3003 W CASINO RD BLDG 40-23 Geotech Report 2025-01-22

borings, the existing moisture content is near the optimum content required for compaction. Therefore, compaction of the soils within the building and other areas may be achievable in dry conditions. We expect that most of the excavation can be accomplished with conventional excavating equipment, such as trackhoes. However, some ripping or use of breaker hammers may be required. The contractor should be prepared for occasional cobbles and boulders in the site soils. The contractor should be prepared to remove these materials. We recommend temporary cut slopes be inclined at 1H:1V (horizontal to vertical) or flatter depending on localized sloughing where cleaner backfill soils may be encountered. Steeper slopes up to 3/4H:1V may be feasible in the very dense glacial till soils, provided the geotechnical engineer confirms soil conditions are consistent and evaluates the stability of the slopes during construction. Neat cuts in the native till soils are feasible for shallow foundation elements. We recommend a geotechnical engineer from our firm evaluate the exposed soils following removal of the existing slab to confirm that subsurface soils are as anticipated and prepared as recommended in this report. 5.2. Earthquake Engineering 5.2.1. Seismicity The Puget Sound region is located at 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 overridingthe subducting Juan de Fuca Plate. The interaction of these two plates results in three potential seismic source zones: (1) a shallow crustal source zone; (2) the Benioff source zone; and (3) the CSZ interplate source zone. The shallow crustal source zone is used to characterize shallow crustal earthquake activity within the North American Plate at depths ranging from 3 to 19 miles below the ground surface (bgs). The closest known fault is the South Whidbey Island Fault. Washington Department of Resources Geological Survey and the United States Geological Survey (USGS) both recently (2021) updated their late Quarternary faults in western Washington and have the same traces. Inferred faults are mapped roughly 11/2 miles northwest and 1 mile southwest of the project. Inferred faults are those for which Quarternary-age deformation is suspected, but for which insufficient evidence has been gathered to support this determination. The Benioff source zone is used to characterize intraplate, intraslab or deep subcrustal earthquakes. Benioff source zone earthquakes occur within the subducting Juan de Fuca Plate at depths between 20 and 40 miles. In recent years, three large Benioff source zone earthquakes occurred that resulted in some liquefaction in loose alluvial deposits and significant damage to some structures. The first earthquake, which was centered in the Olympia area, occurred in 1949 and had a Richter magnitude of 7.1. The second earthquake, which was centered between Seattle and Tacoma, occurred in 1965 and had a Richter magnitude of 6.5. The third earthquake, which was located in the Nisqually valley north of Olympia, occurred in 2001 and had a Richter magnitude of 6.8. The CSZ interplate source zone is used to characterize rupture of the convergent boundary between the subducting Juan de Fuca Plate and the overriding North American Plate. The depth of CSZ earthquakes is greater than 40 miles. No earthquakes on the CSZ have been instrumentally recorded; however, through GEOENGINEER'00' June 15,2022 " Page4 Me Na.'012Q-244-p2