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Kirkland | Tacoma | Mount Vernon
425-827-7701 | www.aesgeo.com
October 4, 2024
Project No. 20240196E001
Renton School District No. 403
7812 South 124th Street
Seattle, Washington 98178
Attention: Andrew Hollenback
Subject: Limited Geotechnical Engineering Study
Renton School District Transportation Facility New Parking Lot
420 Park Avenue North
Renton, Washington
References: Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report,
New Transportation Center, 1220 North 4th Street, Renton, Washington, AESI
Project No. 20070040, March 20, 2007
Geologic Map of the Renton Quadrangle, King County, Washington, Mullineaux,
D.R., 1965
Dear Andrew Hollenback:
As requested, Associated Earth Sciences, Inc. (AESI) has completed a site reconnaissance,
subsurface exploration, and limited geotechnical engineering study for the proposed new parking
lot at the Renton School District Transportation Facility. We are familiar with the site having
completed the site exploration and geotechnical design report (AESI, March 20, 2007) as well as
providing observation and testing services during construction of the existing transportation
facility in 2007.
This study was completed in general accordance with our scope of work and cost proposal dated
August 13, 2024. We were authorized to proceed by means of District Purchase Order
#2012300302. This limited study was completed in accordance with local standards of practice
in the field of geotechnical engineering at the time it was completed. No other warranty, express
or implied, is made.
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 2
SITE AND PROJECT DESCRIPTION
The proposed project will include converting the currently undeveloped northwest corner of the
overall transportation facility property located at 420 Park Avenue North in Renton, Washington,
as shown on the “Vicinity Map” (Figure 1), to a new parking lot measuring some 105 feet
east-west by 200 feet north-south. This portion of the property previously had two buildings
constructed which were demolished by 2020. The topography across the property is flat with
total grade change north to south of just a few feet.
Based on our review of conceptual site plans dated April 5, 2024, provided by Jacobson
Consulting, we understand that the new parking lot will be close to existing grades, paved in
asphalt with planters and landscaping providing separation from parking rows and the city
right-of-way. The parking lot will also be provided with on-site lighting.
SUBSURFACE EXPLORATIONS AND CONDITIONS
Our field study included excavating two exploration pits on September 13, 2024. The various
types of sediments, as well as depths where characteristics of the sediments changed, are
indicated on the exploration logs presented in Appendix A. Our exploration locations were
approximately determined on the property by measuring from known site features. The site and
approximate locations of the subsurface explorations referenced in this study are presented on
the “Site and Exploration Plan” (Figure 2).
The conclusions and recommendations presented in this letter-report are based, in part, on the
exploration pits completed for this study. The number, locations, and depths of the explorations
were completed within site and budgetary constraints. Because of the nature of the exploratory
work below ground, extrapolation of subsurface conditions between explorations is necessary. It
should be noted that differing subsurface conditions may sometimes be present due to the
random nature of deposition and the alteration of topography by past grading and/or filling. The
nature and extent of variations between the field explorations may not become fully evident until
construction. If variations are observed at that time, it may be necessary to re-evaluate specific
recommendations in this letter-report and make appropriate changes.
Previous Explorations
AESI completed a geotechnical study in 2007 during the design phase of the existing
transportation center. The 2007 study included six exploration borings which generally agree
with the geologic findings of the current study. Copies of the 2007 exploration logs are included
in Appendix A, along with logs of exploration pits recently completed for the current project
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 3
proposed. Generally, the 2007 study found fill within the surface 3 feet, underlain by soft/loose,
recent, Quaternary Holocene alluvium deposits.
We reviewed field reports from the construction phase of the current transportation center
which indicated that parking area improvements consisted of a 12-inch-deep overexcavation,
placement of Mirafi 500X, and replacement with 2-inch clean crushed rock. Visual on-site
reconnaissance indicates that the near-surface remedial improvements were effective in
providing structural support of the paved parking areas. We observed no concerning settlement
or cracking, and the paved surface appeared to be performing well.
New Exploration Pits
For the current study, two new exploration pits were completed by Northwest Excavating, an
independent firm working under subcontract to AESI, at the locations shown on Figure 2. The pits
were completed using a 2.5-foot-wide, toothed bucket on a E50, tracked Bobcat Excavator. The
pits permitted direct, visual observation of subsurface conditions. During excavation, samples
were generally collected upon observation of a lithology change with depth. The excavation
process was continuously observed and logged by a geologist from our firm. After logging the
exposed soils, pits were backfilled with the excavated soil and lightly tamped with the excavator
bucket. The various types of materials and sediments encountered in the explorations, as well as
the depths where characteristics of these materials changed, are indicated on the exploration
logs included in Appendix A. The depths indicated on the logs where conditions changed may
represent gradational variations between sediment types in the field.
