21-101265-SU-Geotechnical (Soils) Report-03-31-2021-V1
13705 Bel-Red Road, Bellevue, Washington 98005
Phone 425/649-8757 · Fax 425/649-8758
October 2, 2020 G-5261
Mr. Hamid Korasani
Sazei Design Group
6608 110th Ave NE
Kirkland, WA 98033
Subject: GEOTECHNICAL REPORT
PROPOSED VODA HOUSE SHORT PLAT
35027 – 21ST AVE SW
FEDERAL WAY, WASHINGTON
Dear Mr. Korasani:
In accordance with our September 1, 2020 contract with you we have prepared the following
geotechnical report for the proposed development.
SITE AND PROJECT DESCRIPTION
The project site consists of a 0.68-acre lot at the subject address as shown on the attached Plate 1
- Vicinity Map. The lot is currently developed with a single-family residence and accessory
structures and it is accessed via a gravel driveway running west from 21st Ave SW. It is bounded
by residentially zoned properties and associated roads.
Based upon information provided by the project owner we understand that the proposed
development at the subject lot will consist of subdividing the lot into two lots for eventual
residential development. The name of the short plat is proposed to be the Voda_House Short
Plat. A site plan indicating the short plat/development configuration has not been provided. We
understand that a new house will be developed at the existing yard area west of the existing
residence.
GEO Group Northwest has prepared the attached Plate 2 – Site Plan using the online King
County Imap in order to illustrate the current development and topographic conditions at the
subject site. In general, the site has level to slightly sloping topography with slopes facing
toward the east.
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 2
GEO Group Northwest, Inc.
GEOLOGIC CONDITIONS
The geologic map1 for the site indicates that the subject site is underlain by Quaternary-age
Vashon Till (Qvt). Vashon Till typically consists of a mixture of silt, sand and gravel which was
deposited and overridden by glacial ice during the most recent period of glaciation, the Vashon
Stade.
SUBSURFACE CONDITIONS
GEO Group Northwest explored the subsurface soil and groundwater conditions by excavating
three test pits labeled TP-1 through TP-3 on September 17, 2020. The approximate test pit
locations are shown on the attached Plate 2 – Site Plan.
The soils encountered at the test pits consist of the competent medium dense to dense Vashon
Till soils. Soils are primarily fine sandy SILT with varying amounts of gravel, cobbles and
boulders. At the test pits TP-1 and TP-3 near the base of the pits gravelly silty SAND and
gravelly SAND with some silt was encountered. It is possible that these soils are the underlying
Advance Outwash soil deposit. But they may also be a sandier zone of glacial till. Loose soils
were not encountered at the test pits.
Groundwater seepage was not encountered at the test pits.
The results of our subsurface investigation are shown on the attached Appendix A – Test Pit
Logs and USCS Soil Legend.
SEISMIC DESIGN CRITERIA
Based upon the subsurface investigation it is our opinion that the overlying 100-foot thickness of
soils at the project site may be characterized as Site Class C soil (Very Dense Soil and Soft
Rock) and may be designed accordingly for seismic loads per the IBC. According to an online
Seismic Hazard tool the seismic coefficients are as follows:
Ss = 1.293g S1 = 0.499g
1 “Geologic Map of the Des Moines 7.5’ Quadrangle, King County, Washington”, USGS, Booth et al, 2004.
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 3
GEO Group Northwest, Inc.
CONCLUSIONS AND RECOMMENDATIONS
Based upon our subsurface investigation the proposed development is acceptable for the subject
site soil conditions. We recommend that the new building is constructed to bear on top of the
underlying native and competent medium dense to dense Vashon Till site soils or compacted
structural fills placed on top of the competent soils. Additionally, we recommend that all
foundation subgrades are approved by GEO Group Northwest at the time of construction, prior
to foundation pour(s). If loose soils are encountered at building foundation locations then a
program of over-excavation and replacement with compacted structural fill shall be implemented
and approved by GEO Group Northwest. Based upon our subsurface investigation it is
anticipated that competent medium dense to dense Vashon Till soils are present at typical
foundation subgrade elevations (18-inches below ground surface).
