8925 AIRPORT RD MUKILTEO SCHOOL DISTRICT 2022-01-19

GeoTest Services, Inc. February 26, 2018 Mukilteo SD —Wall Design, Everett, Washington Job No. 18-0068 of the walls was analyzed assuming a peak horizontal ground acceleration of 0.24g for a seismic event with a probability of exceedance of 10 percent in a 50 year period (USGS 2015). The horizontal forces developed during earthquake shaking were represented by a "pseudo -static" seismic coefficient, kh. The horizontal acceleration used in seismic stability analysis for natural soil slopes is typically assumed to be one -have (for non - yielding walls) of the free -field acceleration. For sake of conservatism, the seismic coefficient used in our stability analysis of the walls was 0.15g. The analyses indicated that the global stability meets or exceeds the minimum recommended factors of safety of 1.5 and 1.1 for static and seismic loading, respectively. Stormwater Infiltration Potential Based on the laboratory test results, our field observations, and our experience with similar projects in the area, it is our opinion that the project site is not favorable for stormwater infiltration facilities. The native soils consist of dense to very dense, glacially consolidated, till that is considered a low -permeability soil. Evidence of the low - permeability of the soil is demonstrated by the presence of the wetland that exists in close proximity to where the retaining wall is located. In a similar fashion, the soil underlying drive paths and parking area are not considered suitable due to the presence of historic fill soil underlying the pavement. Per the 2012 Washington State Department of Ecology Stormwater Management Manual for Western Washington (SMMWW), "waste fill materials shall not be used as infiltration soil media nor shall such media be placed over uncontrolled or non -engineered fill soils." Per our field findings, all 3 test pits exhibited either non -engineered fill or waste material. Thus, infiltration does not appear feasible within existing fill soils behind the proposed retaining wall. GTS does not recommend infiltration in front of the wall due to a "restrictive layer" designation per the manual and the wetland that indicates poor drainage and shallow groundwater in the area. Alternative approaches should be considered to address stormwater concerns. Closure The retaining wall design presented in this report was developed based on generally accepted modular block gravity retaining wall design practices for the anticipated site conditions based on information collected from the site, our judgment and experience. The subsurface conditions below the proposed wall alignment are assumed to be suitable to support new modular block gravity retaining wall. However, contingencies in the project construction budget and schedule should be considered if the retaining wall design, as specified in this report, needs to be modified to fit unanticipated subsurface conditions encountered during construction. Limitations Our work has been performed in a manner consistent with that level of care and skill ordinarily exercised by members of the profession currently practicing under similar conditions in this area. GeoTest Services has prepared this report for the exclusive use of Harmsen & Associates, Inc. and their representatives for specific application to the construction of Page 6of8