Disturbed soil samples were selected from the pits, placed in moisture-tight containers, and
transported to AESI’s laboratory for further visual classification and testing, as necessary. The
exploration logs in Appendix A are based on the field observations and inspection of the samples.
SUBSURFACE CONDITIONS
Subsurface conditions at the project site were inferred from our field explorations accomplished
for this study, visual reconnaissance of the site, and review of selected applicable geologic
literature.
Published Geologic and Soils Map
Review of the regional geologic map, titled “Geologic Map of the Renton Quadrangle, King
County, Washington,” U.S. Geological Survey, Geologic Quadrangle Map GQ-405, scale 1:24,000,
prepared by D.R. Mullineaux (1965) indicates that the subject property is underlain by
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 4
widespread artificial fill with Quaternary alluvium deposited by the ancestral Cedar River mapped
nearby. Review of the U.S. Department of Agriculture (USDA) Natural Resources Conservation
Service (NRCS) Web Soil Survey indicates that the soils in the vicinity of the site consist
predominately of Urban Land. The regional mapping is consistent with our subsurface
observations.
Stratigraphy
As shown on the field logs, our explorations in the area of the planned parking lot generally
encountered native, Holocene alluvium at shallow depths, overlain by about 1 to 3 feet of fill and
topsoil. The following section presents more detailed subsurface information organized from the
youngest (shallowest) to the oldest (deepest) sediment types. Copies of our exploration logs are
included in Appendix A.
Sod and Topsoil
Both exploration pits in the current project area encountered up to 10 inches of sod and topsoil;
the sod and topsoil are not suitable for structural support and should be stripped from structural
areas.
Fill
Existing fill was encountered in both explorations; EP-1 was mixed with topsoil to a dept of about
10 inches and in EP-2 there was up to about 2 feet of variable fill under the topsoil, where the
new parking lot will be constructed. Existing fill generally consisted of moist, gray, fine to coarse
sandy, fine to coarse gravel with some plastic and brick debris. Excavation through fill was difficult
for the contractor’s equipment and appeared firm. Fill in EP-2 extended to approximately a depth
of about 3 feet below the ground surface. Existing fill is also expected to be found around buried
utilities and structures, in areas of past grading, and beneath landscape improvements.
Where existing fill is found to be firm and unyielding during a proof-roll with a fully loaded,
tandem-axle dump truck, it may be suitable for structural support of the paved parking area. We
recommend remedial preparation of surfaces where fill is not encountered to be consistent with
preparation in areas where fill is encountered. Excavated existing fill may be suitable for reuse in
structural fill applications provided it is free of excessive organic material and other deleterious
material, at proper moisture content, and is allowed by project plans and specifications.
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 5
Holocene Alluvium
Sediments encountered below the fill and topsoil generally consisted of interbedded clean sand,
silty sand, and silt with some woody debris present. We interpret these sediments to be
representative of alluvium deposited in former channels of the Cedar River. The alluvium extends
past the depth of our deepest exploration pit of 11 feet below current ground surface. This
concurs with the 2007 study which indicates a maximum boring depth of 95 feet below surface
elevation. In general, the alluvium is very loose/soft to medium dense at depths shallower than
75 feet, as indicated by the 2007 study.
Remedial action is recommended in planned paved areas to mitigate compressive settlement
within the loose/soft soil.
In general, the Holocene alluvium where moisture content is within the compactable range is
considered suitable for reuse as structural fill. It should be noted that where soils are above their
optimum moisture content for compaction, their reuse as structural fill during all but the driest
times of the year will be difficult. However, it should be noted that the existing alluvial soil was
observed to have a high silt content and is considered moisture-sensitive.
Hydrology
No groundwater was encountered during our current phase of exploration to depths of
approximately 11 feet below the ground surface, or approximately elevation 25 feet (surface
elevation 36). Groundwater is expected to be present at depth within the Holocene alluvium as
an unconfined local aquifer. Shallow perched water may be present within the alternating silt
and sand interbeds.
During our previous study (2007, referenced above), groundwater was encountered at
approximately 10 feet below the surface, or approximately elevation 25 (surface elevation 35).
To our knowledge, the project specified no deep cuts that might encounter the regional
groundwater aquifer.
