HomeMy WebLinkAboutRS_Geotechnical_Report_Part_1_Page_Set_211222_v1DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
September 15, 2021
The Home Depot
2455 Paces Ferry Road, C19
Atlanta, GA 30339
® PLANNING DIVISION RECEIVED
12/22/2021 JDing
Attn: John R. Foy -Manager/Field Construction
P: (678) 764-2837
E: john_r_foy@homedepot.com
lrerracan
Re: Geotechnical Engineering Report -DRAFT Addendutn Letter No. 1
Former Sam's Club
South Grady Way and Talbot Road
Renton, WA
Terracon Project No. 81195216
Dear Mr. Foy:
This DRAFT addendum letter serves as an update to the existing geotechnical engineering report
developed by Terracon (i.e. formerly Zipper Zeman & Associates; ZZA) dated December 6, 2002
(see attached). This addendum letter was developed consistent with our proposal dated July 13,
2021. Presented in this letter are the following:
•Liquefaction hazard analysis results performed using current methods with previously
performed boring logs (ZZA; Terra Associates; GeoEngineers)
a Considerations for adding piles to the existing building and new piles to building additions
•Preliminary recommendations for downdrag loads on existing piles as a result of post
liquefaction settlement
•Preliminary lateral pile recommendations for use in LPile analyses performed by the
structural engineer to assess adequacy of existing piles
•Overview of geotechnical data gaps and recommended additional analyses to support the
proposed development (see attached proposed site exploration plan)
a Conceptual geologic cross sections: A-A', B-B', C-C', and D-D'
•Photography log from a site visit performed August 26, 2021 is also attached.
This addendum letter should be used with the attached geotechnical engineering report. Based
on review of the construction plans and drawings, the building is support by 18-inch diameter
augercast piles founded in at least 2 feet of bedrock. The augercast pile schedule is presented in
the photography log that is attached at the end of this letter. Floor slabs are supported by grade
beams with piles spaced approximately 20 feet. Each column is supported by a pile cap with two
piles. Additional single augercast piles support the grade beams for the floor slab. The allowable
pile capacity presented in the plan set is 75 tons per pile. Photos of a portion of the plan set are
included in the attached photography log.
Terracon Consultants. Inc. 21905 64 1� Ave. W. Suite 100 Mountlake Terrace. WA 98043
P (42"5) 771 3304 F (425) 771 3549 www.terracon.com
Environmental •Facilities •Geotechnical •Materials
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Geotechnical Engineering Report -DRAFT Addendum Letter No. 1
Former Sam's Club• Renton, WA
September 15, 2021 Terracon Project No. 81195216
UPDATED SEISMIC CONSIDERATIONS
lrerracan
The existing geotechnical engineering report for the project was originally published while the
International Building Code (IBC) 1997 was the current code. Since that time. IBC 2018 has been
adopted by Washington State, which references ASCE 7-16. This new code introduces different
ground motion parameters than the previous version, including an increase in the peak ground
acceleration (PGA) which is used for liquefaction evaluations. Updated seismic design
parameters are presented in the table below:
Description
Site Class
Site Latitude
Site Longitude
.§s_ -Short Period Soectral Acceleration. Site Class E 2
51 -1-Second Period Soectral Acceleration Site Class E 2
PGA -ASCE 7, Peak Ground Acceleration2
.-Ee.GA -Peak Ground Acceleration Site Coefficient 2
PGAM -Site-modified geak around acceleration2
Value
F
47.4727
-122.2061
1.431 g
0.488 g
0.609 g
1.1
0.67
The I BC requires a site profile extending to a depth of 100 feet for seismic site classification. Borings were
performed to the depth range of 50 to 100 feet where blow counts were used to define the site class.
2.These values were obtained using online seismic design maps and tools provided by ATC
(https://hazards. atcouncil .org/}.
LIQUEFACTION
The subsurface conditions are described and presented in the geotechnical report. In general,
subsurface conditions consist of fill that includes some compacted structural fill near the surface
underlain by uncompacted fill that includes very loose to loose coal mine tailings, cinders, and
sandstone and shale fill. The uncompacted unit of fill is underlain by alluvial soil consisting of very
loose to loose sand and silty sand, very soft to soft clay, silty clay, clayey silt, silt, and peat.
Sandstone bedrock underlies the alluvial soil at depths ranging from 19 to 11 O feet below the
ground surface that existed at the time the subsurface explorations were performed. The bedrock
is described as highly weathered but becomes more competent with depth.
Liquefaction was evaluated for select, full-depth borings (i.e. advanced to bedrock) from the
geotechnical report using the liquefaction triggering methods proposed by Idriss & Boulanger
(2014). The borings selected for analysis were based on the boring location, depth, and available
data. In general, the liquefiable unit is observed to extend from the water table to denser alluvium
or cohesive soils. Liquefaction is assumed to occur no deeper than 60 feet bgs. The estimated
Responsive Resourceful • Reliable 2
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Geotechnical Engineering Report -DRAFT Addendum Letter No. 1
Former Sam's Club• Renton, WA lrerracan
September 15, 2021 Terracon Project No. 81195216
range of liquefiable soils and the resulting free-field post-liquefaction settlement for the design
level earthquake is presented in the table below:
1 Locations
B-3
B-5
B-6
B-7
B-8
B-10
Depth Range of Liquefiable Unit Estimated Total Settlement
(feet bgs) (inches)
10 to 50 20
10 to 43 18
10to 38 16
10 to 40; 55 to 60 18
10 to 28; 38 to 48 18
10 to 43; 50 to 56 26
1 Boring performed by ZZA that are presented in geotechnical engineering report.
'2 Below existing ground surface (bgs) at the time of the explorations.
3 Field-field estimate.
;
Although the borings selected were located within the building footprint, similar magnitudes of
settlement should be expected for pavement areas. Smaller (but still significant) earthquakes
could result in less settlement than the estimated total liquefaction as a result of the design
earthquake.
Considerations for differential settlement should be made for any areas of the planned
development that are not pile supported. Over a span of 40 feet, we estimate the differential
settlement will be on the order of 1 to 9 inches (per ASCE 7-16, Table 12.13-3). The differential
settlement limit allowed by the code for preserving life safety is 4� inches. The estimated
differential settlement exceeds the code limit; therefore, mitigation (i.e. pile support) is considered
to be necessary.
DEEP FOUNDATIONS
Conceptual Geologic Cross Sections
The existing soil borings from the geotechnical engineering report and reports from others were
used to generate conceptual geologic cross sections. The geologic cross sections are intended
to conceptually present the extent of liquefiable soils and the sandstone bedrock contact. The
cross sections can be used to evaluate pile stability with other recommendations presented in this
report addendum. The cross sections are presented at this end of this letter and will be updated
following the additional geotechnical explorations recommended herein and subsequent
addendum letter No. 2.
Responsive Resourceful Reliable 3
Geotechnical Engineering Report –DRAFT Addendum Letter No. 1
Former Sam’s Club Renton, WA
September 15, 2021 Terracon Project No. 81195216
Responsive Resourceful Reliable 4
Existing Pile Design and Performance Observations
As mentioned previously, the building is supported by 18-inch diameter augercast piles founded
in at least 2 feet of bedrock. These piles are designed with an allowable capacity of 75 tons. This
translates to an allowable end bearing capacity of 85 kips per ft2. A typical factor of safety of 3 is
assumed.
Based on our site visit and review of exterior walls, columns, and floor slabs, the building appears
to be in good condition with no areas of significant cracking or structural distress noted on the
exterior walls or floor slabs. We surmise that the existing pile foundation appears to be performing
well for the near 20-year old building. Given this apparent satisfactory performance, similar pile
sizes and capacities could be assumed for additional piles. However, pile installation within the
building may be limited, alternative pile methods such as segmental torque-down piles, in addition
to augercast piles, should be considered.
Pile Downdrag
The phenomenon of liquefaction can have a negative impact on pile capacity due to the induced
vertical loading from ground subsidence, or liquefaction-induced settlement. The zone of
liquefaction, as it settles, drags with it the overlying soil overburden. Rather than providing added
vertical capacity via side friction, the overlying non-liquefiable soil and liquefiable soil impart a
load on the pile. The downdrag load; however, is a function of the liquefiable zone thickness,
overburden soil thickness, and soil overburden composition. In general, the soil overburden
thickness is roughly 10 feet while the composition is relatively variable. For purposes of this
addendum letter, an average condition (thickness and composition) is assumed for estimating the
downdrag load. The table below presents a range of values that can be assumed for structural
analysis of pile capacity:
Depth Range of Liquefiable Unit
feet bgs)1,2
Estimated Downdrag Load per Pile (kips)
inches)1,2
10 to 20 18
10 to 30 33
10 to 40 50
10 to 50 68
10 to 60 85
1.For values between the ranges presented, linearly interpolate.
2.See the attached conceptual geologic cross sections for evaluation of other scenarios.
As discussed in the Geotechnical Data Gaps section, additional explorations are recommended
along the building perimeter to improve our understanding of the depth to bedrock which will
facilitate estimation of downdrag and planning of pile embedment depths. Piles terminated short
of the bedrock, or in weathered bedrock, may not achieve the recommended design capacities.
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
DocuSfgn Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Geotechnical Engineering Report -DRAFT Addendum Letter No. 1
Former Sam's Club• Renton, WA lrerracan
September 15, 2021 Terracon Project No. 81195216
The values presented in the table above are considered preliminary and are for planning purposes
only. These values will be updated following completion of the soil borings as part of the
recommended additional explorations and subsequent addendum letter No. 2.
LATERAL PILE LOADING
The following table lists input values for use in LPILE analyses. LPILE estimates values of kh and
Eso based on strength: however, non-default values of kh shoulq be used where provided,
particularly for the sand strata. Since deflection or a service limit criterion will most likely control
lateral capacity design, no safety/resistance factor is included with the parameters.
Stratigraphy 1
No. Material
1 Fill (above
WT)
Liquefiable 2 Zone
Non-
3 Uquefiable
Alluvial Soil
4 Sandstone
L-Pile
Soil
Model
Sand
(Reese)
Liquefied
Sand
(Rollins)
Soft Clay
(Matlock)
Weak
Rock
Depth to Bottom
of Stratum below
existing ground
surface (ft)
10
30 to 60
60 to 105
--
C (psf)
0 30° -
0 30°
1,200 0
28,000 ---
y(pcf)
Eso K (pci)
120 ---25
1'20 ----
125 0.01 125
145 0.001 1,000
1 See Subsurface Profile in the geotechnical ehgineering report provided by llA for more details on
Stratigraphy.
2 Definition of Terms:
c: Cohesive intercept
�: Internal friction angle,
y.Moist unit weight
£5o: Non-default E50 strain
K: Horizontal modulus of subgrade reaction
3.Buoyant unit weight values should be used below water table (i.e. y less 62.4 pcf)----------------
When piles are used in groups, the lateral capacities of the piles in the second, third, and
subsequent rows of the group should be reduced as compared to the capacity of a single,
independent pile. Guidance for applying p-multiplier factors to the p values in the p�y curves for
each row of pile foundations within a pile group are as follows:
Responsive Resourceful Reliable 5
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Geotechnical Engineering Report -DRAFT Addendum Letter No. 1
Former Sam's Club• Renton, WA lrerracan
September 15, 2021 Terracon Project No. 81195216
D D D D Lateral Load D D D D
D D D D
Third & Second Front
Subsequent Row Row Row.s •Front row: Pm = 0.8;•Second row: Pm = 0.4•Third and subsequent row: Pm = 0.3 .
For the case of a single row of piles supporting a laterally loaded grade beam, group action for
lateral resistance of piles would need to be considered when spacing is less than three pile
diameters (measured center-to-center).
GEOTECHNICAL DATA GAPS
Bedrock Depth
Existing geotechnical engineering reports for the site include the ZZA report (attached) and
reports by GeoEngineers and Terra Associates in 1999 and 2000, respectively. Collectively, there
are 27 soil borings within 50 feet of the building; however, not all of the borings were advanced to
bedrock. Along the northwest and southwest sides of the building, where the lumber rack and
garden center expansions are proposed, respectively, additional explorations are necessary to
understand depth to bedrock variations. Therefore, full-depth soil borings that penetrate into
bedrock are recommended. The existing and proposed soil boring locations are included as an
attachment to this letter.
Floor Slab Evaluation
Per the construction plans, the design floor slab thickness is 5 inches and is underlain by a
methane barrier system. Per the recommendations in the geotechnical engineering report, the
native soils are to be overlain by 12 inches of sand/gravel fill with possible use of a woven
geotextile. Over the fill should be a methane/water vapor barrier followed by 6 inches of capillary
break. It is not known if the layering beneath the slab was constructed as recommended in the
geotechnical engineering report. Terracon proposes the following:
Perform a ground penetrating radar (GPR) survey of the building slab to estimate rebar
spacing, slab thickness, and presence of voids that may exist beneath the slab.
•Where voids are identified, perform up to 6 cores to characterize the extent of the voids
(if present)
Ro5ponslvo • Rnsourc oful • Reltablo 6
Geotechnical Engineering Report -DRAFT Addendum Letter No. 1
Deliverable
LIMIT AT IONS
Geotechnical Engineering Report –DRAFT Addendum Letter No. 1
Former Sam’s Club Renton, WA
September 15, 2021 Terracon Project No. 81195216
Responsive Resourceful Reliable 8
impact construction costs. Any parties charged with estimating construction costs should seek
their own site characterization for specific purposes to obtain the specific level of detail necessary
for costing. Site safety, and cost estimating including, excavation support, and dewatering
requirements/design are the responsibility of others. If changes in the nature, design, or location
of the project are planned, our conclusions and recommendations shall not be considered valid
unless we review the changes and either verify or modify our conclusions in writing.
We appreciate the opportunity to be of service to you on this project. If you have any questions
concerning this letter, or if we may be of further service in the meantime, please contact us.
Sincerely,
Terracon Consultants, Inc.
DRAFT
Zachary L. Koehn, P.E. Dennis R. Stettler, P.E.
