grealish geotechnical evaluationBergq ist
27207 8th Avenue S
Des Moines, Washington 98198
Engineering Services
PO, Box 13309
Des Moines, Washington 98198
Phone: 253.941.9399, Fax: 253,941.9499, e-mail: RBergqu510@aol,com
December 31, 2004
Mr. Morgan Llewellyn
PO Box 902
Kent, Washington 98035
Re: Geotechnical Engineering Services
212 SW 292"1 Street
Federal Way, Washington
King County Parcel Number: 1196000121
BES Project Number: 200424, Report 1
Dear Mr. Llewellyn:
This report presents the results of our preliminary geotechnical evaluation for the
proposed construction of a new residence at the site of the existing residence at 212 SW
292"1 Street in Federal Way, Washington. The location of the site is shown on the Vicinity
Map on page Al of this report. The geotechnical evaluation was performed by Bergquist
Engineering Services (BES) to provide information regarding:-
• topographic features on the site,
• geologic setting of the site,
• readily identifiable geotechnical constraints to the project,
• estimated stability of the slope,
• preliminary foundation recommendations, and
• drainage recommendations.
You authorized our work on November 18, 2004 by signing and retuming a copy of
BES Proposal Number 1302004, dated November 8, 2004.
GECTECHNiCAL ENGINEERING AND CONSTRUCTION INSPECTICN
Proposed Residence BES Project Number: 200424, Report 1
Federal Way, Washington December 31, 2004
The proposed project involves design and construction of multi -story, wood -framed
house with a concrete foundation that forms a daylight basement. The building footprint will
be about the size and location of the existing footprint. Site constraints include the slope to
the north and property boundaries to the west and east.
Actual foundation loads were not provided at the time this report was prepared.
Therefore, we have assumed that individual column loads will not exceed 120 kips and
continuous wall loads will not exceed four (4) kips per lineal foot. if the actual foundation
loads are greater than those stated herein, the geotechnical engineer must be notified to
determine whether the recommendations presented herein require revision.
The scope of services included a reconnaissance of the site by the geotechnical
engineer, a review of geologic literature and previous reports prepared by us, and field
measurement of the existing slopes. The data were then evaluated by the geotechnical
engineer relative to the proposed construction. The engineering recommendations and
advice presented in this report have been made in accordance with generally accepted
geotechnical-engineering practices in the area.
The geometry of the slopes at the locations analyzed for stability were obtained from
site cross -sections developed R. Bergquist using a cloth tape, hand clinometer, Brunton
compass, and methodology outlined in Williamson, Neal and Larson (1991).
The slope stability analyses for this study were performed using XSTABL software
developed by Interactive Software Designs, Inc. XSTABL performs a two dimensional limit
equilibrium analysis to compute the factor of safety for a layered slope according to either:
(1) General Limit Equilibrium (GLE) Method, (2) Janbu's Generalized Procedure of Slices
(GPS), (3) Simplified Bishop, or (4) Simplified Janbu. XSTABL calculates a FOS (factor of
safety), which is the result of dividing the total forces supporting the slope by the total forces that
are tending to destabilize the slope. If the FOS is greater than 1.00, the slope is considered
stable; if the FOS is less than 1.00, the slope is considered unstable. A FOS of 1.00 indicates
the slope is in perfect equilibrium. Because of the uncertainty regarding pertinent soil
parameters used in the analyses and the variability of soil, slope, and ground water conditions,
a non -seismic FOS greater than 1.25 is generally recommended for residential construction.
Bergquist Engineering Services Page 2 of 6
Proposed Residence BES Project Number: 200424, Report i
Federal Way, Washington December 31, 2004
The information presented in this section was gathered by BES personnel for
preliminary geotechnical engineering purposes only. This site characterization was not
intended to address the presence or likelihood of contamination on or around the site.
Specialized methods and procedures, which were not a part of this scope of services, are
required for adequate final geotechnical design recommendations and environmental site
assessment.
From the north edge of 212 SW 292'' Street, the ground surface slopes down gently to the
south side of the existing residence. The ground surface drops about five feet from the south to north
sides of the existing house. There is a narrow, nearly level bench between the house and the crest of
the slope. The ground surface slopes down at an average slope of about 50 percent. There are
several nearly flat benches in the slope caused by the switchback, walking path that traverses the
slope. The ground surface flattens for a distance of about 15 feet where a three-foot tail, concrete
retaining wall forms a vertical slope to the beach. The beach slopes gently to the water's edge. The
slope is mostly covered with a ground cover consisting predominantly of ivy.
The site, according to readily available geologic literature, is comprised predominantly
of Vashon till except for the beach deposits at the bottom of the slope. The Vashon till is
characterized as a dense to very dense mixture of sand and silt with varying quantities of
boulders, cobbles, gravel, and clay. The beach deposits are derived mostly from material in
the adjoining bluffs and consist of loose to medium dense gravel, sand, silt, and clay.
