REVIEWED BLD2024-0452+Geotechnical_Report+4.5.2024_11.29.25_AM+4179220------- - - - - - -
Reviewed by
City of Edmonds ;
1
1 Planning Division,
1 1
---------------
NELSON GEOTECHNICAL
ASSOCIATES. INC.
February 23, 2024
Ms. Lisa Johnston
VIA Email: samlijohn@icloud.com
BLD2024-0452
RECEIVED
Apr 082024
17311-13511 Ave. N.E. Suite A-500
Woodinville, WA 98072
(425)486-1669
www.nelsongeotech.com
REVIEWED
BY
CITY OF EDMONDS
Geotechnical Engineering Evaluation
Johnston Talbot Road Retaining Wall Stabilization
17134 Talbot Road
Edmonds, Washington
NGA File No. 102791324
INTRODUCTION
This letter presents the results of our geotechnical engineering evaluation for the recently constructed
cast -in -place concrete retaining wall at your residence property located at 17134 Talbot Road in
Edmonds, Washington, as shown on the Vicinity Map in Figure 1. The purpose of this study is to observe
the existing site conditions and based on previous soil explorations performed within the site, provide
recommendations to underpin and restrain the existing retaining wall due to the proximity of wall in
relationship to the steep slope below.
We previously prepared a geotechnical evaluation dated March 19, 2018 for a proposed deck structure
located between the existing residence and steep west -facing slope within the very western portion of
the property. We understand that the proposed deck has since been constructed. In addition, we
understand that a new cast -in -place retaining wall was also constructed along the top of the steep slope
to level out the yard area between the residence and steep slope. We were informed that the retaining
wall construction was done without a permit, and we have been requested by you to provide
supplemental geotechnical opinions and recommendations for improving retaining wall performance.
The site layout within the vicinity of the residence and the locations of our explorations are shown on the
Schematic Site Plan in Figure 2.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 2
SCOPE
The purpose of this study is to explore and characterize the site surface and subsurface conditions, and
provide our opinion regarding the on -going settlement issues.
Specifically, our scope of services included the following:
1. Review the project plans, as well as the previous geotechnical report.
2. Visit the site to observe the existing site conditions within the vicinity of the retaining
wall.
3. Map the conditions on the steep slope area and evaluate current slope stability
conditions.
4. Provide supplemental geotechnical consultation services for the retaining wall
stabilization prior to construction, as requested.
5. Document our opinions and recommendations in a written geotechnical letter.
6. Provide additional geotechnical consultation for the project, as requested by you or your
representatives.
SITE CONDITIONS
Surface Conditions
The property consists of a rectangular -shaped parcel covering approximately 0.34 acres. The site is
occupied by a multi -level single-family residence and a detached garage within the central and
southeastern portion of the property, respectively. The site layout is shown on the Schematic Site Plan in
Figure 2. The ground surface within the upper central and southeastern portion of the property slopes
gently down from the southeast to the northwest. A steep northwest -facing slope is located below and
to the northwest of the existing residence that descends from the recently constructed concrete cast -in -
place retaining wall to the railroad tracks below at gradients of 21 to 45 degrees (38 to 100 percent grade),
as shown on Cross -Section A -A', presented in Figure 3. The slope is generally covered with grass,
brambles, and young trees. The site is bordered to the west and east by moderately -spaced residential
properties, to the south by a private access road from Talbot Road, and to the north by Burlington -
Northern Santa Fe Railroad Right -of -Way. Site conditions within the property appear to have remained
relatively unchanged since our previous site visit, outside of recently constructed concrete retaining wall
and yard grading activities.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 3
We did not observe seepage on the sloping portions of the site during our past site visit on February 15,
2018 or recent site visit on January 30, 2024. However, as a result of the uncontrolled stormwater flows
within the neighboring property to the south being directed to the steep west -facing slope area, we
observed a deep incised erosion channel within the central portion of the slope. The erosion channel
begins about 10 to 12 feet below the top of the steep slope and extends vertically approximately 20 to 25
feet down to the base of the erosion area within the steep slope area. The erosion area then extends
below and to the west toward the toe of the steep slope. The steep slope area is generally vegetated
within underbrush and sparse young trees. We did not observe signs of recent movement or significant
erosion on the site slopes outside of the existing erosion channel observed immediately below the failed
stormwater system. We understand that the uncontrolled flows have since been tightlined and directed
to flow into an HDPE stormwater pipe that descends the steep slope within the neighboring property to
the south.
