HomeMy WebLinkAboutRS_Geotech_Report_220926_v1Geotechnical Engineering Report
Proposed Tenant Improvements
Building Addition
621 SW Grady Way
Renton, WA 98057
Parcel Nos. 3340404730, 3340404805
September 26, 2022
prepared for:
Washington State Auto Dealers Association
Attention: Vicki Fabre
P.O. Box 58170
Seattle, WA 98318
prepared by:
Migizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
(253) 537-9400
MGI Project Z0363
i
TABLE OF CONTENTS
Page No.
1.0 SITE AND PROJECT DESCRIPTION .............................................................................................. 1
2.0 EXPLORATORY METHODS ............................................................................................................ 2
2.1 Test Pit Procedures ................................................................................................................ 3
3.0 SITE CONDITIONS ............................................................................................................................ 3
3.1 Surface Conditions ................................................................................................................. 3
3.2 Soil Conditions ....................................................................................................................... 4
3.3 Groundwater Conditions ...................................................................................................... 5
3.4 Liquefaction Potential ........................................................................................................... 5
3.5 Seismic Conditions ................................................................................................................. 5
3.6 Infiltration Conditions ........................................................................................................... 7
4.0 CONCLUSIONS AND RECOMMENDATIONS............................................................................ 7
4.1 Site Preparation ...................................................................................................................... 8
4.2 Spread Footings .................................................................................................................... 10
4.3 Slab-On-Grade Floors .......................................................................................................... 12
4.4 Drainage Systems ................................................................................................................. 12
4.5 Structural Fill ........................................................................................................................ 13
5.0 RECOMMENDED ADDITIONAL SERVICES ............................................................................. 14
6.0 CLOSURE ........................................................................................................................................... 14
List of Tables
Table 1. Approximate Locations and Depths of Explorations ............................................................................. 2
Table 2. Seismic Design Parameters ........................................................................................................................ 6
List of Figures
Figure 1. Topographic and Location Map
Figure 2. Site and Exploration Plan
Figure 3. Kleinfelder Site and Exploration Plan
APPENDIX A
Soil Classification Chart and Key to Test Data .................................................................................................. A-1
Logs of Test Pit Explorations TP-1 through TP-3 .................................................................................... A-2…A-4
APPENDIX B
Kleinfelder Exploration Logs
Page 1 of 14
MIGIZI GROUP, INC.
PO Box 44840 PHONE (253) 537-9400
Tacoma, Washington 98448 FAX (253) 537-9401
September 26, 2022
Washington State Auto Dealers Association
P.O. Box 58170
Seattle, WA 98318
Attention: Vicki Fabre
Subject: Geotechnical Engineering Report
Proposed Tenant Improvements – Building Addition
621 SW Grady Way
Renton, WA 98057
Parcel Nos. 3340404730, 3340404805
MGI Project Z0363
Dear Mr. Fabre:
Migizi Group, Inc. (MGI) is pleased to submit this report describing the results of our geotechnical
engineering evaluation of the proposed tenant improvements to the Washington State Auto
Dealers Association (WSADA) building located at the southeast corner of SW Grady Way and
Raymond Ave SW in the King County city of Renton, WA.
The undersigned, James Brigham P.E., while working under the umbrella of E3RA, Inc.,
previously prepared a Geotechnical Letter Report for the original building construction at the
subject property, dated April 4, 2012. MGI also prepared a Geotechnical Engineering Report for the
parking lot expansion in the adjacent previously vacant lot southeast of the subject property,
being dated October 26, 2018. Copies of these evaluations, complete with soil exploration logs,
were made available to us for review with our current study.
This report has been prepared for the exclusive use of WSADA and their consultants, for specific
application to this project, in accordance with generally accepted geotechnical engineering
practice.
1.0 SITE AND PROJECT DESCRIPTION
The project site consists of two contiguous tax parcels immediately southeast of the intersection
between SW Grady Way and Raymond Ave SW toward the southwest corner of the city limits of
the Renton, WA as shown on the enclosed Topographic and Location Map (Figure 1). The subject
property is located within an industrial/commercial area northwest of the intersection between
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 2 of 14
SR 167 and I-405 along the historic floodplain of the Black and Green Rivers. The northernmost
of the subject parcels contains the primary WSADA office building originally constructed in 2013.
The remainder of this parcel and the entirety of the southern parcel is occupied by asphalt
pavement for parking facilities and drive lanes. The combined areal extent of these parcels is
approximately 1.07-acres.
Improvement plans involve the construction of a new 2-story addition to the northwest corner of
the existing building. The newly added space will be 695 sf for both the first and second floors,
which totals 1,390 sf. Specifically, a new stairwell, elevator shaft, and storage rooms will make
up the addition. The addition is to be built with slab-on-grade at the existing grade of the site.
2.0 EXPLORATORY METHODS
We explored surface and subsurface conditions at the project site on April 27, 2018 for our
evaluation of the proposed parking addition and a follow-up site visit to observe existing
conditions was conducted on September 9, 2022. Our exploration and evaluation program
comprised the following elements:
• Surface reconnaissance of the site,
• Three test pit explorations (designated TP-1 through TP-3) advanced on April 27, 2018,
• A review of the Geotechnical Letter Report prepared by E3RA, Inc., dated April 4, 2012,
• A review of the subsurface explorations advanced onsite by Kleinfelder on January 11,
2006 and on February 7, 2006, and
• A review of published geologic and seismologic maps and literature.
