REVIEWED RESUB 2-BLD2024-0257+Geotechnical_Report+9.6.2024_6.27.26_PM+4486166NELSON GEOTECHNICAL
ASSOCIATES. INC.
March 1, 2024
Daniel Bockretsion
Via Email: danielbockretsion@gmail.com
Geotechnical Engineering Evaluation
Bockretsion Residential Redevelopment
1520 —10`h Place North
Edmonds, Washington
NGA File No. 1497924
Dear Daniel:
BLD2024-0257
17311-135th Ave. N.E. Suite A-500
Woodinville, WA 98072
(425) 486-1669
www.nelsongeotech.com
We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation —
Bockretsion Residential Development —1520 - 10' Place North — Edmonds, Washington." This report
summarizes our observations of the existing surface and subsurface conditions within the property and
provides general recommendations for the proposed site development. Our services were completed in
general accordance with the proposal signed by you on January 26, 2024.
The property is square in shape and covers 0.26 acres in area. It is currently occupied by a single-family
residence in the central to northwestern portion. The property is bordered by 101h Place North to the
northwest and neighboring residences on all other sides. Topographically, the site is relatively level to
gently sloping down to the northwest. We understand the plans for development include demolishing
the existing residence and constructing a new single-family residence. We have been requested to
provide this report for determination of geological hazards affecting the site, as well as development
considerations for the proposed residential redevelopment.
Specific grading and stormwater plans have not been developed, but we understand that stormwater
may be directed to on -site infiltration systems, if feasible. As part of our study, we propose to collect
samples and determine the infiltration rate based on the Department of Ecology's 2019 Stormwater
Management Manual for Western Washington (2019 SWMMWW). According to this manual, we
understand that long-term design infiltration rates for this site are to be determined by performing on -
site infiltration testing consisting of the Small Pilot Infiltration Test (PIT).
We explored the site subsurface soil conditions with one hand auger and five trackhoe excavated test pit
explorations on February 15, 2024. Our explorations indicated that the site was generally underlain by
competent, native recessional outwash soils at relatively shallow depths, below a surficial layer of
undocumented fill and/or topsoil.
It is our opinion that the planned development is feasible from a geotechnical standpoint, provided that
our recommendations are incorporated into the design and construction of this project. We recommend
that the future residence structures be founded on medium dense or better native recessional outwash
soils for bearing capacity and settlement considerations. These soils should generally be encountered
approximately 0.5 to 1.5 feet below the existing ground surface. However, deeper areas of loose soil
and/or undocumented fill could also exist within unexplored areas of the site. If undocumented fill is
encountered in unexplored areas of the site, it should be removed and replaced with structural fill for
foundation and pavement support.
RESUB
Sep 09 2024
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
NGA File No. 1497924
March 1, 2024
Summary — Page 2
In the attached report, we have also provided general recommendations for site grading, slabs -on -
grade, structural fill placement, utilities, pavement, erosion control, and drainage. We should be
retained to review and comment on final development plans and observe the earthwork phase of
construction. We recommend that Nelson Geotechnical Associates (NGA) be retained to review the
geotechnical aspects of the project plans prior to construction. We also recommend that NGA be
retained to provide monitoring and consultation services during construction to confirm that the
conditions encountered are consistent with those indicated by the explorations, to provide
recommendations for design changes should the conditions revealed during construction differ from
those anticipated, and to evaluate whether or not earthwork and foundation installation activities
comply with contract plans and specifications.
It has been a pleasure to provide service to you on this project. Please contact us if you have any
questions regarding this report or require further information.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
Khaled M. Shawish, PE
Principal
TABLE OF CONTENTS
INTRODUCTION..........................................................................................................1
SCOPE........................................................................................................................ 2
SITECONDITIONS....................................................................................................... 2
SurfaceConditions....................................................................................................... 2
Subsurface Conditions.................................................................................................. 3
Hydrogeologic Conditions............................................................................................ 4
SENSITIVE AREA EVALUATION....................................................................................4
SeismicHazard............................................................................................................. 4
ErosionHazard............................................................................................................. 5
CONCLUSIONS AND RECOMMENDATIONS.................................................................. 5
General......................................................................................................................... 5
Erosion Control and Slope Protection Measures......................................................... 6
Site Preparation and Grading....................................................................................... 6
Temporary and Permanent Slopes............................................................................... 7
Foundations.................................................................................................................. 8
StructuralFill................................................................................................................ 9
Slab-on-Grade............................................................................................................ 10
Pavements.................................................................................................................. 10
Utilities....................................................................................................................... 11
SiteDrainage.............................................................................................................. 11
CONSTRUCTION MONITORING................................................................................. 12
USEOF THIS REPORT................................................................................................ 13
LIST OF FIGURES
Figure 1— Vicinity Map
Figure 2 — Site Plan
Figure 3 — Soil Classification Chart
Figures 4 and 5 — Log of Exploration
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
1520 — 10th Place North
Edmonds, Washington
INTRODUCTION
This report presents the results of our geotechnical engineering investigation and evaluation of the
planned Bockretsion Residential Development project in Edmonds, Washington. The property is located
at 1520 — 10th Place North in Edmonds, Washington as shown on the Vicinity Map in Figure 1. The
parcel number is 27031300409000. The purpose of this study is to explore and characterize the site's
surface and subsurface conditions and to provide geotechnical recommendations for the planned site
development.
