REDMOND CENTRAL CONNECTOR ď Ž Redmond, Washington
including the original railroad line construction, construction of the roads and utilities that cross the corridor, site remediation activities at the old King County Shops site west of Leary Way, and, most recently, by the installation of the Downtown Redmond Stormwater Collection Pipeline. Because of this past disturbance, subsurface conditions in the borings may not accurately represent the current subsurface conditions at the proposed infiltration facilities. The design infiltration rate for an individual facility will depend on the type of facility. Long Term Design Infiltration Rates for infiltration pipes and trenches must incorporate a correction factor to account for site variability, maintenance to prevent siltation and bio clogging, etc. As discussed in greater detail below, Long Term Infiltration Rates for rain gardens that incorporate an amended soil (bioretention soil) layer must be the lower of the long-term rate of the amended soil or the initial (Short Term) infiltration rate of the underlying (native) soil. Correction factors need not be applied to the Short Term Infiltration Rate of the native soil. Based on the boring data, the surficial soils along the majority of the RCC corridor likely have an uncorrected, Short Term Infiltration Rate of 8 inches per hour, in our opinion. Sandy gravel is locally present below the site; however, it is not pervasive or continuous. If an infiltration pipe is completed in a sandy gravel deposit, the design long-term infiltration rate can be raised to 10 inches per hour. Based on discussions with the design team, we understand that the project infiltration pipes are intended to be sited in the sandy gravel and that they will be designed using a long-term infiltration rate of 10 inches per hour. We also understand that the subsurface conditions at each pipe will be explored prior to construction to verify the presence of sandy gravel soils and that infiltration tests will be performed to verify the design infiltration rate. We concur with this approach. For bioretention areas (rain gardens), Section 7.7 of Appendix C of the 2005 Ecology Manual, Subsection 7.7.4 Infiltration Rate Determinations, states that the assumed infiltration rate for the bioretention area must be the lower of the long-term rate of the imported soil (amended soil) or the initial (short-term) infiltration rate of the underlying (native) soil. This section also states that the underlying native soil does not need a (infiltration rate) correction factor because the overlying imported soil protects it. Accordingly, we propose that LID improvements which incorporate an amended soil layer be designed using the lower of the amended soil long-term infiltration rate or the short-term infiltration rate of the native soil based on values determined using the USDA soil textural classification. We recommend that the Long Term Design Infiltration Rate for the majority of the rain gardens be assumed to be 2½ inches per hour; this corresponds to our estimated infiltration rate for a typical amended/bioretention soil. The soils near the rain garden at Station 13+46 consist of silt that has an estimated Long Term Design Infiltration Rate of less than ½ inches per hour. The nearest boring to the rain garden at Station 57+77 is about 175 feet to the east. The soils in that boring suggest a Short Term Infiltration rate of 2 inches per hour. Because this rate is lower than the estimated infiltration rate of the amended/bioretention soil, it should be considered the design infiltration rate for this rain garden.
March 13, 2012 | Page 13 File No. 0500-172-01