We recommend that the following recommendations and design parameters be incorporated into
the design for the development.
Site Preparation and General Earthwork
The proposed development areas should be stripped and cleared of surface vegetation, organic
soils (topsoil), loose soils and fill debris.
Silt fences should be installed around areas disturbed by construction activity to prevent
sediment-laden surface runoff from being discharged off-site. Exposed soils that are subject to
erosion should be compacted and covered with plastic sheeting. Stockpiled soils should be
covered with plastic sheeting if work is done during wet weather in order to mitigate off-site
sedimentation risks.
The site soils include relatively fine-grained silty soils which are moisture sensitive. Therefore,
if grading work occurs during wet weather there are risks related to soil softening as a result of
construction equipment. Yielding soils should be removed from structural subgrades (footing
and slab areas). If a significant portion of the work may occur during the wet season (Nov-April)
then it may be beneficial to construct a working pad consisting of crushed rock immediately
following the building pad excavation. We recommend that a construction pad for the building
consist of a minimum 6-inch thickness of clean crushed rock with a layer of filter fabric such as
Mirafi 500X placed on top of the non-yielding and competent native site soils. Alternatively, the
crushed rock may have a minimum thickness of 12-inches without the layer of filter fabric.
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 4
GEO Group Northwest, Inc.
Temporary Excavation Slopes and Permanent Slopes
Under no circumstances should temporary excavation slopes be greater than the limits specified
in local, state and national government safety regulations. Temporary cuts greater than four feet
in height should be sloped at an inclination no steeper than 1H:1V (Horizontal:Vertical) in the
overlying loose soils. Excavations within the underlying dense to very dense and cemented silty
soils (un-weathered Vashon Till) may be sloped having inclinations of no steeper than 1H:2V
provided seepage is not encountered. If seepage is encountered at the excavation, then the
excavation work should be halted and the temporary excavation slopes should be evaluated by
GEO Group Northwest prior to proceeding to a deeper level.
We recommend that permanent graded slopes shall be sloped no steeper than 3H:1V and that any
fills placed at these areas where inclinations are steeper than 4H:1V are compacted to meet the
structural fill compaction requirements.
Structural Fill
All fill material used to achieve design site elevations below the building areas and below non-
structurally supported slabs, parking lots, sidewalks, driveways, and patios, should meet the
requirements for structural fill. During wet weather conditions, material to be used as structural
fill should have the following specifications:
1. Be free draining, granular material containing no more than five (5) percent fines (silt and
clay-size particles passing the No. 200 mesh sieve);
2. Be free of organic material and other deleterious substances, such as construction debris
and garbage;
3. Have a maximum size of three (3) inches in diameter.
All fill material should be placed at or near the optimum moisture content. The optimum
moisture content is the water content in soil that enables the soil to be compacted to the highest
dry density for a given compaction effort.
Based upon our subsurface investigation the overlying site soils consist of silty soils. The site
soils are moisture sensitive and meeting the compaction requirements may be difficult during
wet weather. These soils may be used for structural fill provided that they are compacted to meet
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 5
GEO Group Northwest, Inc.
the compaction criteria noted below. It may be beneficial to import a granular fill material
meeting the specifications noted above, especially if work is to be performed during a period of
wet weather.
Structural fill should be placed in thin horizontal lifts not exceeding ten inches in loose thickness.
Structural fill under building areas (including foundation and slab areas) and behind earth
reinforced retaining walls (segmental block retaining walls and geo-grid reinforced rockery
walls), should be compacted to at least 92 percent of the maximum dry density, as determined by
ASTM Test Designation D-1557-91 (Modified Proctor).
Structural fill under driveways, parking lots and sidewalks should be compacted to at least 90
percent maximum dry density, as determined by ASTM Test Designation D-1557-91 (Modified
Proctor). Fill placed within 12-inches of finish grade should meet the 92% requirement.
We recommend that GEO Group Northwest, Inc., be retained to evaluate the suitability of
structural fill material and to monitor the compaction work during construction for quality
assurance of the earthwork.
