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SKJVgeotech4.06.pdfAssociated E%A Sciences, Inc. April 10, 2006 Project No. KE060165A Soundview Homebuilders 902 e Avenue South Edmonds, Washington 98020 Attention: Mr. Ron Den Adel ADM Q6-32 RECEIVE® JUN -0.8 2006 MIF-.oPMENT SERVICES C-rR. CITY of EDM01q1)5 Subject: Subsurface Exploration and Geotechnical Engineering Letter- e o 17039 76'h Avenue West - Lot 3 R E C EH E D Edmonds, Washington JUN 14 2006 Dear Mr. Den Adel: EECE Associated Earth Sciences, Inc. (AESI) is pleased to present this letter -report su mmrizing the results of our subsurface exploration and providing design criteria for foundations and rockeries for the proposed home to be built on Lot 3 of the subject property. A geotechnical report is required for this property based on Section 19.10 of the City of Edmonds Municipal Code for property with slope inclinations exceeding 15 percent, and City of Edmonds review comments issued for your project on December 28, 2005. Subsurface Exploration We completed one exploration pit (EP -1) in the building envelope for Lot 3 on January 2, 2006. We encountered 12 inches of topsoil over 1 foot of loose silty sand with roots overlying brown grading to gray, medium stiff grading to hard, sandy silt to the maximum depth explored of 10 feet (approximate elevation 118). Elevation estimates are based on plotting the exploration pit location on a site survey provided by you (Figure 1). Very slow, perched ground water seepage was observed at a' depth of 2 feet within exploration pit EP -1. The seepage depth represents the contact between the loose surficial sediments and underlying denser sediments. Samples were collected at each strata change. We interpreted the sand to be Vashon advance outwash and the silt to be pre -Fraser sediments consistent with those mapped on the "Composite Geologic Map of the Sno King Area, " Booth et a1., 2004. Advance outwash consists of sand that has been deposited in front of an advancing glacial ice sheet and has been overridden by several thousand feet of glacial ice. Pre -Fraser fine-grained sediments are sediments deposited in a large proglacial lake formed by ice dams that blocked the outlet to the Pacific Ocean prior to the advance of the ice sheet that eventually covered the Puget Sound area during the Fraser Glaciation. As such, this material has also been over -consolidated by the weight of glacial ice. — 11 1 err - — i r.:GL a_------ C._.«_ 4 M. 7!:..1.1,,..,1 [YIA ilQrk:12 .131 fA741 977_77(11 • F 1 (!7771 R77_Kb.7ls Recommendations and Design Criteria Erosion Mitigation Measures The site slopes are defined as Erosion Hazard Areas by the City of Edmonds. The surface sediments underlying the site generally contain silt and fine sand and will be sensitive to erosion and moisture, especially in the sloping portions of the site. In order to reduce the amount of sediment transport off the site during construction, the following recommendations should be followed. 1. Temporary silt fencing should be placed around the lower perimeter of all cleared areas) and at the base of sloped areas. The fencing should be periodically inspected and maintained, as necessary, to ensure proper function. 2. To the extent practical, earthwork -related construction should proceed during the drier periods of the year, and disturbed areas should be revegetated as soon as possible. Temporary erosion control measures should be maintained until permanent erosion control measures are established. 3. Areas stripped of vegetation during construction should be mulched and hydroseeded, replanted as soon as possible, or otherwise protected. During winter construction, hydroseeded areas should be covered with clear plastic to facilitate grass growth. 4. If excavated soils are to be stockpiled on the site for reuse, measures should be taken to reduce the potential for erosion from the stockpile. These could include, but are not limited to, covering the pile with plastic sheeting, the use of low stockpiles in flat areas, and the use of straw bales/silt fences around stockpile perimeters. 5. To the extent necessary, interceptor swales with rock check dams should be constructed to divert storm water from construction areas and to route collected storm water to an appropriate discharge location. 6. A rock construction entrance should be provided to reduce the amount of sediment transported off-site on truck tires. 