3800 RIVERFRONT BLVD Geotech Report 2023-05-04

May 13, 2019 HWA Project No. 2015-061-21 Soil Unit Liquefaction Potential HWA has assessed the liquefaction susceptibility of the various subsurface soil units based on the available data, including blow counts, material constituencies,plasticity indexes, organic content and moisture contents. Details associated with our conclusions for each unit are proved below. Landfill Refuse: HWA has evaluated the potential for liquefaction to occur within the refuse across the site. Unlike conventional soil, liquefaction within refuse is not a well-studied occurrence. HWA conducted an extensive white paper study and was not able to find any research papers that provide insight into the liquefaction potential of saturated landfill refuse. HWA believes that the potential for liquefaction within landfill refuse is highly dependent on the consistency of the refuse material. We expect that saturated refuse consisting predominantly of granular soil or daily cover material may undergo some initiation of liquefaction as a result of the design earthquake. However, we expect that refuse consisting of municipal solid waste and construction debris will not undergo liquefaction as a result of the design earthquake. The highly porous, non-homogenous, and interconnected nature of the municipal solid waste is such that we do not believe that sufficient cyclical pressure can build up to cause liquefaction. To gain a third party opinion on our expectations with respect to refuse liquefaction potential, HWA spoke with Rudolph Bonaparte, Ph.D., P.E., D.GE.. Dr. Bonaparte is an expert on the geotechnical stability of waste fills. Dr. Bonaparte concurred with our assessment of the non-liquefiable nature of municipal refuse materials, and was unaware of any paper written on the subject at the time of our meeting. Therefore, for the purposes of our analysis, HWA is considering the saturated refuse across the site to not be susceptible to seismic induced liquefaction. Alluvial Soils: The alluvial soils on the site are consisted of three primary material types, Peat, Silt/Clays and Sands. The liquefaction susceptibility of each soil type is described below. Peat: The Peat consist of soft organic debris and is not considered to be liquefiable due to its fibrous nature. Silt/Clay: The Silt and Clay soils we observed in our explorations were generally plastic often with plasticity Indexes in excess of 20. HWA has observed near surface outcroppings of these soil, across the site, during past construction activities. Where observed, the nature of these soils consisted of predominantly plastic silts with small pockets of organic material and low plasticity silt. The boring logs completed by GeoEngineers included limited lab testing. However, Everett Riverfront Development Addendum 2 HWA GeoSciences Inc.