Table 1 (below) summarizes the approximate functional locations and termination depths of our
subsurface explorations and Figure 2 (attached) depicts their approximate relative locations. The
following sections describe the procedures used for excavation of the test pits.
TABLE 1
APPROXIMATE LOCATIONS AND DEPTHS OF EXPLORATIONS
Exploration Functional Location
Termination
Depth
(feet)
TP-1
TP-2
TP-3
Roughly central to the western third of the southern parcel
Roughly the middle-most point of the southern parcel
Roughly central to the eastern third of the southern parcel
10
3
10
The specific numbers and locations of our explorations were selected in relation to the existing
site features, under the constraints of surface access, underground utility conflicts, and budget
considerations.
It should be realized that the explorations performed and utilized for this evaluation reveal
subsurface conditions only at discrete locations across the project site and that actual conditions
in other areas could vary. Furthermore, the nature and extent of any such variations would not
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 3 of 14
become evident until additional explorations are performed or until construction activities have
begun. If significant variations are observed at that time, we may need to modify our conclusions
and recommendations contained in this report to reflect the actual site conditions.
2.1 Test Pit Procedures
Our exploratory test pits were excavated with a rubber-tracked mini-excavator operated by an
excavation contractor under subcontract to MGI. A geologist from our firm observed the test pit
excavations, collected soil samples, and logged the subsurface conditions.
The enclosed test pit logs indicate the vertical sequence of soils and materials encountered in our
test pits, based on our field classifications. Where a soil contact was observed to be gradational
or undulating, our logs indicate the average contact depth. We estimated the relative density and
consistency of the in-situ soils by means of the excavation characteristics and the stability of the
test pit sidewalls. Our logs also indicate the approximate depths of any sidewall caving or
groundwater seepage observed in the test pits. The soils were classified visually in general
accordance with the system described in Figure A-1, which includes a key to the exploration logs.
Summary logs of our explorations are included as Figure A-2 through A-4.
3.0 SITE CONDITIONS
The following sections present our observations, measurements, findings, and interpretations
regarding surface, soil, groundwater, seismic and infiltration conditions, and liquefaction
potential.
3.1 Surface Conditions
As previously indicated, the project site consists of two contiguous tax parcels immediately
southeast of the intersection between SW Grady Way and Raymond Ave SW toward the
southwest corner of the city limits of the Renton, WA. The subject property is located within an
industrial/commercial area northwest of the intersection between SR 167 and I-405 along the
historic floodplain of the Black and Green Rivers. The northernmost of the subject parcels
contains the primary WSADA office building originally constructed in 2013. The remainder of
this northern parcel and the entirety of the southern parcel is occupied by asphalt pavement for
parking facilities and drive lanes. The combined areal extent of these parcels is approximately
1.07-acres. The project area is bound on the east by the Renton West Veterinary Hospital and
Perma Dry Waterproofing and Drainage, and on the south by Bell Electronics.
Topographically, the project area is relatively level with minimal grade change being observed
over its full extent. Vegetation onsite is minimal with the majority of the site being paved,
comprising ornamental trees and/or shrubs in designated landscaping strips.
No hydrologic features were observed on site such as seeps, springs, ponds, or streams, nor was
there evidence of surface hydrology present. The course of the rerouted Black River is
approximately 1,500 feet west of the project area.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 4 of 14
3.2 Soil Conditions
During our evaluation of the parking lot expansion, we observed subgrade conditions in three
test pits across the southernmost parcel identified as parcel number 3340404805. These
explorations revealed structural fill and construction-related debris such as concrete, asphalt, and
brick in a dense condition at surface elevations, down to approximately 2½ to 4½ feet below
existing grade. This material overlies native alluvial flood plain deposits generally comprising
mottled silt, though a thin sandy zone was observed in test pit exploration TP-1. Mottled silt was
observed through the termination of test pit explorations TP-1 and TP-3; a maximum depth of
10 feet below ground surface. Native soils were all poorly consolidated and oversaturated. While
excavating TP-2, a block of concrete prevented further excavation beyond 3 feet deep and the test
pit was terminated.
In the Geotechnical Letter Report prepared by E3RA, Inc., they referenced subsurface explorations
previously performed by Kleinfelder across the parent property immediately to the north (parcel
number 3340404730), as highlighted in the attached Figure 3. In total, they performed 6 test pit
explorations and 2 auger borings; with the maximum depth explored being 44 feet below existing
grade. In general, Kleinfelder observed similar subsurface conditions, with existing fill material
and fine-grained alluvium being observed in close proximity to existing grade. Granular, sandy
soils are not encountered until a depth of 20 to 25 feet below existing grade. Kleinfelder boring
logs and test pit logs are attached as Appendix B.
In the Geologic Map of the Renton Quadrangle, King County, Washington, as prepared by the
Department of the Interior United States Geological Survey (USGS) (1965), the project site is
mapped as containing Qaw, or Quaternary Alluvium associated with the flood plains of the
White and Green Rivers. The upper part of these deposits are mostly clayey silt and fine sand,
locally peaty, being 10 to 20 feet thick near Kent, thickening to 30 to 40 feet near Tukwila. The
lower part of these deposits is mostly medium and coarse sand and can reach thicknesses of up
to 75 feet. An excerpt from this publication is presented below:
Excerpt from the Geologic Map of the Renton Quadrangle, King County, WA (USGS) (1965)
Project Site
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 5 of 14
The National Cooperative Soil Survey (NCSS) for the King County Area classifies soils onsite as
Ur-Urban Land, surrounded by minor soil units of sand and silt loam. This soil series reportedly
formed along alluvial flood plains and comprise sandy loam, silt loam, silty clay loam and sand.
