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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 �o �'JST R� 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