Project Engineer Senior Engineering Consultant
Attachments:
Exploration Site Plan (includes existing and proposed explorations)
Conceptual Geologic Cross Sections (A-A’, B-B’, C-C’, D-D’)
Photography Log
ZZA Geotechnical Engineering Report (2002)
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371 F2EBC8E65
EXPLORATION PLAN AND CROSS SECTIONS
Home Depot-Sam's Club Site• Renton, WA
September 14, 2021 • Terracon Project No. 81195216
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AERIAL PHOTOGRAPHY PROVIDED
BY MICROSOFT BING MAPS
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DocuSlgn Envelope ID: 6'314EAD5-0BEB-42DD,88C0-371 F2EBC8E65
GRADE A
EXISTING
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POTENTIALLY LIQUIFIABLE SOIL
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POTENTIALLY �IQUIFIABLE
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SCALE IN FEET
HORIZONTAL. 1' = 30'
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NOTES:
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B-1 BORING NUMBER STRATAGRAPHIC CONTACT 1.PRESENTED ARE POSSIBLE REPRESENTATIONS OF SUBSURFACE CONDITIONS. FOLLOWING
--?---?--INFERRED STRATAGRAPHIC CONTACT
STRONG GROUND MOTIONS. THE STRATIGRAPHY PRESENTED SHOULD BE CONSIDERED
CONCEPTUAL IN NATURE AND ARE SUBJECT TO CHANGE PENDING ADDITIONAL EXPLORATIONS
2. BASED ON ASCE 7-16 CODE AND CURRENT STANDARD OF PRACTICE FOR LIQUEFACTION
EVALUATION.
10
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-10
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SANDSTONE ZK P..,.ciN• 811%216 11erracon CONCEPTUAL GEOLOGIC CROSS SECTION A • A'
BORING END DEPTH
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EXHIBIT
1
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10010-90-100-110-120-130-80-
80-70-60-50-40-30-20-
10010-90-100-110-120-130-80-
80-70-60-50-40-30-20-
10010-90-100-110-120-130-80-
Photography Log
Home Depot -Sam's Club Site • Renton, WA
September 15, 2014 • Terracon Project No. 81195216
PHOTOGRAPHY LOG
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PHOTOGRAPHY LOG 1 of 6
Photography Log
Home Depot – Sam’s Club Site Renton, WA
September 15, 2014 Terracon Project No. 81195216
Responsive Resourceful Reliable PHOTOGRAPHY LOG 2 of 6
Typical perimeter wall and column
Loading dock area from inside the building
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Photography Log
Home Depot – Sam’s Club Site Renton, WA
September 15, 2014 Terracon Project No. 81195216
Responsive Resourceful Reliable PHOTOGRAPHY LOG 3 of 6
Typical interior column. Minor slab cracking.
Slab cracking within loading dock area
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Photography Log
Home Depot – Sam’s Club Site Renton, WA
September 15, 2014 Terracon Project No. 81195216
Responsive Resourceful Reliable PHOTOGRAPHY LOG 4 of 6
Loading dock area from outside the store
Southwest side of store
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Photography Log
Home Depot – Sam’s Club Site Renton, WA
September 15, 2014 Terracon Project No. 81195216
Responsive Resourceful Reliable PHOTOGRAPHY LOG 5 of 6
General notes for foundations and slabs from construction plan set
Sample of augercast pile layout from construction plan set
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Photography Log
Home Depot – Sam’s Club Site Renton, WA
September 15, 2014 Terracon Project No. 81195216
Responsive Resourceful Reliable PHOTOGRAPHY LOG 6 of 6
Augercast pile schedule from construction plan set
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DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371 F2EBC8E65
Zipper Zeman Associates, Inc.
Geotechnical and Environmental Consultants
PacLand
1144 Eastlake Avenue E., Suite 601
Seattle, Washington 98109
Attention: Mr. Joe Geivett, P .E.
J-1470
December 6, 2002
Subject: Subsurface Exploration and Geotechnical Engineering Evaluation
Proposed Retail Development
S.Grady Way and Talbot Road
Renton, Washington
Dear Mr. Geivett:
This report presents the results of our subsurface exploration and geotechnical engineering
evaluation for the above-referenced project. The authorized scope of services for this project
consisted of our field exploration programs for the slope stability analysis and site evaluation,
field and laboratory testing, geotechnical engineering analyses, and preparation of this report. Our
services were completed in accordance the scopes presented in our Proposal for Subsurface
Exploration and Geotechnical Engineering Services, Slope Stability Analysis, and Proposed
Retail Development (P-1673) dated September 3, 2002 and September 13, 2002, respectively.
Written authorization to proceed with this project was provided by PacLand on September 19, 2002.
The purpose of this evaluation was to establish general subsurface conditions at the site
from which conclusions and recommendations for foundation design, pavement design, and
general earthwork construction for the project could be formulated. In the event that there are any
changes in the nature, design, elevation, or location of the proposed structure, the conclusions and
recommendations contained in this report should be reviewed by Zipper Zeman Associates, Inc.
(ZZA) and modified, as necessary, to reflect those changes. This report has been prepared in
accordance with generally accepted geotechnical engineering practice for the exclusive use of
Pacific Land Design and their agents for specific application to this project.
EXECUTIVE SUMMARY
The following is a brief summary outline of the geotechnical conclusions and
recommendations for this project. The summary should be read in complete context with the
accompanying report for proper interpretation.
Review of Existing Literature
•We reviewed two geotechnical reports completed for the project site that _ were provided to us
by the property owner. In October 1999, a report was prepared by GeoEngineers titled
Geotechnical Engineering Services, Proposed Home Depot Development. Another report by
18905 33rd Avenue W., Suite 117 Lynnwood, Washington 98036 ( 425)771-3304
Proposed Retail Development
S. Grady Way and Talbot Road
Renton,Washington
J-1470
December 6, 2002
Page 2
Terra Associates, Inc. was prepared in September 2000 and was titled Geotechnical Report,
Southpoint Corporate Center.
Subsurface Conditions
The subsurface evaluation consisted of completing 43 hollow-stem auger and mud rotary
borings, and 3 electric cone probes across the project site, as shown on Figure 1, the Site and
Exploration Plan.Of the 43 borings, 6 were completed in a separate phase in order to
evaluate the stability of a proposed alteration to protected slopes along the east side of the site.
The slopes were man-made as a result of placing coal mine tailings on the site.
A large portion of the site is currently covered with asphalt pavement and structural concrete
floors. Elsewhere, the surface is covered with gravel. Surficial site soils typically consist of
4Y2 to more than 11Y2 feet of very loose to loose, moist, wet, and saturated, brown to black,
coal, cinders, sandstone, and shale fill. Limited topsoil of variable thickness should be
expected in areas that are not currently developed.
In general, the fill materials are underlain by alluvial soils consisting of very soft to soft peat,
clay, silty clay, clayey silt and silt, as well as very loose to loose sandy silt and sand with
varying proportions of silt and gravel. Sandstone bedrock was encountered at depths ranging
from as shallow as 19 feet to greater than 110 feet. These generalizations should be used in
conjunction with the attached exploration logs.
Groundwater depths varied across the site from 3 to 12 feet at the time of completing the
explorations. The elevations of the groundwater levels vary between 23 and 34 feet with the
highest groundwater elevation occurring near a small pond that is between the outflow from
two discharge pipes at the base of the slope along the east side of the site and the north end of
the aqueduct. Groundwater levels, including quantity and duration of flow, should be expected
to fluctuate throughout the year due to on- and off-site factors.
Site Preparation
Topsoil,if encountered in undeveloped areas, should be completely stripped and removed
from the building pad and parking lot areas. Stripping should also include the removal of
existing asphalt pavement, asphalt and concrete rubble, and vegetation that consists primarily
of limited brush and trees.
The proposed 3H:1V permanent slope that will be created after removing the lobe of coal
mine tailings on the east side of the site appears feasible, based upon our slope stability
analyses. However, in order to maintain adequate slope stability safety factors, we
recommend that a series of groundwater collection pipes be installed above the sandstone
bedrock contact in order to limit the build-up of perched groundwater in the remaining loose
fill materials that will comprise the finished slope.
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S. Grady Way and Talbot Road
Renton,Washington
J-1470
December 6, 2002
Page 3
All asphalt and concrete should be removed prior to placing fill in low areas. Existing asphalt
and gravel surfacing should be left in place wherever possible to protect the site from
construction traffic and provide laydown areas.
Pile foundations beneath the existing structural slabs on site should be cut off a minimum of 3
feet below slab and pavement subgrade elevations. Other concrete foundation elements, slabs,
and walls should be removed and disposed or crushed for reuse as structural fill.
Exposed soils will likely consist of moist to wet coal tailings. As such they should be
considered susceptible to disturbance from construction traffic. Existing fill soils (the entire
site) should be covered with a minimum of 12 inches of pit-run sand and gravel, crushed
recycled concrete, or other approved granular material to protect the sensitive subgrade.
Existing underground utilities should be removed or grouted in place. Excavations created in
order to remove the utilities should be backfilled with compacted structural fill. Deeper
underground structures, such as manholes, should also be backfilled with structural fill, lean-
mix concrete, or controlled density fill.
Depending on the groundwater levels at the time of construction, dewatering may be
necessary to lower groundwater levels if utility excavations or other underground structures
extend below the shallow groundwater table.
Peat should be expected in some of the deeper utility excavations and should be
overexcavated and replaced when encountered.
Structural Fill
All fill used to raise grades should be compacted to a minimum 95 percent of the modified
Proctor maximum dry density.
It is our opinion that all of the existing coal tailings fill on site should be considered
unsuitable for reuse as structural fill. Random areas of silty sand will likely be encountered
and would likely be suitable for reuse as structural fill, although it is not possible to quantify
the amount of this material. Granular material immediately below existing pavements and
slabs should also be considered suitable for reuse.
The parking lots and building pad should be covered with a minimum of one foot of pit-run
snad and gravel or equivalent.
Common fill used for general grading below the upper foot should have less than 15 percent
fines passing the U.S. No. 200 sieve. During periods when wet weather construction is
necessary, we recommend that import fill materials consist of pit-run sand and gravel or
crushed recycled concrete with less than 5 percent fines.
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S.Grady Way and Talbot Road
Renton,Washington
Utilities
J-1470
December 6, 2002
Page 4
Existing on-site,underground utilities should be removed, relocated or properly abandoned in
place in order to prevent possible future settlement problems. All existing underground
utilities should be decommissioned, abandoned, or backfilled in accordance with all
applicable State and local regulations.
We anticipate that most utility subgrades will consist of very loose to loose coal tailings fill or
possibly soft silt, loose silty sand, or peaty soils. Soils deemed unsuitable for utility support
should be overexcavated a minimum of 12 inches in order to develop a firm, uniform base.
Where peat is encountered, we recommend that the entire thickness of the material be
removed and replaced.
Existing on-site soils are considered unsuitable for utility trench backfill.
The two existing mine runoff drain pipes that enter the east side of the site will be tightlined
across a portion of the site. We recommend that the company or agency who owns or is
responsible for their maintenance be determined in order to coordinate a long-term
maintenance and inspection program. We further recommend that the peak flow in the drain
pipes be determined in order to size the proposed tightline pipe. This should likely be done in
the late winter or spring when groundwater would be anticipated to be at its highest.
Building Foundations
Based upon the soil conditions encountered, we recommend that augercast pile foundations be
used to support the proposed building. We recommend using I8-inch diameter piles with
allowable axial compressive capacities of 75 tons, provided the piles penetrate a minimum of
2 feet into the sandstone bedrock or extend to a maximum of 85 feet below the pile caps.
Building Floor Slab
Based upon a finish floor elevation of 37 feet, grading across most of the building pad will
vary from a fill of up to about 3 feet to a cut of up to about 4 feet.However where the lobe of
coal mine tailings is present along the east side of the site, cuts of up to about 20 feet will be
necessary. Due to the presence of the very loose to loose coal fill and the potential for
liquefaction of the underlying native soils, we recommend that the floor slab be pile
supported.
Subgrade compaction may be difficult to achieve because of the existing very loose coal fill.
Instead, it may become necessary to proofroll the subgrade with a loaded dump truck or other
suitable heavy equipment to reveal areas of soft or pumping soils.Overexcavated materials
should be replaced with non-organic compacted structural fill. The same process should be
completed in cut areas of the building pad once the cuts have been completed.
A woven geotextile (as necessary) and a minimum of 12 inches of pit-run sand and gravel fill
should be placed above floor subgrade soils and be compacted to a minimum of 95 percent of
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S.Grady Way and Talbot Road
Renton,Washington
J-1470
December 6, 2002
Page 5
the modified Proctor maximum dry density. We recommend that a durable methane/water
vapor barrier be placed between the 12 inches of granular structural fill and the capillary
break. A minimum 6-inch thick capillary break layer consisting of free-draining aggregate
should be placed over the methane gas barrier.
We recommend that the building be underlain with a passive methane gas venting system that
is installed in the 12 inches of granular soil below the methane gas barrier and be routed to the
outside of the building.Confined spaces and underground structures should also be vented.
Light Pole Foundations
Due to the loose fill conditions on site, we recommend that the parking lot light poles and
large signs be supported on augercast piles.It may be possible to consider other pole support
options, such as overexcavating the poor soils around the pole foundation and replacing it
with compacted structural fill or placing the light pole in a larger diameter steel pipe to
effectively increase the diameter of the foundation.
Drainage
A perimeter footing system is recommended for the proposed structure due to the depth to
groundwater at the time of our explorations relative to the proposed finish floor elevation.
Retaining Walls
Cast-in-place concrete walls should be supported on augercast piles.
Backfilled subsurface walls should be designed using equivalent fluid pressures of 35 and 55
pcf for active and at-rest loading conditions,respectively.Surcharge pressures from
backslopes,traffic, and floor loads should be added to the earth pressures.
Walls should be backfilled with a minimum of 18 inches of free-draining granular structural
fill that communicates with a footing drain or weepholes at the base of the wall.
Subsurface Walls
Relatively shallow groundwater levels should be expected across the eastern portion of the
site. The highest groundwater elevation at the time of drilling was approximately 34 feet and
occurred along the toe of the slope along the east side of the site.Waterproofing systems
should prevent moisture migration through the walls, floors, and construction joints as
necessary to satisfy the owners requirements.
Subsurface walls and floor slabs should be designed to resist hydrostatic lateral and uplift
forces,additive to the lateral earth pressure. Along the east side of the site,structures that
extend below elevation 34 feet should be designed for hydrostatic forces.
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S. Grady Way and Talbot Road
Renton,Washington
East Slope Retaining Wall
J-1470
December 6, 2002
Page 6
A cut of approximately 18 feet will be necessary in the slope along the east side of the site in
order to provide access around the southeast comer of the proposed building.Permanent
shoring using soldier piles set in drilled holes that extend into the native sandstone and
retained by tieback anchors is recommended for the proposed wall.Soldier pile drilling
within the water-bearing sand deposits will likely require stabilizing the holes because the
saturated sands are very loose to loose. A program to maintain stabilized soldier pile holes
should be the responsibility of the contractor.The contractor should also be required to have
the capability to case holes when required.
