3800 RIVERFRONT BLVD Geotech Report 2023-05-04

r May 13, 2019 HWA Project No. 2015-061-21 the silt soils noted in GeoEngineers borings are not anticipated to be substantially different that those observed in HWA's borings. As a result, we expect the liquefaction potential of the alluvial silt to be low. Where pockets of low plasticity silts are present, some liquefaction within the layer may occur. However, we expect this liquefaction to be isolated and discontinuous in nature. Therefore, we do not expect that the occurrence of liquefaction within the alluvial silt will result in continuous liquefiable layers that will affect lateral spread potential of the site. Sand: The alluvial sands were observed to have variable densities across the site. In general, the alluvial sands observed between the leachate collection trench and the Snohomish River were noted to have relatively low density. Therefore, our analysis suggests that liquefaction of portions of the alluvial sands is expected as a result of the deign earthquake. However, it should be noted that the SPT blow counts recorded near the riverbank may have been understated due to the presence of heave during drilling. This is further supported by the sheet pile driving rates observed during the installation of Lift Station 33. During installation of sheet pile shoring, at lift Station 33, significant decrease in the rate of sheet pile advancement was observed once the sheets extended into the alluvial sands at that site. The observed rate of sheet pile advancement was not indicative of soils that would liquefy during the design earthquake. For the purposes of the current lateral spread evaluation, the extent and geometry of liquefaction was determined using the SPT blow counts recorded in the available borings. Given all available information, we believe this is a conservative assumption. Liquefaction Analysis Liquefaction is a temporary loss of soil shear strength due to earthquake shaking. Loose, saturated, cohesionless soils are highly susceptible to earthquake-induced liquefaction. Recent experience and research have shown that certain silts and low-plasticity clays are also susceptible (PI<18). Primary factors controlling the development of liquefaction include the intensity and duration of strong ground motions, the characteristics of subsurface soils, in-situ stress conditions and the depth to groundwater. To evaluate the liquefaction susceptibility of the soils along the project alignment, the simplified procedure originally developed by Seed and Idriss (1971) and updated by Youd et. Al. (2001) and Andrus and Stokoe (2000), was used. For this site the PGAm used to evaluate liquefaction is 0.547g. The analyses indicate that several zones of loose to medium dense sand are expected to liquefy during the design level event. HWA has revised the liquefaction analysis, with respect to previous reports, to depict more accurate conditions for this addendum utilizing available lab data, including fines contents, organic soil conditions and plasticity indexes. The revised geometry of the liquefiable soil layers, along cross section B-B' and C-C', are shown Figures 2 and 3, attached to this addendum. Everett Riverfront Development Addendum 3 HWA GeoSciences Inc.