The Vegas Tunnel Constructors joint venture carried out the push out and lowering of the 30.5m tall, 9.7m diameter intake structure over the course of three days. The 1,250t intake, which was constructed on an assembly of barges and suspended in a ‘moon pool’ by four 500 Mton strand jacks, was floated from the marine staging area out into position over one of the deepest locations in Lake Mead.
Lake Mead is the largest manmade reservoir in the US and was created by the construction of Hoover Dam in the 1930s. Prolonged drought conditions in the US Southwest have strained the lake and reduced its level by more than 30m in the last eight years, down to a low of 45% capacity.
Arup developed the design concept for the intake structure to reduce risk, cost and schedule. The marine engineering division designed the intake in two parts: a lower chamber constructed of reinforced concrete and a riser section out of stainless steel. The design used the natural buoyancy of the structure to decrease the overall barge and suspension loads.
The project’s TBM launched in early December and has completed 55 rings or 95m. It is now been stopped to lower and assemble the last of the machine’s 15 gantries. During this period, probe drilling and geological exploration works have been completed, with restart of the TBM planned for the end of February.
Now that the intake has been lowered into place, preparation is under way for the 9,200m3 tremie concrete pour to lock the intake structure within the excavation. The continuous pour over seven to ten days will use four barges with eight concrete trucks apiece.
Currently, docking of the TBM with the intake is scheduled for October 2013 with final project completion in May 2014.
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