10632 7TH AVE SE Geotech Report 2025-07-28

I June 6 2008 Serenity Meadows L&A Job No. 8A017 Page 16 evaporate into the ambience, be absorbed by vegetation planted in the rain gardens and infiltrate into the ground. An overflow pipe is to be provided in each rain garden to flow into a trench drain and to be released into the wetland buffer zone at the western end of the site. Test pits excavated on the site encountered fresh Vashon till soil unit in the eastern portion and advance outwash soil unit at shallow depth in the western portion of the to be developed area of the site. These soil units are also classified as Alderwood gravelly sandy loam and Everett gravelly sandy loam, respectively,by the SCS (Soil Conservation Service)Classification system, as shown in Soil Survey of Snohomish County Area, Washington. Table 14 in this publication .� lists the permeability to be 2.0 to 6.0 iph (inches per hour) for Alderwood gravelly sandy loam, and 6.0 to 20.0 iph for Everett gravelly sandy loam. We recommend that an infiltration rate of 0.5 iph and 5.0 iph be used for design of rain gardens excavated into the fresh Vashon till soil unit(Alderwood gravelly sandy loam) and the advance outwash soil unit(Everett gravelly sandy loam), respectively. The above recommended design infiltration rates include a factor of safety of about 4.0. The approximately boundary of the two soil units is shown on Plate 2 —Site and Exploration Location Plan. During Roadways and Building g Lots Grading The onsite surficial fine-grained surficial soils in the eastern portion of the to be developed area of the site are of moderately low permeability. They can be saturated quickly and may cause surface runoff and groundwater seepage during periods of heavy rain. To minimize potential difficulties for site grading work due to storm runoff during construction, one or more lines of ditches or interceptor bench drains should be installed, as required, on the uphill sides of the site to intercept storm runoff and near-surface perched groundwater. Water thus collected should be direct into temporary storage and settling ponds before being released into a nearby storm sewer inlet in the street or the adjacent wetland buffer zone. Only clean and clear water should be released into the storm inlet or the wetland buffer zone. LIU & ASSOCIATES, INC.