HomeMy WebLinkAboutEx12_Geotechnical_Engineering_Report_2
Corporate Office
17522 Bothell Way Northeast
Bothell, Washington 98011
Phone 425.415.0551 ♦ Fax 425.415.0311
www.riley-group.com
GEOTECHNICAL ENGINEERING REPORT
PREPARED BY:
THE RILEY GROUP, INC.
17522 BOTHELL WAY NORTHEAST
BOTHELL, WASHINGTON 98011
PREPARED FOR:
ANITA WOO
12906 NORTHEAST 25TH PLACE
BELLEVUE, WASHINGTON 98004
RGI PROJECT NO. 2021-123
JEFFERSON TOWNHOMES
1518 JEFFERSON AVENUE NORTHEAST
RENTON, WASHINGTON
SEPTEMBER 1, 2021
Exhibit 12
RECEIVED
Clark Close 10/27/2021
PLANNING DIVISION
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Corporate Office
17522 Bothell Way Northeast
Bothell, Washington 98011
Phone 425.415.0551 ♦ Fax 425.415.0311
www.riley-group.com
September 1, 2021
Anita Woo
12906 Northeast 25th Place
Bellevue, Washington 98004
Subject: Geotechnical Engineering Report
Jefferson Townhomes
1518 Jefferson Avenue Northeast
Renton, Washington
RGI Project No. 2021-123
Dear Ms. Woo:
As requested, The Riley Group, Inc. (RGI) has performed a Geotechnical Engineering Report
(GER) for the Jefferson Townhomes located at 1518 Jefferson Avenue Northeast, Renton,
Washington. Our services were completed in accordance with our proposal dated February
23rd, 2021 and authorized by you on July 9, 2021. The information in this GER is based on
our understanding of the proposed construction, and the soil and groundwater conditions
encountered in the test pits completed by RGI at the site on August 2nd, 2021.
RGI recommends that you submit the project plans and specifications to RGI for a general
review so that we may confirm that the recommendations in this GER are interpreted and
implemented properly in the construction documents. RGI also recommends that a
representative of our firm be present on site during portions of the project construction to
confirm that the soil and groundwater conditions are consistent with those that form the
basis for the engineering recommendations in this GER.
If you have any questions or require additional information, please contact us.
Respectfully submitted,
THE RILEY GROUP, INC.
Eric L. Woods, LG Kristina M. Weller, PE
Project Geologist Principal Geotechnical Engineer
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report i September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
TABLE OF CONTENTS
1.0 INTRODUCTION ............................................................................................................................... 1
2.0 PROJECT DESCRIPTION ............................................................................................................... 1
3.0 FIELD EXPLORATION AND LABORATORY TESTING .......................................................... 1
3.1 FIELD EXPLORATION ................................................................................................................................... 1
3.2 LABORATORY TESTING ................................................................................................................................ 2
4.0 SITE CONDITIONS ........................................................................................................................... 2
4.1 SURFACE .................................................................................................................................................. 2
4.2 GEOLOGY ................................................................................................................................................. 2
4.3 SOILS ....................................................................................................................................................... 2
4.4 GROUNDWATER ........................................................................................................................................ 3
4.5 SEISMIC CONSIDERATIONS ........................................................................................................................... 3
4.6 GEOLOGIC HAZARD AREAS .......................................................................................................................... 4
5.0 DISCUSSION AND RECOMMENDATIONS ................................................................................. 4
5.1 GEOTECHNICAL CONSIDERATIONS ................................................................................................................. 4
5.2 EARTHWORK ............................................................................................................................................. 4
5.2.1 Erosion and Sediment Control ..................................................................................................... 4
5.2.2 Stripping and Subgrade Preparation ............................................................................................ 5
5.2.3 Excavations................................................................................................................................... 6
5.2.4 Structural Fill ................................................................................................................................ 6
5.2.5 Cut and Fill Slopes ........................................................................................................................ 7
5.2.6 Wet Weather Construction Considerations ................................................................................. 8
5.3 FOUNDATIONS .......................................................................................................................................... 8
5.4 RETAINING WALLS ..................................................................................................................................... 9
5.5 SLAB-ON-GRADE CONSTRUCTION ............................................................................................................... 10
5.6 DRAINAGE .............................................................................................................................................. 10
5.6.1 Surface ....................................................................................................................................... 10
5.6.2 Subsurface .................................................................................................................................. 11
5.6.3 Infiltration .................................................................................................................................. 11
6.0 ADDITIONAL SERVICES .............................................................................................................. 11
7.0 LIMITATIONS ................................................................................................................................. 12
LIST OF FIGURES AND APPENDICES
Figure 1 ..................................................................................................................... Site Vicinity Map
Figure 2 ............................................................................................... Geotechnical Exploration Plan
Figure 3 ............................................................................................... Retaining Wall Drainage Detail
Figure 4 ....................................................................................................Typical Footing Drain Detail
Appendix A .......................................................................... Field Exploration and Laboratory Testing
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report ii September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
Executive Summary
This Executive Summary should be used in conjunction with the entire Geotechnical
Engineering Report (GER) for design and/or construction purposes. It should be recognized
that specific details were not included or fully developed in this section, and the GER must
be read in its entirety for a comprehensive understanding of the items contained herein.
Section 7.0 should be read for an understanding of limitations.
RGI’s geotechnical scope of work included the advancement of five test pits to approximate
depths of 7 feet below existing site grades.