The property is square in shape and covers 0.26 acres in area. It is currently occupied by a single-family
residence in the central to northwestern portion. The property is bordered by 10th Place North to the
northwest and neighboring residences on all other sides. Topographically, the site is relatively level to
gently sloping down to the northwest. Vegetation within the site consists of grass yard areas,
landscaping plants, and sparse young to mature trees. We understand the plans for development
include demolishing the existing residence and constructing a new single-family residence. We have
been requested to provide this report for determination of geological hazards affecting the site, as well
as development considerations for the proposed residential redevelopment. The existing site layout is
shown on the Site Plan in Figure 2.
We have also evaluated the feasibility of stormwater infiltration within the site based on the
Washington State Department of Ecology's 2019 Stormwater Management Manual for Western
Washington. According to this manual, laboratory analysis of soil samples collected in the field can be
used to determine the infiltration system design along with long-term design infiltration rates due to the
site being located within recessional outwash soils. We have been requested by you to provide on -site
infiltration testing with a small-scale Pilot Infiltration Test (PIT) in accordance with the aforementioned
manual.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
SCOPE
NGA File No. 1497924
March 1, 2024
Page 2
The purpose of this study is to explore and characterize the site surface and subsurface conditions and
provide general recommendations for site development.
Specifically, our scope of services includes the following:
1. Reviewing available soil and geologic maps of the area as well as other relevant
geotechnical information, as provided.
2. Exploring the subsurface soil and groundwater conditions within the site using trackhoe-
excavated test pits. Excavation services were provided by NGA.
3. Providing long-term design infiltration rates based on on -site Small Pilot Infiltration
Testing (PIT) per the 2019 SWMMWW.
4. Performing laboratory grain -size sieve analysis on soil samples, as necessary.
5. Providing recommendations for earthwork, foundation support, and slabs -on -grade.
6. Providing recommendations for temporary and permanent slopes.
7. Providing recommendations for subsurface utilities and pavement subgrade
preparation, as necessary.
8. Providing our opinion on stormwater infiltration feasibility.
9. Providing recommendations for infiltration/bioretention system installation.
10. Providing general recommendations for site drainage and erosion control.
11. Documenting the results of our findings, conclusions, and recommendations in a written
geotechnical report.
141kM4191L107111111[a]►1.1
Surface Conditions
The property is square in shape and covers 0.26 acres in area. It is currently occupied by a single-family
residence in the central to northwestern portion. The property is bordered by 10th Place North to the
northwest and neighboring residences on all other sides. Topographically, the site is relatively level to
gently sloping down to the northwest. Vegetation within the site consists of grass yard areas,
landscaping plants, and sparse young to mature trees. We did not observe any surface water or seepage
emitting from the site slopes within the site during our visit on February 15, 2024.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
Subsurface Conditions
NGA File No. 1497924
March 1, 2024
Page 3
Geology: The geologic units for this site are shown on Geologic Map of the Edmonds East and Part of the
Edmonds West Quadrangles, Washington, by James P. Minard, (USGS, 1983). The site is mapped on the
border of transitional beds (Qtb), recessional outwash (Qvr), Olympia gravel (Qog), and undivided till
(Qtu). The transitional bed deposits are described as clay, silt, and fine to very fine sand. The
recessional outwash is generally described as mostly stratified sand and gravel with minor silt and clay
layers. The Olympia gravel is described as consisting of stratified, fluvial sand and gravel. The undivided
till is generally described as a non -sorted mixture of clay, silt, pebbles, cobbles, and boulders all in
variable amounts. Our explorations within the site generally encountered surficial topsoil and/or
undocumented fill underlain by medium dense to dense, fine to coarse sand with varying amounts of
gravel, silt, and roots, which we interpreted as native recessional outwash deposits at depth.
Explorations: The subsurface conditions within the site were explored on February 15, 2024, with one
hand auger and five trackhoe excavated test pits extending to depths in the range of 5.0 to 10.0 feet
below the existing ground surface. One of the test pits was utilized for infiltration testing. The
approximate locations of our explorations are shown on the Site Plan in Figure 2.
