3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 respect to a park development, where the consequences of river bank failure is considered to be much less significant to public safety. The figures also show that for a design level earthquake, bank failures (FS < 1.0) are likely to occur(see Figures 10, 13 and 15), for cross-sections A-A', Ic-c', and D-D'). We also presented the post-liquefaction case for the one cross-section A-A' (Figure 11), which indicates a very low value and would imply lateral spreading or flow failure would occur. Both cross-sections C-C' and D-D' have similarly very low factors of safety, reflecting the severe effects of subsurface soil liquefaction on bank stability. River Bank Seismic Slope Stability Our slope stability analyses indicate that failures of the river bank sections will occur under the influence of the design earthquake event. Two modes of failure may be anticipated, independently or concurrently; namely, slump failures induced by seismic ground accelerations and/or riverbank spreading due to liquefaction of alluvial soil. Observations of physical conditions of such failures indicate that the slump failures typically extend for limited distances behind the bank/slope crest; our analyses suggest of the order of 10 to 20 feet. Lateral spreading failures, however, have been observed to extend many tens of feet behind the bank/slope crest and can be physically accompanied by some flow failure deformation as well. To prevent such failure modes, it would be necessary to implement stabilization measures in the bank areas that would either retain the liquefiable soils (e.g.; retaining wall structures, or heavy buttressing with rip rap), or strengthen the soils (e.g. ground improvement with stone columns or deep dynamic compaction) so that they would be capable of resisting both failure modes. In consultation with the Shelter Holdings and the City of Everett, we have expressed our view and recommendation that, in consideration of the costs of implementation of such measures, it is far more cost effective to simply design for a level of stability that will be acceptable under normal static and low to moderate seismic conditions, and also accept the potential future costs of repair of the banks if a major earthquake causes damage. For bank stability analyses in the area of the crane structure, we assumed the existing sheet pile wall will be removed and replaced with a new sheet pile wall structure integrated with a new pile foundation system for the crane structure. 4.4 SETTLEMENT CONSIDERATIONS 4.4.1 Settlement Within the 3-Acre Project Site II I Grade increases are expected across the 3-Acre Park site as part of the proposed development. The loads associated with these grade increases and the proposed structures are expected to induce consolidation settlement of the underlying compressible soils. Further discussion associated with anticipated settlements is provided below. Settlement Mechanism All soils, whether cohesionless or cohesive, will experience settlements immediately after application of loads. Whether or not the settlements will continue with time after the application Final Geotechnical Report-3-Acre Park.docx 11 HWA GeoSciences Inc.