Historical records indicate that there were mineshaft adits in the area of the project site.
Review of the historical documents leads us to suspect that one of the mine openings may be
along the alignment of the existing 48-inch drainpipe that daylights on the project site. We
did not encounter conditions that would indicate the presence of the shafts. However,if a
zone of fill and/or a mine adit exists in the anchor zone of the proposed wall, it may not be
possible to install some of the tiebacks as recommended.
Permanent tiebacks will also be necessary to support the proposed cut. We anticipate that a
single row of tiebacks will be sufficient.However,we would also anticipate that the tiebacks
could extend beyond the limits of the site and into the Benson Road right-of-way.The
feasibility of constructing permanent tiebacks in the right-of-way should be determined.
Tieback anchors should be performance and proof tested. We recommend that all of the
tiebacks be performance tested to 150 percent of the design load and that that a minimum of 2
anchors be proof tested to 300 percent of the design load.
Recommendations for Further Study: We recommend that additional subsurface
explorations be completed in support of the retaining wall design.If a mineshaft is present in
the tieback zone, further definition of the conditions prior to bidding would reduce the
possibility of change orders and delays during construction.Evaluations could consist of
surficial geophysical evaluations using resistivity or magnetics and/or downhole geophysical
methods in predrilled holes. We also recommend that the 48-inch pipe be logged with a
camera to determine its alignment and where it terminates.
Pavement
Based upon compacting the exposed subgrade to a minimum of 95 percent of the modified
Proctor maximum dry density,standard pavement sections should consist of 3 inches of Class
B asphalt over 4 inches of crushed gravel base course over a minimum of 12 inches of pit-run
subbase.Heavy duty pavement sections should consist of 4 inches of Class B asphalt over 4
inches of crushed gravel base course over 12 inches of pit-run subbase.Depending on the
actual level of compaction,it may be necessary to use a geotextile fabric and additional
subbase. This would have to be determined at the time of construction.Asphalt-treated base
ATB) may be substituted for crushed gravel base course (CGBC)at a ratio of
O.75"ATB:l"CGBC.
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S.Grady Way and Talbot Road
Renton,Washington
Infiltration
J-1470
December 6, 2002
Page 7
Due to the composition of the fill materials on site and the anticipated high groundwater levels
across the site, we recommend that infiltration rates be considered negligible.
SITE AND PROJECT DESCRIPTION
The approximate 16 acre project site is located east of the intersection of South Grady
Way and Talbot Road (State Route 515), in the N.W.lf4 of Section 20, Township 23 North, Range
5 East in Renton, Washington. The property is bordered to the north by the Renton City Hall
Building, to the west and south by Talbot Road, to the north and west by South Grady Way, and
to the east by Benson Road and undeveloped land. The site is currently vacant, and with the
exception of an area along Benson Road, the project site is covered with asphalt pavement, gravel
covered areas, and the remains of two structural slabs that supported former buildings that have
been demolished. Slopes and a large lobe of coal mine waste fill on the eastern margin of the site
are primarily covered with blackberry brush and maple trees. The slopes appear to be on the
order of 20 to 30 feet in height and vary in steepness from about 1~H:1V to 2H:1V, or flatter.
Based on topographic information provided to us,it appears that the flatter portion of the site
varies in elevation between approximately 30 and 39 feet.It appears that Benson Road is
approximately 30 feet above the project site. A concrete aqueduct is situated along the toe of the
eastern slope and conducts water that appears to originate from the former Renton Coal Mine.
There are many above- and below-ground utilities at the site, some of which are still live.
High voltage electrical transmission lines also extend across the site.
As a result of past environmental site assessments on the project site, there are numerous
resource protection wells across the site. We estimate that there could be between 30 and 40
wells across the site.
We understand that the proposed development will consist of an approximate 135,000
square foot building with associated parking and landscaping. At the time of preparing this
report, the finish floor elevation is anticipated to be 37.0 feet. We anticipate that the exterior walls
will be constructed of concrete masonry block or steel frame and metal stud, and that steel tube
columns will provide interior roof support. Typical bay spacing between columns and walls is
approximately 50 by 47 feet and exterior columns are typically spaced 47 feet apart. For
purposes of preparing this report, the following structural loads are anticipated:
Interior column gravity load
Estimated maximum gravity load due to severe live loading
Exterior column gravity load
Maximum Column uplift forces from wind
Uniform load on continuous footings
Maximum uniform floor slab live load
Maximum floor slab concentrated load
65 kips
150kips
50 kips
30 kips
1.5-2.0 kips/lineal foot
250 psf
16.0kips
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S.Grady Way and Talbot Road
Renton,Washington
J·1470
December 6, 2002
Page 8
Zipper Zeman Associates should be notified of any deviation from the project description
presented herein to determine its potential effect on the conclusions and recommendations
presented herein.
SUBSURFACE CONDITIONS
The subsurface exploration program completed by ZZA at the project site included 43
hollow-stem auger and mud-rotary borings, the approximate locations of which are presented on
Figure 1, the Site and Exploration Plan, enclosed with this report.Of the 43 borings, 6 were
completed in a separate phase in order to evaluate the stability of a proposed alteration to a lobe
of coal mine waste fill along the east side of the site. We also reviewed the logs of subsurface
explorations completed by GeoEngineers and Terra Associates for previous proposed projects on
the subject site. Copies of the boring logs completed for this evaluation are enclosed in Appendix
A.
The borings electric cone probes completed in the building pad and parking areas for this
study extended to depths ranging from 11'i'2 to 110'i'2 feet below the existing ground surface.
Below the surface, soils typically consisted of 4'i'2 to more than ll'i'2 feet of very loose to loose,
moist, wet, and saturated, brown to black, coal, cinders and shale fill. In general, the coal ranged
in size from silt to gravel sized. In many borings loose silty sand fill that appeared to originate
from the local sandstone formation was also encountered.Underlying the coal mine wastes,
interbedded alluvial soils consisting of very loose to loose silty sand and very soft to soft sandy
silt, peat, organic silt, and clayey silt extended to depths of 15 to 49 feet below grade. The
alluvium graded to medium dense sand with varying proportions of silt and gravel and medium
stiff to stiff sandy silt, silt, organic silt, clayey silt and silty clay that extended to depths of
approximately 18'i'2 to 107 feet below grade. Very dense,weathered sandstone bedrock was
encountered below these materials and extended to the bottom of the borings. The sandstone is
part of the Renton Formation that is also the source of the coal fill encountered on the site. The
sandstone appears to dip relatively steeply to the west.
Borings B-IA through B-6A were completed on the lobe of coal mine waste fill along the
east side of the site.Subsurface conditions consisted of 20 to 36 feet of very loose to loose, damp
to wet, brown and black, coal and cinder fill with random layers of silty sand that originated from
the sandstone bedrock. Because of the steeply dipping sandstone bedrock in the area, borings B-
IA, B-2A, and B-3A, that were completed along the western margin of the fill,encountered 7 to
19 feet of very loose to loose sandy alluvial soils beneath the fill.Sandstone bedrock was
encountered in borings B-IA and B-2A at depths of 29 and 47 feet below existing grades,
respectively. In borings B-4A, B-5A, and B-6A, an approximate 3-foot thick layer of very loose,
wet to saturated, silty sand was encountered between the tailings and the dense sandstone. The
very loose layer was interpreted to be residual soil derived from the weathering of the sandstone.
Dense to very dense sandstone was encountered at depths of 23 to 33 feet below existing grades.
Four borings (B-4, B-9, B-36, and B-37) were completed in the area of the proposed
retaining wall that is situated near the southeast comer of the proposed building. Boring B-4
encountered approximately 7 feet of loose silty sand fill over 8 feet of loose native soils
consisting of sand with varying proportions of silt, gravel and organics. At 15 feet, a 3-foot thick
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
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Renton,Washington
J-1470
December 6, 2002
Page 9
layer of medium dense, silty sand with some gravel, interpreted as highly weathered sandstone
was.encountered. At a depth of approximately l8'i'2 feet, very dense sandstone was encountered.
In borings B-36 and B-37, approximately 4'i'2 to 6Y2 feet of very loose to loose silty sand and coal
tailings fill was encountered. In B-36, interbedded, very loose silty sand, sandy silt, and peaty
organic layers were encountered between 6'i'2 and 13 feet. Between 13 and 19 feet, medium stiff
sandy silt with interbedded silty sand and organics extended to a depth of approximately 19 feet.
At his depth, very dense sandstone was encountered. In B-37, very loose to loose alluvial sand
with varying proportions of silt gravel and peat was generally encountered. However, a 4-foot
thick peat layer was encountered between 8Y2 and 28 feet below grade. Dense grading to very
dense sandstone was encountered at a depth of 28 feet and continued to the bottom of the boring.
Boring B-9 was completed above the site along the edge of Benson Road. Approximately 10 feet
of loose to medium dense, silty sand fill was encountered below the surface. A possible relic,
silty sand topsoil layer.was encountered between 10 and IOY2 feet. Between 1OY2 and 20 feet,
medium dense silty sand was encountered. This material graded to a dense condition and
extended to a depth of about 25 feet. At that depth, the material graded to very dense weathered
sandstone.
The enclosed boring logs should be referred to for more specific information. Figure I,
the Site and Exploration plan includes information regarding the thickness of fill and depth to
bedrock at each of the boring locations.
Groundwater Conditions
Groundwater was encountered at the time of drilling in 39 of the 43 the borings.
Excluding the topographically higher borings, groundwater levels varied in depth across the site
from 3 to 12 feet at the time of completing the explorations. The elevations of the groundwater
levels varied between 23 and 34 feet with the highest groundwater elevation occurring nearest a
small pond that is between the outflow from two discharge pipes at the base of the slope along the
east side of the site and the north end of the aqueduct. Based on information presented by
GeoEngineers, it appears that the observed groundwater levels at the time of drilling coincide
with their observations.
Wet soils were encountered in borings B-4A, B-5A, and B-6A above the sandstone.
Perched groundwater should be expected to develop above the sandstone in areas above the
regional groundwater table. Variations in groundwater conditions should be expected due to
seasonal variations, on and off-site land usage, irrigation, and other factors.
Seismic Criteria
According to the Seismic Zone Map of the United States contained in the 1997 Uniform
Building Code,the project site lies within Seismic Zone 3. The Seismic Zone Factor (Z) for
Seismic Zone 3 is 0.30 that corresponds Seismic Coefficients C,and C,of 0.36 and 0.84,
respectively. Based on soil conditions encountered at the site, the subsurface site conditions are
interpreted to correspond to a seismic soil profile type SE as defined by Table 16-J of the 1997
Uniform Building Code.Soil profile type SE applies to an average soil profile within the top 100
feet consisting predominantly of soft soil characterized by Standard Penetration Test blowcounts
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
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Renton,Washington
J-1470
December 6, 2002
Page 10
less than 15, a shear wave velocity of less than 600 feet per second, and an undrained shear
strength less than 1,000 psf. Some of the near-surface soils are considered to be prone to
liquefaction during a design earthquake with a 10percent probability of exceedance in 50 years.
Results of pH and Resistivity Tt(stin~
Results of the pH and resistivity testing are presented in the following table.
Borlna and Sample Number Depth (Feet)pH Resistivity (ohm-em)
B-2, S-1 5-6'l'2 6.9 4,600
B-5 S-2 5-6'l'2 6.7 9,400
B-8, S-5b 15'l'2 5.6 3,300
B-26 S-2 5-6'l'2 5.6 4500
The electrical resistivity of each sample listed above was measured in the laboratory with
distilled water added to create a standardized condition of saturation. Resistivities are at about
their lowest value when the soil is saturated. Electrical resistivities of soils are a measure of their
resistance to the flow of corrosion currents. Corrosion currents tend to be lower in high resistivity
soils. The electrical resistivity of the soil varies primarily with its chemical and moisture
contents. Typically, the lower the resistivity of native soils, the more likely that galvanic currents
may develop and increase the possibility of corrosion. Based on .laboratory test results, resistivity
values for the near surface native soils varied between 3,300 and 9,400 ohm-ern.Soils with
resistivity values between 2,000 and 10,000ohm-em are generally associated with soils classified
as "mildly to moderately corrosive". The pH of the soils is slightly acidic but is not considered
significant in evaluating corrosivity. Therefore, it is our opinion that Type IIII cement is suitable
for this project. With respect to the need for protection of buried metal utilities, we recommend
that PacLand consult with the manufacturers of specific products in order to determine the need
for protection.
Climate Data
According to the U.S. Department of Commerce, Climatic Atlas of the United States,
1993, the project site lies within thePuget Sound Lowlands Region of Washington. Mean
monthly rainfall varies from a low of 0.96 inches in July to a high of 5.56 inches in December.
Between November and March, there are about 20 days per month where 0.01 inches or more of
rainfall occurs. Normal daily minimum temperatures are above freezing throughout the year.
Mean annual total snowfall is about 12inches.
Weather data from the Western Region Climate Center (WRCC) for Kent, Washington
the nearest weather station) varied slightly from the Climatic Atlas and likely represents a more
accurate representation of the local weather. The greatest mean monthly snowfall occurs in
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates, Inc.
Lynnwood, Washington 98036
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S.Grady Way and Talbot Road
Renton,Washington
J-1470
December 6, 2002
Page 11
January and averages 7.3 inches. Average monthly rainfall and snowfall amounts can be greatly
exceeded as can be seen in the enclosed weather data.
The mean total precipitation for each month at the Kent weather station is:
January:5.73 in.July: 0.85 in.
February:4.32 in.August:1.15 in.
March:3.88 in.September: 1.78 in.
April:2.70 in.October:3.49 in.
May 1.86 in.November: 5.88 in.
June:1.56in.December: 6.00 in.
The WRCC Monthly Total Snowfall, Monthly Total Precipitation, and Monthly Normals
data are also presented in Appendix D.
CONCLUSIONS AND RECOMMENDATIONS
The geologic conditions at the site are considered to be relatively poor from a geotechnical
engineering standpoint. The soil conditions generally consist of very loose to loose fill soils
composed primarily of coal and cinders over very soft to soft peat, silt, clay, clayey silt, and sandy
silt, as well as very loose to loose, wet to saturated silty sand and sand. The fill soils are
considered unsuitable for shallow foundation support and the native peat, silt and clay exhibit
relatively low strength and high compressibility characteristics that makes them susceptible to
consolidation when loaded. Consolidation under normally loaded foundation elements would
produce excessive total and differential settlements of the structure. Additionally, the cleaner,
very loose to loose sands are susceptible to liquefaction during a design earthquake. Liquefaction
susceptible sands were encountered in the borings within the building pad and the resulting
settlement associated with the occurrence of liquefaction could result in relatively large
differential settlements across the building pad. Preloading would not mitigate the liquefaction
potential at the site.