Our subsurface explorations generally correspond with the site classifications prepared by the
USGS and NCSS.
3.3 Groundwater Conditions
We encountered groundwater seepage in two of our three subsurface explorations at a depth of
4 to 4½ feet below existing grade. Kleinfelder, during the advancement of their subsurface
explorations in the earlier part of 2006, encountered seepage at a depth of 6 to 7 feet below existing
grade. Given the fact that explorations conducted onsite were performed within or just outside
of what is generally considered the rainy season (November 1st to March 31st), we do not anticipate
that groundwater will rise much higher than that which was observed. Groundwater levels will
fluctuate with localized geology and precipitation.
3.4 Liquefaction Potential
Liquefaction is a sudden increase in pore water pressure and a sudden loss of soil shear strength
caused by shear strains, as could result from an earthquake. Research has shown that saturated,
loose, fine to medium sands with a fines (silt and clay) content less than about 20 percent are most
susceptible to liquefaction. Poorly consolidated soils encountered below the water table (a depth
of 4 to 7 feet) present a moderate to severe risk for soil liquefaction. Recommendations for
foundation subgrade preparation and construction contained within this report helps mitigate
some of this risk, but the risk for soil liquefaction and resultant post-construction settlement
should still be considered moderate across the project area in the instance of a large-scale seismic
event.
3.5 Seismic Conditions
The site is in the Puget Sound basin, which has experienced several earthquakes. A detailed
description of the regional seismicity is beyond the scope of this report; however, previous
regional earthquakes can be split into two general categories: 1) large earthquakes with a moment
magnitude greater than 8.0 (MW > 8.0) and 2) modest size earthquakes with a moment magnitude
generally less than 7.25 (MW < 7.25). In all cases, the thickness of the soil between the bedrock and
the ground surface can change (usually amplify) the seismically induced ground motions and
therefore the inertial loads acting on surface structures.
“Site Class” is a classification system used by the International Building Code (IBC) and ASCE 7
to provide some insight to the potential for ground motion amplification. The site class is based
on the properties of the upper 100 feet of the soil and rock materials at the site per ASCE 7-16
Section 11.4.8, a ground motion hazard analysis or site-specific response analysis is required for:
1. All structures on Site Class F sites.
2. All seismically isolated structures and structures with damping systems on sites with S1
greater than or equal to 0.6.
3. All structures on Site Class E sites with Ss greater than or equal to 1.0.
4. All structures on Site Class D and E sites with S1 greater than or equal to 0.2.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 6 of 14
Because the site soils are likely to liquefy during design seismic shaking, this site is classified as
Site Class F and ASCE 7-16 Section 11.4.8 requires a site-specific response analysis be performed
unless one of the three Section 11.4.8 exceptions and/or the exception to ASCE 7-16 20.3.1.1
applies. For example:
• Exception 1 to 20.3.1 – If the fundamental vibration period of the structure is equal to or
less than 0.5 seconds, then site response analysis is not required, and the Site Class may
be taken as Site Class D (Stiff Soil) and the corresponding design response spectrum can
be derived from Figure 11.4-1, Table 11.4-1 and Table 11.4-2.
• Exception 3 of Section 11.4.8 – If on Site Class E/F sites S1 is greater than or equal to 0.2 and
structure’s period ‘T’ is greater than or equal to Ts (as defined in the code) and the
equivalent static force procedure is used for design, then site response analysis is not
required.
Typically, Exception 3 of Section 11.4.8 is more restrictive than Exception 1 to Section 20.3. We
recommend that the applicability of these exceptions and the structural design procedure to be
used be determined by the structural engineer.
If the structural engineer determines Exception 1 to 20.3.1 applies, we recommend the design
seismic values provided in Table 2 (below) be used. If requested, we can complete a site-specific
seismic response analysis which might provide reduced seismic demands from the parameters in
Table 2.
TABLE 2
SEISMIC DESIGN PARAMETERS
Parameter Value Basis
Site Class D – Stiff Soil Site specific data
SS 1.447 seismicmaps.org
Fa 1A seismicmaps.org
SMS 1.447 = Fa · SS
SDS 0.964 = 2/3 SMS
S1 0.493 seismicmaps.org
FV 1.81 B, C 2018 IBC
SM1 0.892 B, C = FV · S1
SD1 0.594 B, C = 2/3 SM1
PGA 0.616g seismicmaps.org
PGAM 0.677g seismicmaps.org
T0 -- C Not applicable
TS -- C Not applicable
TL 6 sec seismicmaps.org
Notes:
A. Use the value provided unless the simplified design procedure of ASCE 7 Section 12.14 is used.
If this occurs, please contact our office for more information.
B. Based on Table 1613.2.3(2) of the 2018 IBC – An ASCE 7-16 Chapter 21 analysis has not been
performed.