Spread Footing Foundations
The proposed new building foundations may consist of conventional spread footings bearing on
top of the underlying medium dense to dense competent Vashon Till soils or on compacted
structural fill which is placed and compacted on top of the competent soils. The footings should
not be constructed on top of loose soils. If loose soils are encountered at footing areas then they
should be removed from foundation support areas and the excavation deepened until the
competent soils are encountered. Over-excavation areas may be backfilled with compacted
structural fill provided that it is placed on top of the competent medium dense to dense native
site soils. We recommend that all foundation subgrades are approved by GEO Group Northwest
at the time of construction, prior to the foundation pour, in order to confirm that the subgrades
have been properly prepared.
Individual spread footings may be used for supporting columns and strip footings for bearing
walls. Our recommended minimum design criteria for foundations bearing on the medium dense
to dense competent Vashon Till soils or on compacted structural fill placed on top of these soils
are as follows:
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 6
GEO Group Northwest, Inc.
- Allowable bearing pressure, including all dead and live loads
Medium dense to dense native Vashon Till soils = 2,500 psf
Compacted structural fill on top of the
medium dense to dense Vashon Till soils = 2,500 psf
- Minimum depth to bottom of perimeter footing below adjacent final exterior
grade = 18 inches
- Minimum depth to bottom of interior footings below top of floor slab = 18 inches
- Minimum width of wall footings = 16 inches
- Minimum lateral dimension of column footings = 24 inches
- Estimated post-construction settlement = 1/4 inch
- Estimated post-construction differential settlement; across building width = 1/4
inch
A one-third increase in the above allowable bearing pressures can be used when considering
short-term transitory wind or seismic loads.
Lateral loads can also be resisted by friction between the foundation and the supporting
compacted fill subgrade or by passive earth pressure acting on the buried portions of the
foundations. For the latter, the foundations must be poured "neat" against the existing
undisturbed soil or be backfilled with a compacted fill meeting the requirements for structural
fill. Our recommended parameters are as follows:
- Passive Pressure (Lateral Resistance)
• 350 pcf equivalent fluid weight for compacted structural fill
• 350 pcf equivalent fluid weight for native dense soil.
- Coefficient of Friction (Friction Factor)
• 0.35 for compacted structural fill
• 0.35 for native dense soil
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 7
GEO Group Northwest, Inc.
Drainage Considerations
We recommend that footing drains be constructed at the perimeter of the new foundations and at
the base of all retaining walls. Footing drains are recommended to consist of a minimum 4-inch
diameter perforated rigid PVC pipe laid in a bed of gravel, surrounded with gravel and separated
from finer grained material with a layer of filter fabric. The footing drain pipes should be
tightlined to the stormwater drainage system and downspout drains shall not drain into the
footing drain piping.
Slab-on-Grade Concrete Floors
Slab-on-grade concrete floors may be constructed directly on top of the competent medium
dense to very dense in-situ site soils or on top of compacted structural fills placed on top of the
competent site soils provided that the subgrade is not yielding at the time of concrete pour. Slab-
on-grade floors should not be constructed on top of loose soils or on top of wet yielding soils.
We recommend that we are retained to observe the condition of the slab subgrades prior to the
concrete pour to verify that they consist of medium dense or dense soils and are non-yielding. A
proof-roll by large construction equipment may be used to evaluate the condition of slab
subgrade areas. Over-excavation and replacement with compacted structural fill may be
necessary if loose or yielding soils are encountered at the slab subgrade. If structural fills are to
be placed at these areas then they should be compacted in accordance with the specifications in
the section titled: Structural Fill.
To avoid moisture build-up on the subgrade, slab-on-grade concrete floors should be placed on a
capillary break, which is in turn placed on the prepared subgrade. The capillary break should
consist of a minimum of a six (6) inch thick layer of free-draining crushed rock or gravel
containing no more than five (5) percent finer than the No. 4 sieve.