Environmentally Critical Areas -StelW Slope, Landslide, and Erosion Considerations
The slopes around the base of the fill lobe are relatively steep and are considered sensitive
and/or protected slopes as presented in the Municipal Code of Renton. As such, slopes
categorized as sensitive or protected are also considered to be geologic hazards by the City of
Renton. In order to modify the ridge, it was necessary to evaluate the soils and complete a slope
stability analysis of the proposed modifications to the slope.
Steep slope areas are classified as protected or sensitive. A protected slope is defined as a
hillside, or portion thereof, with an average slope of 40 percent or greater with a minimum
vertical rise of 15 feet. A sensitive slope is a hillside, or portion thereof,of 25 percent to less than
40 percent or and average slope of 40 percent or greater with a vertical rise of less than 15 feet
abutting an average slope of 25 percent to 40 percent.It is our opinion that all of the affected area
would be classified as protected or sensitive.
1890533rd Avenue W., Suite 117
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Renton,Washington
J-1470
December 6, 2002
Page 12
Moderate landslide hazard areas are defined as those areas with slopes between 15 and 40
percent where the surficial soils are underlain by permeable geologic units. High landslide hazard
areas are defined as those areas with slopes greater than 40 percent and areas with slopes between
15 and 40 percent where the surficial soils are underlain by low permeability geologic units.It
appears that slopes are greater than 15 percent and are underlain by both permeable and
impermeable soils. Therefore, it appears that the existing slope would fall within both categories
depending on the subsurface conditions.
The lobe of fill would also be defined as a high seismic hazard area due the hillsides being
comprised of loose fill over alluvium and post-glacial silts and peats.It also meets the definition
of a high erosion hazard area because the slopes are greater than 15percent.
Slope Stability Analysis
A slope stability analysis was performed for the site using the XSTABL5.2 computer
program. The stability analysis was based on a generalized subsurface soil and groundwater
profile through the existing and was developed using the site-specific subsurface data. Two direct
shear test were completed on representative samples obtained in borings B-3A at a depth of 16 to
16~feet and B-5A at a depth of 21 ~to 22 feet. This provided us with strength parameters that
were used in the slope stability analyses.
A topographic and subsurface profile was developed along line A-A'as shown on Figure
1, the Site and Exploration Plan. Based on the relative density, grain size distribution,
depositional history, and the site specific subsurface and laboratory data, it is our opinion that the
friction angle and cohesion values are reasonable estimates of the site soil strength parameters.
SOIL PARAMETERS FOR SLOPE STABILITY ANALYSES
Soil Layer Friction Angle (<I»Cohesion (pst) Moist Unit Weight
pcf)
Coal Tailings 37 0 70
Loose Sand Above
Sandstone 33 0 120
Loose Alluvium 32 0 120
Sandstone 15 3,000 140
Our analysis evaluated both static and dynamic (seismic) conditions for the existing slope
inclinations with and without an inferred perched groundwater table. The USGS Seismic Hazard
Mapping Project earthquake hazard map for the area indicated a peak horizontal bedrock
acceleration of O.32g for an earthquake with a 10 percent exceedance in 50 years. Our analysis
used a dynamic (seismic) horizontal ground acceleration of 0.16g (1/2 the peak acceleration)
conditions for the permanent cut slope inclination of 3H:1V, which is more indicative of the
average ground acceleration during a seismic event of design magnitude. Figure 2, Generalized
Subsurface Profile A-A', presents the subsurface soil and groundwater profile used for our
analysis.
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
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Renton,Washington
J-1470
December 6, 2002
Page 13
The following table presents the results of the static and dynamic stability analysis
conducted for this project.
TABLE 1
RESULTS OF SLOPE STABILITY ANALYSIS
Minimum Minimum
Slope Configuration Static Safety Factor Seismic Safety Factor
3H:IV Permanent Slope 2.1*1.3*
Reduced factors of safety are possible if high groundwater or low shear strength materials are present in the slope.
Based on our analysis, a permanent cut slope inclination of 3H:IV appears to be suitable
for static and seismic conditions. The results of the pseudostatic stability analysis (lowest safety
factor) are presented on Figure 3.
The occurrence of perched groundwater above the sandstone and within the tailings would
reduce the factor of safety and could potentially cause failure. In order to reduce the risk of
groundwater and surface infiltration destabilizing the slope, a subsurface drainage should be
installed to maintain groundwater levels as deep as possible. A series of perforated interceptor
drains on about a 25-foot lateral spacing in a herringbone pattern and connected to a collector
pipe will likely be necessary. The laterals would be connected to a collector pipe installed
essentially down the middle of the proposed slope. We recommend that the laterals be installed on
the sandstone and that the lowest set of interceptor pipes be installed to an invert elevation of 34
feet. We recommend that our firm review the design of the drains and that ZZA monitor their
installation. The final depths and locations will likely require field adjustments based on the
conditions encountered during construction.
Erosion Mitigation
The soils comprising the proposed cut slope are susceptible to erosion by flowing water.
We recommend the following erosion control Best Management Practices be implemented during
construction:
Establish well-defined clearing limits to reduce the amount of vegetation disturbed during
construction;
Place silt control fence downslope of disturbed areas;
Cover excavated slopes with plastic sheeting during rainfall events;
Cover disturbed and graded areas with straw, excelsior blankets, or other appropriate erosion
control materials, combined with seeding or other planting, to promote revegetation. Excelsior
blankets such as Curlex®,jute matting such as Geojute®, or other rolled erosion control
products, installed in accordance with the manufacturer's recommendations, are
recommended for sloping portions of the site disturbed during construction. Such areas
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates.Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
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Renton,Washington
J-1470
December 6, 2002
Page 14
include spots were the existing landscaping waste located at or slightly over the slope break is
removed.
Perched groundwater could daylight on the proposed 3H:1V slope that generally is not evident
on the existing slopes.If groundwater seepage daylights on the slope, it might cause shallow
slumping. These areas,ifit occurs, should be covered with a minimum of 10 inches of riprap.
We recommend that riprap conform to the specifications for Quarry Spalls as presented in
section 9-13.6 of the 2002 WSDOT Standard Specifications.
Foundations Settlement Discussion
There are three modes of potential settlement relative to the soil conditions encountered at
the site. The existing fill soils are comprised primarily of very loose to loose coal fragments in
the size range of silt, sand, and gravel. The composition and relative density of this material
makes it unsuitable for support of shallow foundations. The very soft to soft peat, silt, clay, and
clayey silt are expected to consolidate under the weight of static foundation loads and fill soil
surcharges. Additional, long-term settlements are probable due to secondary compression of
these materials. Finally, seismically-induced liquefaction in the very loose to loose saturated
sandy soils below the water table is also a significant risk. We have estimated that seismically-
induced settlements on the order of 4 to 7 inches could occur within the zones of sand that were
encountered across the building pad.
Because of the subsurface conditions encountered, we recommend that the building be
supported on pile foundations or soils that are deeply mixed with cement. In our opinion, either
option used to support structural loads for the new building would substantially reduce the risk of
excessive post-construction settlement and transmit foundation loads through liquefaction
susceptible soils. We recommend that the floor also be supported on piling or columns of cement-
mixed soil that extends to the bedrock.
Liqyefaction Analysis
As part of this study, we performed a site-specific liquefaction analysis using the methods
developed by Seed and Idriss for the soil conditions encountered in our boring. Liquefaction can
be described as a sudden loss of shear strength due to the sudden increase in porewater pressure
caused by shear waves associated with earthquakes.Based.on our liquefaction analysis, we
estimate that there is a risk that liquefaction would occur at various depths between approximately
10 to 40 feet below the existing ground surface during a design level earthquake event, as
discussed below. Laboratory testing was completed as a part of this liquefaction analysis, the
results of which are attached or indicated on the boring logs, as appropriate.
Based on the Uniform Building Code (UBC) guidelines, seismic analysis should be based
on an event having a 10 percent probability of exceedance in 50 years or return period of
approximately 475 years. According to available historical data, this return period within the
Seattle-Portland area would be associated with an earthquake of approximate Richter magnitude
7.5. According to the United States Geological Survey, the peak ground surface acceleration
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produced by an earthquake of this magnitude will be about 0.3g at the subject site, which
corresponds with the locally accepted acceleration values for fill or alluvial soils.
Using these seismic parameters, we computed safety factors against liquefaction for the
various soil layers below the water table using an analysis method developed by Seed and Idriss.
Our analyses revealed a high probability of liquefaction (safety factors ranging from <1 to 1.3)
within the zones of sand that were encountered at various depths below the water table down to
depths of 40 feet and more.
The report titled Liquefaction Susceptibility for the Des Moines and Renton 7.5-minute
Quadrangles,Washington,(Geologic Map GM-41), prepared by the Washington State
Department of Natural Resources, delineates the site as being underlain by Category I soil
deposits. Category I soil is defined having a high susceptibility to liquefaction. The report
presents quotes from the Mayor and City Engineer of Renton after the 1965, Richter magnitude
6.5, Seattle-Tacoma earthquake. Reportedly, the entire lengths of Burnett Street and 7th Avenue
required filling and paving to repair settling. In some places, the settlement was reported to be as
much as 2 feet. Burnett Street and 7th Avenue are located just north and west of Grady Way,
respectively, and within a few hundred feet of the project site.
Liquefaction could produce surface disturbance in the form of lateral spreading,
subsidence, fissuring, or heaving of the ground surface, which could result in cracking, settling or
tilting of the building. Volumetric strain on the order of ~to 3 percent could be possible in the
liquefiable layers which correlates the potential settlements of about 4 to 7 inches of settlement,
depending on the thickness of liquefiable soils. Due to the potential for liquefaction, as well as
the relatively high settlement potential for shallow foundations, a pile foundation system that
transmits foundation loads to the competent bedrock or dense soils encountered at a depth of
approximately 19to approximately 85 feet is recommended.
Site Preparation
Critical geotechnical considerations on the site include the moisture-sensitive soils
encountered, high groundwater conditions along the east side of the site, the poor quality coal
tailings fill across the site, and the deeper compressible and potentially liquefiable soils. The
design recommendations presented in this report are therefore based on the observed conditions
and on the assumption that earthwork for site grading, utilities, foundations, floor slabs, loading
dock walls, and pavements will be monitored by a qualified geotechnical engineer.
Any existing buried utilities, underground storage tanks or septic tanks on the site should
be removed, relocated, or abandoned, as necessary, in accordance with all local, state and federal
regulations. Localized excavations made for removal of utilities should be backfilled with
structural fill as outlined in the following section of this report. The excavated soils should be
considered unsuitable for reuse as structural fill.
Stripping, excavation, grading, and subgrade preparation should be performed in a manner
and sequence that will provide positive drainage at all times and provide proper control of
erosion. Accumulated water must be removed from subgrades and work areas immediately and
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prior to performing further work in the area.If ponded surface water collects it should be pumped
or drained to provide a suitable discharge location. The site should be graded to prevent water
from ponding in construction areas and/or flowing into excavations. Exposed grades should be
crowned, sloped, and smooth-drain rolled at the end of each day to facilitate drainage if inclement
weather is forecasted. Equipment access may be limited if drainage efforts are not accomplished
in a timely sequence. Project delays and increased costs could be incurred due to the muddy
conditions if a working drainage system is not utilized.
Site preparation will require the removal of limited surface vegetation and organic-rich
topsoil across the site. Based on the conditions encountered in the explorations, we recommend
that all organics, root mats, and topsoil be stripped to an average depth of 6 inches in those areas
where topsoil is present. Additional removal of vegetation and/or organic-rich soils, such as in
areas of heavy vegetation, should be determined by a qualified geotechnical engineer at the time
of grading based on the subgrade material's organic content and stability.
In general, relatively wet conditions prevail in the project area between November and
May. During this period, the existing surficial fill soils could remain relatively wet and unstable.
A relatively high groundwater table along the east side of the site and the probability of cutting
this area down to approximately elevation 35 or 36 feet will expose very loose fill soils that are
currently wet to saturated. The surficial soils are sensitive because of the elevated moisture
contents and will become unstable if they are not protected from construction traffic. In wet
conditions, additional soils will need to be removed and replaced with a coarse crushed or
naturally occurring sand and gravel or crushed recycled concrete mat. Other stabilization
methods such as lime or cement treatment are not recommended due to the high organic content
of the coal tailings fill. Where overexcavation is necessary, it should be confirmed through
monitoring and testing by a qualified inspection firm.
We recommend that site preparation and initial construction activities should be planned
to reduce disturbance to the existing ground surface. The severity of construction problems will
be dependent, in part, on the precautions that are taken by the contractor to protect the moisture
and disturbance-sensitive site soils. Construction traffic should be restricted to dedicated
driveway and laydown areas to prevent excessive disturbance of the parking area and driveway
subgrades.If site stripping and grading activities are performed during extended dry weather
periods, a lesser degree of subgrade stabilization may be necessary. However, it should be noted
that intermittent wet weather periods during the summer months could delay earthwork if soil
moisture conditions become elevated above the optimum moisture content. The use of a working
surface of pit-run sand and gravel, crushed rock, or quarry spalls may be required to protect the
existing soils particularly in areas supporting concentrated equipment traffic.
Prior to placing structural fill in the building pad, the subgrade should compacted to a firm
and unyielding condition, moisture conditions permitting. Alternatively, the building pad should
be covered with a woven geotextile equivalent to Mirafi 600X and a minimum of 12 inches of
select granular structural fill. The building pad may then be raised to the planned finished grade
with compacted structural fill. Subgrade preparation and selection, placement, and compaction of
structural fill should be performed under engineering controlled conditions in accordance with the
project specifications. We recommend that the building pad be surfaced with a minimum of 18
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inches of "select"granular fill, or free-draining crushed ballast or base course, as defined by
Sections 9-03.9(1) and 9-03.9(3), respectively,of the 2002 WSDOT Standard Specifications
Manual. Material considered to be "select" should meet the 2002 WSDOT Standard
Specifications Section, 9-03.14(1), Gravel Borrow, or be approved by the owner's geotechnical
engineer. Haul roads should be constructed by placing a woven geotextile such as Mirafi 600X or
Amoco 2006 over the existing coal tailings with a minimum of 12 inches of select granular fill
placed over the fabric. The fabric should only be placed in areas between the rows of augercast
piles and not where the piles will be drilled.