Based on the information obtained from our subsurface exploration, the site is suitable for
development of the proposed project. The following geotechnical considerations were
identified:
Soil Conditions: The soils encountered during field exploration include up to 6.5 feet of
loose fill comprised of silty sand with some gravel and sand with some gravel and silt over
loose to medium dense silty sand with some gravel, gravelly sand with some silt, and sand
with some silt and gravel.
Groundwater: No groundwater seepage was encountered during our subsurface
exploration.
Foundations: Foundations for the proposed building may be supported on conventional
spread footings bearing on medium dense native soil or structural fill.
Slab-on-grade: Slab-on-grade floors and slabs for the proposed building can be supported
on medium dense native soil or structural fill.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 1 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
1.0 Introduction
This Geotechnical Engineering Report (GER) presents the results of the geotechnical
engineering services provided for the Jefferson Townhomes in Renton, Washington. The
purpose of this evaluation is to assess subsurface conditions and provide geotechnical
recommendations for the construction of a townhome development. Our scope of services
included field explorations, laboratory testing, engineering analyses, and preparation of
this GER.
The recommendations in the following sections of this GER are based upon our current
understanding of the proposed site development as outlined below. If actual features vary
or changes are made, RGI should review them in order to modify our recommendations as
required. In addition, RGI requests to review the site grading plan, final design drawings
and specifications when available to verify that our project understanding is correct and
that our recommendations have been properly interpreted and incorporated into the
project design and construction.
2.0 Project description
The project site is located at 1518 Jefferson Avenue Northeast in Renton, Washington. The
approximate location of the site is shown on Figure 1.
The site is currently occupied by a single-family residence. RGI understands that the existing
structure will be demolished and townhomes will be constructed on the site.
At the time of preparing this GER, building plans were not available for our review. Based
on our experience with similar construction, RGI anticipates that the proposed building will
be supported on perimeter walls with bearing loads of two to six kips per linear foot, and a
series of columns with a maximum load up to 30 kips. Slab-on-grade floor loading of 250
pounds per square foot (psf) are expected.
3.0 Field Exploration and Laboratory Testing
3.1 FIELD EXPLORATION
On August 2nd, 2021, RGI observed the excavation of five test pits. The approximate
exploration locations are shown on Figure 2.
Field logs of each exploration were prepared by the geotechnical technician that
continuously observed the excavation. These logs included visual classifications of the
materials encountered during excavation as well as our interpretation of the subsurface
conditions between samples. The test pit logs included in Appendix A represent an
interpretation of the field logs and include modifications based on laboratory observation
and analysis of the samples.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 2 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
3.2 LABORATORY TESTING
During the field exploration, a representative portion of each recovered sample was sealed
in containers and transported to our laboratory for further visual and laboratory
examination. Selected samples retrieved from the test pits were tested for moisture
content and grain size analysis to aid in soil classification and provide input for the
recommendations provided in this GER. The results and descriptions of the laboratory tests
are enclosed in Appendix A.
4.0 Site Conditions
4.1 SURFACE
The subject site is a rectangular-shaped parcel of land approximately 0.22 acres in size. The
site is bound to the north, east and south by single-family residences, and to the west by
Jefferson Avenue NE.
The existing site is occupied by a single-family residence. The site slopes generally to the
east with an overall elevation difference of approximately 8 feet. The site is vegetated with
grass, decorative plants and shrubs, and several medium- to large-diameter trees.
4.2 GEOLOGY
Review of the Geologic Map of Surficial Deposits in the Seattle 30’ by 60’ Quadrangle,
Washington, by James C. Yount, etc. (1993) indicates that the soil in the project vicinity is
mapped as Vashon till (Qt), which is light to dark gray, nonsorted, nonstratified mixture of
clay, silt, sand, and gravel. However soil encountered at the site matches the description
for Vashon recessional outwash deposit (Qvr) found mapped to the west of the site, which
is poorly to moderately sorted stratified gravel and sand with some silt and clay.
4.3 SOILS
The soils encountered during field exploration include up to 6.5 feet of loose fill comprised
of silty sand with some gravel and sand with some gravel and silt over loose to medium
dense silty sand with some gravel, gravelly sand with some silt, and sand with some silt and
gravel.
More detailed descriptions of the subsurface conditions encountered are presented in the
Test pits included in Appendix A. Sieve analysis was performed on four selected soil
samples. Grain size distribution curves are included in Appendix A.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 3 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
4.4 GROUNDWATER
No groundwater seepage was encountered during our subsurface exploration
It should be recognized that fluctuations of the groundwater table will occur due to
seasonal variations in the amount of rainfall, runoff, and other factors not evident at the
time the explorations were performed. In addition, perched water can develop within
seams and layers contained in fill soils or higher permeability soils overlying less permeable
soils following periods of heavy or prolonged precipitation. Therefore, groundwater levels
during construction or at other times in the future may be higher or lower than the levels
indicated on the logs. Groundwater level fluctuations should be considered when
developing the design and construction plans for the project.
4.5 SEISMIC CONSIDERATIONS
Based on the International Building Code (IBC), RGI recommends the follow seismic
parameters for design.
Table 1 IBC
Parameter 2018 Value
Site Soil Class1 D2
Site Latitude 47.506039 N
Site Longitude -122.179538 W
Short Period Spectral Response Acceleration, SS (g) 1.426
1-Second Period Spectral Response Acceleration, S1 (g) 0.489
Adjusted Short Period Spectral Response Acceleration, SMS (g) 1.426
Adjusted 1-Sec Period Spectral Response Acceleration, SM1 (g) 0.8853
Numeric seismic design value at 0.2 second; SDS(g) 0.951
Numeric seismic design value at 1.0 second; SD1(g) 0.593
1. Note: In general accordance with Chapter 20 of ASCE 7-16. The Site Class is based on the average characteristics of the upper 100 feet
of the subsurface profile.
