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 4 In Hand Auger Three, located at the southwest of the proposed addition, we encountered at least 7.0 feet of mixed silty fine to coarse sand with charcoal and organic debris in a loose to medium dense condition. The exploration terminated at a depth of 7.0 feet after coring through approximately 1.0 foot of hardwood, which suggests a buried stump or large log. Shallow explorations on the northeast -facing ravine slopes revealed approximately 1.0 foot of brown, silty fine to coarse sand with gravel and debris in a loose condition. We interepted this deposit to be colluvial in nature. Underlying the colluvium, in shallow hand holes, we encountered gray -brown silty fine to medium sand with trace gravel in a medium dense condition, which was interpreted to be the glacial advance outwash mapped in the vicinity of the ravine slopes. Hydrogeologic Conditions Groundwater seepage was not encountered in any of our explorations. Additionally, groundwater seepage was not encountered on the northeast -facing ravine slopes during our visit on May 24, 2018. Although signs of colluvial deposition (i.e. slope wash) were observed on the site slopes, signs of persistent erosion from groundwater seepage were not encountered. If groundwater were to be observed within the soils on this site, we would interpret it to be perched water and not a regional groundwater table. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability material. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2018 International Building Code (IBC) standards for seismic site classification for this project. Since dense glacial deposits are interpreted to underlie the site at depth, the site best fits descriptions for Site Class D. 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 on the site caused by this fault zone is considered low, based on best available information. NELSON GEOTECHNICAL ASSOCIATES, INC.