1802 75TH ST SW Geotech Report 2022-10-25

Geotechnical Engineering Report 1f�rr�cor� York Substation n Bothell, Washington September 5, 2014a Terracon Project No. 81125034 4.10 Surface Water Infiltration Considerations Appendix B contains grain size 'distribution curves of samples of the native soils that were retrieved at variable depths below the proposed substation yard grade. Testing and visual observation indicate that the soils generally consist of silty sand with variable gravel content. We evaluated the allowable long-term infiltration rate for the site's receptor soils for five tested samples based on the grain size testing results and correlations between grain size and infiltration rates presented in Table 3.5, Alternative Recommended Infiltration Rates based on ASTM Giadafion Testing presented in the 2010 Snohomish County Drainage Manual; and four samples based on correlations presented in Figure 3.18, Infiltration Rate as a Function of the D 1.0 Size of the Soil for Ponds in Western Washington. The feasibility of infiltrating storm water depends not only on soil permeability but also the proximity of groundwater to the infiltration facility. Sidewall seepage. was observed in Test Pits TP-1, TP-2, TP-3, and TP-4, at depths of 6.5 feet, 5 feet, 3.5 feet, and 6 feet, respectively, and in exploratory Borings B-1 and B-2 at 12.5 feet and 12 feet respectively. As noted in Section 3.3 of this report; the water table appears to grade upwards in elevation from the west to the east side of the site, though at a gentler grade than the ground surface. Thus, the water table is likely to be found at a greater depth toward the east end of the site. We understand that the District will evaluate the contribution of the substation rock and underlying crushed rock base course in terms of their contribution to storm water management. We collected samples of material meeting the criteria for WSDOT Specification 9-03.9(3) Crushed Surfacing (base course gradation). Each sample was compacted to 95 percent of the modified Proctor maximum dry density (ASTM D 1557) and the permeability determined. Test results are summarized below. Summary of Crushed Surfacing Laboratory Testing Supplier I Dry Density Compaction Specific Void Ratio Permeability Location (ASTM R (percent) Gravity (data (irtc.heslhour) 1657) provided by suppliers) Iron Mountain / Granite Falls 119.7 95.4 2.75 0.440 58.3 CEMEX / Everett I 135.9 95.0 2.69 0.235 47.1 It should be noted that the tests were completed on the sample fractions passing the US No. %- inch sieve for compliance with ASTM D 1557. Actually field values will vary slightly from the reported values due to the presence of aggregate larger than 3/-inch and also due to variations in loads. Material placement procedures can also result in aggregate segregation which can produce variable void ratio and permeability values. Responsive a Resourceful m Reliable 20