Direct by Design
Deep CSO interceptor progresses in Alexandria Jonathan Rowland, TunnelTalk
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Underground in Action U
Fig 1. Deep infrastructure to capture CSO in Alexandria • a 37ft x 140ft (11.2m x 42.7m) drop shaft at outfall location 002 • a 35ft x 130ft (10.7m x 39.6m) retrieval/drop shaft at outfall location 001 The 8.75in (222mm) thick steel fibre-reinforced precast concrete lining will be installed in 4ft 6in (1.4m) wide rings of four rhombus-shaped segments plus a key and counter key. The 2ton segments will be manufactured by Bay State Precast at its facility in Brandywine, Maryland, and brought approximately 20 miles to the launch site by truck for transport to the TBM by rubber-tyred vehicles. After excavation, the larger shaft will be fit-out as a permanent pumping station to dewater the tunnel and pump flows from the interceptor, while the smaller shaft will serve as a screening facility. Captured CSO flows will be treated through AlexRenew’s advanced wastewater treatment plant, with treated flows then discharged to the nearby Hunting Creek, a tributary of the Potomac River. n
PROJECT PROGRESS
onstruction of a new CSO storage and conveyance tunnel is underway in Alexandria, Virginia, following an award to design-build contractor Traylor-Shea JV in November 2020. The project aims to capture over 120 million gallons of combined sewer overflows annually from four existing outfalls that currently discharge to the Potomac River and its tributaries. Excavation of the main 2.15 mile (3.47km) drive by a Herrenknecht EPBM is expected to begin in August 2022. The overall project requires completion by 1 July 2025. Notice to proceed on the US$454.4 million contract was provided to Traylor-Shea with design partners Jacobs and Corman in December 2020. Funding is largely provided through long-term debt incurred by project owner, AlexRenew. To minimise sewer rate rises within the aggressive schedule, AlexRenew secured lowinterest loans from the Federal Environmental Protection Agency and the Virginia Clean Water Revolving Loan Fund. Additionally, the project secured $100 million in grants from the Virginia General Assembly, $50 million of which is matching funds from the American Recue Plan Act. Running at depths of 110ft-160ft (33.5m-48.7m), the 12ft i.d. (3.4m) main alignment will run east from the launch shaft, around the southern edge of historic old town Alexandria, before driving north under the Potomac River (Fig 1). A second 6ft diameter x 2500ft (1.8m x 700m) interceptor will be constructed as part of the project, largely within the alignment of an existing sewer, with bypass pumping allowing flow to continue as the new open cut infrastructure is built using a combination of pressed sheet and driver sheet piles. For the main drive, the design-build contractor proposes to use a new 14.7ft (4.5m) diameter Herrenknecht EPBM able to operate at up to 4.5 bar pressure and featuring hyperbaric airlocks for cutterhead maintenance. Geological conditions along the route mostly comprise Potomac Clay, with a 1000ft (305m) stretch through alluvium (Fig 2). A purchase order for the machine was executed in May 2021 with an expected arrival in Alexandria by May 2022. The EPBM will be launched from two shafts, in a figure-ofeight arrangement, comprising a 65ft diameter x 160ft deep (20m x 48.8m) shaft, which will be used to install the trailing gear, and a 35ft x 160ft (10.7m x 48.8m), which will be used as the launch shaft and house a conveyor for muck removal. Shaft excavation will be supported by slurry walls and finished with an inner cast-inplace concrete lining. Two additional deep shafts will be constructed as part of the project (Fig 1):
References
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Alexandria CSO
TBM drive through clay and alluvium
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