DNS BLD2022-0279+Geotechnical_Report+3.3.2022_11.52.46_AM+2715102NGA
Main Office
17311— 135'h Ave NE, A-500
Woodinville, WA 98072
(425) 486-1669 • FAX (425) 481-2510
December 21, 2018
Ms. Lindell Graham
1414 - 91h Avenue North
Edmonds, WA 98020
lindellgghotmail.com
NELSON GEOTECHNICAL
ASSOCIATES, INC.
GEOTECHNICAL ENGINEERS & GEOLOGISTS
Geotechnical Engineering Evaluation
Sundstone Short Plat
1414 - 9th Avenue North
Edmonds, Washington
NGA File No. 1074518
Dear Ms. Graham:
Engineering -Geology Branch
5526 Industry Lane, #2
East Wenatchee, WA 98802
(509) 665-7696 • FAX (509) 665-7692
We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Sundstone
Short Plat — 1414 - 9th Avenue North — Edmonds, Washington." This report summarizes our
observations of the existing surface and subsurface conditions within the site, and provides general
recommendations for the proposed site development. Our services were completed in general accordance
with the proposal signed by you on November 18, 2018.
The level to gently west -sloping site is occupied by a multistory multifamily residential structure on the
eastern portion of the property, and is vacant on the western portion. The property is bordered to the
north by existing residential properties, to the west by 9th Avenue North, to the south by an easement
along Puget Drive (WA-524), and to the east by commercial development. We understand that the 1.18-
acre parcel will undergo a short plat to create at least one additional residential parcel within the vacant
western portion, and that the Sundstone Condominium structure is intended to remain in the east. We
understand that stormwater generated within this site will be directed to on -site infiltration facilities if
feasible. Specific grading and stormwater plans were not available when this report was prepared, but
based on conversations with you, we understand that stormwater may be directed to on -site infiltration
facilities, if feasible. In addition to providing recommendations for the development of the new single-
family residence(s), we have been requested to evaluate the infiltration capacity of the site soils. The City
of Edmonds utilizes the 2014 WSDOE Stormwater Management Manual for Western Washington to
determine the design of infiltration facilities. According to this manual, on -site infiltration testing
consisting of the small Pilot Infiltration Test (PIT) is used to determine the long-term design infiltration
rates.
We performed four test pit explorations throughout the property, one of which we utilized for our small-
scale pilot infiltration testing (PIT). Our explorations indicated that the site was underlain by surficial
undocumented fill with competent, native soils at depth.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Summary — Page 2
It is our opinion that the proposed site development is feasible from a geotechnical engineering
standpoint, provided that our recommendations for site development are incorporated into project plans.
In general, the native soils underlying the site should adequately support the planned structures.
Foundations should be advanced through any loose soils down to the competent native bearing material
interpreted to underlie the site, for bearing capacity and settlement considerations. These soils should
generally be encountered approximately two to three feet below the existing ground surface, based on our
explorations. If loose soils or undocumented fill are encountered in unexplored areas of the site, they
should be removed and replaced with structural fill for foundation and pavement support. Final
stormwater plans have also not been developed, but we understand that on -site infiltration is being
considered for this site. Based on our onsite testing, it is our opinion that the onsite soils are not
conducive to traditional methods of stormwater infiltration, however low -impact design systems may be
feasible. The subsurface soils generally consisted of surficial undocumented fill soils underlain by silty
fine to medium sand with varying amounts of gravel and iron -oxide weathering that we interpreted as
native glacial soils at relatively shallow depths. We recommend that any low impact stormwater
infiltration systems be designed in accordance with the 2014 Department of Ecology Stormwater
Management Manual for Western Washington.
In the attached report, we have also provided general recommendations for site grading, slabs -on -grade,
structural fill placement, retaining walls, erosion control, and drainage. We should be retained to review
and comment on final development plans and observe the earthwork phase of 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 the work 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.
nowl,
a
Khaled M. Shawish, PE
Principal Engineer
TABLE OF CONTENTS
INTRODUCTION............................................................................................................. I
SCOPE............................................................................................................................... I
SITECONDITIONS......................................................................................................... 2
SurfaceConditions....................................................................................................... 2
SubsurfaceConditions.................................................................................................. 2
Hydrogeologic Conditions........................................................................................... 3
SENSITIVE AREA EVALUATION............................................................................... 3
SeismicHazard............................................................................................................. 3
ErosionHazard............................................................................................................. 4
CONCLUSIONS AND RECOMMENDATIONS.......................................................... 4
General.........................................................................................................................