The engineering properties of the subsurface soils used in the stability analysis are
presented in Table 1. These values are based on experience and on the publication entitled
Geotechnical Properties of Geologic Materials by Jon W. Koloski, Sigmund D. Schwartz, and
Donald W. Tubbs. The values used were intentionally selected at the low end of the range.
TARI F 1- ENGINEERING PROPERTIES USED IN STABILITY ANALYSIS
SOIL UNIT
Moist Unit Weight I
Cohesion (C)
Angle of Inienal Friction (0)
Beach deposits
120 pcf
0 psf
28"
Vashon Till
136 pcf
1500 psf
360
The slope was modeled with uniform foundation loads of 1,000 psf over the footprint of
the proposed building and 200 psf over the footprint of the existing garage. The program was
instructed to calculate the FOS for 1600 potential shear surfaces during each iteration using
Bergquist Engineering Services Page 3 of 6
Proposed Residence
Federal Way, Washington
BES Project Number: 200424, Report 1
December 31, 2004
one of the described methods. The attached graphs (See pages A3 and A9) show the locations
of the weakest surfaces analyzed within the slope segment selected for analysis, with the surface
having the lowest FOS highlighted.
The estimated minimum factors of safety for the existing slope with building loads are
tabulated below (See pages A3 through Al6).
TABLE 2: MINIMUM FACTORS OF SAFETY WITHOUT EARTHQUAKE LOADS
Cross Section Azimuth 0'
Estimated Minimum Factor 3.684
of Safety
TABLE 3: MINIMUM FACTORS OF SAFETY WITH EARTHQUAKE LOADS
Cross Section Azimuth 0°
Estimated Minimum Factor 1.691
of Safety
Based on the stability analysis, the proposed residence should be stable if construc`ed over or
south of the footprint of the existing residence. Conventional, shallow, spread footing foundations are
expected to be suitable at this site based on the geology of the site. Generally, allowable bearing
capacities of these soils are at least 2,000 psf with settlements estimated to be less than one -inch.
The recommended minimum dimension for continuous wall footings is 18 inches and for
individual column footings, the recommended minimum dimension is 24 inches. Footings
near and parallel to the slope crest should be at least 30 inches below the existing grade.
Lateral earth pressures typically used for design of foundations, or short retaining walls
with level backfill and without hydrostatic pressures or surcharge loads, may be calculated
using the equivalent fluid pressures presented below:
Active: Undisturbed subsoils 40 psf/ft.
Compacted granular soils 35 psf/ft.
Passive: Continuous footings 250 psf/ft.
Column footings 300 psf/ft.
Bergquist Engineering Services Page 4 of 6
Proposed Residence BES Project Number: 200424, Report 1
Federal Way, Washington December 31, 2004
Footings, stem walls, and retaining walls must be reinforced to reduce the potential for
distress caused by differential foundation movements. A qualified engineer must determine
the size, quantity, and location of reinforcement. The geotechnical engineer must inspect all
footing excavations at this site prior to placement of construction forms and reinforcement
steel.
Good drainage is critical to the performance of earth -supported structures such as
foundations and retaining walls. Therefore, construction grades and final site grades must be
designed to prevent water from flowing over the slope or from ponding in areas on or
adjacent to foundations, slabs -on -grade, and pavements. Infiltration of water into foundation
and utility excavations shall be prevented during construction and throughout the life of the
project.
All major earthwork for this project shall be completed during the dry, summer
months. The excavations and slopes shall be stabilized before the onset of wet, winter
weather.
The engineering properties of the subsurface soils are based experience and on
assumptions. The actual engineering properties shall be determined by an adequate
subsurface exploration.
The recommendations presented in this report rely on adequate observation and
testing of construction materials and procedures by the geotechnical engineer or his qualified
representative. At a minimum, the testing program must include:
• Observation and review of site clearing and review of all foundation excavations to
evaluate whether actual conditions are consistent with those encountered during
exploration.
• Observation and testing of placement and compaction of all fill and backfill
materials to evaluate compliance with specifications.
• Field inspection and laboratory testing of materials and field inspection of methods
as required by the appropriate building codes. Typically, this includes inspection
Bergquist Engineering Services Page 5 of 6
Proposed Residence
Federal Way, Washington
BES Project Number: 200424, Report l
December 31, 2004
of placement of reinforcing steel; inspection and testing of portland cement
concrete to evaluate compliance with specifications regarding slump, temperature,
air content, and strength.
If you have any questions, or if we may be of further service, please contact us.
A. G" og W►sy��L;;
.d,P 3.03.77
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ssI�NALti�G�
Sincerely, -
Bergquist Engineering Services
Richard A. Bergquist, P.E.
Principal
EXPIRES r/l6
Attachments: Al Vicinity Map
A2 Site Plan
A3-Al 7 Computer Graphs of Stability Analysis
Bergquist Engineering Services Page 6 of 6