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Geotechnical Engineering Report
Proposed Short Plat
16654 113th Ave SE
Renton, Washington 98055
P/N 0088000930
January 24, 2019
prepared for:
Sang Tran
16654 113th Ave SE
Renton, Washington 98055
prepared by:
Migizi Group, Inc.
PO Box 44840
Tacoma, Washington 98448
(253) 537-9400
MGI Project P1239-T18
i
TABLE OF CONTENTS
Page No.
1.0 SITE AND PROJECT DESCRIPTION .............................................................................................. 1
2.0 EXPLORATORY METHODS ............................................................................................................ 2
2.1 Test Pit Procedures ................................................................................................................ 2
3.0 SITE CONDITIONS ............................................................................................................................ 2
3.1 Surface Conditions ................................................................................................................. 2
3.2 Soil Conditions ....................................................................................................................... 3
3.3 Groundwater Conditions ...................................................................................................... 3
3.4 Seismic Conditions ................................................................................................................. 4
3.5 Liquefaction Potential ........................................................................................................... 4
3.6 Infiltration Conditions ........................................................................................................... 4
4.0 CONCLUSIONS AND RECOMMENDATIONS............................................................................ 4
4.1 Site Preparation ...................................................................................................................... 5
4.2 Spread Footings ...................................................................................................................... 7
4.3 Slab-On-Grade Floors ............................................................................................................ 9
4.4 Drainage Systems ................................................................................................................... 9
4.5 Structural Fill ........................................................................................................................ 10
4.6 Asphalt Pavement ................................................................................................................ 11
5.0 RECOMMENDED ADDITIONAL SERVICES ............................................................................. 12
6.0 CLOSURE ........................................................................................................................................... 13
List of Tables
Table 1. Approximate Locations and Depths of Explorations ............................................................................. 2
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-2 .......................................................................................................... A-2…A-3
Page 1 of 13
MIGIZI GROUP, INC.
PO Box 44840 PHONE (253) 537-9400
Tacoma, Washington 98448 FAX (253) 537-9401
January 24, 2019
Sang Tran
16654 113th Ave SE
Renton, Washington 98055
Subject: Geotechnical Engineering Report
Proposed Short Plat
16654 113th Ave SE
Renton, Washington 98055
P/N 0088000930
MGI Project P1239-T18
Dear Mr. Tran:
Migizi Group, Inc. (MGI) is pleased to submit this report describing the results of our geotechnical
engineering evaluation for the proposed 3 lot short plat located at 16654 113th Ave SE in Renton,
Washington.
This report has been prepared for the exclusive use of Sang Tran, and his 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 rectangular shaped parcel encompassing approximately 0.5 acres
located on the corner of SE 168th St and 113th Ave SE in Renton, Washington, as shown on the
enclosed Topographic and Location Map (Figure 1). The parcel has an existing home, detached
garage, driveway, and associated utilities. The site is covered in mowed lawn and slopes to the
south at approximately 5 to 10 percent.
Improvement plans involve platting the parcel into three separate lots, removing the garage, and
constructing two new homes.
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2.0 EXPLORATORY METHODS
We explored surface and subsurface conditions at the project site on December 13, 2018. Our
exploration and evaluation program comprised the following elements:
• Surface reconnaissance of the site;
• Two test pits (designated TP-1 and TP-2), advanced on December 13, 2018; and
• A review of published geologic and seismologic maps and literature.
Figure 2 depicts the approximate relative locations of the test pits. The following sections describe
the procedures used for excavation of test pits.
The specific numbers and locations of our explorations were selected in relation to the existing
site features, under the constraints of surface access, underground utility conflicts, and budget
considerations.
It should be realized that the explorations performed and utilized for this evaluation reveal
subsurface conditions only at discrete locations across the project site and that actual conditions
in other areas could vary. Furthermore, the nature and extent of any such variations would not
become evident until additional explorations are performed or until construction activities have
begun. If significant variations are observed at that time, we may need to modify our conclusions
and recommendations contained in this report to reflect the actual site conditions.
2.1 Test Pit Procedures
Our exploratory test pits were excavated with a rubber-tracked mini-excavator operated by an
excavation contractor under subcontract to MGI. 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 each
test pit, based on our field classifications. Where a soil contact was observed to be gradational or
undulating, our logs indicate the average contact depth. We estimated the relative density and
consistency of the in-situ soils by means of the excavation characteristics and the stability of the
test pit sidewalls. Our logs also indicate the approximate depths of any sidewall caving or
groundwater seepage observed in the test pits. The soils were classified visually in general
accordance with the system described in Figure A-1, which includes a key to the exploration logs.