4
ErosionControl............................................................................................................ 5
Site Preparation and Grading....................................................................................... 5
Temporary and Permanent Slopes............................................................................... 6
Foundations.................................................................................................................. 7
RetainingWalls............................................................................................................ 8
StructuralFill................................................................................................................ 9
Slab-on-Grade.............................................................................................................. 9
Pavements...................................................................................................................
10
Utilities.......................................................................................................................
10
SiteDrainage..............................................................................................................
10
CONSTRUCTION MONITORING............................................................................. 12
USE OF THIS REPORT................................................................................................ 12
LIST OF FIGURES
Figure 1
— Vicinity Map
Figure 2
— Site Plan
Figure 3
— Soil Classification Chart
Figure 4
— Test Pit Logs
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
1414 - 9th Avenue North
Edmonds, Washington
INTRODUCTION
This report presents the results of our geotechnical engineering investigation and evaluation of the
planned Sundstone Short Plat Development project in the Edmonds, Washington. The project site is
located at 1414 - 9th Avenue West, as shown on the Vicinity Map in Figure 1. 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. For our use in preparing this report, we have been
provided with an undated and untitled preliminary site plan showing the existing and proposed conditions
within the site.
The property is currently occupied by a multi -level multi -family residential structure within the eastern
portion of the site, while the subject western portion of the site is undeveloped. We understand the
proposed developments within the site will include short platting the property and constructing a new
residential structure within the lower western parcel. Topography within the subject portion of the site
slopes gently from east to west. Vegetation generally consists of grass -covered yard areas and scattered
young to mature trees. Final development and grading plans have not been prepared at the time this
report was issued. Final stormwater plans have also not been developed, however, we understand that
stormwater may be directed to on -site infiltration systems, if feasible. The existing and proposed site
layout is shown on the Site Plan in Figure 2.
SCOPE
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:
Review available soil and geologic maps of the area.
2. Explore the subsurface soil and groundwater conditions within the site with track -
mounted backhoe-excavated test pits. Excavation services were provided by NGA.
3. Perform laboratory grain -size sieve analysis on soil samples, if necessary.
4. Provide recommendations for earthwork, foundation support, and slabs -on -grade.
5. Provide recommendations for temporary and permanent slopes.
6. Provide recommendations for pavement subgrade.
7. Provide recommendations for site drainage and erosion control.
8. Provide our opinion on the feasibility of infiltration for the onsite soils.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 2
9. Provide long-term design infiltration rates based on on -site Pilot Infiltration Testing (PIT)
per the 2014 WSDOE Manual. One test to be performed within each proposed property.
10. Provide recommendations for infiltration system installation.
11. Document the results of our findings, conclusions, and recommendations in a written
geotechnical report.
SITE CONDITIONS
Surface Conditions
The overall property consists of an approximately 1.18-acre parcel with a multi -family residential
structure within the eastern portion of the site and vacant undeveloped land to the west. We understand
the proposed improvements will consist of dividing the property into two parcels and constructing a
residential structure within the western parcel. The proposed western parcel is vegetated with grass lawn
and few scattered young to mature trees. The ground surface slopes gently from east to west. The subject
site is bound to the east by the multi -family structure, to the north by a residential property, to the south
by Puget Drive, and to the west by 9t' Avenue North. We did not observe surface water throughout the
site during our visit on December 3, 2018.
Subsurface Conditions
Geology: The site is mapped on the Geologic . map of the Edmonds East and part of the Edmonds West
quadrangles, Washington, by James P. Minard (US Geological Survey, 1983). The site is mapped as till
and transitional beds (Qtu and Qtb, respectively). Till deposits generally consists of a compact non -sorted
mixture of clay, silt, sand, pebbles, and boulders. Transitional beds are described as thin bedded clay, silt,
and very fine to fine sand with some layers of peaty sand and gravel lower in the unit. Our explorations
typically encountered undocumented fill underlain by silty fine to medium sand with varying amounts of
gravel consistent with the description of native till deposits at depth.
Explorations: The subsurface conditions within the site were explored on December 3, 2018 by
excavating four test pits to approximate depths in the range of 4.0 to 5.5 feet below the existing ground
surface using a mini-trackhoe. 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 test pits.
The soils were visually classified in general accordance with the Unified Soil Classification System,
presented in Figure 3. The logs of our test pits are attached to this report and are presented as Figure 4.
