PLN REVIEWED Geotechnical report copyBLD2020-0623
NELSON GEOTECHNICAL
ASSOCIATES. INC.
February 27, 2020
Mr. Jorge A. Florez
c/o Mr. Rick Nissen
16000 — 75`h Place West
Edmonds, Washington
VIA Email: blackbearll@comcast.net
Geotechnica) Engineering Evaluation
Florez Retaining Wall Removal
16000 — 75`h Place West
Edmonds, Washington
NGA File No. 1158420
Dear Mr. Florez:
17311-1351h Ave. N.E. Suite A-500
Woodinville, WA 98072
(425) 486-1669
www.nelsongeotech.com
RECEIVED
DATE. 6 . 19 . 2 0
CITY OF EDMONDS
DEVELOPMENT SERVICES
DEPARTMENT
This letter summarizes the results of our geotechnical engineering evaluation for the removal of the
partially constructed concrete retaining walls at the residential property located at 16000 — 75th Place
West in Edmonds, Washington, as shown on the Vicinity Map in Figure 1.
INTRODUCTION
The subject site is a trapezoidal -shaped parcel approximately 0.46 acres in area. The property is
currently occupied by a gated single-family residence with associated paved driveway areas, a grass
yard area, and landscaping throughout the site. The property is bordered to the east by 75`� Place
West, to the north and south by existing residential development, and to the western lower property
line by the BNSF railroad. Localized steep slope hazards are mapped on the property by the City of
Edmonds, and the site itself is located within the Meadowdale Landslide Complex, as designated by the
City of Edmonds. The entire property steps down moderately westward toward the railroad tracks,
with the subject retaining walls situated above and to the east of the BNSF tracks, retaining the soils on
the hillside. We understand that prior to the new wall construction, a previous wall consisting of a
failing stacked timber wall, was removed and replaced with the currently partially -constructed concrete
tiered retaining wall. We also understand that the new wall has encroached upon the BNSF railroad
easement, and you have requested a geotechnical engineering evaluation in support of permitting the
removal of the retaining wall, stabilizing, and restoring the slopes within the easement to their original
condition.
Geotechnical Engineering Evaluation
Florez Retaining Wall Removal
Edmonds, Washington
SCOPE
NGA File No 1158420
February 27, 2020
Page 2
The purpose of this study is to explore and characterize the site subsurface conditions, and provide our
opinions and recommendations regarding the existing slope stability and proposed retaining wall.
Specifically, our scope of services included the following:
1. Review available soil and geologic maps of the area.
2. Explore the subsurface soil and groundwater conditions within the vicinity of the
existing walls with hand auger explorations.
3. Qualitatively map the conditions of the site slopes in the vicinity of the retaining wall.
4. Perform grain -size sieve analyses on soil samples, as necessary.
5. Provide opinions and conclusions on the construction of the present wall structures.
6. Provide recommendations for removing the existing retaining wall from the easement
and restoring and stabilizing the slope.
7. Provide recommendations for site drainage and erosion control.
8. Document the results of our findings, conclusions, and recommendations in a written
geotechnical letter.