2. Note: ASCE 7-16 require a site soil profile determination extending to a depth of 100 feet for seismic site classification. The current
scope of our services does not include the required 100 foot soil profile determination. Test pits extended to a maximum depth of 7 feet,
and this seismic site class definition considers that similar soil continues below the maximum depth of the subsurface exploration.
Additional exploration to deeper depths would be required to confirm the conditions below the current depth of exploration.
3. Note: In accordance with ASCE 11.4.8, a ground motion hazard analysis is not required for the following cases:
• Structures on Site Class E sites with SS greater than or equal to 1.0, provided the site coefficient Fa is taken as equal to that of
Site Class C.
• Structures on Site Class D sites with S1 greater than or equal to 0.2, provided that the value of the seismic response coefficient
Cs is determined by Eq. 12.8-2 for values of T ≤ 1.5Ts and taken as equal to 1.5 times the value computed in accordance with
either Eq. 12.8-3 for TL ≥ T > 1.5Ts or Eq. 12.8-4 for T > TL.
• Structures on Site Class E sites with S1 greater than or equal to 0.2, provided that T is less than or equal to Ts and the equivalent
static force procedure is used for design.
The above exceptions do not apply to seismically isolated structures, structures with damping systems or structures designed using the
response history procedures of Chapter 16.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 4 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength
due to an increase in water pressure induced by vibrations from a seismic event.
Liquefaction mainly affects geologically recent deposits of fine-grained sands that are
below the groundwater table. Soils of this nature derive their strength from intergranular
friction. The generated water pressure or pore pressure essentially separates the soil grains
and eliminates this intergranular friction, thus reducing or eliminating the soil’s strength.
RGI reviewed the results of the field and laboratory testing and assessed the potential for
liquefaction of the site’s soil during an earthquake. Since the site is underlain by medium
dense soils and lacks an established shallow groundwater table, RGI considers that the
possibility of liquefaction during an earthquake is low.
4.6 GEOLOGIC HAZARD AREAS
Regulated geologically hazardous areas include erosion, landslide, earthquake, or other
geological hazards. Based on the definition in the Renton Municipal Code, the site does not
contain geologically hazardous areas.
5.0 Discussion and Recommendations
5.1 GEOTECHNICAL CONSIDERATIONS
Based on our study, the site is suitable for the proposed construction from a geotechnical
standpoint. Foundations for the proposed building can be supported on conventional
spread footings bearing on competent native soil or structural fill. Slab-on-grade floors can
be similarly supported.
Detailed recommendations regarding the above issues and other geotechnical design
considerations are provided in the following sections. These recommendations should be
incorporated into the final design drawings and construction specifications.
5.2 EARTHWORK
The earthwork is expected to include excavating and backfilling the building foundations
and preparing slab subgrades.
5.2.1 EROSION AND SEDIMENT CONTROL
Potential sources or causes of erosion and sedimentation depend on construction
methods, slope length and gradient, amount of soil exposed and/or disturbed, soil type,
construction sequencing and weather. The impacts on erosion-prone areas can be reduced
by implementing an erosion and sedimentation control plan. The plan should be designed
in accordance with applicable city and/or county standards.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 5 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
RGI recommends the following erosion control Best Management Practices (BMPs):
Scheduling site preparation and grading for the drier summer and early fall months
and undertaking activities that expose soil during periods of little or no rainfall
Retaining existing vegetation whenever feasible
Establishing a quarry spall construction entrance
Installing siltation control fencing or anchored straw or coir wattles on the downhill
side of work areas
Covering soil stockpiles with anchored plastic sheeting
Revegetating or mulching exposed soils with a minimum 3-inch thickness of straw
if surfaces will be left undisturbed for more than one day during wet weather or
one week in dry weather
Directing runoff away from exposed soils and slopes
Minimizing the length and steepness of slopes with exposed soils and cover
excavation surfaces with anchored plastic sheeting
Decreasing runoff velocities with check dams, straw bales or coir wattles
Confining sediment to the project site
Inspecting and maintaining erosion and sediment control measures frequently (The
contractor should be aware that inspection and maintenance of erosion control
BMPs is critical toward their satisfactory performance. Repair and/or replacement
of dysfunctional erosion control elements should be anticipated.)
Permanent erosion protection should be provided by reestablishing vegetation using
hydroseeding and/or landscape planting. Until the permanent erosion protection is
established, site monitoring should be performed by qualified personnel to evaluate the
effectiveness of the erosion control measures. Provisions for modifications to the erosion
control system based on monitoring observations should be included in the erosion and
sedimentation control plan.
5.2.2 STRIPPING AND SUBGRADE PREPARATION
Stripping efforts should include removal of pavements, vegetation, organic materials, and
deleterious debris from areas slated for building, pavement, and utility construction. The
Test pits encountered 6 to 8 inches of topsoil and rootmass. Deeper areas of stripping may
be required in heavily vegetated areas of the site.
Subgrade soils that become disturbed due to elevated moisture conditions should be
overexcavated to reveal firm, non-yielding, non-organic soils and backfilled with
compacted structural fill. In order to maximize utilization of site soils as structural fill, RGI
recommends that the earthwork portion of this project be completed during extended
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 6 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
periods of warm and dry weather if possible. If earthwork is completed during the wet
season (typically November through May) it will be necessary to take extra precautionary
measures to protect subgrade soils. Wet season earthwork will require additional
mitigative measures beyond that which would be expected during the drier summer and
fall months.
