REVIEWD BLD Geotech ReportNGA
Main Office
17311— 135'" Ave NE, A-500
Woodinville, WA 98072
(425) 486-1669 • FAX (425) 481-2510
March 21, 2019
Mr. Chris Pickering
VIA Email: crispygreen e,comcast.net
NELSON GEOTECHNICAL
ASSOCIATES, INC.
GEOTECHNICAL ENGINEERS & GEOLOGISTS
Engineering -Geology Branch
5526 Industry Lane, #2
East Wenatchee, WA 98802
(509) 665-7696 • FAX (509) 665-7692
Geotechnical Engineering Evaluation
Pickering 6" Avenue South Townhome Residential Development
302 - 6" Avenue South
Edmonds, Washington
NGA Project No. 1083719
Dear Mr. Pickering:
We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Pickering
6t" Avenue South Townhome Residential Development — 302 - 6' Avenue South — 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 January 23, 2019.
The relatively level to gently sloping, rectangular -shaped subject site is currently occupied by a single-
family residence and detached garage structure within the northern and southwestern portions of the site,
respectively. The property is bordered to the west and south by existing residential properties, to the
north by Maple Street, and to the east by 61" Avenue South. We understand the proposed improvements
within the site will consist of removing the existing structures and constructing a new three -unit
townhome residence structure. Specific grading and stormwater plans were not available when this report
was prepared, however 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 residential structure,
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 monitored the excavation of four test pit explorations throughout the property, two of which we
utilized for our small-scale PIT testing. Our explorations indicated that the site was underlain by surficial
undocumented fill with competent, native glacial soils at depth.
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 competent bearing native glacial soils underlying the site should adequately support the
planned structures. Foundations should be advanced through any loose and/or undocumented fill soils
down to the competent native bearing material for bearing capacity and settlement considerations. These
soils should generally be encountered approximately five 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
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Summary — Page 2
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 WSDOE 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.
M
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............................................................................................................
10
Pavement Subgrade and Other Hard Surfaces...........................................................
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
Figure 4 — Test Pit Logs
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Pickering 6' Avenue South Townhome Residential Development
302 - 6th Avenue South
Edmonds, Washington
INTRODUCTION
This report presents the results of our geotechnical engineering investigation and evaluation of the
planned 6'1i Avenue South Residential Development project in the Edmonds, Washington. The project
site is located at 302 - 6'' Avenue South, 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 preliminary site plan titled "Pickering Townhomes," showing the
proposed conditions within the site.
The property is currently occupied by a single-family residence and detached garage structure within the
northern and southwestern portions of the site, respectively. The remaining portions of the property are
occupied by a concrete driveway and a grass -covered yard area within the western and southern portions
of the site, respectively. The site is generally level to gently sloping from the south to the north. We
understand that the proposed development within the site will include constructing a new three -unit
townhome structure. 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:
1. Review available soil and geologic maps of the area.
2. Explore the subsurface soil and groundwater conditions within the site with trackhoe-
excavated test pits. Trackhoe was provided by the client.
3. Conduct laboratory analyses on selected soil samples, as needed.
4. Provide recommendations for earthwork, foundation support, and slab on grade
subgrades.
5. Provide recommendations for pavement subgrade preparation.
6. Provide recommendations for temporary and permanent slopes.
7. Determine feasibility of on -site stormwater infiltration.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 2
8. Provide long-term design infiltration rates based on two on -site Small Scale Pilot
Infiltration Test (PIT) per the 2014 WSDOE Stormwater Management Manual for
Western Washington. Location and depth of test was determined by civil engineer.
Water for the test was secured by client.
9. Provide recommendations for infiltration system installation.
10. Provide recommendations for site drainage and erosion control.
11. Document the results of our findings, conclusions, and recommendations in a written
geotechnical report.
SITE CONDITIONS
Surface Conditions
The subject site consists of a rectangular -shaped parcel covering approximately 0.15 acres. The site is
generally level to gently sloping from the south to the north and is currently occupied by a single-family
residence, a detached garage and a concrete driveway within the northern, southwestern, and western
portions of the site, respectively. The southeastern and central portions of the property generally consist
of grass -covered yard areas, landscaping plants, and scattered trees. The site is bound to the north by
Maple Street, to the east by 6' Avenue South, and to the south and west by existing residential properties.
We did not observe surface water throughout the site during our visit on February 21, 2019.
Subsurface Conditions
Geology: The geologic units for the overall site are shown on Geologic map of the Edmonds East and
part of the Edmonds West quadrangles, Washington, by Minard, J.P. (USGS, 1983). The site is mapped
as Transitional Beds (Qtb) with Vashon till (Qvt) and Advance Outwash (Qva) in the near vicinity. The
transitional beds seem to grade upward into the base of the overlying advance outwash at some localities
and consist of clay, silt and fine to very fine sand. The Till is generally described as a non -sorted mixture
of clay, silt, sand, pebbles, cobbles, and boulders, all in variable amounts. The advance outwash is
described as mostly clean gray pebbly sand. Our explorations typically encountered surficial
undocumented fill underlain by silty fine to medium sand with varying amounts of gravel, which we
interpreted as native glacial till soils at depth.
