed Single -Family Residence
17125 69th Place West
Edmonds, Washington
N
A
SITE PLAN
FIGURE 2
APPENDIX C
Exploration Logs
Unified Soil Classification System (USCS)
MAJOR DIVISIONS
SYMBOL
TYPICAL DESCRIPTION
Clean Gravels
GW
Well -graded gravels, gravels, gravel -sand mixtures, little or no fines
Gravels
(more than 50%
(less than 5%
fines)
GP
Poorly graded gravels, gravel -sand mixtures, little or no fines
COARSE
GRAINED
SOILS
of coarse fraction
retained on No. 4
sieve)
Gravels with
Fines
(more than 12%
fines)
GM
Silty gravels, gravel -sand -silt mixtures
GC
Clayey gravels, gravel -sand -clay mixtures
(more than 50%
retained on
Clean Sands
;•; sw
Well -graded sands, gravelly sands, little or no fines
No. 200 sieve)
Sands
(50% or more
of coarse fraction
(less than 5%
fines)
sP
Poorly graded sand, gravelly sands, little or no fines
passes the No. 4
sieve)
Sands with
Fines
sM
Silty sands, sand -silt mixtures
(more than 12%
fines)
sc
Clayey sands, sand -clay mixtures
ML
Inorganic silts of low to medium plasticity, sandy silts, gravelly silts,
FINE GRAINED
(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
Organic rganic
oL
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
elastic silt
CH
Inorganic clays of medium to high plasticity, sandy fat clay,
or gravelly fat clay
Organic
OHOrganic
clays of medium to high plasticity, organic silts
HIGHLY ORGANIC
SOILS
Primarily organic matter, dark in color,
and organic odor
PT
Peat, humus, swamp soils with high organic content (ASTM D4427)
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 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 (o.o8 mm)
Sand
-Fine
#200 to #40 (o.o8 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)
1 Moisture Content Definitions 1
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, WA 98028
Soil Classification Chart
Figure Ci
(206) 331-1097
_
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Log of Hand Boring HB-1
Date: March 3, 2020
Depth: 6'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
oo
4
Moisture Content (%)
Plastic Liquid
L
_ >
a OU
-
U
a
Limit Limit
t
o
t
LO
Material Description
DCP N-Value
o
m
U
o
?
G
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
211
--
SM
---------------------------------------------
Loose to medium dense, silty -fine to fine grained sand with gravel,
— 1
mottled yellowish brown to grayish brown.
(Weathered Glacial Till)
—2
—3
SM
Dense to very dense, silty -fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
—5
End of Hand Boring 6'
—7
—8
—9
— 10
12
Cobalt Geosciences, LLC
P.O. Box 82243 Proposed Single Family Residence
Kenmore, wa 98028 1 ]125 69th Place West Hand Boring
(206) 331-1097 Log
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Log of Hand Boring HB-2
Date: March 3, 2020
Depth: 6'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
oo
4
Moisture Content (%)
Plastic Liquid
L
_ >
a OU
-
U
a
Limit Limit
t
o
t
LO
Material Description
DCP N-Value
o
m
U
o
?
G
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
--
SM
---------------------------------------------
Loose to medium dense, silty -fine to fine grained sand with gravel,
— 1
mottled yellowish brown to grayish brown.
• j
(Weathered Glacial Till)
—2
— 3
•
SM
Dense to very dense, silty -fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
—4
—5
End of Hand Boring 6'
—7
—8
—9
— 10
12
Cobalt Geosciences, LLC
P.O. Box 82243 Proposed Single Family Residence
Kenmore, wa 98028 17125 69th Place West Hand Boring
(2o6) 331-1097 Log
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Log of Hand Boring HB-3
Date: March 3, 2020
Depth: 6'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
oo
4
Moisture Content (%)
Plastic Liquid
L
_ >
a OU
-
U
a
Limit Limit
t
o
t
LO
Material Description
DCP N-Value
o
m
U
o
?
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
--
SM
---------------------------------------------
Loose to medium dense, silty -fine to fine grained sand with gravel,
— 1
mottled yellowish brown to grayish brown.
• j
(Weathered Glacial Till)
—2
---
—3
----
--
SM
---------------------------------------------
Dense to very dense, silty -fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
—4
—5
End of Hand Boring 6'
—7
—8
—9
— 10
12
Cobalt Geosciences, LLC
P.O. Box 82243 Proposed Single Family Residence
Kenmore, wa 98028 17125 69th Place West Hand Boring
(2o6) 331-1097 Log
www.cobaltgeo.com Edmonds, Washington
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Log of Hand Boring HB-4
Date: March 3, 2020
Depth: 6'
Initial Groundwater: None
Contractor:
Elevation: N/A
Sample Type: Grab
Method: Hand Auger
Logged By: PH Checked By: SC
Final Groundwater: N/A
oo
4
Moisture Content (%)
Plastic Liquid
L
_ >
a OU
-
U
a
Limit Limit
t
o
t
LO
Material Description
DCP N-Value
o
m
U
o
?
0 10 20 30 40 5
Vegetation/Topsoil
---
----
--
211
--
SM
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Loose to medium dense, silty -fine to fine grained sand with gravel,
— 1
mottled yellowish brown to grayish brown.
(Weathered Glacial Till)
—2
3
----
---I
--
---------------------------------------------
SM
Dense to very dense, silty -fine to fine grained sand with gravel,
grayish brown, moist. (Glacial Till)
—4
—5
End of Hand Boring 6'
—7
—8
—9
— 10
12
Cobalt Geosciences, LLC
P.O. Box 82243 Proposed Single Family Residence
Kenmore, wa 98028 17125 69th Place West Hand Boring
(2o6) 331-1097 Log
www.cobaltgeo.com Edmonds, Washington
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