5.2.3 EXCAVATIONS
All temporary cut slopes associated with the site and utility excavations should be
adequately inclined to prevent sloughing and collapse. The site soils consist of medium
dense silty sand with some gravel, gravelly sand with some silt, and sand with some gravel
and silt.
Accordingly, for excavations more than 4 feet but less than 20 feet in depth, the temporary
side slopes should be laid back with a minimum slope inclination of 1.5H:1V
(Horizontal:Vertical). If there is insufficient room to complete the excavations in this
manner, or excavations greater than 20 feet in depth are planned, using temporary shoring
to support the excavations should be considered. For open cuts at the site, RGI
recommends:
No traffic, construction equipment, stockpiles or building supplies are allowed at
the top of cut slopes within a distance of at least five feet from the top of the cut
Exposed soil along the slope is protected from surface erosion using waterproof
tarps and/or plastic sheeting
Construction activities are scheduled so that the length of time the temporary cut
is left open is minimized
Surface water is diverted away from the excavation
The general condition of slopes should be observed periodically by a geotechnical
engineer to confirm adequate stability and erosion control measures
In all cases, however, appropriate inclinations will depend on the actual soil and
groundwater conditions encountered during earthwork. Ultimately, the site contractor
must be responsible for maintaining safe excavation slopes that comply with applicable
OSHA or WISHA guidelines.
5.2.4 STRUCTURAL FILL
RGI recommends fill below the foundation and floor slab, behind retaining walls, and below
pavement and hardscape surfaces be placed in accordance with the following
recommendations for structural fill. The structural fill should be placed after completion of
site preparation procedures as described above.
The suitability of excavated site soils and import soils for compacted structural fill use will
depend on the gradation and moisture content of the soil when it is placed. As the amount
of fines (that portion passing the U.S. No. 200 sieve) increases, soil becomes increasingly
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 7 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
sensitive to small changes in moisture content and adequate compaction becomes more
difficult or impossible to achieve. Soils containing more than about 5 percent fines cannot
be consistently compacted to a dense, non-yielding condition when the moisture content
is more than 2 percent above or below optimum. Optimum moisture content is that
moisture that results in the greatest compacted dry density with a specified compactive
effort.
Non-organic site soils are only considered suitable for structural fill provided that their
moisture content is within about two percent of the optimum moisture level as determined
by American Society of Testing and Materials D1557-09 Standard Test Methods for
Laboratory Compaction Characteristics of Soil Using Modified Effort (ASTM D1557).
Excavated site soils may not be suitable for re-use as structural fill depending on the
moisture content and weather conditions at the time of construction. If soils are stockpiled
for future reuse and wet weather is anticipated, the stockpile should be protected with
plastic sheeting that is securely anchored. Even during dry weather, moisture conditioning
(such as, windrowing and drying) of site soils to be reused as structural fill may be required.
The site soils are moisture sensitive and may require moisture conditioning prior to use as
structural fill. If on-site soils are or become unusable, it may become necessary to import
clean, granular soils to complete site work that meet the grading requirements listed in
Table 2 to be used as structural fill.
Table 2 Structural Fill Gradation
U.S. Sieve Size Percent Passing
4 inches 100
No. 4 sieve 22 to 100
No. 200 sieve 0 to 5*
*Based on minus 3/4 inch fraction.
Prior to use, an RGI representative should observe and test all materials imported to the
site for use as structural fill. Structural fill materials should be placed in uniform loose layers
not exceeding 12 inches and compacted to 95 percent of the maximum dry density. The
soil’s maximum density and optimum moisture should be determined by ASTM D1557.
Placement and compaction of structural fill should be observed by RGI.
5.2.5 CUT AND FILL SLOPES
All permanent cut and fill slopes should be graded with a finished inclination no greater
than 2H:1V. Upon completion of construction, the slope face should be trackwalked,
compacted and vegetated, or provided with other physical means to guard against erosion.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 8 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
All fill placed for slope construction should meet the structural fill requirements as
described this section.
Final grades at the top of the slopes must promote surface drainage away from the slope
crest. Water must not be allowed to flow in an uncontrolled fashion over the slope face. If
it is necessary to direct surface runoff towards the slope, it should be controlled at the top
of the slope, piped in a closed conduit installed on the slope face, and taken to an
appropriate point of discharge beyond the toe of the slope.
5.2.6 WET WEATHER CONSTRUCTION CONSIDERATIONS
RGI recommends that preparation for site grading and construction include procedures
intended to drain ponded water, control surface water runoff, and to collect shallow
subsurface seepage zones in excavations where encountered. It will not be possible to
successfully compact the subgrade or utilize on-site soils as structural fill if accumulated
water is not drained prior to grading or if drainage is not controlled during construction.
Attempting to grade the site without adequate drainage control measures will reduce the
amount of on-site soil effectively available for use, increase the amount of select import fill
materials required, and ultimately increase the cost of the earthwork phases of the project.
Free water should not be allowed to pond on the subgrade soils. RGI anticipates that the
use of berms and shallow drainage ditches, with sumps and pumps in utility trenches, will
be required for surface water control during wet weather and/or wet site conditions.
5.3 FOUNDATIONS
Following site preparation and grading, the proposed building foundation can be supported
on conventional spread footings bearing on competent native soil or structural fill. Loose,
organic, or other unsuitable soils may be encountered in the proposed building footprint.