The recently constructed concrete retaining wall was constructed along the top of the steep northwest -
facing slope. The overall exposed wall height ranges from 40- to 46-inches. The wall appears to be
retaining approximately 30-inches of the yard area above. The wall foundation appears to have been
constructed near the existing ground surface elevation with minimal to no actual embedment. We were
not able to observe the retaining wall drainage behind the wall but observed that the wall had a weep
hole along the southern portion of the wall and a drain clean out behind the northern portion of the
retaining wall.
Subsurface Conditions
Geology: The geologic units for this area are shown on the map titled, Geologic Map of the Edmonds East
and part of the Edmonds West Quadrangles, Washington, by J.P. Minard (USGS, 1983). The project site is
mapped as surficial deposits dating prior to the Fraser Glaciation, consisting of the Whidbey Formation
(Qw), with nearby mapped deposits of Vashon Stade glacial till of the Fraser Glaciation (Qvt) and modified
land associated with the railway. The Whidbey Formation is described as interglacial, bedded, compact,
medium to coarse -grained sand. Glacial till is generally described as a compact diamict—or mixture —of
poorly sorted silt, sand, and gravel with scattered cobbles and boulders. Our previous explorations
performed within the property generally encountered surficial undocumented fill underlain by medium
dense or better gray -brown fine to coarse sand with silt and gravel, generally consistent with the
description of the Whidbey Formation at depth. We also observed similar soil within the exposures on
the steep slope below.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 4
Explorations: The subsurface conditions within the site were previously explored on February 15, 2018
by performing five hand-augered explorations. The approximate locations of our explorations are shown
on the Schematic Site Plan in Figure 2. A geologist from NGA was present during the explorations,
examined the soils and geologic conditions encountered, obtained samples of the different soil types, and
maintained logs of the explorations.
The soils were visually classified in general accordance with the Unified Soil Classification System,
presented in Figure 3. The logs of our explorations are attached to this report and are presented as Figure
4. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed
description of the subsurface conditions, the previous exploration logs should be reviewed.
In all of the hand auger explorations, we encountered approximately 1.0 to 3.2 feet of surficial colluvium
and/or undocumented fill soils. Underlyin the surficial colluvium and/or undocumented fill soils, we
encountered medium dense or better silty fine to medium sand with gravel that we interpted as native
Whidbey Formation deposits. All of the hand auger explorations were terminated within the native
Whidbey formation deposits at depths in the range of 3.0 to 5.6 feet below the existing ground surface.
Hydrogeologic Conditions
Groundwater seepage was not encountered in our previous explorations. If groundwater is encountered
on this site, we would interpret this water to be perched water. Perched water occurs when surface water
infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability
materials. 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) and the ASCE 7-16 for seismic site classification
for this project. Since competent glacial soils were encountered at depth within the subject site, the site
conditions best fit the IBC description for Site Class D.
Table 1 below provides seismic design parameters for the site that are in conformance with the 2018
IBC, which specifies a design earthquake having a two percent probability of occurrence in 50 years
(return interval of 2,475 years), and the 2014 USGS seismic hazard maps.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Johnston Talbot Road Retaining Wall Stabilization
Edmonds, Washington
Table 1— ASCE 7-16 Seismic Design Parameters
NGA File No. 10279624
February 23, 2024
Page 5
Site Class
Spectral Acceleration
Spectral Acceleration
Site Coefficients
Design Spectral
at 0.2 sec. (g)
at 1.0 sec. (g)
Response
SS
Sl
Parameters
Fa
Fv
Sos
Soi
D
1.314
0.466
1.00
Null
0.876
Null
The spectral response accelerations were obtained from the USGS Earthquake Hazards Program
Interpolated Probabilistic Ground Motion website (2014 data) for the project latitude and longitude.
