909 51ST PL SW Geotech Report 2022-10-25

Geotechncial Engineering Evaluation Garcia Property Additions Everett, Washington NGA Project No. 1040218 June 22, 2018 Page 8 Site Preparation and Grading Site preparation should consist of the removal of the any topsoil or organic fill from the development area. The stripped material should be hauled off site. The stripped material should not be stockpiled near the slope or be used for structural fill. With granular soils exposed at the ground surface, the site should be prepared for access for equipment. The contractor installing the piles should be consulted regarding site preparation for equipment access. Any disturbed areas should be restored after construction. The site soils are considered to be moisture -sensitive and will disturb easily when wet. We recommend that construction take place during the drier summer months if possible. However, if construction takes place during the wet season, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls on exposed subgrades, construction traffic areas, and paved areas prior to placing structural fill. Wet weather grading will also require additional erosion control and site drainage measures. Structure Setbacks Uncertainties related to building along steep slopes are typically addressed by the use of building setbacks. The purpose of the setback is to establish a "buffer zone" between the structure and the top of the slope so that ample room is allowed for normal slope recession during a reasonable life span of the structure. In a general sense, the greater the setback distance, the lower the risk of slope failures impacting the structure. From a geological standpoint, the setback dimension is based on the slope's physical characteristics, such as slope height, surface angle, material composition, and hydrology. Other factors such as historical slope activity, rate of regression, and the type and desired life span of the development are important considerations as well. Top -of -Slope buffers of 25 feet for the ravine slopes are applicable to the site, as outlined by the 2018 IBC section 1808.7, and by EMC 19.37.080.B2. Based upon the conditions described above, it is our opinion that the potential for shallow sloughing -type failures exists on the steep slope. As a modification to the buffer, given the conditions observed on the slopes, we recommend that the residence be set back a minimum of 15 feet from the top of the steep northeasterly -facing slopes. To reduce the risk of potential slope failures of affecting the structure, the structure should be supported on pin pile foundations extending deep enough to provide a 25-foot effective setback from the face of the slope to the bottom of the pin piles. The pin piles should all be tied together via grade beams structurally connected to the residence. NELSON GEOTECHNICAL ASSOCIATES, INC.