11831 AIRPORT RD WHOLE SITE FILE Geotech Report 2022-10-25

Groundwater Conditions As discussed previously,a seasonally perched groundwater condition should be anticipated throughout the site. Water infiltrating through the fill and upper weathered till zone is often perched above the dense, unweathered till. Observed groundwater conditions are summarized for each segment in the previous sections but represent a short-term condition.Groundwater observed during drilling is often inaccurate due to the drilling techniques and the limited time the hole is left open. CONCLUSIONS AND RECOMMENDATIONS Summary of Geotechnical Considerations We conclude that the planned improvements can be successfully completed from a geotechnical perspective, provided the considerations and recommendations presented in this report are incorporated into the project planning and design. 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. • Cut slopes may be supported with SE walls, gravity wall systems, or cast-in-place (CIP) concrete walls. Some short cut slopes may be faced with rockeries,with certain limitations. ■ New fill embankments may be supported with SE walls,gravity wall systems, or CIP concrete walls. • Walls 1 and 2 are planned as a CIP concrete walls at this time. ■ Most of the on-site soils observed in the explorations contain a high percentage of fines(silt and clay). Based on the results of the laboratory moisture content tests, portions of the on-site soils may be suitable for reuse as fill during dry weather only. Imported gravel borrow will be necessary during wet weather and when the existing soils are too wet to achieve compaction.We recommend the suitability of the exposed soils be evaluated during construction when they are exposed and a contingency be planned to use import soils. • New station retaining walls should be supported on structural fill or the existing subgrade soils compacted to 95 percent of the maximum dry density(MDD) per ASTM D-1557. ■ Light weight fill may be necessary for the new station pad to be located on north-bound Airport Road north of 100th Street SW to limit loading on the existing block wall. These and other geotechnical considerations and recommendations are discussed further in the following sections of this report. Earthquake Engineering Design Earthquake Parameters Seismic design of the walls should be completed using the design criteria presented in the WSDOT Geotechnical Design Manual (GDM) and AASHTO specifications. For this alignment, the recommended parameters for Site Class,peak ground acceleration(PGA),short period spectral response acceleration(Ss), 1-second period spectral response acceleration(Si),and Seismic Coefficients FPGA, FA and Fv are presented below in Table 2. The values presented below are based on the 2002 USGS National Seismic Hazards Mapping project for determining a peak ground (bedrock) acceleration coefficient for design for an GEOENGINEERS_ April 14,2017 , Page 10 File No.2095-032-00