A geologist from NGA was present during the explorations, examined the soils and geologic conditions
encountered, obtained samples of the different soil types, and maintained logs of the explorations. The
soils were visually classified in general accordance with the Unified Soil Classification System, presented
in Figure 3. The logs of our explorations are attached to this report and are presented as Figures 4 and
5. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed
description of the subsurface conditions, the exploration logs should be reviewed.
At the surface of all of our explorations, we generally encountered 0.5- to 1.5-feet of surficial
topsoil/undocumented fill. Underlying the surficial topsoil, we encountered orange -brown, fine to
medium sand with silt, roots, and iron -oxide weathering, which we interpreted as weathered recessional
outwash deposits. Underlying the weathered recessional outwash, we encountered gray, fine to coarse
sand with varying amounts of gravel and roots, which we interpreted as unweathered recessional
outwash deposits. All of our explorations were completed within the unweathered recessional outwash
deposits at depths in the range of 5.0- to 10.0-feet below the existing ground surface.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
Hydrogeologic Conditions
NGA File No. 1497924
March 1, 2024
Page 4
We did not encounter groundwater within our explorations. If groundwater is encountered during
construction, we would interpret this water as perched water. Perched water occurs when surface
water infiltrates. Perched water does not represent a regional groundwater "table" within the upper less
pervious soil horizons. Perched water tends to vary spatially and is dependent upon the amount of
rainfall. We would expect the amount of perched groundwater to decrease during drier times of the
year and increase during wetter periods.
SENSITIVE AREA EVALUATION
Seismic Hazard
We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project.
Since competent glacial soils are inferred to underlie the site at depth, the site conditions best fit the
IBC description for Site Class D.
Table 1 below provides seismic design parameters for the site that are in conformance with the
2018 IBC, which specifies a design earthquake having a two percent probability of occurrence in 50
years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps.
Table 1— 2018 IBC Seismic Design Parameters
Site Class
Spectral Acceleration
Spectral Acceleration
Site Coefficients
Design Spectral
at 0.2 sec. (g)
at 1.0 sec. (g)
Response
S,
S1
Parameters
Fa
Fv
Sps
Sol
D
1.295
0.457
1.000
Null
0.863
Null
The spectral response accelerations were obtained from the USGS Earthquake Hazards Program
Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude.
Hazards associated with seismic activity include liquefaction potential and amplification of ground
motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the
groundwater table. It is our opinion that the medium dense or better glacial deposits interpreted to
underlie the site have a low potential for liquefaction or amplification of ground motion.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
Erosion Hazard
NGA File No. 1497924
March 1, 2024
Page 5
The criteria used for determination of the erosion hazard for affected areas include soil type, slope
gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative
cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil
Survey of Snohomish County Area, Washington, by the Natural Resources Conservation Service (NRCS),
classifies the development portions of the site as Alderwood-Urban land complex, 2 to 8 percent slopes.
The erosion hazard for these soils is listed as moderate. Based on our observations and the material
encountered, we would interpret this site as having a low to moderate erosion hazard where the
surficial soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas
where the site is not disturbed.
CONCLUSIONS AND RECOMMENDATIONS
General
It is our opinion that the planned development within the site is generally feasible from a geotechnical
standpoint. Our explorations indicated that the site was generally underlain by competent native glacial
till soils below a surficial layer of topsoil and undocumented fill. The glacial till soils encountered at
depth should provide adequate support for foundation, slab, and pavement loads. We recommend that
the new residence structures be founded on medium dense or better native bearing recessional
outwash soils for bearing capacity and settlement considerations. These soils should generally be
encountered approximately 0.5- to 1.5-feet below the existing ground surface based on our
explorations. However, deeper areas of loose soil and/or undocumented fill could also exist within
unexplored areas of the site. If undocumented fill is encountered in unexplored areas of the site, it
should be removed and replaced with structural fill for foundation and pavement support. We
recommend that NGA be retained to review proposed grading plans once they are developed and
allowed to provide alternative foundation support recommendations as needed.
We performed on -site infiltration testing based on the 2019 Department of Ecology Stormwater Manual
for Western Washington. The onsite testing consisted of performing one small-scale PIT within
Infiltration Test Pit One. Test procedures and results are detailed in the next sections. However, based
on our observations, results of the onsite testing, and the sandy nature of the native recessional
outwash deposits that underlie the site, it is our opinion that the on -site native soils encountered at
depth are conducive for traditional stormwater infiltration systems. More detailed information on our
infiltration tests and drainage recommendations can be found in the Stormwater Infiltration and Site
Drainage subsections of this report.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
NGA File No. 1497924
March 1, 2024
Page 6
The soils encountered on this site are considered moisture -sensitive and will disturb easily when wet. To
lessen the potential impacts of construction on the slopes and to reduce cost overruns and delays, we
recommend that construction take place during the drier summer months. If construction takes place
during the rainy months, additional expenses and delays should be expected. Additional expenses could
include the need for placing erosion control and temporary drainage measures to protect the slopes, the
need for placing a blanket of rock spalls on exposed subgrades, and construction traffic areas prior to
placing structural fill, and the need for importing all-weather material for structural fill.