If unsuitable soils are encountered, they should be overexcavated and backfilled with
structural fill. If loose soils are encountered, the soils should be moisture conditioned and
compacted to a firm and unyielding condition. The foundation design value assumes the
foundation is supported on at least two feet of medium dense native soil or structural fill.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 9 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
Table 3 Foundation Design
Design Parameter Value
Allowable Bearing Capacity 2,000 psf1
Friction Coefficient 0.30
Passive pressure (equivalent fluid pressure) 250 pcf2
Minimum foundation dimensions Columns: 24 inches
Walls: 16 inches
1. psf = pounds per square foot
2. pcf = pounds per cubic foot
The allowable foundation bearing pressures apply to dead loads plus design live load
conditions. For short-term loads, such as wind and seismic, a 1/3 increase in this allowable
capacity may be used. At perimeter locations, RGI recommends not including the upper 12
inches of soil in the computation of passive pressures because they can be affected by
weather or disturbed by future grading activity. The passive pressure value assumes the
foundation will be constructed neat against competent soil or backfilled with structural fill
as described in Section 5.2.4. The recommended base friction and passive resistance value
includes a safety factor of about 1.5.
Perimeter foundations exposed to weather should be at a minimum depth of 18 inches
below final exterior grades. Interior foundations can be constructed at any convenient
depth below the floor slab. Finished grade is defined as the lowest adjacent grade within 5
feet of the foundation for perimeter (or exterior) footings and finished floor level for
interior footings.
With spread footing foundations designed in accordance with the recommendations in this
section, maximum total and differential post-construction settlements of 1 inch and 1/2
inch, respectively, should be expected.
5.4 RETAINING WALLS
If retaining walls are needed in the building area, RGI recommends cast-in-place concrete
walls be used. The magnitude of earth pressure development on retaining walls will partly
depend on the quality of the wall backfill. RGI recommends placing and compacting wall
backfill as structural fill. Wall drainage will be needed behind the wall face. A typical
retaining wall drainage detail is shown in Figure 3.
With wall backfill placed and compacted as recommended, and drainage properly installed,
RGI recommends using the values in the following table for design.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 10 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
Table 4 Retaining Wall Design
Design Parameter Value
Allowable Bearing Capacity 2,000 psf
Active Earth Pressure (unrestrained walls) 35 pcf
At-rest Earth Pressure (restrained walls) 50 pcf
For seismic design, an additional uniform load of 7 times the wall height (H) for
unrestrained walls and 14H in psf for restrained walls should be applied to the wall surface.
Friction at the base of foundations and passive earth pressure will provide resistance to
these lateral loads. Values for these parameters are provided in Section 5.3.
5.5 SLAB-ON-GRADE CONSTRUCTION
Once site preparation has been completed as described in Section 5.2, suitable support for
slab-on-grade construction should be provided. RGI recommends that the concrete slab be
placed on top of medium dense native soil or structural fill. Immediately below the floor
slab, RGI recommends placing a four-inch thick capillary break layer of clean, free-draining
sand or gravel that has less than five percent passing the U.S. No. 200 sieve. This material
will reduce the potential for upward capillary movement of water through the underlying
soil and subsequent wetting of the floor slab. Where moisture by vapor transmission is
undesirable, an 8- to 10-millimeter thick plastic membrane should be placed on a 4-inch
thick layer of clean gravel.
For the anticipated floor slab loading, we estimate post-construction floor settlements of
1/4- to 1/2-inch.
5.6 DRAINAGE
5.6.1 SURFACE
Final exterior grades should promote free and positive drainage away from the building
area. Water must not be allowed to pond or collect adjacent to foundations or within the
immediate building area. For non-pavement locations, RGI recommends providing a
minimum drainage gradient of 3 percent for a minimum distance of 10 feet from the
building perimeter. In paved locations, a minimum gradient of 1 percent should be
provided unless provisions are included for collection and disposal of surface water
adjacent to the structure.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 11 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
5.6.2 SUBSURFACE
RGI recommends installing perimeter foundation drains. A typical footing drain detail is
shown on Figure 4. The foundation drains and roof downspouts should be tightlined
separately to an approved discharge facility. Subsurface drains must be laid with a gradient
sufficient to promote positive flow to a controlled point of approved discharge.
5.6.3 INFILTRATION
RGI evaluated the potential to infiltrate stormwater runoff at the site. An infiltration test
was completed in test pit TP-5 at a depth of approximately 5 feet below existing grade in
general accordance with a Small-Scale Pilot Infiltration Test described in the 2017 City of
Renton Surface Water Design Manual (CRSWDM).
Table 5 Infiltration Rates
Test Location Test Depth
Field Measured
Rate
(Inches per hour)
Design Rate
(Inches per hour)
TP-5 5 4.08 1.6
CRSWDM correction factors were applied to the field measured rate of 4.08 inches per
hour.
Idesign = Imeasured x Ftesting x Fplugging x Fgeometry
Correction factors of 0.5 (Ftesting) for the PIT test method and 0.8 (Fplugging) for loamy sand
loams were applied to the field measured rate to estimate the long-term design
infiltration rate. We assumed Fgeometry = 1. The application of the correction factors
yield a long-term design rate (Idesign) of 1.6 inches per hour. This rate can be used to
design a site specific infiltration element.
6.0 Additional Services
RGI is available to provide further geotechnical consultation throughout the design phase
of the project. RGI should review the final design and specifications in order to verify that
earthwork and foundation recommendations have been properly interpreted and
incorporated into project design and construction.