C. More detailed seismic design criteria are available upon request. Please contact MGI for more
information.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 7 of 14
3.6 Infiltration Conditions
As indicated in the above Soil Conditions and Groundwater Conditions sections of this report,
densely consolidated fill soils are encountered at surface elevations across the project area
generally containing construction debris such as concrete, asphalt and brick. This material
extends 2½ to 4½ feet below existing grade transitioning to native, alluvial soils generally
comprising mottled silt through a depth of 10 feet. The soil conditions encountered onsite was
further corroborated by past Kleinfelder explorations which observed similar subgrade
conditions. Additionally, groundwater levels were encountered 4 to 4½ feet below existing grade
across the project area.
As per the 2017 City of Renton Surface Water Design Manual (RSWDM), full infiltration requires
native soils to consist of medium sands or better, whereas limited infiltration can utilize loamy
sands, sandy loams, and loams. Fill material, silt and clay loams, and cemented till (hardpan) are
not suitable for infiltration. Given the geologic conditions present within the project area, we do
not interpret full or limited infiltration as being feasible for this project, and we do not recommend
utilizing pervious pavements or bioretention. Site produced stormwater should be diverted to
an existing storm system.
4.0 CONCLUSIONS AND RECOMMENDATIONS
Improvement plans involve the construction of a new 2-story addition to the northwest corner of
the existing building. The newly added space will be 695 sf for both the first and second floors
which totals 1,390 sf. Specifically, a new stairwell, elevator shaft, and storage rooms will make
up the addition. The addition is to be built with slab-on-grade at the existing grade of the site.
We offer the following recommendations:
• Feasibility: Based on our field explorations, research and evaluations, the proposed
improvements and pavements appear feasible from a geotechnical standpoint.
• Foundation Options: Over-excavation of spread footing subgrades to a depth of 3 to 5 feet
and construction of structural fill bearing pads will be necessary for foundation support
of the proposed addition. If foundation construction occurs during wet conditions, it is
likely that a geotextile fabric and/or a packed layer of quarry spall rock, placed between
bearing pads and native soils, will also be necessary. Recommendations for spread
footings are provided in Section 4.2.
• Floor Options: Based on explorations across the site, we recommend that floor sections
be over-excavated to a minimum depth of 1½ feet, then placement of suitable and
properly compacted structural fill as a floor subbase. Recommendations for slab-on-grade
floors are included in Section 4.3. If floor construction occurs during wet conditions, it is
likely that a geotextile fabric and/or a packed layer of quarry spall rock, placed between
bearing pads and native soils, will also be necessary. Fill underlying floor slabs should be
compacted to 95 percent (ASTM:D-1557).
• Pavement Sections: After removal of any organic-rich materials underlying pavement
areas, we recommend a conventional pavement section comprising an asphalt concrete
pavement over a crushed rock base course over properly prepared (compacted) subgrade
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 8 of 14
or granular subbase. Given the relative loose/soft condition of native onsite soils, we
recommend an over-excavation in proposed concrete and asphalt areas of 2 feet, with the
placement and compaction of a suitable structural fill subbase.
All soil subgrades below 2 feet should be thoroughly compacted then proof-rolled with a
loaded dump truck or heavy compactor. Any localized zones of yielding subgrade
disclosed during this proof-rolling operation should be over-excavated to a depth of
12 inches and replaced with a suitable structural fill material.
• Geologic Hazards: During our site reconnaissance, advancement of subsurface
explorations, and general evaluation of the proposed development, we did not observe
any erosional, landslide, seismic, settlement, or other forms of geologic hazards within the
subject property. Given this fact, we recommend that no buffers, setbacks, or other forms
of site restraints be implemented to address these potential hazards.
The following sections present our specific geotechnical conclusions and recommendations
concerning site preparation, spread footings, slab-on-grade floors, drainage systems, and
structural fill. The Washington State Department of Transportation (WSDOT) Standard
Specifications and Standard Plans cited herein refer to WSDOT publications M41-10, Standard
Specifications for Road, Bridge, and Municipal Construction, and M21-01, Standard Plans for Road,
Bridge, and Municipal Construction, respectively.
4.1 Site Preparation
Preparation of the project site should involve erosion control, temporary drainage, clearing,
stripping, excavations, cutting, subgrade compaction, and filling.
Erosion Control: Before new construction begins, an appropriate erosion control system should
be installed. This system should collect and filter all surface water runoff through silt fencing.
We anticipate a system of berms and drainage ditches around construction areas will provide an
adequate collection system. Silt fencing fabric should meet the requirements of WSDOT Standard
Specification 9-33.2 Table 6. In addition, silt fencing should embed a minimum of 6 inches below
existing grade. An erosion control system requires occasional observation and maintenance.
Specifically, holes in the filter and areas where the filter has shifted above ground surface should
be replaced or repaired as soon as they are identified.
Temporary Drainage: We recommend intercepting and diverting any potential sources of surface
or near-surface water within the construction zones before stripping begins. Because the selection
of an appropriate drainage system will depend on the water quantity, season, weather conditions,
construction sequence, and contractor's methods, final decisions regarding drainage systems are
best made in the field at the time of construction. Based on our current understanding of the
construction plans, surface, and subsurface conditions, we anticipate that curbs, berms, or ditches
placed around the work areas will adequately intercept surface water runoff.
Clearing and Stripping: After surface and near-surface water sources have been controlled, sod,
topsoil, and root-rich soil should be stripped from the site. The majority of the site is paved, and
minimal to no stripping is anticipated for this project.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 9 of 14
Site Excavations: Based on our field explorations, we anticipate that excavations will encounter
dense fill soils in close proximity to existing grade, and loose/soft fine-grained alluvial soils with
depth. This material can be easily excavated utilizing standard excavation equipment.