9. Provide geotechnical consultation services during our involvement with the project
SITE CONDITIONS
Surface Conditions
The property is generally a trapezoid -shaped parcel covering approximately 0.46 acres in area. The
property is currently occupied by a gated single-family residence with associated paved driveway areas,
a grass yard area, and landscaping throughout the site. The property is bordered to the east by 751h
Place West, to the north and south by existing residential development, and to the western lower
property line by the BNSF railroad. Localized steep slope hazards are mapped on the property by the
City of Edmonds, and the site itself is located within the Meadowdale Landslide Complex, as designated
by the City of Edmonds. The entire property steps down moderately westward toward the railroad
tracks, with the subject retaining wall situated above and to the east of the BNSF tracks to retain the
soils on the hillside.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No 1158420
Florez Retaining Wall Removal February 27, 2020
Edmonds, Washington Page 3
The subject retaining wall consists of a 3.0 and 3.5-foot tall tiered concrete wall system, which has been
partially completed. The ground surface within the site slopes moderately to steeply from the upper
eastern portion of the site down to the west, terraced by paths, rockeries, and retaining walls, including
the subject retaining wall. Cross -sections of the slope within the vicinity of the retaining walls
illustrating the gradients of the slope and our exploration locations are shown on Cross -Section A -A' and
B-B' in Figures 3 and 4. Generally, soils within the vicinity of the tiered retaining wall are bare and
covered with little vegetation except some lawn. We did observe surface water ponding in the ditch
along the BNSF railroad, and slight seepage emitting from the slope just below the tilted timber
retaining wall, shown on Cross -Section A -A' in Figure 3, during our site visit on February 17, 2020.
Subsurface Conditions
Geology: The Geologic map of the Edmonds East and part of the Edmonds West quadrangles,
Washington (MF-1541), by James P. Minard, (USGS, 1983) was reviewed for this site. The site is
mapped as Quaternary glacial deposits of Whidbey Formation (Qw), but is also located within the
Meadowdale Landslide Complex, which can contain old landslide debris. The Whidbey Formation is
described as medium bedded, brown to gray sand, silt, and clay. From our explorations, it appears that
the onsite soils fit the description of the mapped Whidbey Formation glacial deposits.
Explorations: The subsurface conditions within the site were explored with four hand auger
explorations located in the general vicinity of the retaining wall west of the existing residence. The
approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist
from Nelson Geotechnical Associates, Inc. (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 5. The logs of our explorations are attached to this letter and
presented as Figure 6. We present a brief summary of the subsurface conditions in the following
paragraphs. For a detailed description of the subsurface conditions, the exploration logs should be
reviewed.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No 1158420
Florez Retaining Wall Removal February 27, 2020
Edmonds, Washington Page 4
At the surface of the hand augers, we encountered approximately 2.5 to 5.0 feet of dark brown to
brown silty fine to medium sand with organics, wood debris, various amounts of gravel in a loose
condition, which we interpreted as undocumented fill soils. Underlying the upper surficial fill soils in all
our explorations, we encountered brown gray to blue gray silty fine to medium sand with gravel
interbedded with silt in a loose to medium dense or better condition, which we interpreted as native
glacial soils. All of our hand auger explorations met refusal within the native glacial soils at depths in
the range of 3.0 to 8.5 feet below the existing ground surface.
Hydrogeologic Conditions
Slight to heavy groundwater seepage was encountered within every exploration. We interpreted this
water to be perched water. Perched water occurs when surface water infiltrates through less dense,
more permeable soils, and accumulates on top of a relatively low permeability material. 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 soft soils are interpreted to underlie the site at depth, the site best fits the IBC description for Site
Class E.
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 dense glacial deposits interpreted to underlie the site have
a low to moderate potential for liquefaction or amplification of ground motion.
The property is located within the designated 'Meadowdale Landslide Complex,' which has experienced
prevalent documented failures, and was augmented by extensive monitoring and drainage
improvements by the City of Edmonds over the past four decades. Weathered and disturbed surface
soils on the steep slopes have the potential for shallow sloughing failures triggered by severe weather
or seismic events. Such events should not affect the planned retaining wall removal or slope
restoration, provided the slopes are restored and erosion control methods are implemented as
described in this letter.
NELSON GEOTECHNICAL ASSOCIATES, INC. it
Geotechnical Engineering Evaluation NGA File No 1158420
Florez Retaining Wall Removal February 27, 2020
Edmonds, Washington Page 5
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
Natural Resources Conservation Service (NRCS) of the Snohomish County area was reviewed. The soil in
the development portions of the site is listed as Alderwood-Everett gravelly sandy foams, 25 to 70
percent slopes, and lists the erosion hazard as severe, likely due to the steepness of the site slopes.