Hazards associated with seismic activity include liquefaction potential and amplification of ground
motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the
groundwater table. It is our opinion that the medium dense or better glacial deposits interpreted to
underlie the site and nearby vicinity have a low potential for liquefaction or amplification of ground
motion.
Erosion Hazard
The criteria used for determination of erosion hazard areas include soil type, slope gradient, vegetation
cover, and groundwater conditions. The erosion sensitivity is related to the 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 Soil Conservation Service (SCS), was reviewed to determine
the erosion hazard of the on -site soils. The site surface soils were classified using the SCS classification
system as Alderwood-Urban land complex, 8 to 15 percent slopes in highland areas, and Alderwood-
Everett gravelly sandy loams, 25 to 70 percent slopes along the steeply sloping portion of the property.
These soils are listed as having a slight to severe erosion hazard, mainly attributed to the gradient of
slopes.
Landslide Hazard/Slope Stability
The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater
conditions. The ground surface within the upper central and southeastern portion of the property slopes
gently down from the southeast to the northwest. A steep northwest -facing slope is located to the
northwest of the existing residence that descends from the recently constructed cast -in -place retaining
wall and grass covered yard area to the railroad tracks below at gradients of 21 to 45 degrees (38 to 100
percent grade), as shown on Cross -Section A -A', presented in Figure 3. We did not observe evidence of
past significant erosion or sloughing within the site slopes during our site visit. We also did not observe
groundwater seepage emitting from the slopes during our visits.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 102791324
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 6
Relatively shallow failures as well as surficial erosion are natural processes and could occur in the loose
surficial soil on the site slopes, as has happened on this slope and surrounding vicinity. It is our opinion
that while there is ongoing potential for erosion, soil creep, and shallow failures within the loose surficial
soils and undocumented fill on the site slopes, the potential for deep-seated slope failure under current
site conditions is low. Proper site grading and drainage as well as slope maintenance, as recommended
in this letter, should help maintain current stability conditions.
CONCLUSIONS AND RECOMMENDATIONS
General
Based on our observations and our understanding of the existing site conditions, it is our opinion that the
recently constructed cast -in -place retaining wall was not adequately designed or constructed from a
geotechnical standpoint due to the wall foundations not being adequately embedded and not providing
an adequate setback from the top of the steep slope. To stabilize the recently constructed cast -in -place
concrete retaining wall, we recommend that the wall foundation elements be underpinned with 2-inch
diameter steel pipe piles. Due to the retaining wall being situated directly above the steep northwest -
facing slope, we also recommend that the retaining wall be laterally restrained using mechanical helical
anchor tiebacks. This is further discussed in the Retaining Wall Stabilization Improvements subsection of
this letter. Your structural engineer should determine the extent of foundation areas to be underpinned
and overall layout of the underpinning and tieback systems. We should review underpinning plans prior
to implementation.
In our opinion the planned earthwork associated with the retaining wall stabilization improvements
should be minor and should not adversely impact the existing stability conditions of the steep northwest -
facing slope.
In addition to the foundation improvements, we recommend that the drainage system behind the
retaining wall be investigated and improved, as necessary. Such improvements should include confirming
the placement of footing drains behind the retaining wall and tighlining to a permanent discharge system.
This is further discussed in the Drainage Improvements subsection of this letter.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 7
Erosion Control and Slope Protection
Water runoff should not be allowed to reach the slope. The on -site soils are listed as having a slight to
severe potential for erosion, depending 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 excavations and the steep slopes. If any vegetation is removed from the
area above the slope, silt fences or straw bales should be erected to prevent muddy water from flowing
over the site slopes. Disturbed areas should be replanted with vegetation at the end of construction and
the vegetation should be maintained until it is established.
Runoff generated within the site, including roof downspouts, yard areas, and hard surfaces should be
collected into catch basins and yard drains and tightlined into an approved stormwater management
system. Under no circumstances should runoff be allowed to concentrate on the slope either during
construction or after construction has been completed.