Dewatering: We anticipate that site excavations will encounter groundwater at shallow depths
during periods of extended precipitation. In addition, if excavations of trench lines are left open
for an extended period, slow seepage of groundwater may be observed. If groundwater is
encountered during regular earthwork activities, we anticipate that an internal system of ditches,
sumpholes, and pumps will be adequate to temporarily dewater shallow excavations.
Temporary Cut Slopes: At this time, final designs and construction sequencing have not been
completed. To facilitate project planning we provide the following general comments regarding
temporary slopes:
• All temporary soil slopes associated with site cutting or excavations should be adequately
inclined and covered in plastic sheeting to prevent sloughing and collapse,
• Temporary cut slopes in site soils should be no steeper than 1½H:1V, and
• Temporary slopes should conform to Washington Industrial Safety and Health Act
(WISHA) regulations.
These general guidelines are necessarily somewhat conservative (steeper temporary slopes may
be possible). As the project progresses, temporary grading plans are developed, final site features
are better defined, and a contractor is engaged, MGI may modify these general guidelines to allow
steeper slopes.
Subgrade Compaction: Exposed subgrades for the foundations of the planned addition should
be compacted to a firm, unyielding state before new concrete or fill soils are placed. Any localized
zones of looser granular soils observed within a subgrade should be compacted to a density
commensurate with the surrounding soils. In contrast, any organic, soft, or pumping soils
observed within a subgrade should be over-excavated and replaced with a suitable structural fill
material.
Site Filling: Our conclusions regarding the reuse of onsite soils and our comments regarding wet-
weather filling are presented subsequently. Regardless of soil type, all fill should be placed and
compacted according to our recommendations presented in the Structural Fill section of this
report. Specifically, building pad fill soil should be compacted to a uniform density of at least
95 percent (based on ASTM:D-1557).
Onsite Soils: We offer the following evaluation of these onsite soils in relation to potential use as
structural fill:
• Surficial Organic Soil and Organic-Rich Topsoil: Where encountered, surficial organic soils,
like duff, topsoil, root-rich soil, and organic-rich fill soils, are not suitable for use as
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 10 of 14
structural fill under any circumstances, due to high organic content. Consequently, this
material can be used only for non-structural purposes, such as in landscaping areas.
• Alluvial Silt and Silty Sand: The alluvial silty sand that underlies the site is very moisture
sensitive and will be difficult to impossible to reuse during most weather conditions. The
majority of this soil type is currently above the optimum moisture content and will not
compact adequately unless extensively aerated or treated with cement.
Permanent Slopes: All permanent cut slopes and fill slopes should be adequately inclined to
reduce long-term raveling, sloughing, and erosion. We generally recommend that no permanent
slopes be steeper than 2H:1V. For all soil types, the use of flatter slopes (such as 2½H:1V) would
further reduce long-term erosion and facilitate revegetation.
Slope Protection: We recommend that a permanent berm, swale, or curb be constructed along
the top edge of all permanent slopes to intercept surface flow. Also, a hardy vegetative
groundcover should be established as soon as feasible, to further protect the slopes from runoff
water erosion. Alternatively, permanent slopes could be armored with quarry spalls or a
geosynthetic erosion mat.
4.2 Spread Footings
In our opinion, conventional spread footings will provide adequate support for the proposed
addition, if the subgrades are properly prepared. Due to the soft soils that underlie the site, over-
excavation of spread footing subgrades, to a depth of 3 to 5 feet, and the construction of structural
fill bearing pads, will be necessary for foundational support of the new addition. We offer the
following comments and recommendations for spread footing design.
Footing Depths and Widths: For frost and erosion protection, the bases of all exterior footings
should bear at least 18 inches below adjacent outside grades, whereas the bases of interior
footings need bear only 12 inches below the surrounding slab surface level. To reduce post-
construction settlements, continuous (wall) and isolated (column) footings should be at least 18
and 24 inches wide, respectively.
Bearing Subgrades: Footings should bear on medium dense or denser, undisturbed native soils
or properly compacted structural fill which bears on undisturbed medium dense to very dense
native soils. Structural fill bearing pads, 3 to 5 feet thick and compacted to a density of at least
95 percent (based on ASTM: D-1557), should underlie spread footings for the proposed
construction. If foundation work occurs during wet conditions, it is possible that a geotextile
fabric, placed between the bearing pad and native soil, will be necessary. Refer to the Structural
Fill section of this report.
In general, before footing concrete is placed, any localized zones of loose soils exposed across the
footing subgrades should be compacted to a firm, unyielding condition, and any localized zones
of soft, organic, or debris-laden soils should be over-excavated and replaced with suitable
structural fill. Structural fill bearing pads should be compacted to a density of at least 95 percent
(based on ASTM: D-1557).
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 11 of 14
Lateral Over-excavations: Because foundation stresses are transferred outward as well as
downward into the bearing soils, all structural fill placed under footings, should extend
horizontally outward from the edge of each footing. This horizontal distance should be equal to
the depth of placed fill. Therefore, placed fill that extends 3 feet below the footing base should
also extend 3 feet outward from the footing edges.
