3535 RIVERFRONT BLVD Geotech Report 2023-02-22

September 24, 2018 HWA Project No. 2015-061-21 differential settlement has occurred. To accommodate differential settlement, we recommend that expansion joints be installed in long linear concrete structures such as seating benches. 4.6 STORMWATER MANAGEMENT It is our understanding that onsite infiltration of stormwater generated as part of this project may be desirable. However, the explorations conducted across the site indicated that the site is underlain by layers of fine-grained alluvium and organic silt deposits that are known to have a relatively low permeability; therefore, the on-site soils are considered not suitable for stormwater infiltration. Furthermore, discharging of stormwater over the existing riverbank, without bank stabilization, could increase slope instability and erosion. To limit effects on the riverbank we recommend that stormwater be routed to a dispersion faciality to the south of the project site. This dispersion facility should be located at least 100 feet from the bank of the river with heavily vegetated flow path between the dispersion facility and the riverbank. 4.7 IMPROVEMENTS FOR CRANE AND VIEWING PLATFORM 4.7.1 Crane Foundation Rehabilitation of the crane structure will likely require removal of the existing structure and preparation of a new foundation system. We recommend driven, closed-end, steel pipe piles for support of the improved or new crane foundation. At this site, closed-end pipe piles can be driven to achieve ultimate capacities approaching the structural capacity of the steel section. For preliminary design purposes, we suggest 16-inch to 18-inch diameter, 1/2-inch wall, steel pipe sections, which we anticipate will penetrate to the order of 80 to 90 feet below existing grades to develop the ultimate capacities. From our experience with other piles on the Everett Riverfront site, these piles will be capable of developing nominal (ultimate) capacities of 400 to 600 kips. For Load and Resistance Factor Design (LRFD), resistance factors (0) of 0.65, 1.0, and 1.0 may be used for strength limit state, service limit state, and extreme limit state, respectively. Note that piles will be required to be driven to at least 60 feet bgs to penetrate through potentially liquefiable materials, and achieve sufficient resist downdrag qe support to es st dow d ag and structural loading conditions. An advantage of closed-end pipe piles is they can be readily inspected immediately after driving to verify their subsurface alignment and that no damage has occurred within the shaft or at the pile tip. If additional structural stiffness is needed, closed-end steel pipe piles can be filled with a reinforcing cage (or center bar) and structural concrete. Downdrag Loading on Vertical Piles Downdrag loading occurs when soil settlement relative to pile settlement is sufficient enough to mobilize side friction in the downward direction(negative side resistance). Very small relative downward movements between the soil and the pile will fully mobilize negative side resistance or downdrag. Hence, downdrag loading must be considered in design of the crane piles due to Final Geotechnical Report-3-Acre Park.docx 19 HWA GeoSciences Inc.