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Location: Portland, Oregon
Owner: City of Portland Bureau of Environmental Services
Client: Parsons Brinckerhoff
00The Westside CSO tunnel is a deep, soft
ground tunnel 14 feet in diameter that extends along the west side of
the Willamette River in downtown Portland. At the north end of the tunnel’s
alignment, it crosses beneath the Willamette and terminates at the Swan
Island Pump Station. Due to the risks associated with the ground conditions,
depth, and size of the shafts located on either side of the river, the
City of Portland Bureau of Environmental Services, through a subconsultant
agreement with the prime design firm, retained Jacobs Associates to provide
preliminary and final (detail) design of the two shafts on each side of
the river crossing. The two shafts, both required to be greater than 150-feet
deep, are located in saturated, loose to very dense glacial deposits of
granular soil with a near-surface groundwater table. The pump station
shaft, which requires an inside diameter of 144 feet, is considered the
largest shaft ever constructed at this depth in such difficult ground
conditions. The Nicolai Shaft, across the river from the pump station
shaft, is required to have an inside diameter of 60 feet. Due to the nature
of the soils and the dimensions required, these shafts are being designed
as diaphragm slurry walls with jet grout cut-off walls.
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