IMPREGILO has been awarded a contract to build a hydraulic tunnel in Abu Dhabi as part of the third and final lot of the Strategic Tunnel Enhancement Programme (STEP) programme.

Under the terms of the Dh734.5-million ($200-million) contract, awarded by the Abu Dhabi Sewerage Services Company (ADSSC), Impregilo will construct an additional 10 km of the 40-km hydraulic tunnel, with an excavated section of approximately 7 m.

Using gravity, the tunnel will collect waste water and convey it to a treatment plant in Al Wathba. The company is already building the first 15 km of the tunnel under a $243-million contract it won in September 2009. In addition to the tunnel, the project involves construction of four access shafts to depths of between 60 m and 80 m, as well as the links to the main tunnel.

The work will be completed in just over three years.

The project presents significant technical and organisational complexities. It involves simultaneous use of two earth pressure balance tunnel-boring machines for pressures up to eight bar, to bore and simultaneously line the tunnel with prefabricated concrete elements.

This is the first time mechanised boring technology has been used in Abu Dhabi. Both the hydraulic tunnel and the access shafts will be lined with a special membrane to ensure the concrete structures are fully protected from the aggressive environment, for a minimum period of 100 years.