Erosion Control and Slope Protection Measures
The erosion hazard for the on -site soils is considered to be moderate to severe but the actual hazard will
be dependent on how the site is graded and how water is allowed to concentrate. Best Management
Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be
protected from erosion. Erosion control measures may include diverting surface water away from the
stripped or disturbed areas. Silt fences and/or straw bales should be erected to prevent muddy water
from leaving the site or flowing over the slopes. Stockpiles should be covered with plastic sheeting
during wet weather and stockpiled material should be placed near site slopes. Disturbed areas should
be planted as soon as practical, and the vegetation should be maintained until it is established. The
erosion potential for areas not stripped of vegetation should be low.
Site Preparation and Grading
After erosion control measures are implemented, site preparation should consist of stripping any loose
soils and undocumented fill to expose medium dense or better native soil in foundation, slab -on -grade,
and pavement areas. The stripped materials should be removed from the site or stockpiled for later use
as landscaping fill. Based on our observations, we anticipate stripping depths of 0.5- to 1.5-feet within
the proposed development areas. However, additional stripping may be required if areas of deeper
undocumented fill and/or loose soil are encountered in unexplored areas of the site.
If the ground surface, after site stripping, should appear to be loose, it should be compacted to a non -
yielding condition. Areas observed to pump or weave during compaction should be over -excavated and
replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in any slab
areas, the loose soils should be removed and replaced with rock spalls or granular structural fill. If
significant surface water flow is encountered during construction, this flow should be diverted around
areas to be developed, and the exposed subgrades should be maintained in a semi -dry condition.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1497924
Bockretsion Residential Development March 1, 2024
Edmonds, Washington Page 7
This site is underlain by moisture -sensitive soils. Due to these conditions, special site stripping and
grading techniques might be necessary, especially if grading is attempted in wet weather. These could
include using large excavators equipped with wide tracks and a smooth bucket to complete site grading
and promptly covering exposed subgrades with a layer of crushed rock for protection. If wet conditions
are encountered or construction is attempted in wet weather, the subgrade should not be compacted as
this could cause further subgrade disturbance. In wet conditions, it may be necessary to cover the
exposed subgrade with a layer of crushed rock as soon as it is exposed to protect the moisture sensitive
soils from disturbance by machine or foot traffic during construction. The prepared subgrade should be
protected from construction traffic and surface water should be diverted around prepared subgrade.
Shallow groundwater, if encountered, should be intercepted with cut-off drains and routed around the
planned grading area, or the groundwater should be controlled with sump -pumps or dewatering
systems. Failure to follow these recommendations could cause erosion and failures on the slopes, as
well as result in inadequate subgrades.
The site soils are considered to be moisture -sensitive and will disturb easily when wet. We recommend
that construction take place during the drier summer months if possible. However, if construction takes
place during the wet season, additional expenses and delays should be expected due to the wet
conditions. Additional expenses could include the need for placing a blanket of rock spalls on exposed
subgrades, construction traffic areas, and paved areas prior to placing structural fill. Wet weather
grading will also require additional erosion control and site drainage measures. NGA should be retained
to evaluate the suitability of all on -site and imported structural fill material during construction.
Temporary and Permanent Slopes
Temporary cut slope stability is a function of many factors, including the type and consistency of soils,
depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open, and the
presence of surface or groundwater. It is exceedingly difficult under these variable conditions to
estimate a stable, temporary, cut slope angle. Therefore, it should be the responsibility of the
contractor to maintain safe slope configurations at all times as indicated in OSHA guidelines for cut
slopes.
The following information is provided solely for the benefit of the owner and other design consultants
and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility
for job site safety. Job site safety is the sole responsibility of the project contractor.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1497924
Bockretsion Residential Development March 1, 2024
Edmonds, Washington Page 8
For planning purposes, we recommend that temporary cuts in the upper undocumented fill soils be no
steeper than 2 Horizontal to 1 Vertical (2H:1V). Temporary cuts in the competent native recessional
outwash soils encountered at depth should be no steeper than 1.5H:1V. If temporary cut excavations
are not able to achieve the recommended inclinations, we recommend that the cuts be temporarily
shored. If significant groundwater seepage or surface water flow were encountered, we would expect
that flatter inclinations would be necessary. We recommend that cut slopes be protected from erosion.
