2710 14TH ST BLDG A Geotech Report 2023-12-29

■ The on -site soils generally contain a high percentage of fines (silt and clay) ranging from 36 to 98 percent based on our laboratory tests and are highly moisture sensitive. Therefore, reuse of on -site soils should only be planned in the normal dry season (June through September). ■ We anticipate that long-term design infiltration rates will be less than 0.2 inches per hour within the native glacial till and glaciolacustrine deposits. On -site infiltration testing will be needed if infiltration facilities are planned as part of the project. These geotechnical considerations are discussed in greater detail, and conclusions and recommendations for the geotechnical aspects of the project are presented in the following report sections. 4.1. Earthquake Engineering 4.1.1. Seismicity The Puget Sound area is located near the convergent continental boundary known as the Cascadia Subduction Zone (CSZ), which extends from mid -Vancouver Island to Northern California. The CSZ is the zone where the westward advancing North American Plate is overriding the subducting.luan de Fuca Plate. The interaction of these two plates results in two potential seismic source zones: (1) the Benioff source zone and (2) the CSZ interplate source zone. A third seismic source zone, referred to as the shallow crustal source zone, is associated with the north -south compression resulting from northerly movement of the Sierra Nevada block of the North American Plate. Shallow crustal earthquakes occur within the North American Plate to depths up to 15 miles. Shallow earthquakes in the Puget Sound region are expected to have durations ranging up to 60 seconds. Four magnitude 7 or greater known shallow crustal earthquakes have occurred in the last 1,100 years in the Cascadia region; two of these occurred on Vancouver Island and two in Western Washington. The northeast -southwest trending Southern Whidbey Island fault zone is mapped approximately 6 miles southwest of the site. The Benioff zone is characterized as being capable of generating earthquakes up to magnitude (M) 7.5. The Olympia 1949 (M = 7.1), the Seattle 1965 (M = 6.5) and the Nisqually 2001 (M = 6.8) earthquakes are considered to be Benioff zone earthquakes. The recurrence interval for large earthquakes originating from the Benioff source zone is believed to be shorter than for the shallow crustal and CSZ source zones; on average, damaging Benioff zone earthquakes in Western Washington occur every 30 years or so. The CSZ is considered as being capable of generating earthquakes of magnitudes 8 to 9. No earthquakes on the CSZ have been instrumentally recorded; however, through the geologic record and historical records of tsunamis in Japan, it is believed that the most recent CSZ event occurred in the year 1700. Recurrence intervals for CSZ interplate earthquakes are thought to be on the order of 400 to 600 years. 4.1.2.Seismic Hazards We evaluated the site for seismic hazards including liquefaction, lateral spreading and fault rupture. Our evaluation indicates the site does not have liquefiable soils present and, therefore, also has little to no risk of liquefaction -induced ground disturbance including lateral spreading. There are no mapped faults in the immediate vicinity of the site, with the exception of the Southern Whidbey Island fault zone mapped approximately 6 miles southwest of the site. Our opinion is that there is a low risk of fault displacement resulting in ground rupture at the surface. GMENGINEERS� October3O,2019 Pages ' File No 21288-00200