Subgrade Observation: All footing subgrades should consist of firm, unyielding, native soils or
structural fill materials that have been compacted to a density of at least 95 percent (based on
ASTM:D-1557). Footings should never be cast atop loose, soft, or frozen soil, slough, debris,
existing uncontrolled fill, or surfaces covered by standing water.
Bearing Pressures: In our opinion, for static loading, footings that bear on dense, properly
prepared bearing pads can be designed for maximum allowable soil bearing pressures listed in
the following table:
Bearing Pad Thickness (feet) Allowable Bearing Pressure (psf)
3 1,500
4 2,000
5 2,500
A one-third increase in allowable soil bearing capacity may be used for short-term loads created
by seismic or wind related activities.
Footing Settlements: Assuming that structural fill soils are compacted to a dense or denser state,
we estimate that total post-construction settlements of properly designed footings bearing on
properly prepared subgrades will not exceed 1 inch, under static conditions. Differential
settlements for comparably loaded elements may approach one-half of the actual total settlement
over horizontal distances of approximately 50 feet.
Footing Backfill: To provide erosion protection and lateral load resistance, we recommend that
all footing excavations be backfilled on both sides of the footings and stem walls after the concrete
has cured. Either imported structural fill or non-organic onsite soils can be used for this purpose,
contingent on suitable moisture content at the time of placement. Regardless of soil type, all
footing backfill soil should be compacted to a density of at least 90 percent (based on ASTM:D-
1557).
Lateral Resistance: Footings that have been properly backfilled as recommended above will resist
lateral movements by means of passive earth pressure and base friction. We recommend using
an allowable passive earth pressure of 250 psf and an allowable base friction coefficient of 0.35
for both soil types.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 12 of 14
4.3 Slab-On-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used in the proposed addition if the
subgrades are properly prepared. We offer the following comments and recommendations
concerning slab-on-grade floors.
Floor Subbase: For the proposed addition, we recommend over-excavation of slab-on-grade floor
subgrades to a minimum depth of 1½ feet, then placement of properly compacted structural fill
as a floor subbase. If floor construction is to occur during wet conditions, it is likely that a
geotextile fabric and/or compacted layer of quarry spall rock, placed between the structural fill
floor subbase and native soils, will be necessary. All subbase should be compacted to a density
of at least 95 percent (based on ASTM:D-1557).
Capillary Break and Vapor Barrier: To retard the upward wicking of moisture beneath the floor
slab, we recommend that a capillary break be placed over the subgrade. Ideally, this capillary
break would consist of a 4-inch-thick layer of pea gravel or other clean, uniform, well-rounded
gravel, such as “Gravel Backfill for Drains” per WSDOT Standard Specification 9-03.12(4).
Alternatively, angular gravel or crushed rock can be used if it is sufficiently clean and uniform to
prevent capillary wicking. In addition, a layer of plastic sheeting (such as Crosstuff, Moistop, or
Visqueen) be placed directly between the capillary break and the floor slab to prevent ground
moisture vapors from migrating upward through the slab. During subsequent casting of the
concrete slab, the contractor should exercise care to avoid puncturing the vapor barrier
4.4 Drainage Systems
We offer the following recommendations and comments for drainage design for construction
purposes.
Perimeter Drains: We recommend that the proposed addition, where applicable, be encircled
with a perimeter drain system to collect seepage water. This drain should consist of a 4-inch-
diameter perforated pipe within an envelope of pea gravel or washed rock, extending at least
6 inches on all sides of the pipe, and the gravel envelope should be wrapped with filter fabric to
reduce the migration of fines from the surrounding soils. Ideally, the drain invert would be
installed no more than 8 inches above the base of the perimeter of the foundation.
Discharge Considerations: If possible, all perimeter drains should discharge to a municipal storm
drain, or other suitable location by gravity flow. Check valves should be installed along any
drainpipes that discharge to a sewer system, to prevent sewage backflow into the drain system.
Runoff Water: Roof-runoff and surface-runoff water should not discharge into the perimeter
drain system. Instead, these sources should discharge into separate tightline pipes and be routed
away from the buildings to a storm drain or other appropriate location.
Grading and Capping: Final site grades should slope downward away from the building so that
runoff water will flow by gravity to suitable collection points, rather than ponding near the
building. Ideally, the area surrounding the building would be capped with concrete, asphalt, or
low-permeability (silty) soils to minimize or preclude surface-water infiltration.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 13 of 14
4.5 Structural Fill
The term "structural fill" refers to any material placed under foundations, retaining walls, slab-
on-grade floors, sidewalks, pavements, and other structures. Our comments, conclusions, and
recommendations concerning structural fill are presented in the following paragraphs.
Materials: Typical structural fill materials include clean sand, gravel, pea gravel, washed rock,
crushed rock, well-graded mixtures of sand and gravel (commonly called "gravel borrow" or "pit-
run"), and miscellaneous mixtures of silt, sand, and gravel. Recycled asphalt, concrete, and glass,
which are derived from pulverizing the parent materials, are also potentially useful as structural
fill in certain applications. Import soils used for structural fill should not contain any organic
matter or debris, nor any individual particles greater than 4 inches in diameter.
Fill Placement: Clean sand, gravel, crushed rock, soil mixtures, and recycled materials should be
placed in horizontal lifts not exceeding 8 inches in loose thickness, and each lift should be
thoroughly compacted with a mechanical compactor.