If earthwork takes place during freezing conditions, all exposed subgrades should be
allowed to thaw and then be recompacted prior to placing subsequent lifts of structural fill or
foundation components. Alternatively, the frozen material could be stripped from the subgrade to
reveal unfrozen soil prior to placing subsequent lifts of fill or foundation components. The frozen
soil should not be reused as structural fill until allowed to thaw and adjusted to the proper
moisture content, which may not be possible during winter months.
Structural Fill
All structural fill should be placed in accordance with the recommendations presented
herein. Prior to the placement of structural fill, all surfaces to receive fill should be prepared as
previously recommended in Site Preparation section of this report.
Structural fill includes any fill material placed under footings, pavements, or other
permanent structures or facilities. The existing surficial fill soils should be considered unsuitable
for reuse as structural fill. Limited zones of silty sand may be encountered in the large lobe of
coal tailings fill on the east side of the site and should be considered suitable for reuse as
structural fill. However, it appears that the majority of the lobe consists of coal.
It appears that material used as structural fill will need to be imported. On-site soils
considered suitable for reuse appear to be limited to the base course material beneath the existing
asphalt pavement and limited pockets of silty sand that is layered in the coal tailings fill.
Materials typically used for import structural fill include clean, well-graded sand and gravel ("pit
run"), clean sand, various mixtures of sand, silt and gravel, and crushed rock. Recycled concrete,
if locally available, is also useful for structural fill provided the material is thoroughly crushed to
a well-graded, 2-inch minus product. Structural fill materials should be free of deleterious,
organic, or frozen matter and should contain no chemicals that may result in the material being
classified as "contaminated".
Import structural fill for raising site grades can consist of a combination of "common" and
select granular" material. "Common" structural fill consists of lesser quality, more moisture-
sensitive soil, such as the soils encountered at the project site, that is free of organics and
deleterious materials, is compactable to a firm and unyielding condition, and meets the minimum
specified compaction levels. We recommend that common structural fill meet the requirements
of the 2002 WSDOT Standard Specifications Section, 9-03.14(3), Common Borrow.
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Select"granular fill consists of free-draining naturally occurring, crushed aggregate, or
quarry spalls. Select fill is generally used when less moisture sensitive material is needed for
structural fill applications."Select"structural fill should meet the requirements of the 2002
WSDOT Standard Specifications Sections, 9-03.12(2), Gravel Backfill for Walls or 9-03.14(1),
for Gravel Borrow.
Structural fill should be placed in lifts not exceeding 8 inches in loose thickness.
Individual lifts should be compacted such that a minimum density of at least 95 percent of the
modified Proctor (ASTM:D-1557) maximum dry density is achieved. Higher compaction levels
should be achieved where called for in the project specifications of the local development
standards. Subgrade soils below pavement areas and all base course materials should also be
compacted to a minimum of 95 percent of the Modified Proctor maximum dry density. The top
12 inches of compacted structural fill should have a maximum 2-inch particle diameter and all
underlying fill a maximum 6-inch diameter unless specifically recommended by the geotechnical
engineer and approved by the owner. We recommend that a qualified geotechnical engineer from
ZZA be present during the placement of structural fill to observe the work and perform a
representative number of in-place density tests. In this way, the adequacy of earthwork may be
evaluated as grading progresses.
The suitability of soils used for structural fill depends primarily on the gradation and
moisture content of the soil when it is placed. As the fines content (that portion passing the U.S.
No. 200 sieve)of a soil increases, it becomes increasingly sensitive to small changes in moisture
content and adequate compaction becomes more difficult or impossible to achieve. We therefore
recommend that grading activities be scheduled for the driest time of year in consideration of the
moisture-sensitive nature of the site soils. Adjusting the moisture content of the site soils during
the wetter and colder months between November and March would be much more difficult to
accomplish.If inclement weather or soil moisture content prevent the use of imported common
borrow material as structural fill, we recommend that use of "select"granular fill be considered.
It should be noted that the placement of structural fill is in many cases weather-dependent and
delays due to inclement weather are common even when using "select"granular fill.
Reusing wet or excessively over-optimum on-site or import soils for structural fill would
necessitate treatment of the soils to reduce the moisture content to a level adequate for
compaction..In the summer, air drying is commonly incorporated.When air drying is not
feasible, kiln dust admixtures are typically used to increase the workability of the wet soils to a
level where the soils can be compacted. The admixtures are extremely alkaline and can increase
the pH of the soil mixture. Before such admixtures are considered, we recommend that their use
be submitted to the appropriate overseeing agency since some jurisdictions are putting restrictions
on their use, in particular kiln dust.If moisture conditioning of the soils is required to increase the
moisture content of dry-of-optimum soils, we recommend that the soils be uniformly blended
with the added moisture.
Based upon the nature of the existing fill soils, it is our opinion that the subgrade soils
exhibit a low potential for swelling. However because the surficial fill soils consist primarily of
coal, we anticipate that the material left in place could generate methane over time.
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Excess soils may require stockpiling for extended periods of time before they can be used.
It is recommended that all stockpiled soils intended for reuse as structural fill be protected with
anchored polyethylene sheet plastic strong enough to withstand local wind conditions.
Utility Trenchin~and Backfillin~
Existing on-site, underground utilities should be removed, relocated or properly
abandoned in place in order to prevent possible future settlement problems. All existing
underground utilities should be decommissioned, abandoned, or backfilled in accordance with all
applicable State and local regulations. Alternatively, abandoned utilities may be grouted in place.
If the trench backfill materials above the existing utilities consist of coal tailings fill, we
recommend that it be considered unsuitable for reuse as structural fill anywhere on site.If any
existing utilities are to be preserved, grading operations must be carefully performed so as to not
disturb or damage the existing utility.
We anticipate that most utility subgrades will consist of very loose to loose coal tailings
fill or possibly soft silt, loose silty sand, or peaty soils. Soils deemed unsuitable for utility support
should be overexcavated a minimum of 12inches in order to develop a firm, uniform base. Where
peat is encountered, we recommend that the entire thickness of the material be removed and
replaced. The replacement fill will be difficult to compact due to groundwater seepage and/or the
underlying soft, native soils. Where possible, the structural fill used to replace overexcavated
soils should be compacted as specified and as recommended in this report. Where water is
encountered in the excavations, it should be removed prior to fill placement. Alternatively, clean
less than 1 percent fines) quarry spalls could be used for backfill below the water level. We
recommend that utility trenching, installation, and backfilling conform to all applicable federal,
state, and local regulations such as OSHA for open excavations.
In boring B-37, approximately 4 feet of peat was encountered at a depth of about 812 feet
below existing grade or about elevation 33Y2 feet. This is in the area of an alignment for a 4-foot
diameter pipe that will convey mine runoff along the toe of a permanent retaining wall. We
understand that the pipe may be covered with as little as one foot of soil. Given the proposed cut
of about 5 to 6 feet in the area of boring B-37, it appears that the pipe invert will be situated in the
middle of the peat.It is our opinion that the peat is not suitable for support of the pipe and should
therefore be overexcavated and replaced with compacted structural fill.It appears that dewatering
will be necessary to accomplish this since groundwater was encountered approximately 8 feet
below grade at the time of drilling. We recommend that similar measures be taken for all deep
utilities and structures, such as manholes and vaults, when peat or otherwise unsuitable materials
are encountered. We recommend that trench excavation and preparation for all utilities be
completed in general accordance with WSDOT Standard Specification 7-08.
Existing on-site soils are considered unsuitable for utility trench backfill. Instead,
imported soils that can be compacted to the minimum recommended levels should be used taking
into consideration the surrounding soil and groundwater conditions at the time of construction.
Pipe bedding and cover should be placed according to utility manufacturer's recommendations
and local ordinances. Generally, it is recommended that a minimum of 4 inches of bedding
material be placed in the trench bottom. All bedding should conform to the specifications
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presented in Section 9-03.12(3)of the WSDOT Standard Specifications Manual or be approved
by the owners'geotechnical representative based upon specific conditions encountered at the site.
All excavations should be wide enough to allow for compaction around the haunches of pipes and
underground tanks. Otherwise, materials such as controlled density fill or pea gravel could be
used to eliminate the compactive effort required.
Backfilling for the remainder of the trenches could be completed utilizing common fill or
select granular fill, depending on soil moisture and weather conditions, as well as groundwater
levels. Compaction of backfill material should be accomplished with soils within ±2 percent of
their optimum moisture content in order to achieve the minimum specified compaction levels set
forth in this report and project specifications. However, initial lift thickness could be increased to
levels recommended by the manufacturer to protect utilities from damage by compacting
equipment. For planning purposes, we recommend that all native soils be considered unsuitable
for reuse as structural fill.
Filtered sump pumps placed in the bottoms of excavations or other conventional
dewatering techniques are anticipated to be suitable for dewatering excavations that terminate
above the water table,if seepage is encountered. Pumped dewatering well systems would likely
be required to facilitate excavations below the water table.
Pre-bid test pits could assist in evaluating the most economical means of site excavation.
Relatively flat slopes, benching, or temporary bracing may be needed. Conventional trench box
shoring is also an option for the project.
Terrworary and Permanent Slopes
Temporary slope stability is a function of many factors, including the following:
The presence and abundance of groundwater;
The type and density of the various soil strata;
The depth of cut;
Surcharge loadings adjacent to the excavation;
The length of time the excavation remains open.
It is exceedingly difficult under the variable circumstances to pre-establish a safe and
maintenance-free"temporary cut slope angle. Therefore, it should be the responsibility of the
contractor to maintain safe temporary slope configurations since the contractor is continuously at
the job site, able to observe the nature and condition of the cut slopes, and able to monitor the
subsurface materials and groundwater conditions encountered.It may be necessary to drape
temporary slopes throughout the site with plastic sheeting or other means to protect the slopes
from the elements and minimize sloughing and erosion. Unsupported vertical slopes or cuts
deeper than 4 feet are not recommended if worker access is necessary. The cuts should be
adequately sloped, shored, or supported to prevent injury to personnel from local sloughing and
spalling. The excavation should conform to applicable federal, state, and local regulations.
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We recommend that all permanent slopes constructed in the coal tailings be designed at a
3H:1V (Horizontal:Vertical) inclination or flatter. Temporary slopes should be excavated at an
inclination no steeper than 2H:1V. Where wet or saturated coal tailings are exposed, temporary
and permanent slope angles flatter than those may be necessary.
Permanent structural fill placed on existing slopes steeper than 5H:1V
Horizontal:Vertical) should be keyed and benched into natural soils of the underlying slope. We
recommend that the base downslope key be cut into undisturbed native soil. The key slot should
be at least 8 feet wide and 3 feet deep. The hillside benches cut into the native soil should be at
least 4 feet in width. The face of the embankment should be compacted to the same 95 percent
relative compaction as the body of the fill. This may be accomplished by overbuilding the
embankment and cutting back to the compacted core. Alternatively, the surface of the slope may
be compacted as it is built, or upon completion of the embankment fill placement.
Shorinlj Desiljn Criteria
Development of the site will require the construction of a permanent retaining wall near
the southeast comer of the proposed building. The exposed maximum height of the wall will be
approximately 18 feet. However, we understand that a 4-foot diameter pipe will be installed
along the base of the wall to convey mine runoff that currently is routed to the site and then
through an aqueduct to the south end of the site. Below the pipe, the native soils will consist of
very loose sand and silty sand, as well as soft peat, and silt. Sandstone bedrock was encountered
about 19 to 24 feet below the existing ground surface in the area of the wall. Based on the
subsurface conditions encountered at the site we recommend that the retaining wall consist of
soldier pile shoring with permanent tiebacks. A permit to allow the permanent tiebacks in the
Benson Road right-of-way will likely be necessary for a tieback-supported system.If permanent
tiebacks are not permitted, it will be necessary to consider designing a cantilevered soldier pile
wall or a temporarily tied-back wall that is integrated with a pile-supported concrete retaining
wall.
The lateral movement of soil and shoring surrounding the excavation will cause varying
degrees of settlement of streets and sidewalks adjacent to the excavation. The settlement-
sensitivity and importance of any adjacent structures and improvements need to be considered
when selecting appropriate shoring system and design criteria. The excavation will be near
Benson Road that contains utilities that may be settlement-sensitive.
The shoring design criteria presented in this report should be used by the structural
engineer and contractor to design an appropriate system. The shoring system design should be
reviewed by Zipper Zeman Associates, Inc. for conformance with the design criteria presented in
this report.It is generally not the purpose of this report to provide specific criteria for construction
methods, materials, or procedures.It should be the responsibility of the shoring subcontractor to
verify actual soil and groundwater conditions at the site and determine the construction methods
and procedures needed for installation of an appropriate shoring system.
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Lateral Earth Pressures and Movement
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The design of shoring is conventionally accomplished using empirical relationships and
apparent earth pressure distributions. These earth pressure distributions or envelopes do not
represent the precise distribution of earth pressures but rather constitute hypothetical pressures
from which tieback loads can be calculated which would not likely be exceeded in the excavation.
Additionally, pressures must be selected adjacent to existing settlement-sensitive utilities that will
tend to limit deflections, both vertical and horizontal.
Design of temporary shoring could be based on either "active"or "at-rest"lateral earth
pressures, depending on the degree of deformation of the shoring that can be tolerated. Shoring
which is free to deform on the order of 0.001 to 0.002 times the height of the shoring is
considered to be capable of mobilizing active earth pressures. This lateral deformation is likely to
be accomplished by vertical settlement of up to roughly 0.005 times the height of the shoring,
which may extend back from the side of the cut a distance equal to roughly the height of the cut.
Lesser degrees of settlement may also occur within a setback extending twice as far back. A
greater amount of lateral deformation could lead to greater vertical settlements behind the wall.If
no structural elements are located within this zone, or if any structural elements within the zone
are considered to be insensitive to this degree of settlement, then it would be appropriate to design
utilizing active earth pressures. An assumed "at-rest"earth pressure condition theoretically
assumes no movement of the soil behind the shoring; however, some settlement should
realistically be anticipated due to construction practices and/or the fact that it is not possible to
construct a perfectly stiff shoring system.