Based on our experience in the area and the material encountered, we would interpret this site as
having a moderate to severe erosion hazard where the surficial soils are exposed. It is our opinion that
the erosion hazard for site soils should be low to moderate in areas where the site is not disturbed.
Landslide Hazard/Slope Stability
The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater
conditions. ,
Localized steep slope hazards are mapped on the property by the City of Edmonds, and the site itself is
located within the Meadowdale Landslide Complex, as designated by the City of Edmonds. The entire
property steps down moderately westward toward the railroad tracks, with the subject retaining wall
situated above and to the east of the BNSF tracks to retain the soils on the hillside. The subject
retaining wall consists of a 3.0 and 3.5-foot tall tiered concrete wall system, which has been partially
completed. The ground surface within the site slopes moderately to steeply from the upper eastern
portion of the site down to the west, terraced by paths, rockeries, and retaining walls, including the
subject retaining wall. Cross -sections of the slope within the vicinity of the retaining walls illustrating
the gradients of the slope and our exploration locations are shown on Cross -Section A -A' and B-B' in
Figures 3 and 4.
The property is located within the designated'Meadowdale Landslide Complex,' which has experienced
prevalent documented failures, and was augmented by extensive monitoring and drainage
improvements by the City of Edmonds over the past four decades. Weathered and disturbed surface
soils on the steep slope have the potential for shallow sloughing failures triggered by severe weather or
seismic events. Relatively shallow failures, as well as surficial erosion, are natural processes and could
occur in the loose surficial and/or undocumented fill soils on the site slopes.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No 1158420
Florez Retaining Wall Removal February 27, 2020
Edmonds, Washington Page 6
It is our opinion that while there is 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. Based on geologic mapping and observations of surface features at this
and adjoining sites, the risk of failure does not appear to be greater than what would be expected on
similar sites between the beach and top terrace within this area of the City of Edmonds.
Proper restoration and 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 explorations and observations of the current site conditions, it is our opinion, from a
geotechnical standpoint, that the removal of the retaining walls within the BNSF easement is feasible,
provided the recommendations within this letter are followed.
The property is located within the designated'Meadowdale Landslide Complex,' which has experienced
prevalent documented failures, and was augmented by extensive monitoring and drainage
improvements by the City of Edmonds over the past four decades. Weathered and disturbed surface
soils on the steep slope have the potential for shallow sloughing failures triggered by severe weather or
seismic events. It is our opinion that there is potential for erosion, soil creep, and shallow failures
within loose surficial soils on the slope. Proper retaining wall removal, site grading, stabilization, and
drainage should help maintain current stability conditions. In addition, the potential may exist for deep-
seated failures to occur at this site. However, based on geologic mapping and observations of surface
features at this and adjoining sites, the risk of failure does not appear to be greater than what would be
expected on similar sites between the beach and top terrace within this area of the City of Edmonds.
Further explorations, such as deep soil borings, would be required to refine the risk of deep-seated
slope failures.
Our explorations indicated that competent native bearing glacial soils generally underlie the wall at
depth, beneath a mantle of undocumented fill, from the construction of the retaining wall. We
encountered a significant depth of undocumented fill soils behind and beneath the retaining walls
within our explorations, as illustrated in the cross sections. We recommend that the undocumented fill
soils be removed from the existing retaining wall alignments, and the retaining wall then be
demolished.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Florez Retaining Wall Removal
Edmonds, Washington
NGA File No 1158420
February 27, 2020
Page 7
All debris, including the organic -rich topsoils and demolished wall, should be promptly removed and
hauled away from the site. We should note that deeper areas of unsuitable undocumented fill soils
may be encountered in unexplored areas of the site. Detailed recommendations are included within
the Erosion Control and Slope Protection subsection of this report. After the loose organic rich soils
and retaining wall have been removed, the slope should be graded to no steeper than 4H:iV and
covered with erosion control material. Recommendations for permanent slopes are included within the
Temporary and Permanent Slopes subsection.
