7000 HARDESON RD Geotech Report 2022-10-25

Geotechnical Engineering Report Community Transit Merrill Creek Operating Base 7100 Hardeson Road Everett, Washington February 28, 2018 RN File No, 3224-002A Page 7 For planning purposes, we recommend that temporary cuts in the near-surface fill soils be no steeper than 1.5 Horizontal to 1 Vertical (1.5H:1V), If groundwater seepage is encountered, we expect that flatter inclinations would be necessary. We recommend that cut slopes be protected from erosion. Measures taken may include covering cut slopes with plastic sheeting and diverting surface runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper than 4 feet, if worker access is necessary. We recommend that cut slope heights and inclinations conform to local and WISHAIOSHA standards, Drainage We recommend that runoff from the pavement be collected and routed to an appropriate storm water discharge system. The finished ground surface should be sloped at a gradient of 5 percent minimum for a distance of at least 10 feet away from the buildings, or to an approved method of diverting water from the foundation, per iBC Section 1804.4. Surface water should be collected by permanent catch basins and drain lines, and be discharged into a storm drain system. We have reviewed the Storm Drainage Plan and Details on plan sheets SD11 and SDD04. In our opinion, the 4-inch diameter perforated PVC pipe is spaced at appropriate intervals to be sufficient to prevent water from ponding on the surface of the pavement. Infiltration and Dispersion We understand that the City of Everett has adopted the 2014 Department of Ecology Stormwater Management Manual for Western Washington (DOE). The manual presents Best Management Practices(BMPs) that can be implemented to meet the project Minimum Requirements. The manual also refers to the 2012 Low Impact Development Technical Guidance Manual for Puget Sound (LID) for further design criteria of these BMPs. We understand the project is subject to Minimum Requirements #1 through #5. Minimum Requirement#5 requires BMPs be considered in the order listed in List#1 in Volume 1 of the DOE. List#1 presents BMPs to consider for Lawn and Landscaped Areas, Roofs and Other Hard Surfaces. Since this is a pavement rehabilitation project, we are addressing only the `Other Hard Surfaces' section. These BMPs include Full Dispersion, Permeable Pavement, Rain Gardens, Bioretention and Sheet Flow Dispersion. We address the infeasibility criteria for each BMP below. Full Dispersion BMP T5.30: According to DOE Section 5.3 of Volume V, "Full dispersion applies to sites (or sub-areas of sites) with a maximum of 10% effective impervious area that is dispersed through 65% of the site maintained in natural vegetation." With the exception of a few landscaped islands, the site is covered with impervious surface and does not meet the required percentage of native vegetated surface. In addition, the minimum design requirements for residential projects section on page 5-34 of Volume V states "Runoff must be dispersed into the native area in accordance with one or more of the dispersion devices, and in accordance with the design criteria and limits for those devices, cited in this BMP. A native vegetation flowpath of at least 100 feet in length (25 for sheet flow from a non-native pervious surface) Robinson Noble, Inc.