7000 HARDESON RD 2020-06-05 - Laserfiche WebLink

Geotechnical Engineering Report Community Transit Merrill Creek Operating Base 7100 Hardeson Road Everett, Washington February 28, 2018 RN File No, 3224-002A Page 8 must be available along the flowpath that runoff would follow upon discharge from a dispersion devices cited in this BMP. The native vegetated flowpath must meet all of the following criteria: • The flowpath must be over native vegetated surface.".,. In our opinion sufficient native vegetated surface is not available on this site. It is therefore infeasible to implement this BMP. Permeable Pavement BMP 15.15: We have selected several infeasibility criteria including the following from DOE Volume V pages 5-18 through 5-21: a "Where infiltrating and ponded water below new permeable pavement area would compromise adjacent impervious pavements." o We observed standing water and seepage throughout the site during an extended period of dry weather. Interviews with staff personnel indicate there is seepage all times of the year. In our opinion, infiltrating water through permeable pavement would further compromise the adjacent impervious pavement. • "Where fill soils are used that can become unstable when saturated." o The entire site is underlain by undocumented fill that varies considerably in gradation and density/consistency. Some of the fill is in a loose condition and some of the fill contains clay. In our opinion, the clay and loose material as well as the overall varying nature of the existing fill provide potential conditions for instability when saturated. • "Where installation of permeable pavement would threaten the safety or reliability of pre-existing underground utilities, pre-existing underground storage tanks, or pre- existing road subgrade." o For the reasons stated above, saturating the pre-existing pavement subgrade would provide conditions that would threaten the reliability of the existing subgrade to support the asphalt pavement. In our opinion, this infeasibility criteria is met. • "Where seasonal high groundwater or an underlying impermeable/low permeable layer would create saturated conditions within one foot of the bottom of the lowest gravel base course," o As shown in the photos below, saturated conditions exist within one foot of the bottom of a potential gravel base course. In addition, the DOE states "A local government may designate geographic areas within which permeable pavement, or certain types of permeable pavement, may be designated as infeasible due to year-round, seasonal or periodic high groundwater conditions, or due to inadequate infiltration rates....The types of pertinent data include, but are not limited to: • Standing water heights or evidence of recent saturated conditions in observation wells, test pits, test holes, and well logs." Robinson Noble, Inc.