A joint venture led by Lane, a unit of Italy’s Webuild Group, has won a US$1bn (£743M) contract to construct a deep tunnel beneath Pittsburgh, Pennsylvania, intended to reduce sewage overflows into the city’s three rivers.

The Ohio River Tunnel is the first major project of the Allegheny County Sanitary Authority’s (Alcosan’s) regional tunnel system, part of its wider Clean Water Plan to curb combined sewer overflows (CSOs) that occur during heavy rainfall.

CSOs are a long-standing problem in older US cities where stormwater and sanitary sewers share the same pipes. During intense storms the system can be overwhelmed, leading to untreated discharges into waterways.

Alcosan’s tunnel system will aim to store and convey those excess flows for treatment when capacity allows, a strategy already used in other US cities such as Boston and Washington, D.C.

The contract, announced on Thursday, 3 June, was awarded to a 50:50 joint venture (JV) between Lane and Pennsylvania-based Brayman Construction Corporation.

Engineered as an “underground river”, the scheme will capture excess stormwater and sewage flows, convey them into a deep storage tunnel and then pump them to an upgraded treatment plant, preventing direct discharges into the Allegheny, Monongahela and Ohio rivers.

The tunnel will sit roughly 36m to 45m beneath ground level.

The project comprises roughly 8km of tunnels and multiple shafts. The main hydraulic tunnel will be more than 5m in diameter and extend for approximately 6km. It will be excavated with a tunnel-boring machine (TBM).

Two smaller connectors, the Chartiers Creek Tunnel and the Saw Mill Run Tunnel, will be roughly 4m in diameter and about 1.3km and 500m long respectively.

The contract also includes surface works such as flow regulators, two technical buildings and a new river outfall, which the JV said will be integrated with a minimal urban footprint.

Alcosan’s Clean Water Plan calls for a series of deep tunnels, upgraded pumping and treatment facilities. The Ohio River Tunnel is seen as the first of three planned major tunnels in the programme where the others will serve the Allegheny and Monongahela rivers.

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