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

CENTRAL UTILITY PLANT PARKING LOT Everett,Washington may be placed over soft subgrade areas to limit the thickness of structural fill required to bridge soft,yielding areas. Asphalt Pavement In light-duty pavement areas (for example, automobile parking), we recommend a pavement section consisting of at least a 2-inch-thick layer of 1/2-inch HMA (PG 58-22) conforming to Sections 5-04 and 9-03 of the WSDOT Standard Specifications, over a 4-inch-thick layer of densely compacted crushed rock base course conforming to Section 9-03.9(3) of the WSDOT Standard Specifications. In heavy-duty pavement areas (for example, truck traffic and materials delivery areas) around the building, we recommend a pavement section consisting of at least a 3-inch-thick layer of 1/2-inch HMA (PG 58-22) over a 4-inch-thick layer of densely compacted crushed rock base course. We recommend that proof-rolling of the compacted base course be observed by a representative from our firm prior to paving. Soft or yielding areas observed during proof-rolling may require over-excavation and replacement with compacted structural fill. The pavement sections recommended above are based on our experience with similar developments. Thicker asphalt sections may be needed based on the actual traffic data, intended use and performance expectations. Stormwater Infiltration Recommended Infiltration Values Two methods were used to evaluate an appropriate design (long-term) infiltration rate for the soils encountered in the test pit explorations. The two methods consist of correlations based on United States Department of Agriculture (USDA) soil textural classification and ASTM gradation testing, as discussed in Section 3.3.6 of the Stormwater Management Manual for Western Washington (Ecology, 2005). The following table presents a summary of the subsurface soil and groundwater conditions encountered in selected test pits completed in the area planned for parking, and the estimated infiltration rate based on the USDA textural class and on ASTM laboratory gradation testing. Sample USDA Approximate Estimate of Infiltration Test ASTM Dlo z hourInches Pit Depth Classification Textural mm Groundwater Rate( / ) (feet) Class ( ) Depth(ft)1 USDA ASTM TP-8 5 ML Clay <0.002 Not Encountered — 0.2 TP-9 4.5 ML Clay <0.002 Not Encountered — 0.2 Notes: 'Depth to perched groundwater below existing ground surface, measured at time of the excavation of the test pits. 'Infiltration rates shown for three different methodologies(Ecology,2005). The existing soils encountered in areas where infiltration is being considered consist of either loose silty sand with variable gravel content or stiff to hard silt and clay (glacially consolidated soil). The soils within the likely infiltration layer contain a variable percentage of fines, which can limit the infiltration capacity. GEOENGINEERS October 17,2012 Page 9 File No.10738-010-00