1901 W MUKILTEO BLVD Geotech Report 2022-10-25

Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 11 the design capacity. Piles that do not meet this minimum embedment criterion should be rejected, and replacement piles should be driven after consulting with the structural engineer regarding the new pile locations. The piles should be spaced a minimum of three feet apart to avoid a grouping effect on the piles. Due to the relatively small slenderness ratio of pin piles, maintaining pin pile confinement and lateral support is essential in preventing pile buckling. Pin piles should be suitably embedded into the reinforced concrete. The structural engineer should design the connections of the piles to the foundations. Vertically driven pin piles do not provide meaningful lateral capacity. All lateral loads should be transferred back to the existing residence via grade beams. Due to the rigid pile support, friction between the foundation and subgrade soil should not be considered as resisting lateral pressures on this structure. Retaining Wall Design and Construction General Any proposed new retaining wall, for protection of the top of the slope, should consist of minimum 6-inch driven steel "H"piles (W6x25) bridged using pressure-treated timber lagging and anchored back into the slope using grouted tieback anchors or mechanical anchors such as helical anchors. The new retaining wall should be generally located along the existing top of the steep slope along the entire width of the property. The final extent and heights of the new retaining wall should be determined based on an accurate survey of the property. However, we recommend that the exposed portion of the new wall not exceed eight feet,but be no less than four to six feet. The retaining wall should be designed by an experienced structural engineer licensed in the State of Washington. The wall should be designed to resist an active pressure acting on the piles and lagging for design of the piles and should be calculated based on a triangular pressure distribution equivalent to that exerted by a fluid with a density of 60 pcf. This value is based on loads from the undocumented fill and loose soils that will remain behind the wall. A uniform surcharge of 8H should be applied to the wall design to account for seismic loading,where H is the total height of the wall. The above active load should be applied on the full center-to-center pile spacing above the base of the exposed portion of the wall. A 50 percent reduction of this value can be used for the purpose of designing the wall lagging. The active pressure should be entirely resisted by the tieback anchors. Passive resistance on the below grade portion of the piles should not be included in the design. NELSON GEOTECHNICAL ASSOCIATES, INC.