Explorations: The subsurface conditions within the site were explored on February 21, 2019 by
excavating four test pits to approximate depths in the range of 7.6 to 9.0 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 3
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 paragraph. For a detailed
description of the subsurface conditions, the logs of the test pits should be reviewed.
At the surface of Infiltration Pits 1 and 2 and Test Pits 1 and 2, we generally encountered 4.8 to 5.5 feet of
loose, dark brown to orange -brown, silty fine to medium sand with gravel, roots, and organics, which we
interpreted as undocumented fill soils. Underlying the fill soils in each of our explorations, we generally
encountered gray, silty fine to medium sand with gravel, cobbles, and iron -oxide staining, which we
interpreted as native glacial soils. Infiltration Pit 1 and 2 and Test Pit 1 and 2 were terminated within the
native glacial till soils at depths in the range of 7.6 to 9.0 feet below the existing ground surface.
Hydrogeologic Conditions
We encountered varying degrees of groundwater seepage within Infiltration Pit 1 and Infiltration Pit 2 at
depths in the range of 7.0 to 8.0 feet below the existing ground surface. We interpret the water observed
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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Pickering 61 Ave South Residential Development
Edmonds, Washington
NGA File No. 1083719
March 21, 2019
Page 4
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
SS
S1
Parameters
Fa
Fv
SDs
SDI
D
1.269
0.497
1.000
1.503
0.846
0.498
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 native 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 (NRCS)
was reviewed to determine the erosion hazard of the on -site soils. The surface soils for this site were
mapped as Everett very gravelly sandy loam, 15 to 30 percent slopes. The erosion hazard for this material
is listed as moderate. 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 planned development is feasible from a geotechnical standpoint. Our
explorations indicated that the site is generally underlain by competent native bearing glacial 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 glacial soils, or structural fill extending
to these soils. The medium dense or better native glacial soils should typically be encountered
approximately five 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
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 5
condition would require additional excavations in foundation, slab, and pavement areas to remove the
unsuitable soils. We have provided detailed recommendations regarding slab -on -grade, pavement and
hard surfacing in the Slab -on -Grade and Pavement subgrade and Other Hard Surfaces subsections of
this report.
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 moderate 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 glacial 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 approximately five 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 6
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
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 takes 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 native on -site soils may be
suitable for use as structural fill, depending on the moisture content of the soil at the time of construction.
It is our opinion that the undocumented fill soils encountered within the site are not suitable for structural
fill. 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:1V). Temporary cuts in the competent native glacial soils at
depth should be no steeper than 1.5H:1V. 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
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 7
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.
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 five 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 footings 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 2,500 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 8
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.
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 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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 9
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.
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 native 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. In our opinion, the surficial undocumented fill soils are not suitable for use as structural fill.
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
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 10
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. However, undocumented fill soils may be encountered at slab
subgrade elevations. If some future slab settlement and/or cracking cannot be tolerated, we recommend
that at a minimum, the slab subgrade be overexcavated by two feet and the overexcavation backfilled with
11/4-inch crushed rock compacted to structural fill specifications. The resulting overexcavation subgrade
should be compacted to an unyielding condition prior to placement of the crushed rock backfill.
Additional reinforcement and doweled cold joints should be incorporated into the slab design.
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.
Pavement Subgrade and Other Exterior Hard Surfaces
Pavement and walkway subgrade preparation should be completed as recommended in the Site
Preparation and Grading and Structural Fill subsections of this report. Depending on tolerance to
cracking, we recommend that at least the upper two feet of the existing material be removed and replaced
with granular structural fill or crushed rock. The subgrades should be proof -rolled with a heavy, rubber -
tired piece of equipment, to identify soft or yielding areas that may require repair prior to placing any
structural fill and prior to placing the pavement base course. We should be retained to observe the proof -
rolling and to recommend repairs prior to placement of the asphalt or hard surfaces. The hard surface
section should be thickened and reinforced with rebar where applicable to further reduce the effects of
settlement due to the loose/soft soils, but potential long-term cracking should still be expected if any
undocumented fill is left in place below the upper crushed rock backfill.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington 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% of the modified proctor as described in the Structural Fill subsection of this
report. Trench backfill should be compacted to a minimum of 95% of the modified proctor maximum dry
density. Trenches located in non-structural areas and five feet below roadway subgrade should be
compacted to a minimum 90% of the maximum dry density. The trench backfill should be tested.
Site Drainage
Infiltration: The subsurface soils generally consisted of undocumented fill soils underlain by silty sand
with varying amounts of gravel that we interpreted as native glacial deposits to the depths explored. The
2014 WSDOE Stormwater Management Manual for Western Washington was utilized to determine the
long term design infiltration rate of the site soils. In accordance with this manual, on -site infiltration
testing consisting of the Small Scale Pilot Infiltration Test (Small PIT) was used to determine the long-
term design infiltration rates.
We conducted two Small PITs within Infiltration Pit 1 and Infiltration Pit 2 located within the
northeastern and central portions of the site respectively, as shown on the attached Site Plan in Figure 2.
