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Peanuts and Tunnel Stuffing – The Brightwater Conveyance System East Contract

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Author D N Adams, J J Johnson, M E Trim and W Cranston
ID P200803031

Description

The Brightwater conveyance system is located near Seattle, Washington in the
north-western United States. The East Contract portion of the system consists of
a 4.3 km long, 5.9 m diameter tunnel in soft ground and two large shafts
constructed in challenging soil conditions. The two shafts are situated
side-by-side in an urban valley. One will house an influent structure to direct
incoming sewage and outgoing treated effluent, while the other will house a 645
megalitre per day (ML/d) influent pump station.


The tunnel contains four pipelines: two force mains to convey untreated
wastewater eastward from the new pump station to a new treatment plant, one
gravity-pressure pipeline to convey treated effluent westward to a new outfall
in Puget Sound, and one high-pressure pipeline to convey reclaimed water from
the treatment plant to downstream distribution points. The shafts and the tunnel
presented many design challenges.


Groundwater is near the surface in the area surrounding the shafts, and
compressible peat and clay soils preclude dewatering during construction. These
and other challenges were encountered during design of the pump station shaft
and multiple-pipe tunnel. The challenges were addressed by a design that
includes a dual intersecting circular cell shaft (referred to as ‘the peanut’)
and a single tunnel into which stacked pipes will be ‘stuffed’. This paper
discusses the design approach and design details for both
elements.