We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed
description of the subsurface conditions, the logs of the test pits should be reviewed.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 3
At the surface of each exploration we generally encountered 1.7 to 2.5 feet of dark brown to brown, silty
fine to medium sand with varying amounts of gravel, roots, and organics, which we interpreted as topsoil
and/or undocumented fill soils. Underlying the fill soils and topsoil we generally encountered medium
dense or better orange -brown to gray, silty fine to medium sand with varying amounts of gravel and iron -
oxide staining, which we interpreted as native till deposits at depth. Infiltration Pit 1 and Test Pits 1
through 3 terminated at respective depths of 4.0, 5.0, 5.5, and 4.0 feet below the existing ground surface.
Hydrogeologic Conditions
We did not encounter groundwater within our explorations throughout the site. If groundwater is
encountered during construction we would interpret this as a perched groundwater condition. Perched
water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on
top of relatively low permeability materials. The more permeable soils consist of the topsoil/weathered
soils and undocumented fill. The low permeability soil consists of relatively silty native deposits.
Perched water does not represent a regional groundwater "table" within the upper 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 2% 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
%
S1
Parameters
Fa
Fv
SDs
SDI
D
1.278
0.501
1.000
1.5
0.852
0.501
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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 4
The site is located within the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of
seismicity within central Puget Sound stretching from the Strait of Juan de Fuca to North Bend.
Information published in 2013 by the Washington State Department of Natural Resources suggests the
SWIFZ last ruptured less than 2,700 years ago, and that the fault zone can produce a M7.5 earthquake. In
our opinion, the possibility of faulting ground rupture caused by this fault zone is considered low.
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 deposits interpreted to underlie the
site have a low potential for liquefaction or amplification of ground motion.
Erosion Hazard
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 (MRCS)
was reviewed to determine the erosion hazard of the on -site soils. The surface soils for this site were
mapped as Alderwood-Urban land complex, 2 to 8 percent slopes. The erosion hazard for this material is
listed as slight. This site is relatively level to gently sloping and there are no steep slopes on the property.
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 site planned development is feasible from a geotechnical standpoint. Our
explorations indicated that the site is generally underlain by competent native bearing soils at relatively
shallow depths. The native soils encountered at depth should provide adequate support for foundation,
slab, and pavement loads. We recommend that the planned structure be designed utilizing shallow
foundations. Footings should extend through any loose soil or undocumented fill soils and be founded on
the underlying medium dense or better native bearing soil, or structural fill extending to these soils. The
medium dense or better native glacial soils should typically be encountered approximately two to three
feet below the existing surface, based on our explorations. We should note that localized areas of deeper
unsuitable soils and/or undocumented fill could be encountered at this site. This condition would require
additional excavations in foundation, slab, and pavement areas to remove the unsuitable soils.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 5
Based on the results of our infiltration testing and soil explorations throughout the site, it is our opinion
that the onsite native soils are not conducive for traditional methods of stormwater infiltration. This is
further discussed in the Site Drainage section of this report.
Erosion Control
The erosion hazard for the on -site soils is interpreted to be slight for exposed soils, but actual erosion
potential 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. Disturbed areas should be planted as soon as practical and the vegetation
should be maintained until it is established. The erosion potential of areas not stripped of vegetation
should be low.
Site Preparation and Grading
After erosion control measures are implemented, site preparation should consist of stripping the topsoil,
undocumented fill and loose soils from foundation, slab, pavement areas, and other structural areas, to
expose medium dense or better native bearing soils. The stripped soil should be removed from the site or
stockpiled for later use as a landscaping fill. Based on our observations, we anticipate stripping depths of
two to three feet, depending on the specific locations. However, additional stripping may be required if
areas of deeper undocumented fill and/or loose soil are encountered in unexplored areas of the site.
After site stripping, if the exposed subgrade is deemed loose, it should be compacted to a non -yielding
condition and then proof -rolled with a heavy rubber -tired piece of equipment. Areas observed to pump or
weave during the proof -roll test should be reworked to structural fill specifications or over -excavated and
replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in the
pavement 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.
If wet conditions are encountered, alternative site stripping and grading techniques might be necessary.
These could include using large excavators equipped with wide tracks and a smooth bucket to complete
site grading and covering exposed subgrade 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
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 6
disturbance by machine or foot traffic during construction. The prepared subgrade should be protected
from construction traffic and surface water should be diverted around areas of prepared subgrade.
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. Some of the on -site soils may be suitable
for use as structural fill, depending on the moisture content of the soil at the time of construction. 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.