The surficial soils encountered on this site are considered moisture -sensitive and may disturb easily
when wet. To lessen the potential impacts of construction and to reduce cost overruns and delays, we
recommend that de -construction take place during dry weather, if possible. If de -construction takes
place during the rainy months, additional expenses and delays should be expected. These extra
expenses could include additional erosion control and temporary drainage measures to protect the
proposed retaining wall removal and restoration area.
Erosion Control and Slope Improvements
The on -site soils can have a high 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 wall removal should be protected from erosion. Measures taken may include
diverting surface water away from the stripped areas and bare soils. Silt fences or straw bales should
be erected to prevent muddy water from flowing over the site slopes or onto neighboring properties.
Disturbed areas should be replanted with vegetation at the end of construction. The final graded slope
should be stabilized by plantings and erosion control measures consisting of heavy-duty jute netting or
coconut coir grid. The matting should be staked at the top of the slope with 2- to 3-foot long metal
rebar that has a metal "T" welded to the end. The mat should be staked to the surface every five feet.
After the matting is placed, we recommended that deep-rooted vegetation be planted on the slope and
grass seed be placed to re-establish vegetation growth. The vegetation should be maintained until
established. In general, we recommend a mixture of 25 percent each of the following vegetation:
Snowberry (Symphoricarpos albus), Nootka rose (Rosa nutkana), Ocean Spray (Holodiscus discolor), and
Oregon -grape (Manhonia nervosa). Final grading should incorporate appropriate erosion control
measures to route stormwater runoff away from sloping areas and to appropriate discharge locations.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation
Florez Retaining Wall Removal
Edmonds, Washington
NGA File No 1158420
February 27, 2020
Page 8
Runoff generated within the site, including roof downspouts, yard areas, and any other 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 flow over the site slopes
either during construction or on a permanent basis 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 steep 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 slopes.
USE OF THIS LETTER
NGA has prepared this letter for Mr. Jorge A. Florez and his agents, for use in the removal of the
retaining wall on this site only. This letter is a specific evaluation of the retaining wall area and should
not be considered an evaluation of the entire site. 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
letter for consideration in design. There are possible variations in subsurface conditions between the
explored and unexplored areas and also with time. Our letter, 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 prior to construction and to monitor wall
installation. These additional services are intended 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 construction activities comply with specifications. We should be contacted a minimum of one
week prior to construction activities and could attend pre -construction meetings if requested.
NELSON GEOTECHNICAL ASSOCIATES, INC.
Geotechnical Engineering Evaluation NGA File No 1158420
Florez Retaining Wall Removal February 27, 2020
Edmonds, Washington Page 9
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 letter
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 1158420
Florez Retaining Wall Removal February 27, 2020
Edmonds, Washington Page 10
We appreciate the opportunity to be of service to you. If there are any questions concerning this letter,
or if we can provide additional services, please call.
Sincerely,
NELSON GEOTECHNICAL ASSOCIATES, INC.
Katelyn S. Brower, GIT
Staff Geologist
Khaled M. Shawish, PE
Principal
KSB: KMS:dy
Six Figures Attached
NELSON GEOTECHNICAL ASSOCIATES, INC
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UNIFIED SOIL CLASSIFICATION SYSTEM
GROUPSYMBOL
MAJOR DIVISIONS
GROUP NAME
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
SOILS
RETAINED ON
NO.4 SIEVE
WITH FINES
GC
CLAYEY GRAVEL
SAND
CLEAN
SW
WELL -GRADED SAND, FINE TO COARSE SAND
SAND
$P
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
SC
CLAYEY SAND
WITH FINES
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.
c
Project Number
NELSON GEOTECHNICAL
No.
Date
Revision
By
CK
1158420
Florez Retaining Wall
N
ASSOCIATES,
INC.
