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Geotechnical Engineering Report
Proposed Talbot Gardens Short Plat
4827 Talbot Rd S
Renton, Washington 98055
P/N 3123059022
October 11, 2017
Revised August 30, 2018
prepared for:
Sapphire Homes
Attention: Troy Schmeil
16834 SE 43rd St
Bellevue, Washington 98006
prepared by:
Migizi Group, Inc.
PO Box 44840
Tacoma, Washington 98448
(253) 537-9400
MGI Project P1389-T18
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 ....................................................................................................................... 3
3.3 Groundwater Conditions ...................................................................................................... 4
3.4 Infiltration Conditions and Infiltration Rate ...................................................................... 4
3.5 Seismic Conditions ................................................................................................................. 6
3.6 Liquefaction Potential ............................................................................................................ 6
4.0 CONCLUSIONS AND RECOMMENDATIONS ............................................................................ 7
4.1 Site Preparation ...................................................................................................................... 8
4.2 Spread Footings .................................................................................................................... 10
4.3 Slab-On-Grade-Floors .......................................................................................................... 11
4.4 Asphalt Pavement ................................................................................................................ 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. Laboratory Test Results for Non-Organic Onsite Soils .......................................................................... 5
List of Figures
Figure 1. Topographic and Location Map
Figure 2. Site and Exploration Plan
APPENDIX A
Soil Classification Chart and Key to Test Data .................................................................................................. A-1
Logs of Test Pits TP-1 through TP-5 .......................................................................................................... A-2…A-6
MIGIZI GROUP, INC.
PO Box 44840 PHONE (253) 537-9400
Tacoma, Washington 98448 FAX (253) 537-9401
October 11, 2017
Revised August 30, 2018
Sapphire Homes
16834 SE 43rd St
Bellevue, Washington 98006
Attention: Troy Schmeil
Subject: Revised Geotechnical Engineering Report
Proposed Talbot Gardens Short Plat
4827 Talbot Rd S
Renton, Washington 98055
P/N 3123059022
MGI Project P1389-T18
Dear Mr. Schmeil:
Migizi Group, Inc. (MGI) is pleased to submit this revised report describing the results of our
geotechnical engineering evaluation of the proposed residential development in Renton,
Washington. It is our understanding that since our initial iteration of this report, the subject
property has changed hands, and that a new design scheme is being implemented. Additional soils
information is needed in order to execute the new design.
This report has been prepared for the exclusive use of Sapphire Homes, 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 a 1.67-acre, residential parcel located immediately southwest of the
intersection between S 48th St and Talbot Rd S in Renton, Washington, as shown on the enclosed
Topographic and Location Map (Figure 1). The subject property is situated in a densely populated
residential area located towards the south end of the city limits of Renton. A single-family
residence, originally constructed in 1936, and accompanying detached garage and shed building
occupy the central portion of the site, with the northeast corner of the project area containing
extensive paved surfaces for overflow parking. The remainder of the parcel is occupied by tall
grasses, and various forms of vegetation.
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Improvement plans involve the clearing/stripping of the property, demolition of the existing
structures, and development of the site for residential purposes. The proposed development will
result in approximately 20 buildable lots and contain a primary access road which travels east-west
across the length of the property, following the general course of the existing driveway. A
supplemental alleyway will also be introduced as part of the proposed development, traveling
south from S 48th Street, spanning the full length of the property along this orientation. The
southwest corner of the subject property will be utilized as an Open Space / Tree Retention Tract,
and will also house stormwater retention facilities, if feasible.
2.0 EXPLORATORY METHODS
We explored surface and subsurface conditions at the project site on September 5, 2017 and July 24,
2018. Our exploration and evaluation program comprised the following elements:
• Surface reconnaissance of the site;
• Five test pit explorations (designated TP-1 through TP-5) advanced on September 5,
2017 and July 24, 2018;
• Two grain-size analyses performed on soil samples collected from our subsurface
explorations; and
• A review of published geologic and seismologic maps and literature.
Table 1 summarizes the approximate functional locations and termination depths of our subsurface
explorations, and Figure 2 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
TP-4
TP-5
Northwest corner of the project area
West of the existing residence
Southeast corner of the project area
Southwest corner of the project area, in proposed stormwater retention area
Immediately northeast of test pit exploration TP-4
10
10
10
15
15
The specific number 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
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.