For planning purposes, we recommend that temporary cuts in the upper undocumented fill soils be no
steeper than 2 Horizontal to 1 Vertical (2H:IV). Temporary cuts in the competent native soils at depth
should be no steeper than 1.5H:1 V. 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:IV. 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 7
Foundations
Conventional shallow spread foundations should be placed on medium dense or better native bearing
soils, or be supported on structural fill or rock spalls extending to those soils. Medium dense soils should
be encountered approximately two to three feet below ground surface based on our explorations. Where
undocumented fill or less dense soils are encountered at footing bearing elevation, the subgrade should be
over -excavated to expose suitable bearing soil. The over -excavation may be filled with structural fill, or
the footing may be extended down to the competent native bearing soils. If footings are supported on
structural fill, the fill zone should extend outside the edges of the footing a distance equal to one half of
the depth of the over -excavation below the bottom of the footing.
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.
For foundations constructed as outlined above, we recommend an allowable design bearing pressure of
not more than 2,000 pounds per square foot (psf) be used for the design of footings founded on the
medium dense or better native bearing soils or structural fill extending to the competent native bearing
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 %2-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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 8
Retaining Walls
Specific grading plans for this project were not available at the time this report was prepared, but
retaining walls may be incorporated into project plans. In general, the lateral pressure acting on
subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount
of lateral wall movement which can occur as backfill is placed, wall drainage conditions, and the
inclination of the backfill. For walls that are free to yield at the top at least one thousandth of the height
of the wall (active condition), soil pressures will be less than if movement is limited by such factors as
wall stiffness or bracing (at -rest condition). We recommend that walls supporting horizontal backfill and
not subjected to hydrostatic forces, be designed using a triangular earth pressure distribution equivalent to
that exerted by a fluid with a density of 40 pcf for yielding (active condition) walls, and 60 pcf for non -
yielding (at -rest condition) walls. A seismic design loading of 8H should also be included in the wall
design, where H represents the total height of the wall.
These recommended lateral earth pressures are for a drained granular backfill and are based on the
assumption of a horizontal ground surface behind the wall for a distance of at least the height of the wall,
and do not account for surcharge loads. Additional lateral earth pressures should be considered for
surcharge loads acting adjacent to walls and within a distance equal to the height of the wall. This would
include the effects of surcharges such as traffic loads, floor slab loads, slopes, or other surface loads. We
could consult with the structural engineer regarding additional loads on retaining walls during final
design, if needed.
The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil, and
by passive resistance acting on the below -grade portion of the foundation. Recommendations for
frictional and passive resistance to lateral loads are presented in the Foundations subsection of this
report.
All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report.
Care should be taken to prevent the buildup of excess lateral soil pressures due to over -compaction of the
wall backfill. This can be accomplished by placing wall backfill in 8-inch loose lifts and compacting the
backfill with small, hand -operated compactors within a distance behind the wall equal to at least one-half
the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower
compactive energy of the hand -operated equipment. The recommended level of compaction should still
be maintained.
Permanent drainage systems should be installed for retaining walls. Recommendations for these systems
are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to
evaluate the proposed wall drain backfill material and observe installation of the drainage systems.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 9
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.
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 more granular on -site soils may be suitable for use as structural fill,
but this will be highly dependent on the moisture content of these soils 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. We recommend that the capillary break be hydraulically connected to the footing
drain system to allow free drainage from under the slab. 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 10
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.
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% 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% of the
maximum dry density. The trench backfill compaction should be tested.
Site Drainage
Infiltration: We conducted a Small PIT within Infiltration Pit 1, located within the northwest portion of
the site as shown on the attached Schematic Site Plan in Figure 2. The test was conducted within a pit
that measured 4.0-feet long by 3.0-feet wide by 4.0-feet deep. The pit was filled with 12-inches of water
at the beginning of the day and we began the soaking period of the PIT for approximately 6 hours. At this
time, the water flow rate into the hole was monitored with a Great Plains Industries (GPI) TM 075 water
flow meter for the pre-soak period.
After the 6-hour soaking period was completed, the water level was maintained at approximately 12-
inches for one hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 0.02
gallons per minute (1.20 gallons per hour). This equated to an approximate infiltration rate of 0.182
inches per hour. The water was shut off after the steady-state period and monitored at least every 15
minutes for one hour. After 60 minutes, the water level within the pit dropped approximately 0.125
inches, resulting in a measured infiltration rate of 0.125 inch per hour.
In accordance with the Table 3.5 of the Department of Ecology 2014 SWMMWW, correction factors of
0.8, 0.5, and 0.9 for CFv, CFt, CFm, respectively were applied to the field measured infiltration rate of
0.125 inches per hour, obtained from the falling -head portion of the testing in Infiltration Pit 1. A total
correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head
portion of the test to determine the long-term design infiltration rate.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 11
Using the above correction factor, we calculated a long-term design infiltration rate of approximately
0.045 inches per hour. In our opinion, a long-term design infiltration rate of 0.045 inches per hour could
be utilized to design low -impact infiltration systems, such as rain gardens, bioswales, or permeable
pavements within the native, silty fine to medium sand with gravel found on this site at depth. We
recommend these systems be sized and designed in accordance with the 2014 Department of Ecology
Stormwater Management Manual for Western Washington and in conjunction with the provided long-
term design infiltration rate of 0.045 inches per hour. Additionally, due to the very low infiltration rate we
recommend incorporating an overflow component into any infiltration systems within the site, which
should be directed towards an approved point of discharge.