All deep excavations do invite a certain amount of risk. Since the selection of shoring
techniques and criteria affect the level of risk, we recommend that the final selection of shoring
design criteria be made by the owner in conjunction with the structural engineer and other design
team members. The project shoring walls could be designed using active pressures, provided
lateral movement and vertical settlement to the degree described above is considered tolerable.
The anticipated lateral and vertical movements of l-inch or less with active earth pressures are
typically tolerable for streets and buried utilities.
For the case of a cantilevered shoring system, or shoring with only one level of internal or
external bracing, the applied lateral pressure would be represented by a triangular pressure
distribution termed an equivalent fluid density. Figure 4 of this report illustrates the
recommended pressure distribution. We recommend an active pressure equivalent fluid density
of 36 pounds per cubic foot (pet) for these conditions. Additional lateral pressure should be
added to this value to model surcharges such as street or construction loads, or existing
foundation and floor loads. For design of shoring for assumed "at-rest"earth pressure conditions,
with cantilever piles or one level of bracing, we recommend using an equivalent fluid density of
50 pcf.
As noted on Figure 4, a backslope surcharge is recommended for shoring. The backslope
surcharge load is added to the height of the excavation as an equivalent soil height of H/4,where
H is the height of the wall in feet. We also recommend applying a uniform seismic pressure of
16H to the shoring wall.It appears that Benson Road is more than 25 feet away from the wall and
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should not impose a traffic surcharge. Other surcharge pressures acting on the shoring can be
determined by the methods shown on Figure 5, and should be added to the lateral earth pressures
as discussed above.
Soldier Piles
Soldier piles for shoring are typically set in pre-angered holes and backfilled with lean or
structural concrete. Vertical loads on such piles could be resisted by a combination of friction and
end bearing below the base of the excavation. We recommend neglecting the side friction
throughout the loose and soft native soils and using a value of 2,000 psf in the sandstone. An
allowable end-bearing value of 30 ksf (kips per square foot) can be used for soldier piles
embedded at least 5 feet into the sandstone. The above values include a factor of safety of 1.5.
Embedment depth of soldier piles below final excavation level must be designed to
provide adequate lateral or "kick out" resistance to horizontal loads below the lowest strut or
tieback level. For design, the lateral resistance may be computed on the basis of the passive
pressure presented on Figure 4, acting over twice the diameter of the concreted soldier pile
section or the pile spacing, whichever is less. We recommend that the passive resistance within
the upper loose and soft soils be neglected and that an allowable passive resistance of 1,200 psf be
used for that portion of the pile embedded in the sandstone.
If excessive ground loss is allowed to occur during pile installation, increased settlement
of the areas retained by the shoring would be more likely to occur. Soldier pile drilling is
anticipated to extend through water-bearing coal tailings and native sand layers. Casing is
recommended for these drilling conditions, in order to prevent caving. The contractor should be
responsible for installation of casing, or using alternate means at their discretion, to prevent
caving and loss of ground during pile drilling.
We recommend lagging, or some other form of protection, be installed in all areas. Due to
soil arching effects, lagging may be designed for 50 percent of the lateral earth pressure used for
shoring design. Prompt and careful installation of lagging will reduce potential loss of ground.
The requirements for lagging should be made the responsibility of the shoring subcontractor to
prevent soil failure, sloughing and loss of ground and to provide safe working conditions. We
recommend all void space between the lagging and soil be backfilled. The backfill should be
free-draining in order to prevent the build-up of hydrostatic pressure behind the wall. A
permeable sand slurry or pea gravel should be considered for lagging backfill.If the lagging is
exposed for the life of the wall, we recommend that it consist of concrete.If the wall is faced
with a protective layer of concrete, the lagging should be adequately treated to resist rot.
Lateral Support and Tiebacks
Lateral support for the shored wall should be provided by tieback anchors. We anticipate
that the anchors will be drilled into competent sandstone bedrock. However, historical records
indicate that there were mineshaft adits in the area of the project site. Review of the historical
documents leads us to suspect that one of the mine openings may be along the alignment of the
existing 48-inch drainpipe that daylights on the project site. We did not encounter conditions that
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates.Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
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J-1470
December 6, 2002
Page 24
would indicate the presence of the shafts. However,if a zone of fill and/or a mineshaft exists in
the anchor zone of the proposed wall, it may not be possible to install some of the tiebacks as
recommended. We recommend that additional subsurface explorations be completed in support of
the retaining wall design.If a mineshaft is present in the tieback zone, further definition of the
conditions prior to bidding would reduce the possibility of change orders and delays during
construction. Evaluations could consist of surficial geophysical evaluations using resistivity or
magnetics and/or downhole geophysical methods in predrilled holes. We also recommend that
the 48-inch pipe be logged with a camera to determine its alignment and where it terminates.
The anchor portion of the tieback must be fully located a sufficient distance behind the
retained excavation face to develop resistance within the stable soil mass. We recommend the
anchorage be attained behind an assumed failure plane that is formed by a 60° angle from the base
of the excavation and set back from the retained excavation face for a horizontal distance of one-
fourth the height of the soldier pile above the bottom of the excavation. The zone in front of the
above-described plane is called the "no-load zone". The unbonded portion of the tieback anchor
should extend entirely through the no-load zone, and should be a minimum of 15 feet in length.
The anchor portion of the tieback should be a minimum length of 12 feet. All tieback holes
within the no-load zone should be immediately backfilled. The sole purpose of the backfill is to
prevent possible collapse of the holes, loss of ground and surface subsidence. We recommend
that the backfill consist of sand or a non-cohesive mixture. Sand/cement grout could be utilized
only if some acceptable form of bond-breaker (such as plastic sheathing) is applied to the tie-rods
within the no-load zone.
Anchor holes should be drilled in a manner that will minimize loss of ground and not
disturb previously installed anchors. Caving will likely occur in the coal tailings above the
sandstone and will likely require the use of casing. Caving could also occur if wet or saturated
zones are encountered. Drilling with and grouting through a continuous-flight auger or a casing
would reduce the potential for loss of ground.
Using the design values presented herein is dependent on a well-constructed anchor. We
recommend that concrete be placed in the drilled tieback anchor hole by tremie methods such as
pumping through a hose placed in the bottom of the hole or pumping through the center of a
continuous-flight auger. In this way, the grout is forced up through the anchor zone under
pressure, with the resulting anchor more likely to be continuous. The grout should not be placed
into the anchor zone by simple gravity methods such as flowing down a chute. We recommend
that Zipper Zeman Associates, Inc. monitor all tieback installation.
With a low-pressure grouted tieback shoring system, we estimate an allowable concrete-
sandstone adhesion of 3,000 pounds per square foot (psf) is recommended. We recommend that
all anchors be located at least 10 feet below ground surface. For high-pressure grouted or
secondary grouted anchors, the adhesion is highly dependent on grouting procedures. For
planning purposes, a four-inch diameter pressure-grouted tieback can be assumed to have the
same capacity per lineal foot as a 12-inch diameter low-pressure grouted (augered) tieback, or
roughly 9 kips per lineal foot.
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Tieback Testing and Lockoff
J·1470
December 6, 2002
Page 25
All permanent tieback anchors should be performance tested to at least 150 percent of
design capacity prior to lockoff. Performance testing should include increasing the load on the
tieback to the test load in five increments. Each increment is to be held long enough to obtain a
stable measurement of tieback deflection, and the 150percent load is to be held until five minutes
elapse with less than 0.01 inch of creep movement. The anchors should then be locked off at 80
percent of design load.
Tieback adhesion capacities presented in this report are estimates based on soil conditions
encountered in the borings. The final adhesion capacity for each anchor installation method and
soil type should be determined by field tests. We recommend that at least two, 300-percent
tieback proof tests be completed prior to installing production tiebacks for each soil type and
installation method.Proof tests should consist of applying the load in eight increments to the test
load, with each increment held until 5 minutes elapses with less than 0.01 inch of creep. The 300-
percent load should be held until 30 minutes elapse with less than 0.0I inch of creep.
Acceptance criteria for tieback tests should include all of the following:
1. Hold maximum test load for required duration with less than 0.0 l-inch of creep;
2. Linear or near-linear plot of unit anchor stress and movement, with creep movement less than
0.08 inches per log cycle of time;
3. Total movement during performance test loading, from 50 to 150 percent of design load,
exceeds 80 percent of theoretical elastic elongation of unbonded tendon length;
4. Total movement during test loading, does not exceed theoretical elastic elongation of
unbonded tendon length plus 50 percent of bond length;
5. Performance of the anchor head/pile connection acceptable to the structural engineer.
Failure of an anchor to meet the required test acceptance criteria should be brought to the
attention of the structural engineer. In most cases, where total anchor movement is within
tolerable ranges, a reduced capacity will be assigned to the subject tieback.If total anchor
movement is in excess of 6 inches, we recommend that the anchor be abandoned and replaced.
Shoring Monitoring
Any time an excavation is made below the level of existing buildings, utilities or other
structures, there is risk of damage even if a well-designed shoring system has been planned. We
recommend, therefore, that a systematic program of observations be conducted on adjacent
facilities and structures. We believe that such a program is necessary for two reasons. First, if
excessive movement is detected sufficiently early, it may be possible to undertake remedial
measures that could prevent serious damage to existing facilities or structures. Second, in the
unlikely event that problems do arise, the responsibility for damage may be established more
equitably if the cause and extent of the damage are better defined. Monitoring can consist of
conventional survey monitoring of horizontal and vertical movements.
18905 33rd Avenue W., Suite 117
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Lynnwood,Washington 98036 (425)771-3304
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J-1470
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Page 26
The monitoring program should include measurements of the horizontal and vertical
movements of the retained improvements and the shoring system itself. At least two reference
lines should be established adjacent to the excavation at horizontal distances back from the
excavation space of about 1/3H and H, where H is the final excavation height. Monitoring of the
shoring system should include measurements of vertical and horizontal movements at the top of
each soldier pile.If local wet areas are noted within the excavation, additional monitoring points
should be established at the direction of the soils engineer. Reference points for horizontal
movement should also be selectively placed at various tieback levels as the excavation progresses.
The measuring system used for shoring monitoring should have an accuracy of at least
O.Ol-foot.All reference points on the existing structures should be installed and readings taken
prior to commencing the excavation. All reference points should be read prior to and during
critical stages of construction. The frequency of readings will depend on the results of previous
readings and the rate of construction. As a minimum, readings should be taken about once a week
throughout construction until the excavation is completed. A registered surveyor should complete
all readings and the data should be reviewed by the geotechnical engineer.
Building Foundations
We recommend that the proposed building be supported on pile foundations due to the
risk of settlements that exceed the maximums presented in the Geotechnical Investigation and
Report Requirements.We recommend that foundation support be provided by augercast piles,
although other pile options such as timber or pipe piles could be considered.If steel piles are
considered, the effects of corrosion will need to be taken into account. We can provide
recommendations for alternative pile options,if requested.
As noted in the Subsurface Conditions section of this report, the thickness of coal tailings
fill, compressible soils, and potentially liquefiable soils, and the depth to sandstone bedrock varies
across the site. In general, the depth to sandstone bedrock varies from about 19 to more than 110
feet below existing grades. We anticipate that the auger will be able to slightly penetrate the
bedrock as it appears to be moderately to highly weathered at the contact.It is our opinion that
piles can achieve the recommended allowable capacities with a maximum length of 85 feet.
We understand that the former buildings on site were timber pile supported.Of the two
pile supported buildings, the proposed building envelopes one entire building and a portion of
another. Therefore, we recommend that the layout of the piles take into consideration the location
of the existing piles. We also recommend that the location of the piles be surveyed in order to
modify the layout of the new piles before construction begins.
An augercast pile is formed by drilling to an appropriate pre-determined depth with a
continuous-flight, hollow-stem auger. Cement grout is then pumped down the stem of the auger
under high pressure as the auger is withdrawn. The final result is a cast-in-place pile.
Reinforcing can be lowered into the unset concrete column to provide lateral and/or tension
capabilities.
18905 33rd Avenue W., Suite 117
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Lynnwood,Washington 98036 (425) 771-3304
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Pressure grouting methods typically result in a grout column diameter in excess of the
nominal diameter of the drilled hole. The soft and loose soils on the site could provide difficult to
augercast pile construction due to grout loss into the loose/soft soil strata. We anticipate grout
volumes within the soil column could average about 150to 180percent of the theoretical volume
of drilled holes. The contractor should be required to stagger the pile grouting and drilling
operations, such that all completed piles within 10 feet of the pile being drilled have set for at
least 24 hours. Greater spacings may become necessary due to the length of the piles and should
be determined at the time of construction.
Augercast piles would gain their vertical compressive capacity mainly from end-bearing
on bedrock or end-bearing and skin friction in soils below the liquefiable zone where bedrock is
not encountered. Vertical uplift pile capacity will develop as a result of side friction between the
pile and the adjacent soil in addition to the weight of the pile. Due to the variable depth of the
bedrock, augercast piles will likely vary in length from about 20 to 85 feet. Recommended
augercast pile capacities are presented in Table I below. The vertical compressive pile capacities
presented assume that adjacent piles are located at least three pile diameters apart and that the
piles supported on the sandstone bedrock are embedded a minimum of 2 feet into the rock.
Lateral pile capacities are also presented in Table 1 for l8-inch diameter piles. The allowable
lateral capacities are based on fixed- and free-head conditions and limiting the deflection to 12
inch.
Because augercast piles are drilled, obstacles such as rocks, utilities, foundations and other
man-placed debris in the subsurface can cause difficult installation conditions.It is possible that
obstacles encountered during drilling the piles would require relocation of piles at the time of
construction if impenetrable obstacles are encountered at planned pile locations.It may be
necessary to periodically remove the pile auger from the holes during drilling in order to verify
depths of the various soil types, and penetration into the bearing soil layer.
We understand that the proposed building will be designed for the typical structural loads
as presented to ZZA in the Geotechnical Investigation Specifications and Report Requirements.
Based upon these values, as well as the conditions that could develop during a liquefaction event,
we have developed allowable compressive and uplift capacities for l8-inch diameter augercast
piles. The recommended pile lengths and associated allowable capacities are presented in Table I
below. The allowable capacities may be increased by one-third to resist short-term transient
forces.If the piles are spaced closer than three pile diameters, the allowable capacities should be
reduced. The reduction factor will be based on the actual center to center pile spacing and the
configuration of the group.