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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. An engineering 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 our exploration logs. Summary logs of our
explorations are included as Figures A-2 through A-6.
3.0 SITE CONDITIONS
The following sections present our observations, measurements, findings, and interpretations
regarding surface, soil, groundwater, and infiltration conditions.
3.1 Surface Conditions
As previously indicated, the project site consists of a fully developed, 1.67-acre residential parcel
located immediately southwest of the intersection between S 48th St and Talbot Rd S in Renton,
Washington. The project area is roughly square-shaped, spanning approximately 270 feet along its
primary boundaries. The central portion of the site is occupied by an existing single-family
residence and accompanying detached garage and shed building. Expanded asphalt pavements
occupy the northeast corner of the subject property. The primary driveway enters the site from the
east along Talbot Rd S, looping to the north to access the aforementioned expanded parking area.
Vegetation consists primarily of lawn grass immediately east and west of the existing structures. A
thick growth of alder and fir trees are located towards the southern and western margins of the
project area, containing a dense understory of fern, blackberry bushes, and other brush.
Topographically, the subject property is gently sloped, generally descending from east to west at
gradients of less than 10 percent; with a total elevation change of ± 25 feet being observed over the
extent of the parcel. The subject parcel is situated along the eastern valley wall of the Duwamish
Valley.
No hydrologic features were observed on site, such as seeps, springs, ponds and streams.
3.2 Soil Conditions
Our test pit explorations revealed relatively consistent subgrade conditions across the project area,
generally consisting of a surface mantle of sod/topsoil, underlain by native, Vashon-aged glacial
soils.
Renton, and the larger Puget Sound area in general, has been glaciated a number of times over the
last 2.4 million years. The most recent of these glacial events, the Vashon Stade of the Fraser
Sapphire Homes – Talbot Gardens Short Plat, Renton, WA October 11, 2017 / Revised August 30, 2018
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Glaciation, receded from this region approximately 13,500 years ago. The majority of near surface
soils encountered within the Renton area are either directly associated with, or have been physically
altered by the Vashon glacial event. Glacial till is typically described as being a compact, coherent
mixture of gravel, silt, clay and sand-sized clasts deposited along the base of glacial ice during a
period of localized advancement. This material is generally encountered in a compact relative
consistency given the fact that it was overridden by the ice mass shortly after deposition, and is
commonly underlain by advance outwash soils. Advance outwash is resultant of pro-glacial rivers
and streams which carried sediment ahead of the advancing ice mass.
In general, our test pit explorations encountered glacial till soils within 12 inches of existing grade.
This material was continuous through a depth of approximately 8 feet below existing grade, being
comprised of silty sand with some gravel, to gravelly silty sand. As encountered on site, glacial till
soils were highly weathered and heavily mottled; indicating poor surface drainage. From a depth
of 8 feet to 10 feet below existing grade, the termination depth of three of our subsurface
explorations, we encountered advance outwash deposits. Advance outwash is typically comprised
of densely consolidated, relatively clean, sands and gravel. However, on site, this soil group was
largely comprised of alternating lenses of fine sand and silty sand; with an average relative fines
content (percent silt and clay) between 25 to 30 percent. This soil group was found to extend to a
depth of ± 13 feet below existing grade; transitioning to a more homogenous, relatively impervious,
dense, silty sand with some gravel.
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 Qvt, or Vashon-aged glacial till. Our subsurface explorations generally
correspond with the geologic mapping prepared by the USGS.
The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata
encountered in our subsurface explorations.
3.3 Groundwater Conditions
We did not encounter groundwater in any of our subsurface explorations, which extended to a
depth upwards of 15 feet below existing grade. Given the fact that our explorations were
performed outside of what is generally considered the rainy season (November 1 to March 31),
groundwater could rise higher than that which we observed. However, given the
geologic/topographic setting of the project area, we do not anticipate that groundwater will rise
high enough to adversely affect the proposed development. Seasonally perched groundwater will
likely be encountered on site at relatively shallow depths during periods of extended precipitation,
given the presence of low permeability till soils along the subsurface.