Summary logs of the explorations are included as Figures A-2 and A-3.
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 rectangular shaped parcel encompassing
approximately 0.5 acres located on the corner of SE 168th St and 113th Ave SE in Renton,
Washington. The parcel has an existing home, detached garage, driveway, and associated
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utilities. The site is covered in mowed lawn and slopes to the south at approximately 5 to
10 percent.
The topography of the site is gently sloping to the south.
Vegetation onsite includes primarily mowed lawn with scattered shrubs near the home.
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 glacial till 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
Glaciation, receded from this region approximately 13,500 years ago. The majority of near surface
soils encountered within the Tacoma 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.
Both of our test pit explorations terminated in undisturbed heavily mottled glacial till at a depth
of 8 feet, overlain by varying amounts of topsoil or silty sands with gravel. Exploration TP-1
encountered weathered till beneath 1½ feet of brown silty sand with gravel, transitioning to
unweathered till. Exploration TP-2 encountered approximately 1 foot of brown silty sand with
gravel, and unweathered till from 1 foot through the termination of the test pit; a depth of 8 feet.
In the Geologic Map of the Renton 7.5’ Quadrangle, King County, Washington, as prepared by the
United States Department of the Interior (USDI) (1965), the project site is mapped as containing
Qgt, ablation glacial till over lodgment glacial till. This soil group reportedly contains clay, silt,
sand, and gravel, and is often unstratified and highly compact. Our subsurface observations
generally conform with the classifications performed by the USDI.
The enclosed exploration logs (Appendix A) provide a detailed description of the soil strata
encountered in our subsurface explorations.
3.3 Groundwater Conditions
At the time of our reconnaissance and subsurface explorations (December 13, 2018), we
encountered groundwater seepage at the top of the impermeable layer of glacial till deposits
approximately 2 feet and 3 feet below existing grades.
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3.4 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 C, as defined by
Table 20.3-1 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 = 1.401 g SMS = 1.401 g SDS = 0.934 g
S1 = 0.522 g SM1 = 0.678 g SD1 = 0.452 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 1.4g and Sa at a period of
1.0 seconds is 0.52g.
The Design Response Spectrum from the same website, using the same IBC information and Risk
Category, Sa at a period of 0.2 seconds is 1.4 and Sa at a period of 1.0 seconds is 0.68g.
3.5 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. Subsurface explorations performed within the confines of the project
area did not encounter any saturated, loosely consolidated sandy soils, and we interpret the
potential for seismically induced liquefaction to be low within the subject property.
3.6 Infiltration Conditions
As indicated in the Soil Conditions section of this report, and corresponding exploration logs, the
site is underlain at depth by relatively impermeable glacial till soils. Variable thicknesses of
topsoil overlay heavily mottled glacial till which is considered an impermeable layer.
4.0 CONCLUSIONS AND RECOMMENDATIONS
Improvement plans involve the platting of the parcel into three separate parcels. We offer the
following recommendations:
• Feasibility: Based on our field explorations, research, and evaluations, the
proposed structures and pavements appear feasible from a geotechnical
standpoint.
• Foundation Options: Foundation elements should be constructed on medium
dense or denser undisturbed native soils, or on structural fill bearing pads that
extend down to medium dense or denser native soils. We anticipate that adequate
bearing soils will be encountered at relatively shallow depths (less than 2 feet)
across much of the proposed building area. We do not foresee the need for any
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localized over-excavations. Recommendations for Spread Footings are provided
in Section 4.2.
• Floor Options: Floor sections should bear on medium dense or denser native soils
or on properly compacted structural fill that extends down to medium dense or
denser native soils. We anticipate that adequate bearing soils will be encountered
at relatively shallow depths (less than 2 feet) at the proposed building area, and
we do not foresee the need for a granular fill subbase. 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
the soil conditions exposed during 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: As indicated in the Soil Conditions section of this report,
and corresponding exploration logs, the site is underlain at depth by relatively
impermeable, glacial till soils and infiltration is not recommended for this site.
• 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, 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
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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 explorations and field
observations indicate that the topsoil horizon ranges in thickness between 4 to 6 inches across the
project area.
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, moderately consolidated glacial soils which can be readily excavated using
standard excavation equipment.
