The work and the technique: Lake Mead Intake Hydraulic Tunnel, USA

In March 2008 Webuild began work on the new purification plant which involved the construction of a hydraulic tunnel and an Intake to be installed on the bottom of Lake Mead at a depth of 262.128 metres.

The two existing Intakes, built on the side of the artificial lake, the first one at 320 ASL, would soon have been unusable due to the continuous reduction of the level of water that in 2014 reached 329 metres, the lowest in its history.

Making use of explosives, a housing cavity was created in the bottom of the lake, the base of which was leveled with underwater concrete casting using the contractor process. The new Intake was therefore installed: a combined steel and concrete structure, 29 metres in height with an internal diameter of 9.15 metres and a weight of 1,250 tons. A further jet of concrete allowed to fill the hole and seal the entire structure.

Concomitantly, a 185-m-deep shaft was excavated on the lake shore. The connection between the shaft and the new Intake in the center of the reservoir was the project’s real technological challenge: a tunnel measuring 4.6 km in length and an excavation diameter of 7.22 m running under the bed of one of the largest man-made lakes in the United States. 
The tunnel was built by advancing a special TBM capable of working at a hydrostatic pressure that reached a peak of 14 bar, far higher than the traditional average (1/3 bar). 

Despite the geological complexity of the terrain and a sustainable margin of error of only 30 cm in in the alignment between the tunnel and the Intake, the connection was successfully completed in December 2014, setting a world engineering record.