The slope protection measures may include covering cut slopes with plastic sheeting and diverting
surface runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper
than four feet if worker access is necessary. We recommend that cut slope heights and inclinations
conform to appropriate OSHA/WISHA regulations.
Permanent cut and fill slopes should be no steeper than 2H:1V. However, flatter inclinations may be
required in areas where loose soils are encountered. Permanent slopes should be vegetated, and the
vegetative cover maintained until established.
Foundations
Conventional shallow spread foundations should be placed on undisturbed medium dense or better
native glacial till soils. Medium dense to dense soils should be encountered roughly 0.5- to 1.5-feet
below the ground surface based on our explorations within the development area; however, deeper
loose soils may be encountered in unexplored areas of the site. Where undocumented fill or less dense
soils are encountered at footing bearing elevation, the subgrade should be over -excavated to expose
suitable bearing soil.
Footings should extend at least 18 inches below the lowest adjacent finished ground surface for frost
protection and bearing capacity considerations. Foundations should be designed in accordance with the
2018 IBC. Footing widths should be based on the anticipated loads and allowable soil bearing pressure.
Water should not be allowed to accumulate in footing trenches. All loose or disturbed soil should be
removed from the foundation excavation prior to placing concrete.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
NGA File No. 1497924
March 1, 2024
Page 9
For foundations constructed as outlined above, we recommend an allowable design bearing pressure of
no more than 2,000 pounds per square foot (psf) be used for the design of footings founded on the
medium dense or better native glacial soils or structural fill extending to the competent native material.
The foundation bearing soil should be evaluated by a representative of NGA. We should be consulted if
higher bearing pressures are needed. Current IBC guidelines should be used when considering increased
allowable bearing pressure for short-term transitory wind or seismic loads. Potential foundation
settlement using the recommended allowable bearing pressure is estimated to be less than 1-inch total
and %-inch differential between adjacent footings or across a distance of about 20 feet, based on our
experience with similar projects.
Lateral loads may be resisted by friction on the base of the footing and passive resistance against the
subsurface portions of the foundation. A coefficient of friction of 0.35 may be used to calculate the base
friction and should be applied to the vertical dead load only. Passive resistance may be calculated as a
triangular equivalent fluid pressure distribution. An equivalent fluid density of 200 pounds per cubic
foot (pcf) should be used for passive resistance design for a level ground surface adjacent to the footing.
This level surface should extend a distance equal to at least three times the footing depth. These
recommended values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values
for frictional and passive resistance, respectively. To achieve this value of passive resistance, the
foundations should be poured "neat" against the native medium dense soils or compacted fill should be
used as backfill against the front of the footing. We recommend that the upper one foot of soil be
neglected when calculating the passive resistance.
Structural Fill
General: Fill placed beneath foundations, pavement, or other settlement -sensitive structures should be
placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods
and standards, and is monitored by an experienced geotechnical professional or soils technician. Field
monitoring procedures would include the performance of a representative number of in -place density
tests to document the attainment of the desired degree of relative compaction. The area to receive the
fill should be suitably prepared as described in the Site Preparation and Grading subsection prior to
beginning fill placement.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
NGA File No. 1497924
March 1, 2024
Page 10
Materials: Structural fill should consist of a good quality, granular soil, free of organics and other
deleterious material, and be well graded to a maximum size of about three inches. All-weather fill
should contain no more than five -percent fines (soil finer than U.S. No. 200 sieve, based on that fraction
passing the U.S. 3/4-inch sieve). Some of the on -site soils may be suitable for use as structural fill, but
this will depend on moisture content at the time of construction. We should be retained to evaluate all
proposed structural fill material prior to placement.
Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling
should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and
be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building
areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry
density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557
Compaction Test procedure. The moisture content of the soils to be compacted should be within about
two percent of optimum so that a readily compactable condition exists. It may be necessary to over -
excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All
compaction should be accomplished by equipment of a type and size sufficient to attain the desired
degree of compaction and should be tested.
Slab -on -Grade
Slabs -on -grade should be supported on subgrade soils prepared as described in the Site Preparation and
Grading subsection of this report. We recommend that all floor slabs be underlain by at least six inches
of free -draining gravel with less than three percent by weight of the material passing Sieve #200 for use
as a capillary break. A suitable vapor barrier, such as heavy plastic sheeting (6-mil minimum), should be
placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to
cover the vapor barrier. This sand layer may be used to protect the vapor barrier membrane and to aid
in curing the concrete.