Protection of the slope areas should be performed as required by the City of Edmonds. Specifically, we
recommend that the site slopes not be disturbed or modified through placement of any fill or removal of
the existing vegetation. No material of any kind, such as excavation spoils, lawn clippings, debris, and soil
stockpiles, should be placed on or near the slope. Any sloping areas disturbed during construction should
be planted as soon as practical to reduce the potential for erosion. The new vegetation should be
maintained until it is established. Replacement of vegetation should be performed in accordance with
the City of Edmonds code. Under no circumstances should water be allowed to concentrate on the slope.
Temporary Excavations
Some excavations may be needed to reach final pile locations. 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 water 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 since
they are continuously at the job site, able to observe the soil and groundwater conditions encountered,
and able to monitor the nature and condition of the cut slopes.
The following information is provided solely for the benefit of the owner and other design consultants
and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for
job site safety. Job site safety is the sole responsibility of the project contractor.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 8
For planning purposes, we recommend that temporary cuts in the on -site soils be no steeper than 2
Horizontal to 1 Vertical (2H: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
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.
Retaining Wall Stabilization Improvements
General: We recommend that the recently constructed cast -in -place retaining wall be supported on a
deep foundation system to transfer structure loads down into the underlying competent native bearing
materials to limit future settlement. Structural brackets should be used to attach the pin piles/helical
anchors to the existing foundations. The structural engineer should design the new foundation supports
and determine the location of the supports based on the recommendations provided in this letter. A
qualified contractor specializing in foundation retrofit should be retained to complete the repairs.
Considering the limited site access conditions, it is our opinion that the most feasible foundation
underpinning systems are 2-inch diameter pin piles driven to refusal using a hand operated 140-pound
jackhammer or hydraulically driven helical anchors. Extreme care should be taken during the proposed
repairs as to not impact existing utilities within the vicinity of the residence.
Our explorations encountered loose undocumented fills within the planned areas to be repaired. If large
objects or debris are present within the fill, there is a possibility that this material may obstruct some piles
or anchors at shallow depths. There should be contingencies in the budget and design for
additional/relocated piles/anchors that may be obstructed by possible debris in the fill. We recommend
that the foundation design of the concrete cast -in -place retaining wall and utilities within the yard area
should be verified by the contractor and measurements made in the field at the time of underpinning
installation to ensure the underpinning and tieback elements do not encounter the existing foundations
or any underground utilities. We understand that an existing sewer main line runs north to south within
the yard area above the cast -in -place retaining wall. Test piles may need to be driven along the lower
downslope side of the structure to confirm the approach prior to finalizing the overall underpinning
design.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 9
----------------
Due to the rigid pile/anchor support, friction between the foundation and subgrade soil should not be
considered for resisting lateral pressures on this structure. Also, passive resistance acting on the below -
grade portion of the foundation should not be used to resist lateral pressures. We recommend that all
lateral loads acting on the retaining wall be restrained by helical anchor tieback anchors. The helical
tieback anchors should be installed through the concrete cast -in -place retaining wall.
We recommend that the existing concrete retaining wall be analyzed utilizing a triangular earth pressure
distribution equivalent to that exerted by a fluid with a density of 60 pcf. A seismic design loading of 8H
in psf should also be included in the wall design where "H" is 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 subsurface height of the wall,
and do not account for surcharge loads. Additional lateral earth pressures should be considered for
surcharge loads acting adjacent to subsurface walls and within a distance equal to the subsurface 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. We should be retained to review final plans and to monitor installation
of the pin piles/anchors during construction.
Pin Piles: For 2-inch diameter pipe piles driven to refusal using a hand-held, 140-pound jackhammer, we
recommend a ree tons tor The refusal criterion for this
pile and hammer size is defined as less than one inch of movement during 60 seconds of continuous
driving. We recommend using galvanized extra strong (Schedule 80) steel pipe for the 2-inch pin piles.
