Ohio River Tunnel: delivering ALCOSAN's first CSO tunnel in Pittsburgh

Transforming Pittsburgh's water infrastructure

The Ohio River Tunnel is the first of three CSO interception tunnels implemented under Allegheny County Sanitary Authority’s (ALCOSAN) Clean Water Plan. This initiative aims to reduce CSOs by seven billion gallons annually and eliminate sanitary sewage overflows for 83 Allegheny County communities, including the City of Pittsburgh.

The completed tunnel will stretch along the north shore of the Ohio River from the 16th Street Bridge to the expanded Northside wastewater treatment plant with a river crossing to McKees Rocks Borough and another to the West End of Pittsburgh. Delivering the project in a dense urban environment requires careful coordination across multiple neighborhoods and constrained sites while maintaining the permanent system 150 feet (46 m) below ground, minimizing impacts on the community.

As the prime consultant for all phases of the Ohio River Tunnel project, we drew on a global team of multidisciplinary experts to deliver ALCOSAN’s first regional wet weather tunnel design. Our approach integrated lessons from similar major tunnel programs such as Thames Tideway in the UK and Project Clean Lake in Cleveland, OH. The work spanned eight non-contiguous sites across multiple jurisdictions, each presenting unique technical, regulatory, property and community challenges. Given the constraints of an urban setting, we coordinated extensively with stakeholders, utilities, property owners and construction planners to facilitate successful delivery.

Tunnel design

The Ohio River Tunnel consists of 4.9 miles (7.9 km) of segmentally lined deep rock tunnels, with diameters ranging from 14 to 18 feet (4.3 to 5.5 m), as well as smaller dewater and flow connector tunnels. Tunnel boring machines (TBMs) with pressurized faces were selected for their ability to manage excessive groundwater and high hydrostatic pressures, especially during the subaqueous excavation under the Ohio River. The design addressed the challenges of a tight urban environment, including adjacent transportation infrastructure, historic structures, utilities and limited staging areas. Key components included:

• Eight flow drop and access shafts between 25 and 68 feet (8 and 21 m) in diameter and 430 to 568 feet (131 to 173 m) deep
• Six large flow regulators
• Two tunnel relief outfalls
• Two reconstructed outfalls
• Two flow control buildings

Diaphragm/slurry and secant pile walls will be used to construct the deep shafts while secant pile and steel sheeting will be used to construct the near-surface facility structures. We also designed temporary cofferdams to facilitate the construction of the two new outfalls and the reconstruction of the two existing outfalls.

In addition to providing surge relief and future tunnel access, the TBM launch shaft will also serve as the launch point for the future Allegheny River Tunnel and retrieval shaft for the Monongahela River Tunnel. We supported planning for significant electrical demand during TBM operations, helping align utility coordination, power upgrades and construction sequencing. We also coordinated logistics to account for traffic, emergency access and overlapping infrastructure projects.

The final design included computational fluid dynamics modeling and dynamic surge analysis for the tunnels, shafts and near-surface facilities. The Ohio River Tunnel is designed for a conveyance capacity of up to 250 million gallons per day. All near-surface facility sites are within Federal Emergency Management Agency (FEMA) defined floodways or 100-year floodplains, requiring flood protection measures to be taken during construction as well as in the design of the permanent structures.

Advanced geotechnical modeling

We performed a comprehensive multi-phase geotechnical investigation program in support of the Ohio River Tunnel final design and to complement the previous explorations that had been performed under ALCOSAN's Preliminary Planning Program. This program included desktop study work; survey of existing wells and field rock outcrop mapping; borehole drilling and sampling on land and in the Ohio River; field testing, including geophysical and packer testing; installation of observation wells and vibrating wire piezometers; and soil and rock laboratory testing.

We used a digital field tracking system during field work to standardize the quality control process and enhance efficiency through real-time data collection and monitoring. Using advanced geotechnical modeling software, we visualized the subsurface conditions, reducing uncertainty and supporting design coordination.

Working with stakeholders

The permitting process involved over 100 permit applications across federal, state and local agencies. Over four years, we interfaced with municipal, county, state and federal regulatory entities, as well as private power, gas transmission, and telecommunication companies, navigating varied compliance requirements and overlapping permit conditions.

We supported negotiations for surface, subsurface and subterranean rights for access, staging and long-term operations while managing utility relocations and regulator connections. Design refinements helped to reduce conflicts, minimizing disruptions to residents and critical services.

Our team delivered four early action construction packages for the demolition of existing structures and initial site preparation and handled responsible management of excavated materials. We maintained proactive community communication and worked closely with ALCOSAN and the City of Pittsburgh to manage utility disconnections, traffic and site access.

A benchmark for future CSO tunnels

The Ohio River Tunnel sets the standard for the Clean Water Plan’s next two CSO tunnels. Our work  demonstrates how major wet weather infrastructure can be advanced in a dense urban environment through disciplined coordination and excellent design. Our team remains committed to supporting ALCOSAN throughout construction, delivering engineering expertise and guidance at every stage.

Construction begins in 2026 and continues through 2032, followed by the Allegheny River Tunnel (with construction starting in 2029) and the Monongahela River Tunnel (with construction starting in 2031). Together, this three-tunnel system aims to protect sensitive riverfront areas, eliminate CSOs at various outfalls and improve water quality for Pittsburgh and surrounding communities.