Pavements
Pavement subgrade preparation and structural filling where required, should be completed as
recommended in the Site Preparation and Grading and Structural Fill subsections of this report. The
pavement subgrade should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify
soft or yielding areas that require repair. The pavement section should be underlain by a minimum of
six inches of clean granular pit run. We should be retained to observe the proof -rolling and recommend
repairs prior to placement of the asphalt or hard surfaces.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
NGA File No. 1497924
March 1, 2024
Page 11
Utilities
We recommend that underground utilities be bedded with a minimum six inches of pea gravel prior to
backfilling the trench with on -site or imported material. Trenches within settlement sensitive areas
should be compacted to 95 percent of the modified proctor as described in the Structural Fill subsection
of this report. Trenches located in non-structural areas should be compacted to a minimum 90 percent
of the maximum dry density. Trench backfill compaction should be tested.
Site Drainage
Infiltration Testing
General: The 2019 WSDOE Stormwater Management Manual for Western Washington was utilized to
determine the long-term design infiltration rate of the site soils. According to this manual, on -site
infiltration testing consisting of the Small -Scale Pilot Infiltration Test (PIT) was used to determine the
long-term design infiltration rates. The infiltration pit measured approximately 4.0-feet long by 3.0-feet
wide by 3.5-feet deep. Infiltration Test Pit 1 was filled up to a level 12 inches and this level was
maintained for the soaking period for approximately 6 hours. After the 6-hour soaking period was
completed, the water level was maintained at approximately 12-inches while the water flow rate into
the hole was monitored using Great Plains Industries (GPI) TM 050 water flow meter. The flow rate for
infiltration test pit one stabilized at 3.98 gallons per minute (approximately 238.8 gallons per hour)
resulting in an infiltration rate of 31.9 inches per hour. The water was shut off after the steady-state
period and the water level within the pit was monitored every 15 minutes for one hour. After thirty
minutes, the water level in infiltration test pit one had dropped 12 inches. This resulted in an infiltration
rate of 24 inches per hour.
Based on the results of our onsite infiltration testing, it is our opinion that the granular recessional
outwash soils encountered within the site are conducive for traditional stormwater infiltration. We
have selected an overall measured field rate of 24 inches per hour obtained from the falling head
portion of the test within Infiltration Pit 1 be utilized in determining the long-term design infiltration
rate for the infiltration systems within the site. We referenced the 2019 WSDOE Stormwater
Management Manual for Western Washington that applies correction factors to the field measured
infiltration rate to generate a long-term design infiltration rate. Correction factors of 0.50, 0.65, and
0.80 were utilized in this equation for Ftesting, Fgeometry, Fplugging, respectively. Using these correction
factors, we calculated a long-term design infiltration rate of 6.14 inches per hour to be utilized to design
on -site infiltration systems, provided that the base of the system terminates within the native clean
recessional outwash soils encountered at depth.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
NGA File No. 1497924
March 1, 2024
Page 12
NGA should be retained to evaluate the soils exposed in the infiltration system excavations at the time
of construction. This is imperative to ensure that soils exposed in the system can provide the design
infiltration rate.
The stormwater manual recommends a five-foot separation between the base of an infiltration system
and any underlying bedrock, impermeable horizon, or groundwater. We did not encounter any
groundwater to the depths explored throughout the site and as such, we do not anticipate groundwater
to adversely impact the infiltration system if the base of the infiltration system is founded in the native
recessional outwash deposits.
Subsurface Drainage: If groundwater is encountered during construction, we recommend that the
contractor slope the bottom of the excavation and collect the water into ditches and small sump pits
where the water can be pumped out of the excavation and routed into a suitable outlet. We
recommend that the residence down spouts and footing drains be tightlined to an appropriate discharge
location.
We recommend the use of footing drains around structures. Footing drains should be installed at least
one foot below planned finished floor elevation. The drains should consist of a minimum 4-inch-
diameter, rigid, slotted or perforated, PVC pipe surrounded by free -draining material wrapped in a filter
fabric. We recommend that the free -draining material consist of an 18-inch-wide zone of clean (less
than three -percent fines), granular material placed along the back of walls. Washed rock is an
acceptable drain material, or drainage composite may be used instead. The free -draining material
should extend up the wall to one foot below the finished surface. The top foot of soil should consist of
low permeability soil placed over plastic sheeting or building paper to minimize the migration of surface
water or silt into the footing drain. Footing drains should discharge into tightlines leading to an
appropriate collection and discharge point with convenient cleanouts to prolong the useful life of the
drains. Roof drains should not be connected to wall or footing drains.
