10723 19TH AVE SE Geotech Report 2023-04-07

Geotechnical Engineering Evaluation NGA File No. 1014317 Zerzura Townhome Development November 9,2017 Everett,Washington Page 4 The site is located within the South Whidbey Island Fault Zone (SWIFZ): an active, shallow region of seismicity within central Puget Sound stretching from the Strait of Juan de Fuca to North Bend. Information published in 2013 by the Washington State Department of Natural Resources suggests the SWIFZ last ruptured less than 2,700 years ago, and that the fault zone can produce a M7.5 earthquake. In our opinion,the possibility of faulting ground rupture caused by this fault zone is considered low. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the very dense glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. Erosion Hazard The criteria used for determination of erosion hazard areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to the vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington by the Soil Conservation Service (SCS), was reviewed to determine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as Alderwood-Urban land complex, 2 to 8 percent slopes. These soils are listed as having a slight to moderate erosion hazard where soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas where vegetation is not disturbed. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the site is generally compatible with the planned development from a geotechnical standpoint. Our explorations indicated that the site is generally underlain by competent native glacial till soils at relatively shallow depths. The native glacial till soils should provide adequate support for foundation, slab, and pavement loads. We recommend that the planned structures be designed utilizing shallow foundations. Footings should be founded on the underlying medium dense or better native soil, or structural fill extending to these soils. The medium dense or better soil should typically be encountered approximately two to three feet below the existing surface, based on our explorations. However, deeper areas of undocumented fill or loose soils could be encountered in the unexplored areas of the site. This condition, if encountered, would require deeper excavations in foundation, slab, and pavement areas to remove the unsuitable soils. NELSON GEOTECHNICAL ASSOCIATES,INC.