18905 33rdAvenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
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TABLE 1
ALLOW ABLE CAPACITIES OF AUGERCAST PILES
Pile Diameter Estimated Pile Allowable Allowable (Inches) Length (feet) Compressive Uplift Capacity Capacity (tons) (tons)*
18 (in bedrock) 20-30 75 2
18 (in bedrock) 30-40 75 4
18 (in bedrock) 40-50 75 6
18 (in bedrock) 50-60 75 10
18 (in bedrock) 60-70 75 15
18 (in bedrock) 70-80 75 22
18 (in soil) 85 75 36
*Does not include the weight of the pile
J-1470December 6, 2002 Page 28
Allowable Lateral Capacity, fixed head/free head (tons)
6.0/3.0
6.0/3.0
6.0/3.0
6.0/3.0
6.0/3.0
6.0/3.0
6.0/3.0
Based on an assumed modulus of horizontal subgrade reaction of 3 pci in the loose and
soft near-surface soils, the stiffness factor (T) for a fixed- and free-head, 18-inch diameter auger
cast pile was calculated to be 88 inches (7.3 feet). The recommended allowable lateral capacities
are based on limiting deflection to 0.5 inch. We recommend that the reinforcing cages extend a
minimum of 30 feet into each pile, or the full pile length if it is shorter than 30 feet. In addition to
the reinforcing cages, we recommend that a full-length center bar be installed in each pile in order
to develop the allowable uplift capacity.
Some downdrag forces on the piles should be expected to develop over time as the peat
and organic-rich soils consolidate over time. We estimate that forces of up to about 5 · tons could
develop on longest piles that penetrate through the greatest thicknesses of compressible soils.
However, given the 2.5 safety factor applied to the ultimate pile capacities, it does not appear that
the down drag forces will adversely affect the performance of the piles.
Provided the piles are designed in accordance with our recommendations and they are
constructed in accordance with industry standards, we estimate that total settlements will be less
than one inch. Differential settlements are estimated to be less than Yi inch in 40 feet.
The integrity of augercast piles is controlled in the field and can be affected by many
variables. Unlike steel or timber piles with structural characteristics that can be predetermined
during design, the construction of augercast piles must be continuously observed in order to
determine that the piles have been constructed in a manner that will achieve the required design
characteristics. Therefore, we recommend that ZZA provide construction observation services
during the installation of the augercast pile foundations. This will allow us evaluate all of the
variables that go into constructing an augercast pile and determine the adequacy of the piles as they are constructed.
Zipper Zeman Associates, Inc. 18905 33rd Avenue W ., Suite 117 Lynnwood, Washington 98036 ( 425)771-3304
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Methane Gas MitigatiQn
J-1470
December 6, 2002
Page 29
The presence of peat, organic-rich soils and coal tailings at the site can result in the
generation of methane gas as the organics decay. Methane gas will follow the path of least
resistance and has been shown to migrate laterally to find escape paths. It accumulates in pockets
both inside and outside of buildings. Methane can present an explosive hazard if it concentrates in
confined or enclosed spaces within a building, in underground vaults, conduits, and other
collection points. We recommend that a methane barrier system that prevents the passage of
methane gas into the building be provided under the floor slab and that a collection and venting
system be installed below the gas barrier. We recommend that the vapor barrier be installed after
the pile foundations have been constructed but before the capillary break is placed. The venting
system should consist of 4-inch diameter perforated pipes fully enveloped in granular soils that is
routed to the outside of the building.
Further study of the development of methane at the site could be completed to determine
hQW significant the development of methane is at the site. We would recommend that a minimum
of four gas collection wells be installed at the site in order to collect samples of the vapor
generated. The concentration of methane would then be determined in the samples and specific
recommendations could be formulated based on the test results.
Structural FIQors
We recommend that all floor slabs be supported on augercast piles due to the thickness of
very IQQse coal tailings and the risk of liquefaction induced settlements. We recommend that the
slab be supported on a minimum of 12 inches of nonexpansive, granular structural fill compacted
to a minimum of 95 percent of the modified Proctor maximum dry density (ASTM D-1557).
This will provide the SUPPQrt for the augercast pile construction equipment.
We recommend that 6 inches of free-draining granular material be placed over the
building pad to serve as a capillary break. Aggregates similar to those specified in WSDOT 2002
Standard Specifications for Road, Bridge, and Municipal Construction, listed under specifications
9-03.12(4), 9-03.15 or 9-03.16 can be used for capillary break material provided they are
modified to meet the fines content recommendation,Alternatively, we recommend that the
capillary break consist of free-draining aggregate that conforms with ASTM D2321, Table 1,
Classes of Embedment and Backfill Material, Class lA, IB, or II (GW or GP). The fines content
of the capillary break material should be limited tQ 3 percent or less, by weight, when measured
on that portion passing the U.S.NQ.4 sieve. A water vapor barrier is not considered to be
necessary if a methane gas barrier is constructed. After the capillary break is placed, it will be
required to SUPPQrt the reinforcing steel for the structural floor and its SUPPQrts (dobies). We
understand that it is very important to maintain the proper clearance between the structural fill
subgrade and the rebar. Therefore, we recommend that the contractor submit detailed information
in a timely manner about the material they intend to use in order tQ determine its adequacy for the
intended use.
18905 33rd Avenue W., Suite 117
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Lynnwood,Washington 98036 (425) 771-3304
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We recommend that all outdoor slabs and sidewalks supported on a minimum of 12 inches
of nonexpansive,granular structural fill compacted to a minimum of 95 percent of the modified
Proctor maximum dry density (ASTM D-1557).
We have estimated a vertical modulus of subgrade reaction of approximately 150 pounds
per cubic inch for a l2-inch thick layer of granular soil compacted to a minimum of 95 percent of
the modified Proctor maximum dry density.
Conventional Retaining Walls
The lateral soil pressure acting on backfilled walls will primarily depend on the degree of
compaction and the amount of lateral movement permitted at the top of the wall during
backfilling operations.lfthe wall is free to yield at the top an amount equal to at least 0.1 percent
of the height of the wall, the soil pressure will be less than if the wall structurally restrained from
lateral movement at the top. We recommend that an equivalent active fluid pressure of35 pcfbe
used for yielding walls and an at-rest equivalent fluid pressure of 55 pcfbe used for non-yielding
backfilled walls. These equivalent fluid pressures assume the backfill is compacted to
approximately 90 percent of its modified Proctor maximum dry density. We recommend that we
be allowed to review the design values and modify them,if necessary,if they are to be applied to
walls greater than 12 feet in height. For those portions of foundations embedded more than 18
inches below finish surrounding grade, we recommend using an allowable passive earth pressure
of 125 and 250 pcf in the existing loose fill and in structural fill that extends laterally beyond the
limits of the footing a distance of twice the embedment depth,respectively.We recommend
using an allowable base friction coefficient of 0.30.
The above equivalent fluid pressures are based on the assumption of a uniform horizontal
backfill and no buildup of hydrostatic pressure behind the wall.Surcharge pressures due to
sloping ground,adjacent footings, vehicles,construction equipment,etc. must be added to these
values. For loading docks,surcharge loading on the floor slab above the dock will result in a
horizontal,uniformly distributed surcharge on the wall equal to 40 percent of the distributed
vertical loading. We can provide surcharge criteria for other loading conditions behind the
loading dock wall,if requested. We recommend a minimum width of 18 inches of clean,
granular,free-draining material should extend from footing drains at the base of the wall to the
ground surface, to prevent the buildup of hydrostatic forces.Alternatively,weepholes on 4-foot
centers could be constructed at the bases of the wall to provide a drainage path.It should be
realized that the primary purpose of the free draining material is reduction in hydrostatic
pressures.Some potential for moisture to contact the back face in the wall may exist even with
this treatment,which may require more extensive water proofing be specified for walls which
require interior moisture sensitive finishes.
Care should be taken where utilities penetrate through backfilled walls.Minor settlement
of the wall backfill soils can impart significant soil loading on utilities,and some form of flexible
connection may be appropriate at backfilled wall penetrations.
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Drainage Considerations
J-1470
December 6, 2002
Page 31
A perimeter foundation drainage system is recommended for this site due to the proposed
finish floor elevation and the depth to groundwater at the time of our explorations. All retaining
walls should be designed to include drainage systems that drain by gravity to a storm sewer or
other suitable discharge location.
Water from downspouts and surface water should be independently collected and routed
to a suitable discharge location. Final exterior grades should promote free and positive drainage
from the building areas at all times. Water must not be allowed to pond or to collect adjacent to
foundations or within the immediate building area. We recommend that a gradient of at least two
percent for a minimum distance of 10 feet from the building perimeter be provided, except in
paved locations. In paved locations, a minimum gradient of one percent should be provided
unless provisions are included for collection and disposal of surface water adjacent to the
structure.
For design purposes, we recommend using a high groundwater elevation of 34 feet along
the east side of the site. Permanent structures that extend below this elevation should be designed
to resist hydrostatic pressures and should be appropriately waterproofed.
The two existing mine runoff drainpipes that enter the east side of the site will be
tightlined across a portion of the site. We recommend that the company or agency that owns or is
responsible for their maintenance be determined in order to coordinate a long-term maintenance
and inspection program. We further recommend that the peak flow in the drainpipes be
determined in order to size the proposed tightline pipe. This should likely be done in the late
winter or spring when groundwater would be anticipated to be at its highest.
Pavement Design Parameters
The subgrade soils are anticipated to generally consist of very loose to loose coal tailings.
As such, a CBR sample was not collected because it is our opinion that a minimum of one foot of
structural fill will be necessary over the coal. Therefore, we have assumed that the fill will have a
minimum California Bearing Ratio (CBR)of 50 percent. This would be similar to using a pit-run
sand and gravel soil. All soil within the upper one foot of the base course must have pavement
support characteristics at least equivalent to this and must be placed under engineering controlled
conditions. A confirmatory CBR test should be completed on the proposed import road bed
material.
Asphalt Concrete Pavement
It must be recognized that pavement design is a compromise between high initial cost and
little maintenance on one side and low initial cost coupled with the need for periodic repairs. As a
result, the owner will need to take part in the development of an appropriate pavement section.
Critical features which govern the durability of the surface include the level of compaction of the
subgrade, the stability of the subgrade, the presence or absence of moisture, free water and
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J-1470
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Page 32
organics, the fines content of the subgrade soils, the traffic volume, and the frequency of use by
heavy vehicles. Our recommendations are based upon a 20-year design life.
The pavement design recommendations assume that the subgrade and any structural fill
will be prepared in accordance with the recommendations presented in this report. The top 12
inches beneath the pavement surface should be compacted to a minimum of 95 percent relative
compaction, using AASHTO T-180 (ASTM: D1557) as a standard. However, the majority of
the surficial soils consist of coal fill that may be difficult to compact and can break down over
time.
The pavement design recommendations assume that the subgrade and any structural fill
will be prepared in accordance with the recommendations presented in this report. All fill, as
well as the upper 12 inches beneath the pavement surface should be compacted to a minimum of
95 percent relative compaction, using AASHTO T-180 (ASTM:D1557).Specifications for
manufacturing and placement of pavements and crushed top course should conform to
specifications presented in Divisions 5 and 4,respectively,of the 2002 Washington State
Department of Transportation,Standard Specifications for Roads, Bridges, and Municipal
Construction.We recommend that the subbase course material conform to Sections 9-03.9(1),
Ballast, 9-03.10, Aggregate for Gravel Base, 9-03.14(1), Gravel Borrow, 9-01.14(2), Select
Borrow, or 9-03.11 Recycled Portland Cement Concrete Rubble, with the maximum aggregate
size of3 inches. The crushed aggregate base course material conform to Section 9-03.9(3),
Crushed Surfacing Top Course. In lieu of crushed gravel base/top course,asphalt-treated base
ATB) can be substituted. The ATB would provide a more durable wearing surface if the
pavement subgrade areas will be exposed to construction traffic prior to final paving with Class
B asphalt.Production and placement of asphalt should be completed in accordance with Section
5-04 of the WSDOT Standard Specifications.We recommend using a Class B mix as described
in Section 9-03.8(6), Proportions of Materials,of the WSDOT Standard Specifications.ATB
should conform to the specifications of Section 4-06, Asphalt Treated Base of the WSDOT
Standard Specifications.
Recommended Pavement Sections for 20-Year Lifespan
ATB Substitute for
Traffic Asphalt Crushed Top/Base Pit-Run Subbase Crushed Aggregate
Thickness (in.)Course (in.) (Inches)(Inches)*
Heavy 4 4 12 3
Standard 3 4 12 3
ATB:Asphalt Treated Base may be substituted for crushed Top/Base Course beneath Class B asphalt.
Pavement design recommendations assume that the subgrade can be compacted to a
minimum of 95 percent of the modified Proctor maximum dry density and that construction
monitoring will be performed.If the subgrade can only be compacted to 90 percent of the
modified Proctor or 95 percent of the standard Proctor, we recommend that an additional 5 inches
of subbase be added to the pavement section. Continual flexible pavement maintenance along
with major rehabilitation after about 8 to 10 years should be expected to obtain a 20-year service
life.
18905 33rd Avenue W., Suite 117
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J-1470
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Page 33
If possible,construction traffic should be limited to unpaved and untreated roadways, or
specially constructed haul roads.If this is not possible, the pavement design should include an
allowance for construction traffic.
Stabilizing the subgrade with a fabric such as Mirafi 600X or similar may be necessary
during wet weather construction or wet subgrade conditions.Proper geotextile fabrics will
maintain segregation of the subgrade soil and base course materials.If the subgrade soils are
allowed to migrate upwards into the base course, the result would be decreased pavement support.
The use of stabilization fabric will not reduce the necessary base rock thickness,as fabric does not
provide structural strength at such shallow depths.If the subgrade is disturbed when wet,
overexcavation may be required and backfill with import fill.
Concrete pavement
Concrete pavement design recommendations are based on the soil parameters used for the
asphalt pavement design, and an assumed modulus of rupture of 550 psi and a minimum
compressive strength of 4,000 psi for the concrete. For standard and heavy-duty concrete
pavement sections,minimum concrete pavement sections are presented below.
Recommended Base and Subbase Thickness
Traffic Concrete Crushed Base Pit-Run Subbase (in)
Surfacing (in) Course (in)
Heavy 6 4 12
Standard 5 4 12
The materials and construction procedures should be in accordance with WSDOT
Standard Specifications for concrete pavement construction.