Dewatering: Our explorations encountered slow groundwater seepage within 3 feet of the
surface, so it is possible that groundwater may be present in the planned excavations depending
on the time of year and weather conditions. If groundwater is encountered, we anticipate that an
internal system of ditches, sumpholes, and pumps will be adequate to temporarily dewater
shallow 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 foundations of the planned structures should
be compacted to a firm, unyielding state before new concrete or fill soils are placed. Any localized
zones of looser granular soils observed within a subgrade should be compacted to a density
commensurate with the surrounding soils. In contrast, any organic, soft, or pumping soils
observed within a subgrade should be over-excavated and replaced with a suitable structural fill
material.
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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.
• Silty Sands with Gravel: Underlying a surface mantle of sod and topsoil, weathered
glacial till soils were encountered; generally consisting of medium, 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.
• 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.
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.
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4.2 Spread Footings
In our opinion, conventional spread footings will provide adequate support for the proposed
structures, if the subgrades are 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 18
and 24 inches wide, respectively.
Bearing Subgrades: Footings should bear on dense or denser, undisturbed native soils which
have been stripped of surficial organic soils and vigorously surface compacted, or on properly
compacted structural fill which bears on the soils just described. We anticipate that adequate
bearing soils will be encountered at relatively shallow depths (less than 2 feet) across much of the
proposed building area.
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 overexcavated 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 24 inches below the footing base should
also extend 24 inches outward from the footing edges.
Subgrade Observation: All footing subgrades should consist of firm, unyielding, native soils, the
existing medium dense or denser gravelly fill, or structural fill materials that have been
compacted to a density of at least 95 percent (based on ASTM:D-1557). Footings should never be
cast atop loose, soft, or frozen soil, slough, debris, existing uncontrolled fill, or surfaces covered
by standing water.
Bearing Pressures: In our opinion, for static loading, footings that bear on dense properly
prepared subgrades 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.
Footing Settlements: Assuming that structural fill soils are compacted to a dense or denser state,
we estimate that total post-construction settlements of properly designed footings bearing on
properly prepared subgrades will not exceed 1 inch. Differential settlements for comparably
loaded elements may approach one-half of the actual total settlement over horizontal distances
of approximately 50 feet.
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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 for both the glacial outwash on site and an
allowable base friction coefficient of 0.35.
4.3 Slab-On-Grade Floors
In our opinion, soil-supported slab-on-grade floors can be used in structures if the subgrades are
properly prepared. We offer the following comments and recommendations concerning slab-on-
grade floors.
Floor Subbase: Generally, structural fill subbases do not appear to be needed under soil-
supported slab-on-grade floors. Surface compaction of slab subgrades is recommended.
If a subbase is required for some reason, 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).
Alternatively, angular gravel or crushed rock can be used if it is sufficiently clean and uniform to
prevent capillary wicking.
Vapor Barrier: We recommend that a layer of durable plastic sheeting (such as Crosstuff,
Moistop, or Visqueen) be placed directly between the capillary break and the floor slab to prevent
ground moisture vapors from migrating upward through the slab. During subsequent casting of
the concrete slab, the contractor should exercise care to avoid puncturing this vapor barrier.
4.4 Drainage Systems
In our opinion, the new structures should be provided with permanent drainage systems to
reduce the risk of future moisture problems. We offer the following recommendations and
comments for drainage design and construction purposes.
Perimeter Drains: We recommend that buildings be encircled with a perimeter drain system to
collect seepage water. This drain should consist of a 4-inch-diameter perforated pipe within an
envelope of pea gravel or washed rock, extending at least 6 inches on all sides of the pipe, and
the gravel envelope should be wrapped with filter fabric to reduce the migration of fines from
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the surrounding soils. Ideally, the drain invert would be installed no more than 8 inches above
the base of the perimeter footings.
Subfloor Drains: Given the geologic/hydrologic conditions present within the project area, we do
not believe it necessary to install subfloor drains beneath the proposed structure.
Discharge Considerations: If possible, all perimeter drains should discharge to a sewer system
or other suitable location by gravity flow. Check valves should be installed along any drainpipes
that discharge to a sewer system, to prevent sewage backflow into the drain system. If gravity
flow is not feasible, a pump system is recommended to discharge any water that enters the
drainage system.
Runoff Water: Roof-runoff and surface-runoff water should not discharge into the perimeter
drain system. Instead, these sources should discharge into separate tightline pipes and be routed
away from the building to a storm drain or other appropriate location.
Grading and Capping: Final site grades should slope downward away from the buildings so that
runoff water will flow by gravity to suitable collection points, rather than ponding near the
building. Ideally, the area surrounding the building would be capped with concrete, asphalt, or
low-permeability (silty) soils to minimize or preclude surface-water infiltration.