CONSTRUCTION MONITORING
We recommend that we be retained to provide construction monitoring services to evaluate conditions
encountered in the field with respect to anticipated conditions, to provide recommendations for design
changes should the conditions differ from anticipated, and to evaluate whether construction activities
comply with contract plans and specifications.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Bockretsion Residential Development
Edmonds, Washington
USE OF THIS REPORT
NGA File No. 1497924
March 1, 2024
Page 13
NGA has prepared this report for Daniel Bockretsion and associated agents, for use in the planning and
design of the development on this site only. The scope of our work does not include services related to
construction safety precautions and our recommendations are not intended to direct the contractors'
methods, techniques, sequences, or procedures, except as specifically described in our report for
consideration in design. There are possible variations in subsurface conditions between the explorations
and also with time. Our report, conclusions, and interpretations should not be construed as a warranty
of subsurface conditions. A contingency for unanticipated conditions should be included in the budget
and schedule. We recommend that we be retained to review the project plans after they have been
developed to determine that recommendations in the report were incorporated into project plans.
We recommend that NGA be retained to review final plans prior to construction. We also recommend
that NGA be retained to provide monitoring and consultation services during construction to confirm
that the conditions encountered are consistent with those indicated by the explorations, to provide
recommendations for design changes should the conditions revealed differ from those anticipated, and
to evaluate whether or not earthwork and foundation installation activities comply with contract plans
and specifications. We should be contacted a minimum of one week prior to construction activities and
could attend pre -construction meetings if requested.
Within the limitations of scope, schedule, and budget, our services have been performed in accordance
with generally accepted geotechnical engineering practices in effect in this area at the time this report
was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and
opinions are a means to identify and reduce the inherent risks to the owner.
...
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1497924
Bockretsion Residential Development March 1, 2024
Edmonds, Washington Page 14
It has been a pleasure to provide service to you on this project. If you have any questions or require
further information, please call.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
-:)p,� uv�-
Daniel J. O'Dell
Project Geologist
3.01.2024
Khaled M. Shawish, PE
Principal
DJO:FKS:KMS:dy
Five Figures Attached
NELSON GEOTECHNICAL ASSOCIATES, INC.
VICINITY MAP
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Project Number
Alh..IIELSOti GEOTECHIIICAL
No.
Date
Revision
By
CK
Bockretsion Residential
'
1497924
Redevelopment
ASSOCIATES, Inc
1
2/26/24
Original
ABT
ono OX
(Y
Figure 1
J
Vicinity Map
Af°vllle—°e Wenatcheeuse °f°e
17311-135th Ave. NE, A-500 105 PaloSt.
Wood-ille, WA 98072 Wenatchee, WA 98801
Q
y
www.nelsongeotech.com (425) 486-1669 / Fax. 481-2510 (509) 665-7696 / Fax 665-7692
U
U
Site Plan
HA-1
TP-4
I
I '
1
i
LEGEND
— • —
Property line
INF-1
—�—
Number and approximate
I
location of infiltration test pit
TP-1
—�—
Number and approximate
location of test pit
HA-1
—�—
Number and approximate
location of hand auger
0 15 30
Approximate Scale: 1 inch = 15 feet
Reference: Site Plan based on field measurements, observations, and aerial parcel map review
Project Number Bockretsion Residential h. nELson GEOTECHIIICOL No. Date Revision By I cK
1497924 Redevelopment ASSOCIATES, inc 1 2/26/24 Original ABT ono
Site Plan WoodMAI� MC. Wanatchee ice
Figure 2 17311-1351h Ave. NE, A-500 105 Palause St.
Wcotllnville,WA 98072 Wenatchee, WA 98801
www.nelsangeolech.ca1 (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax 665-7692
UNIFIED SOIL CLASSIFICATION SYSTEM
GROUP
MAJOR DIVISIONS
GROUP NAME
SYMBOL
CLEAN
GW
WELL -GRADED, FINE TO COARSE GRAVEL
COARSE-
GRAVEL
GRAVEL
GP
POORLY -GRADED GRAVEL
GRAINED
MORE THAN 50 %
GRAVEL
GM
SILTY GRAVEL
OF COARSE FRACTION
RETAINED ON
SOILS
NO.4 SIEVE
WITH FINES
GC
CLAYEY GRAVEL
SAND
CLEAN
SW
WELL -GRADED SAND, FINE TO COARSE SAND
SAND
SP
POORLY GRADED SAND
MORE THAN 50 %
RETAINED ON
MORE THAN 50 %
NO. 200 SIEVE
OF COARSE FRACTION
SAND
SM
SILTY SAND
PASSES NO. 4 SIEVE
WITH FINES
SC
CLAYEY SAND
FINE -
SILT AND CLAY
ML
SILT
INORGANIC
GRAINED
LIQUID LIMIT
CL
CLAY
LESS THAN 50 %
SOILS
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SILT AND CLAY
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
INORGANIC
MORE THAN 50 %
PASSES
LIQUID LIMIT
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
NO. 200 SIEVE
50 % OR MORE
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES:
1) Field classification is based on visual SOIL MOISTURE MODIFIERS:
examination of soil in general
accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to
the touch
2) Soil classification using laboratory tests
is based on ASTM D 2488-93. Moist - Damp, but no visible water.