Compaction Criteria: Using the Modified Proctor test (ASTM:D-1557) as a standard, we
recommend that structural fill used for various onsite applications be compacted to the following
minimum densities:
Fill Application Minimum Compaction
Footing subgrade and bearing pad
Foundation backfill
Slab-on-grade floor subgrade and subbase
Asphalt pavement base and subbase
Asphalt pavement subgrade (upper 2 feet)
Asphalt pavement subgrade (below 2 feet)
95 percent
90 percent
95 percent
95 percent
95 percent
90 percent
Subgrade Observation and Compaction Testing: Regardless of material or location, all structural
fills should be placed over firm, unyielding subgrades prepared in accordance with the Site
Preparation section of this report. The condition of all subgrades should be observed by
geotechnical personnel before filling or construction begins. Also, fill soil compaction should be
verified by means of in-place density tests performed during fill placement so that adequacy of
soil compaction efforts may be evaluated as earthwork progresses.
Soil Moisture Considerations: The suitability of soils used for structural fill depends primarily
on their grain-size distribution and moisture content when they are placed. As the "fines" content
(that soil fraction passing the U.S. No. 200 Sieve) increases, soils become more sensitive to small
changes in moisture content. Soils containing more than about 5 percent fines (by weight) cannot
be consistently compacted to a firm, unyielding condition when the moisture content is more than
2 percentage points above or below optimum. For fill placement during wet-weather site work,
we recommend using "clean" fill, which refers to soils that have a fines content of 5 percent or less
(by weight) based on the soil fraction passing the U.S. No. 4 Sieve.
WSADA – Proposed Tenant Improvements, 621 SW Grady Way, Renton, WA September 26, 2022
Geotechnical Engineering Report Z0363
Migizi Group, Inc. Page 14 of 14
5.0 RECOMMENDED ADDITIONAL SERVICES
Because the future performance and integrity of the structural elements will depend largely on
proper site preparation, drainage, fill placement, and construction procedures, monitoring and
testing by experienced geotechnical personnel should be considered an integral part of the
construction process. Subsequently, we recommend that MGI be retained to provide the
following post-report services:
• Review all construction plans and specifications to verify that our design criteria
presented in this report have been properly integrated into the design,
• Prepare a letter summarizing all review comments (if required),
• Check all completed subgrades for footings and slab-on-grade floors before concrete is
poured, in order to verify their bearing capacity, and
• Prepare a post-construction letter summarizing all field observations, inspections, and test
results (if required).
6.0 CLOSURE
The conclusions and recommendations presented in this report are based, in part, on the
explorations that we observed for this study; therefore, if variations in the subgrade conditions
are observed at a later time, we may need to modify this report to reflect those changes. Also,
because the future performance and integrity of the project elements depend largely on proper
initial site preparation, drainage, and construction procedures, monitoring and testing by
experienced geotechnical personnel should be considered an integral part of the construction
process. MGI is available to provide geotechnical monitoring of soils throughout construction.
We appreciate the opportunity to be of service on this project. If you have any questions regarding
this report or any aspects of the project, please feel free to contact our office.
Respectfully submitted,
MIGIZI GROUP, INC.
09/26/22
Zach L. Logan, LG James E. Brigham, P.E.
Project Geologist Senior Principal Engineer
APPROXIMATE SITE
LOCATION
P.O. Box 44840
Tacoma, WA 98448
Location Job Number Figure
DateTitle
621 SW Grady Way
Renton, WA 98057
Topographic and Location Map
1
09/19/22
Z0363
APPENDIX A
SOIL CLASSIFICATION CHART AND
KEY TO TEST DATA
CLAYEY GRAVELS, POORLY GRADED GRAVEL-SAND-CLAY
MIXTURES
SILTS AND CLAYSCOARSE GRAINED SOILSMore than Half > #200 sieveLIQUID LIMIT LESS THAN 50
LIQUID LIMIT GREATER THAN 50
CLEAN GRAVELS
WITH LITTLE OR
NO FINES
GRAVELS WITH
OVER 15% FINES
CLEAN SANDS
WITH LITTLE
OR NO FINES
MORE THAN HALF
COARSE FRACTION
IS SMALLER THAN
NO. 4 SIEVE
MORE THAN HALF
COARSE FRACTION
IS LARGER THAN
NO. 4 SIEVE
INORGANIC SILTS, MICACEOUS OR DIATOMACIOUS FINE
SANDY OR SILTY SOILS, ELASTIC SILTS
ORGANIC CLAYS AND ORGANIC SILTY CLAYS OF LOW
PLASTICITY
OH
INORGANIC SILTS AND VERY FINE SANDS, ROCK FLOUR,
SILTY OR CLAYEY FINE SANDS, OR CLAYEY SILTS WITH
SLIGHT PLASTICITY
CH
SILTY GRAVELS, POORLY GRADED GRAVEL-SAND-SILT
MIXTURES
SANDS
SILTS AND CLAYS
Figure A-1
INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY,
GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS,
LEAN CLAYS