3.4 Infiltration Conditions and Infiltration Rate
As indicated in the Soil Conditions section of the report, the site is underlain by low permeability
glacial till soils at shallow depths, extending upwards of 8 feet below existing grade. The extensive
soil mottling encountered in this soil group is indicative of poor surface drainage. Encountered at a
depth of 8 feet, extending through a depth of ± 13 feet, we encountered advance outwash. As
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observed in our explorations, this soil group was largely comprised of alternating lenses of fine
sand and silty sand. Though not ideal for stormwater retention, it is our opinion that this soil group
could support limited infiltration. Additionally, we are of the opinion that groundwater levels are
at a sufficient depth so as to not adversely affect stormwater retention. Our grain-size analysis
indicates that the advance outwash contains an average relative fines (percent silt/clay) content
between 27 to 30 percent.
The results of our soil grain size analyses are presented below, and the attached Soil Gradation
Graphs (Appendix B) display the grain-size distribution of the samples tested.
TABLE 2
LABORATORY TEST RESULTS FOR NON-ORGANIC ONSITE SOILS
Soil Sample, Depth % Coarse
Gravel
% Fine
Gravel
% Coarse
Sand
%
Medium
Sand
% Fine
Sand % Fines D10
TP-1, S-3, 9 feet
TP-2, S-2, 9 feet
TP-4, S-1, 10 feet
TP-5, S-1, 9 feet
19.3
3.3
0.0
0.0
5.7
9.2
3.8
4.5
2.6
4.2
3.5
2.6
7.3
13.3
14.2
12.7
33.8
43.1
52.3
50.0
31.3
26.9
26.1
30.3
-
-
-
-
Design Infiltration Rate
We determined a design infiltration rate for the advance soils on site, by utilizing Section 3.3.6 of
Volume III of the 2014 Stormwater Management Manual for Western Washington (SMWW). This section
determines a design infiltration rate by taking gradational information from sieve analyses, and
inputting this data into equation 1:
log10(Ksat) = -1.57 + 1.90D10 + 0.015D60 – 0.013D90 – 2.08Ffines
Where D10, D60, and D90 are the grain sizes in mm for which 10 percent, 60 percent and 90 percent of
the sample is more fine and Fines is the fraction of the soil (by weight) that passes the #200 sieve. Ksat
is in cm/s, though we, for ease of use, have converted this value to inches per hour.
We recommend that an uncorrected Ksat initial value of 7.1 in/hr be utilized by the design engineer, for
retention facilities utilizing the native advance outwash as the primary infiltrative unit.
The value acquired above is an initial rate. It is recommended that this initial rate be reduced
through correction factors that are appropriate for the design situation to produce a design
infiltration rate.
As per the same section of the SMWW, the Total Correction Factor (CFT):
CFT = CFv x CFt x CFm
Where CFv accounts for site variability and number of locations tested, CFt accounts for errors
associated with the test method, and CFm accounts for potential siltation and biofouling. Utilizing a
Sapphire Homes – Talbot Gardens Short Plat, Renton, WA October 11, 2017 / Revised August 30, 2018
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value of 0.5 for CFv, 0.40 for CFt, and 0.9 for CFm, our recommended CFT = 0.18.
Ksat design = Ksat initial x CFT
We recommend that a Ksat design value of 1.25 in/hr be utilized by the engineer in charge of design of
retention facilities associated with this project. This value is for the native advance outwash soils
which underlie the project area. Retention facility should have an invert elevation of at least 8 feet
below existing grade. A more homogeneous restrictive layer is encountered at a depth of 13 feet in
the proposed stormwater retention area. The presence of this restrictive layer should be evaluated
by the Civil Engineer of Record for this project to determine whether it will adversely affect any
proposed designs. Generally, in the City of Renton, field infiltration tests are performed in order to
determine design infiltration rates for native soils. However, given the relative depth to the only
potential soil group to support stormwater retention onsite, we are of the opinion that the grain size
analysis method is appropriate.
3.5 Seismic Conditions
Based on our analysis of subsurface exploration logs and our review of published geologic maps,
we interpret the onsite soil conditions to generally correspond with site class D, as defined by
Table 30.2-1 in ASCE 7, per the 2015 International Building Code (IBC).