3) Descriptions of soil density or Wet - Visible free water or saturated,
consistency are based on usually soil is obtained from
interpretation of blowcount data, below water table
visual appearance of soils, and/or
test data.
Project Number
nELSOn GEOTEnninl_
No.
Date
Revision
By
cK
Bockretsion Residential
�
2/26/24
Original
ABT
ono
1497924
Redevelopment
OSSOCIATES, inc,
Figure 3
Soil Classification Chart
ui ""135thA°e°""e Wa"a`°hee.S"e
17311-1351h Ave. NE, A-500 105 Palouse St.
Woodinville, WA 98072 Wenatchee, WA 98801
www.nelsongeoiech.com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692
LOG OF EXPLORATION
DEPTH (FEET) USCS SOIL DESCRIPTION
INFILTRATION TEST PIT
ONE
0.0 - 1.5 TOPSOIL / FILL
1.5 - 2.0 SP-SM ORANGE -BROWN, FINE TO MEDIUM SAND WITH SILT, ROOTS, AND IRON -OXIDE WEATHERING
(MEDIUM DENSE, MOIST)
2.0 - 3.5 SP GRAY, FINE TO MEDIUM SAND WITH TRACE GRAVEL AND TRACE ROOTS
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
INFILTRATION TEST PIT WAS COMPLETED AT 3.5 FEET ON 2/15/24
0.0 - 1.5 TOPSOIL / FILL
1.5 - 2.0 SP-SM ORANGE -BROWN, FINE TO MEDIUM SAND WITH SILT, ROOTS, AND IRON -OXIDE WEATHERING
(MEDIUM DENSE, MOIST)
2.0 - 6.0 SP GRAY, FINE TO MEDIUM SAND WITH TRACE GRAVEL AND TRACE ROOTS
(MEDIUM DENSE TO DENSE, MOIST)
6.0 - 10.0 GRAY, FINE TO COARSE SAND WITH GRAVEL (MEDIUM DENSE TO DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 10.0 FEET ON 2/15/24
TEST PIT TWO
111<1m 11101;Y0111WAy11111
1.5 - 2.0
SP-SM
ORANGE -BROWN, FINE TO MEDIUM SAND WITH SILT, ROOTS, AND IRON -OXIDE WEATHERING
(MEDIUM DENSE, MOIST)
2.0 - 6.0
SP
GRAY, FINE TO MEDIUM SAND WITH TRACE GRAVEL AND TRACE ROOTS
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 6.0 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 6.0 FEET ON 2/15/24
TEST PIT THREE
0.0 - 0.5
TOPSOIL / FILL
0.5 - 1.5
SP-SM
ORANGE -BROWN, FINE TO MEDIUM SAND WITH SILT, ROOTS, AND IRON -OXIDE WEATHERING
(MEDIUM DENSE, MOIST)
1.5 - 6.0
SP
GRAY, FINE TO MEDIUM SAND WITH TRACE GRAVEL AND TRACE ROOTS
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 6.0 FEET ON 2/15/24
ABT:DJO
NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 1497924
FIGURE 4
LOG OF EXPLORATION
DEPTH (FEET)
USCS
SOIL DESCRIPTION
TEST PIT FOUR
0.0 - 1.0
TOPSOIL / FILL
1.0 - 2.5
SP-SM
ORANGE -BROWN, FINE TO MEDIUM SAND WITH SILT, ROOTS, AND IRON -OXIDE WEATHERING
(MEDIUM DENSE, MOIST)
2.5 - 6.0
SP
GRAY, FINE TO MEDIUM SAND WITH TRACE GRAVEL AND TRACE ROOTS
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 3.0 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 6.0 FEET ON 2/15/24
HAND AUGER ONE
0.0 - 1.5
TOPSOIL / FILL
1.5 - 3.5
SP-SM
ORANGE -BROWN, FINE TO MEDIUM SAND WITH SILT, ROOTS, AND IRON -OXIDE WEATHERING
(MEDIUM DENSE, MOIST)
3.5 - 5.0
SP
GRAY, FINE TO MEDIUM SAND WITH TRACE GRAVEL AND TRACE ROOTS
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERD
HAND AUGER CAVING WAS NOT ENCOUNTERED
HAND AUGER WAS COMPLETED AT 5.0 FEET ON 2/15/24
ABT:DJO NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 1497924
FIGURE 5