Final pile depths should be expected to vary somewhat and will depend on the depth of the loose material,
the nature of the underlying competent soils. The pin piles should penetrate a minimum of five feet into
the competent native glacial soils below the fill material in order to develop the design capacity. Based
on our explorations, we would recommend that the piles be driven to a minimum depth of approximately
10 feet below the ground surface to achieve the minimum embedment depth into the competent glacial
soils. Piles that do not meet this minimum embedment criterion should be rejected, and replacement
piles should be driven after consulting with the structural engineer regarding the new pile locations. The
piles should be spaced a minimum of two feet apart to avoid a grouping effect on the piles. We should
also be retained to observe pin pile installation during construction.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 10
----------------
Tie-Backs: Due to the retaining wall being situated directly above the steep northwest -facing slope, we
recommend laterally restraining the retaining wall with helical tie -back anchors. The contractor should
determine the torque values required to achieve the desired capacity. However, the anchors should
advance a minimum of 15 feet into the hillside. We recommend that the tie -backs be installed at 15
degrees below horizontal. We recommend that two anchors be tested to a minimum of 200 percent of
the design loads. The soil creep characteristics would be evaluated during these tests. We recommend
that the foundation embedment of the residence and utilities within the yard area be verified by the
contractor and measurements made in the field at the time of tieback installation to ensure tiebacks do
not encounter the existing residence or any underground utilities. We understand that an existing sewer
main line runs north to south within the yard area above the cast -in -place retaining wall. We should be
retained to review anchor designs and proposed installation methods. We should also monitor anchor
testing.
Structural Fill
General: We do not anticipate significant structural fill placement for this project; however, 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 to a firm, non -yielding condition prior to fill placement as described below.
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 structural
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). The use of on -site soils as structural fill is not recommended. We should
be retained to evaluate proposed structural fill material prior to placement.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 11
Fill Placement: Following excavation to final subgrade, placement of structural fill may proceed. All fill
placements 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.
Drainage Improvements
We recommend that all residence downspouts and yard drains be investigated to understand where they
are directed. We recommend that all of the existing roof, footing, yard, and driveway drains associated
with the residence be tightlined to flow into an approved system. During our site visit, we were unable to
confirm the existence of a footing drain behind the retaining wall. If it is determined that no foundation
footing drains exist, we recommend that during the installation of the recommended foundation
improvements an adequate foundation footing drain system be installed behind the retaining wall. The
drain 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. Washed rock is an
acceptable drain material. The free -draining material should extend up one foot below the finished
surface. The top foot of backfill should consist of low permeability soil placed over plastic sheeting or
building paper to minimize the migration of surface water or silt into the footing drain. Footing drains
should discharge into tightlines leading to an 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 recommend that we be retained to provide construction monitoring services during retaining wall
underpinning and tieback installation and drainage improvements to evaluate conditions encountered in
the field with respect to anticipated conditions, to provide recommendations for design changes should
the conditions differ from anticipated, and to evaluate whether construction activities comply with
contract plans and specifications.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No. 10279624
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington Page 12
USE OF THIS REPORT
NGA has prepared this report for Ms. Lisa Johnston, for use in the planning and design of retaining wall
stabilization measures and drainage improvements 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 review project plans as they are being developed. We also
recommend that we 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 NGA File No. 102791324
Johnston Talbot Road Retaining Wall Stabilization February 23, 2024
Edmonds, Washington 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.
Lee S. Bellah, LG
Senior Geologist
2.23.2024
Khaled M. Shawish, PE
Principal Engineer
LSB:KMS:dy
Five Figures Attached
NELSON GEOTECHNICAL ASSOCIATES, INC.
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Top of Slope -3 I 1 1 ' ' & IN
L HA-1
i
Existing Residence
- ♦ .i, IL
Existing Garage.'
W
• i 0
•� •."�, leOf
ss f
ti, • 440 OOV
i
y 3. if 11 Ae. • 1..