Using 2015 IBC information on the USGS Design Summary Report website, Risk Category I/II/III
seismic parameters for the site are as follows:
Ss = 1.394 g SMS = 1.394 g SDS = 0.929 g
S1 = 0.520 g SM1 = 0.780 g SD1 = 0.520 g
Using the 2015 IBC information, MCER Response Spectrum Graph on the USGS Design Summary
Report website, Risk Category I/II/III, Sa at a period of 0.2 seconds is 1.39 g and Sa at a period of
1.0 seconds is 0.78 g.
The Design Response Spectrum Graph from the same website, using the same IBC information and
Risk Category, Sa at a period of 0.2 seconds is 0.93 g and Sa at a period of 1.0 seconds is 0.52 g.
3.6 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. Our explorations did not encounter any saturated, or potentially
saturated granular soils, and we interpret the site as having a low potential for soil liquefaction
during a large-scale seismic event.
Sapphire Homes – Talbot Gardens Short Plat, Renton, WA October 11, 2017 / Revised August 30, 2018
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Migizi Group, Inc. Page 7 of 14
4.0 CONCLUSIONS AND RECOMMENDATIONS
Improvement plans involve the clearing/stripping of the property, demolition of the existing
structures, and development of the site for residential purposes. The proposed development will
result in approximately 20 buildable lots and contain a primary access road which travels east-west
across the length of the property, following the general course of the existing driveway. A
supplemental alleyway will also be introduced as part of the proposed development, traveling
south from S 48th Street, spanning the full length of the property along this orientation. The
southwest corner of the subject property will be utilized as an Open Space / Tree Retention Tract,
and will also house stormwater retention facilities, if feasible. We offer these recommendations:
• Feasibility: Based on our field explorations, research and analyses, the proposed
structures appear feasible from a geotechnical standpoint.
• Foundation Options: Foundation elements for the proposed residences should be
constructed on medium dense or denser undisturbed native soils, or on structural
fill bearing pads extending down to these soils. We anticipate that adequate
bearing soils will be encountered within two to three feet of existing grade.
Recommendations for Spread Footings are provided in Section 4.2.
• Floor Options: Floor sections for the proposed residences should bear on medium
dense or denser native soils or on properly compacted structural fill extending down
to these soils. We anticipate that adequate bearing soils will be encountered within
two to three feet of existing grade. Recommendations for slab-on-grade floors are
included in Section 4.3. Fill underlying floor slabs should be compacted to
95 percent (ASTM:D-1557).
• Pavement Sections: Native, in-situ soil conditions are amenable to the use of soil-
supported pavements. We recommend a conventional pavement section comprised
of an asphalt concrete pavement over a crushed rock base course over a properly
prepared (compacted) subgrade or a granular subbase, depending on subgrade
conditions during pavement subgrade preparation.
All soil subgrades 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.
• Infiltration Conditions: Given the geological conditions encountered on site, we do
not foresee full-infiltration as being feasible for this project. However, limited
infiltration utilizing a system of trenches, likely can be implemented on site, utilizing
the advance outwash soils for stormwater retention. We recommend utilizing a
design infiltration rate of 1.25 inches per hour for this soil group. Invert elevations
for retention facilities should be located at least 8 feet below existing grade. A more
homogeneous restrictive layer is encountered at a depth of 13 feet in the proposed
stormwater retention area. The presence of this restrictive layer should be evaluated
by the Civil Engineer of Record for this project to determine whether it will
adversely affect any proposed designs. Shallower glacial till soils, should be
considered impermeable for design purposes.
Sapphire Homes – Talbot Gardens Short Plat, Renton, WA October 11, 2017 / Revised August 30, 2018
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• 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 of this report present our specific geotechnical conclusions and
recommendations concerning site preparation, spread footings, slab-on-grade floors, asphalt
pavement, 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 3. 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. Our subsurface explorations indicate
that the organic horizon can reach thicknesses of up to 14 inches. Stripping is best performed
during a period of dry weather.
Site Excavations: Based on our explorations, we expect deeper site excavations will predominately
encounter densely consolidated glacial till soils. This soil group can be readily excavated utilizing
standard excavation equipment, though special teeth, or “rippers”, may need to be utilized in order
to rapidly excavate glacial till soils. Shallower excavations will encounter highly weathered, loosely
consolidated soils which can be readily excavated using standard excavation equipment.