It should be noted that fluctuations in the level of the groundwater may occur due to the time of
the year, variations in rainfall, and adjacent river levels. No utilities are anticipated to be installed
at depths where the anticipated groundwater table might be encountered.
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 6
CONCLUSIONS AND RECOMMENDATIONS
Our explorations completed in the location of the proposed new parking lot encountered
loose/soft Holocene alluvium underlying 1 to 3 feet of existing fill which contains up to 10 inches
of topsoil. These same conditions were reported in AESI’s “Subsurface Exploration, Geologic
Hazard, and Geotechnical Engineering Report” dated in 2007. Recommendations in this
letter-report included near-surface remedial improvements for preparation of paved parking
areas.
Given that the adjacent parking areas with similarly described subsurface conditions performed
well with the completion of near-surface remedial improvements, it is our opinion that the site
is suitable for the proposed parking lot development, provided that recommendations contained
herein are properly followed.
Recommendations
Site preparation of the planned paved parking area should include removal of all grass, topsoil,
and any other deleterious surface materials and the remaining roots grubbed to a depth that will
allow at least 12 inches of imported base material under the pavement section. Any loose existing
fill soils below this elevation should be recompacted to a firm and unyielding condition prior to
the placement of subsequent base stabilization.
The finished parking lot subgrade should receive similar near-surface stabilization improvements
as performed in the adjacent transportation center parking lot. Generally, this consists of a
minimum of 12 inches of aggregate (crushed rock) over geotextile reinforcement fabric
(Mirafi 500X equivalent or structurally better) on the stripped surface. Overexcavation to
accommodate the specified thickness of aggregate may be necessary, depending on design
elevations. Surface aggregate shall be comprised of a granular crushed rock or crushed recycled
concrete material, compacted to a firm and unyielding condition prior to asphalt paving.
Once the subgrade elevation is achieved, we recommend performing a proof-roll with a fully
loaded, tandem-axle dump truck on the exposed excavated surface prior to placing geotextile
fabric. This will allow the opportunity for visual performance verification of the exposed
subgrade, and if needed, less invasive access to remedy any concerns that may arise. We
recommend allowing budgetary flexibility should we identify any necessity for additional
near-surface improvements.
After recompaction of the exposed ground, if needed, additional structural fill may be placed to
attain desired grades. Structural fill placed below parking areas should consist of a granular
imported material such as crushed base course or clean chip rock which are free of organics and
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 7
other deleterious materials, placed in maximum 8-inch loose lifts. Each lift should be compacted
to 95 percent of the modified Proctor maximum density using ASTM D-1557 as the standard. In
the case of utility trench filling, the backfill should be placed and compacted in accordance with
applicable codes and standards.
The contractor should note that any proposed fill soils must be evaluated by AESI prior to their
use in fills. This would require that we have a sample of the material 72 hours in advance to
perform a Proctor test and determine its field compaction standard.
Pavement Recommendations
AESI recommends that an engineering stabilization geotextile reinforcement such as Mirafi 500x
(or approved equivalent) be placed over the stripped subgrade. The geotextile should be overlain
with a minimum of 12 inches of 2-inch clean crushed rock or crushed recycled concrete. The
fabric acts as a separation barrier between relatively fine-grained surficial materials on the site
and the load-distributor aggregate. As a separator, it reduces the loss of costly aggregate material
into the subgrade and prevents the upward pumping of contaminating silt into the aggregate.
The high tensile strength and low modulus of elongation of the fabric also act to reduce localized
stress by redistributing the traffic loads over a wider area of subgrade. In addition, the
recommended method of installation (proof-rolling) identifies weak areas, which can be
improved prior to paving.
The majority of the new parking lot will accommodate light vehicle loads from passenger vehicles.
In these light traffic load areas, we recommend a pavement section consisting of 3 inches of
hot-mix asphalt (HMA) underlain by 4 inches of crushed surfacing. The eastern edge of the
parking lot is proposed for a heavy-duty section. In these areas we recommend a minimum of
4 inches of HMA over 4 inches of crushed rock surfacing. The crushed rock will provide improved
and consistent drainage. The crushed rock courses must be compacted to 95 percent of the
maximum density, as determined by ASTM D-1557. All paving materials should meet gradation
criteria contained in the current Washington State Department of Transportation (WSDOT)
Standard Specifications.
CLOSURE
We are available to provide geotechnical engineering and monitoring services during
construction if required. The integrity of the paved parking areas depend on proper site
preparation and construction procedures. In addition, engineering decisions may have to be
made in the field in the event that variations in subsurface conditions become apparent.