We recommend that any proposed infiltration systems be placed as to not negatively impact any proposed
or existing nearby structures and also meet all required setbacks from existing property lines, structures,
and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be
located within proposed fill areas within the site associated with site grading or retaining wall backfill as
such condition could lead to failures of the placed fills and/or retaining structures. We should be retained
to evaluate the infiltration system design and installation during construction.
Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an
approved stormwater collection system. Water should not be allowed to stand in any areas where
footings, slabs, or pavements are to be constructed. Final site grades should allow for drainage away from
the residences. We suggest that the finished ground be sloped at a minimum gradient of three percent, for
a distance of at least 10 feet away from the residences. Surface water should be collected by permanent
catch basins and drain lines, and be discharged into an approved discharge system.
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 and routed into a permanent storm drain.
We recommend the use of footing drains around the 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. Pea gravel is an acceptable drain
material. The free -draining material should extend up the wall to one foot below the finished surface.
The top foot of backfill should consist of impermeable soil placed over plastic sheeting or building paper
to minimize surface water or fines migration into the footing drain. Footing drains should discharge into
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1074518
Sundstone Short Plat December 21, 2018
Edmonds, Washington Page 12
tightlines leading to an approved 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 should be retained to provide construction monitoring services during the earthwork phase of the
project to evaluate subgrade conditions, temporary cut conditions, fill compaction, and drainage system
installation.
USE OF THIS REPORT
NGA has prepared this report for Ms. Lindell Graham and her 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 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 the
work 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
Sundstone Short Plat
Edmonds, Washington
NGA File No. 1074518
December 21, 2018
Page 13
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.
Alex B. Rinaldi, GIT
Staff Geologist II
Maher A. Shebl, PhD, PE, M.ASCE
Senior Engineer
ABR:MAS:dy
Four Figures Attached
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Vicinity Map GEOTECHNICAL ENGINEERS & GEOLOGISTS
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Number and approximate
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Number and approximate
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Scale: 1 inch = 30 feet
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
GM
OF COARSE FRACTION OF
SILTY GRAVEL
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
CH
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 GEOTECHNICAL
No.
Date
Revision
By
CK
1074518
Graham Short Plat
Soil Classification Chart
NGA ASSOCIATES, INC.
GEOTECHNICAL ENGINEERS & GEOLOGISTS
1
12/5/18
Original
DPN
ABR
Figure 3
Woodinville Office East Wenatchee Office
17311-135th Ave. NE, A-500 5526Industry Lane, #2
Woodinville, WA 98072 East Wenatchee, WA 98802
(425) 486-16691 Fax: 481-2510 w.nelsongeotech.com (509) 665-7696 / Fax: 665-7692
LOG OF EXPLORATION
DEPTH (FEET) USC SOIL DESCRIPTION
INFILTRATION PIT
ONE
0.0-0.8 GRASS UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS,
AND ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL)
0.8-2.0 ORANGE -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, WOOD DEBRIS, AND ROOTS
(LOOSE TO MEDIUM DENSE, MOIST) (FILL)
2.0 -4.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 4.0 FEET ON 12/3/2018
TEST PIT ONE
0.0-2.5
DARK BROWN, ORGANIC -RICH SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ROOTS
(LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL)
2.5 - 5.0
SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE IRON -OXIDE STAINING
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 4.0 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 5.0 FEET ON 12/3/2018
TEST PIT TWO
0.0 - 1.7
DARK BROWN TO BROWN, ORGANIC -RICH SILTY FINE TO MEDIUM SAND WITH GRAVEL AND
ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL)
1.7 - 5.5
SM ORANGE -BROWN TO GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE IRON -OXIDE
STAINING, AND ROOTS (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 5.5 FEET ON 12/3/2018
TEST PIT THREE
0.0-1.0
DARK BROWN TO ORANGE -BROWN, ORGANIC -RICH SILTY FINE TO MEDIUM SAND WITH
GRAVEL AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL/FILL)
1.0 -4.0
SM LIGHT BROWN TO GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE IRON -OXIDE
STAINING (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 4.0 FEET ON 12/3/2018
ABR:KMS NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 1074518
FIGURE 4