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Dewatering: We did not encounter groundwater seepage in any of subsurface explorations, which
extended a maximum depth of 15 feet below existing grade. We do not anticipate that groundwater
levels will rise high enough to adversely affect the proposed development; however, seasonally
perched groundwater will likely be encountered at relatively shallow depths across the project area
during extended precipitation given the presence of low permeability till soils across the subsurface.
If groundwater is encountered, we anticipate that an internal system of ditches, sump holes, and
pumps will be adequate to temporarily dewater excavations.
Temporary Cut Slopes: All temporary soil slopes associated with site cutting or excavations should
be adequately inclined to prevent sloughing and collapse. Temporary cut slopes in site soils should
be no steeper than 1½H:1V, and should conform to Washington Industrial Safety and Health Act
(WISHA) regulations.
Subgrade Compaction: Exposed subgrades for the foundation of the proposed residence 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 overexcavated 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 Fill Soils: Where encountered, surficial organic
soils like duff, topsoil, root-rich soil, and organic-rich fill soils are not suitable for use
as 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.
• Glacial Till: Underlying a surface mantle of sod and topsoil, native glacial till soils
were encountered; generally consisting of dense, gravelly silty sand. These soils are
moderately moisture sensitive and will be difficult, if not impossible, to reuse during
wet weather conditions. If reuse is planned, care should be taken while stockpiling
in order to avoid saturation/over-saturation of the material, and moisture
conditioning should be expected.
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.
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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
residences if the subgrade is properly prepared. 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 16 and 24 inches
wide, respectively.
Bearing Subgrades: Footings should bear on medium dense or denser, undisturbed native soils
which have been stripped of surficial organic soils and vigorously surface compacted, or on
properly compacted structural fill bearing pads which extend down to soils described above. We
anticipate that adequate bearing subgrades will be encountered within 1 to 2 feet of existing grade,
within glacial till soils.
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.
Lateral Overexcavations: 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 medium dense or denser,
native, glacial soils can be designed for a maximum allowable soil bearing pressure of 2,000 psf. A
one-third increase in allowable soil bearing capacity may be used for short-term loads created by
seismic or wind related activities.
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Footing Settlements: Assuming that structural fill soils are compacted to a medium 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. 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 stemwalls 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 225 psf and an allowable base friction coefficient of 0.35 for site
soils.
4.3 Slab-On-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used in the proposed residences if the
subgrades are properly prepared. Floor sections for the proposed structures should bear on
medium dense or denser native soils or on properly compacted structural fill which extends down
to soils described above. We anticipate that adequate bearing soils will be encountered within 1 to
2 feet of existing grade. We offer the following comments and recommendations concerning slab-
on-grade floors.
Floor Subbase: Surface compaction of all slab subgrades is recommended. If a subbase is required,
it 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), but clean angular
gravel can be used if it adequately prevents capillary wicking. In addition, a layer of plastic
sheeting (such as Crosstuff, Visqueen, or Moistop) should be placed over the capillary break to
serve as a vapor barrier. During subsequent casting of the concrete slab, the contractor should
exercise care to avoid puncturing this vapor barrier.
Vertical Deflections: Due to elastic compression of subgrades, soil-supported slab-on-grade floors
can deflect downwards when vertical loads are applied. In our opinion, a subgrade reaction
modulus of 250 pounds per cubic inch can be used to estimate such deflections.
Sapphire Homes – Talbot Gardens Short Plat, Renton, WA October 11, 2017 / Revised August 30, 2018
Revised Geotechnical Engineering Report P1389-T18
Migizi Group, Inc. Page 12 of 14
4.4 Asphalt Pavement
Since asphalt pavements will also be used for the proposed communal driveway system, we offer
the following comments and recommendations for pavement design and construction.
Subgrade Preparation: All soil subgrades 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 maximum depth of 12 inches and
replaced with a suitable structural fill material. All structural fill should be compacted according to
our recommendations given in the Structural Fill section. Specifically, the upper 2 feet of soils
underlying pavement section should be compacted to at least 95 percent (based on ASTM D-1557),
and all soils below 2 feet should be compacted to at least 90 percent.