RGI is also available to provide geotechnical engineering and construction monitoring
services during construction. The integrity of the earthwork and construction depends on
proper site preparation and procedures. In addition, engineering decisions may arise in the
field in the event that variations in subsurface conditions become apparent. Construction
monitoring services are not part of this scope of work. If these services are desired, please
let us know and we will prepare a cost proposal.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report 12 September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
7.0 Limitations
This GER is the property of RGI, Anita Woo, and its designated agents. Within the limits of
the scope and budget, this GER was prepared in accordance with generally accepted
geotechnical engineering practices in the area at the time this GER was issued. This GER is
intended for specific application to the Jefferson Townhomes project in Renton,
Washington, and for the exclusive use of Anita Woo and its authorized representatives. No
other warranty, expressed or implied, is made. Site safety, excavation support, and
dewatering requirements are the responsibility of others.
The scope of services for this project does not include either specifically or by implication
any environmental or biological (for example, mold, fungi, bacteria) assessment of the site
or identification or prevention of pollutants, hazardous materials or conditions. If the
owner is concerned about the potential for such contamination or pollution, we can
provide a proposal for these services.
The analyses and recommendations presented in this GER are based upon data obtained
from the explorations performed on site. Variations in soil conditions can occur, the nature
and extent of which may not become evident until construction. If variations appear
evident, RGI should be requested to reevaluate the recommendations in this GER prior to
proceeding with construction.
It is the client’s responsibility to see that all parties to the project, including the designers,
contractors, subcontractors, are made aware of this GER in its entirety. The use of
information contained in this GER for bidding purposes should be done at the contractor’s
option and risk.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
USGS, 2020, Mercer Island, Washington
USGS, 2020, Renton, Washington
7.5-Minute Quadrangle
Approximate Scale: 1"=1000'
0 500 1000 2000 N
Site Vicinity Map
Figure 1
08/2021
Corporate Office
17522 Bothell Way Northeast
Bothell, Washington 98011
Phone: 425.415.0551
Fax: 425.415.0311
Jefferson Townhomes
RGI Project Number:
2021-123-1
Date Drawn:
Address: 1518 Jefferson Avenue Northeast, Renton, Washington 98056
SITE
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
TP-1TP-2TP-3TP-4TP-508/2021Corporate Office17522 Bothell Way NortheastBothell, Washington 98011Phone: 425.415.0551Fax: 425.415.0311Jefferson TownhomesRGI Project Number:2021-123-1Date Drawn:Address: 1518 Jefferson Avenue Northeast, Renton, Washington 98056Figure 2Approximate Scale: 1"=30'0153060N= Test pit by RGI, 08/02/21= Site boundaryGeotechnical Exploration PlanDocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Incliniations)
12" Over the Pipe
3" Below the Pipe
Perforated Pipe
4" Diameter PVC
Compacted Structural
Backfill (Native or Import)
12" min.
Filter Fabric Material
12" Minimum Wide
Free-Draining Gravel
Slope to Drain
(See Report for
Appropriate
Excavated Slope
08/2021
Corporate Office
17522 Bothell Way Northeast
Bothell, Washington 98011
Phone: 425.415.0551
Fax: 425.415.0311
Jefferson Townhomes
RGI Project Number:
2021-123-1
Date Drawn:
Address: 1518 Jefferson Avenue Northeast, Renton, Washington 98056
Retaining Wall Drainage Detail
Figure 3
Not to Scale
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
3/4" Washed Rock or Pea Gravel
4" Perforated Pipe
Building Slab
Structural
Backfill
Compacted
Filter Fabric
08/2021
Corporate Office
17522 Bothell Way Northeast
Bothell, Washington 98011
Phone: 425.415.0551
Fax: 425.415.0311
Jefferson Townhomes
RGI Project Number:
2021-123-1
Date Drawn:
Address: 1518 Jefferson Avenue Northeast, Renton, Washington 98056
Typical Footing Drain Detail
Figure 4
Not to Scale
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
Geotechnical Engineering Report September 1, 2021
Jefferson Townhomes, Renton, Washington RGI Project No. 2021-123
APPENDIX A
FIELD EXPLORATION AND LABORATORY TESTING
On August 2nd, 2021, RGI performed field explorations using a backhoe. We explored
subsurface soil conditions at the site by observing the excavation of five test pits to a
maximum depth of 7 feet below existing grade. The test pit locations are shown on Figure
2. The test pit locations were approximately determined by measurements from existing
property lines and paved roads.
A geologist from our office conducted the field exploration and classified the soil conditions
encountered, maintained a log of each test exploration, obtained representative soil
samples, and observed pertinent site features. All soil samples were visually classified in
accordance with the Unified Soil Classification System (USCS).
Representative soil samples obtained from the explorations were placed in closed
containers and taken to our laboratory for further examination and testing. As a part of the
laboratory testing program, the soil samples were classified in our in house laboratory
based on visual observation, texture, plasticity, and the limited laboratory testing described
below.
Moisture Content Determinations
Moisture content determinations were performed in accordance with ASTM D2216-10
Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil
and Rock by Mass (ASTM D2216) on representative samples obtained from the exploration
in order to aid in identification and correlation of soil types. The moisture content of typical
sample was measured and is reported on the test pit logs.
Grain Size Analysis
A grain size analysis indicates the range in diameter of soil particles included in a particular
sample. Grain size analyses was determined using D6913-04(2009) Standard Test Methods
for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis (ASTM D6913) on four
of the samples.
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
THE RILEY GROUP, INC.