Stormwater Detention
It appears that underground stormwater detention vaults may be constructed on site.If
liquefaction related settlements can not be tolerated, we recommend that the vaults be supported
on augercast piles. Even if the vaults are supported on piles, we recommend that the grading be
completed prior to excavating for the vaults in order to preconsolidate the native soils around the
limits of the vault. We recommend that the area of the vaults be preloaded with a 3-foot
surcharge (using a soil with a unit weight of 125 pet)to reduce the effects of differential
settlements around the perimeter of the vault that would likely be manifested in the asphalt
surfacing over time. This surcharge should extend at least 5 feet beyond the limits of the vault
and be left in place a minimum of 4 weeks. We estimate that the resulting consolidation could be
on the order of 1 inch or more. Based on previous projects with similar conditions,we understand
that settlement of this magnitude may not be acceptable with respect to the possibility of damage
to the pipe connections at the vault.
We recommend that a minimum of two settlement plates be installed on the existing
subgrade at each vault location and that the elevations of the plates be determined prior to the
18905 33rd Avenue W _,Suite 117
Zipper Zeman Associates, Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
Proposed Retail Development
S. Grady Way and Talbot Road
Renton,Washington
J-1470
December 6, 2002
Page 34
placement of any structural fill/surcharge soils. Readings made by a qualified surveyor should be
completed twice a week until the finish subgrade elevation has been achieved. From then on,the
readings should be taken once a week until it is determined that the vault excavation can begin.
The survey information should be provided to ZZA in a timely manner for review.
Because of the loose/soft, wet subgrade conditions below the surface, (even after
preloading) we recommend that vault subgrades be overexcavated a minimum of 18 inches in
order to replace the loose/soft soils with relatively uncompressible granular soils. These materials
would also provide a working surface.If the vaults are not pile supported and peat is encountered
in the bottom of the excavation, we recommend that all of the peat be removed and replaced with
compacted structural fill. Prior to placing the granular fill, we recommend that a geotextile such
as Amoco 1199,Layfield 104F, or similar (with an AOS of70 or less) be placed over the exposed
subgrade except in those areas where the augercast piles will be installed. The fill should be
placed in maximum 6-inch thick lifts and be statically rolled and compacted. Vibratory
compactors should be used with extreme caution as these could soften and disturb the underlying
native soils. Pumped sumps or well points may also be necessary around the perimeter of the
vaults depending on groundwater levels at the time of construction.If groundwater is present, we
recommend that the water level be maintained a minimum of 18 inches below the top of the
gravel pad during construction. We recommend that the structural fill placed over the geotextile
consist of select aggregate as described in the Structural Fill section of this report.
At the time of drilling, the groundwater elevations varied between 23 and 34 feet. Where
applicable, we recommend designing the vault for buoyant forces for that portion that extends
below the interpreted seasonal high groundwater levels.If underground vaults are used and their
locations are determined, we recommend that ZZA be contacted in order to determine if buoyant
forces should be incorporated into their design.
CLOSURE
The conclusions and recommendations presented in this report are based, in part, on the
explorations accomplished for this study. The number, location, and depth of the explorations
were completed within the constraints of budget and site access so as to yield the information to
formulate the recommendations.Project plans were in the preliminary stage at the time of this
report preparation. We therefore recommend that ZZA be provided the opportunity to review the
project plans and specifications when they become available in order to confirm that the
recommendations and design considerations presented in this report have been properly
interpreted and implemented into the project design package.
The integrity of earthwork, structural fill, and foundation and pavement performance
depend greatly on proper site preparation and construction procedures. We recommend that a
qualified geotechnical engineering firm be retained to provide geotechnical engineering services
during the earthwork-related construction phases of the project.If variations in the subsurface
conditions are observed at that time, a qualified engineer would be able to provide additional
geotechnical engineering recommendations to the contractor and design team in a timely manner
as the project construction progresses.
18905 33rd Avenue W., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood,Washington 98036 (425) 771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371 F2EBC8E65
Proposed Retail Development
S.Grady Way and Talbot Road
Renton, Washington
J-1470
December 6, 2002
Page 35
We appreciate the opportunity to have been of service on this project and would be
pleased to discuss the contents of this report or other aspects of this project with you at your
convenience. If you have any questions, please � do not hesitate to call.
Respectfully submitted,
Zipper Zeman Associates, Inc.
Thomas A. Jones, P.E.
Associate
Enclosures: Figure 1 -Site and Exploration Plan
Figure 2 -Generalized Subsurface Profile A-A'
Figure 3 -Pseudostatic Seismic Stability Analysis
.... �,
Figure 4 -Recommended Design Criteria for Shoring, Cantilever or Single Row of
Tiebacks
Figure 5 -Surcharge Pressure Acting on Adjacent Shoring or Subsurface Wall
Appendix A -Field Procedures and Exploration Logs
Appendix B -Laboratory Testing and Classification
Appendix C-Geotechnical Investigation Fact Sheet, Foundation Design Criteria,
Foundation Subsurface Preparation Notes and AASHTO Pavement
Design
Appendix D -Climatic Data
Distribution: PacLand - 7 copies
18905 33rd A venue W ., Suite 117
Zipper Zeman Associates. Inc.
Lynnwood, Washington 98036 (425)771-3304
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
LEGEND:
SB-1
TP-1
0 120 240
APPROXIMATE SCALE IN FEET
APPROXIMATE DUTCH CONE PROBE
LOCATION AND NUMBER
\
\
\
EB GB-1/TB-1 APPROXIMATE BORING LOCATION AND NUMBER OF EX PLORATION
A A'
COMPLETED BY GEOENGINEERS ( ) AND TERRA ASSOCIATES (
GENERALIZED GEOLOGIC
CROSS SECTION
).
[NlRY
• i;P-3.-P-2 /
Zipper Zeman Associates, Inc.
Geotechnical and Environmental Consulting
18905 33rd Avenue West, Suite 117
Lynnwood, Washington 98036
Tele: (425) 771-3304 Fax: (425) 771-3549
-----
Project No: J-1470
Drawn by: J.Duncan
Date: Oct. 2002
Scale: f.Js Noted
Proposed Retail Project
Renton, Washington
FIGURE 1 -SITE ANO EXPLORATION PLAN
Basemap DWG File Provided by PACLAND, dated 9/13/02.
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
A
80
60
40
LL
C
C 20
iii
0
-40
LEGEND:
B-1
OFFSET 4' SOUTH
7
Note:
GB-12
(OFFSET 6' NORTH)
Very dense SANDSTONE
EXPLORATION NUMBER, APPROXIMATE LOCATION, AND
OFFSET FROM PROFILE A-A'
STANDARD PENETRATION RESISTANCE
MEASURED GROUNDWATER LEVEL AT TIME OF DRILLING
OR DATE NOTED
0 20
HORIZONTAL 1"=20'
VERTICAL 1 "=20'
50/4"
50/1"
40
See Figure 1 for location of profile.
------·-· .. --. ---·. ---· --···--· ·-·· -·-.
B-1A (SLOPE EVALUATION)
(OFFSET 4' SOUTH)
EXISTING GROUND
SURFACE
A'
80
B-3 (SITE EVALUATION)60 (OFFSET 20' SOUTH)
Very loose to medium dense 7
3 40 �
ILL)
� -----
4 LL
lnterbedded very loose to loose, 3 .5 "" . ----------Loose, silty SAND ----_ � ----_
5 silty SAND, sandy SILT, and SILT with 10 20 0
4 variable gravel, wood, and organic :;:, CV ------...._debris content. (ALLUVIUM)
Very dense SANDSTONE
-----
.............. .............. '-._ -----�-..........,._
NOTES:
THE STRATA ARE BASED UPON INTERPOLATION
BETWEEN EXPLORATIONS AND MAY NOT REPRESENT
ACTUAL SUBSURFACE CONDITIONS. SI MPLIFIED NAMES
ARE SHOVVN FOR SOIL DEPOSITS, BASED ON
GENERALIZATIONS OF SOIL DESCRIPTIONS.
SEE EXPLORATION LOGS AND REPORT TEXT FOR MORE
DETAILED SOIL ANO GROUNDWATER DESCRIPTIONS.
Zipper Zeman Associates, Inc. Project No: J-1470
Drawn by: J. Duncan
Date: Oct. 2002
Scale: As Noted
Geotechnical and Environmental Consulting
18905 33rd Avenue West, Suite 117
Lynnwood, vyashington 98036
Tele: (425) 771-3304 Fax: (425) 771-3549
-----�.
3 iii 5
5
22 0
8
� 17 �
50/5"
50/1" -20
-40
Renton Retail Slope Stability Analysis
Renton, Washington
FIGURE 2 -GENERALIZED SUBSURFACE
PROFILE A-A'
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371 F2EBC8E65
Pseudostatic Seismic Stability Analysis
J1470A1 10-29-••9:.36
Renton Retail
200
160
a>1200,) '+...........
(/1
X <t: 80 I >-
40
0
0
10 most critical surfaces, MINIMUM BISHOP FOS
40
4
80
Critical Failure Surface
FOS = 1.312
120 160 200
X-AXIS (feet)
2
240
ZIPPER ZEMAN ASSOCIATES, INC.
GEOTECHNICAL AND ENVIRONMENT AL
CONSULTING
Project No. J-1470
Date: September 2002
Scale: Noted
1.312
280
SOIL STRENGTH VALUES
Soil Type I: Loose to medium dense
coal tailings (Fill), 0=37", C=O psf.
Soil Type 2: Interbedded very loose to
loose, silty sand, sandy silt, and silt
with variable gravel, wood, and
organic debris content (Alluvium),
0=32°, C=O.O psf.
Soil Type 3: Loose silty sand
(Residual Soil), 0=33°, C=O psf.
Soil Type 4: Very dense sandstone,
0=15°, C=3000 psf.
320
Renton Retail
Renton, Washington
Pseudostatic Seismic Stability Analysis
Fi�re3
Hs =EQUIVALENT SOIL ~]
I
NOTES:
SURCHARGE FOR BACKSLOPE A
E!!!1 !/1
1. SOIL SURCHARGE "Hs"APPLIES TO
V~-
2H:1V BACKSLOPE SURCHARGE.EXCAVATION BASE
GROUN~Y GR~UND'S~RF:CE -.l /
ADDITIONAL SURCHARGE REQUIRED ~,
ASSUME NORESISTANCEASNOTEDONFIGURE4,AND/OR FOR
APPROXIMATE 2H:1V) /SLOPE ABOVE TOP OF PILE.
t t (fs)
SURFACE
I\2. ACTIVE, AT-REST, AND SURCHARGE
PRESSURE ASSUMED TO ACT OVER tc-t (fs)(qa)
I PILE SPACING ABOVE EXCAVATION ALLOWABLE ALLOWABLE
ttt FRICTION END BEARING
NOLOAD ZONEJ \ \
BASE AND OVER PILE DIAMETER
BELOW EXCAVATION BASE.NATIVE SOIL 0 ksf o ksf
SANDSTONE 2.5ksf 30 ksf
LOCATE ALL 3. PASSIVE PRESSURE ASSUMED TO RECOMMENDED MINIMUM EMBEDMENT
ANCHORS BEHIND \
I
ACT OVER TWICE THE GROUTED DEPTH 5 FEET INTO SANDSTONE
THIS LINE SOLDIER PILE DIAMETER OR THE PILE
V ~
SPACING,WHICHEVER IS SMALLER.
B.VERTICAL CAPACITYOFHPASSIVEPRESSURESINCLUDE
FACTOR OF SAFETY OF ABOUT 1.5.SOLDIER PILE/\.>
TIEBACK ANCHOR
NEGLECT LOOSE/SOFT NATIVE SOILS.
TYP.)<»:
36 (H+D)36 Hs 16 H 4. SEISMIC PRESSURE =16 H, Hs =H/4
5.0 =DEPTH OF EMBEDMENT INTO
SANDSTONE.
FRICTION
I \
6. ALL DIMENSIONS IN FEET.SOIL TYPE ADHESION)
NATIVE SOIL o ksf
SANDSTONE 3.0 ksf (augered)
I 60r
9.0 kif (pressure grouted)
EXCAVATION BASE
VERIFY WITH LOAD TEST 300% OF DESIGN STRESS
I !-H/4-
y/
LEVEL, SEE TEXT.
r >.
l-PROPOSED PROOF TEST TO 150% OF DESIGN ANCHOR LOAD,
J 4-FT DIA. PIPE SEE TEXT.
I -,-r-=-:::::.- - ---..J>ANDSTON~
I C.TENTATIVE ANCHOR PULLOUT___0
RESISTANCEI."I
PASSIVE PRESSURE
ACTIVE PRESSURE 36(H+D)+ 36(Hs) + 16 H 100 0 (psf) in loose/soft native soil
AT-REST PRESSURE 50(H+D) + 50(Hs) + 16 H 1200 0 (psf) in sandstone
A.LATERAL EARTH PRESSURE·NO LOAD ZONE
NOTE:Zipper Zeman Associates, Inc.Project No. J-1470 PROPOSED RETAIL DEVELOPMENT
FIELD VERIFY BACKSLOPE ANGLE
Geotechnical and Environmental Consulting Renton, Washington
Date: Nov. 2002
BETWEEN WALL AND BENSON
18905 33rd Avenue West, Suite 117
ROAD BEFORE DESIGN.Lynnwood,Washington 98036 Drawn by: J.D.
Figure 4:Recommended Design Criteria for
Tele: (425) 771-3304 Fax: (425)771-3549
Shoring Cantilever or Single Row of Tiebacks
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371F2EBC8E65
DocuSign Envelope ID: 6314EAD5-0BEB-42DD-88C0-371 F2EBC8E65
x=mD
II
crh
POINT LOAD
(FOR m > 0.4)
1.77q m2 n2
crh = � • (m 2+ n2)3
(FOR ID < 0.4)
0.28q n 2 crh = � • (0.16+n2)3
BASE OF EXCAVATION q cr' h = crh cos2 (1. Ht) � <>'h
q, lb per tt2
�
PLAN VIEW OF WALL
STRIP LOADING PARALLEL
TO EXCAVATION
crh = 3.9.. (�-sin � cos 2a.)1t
UNIFORM LOAD DISTRIBUTION
(jh = 0.4 q q = VERTICAL PRESSURE in psf
BASE OF EXCAVATION
Zipper Zeman Associates, Inc.
Geotechnical and Environmental Consulting
18905 33rd Avenue West, Suite 117
Lynnwood, Washington 98036
Tele: (425) 771-3304 Fax: (425) 771-3549
Project No. J-1470
Date: Nov. 2002
Drawn by: J.D.
PROPOSED RETAIL DEVELOPMENT
Renton, Washington
Figure 5: Surcharge Pressure Acting on
Adjacent Shoring or Subsurface Wall