3000 41ST ST Geotech Report 2025-12-31

• PNW Home Builders, LLC ES-2598.03 May 2, 2013 Page 3 Native Soil Underlying the fill and topsoil,, native soils were reported to primarily consist of silt and sands (ML, SP, and SM). A dark brown organic silt and peat layer (OL and OH) was reported at four of the five boring locations at depths ranging from 11 to 64.5 feet below existing grades, with a thickness ranging from 3.5 to 11..5 feet. The silt, sand, and organic layers, consistent with younger alluvium and floodplain deposits, were reported extending to the maximum exploration depth of 131.5 feet below existing grades. Moisture contents for the organic silt (OL and OH) reportedly ranged from 39 to 145 percent, with an average of approximately 84 percent. Moisture contents for the native soils (ML, SP, and SM) reportedly ranged from 11 to 102 percent, with an average of approximately 37 percent. Geologic Setting The referenced geologic map resource identifies younger alluvium (Qyal) deposits across the site and the extended area eastward. These deposits occur as present and former stream channel and flood plain deposits. Sediments consist mostly of unconsolidated, stratified, gray to brown clay, silt, and sand, with abundant organic material. Geologic explorations that have been reported on the subject site are consistent with these sediment descriptions. Groundwater Groundwater seepage was reported at all five boring locations at the time of the field work (January 2007), at depths of 3 to 14 feet below existing grades. Seepage should be expected in excavations at this site, particularly during the late fall, winter, spring and early summer months. Groundwater seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and _soil conditions. In general, groundwater flow rates are higher during the wetter, winter months. Liquefaction Hazard Evaluation We reviewed the referenced liquefaction susceptibility map which includes the subject parcel. This resource indicates the site has a moderate to high potential for liquefaction. This rating is based in part on the presence of younger alluvium deposits and the presence of the nearby river valley associated with the Snohomish River. The conditions reported at the boring locations are consistent with younger alluvium and floodplain deposits associated with the nearby river valley. Groundwater was initially reported at three feet below existing grades. Based on these conditions, it is our opinion that the site presents a high susceptibility to liquefaction-induced settlement during a significant seismic event. The general recommendation of high site liquefaction susceptibility is primarily due to the younger alluvium deposits reported at depth. Due to its medium dense nature, the fill material reported in the upper 11 to.20.5 feet of existing grades has a low susceptibility to liquefaction. Earth Solutions NW,LLC