Pavement Materials: For the base course, we recommend using imported washed crushed rock,
such as "Crushed Surfacing Base Course” per WSDOT Standard Specification 9-03.9(3) but with a
fines content of less than 5 percent passing the No. 200 Sieve. Although our explorations do not
indicate a need for a pavement subbase, if a subbase course is needed, we recommend using
imported, clean, well-graded sand and gravel such as “Ballast” or “Gravel Borrow” per WSDOT
Standard Specifications 9-03.9(1) and 9-03.14, respectively.
Conventional Asphalt Sections: A conventional pavement section typically comprises an asphalt
concrete pavement over a crushed rock base course. We recommend using the following
conventional pavement sections:
Minimum Thickness
Pavement Course Parking Areas High Traffic Driveways
and Private Access Roads
Asphalt Concrete Pavement 2 inches 4 inches
Crushed Rock Base 4 inches 8 inches
Granular Fill Subbase (if needed) 6 inches 12 inches
Compaction and Observation: All subbase and base course material should be compacted to at least
95 percent of the Modified Proctor maximum dry density (ASTM D-1557), and all asphalt concrete
should be compacted to at least 92 percent of the Rice value (ASTM D-2041). We recommend that
an MGI representative be retained to observe the compaction of each course before any overlying
layer is placed. For the subbase and pavement course, compaction is best observed by means of
frequent density testing. For the base course, methodology observations and hand-probing are
more appropriate than density testing.
Pavement Life and Maintenance: No asphalt pavement is maintenance-free. The above described
pavement sections present our minimum recommendations for an average level of performance
during a 20-year design life; therefore, an average level of maintenance will likely be required.
Furthermore, a 20-year pavement life typically assumes that an overlay will be placed after about 10
years. Thicker asphalt and/or thicker base and subbase courses would offer better long-term
performance, but would cost more initially; thinner courses would be more susceptible to
“alligator” cracking and other failure modes. As such, pavement design can be considered a
Sapphire Homes – Talbot Gardens Short Plat, Renton, WA October 11, 2017 / Revised August 30, 2018
Revised Geotechnical Engineering Report P1389-T18
Migizi Group, Inc. Page 13 of 14
compromise between a high initial cost and low maintenance costs versus a low initial cost and
higher maintenance costs.
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. Soils used for structural fill should not contain any organic matter or debris,
nor any individual particles greater than about 6 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
Asphalt pavement base
Asphalt pavement subgrade (upper 2 feet)
Asphalt pavement subgrade (below 2 feet)
95 percent
90 percent
95 percent
95 percent
90 percent
Subgrade Observation and Compaction Testing: Regardless of material or location, all structural fill
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.
APPROXIMATE SITE
LOCATION
P.O. Box 44840
Tacoma, WA 98448
Location Job Number Figure
DateTitle
4827 Talbot Rd S
Renton, WA
P/N 3123059022
Topographic and Location Map
1
08/28/18
P1389-T18
APPENDIX A
SOIL CLASSIFICATION CHART AND
KEY TO TEST DATA
LOGS OF TEST PITS
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
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, POORLY 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
SM
SM
SM
SM
0.6
1.5
5.0
8.0
10.0
Sod and topsoil
(SM) Gray fine silty sand (medium dense, damp) (Weathered Glacial Till)
(SM) Light brown mottled silty sand with gravel (dense, damp) (Weathered Glacial Till)
(SM) Gray/brown silty sand with gravel (very dense, moist) (Unweathered Glacial Till)
(SM) Brown silty sand with gravel and intermittent lenses of fine sand (medium dense, moist) (Advanced Outwash)
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 10.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Rubber Tracked Mini Excavator
EXCAVATION CONTRACTOR Paulman GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 9/5/17 COMPLETED 9/5/17
AT TIME OF EXCAVATION ---
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 Monsef Donogh Design Group
PROJECT NUMBER P1056-T17
PROJECT NAME Proposed Talbot Gardens Short Plat
PROJECT LOCATION 4827 Talbot Rd S, Renton, WA 98055
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 10/11/17 10:17 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1056-T17\P1056-T17 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