To reduce moisture vapor transmission through the slab we recommend installing a minimum
10-mil thick vapor retarder, such as Moistop Ultra® 10, by Fortifiber Building Systems Group®,
between the capillary break and concrete floor slab. Moistop Ultra 10 is a polyolefin film with a
water vapor permeance of 0.02 perms. It is puncture and tear resistant, meets ASTM E-1745
Class A, B and C requirements for underslab vapor retarders and is suitable for residential and
commercial applications. Boots are available for sealing around pipes, conduit and other
penetrations. We recommend these be installed in accordance with the manufacturer’s
recommendations.
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 8
GEO Group Northwest, Inc.
ADDITIONAL SERVICES
We recommend that GEO Group Northwest Inc. be retained to perform a general plan review of
the final design and specifications for the proposed development to verify that the earthwork and
foundation recommendations have been properly interpreted and implemented in the design and
in the construction documents. We also recommend that GEO Group Northwest Inc. be retained
to provide monitoring and testing services for geotechnically-related work during construction.
This is to observe compliance with the design concepts, specifications or recommendations and
to allow design changes in the event subsurface conditions differ from those anticipated prior to
the start of construction. We anticipate that geotechnical construction monitoring inspections
may be necessary for the following construction tasks:
1. Foundation and building pad preparation including over-excavation and structural fill
placement at areas where loose soils are encountered;
2. Structural fill placement and compaction;
3. Subsurface drainage installation;
4. Slab-on-grade floor subgrade preparation observation/approval.
LIMITATIONS
This report has been prepared for the specific application to this site for the exclusive use of
Sazei Design Group and their authorized representatives. Any use of this report by other parties
is solely at that party’s own risk.
Our findings and recommendations stated herein are based on field observations, our experience
and judgement. The recommendations are our professional opinion derived in a manner
consistent with the level of care and skill ordinarily exercised by other members of the
profession currently practicing under similar conditions in this area and within the budget
constraint. No warranty is expressed or implied. In the event that soil conditions not anticipated
in this report are encountered during site development, GEO Group Northwest, Inc., should be
notified and the above recommendations should be re-evaluated.
If you have any questions please do not hesitate to contact us.
October 2, 2020 G-5261
Geotechnical Report – 35027 – 21st Ave SW, Federal Way, WA Page 9
GEO Group Northwest, Inc.
Sincerely,
GEO GROUP NORTHWEST, INC.
Adam Gaston
Project Engineer
William Chang, P.E.
Principal
Attachments: Plate 1 – Vicinity Map
Plate 2 – Site Plan
Appendix A – Test Pit Logs and USCS Soil Legend
APPENDIX A
TEST PIT LOGS AND USCS SOIL LEGEND
G-5261
CLEAN
GRAVELS
GW
(little or no
fines)GP
DIRTY
GRAVELS
GM
(with some
fines)GC
CLEAN
SANDS
SW
(little or no
fines)SP < 5% Fine Grained:
GW, GP, SW, SP
DIRTY
SANDS
SM > 12% Fine Grained:
GM, GC, SM, SC
(with some
fines)SC
5 to 12% Fine
Grained: use dual
symbols
Liquid Limit
< 50%ML
Liquid Limit
> 50%MH
Liquid Limit
< 30%CL
Liquid Limit
> 50%CH
Liquid Limit
< 50%OL
Liquid Limit
> 50%OH
Pt
Sieve Size
(mm)Sieve Size
(mm)
SILT / CLAY #200 0.075
SAND 0 - 4 0 -15 Very Loose < 2 < 0.25 Very soft
FINE #40 0.425 #200 0.075 4 - 10 15 - 35 26 - 30 Loose 2 - 4 0.25 - 0.50 Soft
MEDIUM #10 2 #40 0.425 10 - 30 35 - 65 28 - 35 Medium Dense 4 - 8 0.50 - 1.00 Medium Stiff
COARSE #4 4.75 #10 2 30 - 50 65 - 85 35 - 42 Dense 8 - 15 1.00 - 2.00 Stiff
GRAVEL > 50 85 - 100 38 - 46 Very Dense 15 - 30 2.00 - 4.00 Very Stiff
FINE 19 #4 4.75 > 30 > 4.00 Hard
COARSE 76 19
COBBLES
BOULDERS
ROCK
FRAGMENTS
ROCK PLATE A1
CONTENT OF
FINES
EXCEEDS
12%
Cu = (D60 / D10) greater than 6
Cc = (D302 ) / (D10 * D60) between 1 and 3
Cu = (D60 / D10) greater than 4
Cc = (D302 ) / (D10 * D60) between 1 and 3
SILTY & CLAYEY SOILS
Blow
Counts
N
Unconfined
Strength
qu, tsf
Description
Bellevue, WA 98005
NOT MEETING ABOVE REQUIREMENTS
Fax (425) 649-8758Phone (425) 649-8757
Blow
Counts
N
Relative
Density
%
Friction
Angle
N, degree
Description
> 76 mm
>0.76 cubic meter in volume
13240 NE 20th Street, Suite 10
DETERMINE
PERCENTAGES OF
GRAVEL AND SAND
FROM GRAIN SIZE
DISTRIBUTION
CURVE
COARSE GRAINED
SOILS ARE
CLASSIFIED AS
FOLLOWS:SANDS
(More Than Half
Coarse Grains
Smaller Than No.
