The work and the technique: Portland West Side CSO Project, Oregon, USA

The Portland West Side CSO project, commissioned by the City of Portland's Bureau of Environmental Services, involved the renovation and expansion of existing infrastructure with new tunnels and conduits
Work on the project, entrusted to a joint venture between Impregilo - now Webuild Group - and Healy, began in 2002.

The excavations, which came up against major critical geological issues, were carried out using two Multi-Mode hydroshield TBMs, marking their first ever use in the United States.
These TBMs - which are amongst the most innovative models available, used in the event of major changes in the geology along the route of the tunnel - were customised specifically for this project and ultimately proved to be fundamental to its successful completion. Both of the excavators started digging from the Nicolai Shaft: the first moving northwards, under the bed of the Willamette River at a depth of 36 metres, heading towards the Swan Island pumping station, and the second heading south towards Clay Shaft.

In order to construct the network of secondary conduits, which spans a total of 3.4km, the team used a micro-TBM with an adaptable excavation diameter of between 2.25 and 2.75m. Work was completed in September 2006 and the project was fully operational as of that December.