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. Utilizing recycled content may require approval from the Health
Department for placement in an aquifer recharge area. 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.
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Compaction Criteria: Using the Modified Proctor test (ASTM:D-1557) as a standard, we
recommend that structural fill used for various onsite applications be compacted to the following
minimum densities:
Fill Application Minimum
Compaction
Footing subgrade and bearing pad
Foundation backfill
Slab-on-grade floor subgrade and subbase
Asphalt pavement base
Asphalt pavement subgrade (upper 2 feet)
Asphalt pavement subgrade (below 2 feet)
95 percent
90 percent
95 percent
95 percent
95 percent
90 percent
Subgrade Observation and Compaction Testing: Regardless of material or location, all structural
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.
4.6 Asphalt Pavement
Since asphalt pavements will be used for the primary access road and residential driveways, 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
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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 Roadways and High Traffic Areas
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
compromise between a high initial cost and low maintenance costs versus a low initial cost and
higher maintenance costs.
5.0 RECOMMENDED ADDITIONAL SERVICES
Because the future performance and integrity of the structural elements will depend largely on
proper site preparation, drainage, fill placement, and construction procedures, monitoring and
testing by experienced geotechnical personnel should be considered an integral part of the
construction process. Subsequently, we recommend that MGI be retained to provide the
following post-report services:
• Review all construction plans and specifications to verify that our design criteria
presented in this report have been properly integrated into the design;
• Prepare a letter summarizing all review comments (if required);
• Check all completed subgrades for footings and slab-on-grade floors before
concrete is poured, in order to verify their bearing capacity; and
• Prepare a post-construction letter summarizing all field observations, inspections,
and test results (if required).
APPROXIMATE SITE
LOCATION
P.O. Box 44840
Tacoma, WA 98448
Location Job Number Figure
DateTitle
16654 113th Ave SE
Renton, Washington 98055
Topographic and Location Map
1
12/28/18
P1239-T18
APPENDIX A
SOIL CLASSIFICATION CHART AND
KEY TO TEST DATA
LOG 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
GB
S-1
GB
S-2
SM
SM
0.3
1.5
8.0
4 inches topsoil
(SM) Brown silty sand with gravel (medium dense, moist)
(SM) Gray heavily mottled silty sand with gravel (medium dense to very dense, moist) (Till)
No caving observed
Moderate perched groundwater seepage observed at 2 feet
The depths on the test pit logs are based on an average of measurements across the test hole and should be
considered accurate to 0.5 foot.
Bottom of test pit at 8.0 feet.
NOTES
LOGGED BY CRL
EXCAVATION METHOD
EXCAVATION CONTRACTOR Posh LLC GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 12/13/18 COMPLETED 12/13/18
AT TIME OF EXCAVATION 2.00 ft Moderate perched
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
PAGE 1 OF 1
Figure A-2
TEST PIT NUMBER TP-1
CLIENT Sang Tran
PROJECT NUMBER P1239-T18
PROJECT NAME Proposed Short Plat
PROJECT LOCATION 16654 113th Ave SE, Renton, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/28/18 15:28 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1239-T18\P1239-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
1.0
8.0
(SM) Dark brown silty sand with gravel (medium dense, moist)
(SM) Gray heavily mottled silty sand with gravel (medium dense to very dense, moist) (Till)
No caving observed
Moderate perched groundwater seepage observed at 3 feet
The depths on the test pit logs are based on an average of measurements across the test hole and should be
considered accurate to 0.5 foot.
Bottom of test pit at 8.0 feet.
NOTES
LOGGED BY CRL
EXCAVATION METHOD
EXCAVATION CONTRACTOR Posh LLC GROUND WATER LEVELS:
CHECKED BY JEB
DATE STARTED 12/13/18 COMPLETED 12/13/18
AT TIME OF EXCAVATION 3.00 ft Moderate perched
AT END OF EXCAVATION ---
AFTER EXCAVATION ---
TEST PIT SIZEGROUND ELEVATION
SAMPLE TYPENUMBERDEPTH(ft)0.0
2.5
5.0
7.5
PAGE 1 OF 1
Figure A-3
TEST PIT NUMBER TP-2
CLIENT Sang Tran
PROJECT NUMBER P1239-T18
PROJECT NAME Proposed Short Plat
PROJECT LOCATION 16654 113th Ave SE, Renton, Washington
COPY OF GENERAL BH / TP LOGS - FIGURE.GDT - 12/28/18 15:28 - C:\USERS\JESSICA\DESKTOP\TEST PITS AND BORINGS - GINT\P1239-T18\P1239-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