LEGEND
N
L
O
7
• — Property line
HA-1 0 50 100
Number and approximate o
IL
location of hand auger
A A' Approximate Scale: 1 inch = 50 feet N
Approximate location N
L� of cross-section Z
Reference: Site Plan based on field measurements, observations, and aerial parcel map review. o
Project Number .Johnston Talbot Road , nELSOn GEOTECHnICAL No. Date Revision By CK
102791324 Retaining Wall Stabilization ASSOCIATES, inc 1 3/2/18 Original DPN LsB
J
Schematic Site Plan
W.
f oodinville Office Wenatchee Office
Figure 2 17311-135th Ave. NE, A-500 105 Palouse St. y
Woodinville, WA 98072 Wenatchee, WA 98801 �
w.nelsongeotech.co'n (425)486-1669/Fa, '481-2510 (509)665-7696/Fax'665-7692 U
A
~,
Tl
o C
N
Existing House
cam•
New Yard Fill
m
(3 z
Retaining Wall
Proposed Deck
K) 3
Northwest
m
I
Old Fill
I Southeast
80
HA-1 HA-2
80
C
o =3
211
_ ?
cn ov
°
HA-3—Qvt
cis
41
60
HA-4
Qw
60
o C—n
HA-5
v v o
DL7;a
i>
m
43°
o
Puget
v
° 40
>
°'
Sound
Railroad
45°
40
-
_
W
Tracks
3
=
X
30°
M
n 20
20
S.
N O
Q
37°
mo cn
n
aT
o
mC:)
m� N
0
0
_ s (_') =
I I I I I I I
I I I I I I I I I I I I I I I I I
I I I I I I I I I I I I I I I I I I I I I I I I
I I I I I I I I I I I I I I I I I I I I I
I
0
20 40
60 80
100 120
140
'�N n
r
Exploration
Distance (feet)
w
v
°
m
Hand Auger Designation ---> HA-1
o
�
m
v
y
Groundwater Level � 1
NOTES:
During Exploration
1) Stratigraphic conditions are
interpolated between
Geologic Contact ---> ?
_ —?
the explorations. Actual conditions
2) Elevations are arbitrary.
may vary.
Z
moo
(approximate)
Reference: Cross Section is based on field measurements usina a hand-held clinometer and 100-ft taDe measure.
W
OX
C:\Users\LeeBellah\OneDrive - NGA\2024 NGA Project\10279B-24 Johnston Talbot Road Retaining Wall Supp Consult Edmonds\Drafting\CS.dwg
UNIFIED SOIL CLASSIFICATION SYSTEM
GROUP
MAJOR DIVISIONS
GROUP NAME
SYMBOL
CLEAN
GW
WELL -GRADED, FINE TO COARSE GRAVEL
COARSE-
GRAVEL
GRAVEL
GP
POORLY -GRADED GRAVEL
GRAINED
MORE THAN 50 %
GRAVEL
GM
SILTY GRAVEL
OF COARSE FRACTION
RETAINED ON
SOILS
NO.4 SIEVE
WITH FINES
GC
CLAYEY GRAVEL
SAND
CLEAN
SW
WELL -GRADED SAND, FINE TO COARSE SAND
SAND
SP
POORLY GRADED SAND
MORE THAN 50 %
RETAINED ON
MORE THAN 50 %
NO. 200 SIEVE
OF COARSE FRACTION
SAND
SM
SILTY SAND
PASSES NO. 4 SIEVE
WITH FINES
SC
CLAYEY SAND
FINE -
SILT AND CLAY
ML
SILT
INORGANIC
GRAINED
LIQUID LIMIT
CL
CLAY
LESS THAN 50 %
SOILS
ORGANIC
OL
ORGANIC SILT, ORGANIC CLAY
SILT AND CLAY
MH
SILT OF HIGH PLASTICITY, ELASTIC SILT
INORGANIC
MORE THAN 50 %
PASSES
LIQUID LIMIT
CH
CLAY OF HIGH PLASTICITY, FAT CLAY
NO. 200 SIEVE
50 % OR MORE
ORGANIC
OH
ORGANIC CLAY, ORGANIC SILT
HIGHLY ORGANIC SOILS
PT
PEAT
NOTES:
1) Field classification is based on visual SOIL MOISTURE MODIFIERS:
examination of soil in general
accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to
the touch
2) Soil classification using laboratory tests
is based on ASTM D 2488-93. Moist - Damp, but no visible water.
