1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC 2019-07-08

CENTRAL UTILITY PLANT PARKING LOT Everett,Washington Groundwater Conditions Although groundwater was not encountered in the test pits perched groundwater was observed previously in boring GEI-2 at approximate depth of 13 below the existing ground surface. Groundwater observed in boring GEI-2 appeared to be perched on the underlying, less permeable glacially consolidated deposits. Groundwater observations represent conditions observed during the excavation and may not represent the groundwater conditions throughout the year. We anticipate that shallow groundwater seepage will exist in permeable soils above the glacially consolidated soils. Groundwater seepage is expected to fluctuate as a result of season, precipitation and other factors. CONCLUSIONS AND RECOMMENDATIONS Summary 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 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. IN Fill and former topsoil layers are present at the site and may represent unsuitable soils for support of new pavement areas. These soils have supported a parking lot surfaced with crushed rock for several years. 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. • The existing crushed gravel surfacing over the majority of the site can be removed, stockpiled, and reused as structural fill for portions of the subgrade where over excavation of soft areas will be completed, as long as the crushed gravel surfacing is not contaminated with fines from the fill or native soils. • Due to the short service life of the pavement of 10 years, we recommend in light-duty pavement areas (e.g., automobile parking), a pavement section consisting of at least a 2-inch thickness of /-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 a pavement section consisting of at least a 3-inch thickness of 1/2-inch HMA (PG 58-22) over a 4-inch thickness of densely compacted crushed rock base course. GEOENGINEERS October 17,2012 Page3 File No.10738-010-00