Renton School District Transportation Facility Limited Geotechnical
New Parking Lot Engineering Study
October 4, 2024 ASSOCIATED EARTH SCIENCES, INC.
FGR/ld – 20240196E001-002 Page 8
We have enjoyed working with you on this study and are confident these recommendations will
aid in the successful completion of your project. If you should have any questions or require
further assistance, please do not hesitate to call.
Sincerely,
ASSOCIATED EARTH SCIENCES, INC.
Kirkland, Washington
______________________________
Forest Gheen-Regouski Matthew A. Miller, P.E.
Natural Resource Scientist Principal Engineer
Attachments: Figure 1: Vicinity Map
Figure 2: Site and Exploration Plan
Appendix A: Exploration Logs
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COUNTY LOCALE LOCATION
PROJECT NO.DATE FIGURE
19/2420240196E001
RENTON SD TRANSPORATION SITE PARKING EXPANSION
RENTON, WASHINGTON
VICINITY MAP
ESRI, USGS, NATIONAL GEOGRAPHIC,DELORME, NATURALVUE, I-CUBED, GEBCO:ARCGIS ONLINE BASEMAP. WADOT STATEROUTES 24K (12/20). KING CO: PARCELS,ROADS (8/24).
NOTE: LOCATION AND DISTANCES SHOWNARE APPROXIMATE. BLACK AND WHITEREPRODUCTION OF THIS COLOR ORIGINALMAY REDUCE ITS EFFECTIVENESS AND LEADTO INCORRECT INTERPRETATION.
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PROJECT NO.DATE FIGURE
±
210/2420240196E001
RENTON SD TRANSPORATION SITE PARKING EXPANSION
RENTON, WASHINGTON
EXISTING SITE AND
EXPLORATION PLAN
DATA SOURCES/REFERENCES:KING COUNTY: PARCELS, ROADS (8/24). EAGLEVIEW TECHNOLOGIES,INC.: AERIAL IMAGERY (2023). WA DNR LIDAR:KING_COUNTY_WEST_2021, ACQUIRED 4/21, 1.5' CELL SIZE.CONTOURS DERIVED FROM LIDAR.
0 100
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LEGEND
SITE
EXPLORAION TYPE-YEAR
EXPLORATION
BORING
EXPLORATION PIT
CONTOUR 10 FT
CONTOUR 2 FT
PARCEL
APPENDIX A
Exploration Logs
Classifications of soils in this report are based on visual field and/or laboratory observations,
which include density/consistency, moisture condition, grain size, and plasticity estimates
and should not be construed to imply field or laboratory testing unless presented herein.
Visual-manual and/or laboratory classification methods of ASTM D-2487 and D-2488 were
used as an identification guide for the Unified Soil Classification System.
OH
PT
CH
OL
MH
CL
ML
SM
SC
GW
SP
GC
SW
GM
GP
Well-graded gravel
and gravel with sand,
little to no fines
Poorly-graded gravel
and gravel with sand,
little to no fines
Clayey gravel
and clayey gravel
with sand
Silty gravel and silty
gravel with sand
Well-graded sand
and sand with gravel,
little to no fines
Poorly-graded sand
and sand with gravel,
little to no fines
Clayey sand and
clayey sand with
gravel
Organic clay or silt
of low plasticity
Organic clay or silt of
medium to high
plasticity
Peat, muck and other
highly organic soils
Silty sand and
silty sand with
gravel
Silt, sandy silt, gravelly
silt, silt with sand or
gravel
Clay of low to medium
plasticity; silty, sandy, or
gravelly clay, lean clay
Elastic silt, clayey silt,
silt with micaceous
or diatomaceous fine
sand or silt
Clay of high
plasticity, sandy or
gravelly clay, fat clay
with sand or gravel
(1
)
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(2
)
(2
)
(2
)
(2
)
Terms Describing Relative
Density and Consistency
Estimated Percentage Moisture Content
Percentage by Weight
<5
5 to <12
12 to <30
30 to <50
Component Definitions
Component
Trace
Some
Modifier
(silty, sandy, gravelly)
Very modifier
(silty, sandy, gravelly)