4 Sieve)
SILTS
(Below A-Line on
Plasticity Chart,
Negligible
Organic)
CLAYS
(Above A-Line on
Placticity Chart,
Negligible
Organic)
HIGHLY ORGANIC SOILS
NOT MEETING ABOVE REQUIREMENTS
UNIFIED SOIL CLASSIFICATION SYSTEM (USCS)
LEGEND OF SOIL CLASSIFICATION AND PENETRATION TEST
CONTENT
OF FINES
EXCEEDS
12%
ATTERBERG LIMITS BELOW
"A" LINE.
or P.I. LESS THAN 4
ATTERBERG LIMITS ABOVE
"A" LINE.
or P.I. MORE THAN 7
CLAYEY GRAVELS, GRAVEL-SAND-CLAY
MIXTURES
TYPICAL DESCRIPTION LABORATORY CLASSIFICATION CRITERIA
ATTERBERG LIMITS BELOW
"A" LINE
with P.I. LESS THAN 4
ATTERBERG LIMITS ABOVE
"A" LINE
with P.I. MORE THAN 7
U.S. STANDARD SIEVE
SOIL PARTICLE SIZE
FRACTION Passing Retained
GENERAL GUIDANCE OF SOIL ENGINEERING PROPERTIES FROM STANDARD PENETRATION TEST (SPT)
SANDY SOILS
INORGANIC CLAYS OF HIGH PLASTICITY, FAT
CLAYS
ORGANIC SILTS AND ORGANIC SILTY CLAYS OF
LOW PLASTICITY
ORGANIC CLAYS OF HIGH PLASTICITY
PEAT AND OTHER HIGHLY ORGANIC SOILS
CLAYEY SANDS, SAND-CLAY MIXTURES
INORGANIC SILTS, ROCK FLOUR, SANDY SILTS
OF SLIGHT PLASTICITY
INORGANIC SILTS, MICACEOUS OR
DIATOMACEOUS, FINE SANDY OR SILTY SOIL
INORGANIC CLAYS OF LOW PLASTICITY,
GRAVELLY, SANDY, OR SILTY CLAYS, CLEAN
CLAYS
WELL GRADED SANDS, GRAVELLY SANDS,
LIITLE OR NO FINES
POORLY GRADED SANDS, GRAVELLY SANDS,
LITTLE OR NO FINES
SILTY SANDS, SAND-SILT MIXTURES
WELL GRADED GRAVELS, GRAVEL-SAND
MIXTURE, LITTLE OR NO FINES
POORLY GRADED GRAVELS, AND GRAVEL-
SAND MIXTURES LITTLE OR NO FINES
SILTY GRAVELS, GRAVEL-SAND-SILT MIXTURES
> 203 mm
FINE-GRAINED
SOILS
More Than Half
by Weight
Smaller Than No.
