1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC Geotech Report 2022-10-25

CONCLUSIONS AND RECOMMENDATIONS A summary of the primary geotechnical considerations is provided below. The summary is presented for introductory purposes only and should be used in conjunction with the complete recommendations presented in this report. ■ The site is designated as Soil Profile Type C per the 2012 IBC. ■ The soils encountered at the site consist primarily of silty sands and silt. These soils are highly susceptible to disturbance during periods of wet weather. Earthwork should be completed during the drier summer months in order to minimize earthwork costs. ■ Excavation can be completed using temporary cut slopes. ■ The proposed detention vault will need to be designed to resist permanent lateral earth pressures. • Fill and former topsoil layers are present at the site and may represent unsuitable soils for support of new pavement areas.The unsuitability of these materials and the need for removal and replacement will depend on planned site grades,weather conditions during construction,and the consistency of the fill and former topsoil deposits exposed at the planned subgrade elevation. A conservative approach would be to remove these materials and replace them with properly compacted structural fill. Alternatively, these soils can be evaluated during subgrade preparation and localized removal and replacement can be completed,as necessary.The latter approach can potentially reduce project costs; however,will require more flexible measurement and payment contractual provisions. • We recommend in light-duty pavement areas(e.g.,automobile parking),a pavement section consisting of at least a 2-inch thickness of 1/2-inch hot-mix asphalt (HMA) (PG 58-22) per Washington State Department of Transportation (WSDOT) Sections 5-04 and 9-03, over a 4-inch thickness of densely compacted crushed rock base course per WSDOT Section 9-03.9(3). If heavy-duty pavement areas (e.g., truck traffic areas, materials delivery, and forklifts) are required, we recommend the minimum pavement section consist of at least 3 inches of HMA over a minimum of 6 inches of crushed surfacing base course. • Foundations should bear on undisturbed glacially consolidated soils. For shallow foundations bearing directly on undisturbed dense to very dense or very stiff to hard glacially consolidated soils, an allowable soil bearing pressure of 6 kips per square foot(ksf) may be used Our specific geotechnical recommendations are presented in the following sections of this report. Earthquake Engineering Liquefaction Liquefaction refers to the condition by which vibration or shaking of the ground, usually from earthquake forces, results in the development of excess pore pressures in saturated soils with subsequent loss of strength. In general,soils that are susceptible to liquefaction include very loose to medium dense, clean to silty sands that are below the water table.The soils that underlie the proposed improvements have a low risk of liquefaction. GEOENGINEERSZ April 22,2016 Page 3 Rie No.10$38-0IO-01