5801 23RD DR W Geotech Report 2025-07-29

I 111 October 22,2007 Project No.T-5922-1 The ground surface in the existing storm sewer easement along the crest of the steep slope has been graded to slope away from the slope crest and hydroseeded. We did not observe any indications of significant active erosion within the easement,or indications of uncontrolled surface runoff toward the steep slope. 1 3.2 Soils The soils observed in our site explorations generally consist of about 6 to 12 inches of forest duff and organic surficial soils overlying weathered and unweathered glacial till consisting of medium dense to very dense silty sand with varying amounts of fine to coarse gravel. Where observed, the dense to very dense unweathered till generally underlies approximately three to five feet of light brown to reddish-brown, medium dense to dense weathered till. We observed very stiff to hard low-plasticity silt underlying the granular surficial soils at depths of about 6.5 to 10 feet in Test Pits TP-I01,TP-1,and TP-6. The Distribution and Description of Geologic Units in the Mukilteo Quadrangle, Washington, by James P. Minard, dated 1982, shows soils in the planned development area mapped as Vashon till (Qvt). Soils in the Narbeck Creek Ravine are mapped as glacial advance outwash(Qva). The medium dense to very dense silty sand with gravel we observed in our site explorations is generally consistent with the mapping of Vashon till. We observed dense to very dense sand in shallow excavations made on the face of the steep slope, and in exposures along the stream channel at the base of the slope. These soils are consistent with advance outwash deposits. The preceding discussion is intended as a general review of the soil conditions encountered. Detailed descriptions of the soil conditions observed in our site explorations are presented on the Test Pit Logs attached in Appendix A. 3.3 Groundwater We observed very light groundwater seepage at a depth of approximately 10 feet in Test Pit TP-3, excavated during our May 2006 study. The seepage at this location appeared to be perched on a very dense, weakly- cemented zone within a silty gravel with sand unit. We did not observe groundwater seepage in our test pits excavated in September 2007; however, we did observe occasional faint mottling within the weathered till soils, ' indicating that a seasonal,perched water table develops at times above the unweathered till. The development of a perched groundwater table is typical for sites underlain by glacial till, particularly during the wet winter and spring months. However,considering we observed only very light seepage in one of the test pits excavated in Iate spring,it does not appear that a prominent perched groundwater table develops at the site. 3.4 Seismic Site Class Based on the soil conditions observed in our site explorations and our review of local geologic literature, the 2003 International Building Code(IBC) indicates that site class"C"should be used in structural design. f Page No. 3 I