200 Sieve
MAJOR DIVISION GROUP
SYMBOL
76 mm to 203 mm
ORGANIC SILTS
& CLAYS
(Below A-Line on
Placticity Chart)
More Than Half
by Weight Larger
Than No. 200
Sieve
COARSE-
GRAINED SOILS
GRAVELS
(More Than Half
Coarse Grains
Larger Than No. 4
Sieve)
0
10
20
30
40
50
60
0 10 20 30 40 50 60 70 80 90 100 110PLASTICITY INDEX (%)LIQUID LIMIT (%)
CL-ML
CL or OL
MH or OH
OL or ML
A-LinePLASTICITY CHART
FOR SOIL PASSING
NO. 40 SIEVE
7
4
CH or OH
Geotechnical Engineers, Geologists, &
Environmental Scientists
GEO Group Northwest, Inc.
TEST PIT NO. TP-1
LOGGED BY AG TEST PIT DATE:
DEPTH SAMPLE Water OTHER TESTS/
ft. USCS SOIL DESCRIPTION No. % COMMENTS
5
10
15
TEST PIT NO. TP-2
LOGGED BY AG TEST PIT DATE:09/17/2020
DEPTH SAMPLE Water OTHER TESTS/
ft. USCS SOIL DESCRIPTION No. % COMMENTS
5
10
15
TEST PIT LOGS
PROPOSED DEVELOPMENT
35027 - 21ST AVE SW
FEDERAL WAY, WA
DATE 9/16/20 PLATE A2JOB NO. G-5261
09/17/2020
Total depth of test pit = 7 feet below ground surface (bgs)
No groundwater seepage observed
Geotechnical Engineers, Geologists, &
Environmental Scientists
GEO Group Northwest, Inc.
S-1
Total depth of test pit = 6 feet below ground surface (bgs)
No groundwater seepage observed
Tan gravelly fine sandy SILT, dry, medium dense to dense ML
Whitish tan gravelly fine sandy SILT with some cobbles, dry,
cemented below 30" bgs, dense
Tan gravelly fine sandy SILT, moist, dense, cemented below 18" bgsML
S-1
S-2
S-2
Gray cemented gravelly fine sandy SILT, moist, dense to very
dense (unweathered till)
ML
ML
Probe 6-8"
Probe 1"
Probe 1/2"
Probe 1/4"
Probe 3"
Probe 1-2"
Gray with mottling gravelly sandy SILT, moist, cemented, dense to
very dense (unweathered till)
ML
Probe 1/4"
Probe 1/4"
Probe 1"
Probe 1-2"
S-3
becoming gravelly silty SAND below 6' bgs (unweathered till)SM
Probe 1/4"
- occasional cobble at 18" bgs
TEST PIT NO. TP-3
LOGGED BY AG TEST PIT DATE:
DEPTH SAMPLE Water OTHER TESTS/
ft. USCS SOIL DESCRIPTION No. % COMMENTS
5
10
15
TEST PIT NO.
LOGGED BY TEST PIT DATE:
DEPTH SAMPLE Water OTHER TESTS/
ft. USCS SOIL DESCRIPTION No. % COMMENTS
5
10
15
TEST PIT LOGS
PROPOSED DEVELOPMENT
35027 - 21ST AVE SW
FEDERAL WAY, WA
DATE 9/16/20 PLATE A3
09/17/2020
JOB NO. G-5261
Total depth of test pit = 8 feet - 9-inches below ground surface (bgs)
No groundwater seepage observed
Geotechnical Engineers, Geologists, &
Environmental Scientists
GEO Group Northwest, Inc.
S-1
Tan gravelly fine sandy SILT, dry, dense ML
Tan gravelly fine sandy SILT with some cobbles and the occasional
boulder, dry, cemented below 3' bgs, dense
S-2
Gray cemented gravelly fine sandy SILT, moist, dense to very
dense (unweathered till)
ML
ML
Probe 1"
Probe 1/2"
Probe 1/4"
Probe 1/4"
Probe 1"
Probe 1"
S-3
- becoming gravelly silty SAND below 6' bgs (unweathered till)SM
S-4 Probe 1/4"
- becoming gravelly SAND with some silt below 7.5' bgs, dense to
very dense (unweathered till)
SM/
SP