Size Range and Sieve Number
Larger than 12"
Descriptive Term
Smaller than No. 200 (0.075 mm)
3" to 12"
Coarse-
Grained Soils
Fine-
Grained Soils
Density
Very Loose
Loose
Medium Dense
Dense
Very Dense
SPT blows/foot
0 to 4
4 to 10
10 to 30
30 to 50
>50
(3)
0 to 2
2 to 4
4 to 8
8 to 15
15 to 30
>30
Consistency
Very Soft
Soft
Medium Stiff
Stiff
Very Stiff
Hard
SPT blows/foot(3)
Test Symbols
No. 4 (4.75 mm) to No. 200 (0.075 mm)
Boulders
Silt and Clay
Gravel
Coarse Gravel
Fine Gravel
Cobbles
Sand
Coarse Sand
Medium Sand
Fine Sand
Dry - Absence of moisture,
dusty, dry to the touch
Slightly Moist - Perceptible
moisture
Moist - Damp but no visible
water
Very Moist - Water visible but
not free draining
Wet - Visible free water, usually
from below water table
G = Grain Size
M = Moisture Content
A = Atterberg Limits
C = Chemical
DD = Dry Density
K = Permeability
No. 4 (4.75 mm) to No. 10 (2.00 mm)
No. 10 (2.00 mm) to No. 40 (0.425 mm)
No. 40 (0.425 mm) to No. 200 (0.075 mm)
3" to No. 4 (4.75 mm)
3" to 3/4"
3/4" to No. 4 (4.75 mm)
Symbols
Sampler Type and Description
Blows/6" or portion of 6"15
10
20
California Sampler
Ring Sampler
Continuous Sampling
Grab Sample
Portion not recovered
Split-Spoon Sampler (SPT)
Cement grout
surface seal
Bentonite seal
Filter pack with
blank casing
section
Screened casing
or Hydrotip with
filter pack
End cap
ATD
At time
of drilling
Static water
level (date)
(1) Percentage by dry weight(2) Combined USCS symbols used for fines between 5% and 12%(3) (SPT) Standard Penetration Test (ASTM D-1586)(4) In General Accordance with Standard Practice for Description
and Identification of Soils (ASTM D-2488)
Groundwater
depth
i n c o r p o r a t e d
e a r t h s c i e n c e s
a s s o c i a t e d
EXPLORATION LOG KEY FIGURE:A1Blo
c
k
s
\
d
w
g
\
l
o
g
_
k
e
y
2
0
2
2
.
d
w
g
L
A
Y
O
U
T
:
L
a
y
o
u
t
5
-
2
0
2
2
L
o
g
d
r
a
f
t
0
2.5
5
7.5
10
12.5
15
17.5
20
Topsoil / Fill - 10 inches
Moist, dark brown, silty, gravelly, fine to medium SAND; angular gravel (SM).
Holocene Alluvium
Moist, light brown, silty, fine SAND (SM).
Very moist, gray with orange mottling, fine sandy, SILT; micaceous (ML).
Becomes less orange mottling.
Very moist, dark gray, fine to coarse SAND; some organic wood (2 to 4 inches thick); interbeds of gray, silt (SP).
Occasional orange, fine sand.
No seepage. No caving.
Associated Earth Sciences, Inc.
Exploration Pit EP-1
Renton SD Transportation Site Parking Expansion
Renton, WA Date:9/13/2024 Logged By:FGR
20240196E001 Total Depth (ft):11 Approved By:JHS
De
p
t
h
(
f
t
)
Description US
C
S
20
2
4
0
1
9
6
E
0
0
1
10
/
4
/
2
0
2
4
Elev.: »36 ft NAVD88
Sheet: 1 of 1
0
2.5
5
7.5
10
12.5
15
17.5
20
Topsoil - 6 inches
Fill
Slightly moist, gray, silty, fine to coarse sandy, fine to coarse GRAVEL; some plastic; brick debris (SM).
Holocene Alluvium
Very moist, orangish brown and gray, SILT; some interbed (2 to 3 inches thick) of orange, fine sand (ML).
Moist, brown, fine SAND, some silt; discontinuous; interdeposits of gray, silt; bed (1 to 3 inches thick) (SP).
Becomes gray.
Very moist, gray, SILT; beds (0.5 to 2 inches thick) of gray, fine sand, and orange, fine to medium sand (ML).
No seepage. No caving.
Associated Earth Sciences, Inc.
Exploration Pit EP-2
Renton SD Transportation Site Parking Expansion
Renton, WA Date:9/13/2024 Logged By:FGR
20240196E001 Total Depth (ft):11 Approved By:JHS
De
p
t
h
(
f
t
)
Description US
C
S
20
2
4
0
1
9
6
E
0
0
1
10
/
4
/
2
0
2
4
Elev.: »36 ft NAVD88
Sheet: 1 of 1