These tests were performed in The tests were conducted within pits that measured 6.0-feet long by 3.3-
feet wide by 8.5-feet deep and 5.0-feet long by 2.5-feet wide by 8.0-feet deep for Infiltration Pit 1 and 2
respectively. The pits were 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 holes was
monitored with a Great Plains Industries (GPI) TM 075 water flow meter for the pre-soak period.
At the conclusion of the pre-soak period, we observed the water level for an additional hour. During the
pre-soak period and additional hour, we did not observe any water level movement with no additional
water added. Based on the results of our onsite infiltration testing and the relatively dense and silty nature
of the native glacial soils encountered at depth, it is our opinion that traditional methods of stormwater
infiltration are not feasible within the site. Low impact design infiltration systems such as pervious
pavements, rain gardens and/or bio-retention systems may be feasible within the site. This should be
further discussed with the project civil engineer. Any low -impact design systems should be designed with
an incorporated overflow component that is directed to discharge into an approved off -site point of
discharge.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 12
We recommend that any proposed low -impact design 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 the fill areas 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
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.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 1083719
Pickering 61 Ave South Residential Development March 21, 2019
Edmonds, Washington Page 13
USE OF THIS REPORT
NGA has prepared this report for Mr. Chris Pickering and his 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
Pickering 61 Ave South Residential Development
Edmonds, Washington
NGA File No. 1083719
March 21, 2019
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.
Alex B. Rinaldi, GIT
Staff Geologist II
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Lee S. Bellah, LG
Project Geologist
Maher A. Shebl, PhD, PE, M.ASCE
Senior Engineer
ABR:LSB:MAS:dy
Four Figures Attached
NELSON GEOTECHNICAL ASSOCIATES, INC.
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Pickering 6th Avenue South
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1083719
Residential Development
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3/12/19
Original
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Reference: Site Plan based on a plan dated September 8, 2014 titled "Topographic Survey," prepared by Emerald Land Surveying, Inc. b
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Project Number
Pickering 6th Avenue South
NELSON GEOTECHNICAL
No.
Date
Revision
By
CK 6
1
3115116
Original
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GEOTECHNICAL ENGINEERS & GEOLOGISTS
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Figure 2
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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
OF COARSE FRACTION
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
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 GEOTECHNICAL
No.
Date
Revision
By
CK
1083719
Pickering 6th Avenue South
��., ASSOCIATES, INC.
N G A
Residential Development
GEOTECHNICAL ENGINEERS & GEOLOGISTS
1
3/12/19
Original
DPN
ABR
Figure 3
Soil Classification Chart
W°35th Aye. NE, East Wenatchee Office
17311-135th Ave. NE, A-500 5526 Industry Lane, #2
Woodinville, WA 98072 East Wenatchee, WA 98802
(425) 486-1669 / Fax: 481-2510—relsongeotech—an (509) 665-7696 / Fax: 665-7692
LOG OF EXPLORATION
DEPTH (FEET) USC SOIL DESCRIPTION
INFILTRATION PIT
ONE
0.0-5.0 GRASS UNDERLAIN BY DARK BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH
GRAVEL, ROOTS, AND ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (UNDCOUMENTED FILL)
5.0 - 8.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND TRACE IRON -OXIDE
STAINING (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS NOT COLLECTED
MINOR GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 8.0 FEET
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 8.5 FEET ON 2/21/2019
INFILTRATION PIT
TWO
0.0 - 5.5
DARK BROWN TO ORANGE -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS,
ORGANICS, AND TRACE IRON -OXIDE STAINING
(LOOSE TO MEDIUM DENSE, MOIST) (UNDOCUMENTED FILL)
5.5 - 8.0
SM GRAY WELL -CEMENTED SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND TRACE
IRON -OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 7.5 FEET
GROUNDWATER SEEPAGE WAS ENCOUNTERED FROM 7.0 TO 8.0 FEET
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 7.5 FEET ON 2/21/2019
TEST PIT ONE
0.0-4.8
GRASS UNDERLAIN BY DARK BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH
GRAVEL, ROOTS, AND ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (UNDCOUMENTED FILL)
4.8 - 7.6
SM GRAY WELL -CEMENTED SILTY FINE TO MEDIUM SAND WITH GRAVEL
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 7.0 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 7.6 FEET ON 2/21/2019
TEST PIT TWO
0.0 - 5.4
GRASS UNDERLAIN BY DARK BROWN TO ORANGE -BROWN, SILTY FINE TO MEDIUM SAND
WITH GRAVEL, ROOTS, AND ORGANICS
(LOOSE TO MEDIUM DENSE, MOIST) (UNDCOUMENTED FILL)
5.4 - 9.0
SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE COBBLES
(MEDIUM DENSE TO DENSE, MOIST)
SAMPLE WAS COLLECTED AT 9.0 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
TEST PIT CAVING WAS NOT ENCOUNTERED
TEST PIT WAS COMPLETED AT 9.0 FEET ON 2/21/2019
ABR:LSB NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 1083719
FIGURE 4