3) Descriptions of soil density or Wet - Visible free water or saturated,
consistency are based on usually soil is obtained from
interpretation of blowcount data, below water table
visual appearance of soils, and/or
test data.
Project Number
IlELS011 GEOTECHIIICAL
No.
Date
Revision
By
cK
Johnston Talbot Road
'
2/19/24
Original
DPN
LSB
10279B24
Retaining Wall Stabilization
y ASSOCIATES, inc,
Figure 4
Soil Classification Chart
�«� Woodinville Office Wenatchee OHice
311-135th Ave. NE, A-500 105 Palouse St,
Woodinville, WA 98072 Wenatchee, WA 98801
-nalsongeotech,com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692
0
0
LOG OF EXPLORATION
DEPTH (FEET) USC
HAND AUGER ONE
0.0 - 2.5
2.5 - 3.5
3.5-5.0
HAND AUGER TWO
0.0 - 3.2
3.2 - 5.0
HAND AUGER THREE
0.0 - 2.5
2.5 - 3.8
3.8 - 5.6
HAND AUGER FOUR
0.0 - 1.0
1.0 - 3.0
HAND AUGER FIVE
0.0 - 2.0
2.0 - 3.0
SOIL DESCRIPTION
DARK BROWN SILTY FINE TO COARSE SAND WITH SLATE GRAVEL AND TRACE CHARCOAL
(LOOSE, MOIST) (FILL)
SM ORANGE -BROWN SILTY FINE TO COARSE SAND WITH GRAVEL (MEDIUM DENSE, MOIST)
SM GRAY -BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST -WET)
SAMPLES WERE COLLECTED AT 3.2 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
CAVING WAS NOT ENCOUNTERED
HAND AUGER TEST HOLE TERMINATED AT 5.0 FEET ON 2/15/2018
DARK BROWN SILTY FINE TO COARSE SAND WITH GRAVEL AND TRACE CHARCOAL
(LOOSE, MOIST) (FILL)
SM LIGHT GRAY SILTY FINE TO MEDIUM SAND WITH TRACE FINE GRAVEL AND
IRON OXIDATION STAINING (DENSE, MOIST)
SAMPLES WERE COLLECTED AT 3.5 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
CAVING WAS NOT ENCOUNTERED
HAND AUGER TEST HOLE TERMINATED AT 5.0 FEET ON 2/15/2018
DARK BROWN SILTY FINE TO COARSE SAND WITH SLATE GRAVEL AND TRACE CHARCOAL
(LOOSE, MOIST) (FILL)
SM ORANGE -BROWN SILTY FINE TO COARSE SAND WITH GRAVEL (LOOSE, MOIST)
SM GRAY -BROWN SILTY FINE TO COARSE SAND WITH SILT AND GRAVEL AND COBBLES AND
IRON OXIDATION STAINING (MEDIUM DENSE -DENSE, MOIST -WET)
SAMPLES WERE COLLECTED AT 3.5 AND 5.6 FEET
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
CAVING WAS NOT ENCOUNTERED
HAND AUGER TEST HOLE TERMINATED AT 5.6 FEET ON 2/15/2018
SM BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ORGANIC DEBRIS
(LOOSE, MOIST) (COLLUVIUM)
SM GRAY -BROWN SILTY FINE TO COARSE SAND WITH SILT AND GRAVEL
(MEDIUM DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
CAVING WAS NOT ENCOUNTERED
HAND AUGER TEST HOLE TERMINATED AT 3.0 FEET ON 2/15/2018
SM BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ORGANIC DEBRIS
(LOOSE, MOIST) (COLLUVIUM)
SM GRAY -BROWN SILTY FINE TO COARSE SAND WITH SILT AND GRAVEL
(MEDIUM DENSE, MOIST)
SAMPLES WERE NOT COLLECTED
GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED
CAVING WAS NOT ENCOUNTERED
HAND AUGER TEST HOLE TERMINATED AT 3.0 FEET ON 2/15/2018
ABR:LSB NELSON GEOTECHNICAL ASSOCIATES, INC.
FILE NO 102791324
FIGURE 5