E3RA
R-Value
Sieve Analysis
Swell Test
Cyclic Triaxial
Unconsolidated Undrained Triaxial
Torvane Shear
Unconfined Compression
(Shear Strength, ksf)
Wash Analysis
(with % Passing No. 200 Sieve)
Water Level at Time of Drilling
Water Level after Drilling(with date measured)
RV
SA
SW
TC
TX
TV
UC
(1.2)
WA
(20)
Modified California
Split Spoon
Pushed Shelby Tube
Auger Cuttings
Grab Sample
Sample Attempt with No Recovery
Chemical Analysis
Consolidation
Compaction
Direct Shear
Permeability
Pocket Penetrometer
CA
CN
CP
DS
PM
PP
PtHIGHLY ORGANIC SOILS
TYPICAL NAMES
GRAVELS
ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY,
ORGANIC SILTS
WELL GRADED GRAVELS, GRAVEL-SAND MIXTURES
MAJOR DIVISIONS
PEAT AND OTHER HIGHLY ORGANIC SOILS
WELL GRADED SANDS, GRAVELLY SANDS
POORLY GRADED SANDS, GRAVELLY SANDS
SILTY SANDS, POOORLY GRADED SAND-SILT MIXTURES
CLAYEY SANDS, POORLY GRADED SAND-CLAY MIXTURES
POORLY GRADED GRAVELS, GRAVEL-SAND MIXTURES
SOIL CLASSIFICATION CHART AND KEY TO TEST DATA
GW
GP
GM
GC
SW
SP
SM
SC
ML
FINE GRAINED SOILSMore than Half < #200 sieveLGD A NNNN02 GINT US LAB.GPJ 11/4/05INORGANIC CLAYS OF HIGH PLASTICITY, FAT CLAYS
CL
OL
MH
SANDS WITH
OVER 15% FINES
GB
S-1
GP-
GM
SM
SP
ML
0.5
2.5
4.0
10.0
(GP-GM) Gray/brown gravel with silt and sand (dense, moist) (Fill)
(SM) Gray silty sand with gravel and concrete/asphalt/brick debris (dense, moist) (Fill)
(SP) Blue/gray fine to medium sand with gravel (medium dense, moist) (Alluvium)
(ML) Blue/gray silt (very soft, wet) (Alluvium)
Moderate caving observed from 4 to 10 feet
Moderate groundwater seepage observed at 4 feet
The depths on the test pit logs are based on an average of measurements across the test pit and should be
considered accurate to 0.5 foot.
Bottom of test pit at 10.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Rubber Tracked Mini Excavator
EXCAVATION CONTRACTOR Paulman GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 4/27/18 COMPLETED 4/27/18
AT TIME OF EXCAVATION 4.00 ft Moderate seepage
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
10.0
PAGE 1 OF 1
Figure A-2
TEST PIT NUMBER TP-1
CLIENT Sitts & Hill Engineers, Inc.
PROJECT NUMBER P1258-T18
PROJECT NAME Washington State Auto Dealers Association
PROJECT LOCATION 621 SW Grady Way, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 5/16/18 11:29 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1258-T18\P1258-T18 TEST PITS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
GP-
GM
SM
0.8
3.0
(GP-GM) Gray/brown gravel with silt and sand (dense, moist) (Fill)
(SM) Gray silty sand with gravel and concrete/asphalt/brick debris (dense, moist) (Fill)
Refusal at a depth of 3 feet atop a large section of concrete
No caving observed
No groundwater seepage observed
The depths on the test pit logs are based on an average of measurements across the test pit and should be
considered accurate to 0.5 foot.
Bottom of test pit at 3.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Rubber Tracked Mini Excavator
EXCAVATION CONTRACTOR Paulman GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 4/27/18 COMPLETED 4/27/18
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
PAGE 1 OF 1
Figure A-3
TEST PIT NUMBER TP-2
CLIENT Sitts & Hill Engineers, Inc.
PROJECT NUMBER P1258-T18
PROJECT NAME Washington State Auto Dealers Association
PROJECT LOCATION 621 SW Grady Way, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 5/16/18 11:29 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1258-T18\P1258-T18 TEST PITS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
GP-
GM
SM
SM
ML
1.0
3.5
4.5
10.0
(GP-GM) Gray/brown gravel with silt and sand (dense, moist) (Fill)
(SM) Gray silty sand with gravel and concrete/asphalt/brick debris (dense, moist) (Fill)
(SM) Gray silty sand with gravel, wood, glass, plastic and other detritus (loose, wet) (Fill)
(ML) Blue/gray mottled silt (very soft, wet) (Alluvium)
Moderate caving observed from 4.5 to 10 feet
Moderate groundwater seepage observed at 4.5 feet
The depths on the test pit logs are based on an average of measurements across the test pit and should be
considered accurate to 0.5 foot.
Bottom of test pit at 10.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Rubber Tracked Mini Excavator
EXCAVATION CONTRACTOR Paulman GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 4/27/18 COMPLETED 4/27/18
AT TIME OF EXCAVATION 4.50 ft Moderate seepage
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
10.0
PAGE 1 OF 1
Figure A-4
TEST PIT NUMBER TP-3
CLIENT Sitts & Hill Engineers, Inc.
PROJECT NUMBER P1258-T18
PROJECT NAME Washington State Auto Dealers Association
PROJECT LOCATION 621 SW Grady Way, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 5/16/18 11:29 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1258-T18\P1258-T18 TEST PITS.GPJMigizi Group, Inc.
PO Box 44840
Tacoma, WA 98448
Telephone: 253-537-9400
Fax: 253-537-9401
U.S.C.S.GRAPHICLOGMATERIAL DESCRIPTION
APPENDIX B
KLEINFELDER EXPLORATION LOGS