17522 Bothell Way NE
Bothell, WA 98011
PHONE: (425) 415-0551
FAX: (425) 415-0311
GRAIN SIZE ANALYSIS
ASTM D421, D422, D1140, D2487, D6913
PROJECT TITLE Jefferson Townhomes SAMPLE ID/TYPE TP-1
PROJECT NO.2021-123-1 SAMPLE DEPTH 1.5 feet
TECH/TEST DATE LB 8/3/2021 DATE RECEIVED 8/3/2021
WATER CONTENT (Delivered Moisture) Total Weight Of Sample Used For Sieve Corrected For Hygroscopic Moisture
Wt Wet Soil & Tare (gm) (w1)735.6 Weight Of Sample (gm)704.2
Wt Dry Soil & Tare (gm) (w2)704.2 Tare Weight (gm) 15.7
Weight of Tare (gm) (w3)15.7 (W6) Total Dry Weight (gm) 688.5
Weight of Water (gm) (w4=w1-w2) 31.4 SIEVE ANALYSIS
Weight of Dry Soil (gm) (w5=w2-w3) 688.5 Cumulative
Moisture Content (%) (w4/w5)*100 5 Wt Ret (Wt-Tare) (%Retained)% PASS
+Tare {(wt ret/w6)*100}(100-%ret)
% COBBLES 0.0 12.0"15.7 0.00 0.00 100.00 cobbles
% C GRAVEL 26.6 3.0"15.7 0.00 0.00 100.00 coarse gravel
% F GRAVEL 17.7 2.5" coarse gravel
% C SAND 6.8 2.0" coarse gravel
% M SAND 16.9 1.5"15.7 0.00 0.00 100.00 coarse gravel
% F SAND 21.6 1.0" coarse gravel
% FINES 10.3 0.75"198.8 183.10 26.59 73.41 fine gravel
% TOTAL 100.0 0.50" fine gravel
0.375"273.4 257.70 37.43 62.57 fine gravel
D10 (mm)0.075 #4 321.0 305.30 44.34 55.66 coarse sand
D30 (mm)0.38 #10 368.1 352.40 51.18 48.82 medium sand
D60 (mm)7.5 #20 medium sand
Cu 100.0 #40 484.8 469.10 68.13 31.87 fine sand
Cc 0.3 #60 fine sand
#100 607.0 591.30 85.88 14.12 fine sand
#200 633.2 617.50 89.69 10.31 fines
PAN 704.2 688.50 100.00 0.00 silt/clay
DESCRIPTION Gravelly SAND with some silt
USCS SP-SM
Prepared For:Reviewed By:
Anita Woo ELW
0
10
20
30
40
50
60
70
80
90
100
0.0010.010.11101001000
%
P
A
S
S
I
N
G
Grain size in millimeters
12"3" 2" 1".75" .375" #4 #10 #20 #40 #60 #100 #200
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
THE RILEY GROUP, INC.
17522 Bothell Way NE
Bothell, WA 98011
PHONE: (425) 415-0551
FAX: (425) 415-0311
GRAIN SIZE ANALYSIS
ASTM D421, D422, D1140, D2487, D6913
PROJECT TITLE Jefferson Townhomes SAMPLE ID/TYPE TP-1
PROJECT NO.2021-123-1 SAMPLE DEPTH 5.5 feet
TECH/TEST DATE LB 8/3/2021 DATE RECEIVED 8/3/2021
WATER CONTENT (Delivered Moisture) Total Weight Of Sample Used For Sieve Corrected For Hygroscopic Moisture
Wt Wet Soil & Tare (gm) (w1)791.0 Weight Of Sample (gm)759.3
Wt Dry Soil & Tare (gm) (w2)759.3 Tare Weight (gm) 15.7
Weight of Tare (gm) (w3)15.7 (W6) Total Dry Weight (gm) 743.6
Weight of Water (gm) (w4=w1-w2) 31.7 SIEVE ANALYSIS
Weight of Dry Soil (gm) (w5=w2-w3) 743.6 Cumulative
Moisture Content (%) (w4/w5)*100 4 Wt Ret (Wt-Tare) (%Retained)% PASS
+Tare {(wt ret/w6)*100}(100-%ret)
% COBBLES 0.0 12.0"15.7 0.00 0.00 100.00 cobbles
% C GRAVEL 7.7 3.0"15.7 0.00 0.00 100.00 coarse gravel
% F GRAVEL 21.7 2.5" coarse gravel
% C SAND 9.4 2.0" coarse gravel
% M SAND 20.8 1.5"15.7 0.00 0.00 100.00 coarse gravel
% F SAND 30.6 1.0" coarse gravel
% FINES 9.9 0.75"72.6 56.90 7.65 92.35 fine gravel
% TOTAL 100.0 0.50" fine gravel
0.375"150.4 134.70 18.11 81.89 fine gravel
D10 (mm)0.075 #4 233.9 218.20 29.34 70.66 coarse sand
D30 (mm)0.29 #10 303.7 288.00 38.73 61.27 medium sand
D60 (mm)2 #20 medium sand
Cu 26.7 #40 458.1 442.40 59.49 40.51 fine sand
Cc 0.6 #60 fine sand
#100 660.2 644.50 86.67 13.33 fine sand
#200 686.0 670.30 90.14 9.86 fines
PAN 759.3 743.60 100.00 0.00 silt/clay
DESCRIPTION SAND with some gravel and silt
USCS SP-SM
Prepared For:Reviewed By:
Anita Woo ELW
0
10
20
30
40
50
60
70
80
90
100
0.0010.010.11101001000
%
P
A
S
S
I
N
G
Grain size in millimeters
12"3" 2" 1".75" .375" #4 #10 #20 #40 #60 #100 #200
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
THE RILEY GROUP, INC.