7. All storm water from impermeable surfaces, including driveways and roofs, should be tightlined into approved facilities and not be directed onto or above steeply sloping areas. Temporary Cut Slopes Review of the project plans indicate that cuts up to 13 feet high along the east side of the house will be required to obtain the design basement floor elevation. In our opinion, stable construction slopes should be the responsibility of the contractor, and should be determined during construction based on actual soil and ground water conditions encountered. For estimating purposes, we anticipate that temporary, unsupported cut slopes in the advance 2 outwash sand and underlying pre -Fraser silt can be planned at 1HAV (Horizontal:Vertical) or flatter. Temporary, vertical cuts should be no taller than 4 feet. Permanent cut or structural fill slopes should not be steeper than 211: IV. These slope angles are for areas where ground water seepage is not encountered, and assume that surface water is not allowed to flow across the slope faces. If ground or surface water is present when the excavation slopes are exposed, flatter slope angles or shoring may be required. As is typical with earthwork operations, some sloughing and raveling may occur, and cut slopes may have to be adjusted in the field. In addition, WISHA/OSHA regulations should be followed at all times. Engineering Design Criteria Based on our calculations, the basement retaining walls for the structure must be completely founded within the dense to very dense natural soils in order to reduce the risk of slope failure along the sand/silt depositional plane that is characteristic. of the failure mechanism within the Meadowdale Landslide Complex. Interior spread footing foundations, and the west and portions of the north and south walls of the home, can be founded in medium dense to dense, brown or gray sand. For the residential structure founded on the prescribed bearing stratum, spread footings may be designed for an allowable foundation soil bearing pressure of 1,500 pounds per square foot (psf), including both dead and live loads. This bearing pressure is also appropriate for approved granular on-site or imported structural fill, compacted under our observation and testing to at least 95 percent of American Society for Testing and Materials (ASTM):D 1557 and to a firm, non -yielding condition, as determined by the geotechnical engineer or their representative. The on-site sand is appropriate for structural fill use. However, the silt is not considered appropriate for use as structural fill due to its extreme moisture sensitivity. Soils in which the amount of fine-grained material (smaller than the No. 200 sieve) is greater than approximately 5 percent (measured on the minus No. 4 sieve size) should be considered moisture -sensitive. Use of moisture -sensitive soil in structural fills should be limited to favorable dry weather and dry subgrade conditions. If fill containing moderate quantities of silt is placed during wet weather or if proper compaction cannot be obtained, a select material consisting of a clean, free -draining gravel and/or sand should be used. Free -draining fill consists of non-organic soil with the amount of fine-grained material limited to 5 percent by weight when measured on the minus No. 4 sieve fraction and at least 25 percent retained on the No. 4 sieve. These recommendations are predicated on a member of our firm observing footing foundation excavations to determine if the proper bearing capacity has been reached. Anticipated settlement of footings founded as recommended should be on the order of N inch or less, with differential settlement of 1/z inch or less. However, disturbed material not removed from footing trenches prior to footing placement could result in increased settlements. 3 Structural design of the residence should follow 2003 International Building Code (IBC) standards using Site Class "D" as defined in Table 1515.1.1. The 2003 IBC seismic design parameters for short period (Ss) and I -second period (Sr) spectral acceleration values were determined by the latitude and longitude of the project site using the United States Geological Survey (USGS) National Seismic Hazard Mapping Project website (http://earthquakee.usgs.gov/hazmaps/). Based on the more current 2002 data, the USGS website interpolated ground motions at the project site to be 1.22g and 0.438 for building periods of 0.2 and 1.0 seconds, respectively, with a 2 percent chance of exceedance in 50 years. Upslope from the east basement retaining walls, short rockery structures are currently planned. The rockeries should be founded at least 8 feet in horizontal distance from the east basement retaining wall on medium stiff/dense to hard/dense native silt or sand sediments, or on structural fill compacted to at least 95 percent of ASTM:D 1557. The slopes above the rockeries should nQLexceed--2H-4 r. 146iizontally backfilled basement walls that are free to yield laterally at least 0.1 percent of their height may be designed using an equivalent fluid equal to 40 pounds per cubic foot (pef). Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an equivalent fluid of 55 pcf. If parking areas are adjacent to walls, a surcharge equivalent to t of soil should be added to the wall height in determining lateral des' n fo cesw As required by the 2003 IBC, retaining wall design should include a seismic surcharge pressure in addition to the equivalent fluid pressures presented above. Considering the site soils and the recommended wall backfill materials, we recommend a seismic surcharge pressure of 4H and 8H psf, where H is the wall height in feet for the active and at -rest loading conditions, respectively. The seismic surcharge should be modeled as a rectangular distribution with the resultant applied at the midpoint of the wall. The lateral pressures presented above are based on the conditions of a uniform backfill consisting of excavated on-site granular (sandy) soils, or imported granular structural fill compacted to 90 percent of ASTM:D 1557. A higher degree of compaction is not recommended, as this will increase the pressure acting on the walls. A lower compaction may result in settlement of the slab -on -grade or other structures supported above the walls. Thus, the compaction level's critical and must be tested by our firm during placement. Surcharges from adjacent footings or heavy construction equipment must be added to the above values. Lateral loads can be resisted by friction between the foundation and the native, medium stiff/dense to hard/dense soils or supporting structural fill soils, or by passive earth pressure acting on the buried portions of the foundations. The foundations must be backfilled with granular structural fill, compacted to at least 95 percent of the maximum dry density, to achieve the passive resistance provided below. We recommend the following allowable design parameters: • Passive equivalent fluid = 250 pef • Coefficient of friction = 0.35 WU Drainage. Perimeter footing drains should be provided for all retaining walls and footings. It is imperative that. proper drainage be provided so that hydrostatic pressures do not develop against the walls. This would involve installation of a minimum, 1 -foot -wide blanket drain to within 1 foot of finish grade for the full wall height using imported washed gravel against the walls. If a drainage mat is used, it should be installed according to the manufactprer's specifications. The retaining wall detail drawing shown on the building plans we reviewed included this drain. All footing drains should be tightlined separately from roof drains to an approved discharge location. Closure We trust this information meets your current needs additional information, please contact us. Sincerely, ASSOCIATED EARTH SCIENCES, INC. Kirkland, Washington R a s 704 eo\ tsC� Lon N. S dergaard Jon N. Sondergaard, P.G., P.E.G. Principal Engineering Geologist Attachments: Figure 1 - Site and Exploration Plan Exploration Pit Log JNSlid KE060165A1 ProjectsX200601651KE1WP k If you have any questions or require ExpnEs 2 I is 1 Matthew A. Miller, P.E. Associate Engineer , + 170M 76TH AM W. � � GEORCE *1700WARo i MMONM WA \ 17004 78TH RYE W. ! , -4040 ,\\ FX- DRAWACE N. N L7 �- 6:xP1-0 ATtt �1 _401 t fu L HfAWWAY R I 12,048 3E (NEW 17o1�B 74 YESY F", 1 14702 SF. {CFi065) v a. HK AVERAGE EL -11a1 mAX 0(*f : EL. -144.1 c i � r 1 oerl��srraa'oo- / 1,-.31.42'* r -'' e e i � I Q. r j _ x3 13,319 -Ssm (r 1 ! u 1 Ar _ 171f v� w. N swum x4w3m EDWONM Wa A "=3o' FIGURE i Associated Earth seances, Inc. SITE AND EXPLORATION PLAN ® � � EDMONDS SHORT PLAT DATE 4/06 EDMONDS, WASHINGTON PROJ. No. KE060165A LAG OF EXPLORATION PIT NO. EP -1 South Home 9 This fog is part of the reportprepared by Associated Earth Sciences, Inc. (AESI) for the named project and should be 5 read together with that report for complete interpretation. This summary appplies only to the location of this trench at the 0 time of excavation. Subsurface conditions may change at this location with the passage of time. The data presented are n a simplfication of actual conditions encountered. DESCRIPTION 1 2 3 4 5 6 7 8 9 1© 11 12 13 14 15 16 17 18 19 Loose, very moist, brown, silty SAND with roots, mottled. Pre -Fraser Depo Medium stiff, very moist, brown, sandy SILT. Medium stiff to stiff, very moist, brown, sandy SILT, little gravel. Very stiff, moist, gray, sandy SILT, trace gravel and cobbles. Hard below 9'. Bottom of exploration pit at depth 10 feet Slow seepage at 2'. Edmonds Shout Plat Edmonds, WA Associated Earth Sciences, Inc. a project No. KE05957A 9 Logged by: a Approved by: GB ® ® ® 1131fl6 U Y