ML
SM
SM
SM
0.6
2.5
5.0
8.0
10.0
Sod and topsoil
(ML) Gray/brown mottled sandy silt (medium stiff, damp) (Recessional Outwash)
(SM) Gray fine silty sand (medium dense, damp) (Weathered Glacial Till)
(SM) Gray/brown silty sand with gravel (dense, moist) (Unweathered Glacial Till)
(SM) Brown silty sand with some gravel and intermittent lenses of fine sand (medium dense, moist) (Advanced
Outwash)
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 10.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Rubber Tracked Mini Excavator
EXCAVATION CONTRACTOR Paulman GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 9/5/17 COMPLETED 9/5/17
AT TIME OF EXCAVATION ---
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-3
TEST PIT NUMBER TP-2
CLIENT Monsef Donogh Design Group
PROJECT NUMBER P1056-T17
PROJECT NAME Proposed Talbot Gardens Short Plat
PROJECT LOCATION 4827 Talbot Rd S, Renton, WA 98055
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 10/11/17 10:17 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1056-T17\P1056-T17 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
SM
SM
SM
SM
1.2
3.0
7.0
8.0
10.0
Sod and topsoil with small roots
(SM) Gray fine silty sand (medium dense, damp) (Weathered Glacial Till)
(SM) Light brown mottled silty sand with some gravel (medium dense, damp) (Weathered Glacial Till)
(SM) Gray/brown silty sand with some gravel (dense, damp) (Unweathered Glacial Till)
(SM) Brown silty sand with some gravel and intermittent lenses of fine sand (dense, moist) (Advanced Outwash)
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 10.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Rubber Tracked Mini Excavator
EXCAVATION CONTRACTOR Paulman GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 9/5/17 COMPLETED 9/5/17
AT TIME OF EXCAVATION ---
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 Monsef Donogh Design Group
PROJECT NUMBER P1056-T17
PROJECT NAME Proposed Talbot Gardens Short Plat
PROJECT LOCATION 4827 Talbot Rd S, Renton, WA 98055
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 10/11/17 10:17 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1056-T17\P1056-T17 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
GB
S-1
SM
SM
SM
SM
0.8
2.5
8.0
13.0
15.0
Sod and topsoil
(SM) Gray/brown fine silty sand (medium dense, damp) (Weathered Glacial Till)
(SM) Orange/brown mottled fine silty sand with some gravel (dense, moist) (Unweathered Glacial Till)
(SM) Light brown silty sand with some gravel and intermittent lenses of fine sand (medium dense, moist) (Advanced
Outwash)
(SM) Gray fine silty sand with gravel (dense, moist)
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 15.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Steel Tracked Excavator
EXCAVATION CONTRACTOR Dreamline Construction GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 7/24/18 COMPLETED 7/24/18
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
10.0
12.5
15.0
PAGE 1 OF 1
Figure A-5
TEST PIT NUMBER TP-4
CLIENT Sapphire Homes
PROJECT NUMBER P1389-T18
PROJECT NAME Proposed Talbot Gardens Short Plat
PROJECT LOCATION 4827 Talbot Rd S, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 8/28/18 17:10 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1389-T18\P1389-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
GB
S-1
SM
SM
SM
SM
1.0
3.0
8.5
13.0
15.0
Sod and topsoil
(SM) Gray/brown fine silty sand (loose, damp) (Weathered Glacial Till)
(SM) Orange/brown mottled fine silty sand (dense, moist) (Unweathered Glacial Till)
(SM) Light brown silty sand with some gravel and intermittent lenses of fine sand (medium dense, moist) (Advanced
Outwash)
(SM) Gray fine silty sand with gravel (dense, moist)
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 15.0 feet.
NOTES
LOGGED BY ZLL
EXCAVATION METHOD Steel Tracked Excavator
EXCAVATION CONTRACTOR Dreamline Construction GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 7/24/18 COMPLETED 7/24/18
AT TIME OF EXCAVATION ---
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
10.0
12.5
15.0
PAGE 1 OF 1
Figure A-6
TEST PIT NUMBER TP-5
CLIENT Sapphire Homes
PROJECT NUMBER P1389-T18
PROJECT NAME Proposed Talbot Gardens Short Plat
PROJECT LOCATION 4827 Talbot Rd S, Renton, WA
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 8/28/18 17:10 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1389-T18\P1389-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