17522 Bothell Way NE
Bothell, WA 98011
PHONE: (425) 415-0551
FAX: (425) 415-0311
GRAIN SIZE ANALYSIS
ASTM D421, D422, D1140, D2487, D6913
PROJECT TITLE Jefferson Townhomes SAMPLE ID/TYPE TP-1
PROJECT NO.2021-123-1 SAMPLE DEPTH 6.5 feet
TECH/TEST DATE LB 8/3/2021 DATE RECEIVED 8/3/2021
WATER CONTENT (Delivered Moisture) Total Weight Of Sample Used For Sieve Corrected For Hygroscopic Moisture
Wt Wet Soil & Tare (gm) (w1)845.0 Weight Of Sample (gm)807.9
Wt Dry Soil & Tare (gm) (w2)807.9 Tare Weight (gm) 15.6
Weight of Tare (gm) (w3)15.6 (W6) Total Dry Weight (gm) 792.3
Weight of Water (gm) (w4=w1-w2) 37.1 SIEVE ANALYSIS
Weight of Dry Soil (gm) (w5=w2-w3) 792.3 Cumulative
Moisture Content (%) (w4/w5)*100 5 Wt Ret (Wt-Tare) (%Retained)% PASS
+Tare {(wt ret/w6)*100}(100-%ret)
% COBBLES 0.0 12.0"15.6 0.00 0.00 100.00 cobbles
% C GRAVEL 16.2 3.0"15.6 0.00 0.00 100.00 coarse gravel
% F GRAVEL 8.4 2.5" coarse gravel
% C SAND 7.4 2.0" coarse gravel
% M SAND 18.1 1.5"15.6 0.00 0.00 100.00 coarse gravel
% F SAND 40.7 1.0" coarse gravel
% FINES 9.1 0.75"144.3 128.70 16.24 83.76 fine gravel
% TOTAL 100.0 0.50" fine gravel
0.375"185.4 169.80 21.43 78.57 fine gravel
D10 (mm)0.09 #4 210.7 195.10 24.62 75.38 coarse sand
D30 (mm)0.25 #10 269.6 254.00 32.06 67.94 medium sand
D60 (mm)1.1 #20 medium sand
Cu 12.2 #40 413.4 397.80 50.21 49.79 fine sand
Cc 0.6 #60 fine sand
#100 712.9 697.30 88.01 11.99 fine sand
#200 735.8 720.20 90.90 9.10 fines
PAN 807.9 792.30 100.00 0.00 silt/clay
DESCRIPTION SAND with some gravel and silt
USCS SP-SM
Prepared For:Reviewed By:
Anita Woo ELW
0
10
20
30
40
50
60
70
80
90
100
0.0010.010.11101001000
%
P
A
S
S
I
N
G
Grain size in millimeters
12"3" 2" 1".75" .375" #4 #10 #20 #40 #60 #100 #200
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491
THE RILEY GROUP, INC.
17522 Bothell Way NE
Bothell, WA 98011
PHONE: (425) 415-0551
FAX: (425) 415-0311
GRAIN SIZE ANALYSIS
ASTM D421, D422, D1140, D2487, D6913
PROJECT TITLE Jefferson Townhomes SAMPLE ID/TYPE TP-5
PROJECT NO.2021-123-1 SAMPLE DEPTH 4.5 feet
TECH/TEST DATE LB 8/3/2021 DATE RECEIVED 8/3/2021
WATER CONTENT (Delivered Moisture) Total Weight Of Sample Used For Sieve Corrected For Hygroscopic Moisture
Wt Wet Soil & Tare (gm) (w1)787.4 Weight Of Sample (gm)731.8
Wt Dry Soil & Tare (gm) (w2)731.8 Tare Weight (gm) 15.7
Weight of Tare (gm) (w3)15.7 (W6) Total Dry Weight (gm) 716.1
Weight of Water (gm) (w4=w1-w2) 55.6 SIEVE ANALYSIS
Weight of Dry Soil (gm) (w5=w2-w3) 716.1 Cumulative
Moisture Content (%) (w4/w5)*100 8 Wt Ret (Wt-Tare) (%Retained)% PASS
+Tare {(wt ret/w6)*100}(100-%ret)
% COBBLES 0.0 12.0"15.7 0.00 0.00 100.00 cobbles
% C GRAVEL 4.1 3.0"15.7 0.00 0.00 100.00 coarse gravel
% F GRAVEL 13.4 2.5" coarse gravel
% C SAND 4.9 2.0" coarse gravel
% M SAND 14.4 1.5"15.7 0.00 0.00 100.00 coarse gravel
% F SAND 43.3 1.0" coarse gravel
% FINES 20.0 0.75"44.8 29.10 4.06 95.94 fine gravel
% TOTAL 100.0 0.50" fine gravel
0.375"96.9 81.20 11.34 88.66 fine gravel
D10 (mm)#4 140.7 125.00 17.46 82.54 coarse sand
D30 (mm)#10 175.5 159.80 22.32 77.68 medium sand
D60 (mm)#20 medium sand
Cu #40 278.5 262.80 36.70 63.30 fine sand
Cc #60 fine sand
#100 555.2 539.50 75.34 24.66 fine sand
#200 588.4 572.70 79.97 20.03 fines
PAN 731.8 716.10 100.00 0.00 silt/clay
DESCRIPTION Silty SAND with some gravel
USCS SM
Prepared For:Reviewed By:
Anita Woo ELW
0
10
20
30
40
50
60
70
80
90
100
0.0010.010.11101001000
%
P
A
S
S
I
N
G
Grain size in millimeters
12"3" 2" 1".75" .375" #4 #10 #20 #40 #60 #100 #200
DocuSign Envelope ID: E4190986-0960-4AF4-9156-D3C7AC5C2491