I
2120120
Original
DPNJ
KSB
Soil Classification Chart
GEOTECHNICAL ENGINEERS & GEOLOGISTS
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LOG OF EXPLORATION
DEPTH (FEET)
USC SOIL DESCRIPTION
HAND AUGER ONE
0.0-4.2
DARK BROWN TO BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL, ORGANICS, WOOD
DEBRIS (LOOSE, MOIST TO WET) (FILL)
4.2 - 4.8
ORANGE BROWN SILTY FINE TO MEDIUM SAND WITH TRACE GRAVEL (LOOSE, WET)
4.8 - 8.0
SM BROWN TO GRAY SILTY FINE TO MEDIUM SAND WITH GRAVEL (LOOSE, WET)
8.0 - 8.5
SM BROWN TO GRAY SILTY FINE TO MEDIUM SAND WITH GRAVEL
(LOOSE TO MEDIUM DENSE, WET)
SAMPLES COLLECTED AT 4.8. 5.5 AND 8.5 FEET
MODERATE TO HEAVY GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 4.8 FEET
SLIGHT HAND AUGER CAVING WAS ENCOUNTERED FROM 4.8 TO 8.5 FEET
HAND AUGER ONE WAS COMPLETED AT 8.5 FEET ON 2/17/2020
HAND AUGER TWO
0.0-2.5
BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, ORGANICS, AND WOOD
DEBRIS (LOOSE, MOIST TO WET) (FILL)
2.5-4.0
SM GRAY BROWN MOTTLED SILTY FINE TO MEDIUM SAND WITH GRAVEL (LOOSE TO MEDIUM
DENSE, WET)
4.0 - 7.3
SM-ML GRAY TO BLUE GRAY SILT AND SILTY FINE TO MEDIUM SAND WITH TRACE GRAVEL
(MEDIUM DENSE, WET)
SAMPLES COLLECTED AT 2.5 AND 6.0 FEET
MODERATE TO HEAVY GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 2.0 FEET
SLIGHT HAND AUGER CAVING WAS ENCOUNTERED FROM 3.0 TO 7.3 FEET
HAND AUGER TWO WAS COMPLETED AT 7.3 FEET ON 2/17/2020
HAND AUGER THREE
0.0 - 3.0
BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL, TREE ROOTS, ORGANICS, AND WOOD
DEBRIS (LOOSE, MOIST TO WET) (FILL)
3.0 - 5.0
SM BROWN GRAY MOTTLED SILT AND SILTY FINE TO MEDIUM SAND WITH GRAVEL AND IRON
OXIDE STAINING (LOOSE TO MEDIUM DENSE, WET)
5.0 - 7.5
SM-ML GRAY TO BLUE GRAY SILT AND SILTY FINE TO MEDIUM SAND
(LOOSE TO MEDIUM DENSE, WET)
SAMPLES COLLECTED AT 4.0 AND 7.0 FEET
MODERATE TO HEAVY GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 3.0 FEET
SLIGHT HAND AUGER CAVING WAS ENCOUNTERED FROM 5.0 TO 7.5 FEET
HAND AUGER TWO WAS COMPLETED AT 7.5 FEET ON 2/17/2020 -
HAND AUGER FOUR
0.0 - 2.5
BROWN TO GRAY MOTTLED SILTY FINE TO MEDIUM SAND WITH TRACE GRAVELAND ROOTS
(LOOSE MOIST TO WET) (FILL)
2.5 - 3.0
SM GRAY SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, WET)
NO SAMPLES WERE COLLECTED
GROUNDWATER SEEPAGE EMITTING FROM SURFACE
HAND AUGER CAVING WAS NOT ENCOUNTERED
HAND AUGER TWO WAS COMPLETED AT 3.0 FEET ON 2/17/2020
KSB: NELSON GEOTECHN/CAL ASSOCIATES, INC.
FILE NO 1158420
FIGURE 6