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4735 Bayview Lane <br />Everet4 Washington Geotechnical Report <br />Slope Stability <br />Although a computer modeled slope stability analysis was not conducted as a part of this study, <br />based on the observed conditions and underlying dense to very dense, glacially consolidated soils <br />without an adverse ground water table, it is our opinion that the existing slope stability factor of <br />safety for a deep seated, rotational landslide is well in excess of 1.5 under static conditions and 1.2 <br />under dynamic conditions. As with all steep slope areas there is a risk of shallow, colluvial slope <br />movement and soil creep of the outer soil layer. The planned new construction, with no alteration <br />to the steep slope area and no loss of vegetation, will not have any adverse impact to the stability <br />of the slope and the factors of safety will be unchanged from pre -development conditions. In <br />addition, a shallow colluvial landslide, should it ever occur, would only encompass the outer 1 to <br />3 feet and will have no impact to the proposed residence. <br />Foundation Recommendations <br />Based on the plans provided to us, the existing basement and crawlspace will remain and be reused <br />in the remodel. Although we have no direct knowledge of the existing foundation system our <br />visual evaluation of the existing foundation did not find any indications of past failure or <br />significant cracking of the concrete. <br />All new foundations for the additions should be placed atop native bearing soils consisting of <br />medium dense or better, silty, fine to medium sand with gravel and cobbles. These bearing soils <br />were observed at very shallow depth in exploration pits EP-1 and EP-4. However, in EP-5 the <br />bearing soils were overlain by about 3 feet of silty clay sediments. Footings bearing on the medium <br />dense or better granular sediments may be designed for an allowable soil bearing capacity of 2,500 <br />psf. Passive resistance to lateral translation can be obtained from backfill around the footings. <br />The backfill should be compacted to a medium dense or better condition. For foundations that are <br />appropriately backfilled a passive resistance of 250 pcf may be used for design purposes. <br />Additionally, a soil friction factor of 0.45 may be used if additional passive resistance is required. <br />An increase of 1/3 may be used for short term wind or seismic loading. Anticipated settlement <br />over a 20-foot span should be less than 1/2 inch total and 1/4 inch differential for equally loaded <br />foundations. <br />Retaining Walls <br />Any new foundation or retaining walls that are required should be designed using the design <br />parameters provided in the foundation recommendations section above. In addition, the walls <br />should be designed for an equivalent fluid pressure of 35 pcf if unbraced and free to rotate. Braced <br />walls that cannot rotate should be designed for an equivalent fluid pressure of 55 pcf. <br />All walls over 3 feet tall must be effectively drained in order to avoid hydrostatic pressure on the <br />wall. The wall drain can either be a minimum of 12 inches wide of free draining material (< 3 <br />percent fines)j or can be a drainage mat such as MiraDrain or DeltaDrain. The drainage material <br />Gary A. Flowers, PLLC <br />5205 23`tl Avenue W Everett, Washington 98203 206-819-4304 <br />6 <br />