1001 N BROADWAY LEARNING RESOURCE CENTER 2026-01-15

Shallow foundations can be constructed on the dense advance outwash deposits. An allowable bearing pressure of 6,000 pounds per square foot (psf) may be used for native undisturbed advance outwash deposits located at least 5 feet below existing grade.We anticipate that most of the footings will bear in the native undisturbed advance outwash deposits. An allowable bearing pressure of 3,000 psf may be used where imported structural is placed below footings, if needed, that extends to the native undisturbed advance outwash deposits. Conventional slabs-on-grade are considered appropriate and should be underlain by a 6-inch-thick layer of capillary break consisting of clear crushed gravel with negligible fines and sand content. The on-site soils within the upper 5 to 10 feet generally contain a high percentage of fines (17 to 28 percent) and are highly moisture sensitive. Therefore, reuse of on-site soils should only be planned in the normal dry season (June through September). It may be feasible to infiltrate a portion of the site stormwater in areas of the site where the advance outwash deposits were encountered; however, we anticipate that infiltration rates will be less than 0.4 inches per hour. 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 geotechnicai 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 contiinental._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 Juan 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. GMENGINEER July22,2029 Page4 ile:to.5E36-�r�-30